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A pair of mechanically driven small-scale turbocompressors running on gas lubricated bearings have been designed for a two-stage heat pump application functioning under variable operating conditions. Novelty in the present two-stage heat pump system lies in the application of oil-free turbocompressor technology and the introduction of unused sec-ondary heat from various sources. Managing the operational deviations and the secondary heat during off-design heat pump operation is challenging for the turbocompressors. The turbocompressors can potentially exceed their operating range defined by the surge and choke margins, and the maximum rotational speed limit set by the structural and rotor-dynamic considerations. A wide operating range is, therefore, a prerequisite design condition for the turbocompressors. The present paper will guide the readers through different stages of the design process of such turbocompressors sub-jected to various operational and design constraints. Moreover, a stochastic evaluation on the influence of variable operating conditions on the heat pump off-design performance will be detailed.

Domestic heat pumps have been identified as a key-technology to decrease the energy consumption of the domestic sector, which represents 29% of the world final energy consumption. Space & water heating represent more than two third of the domestic sector energy consumption in many countries. Next generation domestic heat pumps powered by oil-free high-speed radial compressors are expected to (a) be more efficient at rating operating points and in partial loads, (b) to match better the energy demand characteristics, (c) to be more compact and lighter, (d) more silent, and (e) to need lower refrigerant charges. As a first step towards the development of such advanced heat pumps, an oil-free twin-stage Air/Water domestic heat pump prototype powered by an oil-free high-speed twin-stage radial compressor has been developed and tested. The heat pump layout corresponds to a two-stage heat pump cycle with an open economizer. The two main functions of the economizer are (a) to separate the liquid from the gas before the second stage compressor inlet and offer a proper mixing process, and (b) to store the liquid refrigerant not located in the heat exchangers at a given time. This key-component has been coupled with the compression unit and improved through incremental experimental steps. The heat pump driven by an oil-free twin-stage radial compressor has been successfully tested at the rating operating point A-7/W35. The performance reached with the prototype is considered very promising and constitutes a breakthrough in the domain. This article describes the experimental setup, the Operating Point (OP) A-7/W35, the methodology applied for data analysis, followed by the results. The issues limiting the performance of the prototype are also identified and briefly documented.

This paper investigates the waste heat recovery potential of internal combustion engines, using organic Rankine cycles running on small-scale radial turbomachinery. ORC are promising candidates for low-grade thermal sources and the use of dynamic expanders yields very compact systems, which is advantageous for automotive applications. As engine coolant and exhaust gases are the major available heat sources, different cycle configurations and working fluids have been investigated to capture them, in both urban and highway car operation. Pareto fronts showing the compromise between net power output and total heat exchange area have been identified for a set of cycle’s variables including turbine inlet conditions and heat exchanger pinches. A preliminary optimization, including only R-1234yf working fluid, shows that a single-source regenerative cycle harvesting the high temperature exhaust gas stream performs averagely better than coolant-driven and dual-source cycles. A more in-depth optimization including eight working fluids as well as aerodynamic and conceptual limitations related to radial turbomachinery and automotive design constraints, finally shows that an ICE exhaust heat recovery ORC could improve the first law efficiency of the driving system by up to 10% when implemented with fluid R-1233zd.

High temperature heat pumps with heat sink temperatures of 100 to 160 °C have great application potential in the food, paper, and chemical industries for heat recovery and process heat generation in drying, sterilization, and evaporation processes. The present study compares the theoretical performance of the low GWP refrigerants R1336mzz(Z), R1224yd(Z), R1234ze(Z), and R1233zd(E) in common one- and two-stage heat pump cycles (IHX, economizer, flash tank, cascade) at different temperature levels. The thermodynamic simulations reveal that, depending on the refrigerant and cycle, a trade-off between system complexity (e.g. control, number of components), efficiency, and volumetric heating capacity (VHC) has to be found. Optimal COPs are reached depending on the refrigerant and its heat outlet temperature. R1336mzz(Z) is suggested as the closest drop-in replacement for R365mfc, while R1224yd(Z), R1234ze(Z), and R1233zd(E) are closer to R245fa in terms of COP and VHC. A two-stage cascade with IHX and R1336mzz(Z) in both stages achieved the highest COP for high temperature lifts.

This paper offers an extensive review of publications dealing with the modeling, the design and the experimental investigation of grooved dynamic gas lubricated bearings. Recent years have witnessed a rise in small-scale and high-speed turbomachinery applications. Besides the well-known gas foil bearings, grooved bearings offer attractive advantages, which unveil their potential in particular at small scale due to the structural simplicity as well as satisfying predictability. This paper starts with a general background of the application of gas lubricated bearings and introduces and compares the different gas bearing topologies. Further, the state-of-the-art modeling of grooved gas lubricated bearings is introduced in a systematic way assessing the advantages and inconveniences of two major approaches, the Narrow Groove Theory (NGT) and direct discretization method. Since NGT method is an elegant and efficient approach to model the complex effects of periodic grooves, a critical section is dedicated to the Narrow Groove Theory. In a second phase different models to include additional physical phenomena such as real gas lubrication, rarefaction or turbulence effects are reviewed. The paper concludes with a critical assessment of the state-of-the-art and indicates potential fields of research that would allow to shed more light into the understanding of these bearings, as well as with some thoughts on the integrated design methodologies of gas bearing supported rotors.

Emerging energy-efficient, oil-free radial centrifugal compressors seem promising for small to mid-capacity air conditioning and refrigeration applications. Compressor design must balance trade-offs among impeller diameter, bearing type, and refrigerant selection. This paper guides refrigerant selection for single-stage systems delivering 10-250 kWtherm of cooling at a representative evaporating temperature of 0°C. The viable oil-free bearing types, the impeller diameter for maximum compressor efficiency and the resulting system efficiency were determined for a set of representative refrigerants with normal boiling temperatures ranging from -51.7°C to +49°C. For residential scale cooling systems, low pressure refrigerants enable high energy efficiency with small size, high-speed centrifugal compressors and vapor-lubricated bearings. For commercial scale cooling systems, medium pressure refrigerants enable high energy efficiency with reasonable size, high-speed centrifugal compressors and vapor-lubricated bearings. For commercial scale cooling systems, low pressure refrigerants enable higher energy efficiency with larger size, high-speed centrifugal compressors and magnetic bearings compared to medium pressure refrigerants with vapor-lubricated bearings. The preferred refrigerant enables combinations of system first and operating costs that are attractive for the intended markets, while meeting equipment reliability, operability and other constraints.

ORC driven Heat Pumps (HP-ORC) offer a promising technology of thermally driven heat pumps (TDHPs) for domestic applications. Recently, a 40 kW HP-ORC unit based on gas supported turbomachinery has been investigated in which the ORC radial turbine drives the HP centrifugal compressor, offering an oil free and high power density solution. The radial compressor and the radial turbine have tip diameters of the order of 20mm and the system has been tested at rotor speeds in excess of 200 krpm with shaft powers up to 2.4 kW, running with R134a. The compressor and the turbine were operated at pressure ratios of up to 2.8 and 4.4, while reaching thermal COP of the order of 1.5 and isentropic compressor and turbine efficiencies in excess of 70%, thus validating the HP-ORC concept based on small-scale turbomachinery. However, performance limitations were encountered due to suboptimal heat exchanger, working fluid and turbomachinery design. This article presents a full thermo-economic optimization of the HP-ORC enabling the identification of the best trade-off Investment Cost/ COP. Several working fluids have then been compared, revealing that a compromise is to be found between best cost/efficiency trade-off, highest performances achievable and technical feasibility. It also showed a thermal COP improvement potential up to 30% compared to the existing experimental system. Finally, the thermo-economic results of the HP-ORC have been compared with other typical TDHPs, such as absorption heat pump.

Motivated by the use of aerodynamic bearings lubricated with high-pressure gases in energy conversion cycles, the Reynolds equation is adapted in order to include effects of real-gas and turbulence. Three geometries (Rayleigh-step slider, plain and herringbone-grooved journal bearing) serve to investigate real-gas effects on the static and dynamic properties with a wide variety of lubricants and nondimensional operating conditions. Computational results show a depreciation of the load capacity of journal bearings, with cases reaching a reduction of 50% with unequally affected force components. Stability can be affected both positively and negatively. Some stability losses reach nearly 100%, while improvements of several orders of magnitude with the grooved bearing are reported. Results are fluid-independent for similar reduced pressure and temperature.

Small-scale, oil-free radial compressors on gas lubricated bearings represent a promising alternative to state-of-the-art compressor technology for driving domestic sized heat pumps. The inherent characteristics of turbocompressors manage to fit the heat pump load while the oil-freeness allows the implementation of advanced heat exchanger technology and the deployment of advanced multi-stage cycles, both validated means to significantly enhance performance and efficiency. The paper presents experimental investigation and results of both electrically and thermally driven 20mm 2kW radial compressor rotating at rotor speeds of up to 210krpm. The compressor stage has been tested in R134a, reaching isentropic efficiencies above 70%. Dynamic, gas lubricated bearings are supporting the high-speed rotors that are lubricated with vapor phase working fluid, thus offering an oil-free and hermetic solution. Insights into the experimental investigation of a 6kW two-stage radial compressor for driving retrofit heat pumps with high temperature lifts are introduced as well. Besides comparing experimental data, the paper also sheds light into critical design aspects related to reduced scale machines and compares them with regards to their impact on efficiency. Identified key issues are (1) aerodynamic challenges related to the severe downscaling of radial compressors such as tip clearance, surface roughness and non-adiabatic operation (2) stable gas bearing technology and (3) challenges related to high power density designs.

This paper presents measurements of the motion amplitudes from flexibly supported Herringbone Grooved Journal Bearing bushings. Measurements taken in four locations on the flexibly supported bearing bushing enable the distinction between the radial and the tilting motion, which allowed to identify an unstable bearing bushing tilting mode. The bushing tilting motion instability can be attributed to the cross-coupled tilting impedance of the gas bearing and its occurrence is characterized by the appearance of a sub-synchronous vibration at the speed of instability onset. The comparison between experimentally obtained data and the prediction of a two degrees of freedom dynamic model that includes the flexible support characteristics, the bushing transverse inertia as well as the gas bearing tilting impedance suggests an excellent agreement on both the Eigenfrequency and the rotor speed of instability onset. Furthermore, the effect of the flexible support tilting stiffness and bearing bushing transverse moment of inertia on the critical bushing tilting speed and onset of tilting instability has been investigated, which led to a design guideline to avoid the onset of an unstable bush tilting mode. In addition, experimental data gathered after the introduction of damping in the flexible support of the bearing bushing suggests a significant increase of the onset speed of the unstable bearing bushing tilting mode.

Hybrid absorption heat pumps (Osenbrück cycle and Hybrid wet compression cycle) are known for high temperature lifts, low-pressure ratios and wide capacity control. They are well suited for processes with the occurrence of significant heat sink and heat source temperature glides. Environmentally friendly working fluids such as ammonia/water mixtures offer advantages for system performance and play an important role for the future of heat pump applications. However, the main challenges of these cycles is the requirement of oil-free compressors suitable for high temperature lifts that may cope with two phase. In this paper a theoretical integration of a novel oil-free co-rotating scroll compressor into a Hybrid absorption heat pump (for cooling and heating) is presented. This compressor has been experimentally characterized and modeled with air and varying rates of water ingestion; the main advantages are that it can operate in both dry and wet conditions without additional internal lubrication. Also, the absence of lubricant leads the oil-free compressor to be used in refrigeration heat pumps. Maximum COP values up to 3.3 for cooling mode and 4.3 for heating mode are determined when the cycle operates with sink temperatures of 45°C/80°C and chilled water temperatures of 20°C/80°C, respectively. Maximum temperature lifts about 66°C for cooling mode and 156°C for heating mode are determined.

In the endeavor towards distributed power systems, not seasonal-dependent micro-power generation technologies are expected to integrate the energy scenario in the years to come. In this context, the Organic Rankine Cycle (ORC) in the (1e10) KWe scale has chronically lacked a suitable expansion device, hindering its market attractiveness. As scroll expanders have been pointed out as strong potential candidates, performance correlations and pre-design maps based on a review and analysis of published experimental data are presented. A dimensionless approach based on the traditional Ns, Ds dimen-sionless numbers stemming from turbomachinery has been chosen for greater generality. In addition, the lubricating oil mass fraction effect on the scroll expander performance has been included. The generated maps contribute to accelerating the pre-design phases at the system and component level with beneficial effects for the overall development process. Basic geometry and size characteristics are considered as well, acknowledging their importance in micro-power embedded applications; these considerations are illustrated in a passenger car waste-heat recovery case study. Findings suggest that optimized scroll expanders may potentially reach very interesting nominal electric isentropic efficiencies (up to 80% for an oil lubricated scroll expander).

Gas foil bearings (GFBs) are a key enabling technology for high-speed turbomachinery. The manufacturing of GFBs relies mainly on sheet metal forming techniques in order to conceive the compliant structure (e.g., bump foil) and the top foil. Such techniques require the development of a special know-how and most importantly, limit the design creativity to what is manufacturable using sheet metal forming. Additive manufacturing (AM) is a disruptive technology in prototyping and fabrication. This paper accesses the feasibility of AM in the fabrication of GFBs using selective laser melting (SLM) technology. A stainless steel GFB is 3D-printed in one piece, including the sleeve, the bump, and top foils. The bearing is assessed geometrically and statically before being tested on a gas bearing test rig, where it supported a ø40 mm rotor (m = 2 kg). The bearing performed similar to a conventional GFB, showing rotordynamically stable and repeatable operation up to 37.5 krpm. Such result highlights the potentials of AM as a viable alternative for foil bearing manufacturing.

In this paper, the influence of tip clearance on the performance of small-scale turbopumps is studied numerically on extensive parameter ranges suitable for organic Rankine cycle applications. A novel and fully parameterized design model is developed and used to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage. Impellers are investigated at different operating conditions, and the accomplished results are analyzed to characterize the performance at design and off-design conditions. The CFD calculations demonstrate that the slip factor is dependent not only on its geometrical parameters, as considered by most correlations, but also on its operating conditions. The slip factor decreased almost linearly as the flow rate increased. In addition, the tip clearance ratio influences the slip factor, and its influence is non-linear. The head rise decreases as the tip clearance ratios increase. However, for radial impellers, a higher head rise was observed for small tip clearance ratios (

A tolerancing method highlighting trade-offs against key design variables of mechanical systems is proposed and applied to herringbone-grooved gas journal bearings. Gas bearings typically suffer from a subsynchronous instability, demanding a very tight tolerance on the bearing clearance and groove depth. Classical optimization techniques look for the most stable design, which does not necessarily lead to most robust design against manufacturing deviations. The proposed method uses a normalized multidimensional lookup table of stability score (critical mass), covering a large design space of gas bearings. It then dimensionalizes the table for a specific rotor–bearing system, highlighting regions of the hyperspace where the system is stable. The hyperspace is sliced into 2D maps and a Monte Carlo method creates windows within the stable domain along the two most critical design variables regarding manufacturing: the bearing clearance and the groove depth. Width and length of the windows represent the manufacturing tolerance allowed for the two parameters to remain stable. A Pareto front of optimum windows in the entire hyperspace is then compiled. It displays the trade-off between the tolerance against deviation in clearance and groove depth, allowing the designer to select a nominal geometry tailored to the available manufacturing methods. A test rotor is analyzed with this method and the effects of pressure, speed, viscosity, radius, mass, and centrifugal growth on manufacturing tolerances are investigated, highlighting that the radius and viscosity have the greatest impact on the robustness.

Potential geometrical deviations in bump foil bearings due to manufacturing uncertainty can have significant effects on both the local stiffness and clearance, and hence, affecting the overall bearing performance. The manufacturing uncertainty of bump type foil bearings was investigated, showing large geometrical deviations, using a developed measurement tool for the formed bump foils. A reduced order foil bearing model was used in a Monte Carlo simulation studying the effect of manufacturing noise on the onset of instability, highlighting the sensitivity of the rotor-bearing system to such manufacturing deviations. It was found that 30% of the simulated cases resulted improvements in stability, the remaining cases underperformed. Attempting to increase the robustness of the bearing, two other compliant structures replacing the classical gen-II bump foils were investigated from a manufacturing perspective. The first is a modified bump type Sinusoidal foil, and the second is the Cantilever beam foil. Consequently, quasi-static load-displacement tests were executed showing deviations in local clearance and stiffness for the classical bump type compliant structure compared to the other designs. It was found that the Cantilever beam foils yield more robustness compared to the bump type foils. Finally, an analytical model for the sequential engagement of the compliant structure is presented and validated with experimental measurements for both bump type and Cantilever structures.

High temperature heat pumps (HTHP) with heat sink temperatures of 100 to 160 °C will increasingly become commercialized in the coming years, especially for industrial drying, sterilization and evaporation processes. In particular, the HFO R1336mzz(Z) and the HCFOs R1233zd(E) & R1224yd(Z) are low GWP replacement refrigerants for R245fa and R365mfc. This study examines the experimental performance of R1336mzz(Z) and R1233zd(E) in a laboratory HTHP with 5 to 10 kW heating capacity. The developed heat pump is single-stage, operates with a variable speed piston compressor, and contains a continuously adjustable internal heat exchanger (IHX) for superheating control. The performance data were measured at 30, 50, and 70 K temperature lift (40 to 80°C heat source, 80 to 150°C heat sink). At W60/W110, the experimental results with R1233zd(E) showed a COP of 2.8 (basic cycle) and 3.1 with IHX integration (+15%). For R1336mzz(Z) a COP of 2.4 and 3.0 was reached (+24%). By increasing the heat sink temperature glide from 5 to 30 K, a further 15% COP increase was achieved.

The design process followed by mechanical engineers is undergoing significant changes with the development of rapid prototyping techniques [1] and of artificial intelligence (A.I.) supported tools [2]. The job market is highly affected by these changes. However, Deininger et al. [3] show that novice designers do only rarely take advantage of the new tools and if they do, their use is unintentional. There is a clear need for mechanical engineers to be trained to use A.I.-based tools. Created in 2016-2017, the ME-403 course in “Applied mechanical design” seeks to help students develop both design and project management skills though a highly applied project-based format. In 2018-2019, the course has been modified to include the use of A.I.-based tools: students first experience the traditional design process and then the A.I.-based process with the same set of requirements. This contribution presents the results of a study based on the data collected during normal teaching and learning activities of this course. Indeed, students submit several group reports, which include design process information, design data (chosen components and dimensions) as well as project management related information. In addition, students answer a “Project Management Skills” questionnaire at the beginning and at the end of the course to get feedback on their skills in five project-management domains (planning, risk-assessment, ethical sensitivity, communication and interprofessional competence). Because the course is currently running, we cannot summarize the findings yet, but we intend to report on: • The evolution of students’ project-management skills, using the pre-post responses to the questionnaire and project-management information extracted from students reports; • The evolution of students’ design process and performance between the traditional and A.I.-based parts of the project, using design information extracted from students reports. Finally, we will describe how these results will inform the teaching of the course for the next occurrence.

The hydraulic design, computational analysis, and experimental investigations of a high-speed small-scale turbopump for mobile waste heat recovery applications based on organic Rankine cycle systems are presented in this paper. Such applications demand high-pressure rise, lightweight, and compact pumping systems with simple construction. The investigated turbopump features an unshrouded 37.75 mm tip diameter single-stage centrifugal pump equipped with eight radial blades, eight splitters blades, and a rectangular axisymmetric volute. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar at a designed rotational speed of 25,000 rpm. Due to uncertainties observed in employing state-of-the-art 1-d methods that are valid for much larger machines, computational fluid dynamics is utilized to obtain a design meeting the specifications. The pump’s performance is evaluated experimentally at different rotational speeds, mass flow rates, and impeller tip clearances. The excellent agreement between experimental data and predictions from computational fluid dynamics validates the design methodology and computational results. The turbopump’s characteristics are then utilized to estimate the possible performance improvement of a target organic Rankine cycle using the novel turbopump instead of a commercial multi-stage centrifugal pump. The comparison suggests that the novel turbopump increases the efficiency of the target organic Rankine cycle by 0.3 % points and decreases its back-work ratio by nearly 50 %. The novel turbopump is approximately ten times more compact compared to commercial systems.

Multi-objective optimization tools are becoming increasingly popu-lar in mechanical engineering and allow decision-makers to better understand the inevitable trade-offs. Mechanical design problems can however combine properties that make the use of optimization more complex: (i) expensive cost functions; (ii) discrete or step-like behavior of the cost functions; and (iii) non-linear constraints. The latter in particular has a great impact on the convergence and the diversity of the obtained Pareto front. In this paper, we present five bi-objective mechanical design optimization problems with various levels of constraint complexity. They are used to rigorously benchmark two common constraint handling strategies (constrained-dominance and penalty function). The results suggest that both strategies have similar performance, and that as constraints become more intricate, convergence to the best-known Pareto front is not guaranteed. Indeed, analyzing the evolution of the hypervolume along generations reveals that the optimizer can get trapped in local optima. A detailed analysis of the obtained Pareto fronts for the proposed problems allows us to qualify the effects of the different constraints.

Small-scale oil-free turbocompressors driven by high-speed electric motors and supported on refrigerant vapor bearings represent a promising technology to increase the performance of domestic heat pumps. Gas bearings enable high rotational speeds with low mechanical losses. Furthermore, turbocompressors can reach high compression efficiencies and offer a compact and lightweight design. This paper evaluates the performance of a heat pump using a small-scale radial turbocompressor rotating on gas bearings. The turbocompressor has been optimized for R134a and achieves pressure ratios ion the order of 1.5 to 3.5 at rotational speeds of 160 to 280 krpm with isentropic efficiencies of up to 75%. The impeller diameter is 15.2 mm. A system model of the heat pump has been programmed in the Engineering Equation Solver (EES) software. The model uses effectiveness-NTU models for the heat exchangers and polynomial fit approximations for the non-dimensional compressor map data. The heat pump produces 4.0 kW of 30°C water from 10°C ground heat with a predicted COP of 8.1 and 53.4% 2nd law efficiency. Simulation results, the design of the turbocompressor impeller, as well as the layout of the experimental setup are presented. First experimental measurement results will be expected in the beginning of 2017. The system serves as a proof of concept before stepping forward to a two-stage heat pump cycle with two turbocompressors reaching heat sink temperatures of 55 to 65°C.

The current paper presents the thermo-economic optimization and comparison, through a multi-objective algorithm, between HP-ORC and the competing single effect absorption heat pump (SEAHP), with the aim to find the best trade-off between investment cost and exergetic efficiency. Heat exchanger (HX) cost, which is the dominant cost driver, is predicted on catalogue-based correlations as a function of heat exchanger areas. The dominating HP-ORC working fluid is R-245fa for all operating conditions, except for hot source temperatures of 180 °C, where R-600a is a better candidate. The resulting Pareto frontiers suggest that, for hot source temperatures up to 120 °C the SEAHP and HP-ORC are highly competitive by yielding similar thermo-economic tradeoffs. For higher hot source temperatures, however, the HP-ORC outperforms the SEAHP both in terms of performance and cost while providing a significantly wider performance range (0.3

The combination of multi-stage heat pump cycles with small-scale oil-free turbocompressor technology running on gas bearings could be a promising way to increase performance in domestic and commercial heat pumps. This paper presents a novel two-stage heat pump system with two heat sources at two different temperature levels using two separate turbocompressors rotating on gas bearings optimized for R134a. The system allows integration of unused heat sources, e.g. solar thermal or waste heat, into heat production with a minimal loss of exergy. The cycle comprises an evaporator for the first heat source, a condenser as heat sink, an open economizer with integrated heat exchanger for the second heat source, and a tube-in-tube suction line heat exchanger (SHX) in the high-pressure for superheating and subcooling. The aim of this study is to evaluate theoretically the performance of this heat pump cycle using a system model programmed in the software EES (Engineering Equations Solver). The simulation assumes steady-state, negligible pressure drops and heat losses, and adiabatic expansion processes. The superheating in the evaporator and the SHX is 5°C, and there is no subcooling in the condenser. The heat exchangers are modeled using effectiveness-NTU models. At the design point, the heating capacity of the condenser is set to 6.5 kW and provides hot water of 55°C. The first heat source is brine of 5°C. The second heat source is water of 30°C and has been designed to provide up to 30% of the total condenser heat capacity. The two turbocompressors are designed specifically to meet the heat pump design point. Presently, one-dimensional (1D) compressor maps are used in the heat pump model. Simulation results show that coefficient of performance (COP) improvements of 20% to 30% are achievable, depending on the source temperature levels of the heat pump cycle and the amount of second heat source added to the system. The COP increases with higher source temperatures, higher second heat source capacity, and lower sink temperature. The pressure ratios are defined by the imposed temperature levels. The mass flow rate of the refrigerant in the first stage is mainly determined by the second heat source capacity, and in the second stage by the heat capacity of the condenser. In future work, this novel heat pump concept will be tested experimentally.

In this paper, the influence of tip clearance on the performance of small-scale turbopumps is studied numerically on extensive parameter ranges suitable for organic Rankine cycle applications. A novel and fully parameterized design model is developed and used to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage. Impellers are investigated at different operating conditions and the accomplished results are analyzed to characterize the performance at design and off-design conditions. The CFD calculations demonstrate that the slip factor is dependent not only on its geometrical parameters as considered by most correlations but also on its operating conditions. The slip factor decreased almost linearly as the flow rate increased. In addition, the tip clearance ratio influences the slip factor and its influence is non-linear. The head rise decreases as the tip clearance ratios increased, however, for radial impellers, the higher head rise was observed for small tip clearance ratios.

This paper presents neural network regression models for predicting the nonlinear static and linearized dynamic reaction forces of spiral grooved gas journal bearings. The partial differential equations (PDEs) are sampled, based on a full factorial and randomly spaced parameter set. Feed-forward neural network (FNN) architectures are developed for modeling the PDEs and therefore replacing the time-consuming discrete and iterative solution procedure used to this date. A significant speed-up factor of >103 in computation time is achieved, compared to solving the PDE numerically. Furthermore, the FNN allows for multi-dimensional interpolation, which makes global system optimization easily possible. This is demonstrated by a real-case rotordynamic system optimization. By using the neural network meta-models, a complete rotordynamic system optimization time reduction of factor 300 is achieved.

Small-scale turbomachines in domestic heat pumps reach high efficiency and provide oil-free solutions, which improve heat-exchanger performance and offer major advantages in the design of advanced thermodynamic cycles. An appropriate turbocompressor for domestic air based heat pumps requires the ability to operate on a wide range of inlet pressure, pressure ratios, and mass flows, confronting the designer with the necessity to compromise between range and efficiency. Further, the design of small-scale direct driven turbomachines is a complex and interdisciplinary task. Textbook design procedures propose to split such systems into subcomponents and to design and optimize each element individually. This common procedure, however, tends to neglect the interactions between the different components leading to suboptimal solutions. The author proposes an approach based on the integrated philosophy for designing and optimizing gas bearing supported, direct driven turbocompressors for applications with challenging requirements with regards to operation range and efficiency. Using experimentally validated reduced order models for the different components an integrated model of the compressor is implemented and the optimum system found via multi-objective optimization. It is shown that compared to standard design procedures, the integrated approach yields an increase of the seasonal compressor efficiency of more than 12 points. Further, a design optimization based sensitivity analysis allows to investigate the influence of design constraints determined prior to optimization such as impeller surface roughness, rotor material, and impeller force. A relaxation of these constrains yields additional room for improvement. Reduced impeller force improves efficiency due to a smaller thrust bearing mainly, whereas a lighter rotor material improves rotordynamic performance. A hydraulically smoother impeller surface improves the overall efficiency considerably by reducing aerodynamic losses. A combination of the relaxation of the three design constraints yields an additional improvement of six points compared to the original optimization process. The integrated design and optimization procedure implemented in the case of a complex design problem thus clearly shows its advantages compared to traditional design methods by allowing a truly exhaustive search for optimum solutions throughout the complete design space. It can be used for both design optimization and for design analysis.

The blade tip clearance loss was studied experimentally and numerically for a micro radial fan with a tip diameter of 19:2mm. Its relative blade tip clearance, i.e., the clearance dividedby the blade height of 1:82mm, was adjusted with different shims. The fan characteristics were experimentally determined for an operation at the nominal rotational speed of 168 krpm with hot air (200°C). The total-to-total pressure rise and efficiency increased from 49 mbar to 68 mbar and from 53% to 64%, respectively, by reducing the relative tip clearance from 7.7% to the design value of 2.2%. Single and full passage computational fluid dynamics simulations correlate well with these experimental findings. The widely-used Pfleiderer loss correlation with an empirical coefficient of 2.8 fits the numerical simulation and the experiments within 2 efficiency points. The high sensitivity to the tip clearance loss is a result of the design specific speed of 0.80, the highly-backward curved blades (17°), and possibly the low Reynolds number. The authors suggest three main measures to mitigate the blade tip clearance losses for smallscale fans: (1) utilization of high-precision surfaced-grooved gas-bearings to lower the blade tip clearance, (2) a mid-loaded blade design, and (3) an unloaded fan leading edge to reduce the blade tip clearance vortex in the fan passage.

The organic Rankine cycle (ORC) system is among the technologies applied to exploit waste heat in vehicles. Since common organic fluids have lower latent heat than water, higher mass flow rates are required to convert the same waste heat, considerably increasing the pump size and energy usage. The weight and size of standard pumps can significantly penalize the benefits of installing waste heat recovery on vehicles. Hence, efficient and compact pumps, such as small-scale turbopumps, would be very welcome for mobile applications. Numerical investigations are conducted on extensive parameter ranges to study their effects on the performance of small-scale turbopumps. A fully parameterized tool is developed to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage and the entire pump. Investigations are conducted under different operating conditions, and the accomplished results are analyzed to characterize the performance under design and off-design conditions. First off, the effect of tip clearance in small-scale turbopumps is studied. The slip factor is suggested to decrease linearly with increasing the flow rate. In addition, the tip clearance ratio influences the slip factor, and its effect is non-linear. The head rise decreases as the tip clearance ratios increase. The numerical data is employed to infer reduced-order models for considering the tip clearance effect in the early-phase design process of small-scale turbopumps. The models predict the CFD data with an average relative deviation of 3.6% and 5.8% for the slip factor and the head loss coefficient, respectively. Likewise, the effects of splitter blades and meridional profiles are studied. The splitters can increase impeller head coefficient and slip factors by 10%-24% depending on the blade outlet angle. At the same time, the total efficiency is not influenced significantly. For an unshrouded impeller with a tip clearance ratio of 0.10, a higher head rise is observed for a placement closer to the suction side of the main blade. Moreover, meridional profiles with a sharper change in the cross-sectional area in the vicinity of the inlet rather than a linear change from inlet to the outlet are found to be more advantageous. The optimization of the splitter blade deviating from the main blade profile is less successful than the optimization of meridional profiles in mitigating the effects of tip clearance, as only a 2.43% improvement in total efficiency is achieved. In a further step, the specific speed-specific diameter diagrams are extended to small-scale high-speed pumps. The CFD computations are conducted across the entire pump stage, and the results are analyzed to characterize the performance of turbopumps in terms of nq, ds, and efficiency. The good agreement between experimental performance characteristics and CFD predictions validates the computational results and reduced-order models. Finally, a high-speed turbopump with a specific speed of 8.9 is studied experimentally. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar. The turbopump's characteristics are utilized to estimate the possible performance improvement of a target ORC. The comparison suggests that the pump increases the efficiency of the ORC by 0.3% and decreases its back-work ratio by nearly 50%. The pump is approximately ten times more compact compared to commercial systems.

The global warming potential (GWP) of working fluids in thermodynamic cycles and their environmental impact have been gaining considerable attention within the recent years. The global objectives on climate protection are becoming increasingly ambitious, whereas the general demand for refrigeration appliances with enhanced cooling capacities is constantly growing all over the world. One promising approach to reconcile these contradictory expectations is the replacement of synthetic working fluids with natural refrigerants. The European Organization for Nuclear Research (CERN) supports the development of green technologies within the upgrades of their future accelerator facilities and Carbon Dioxide (CO2) has been identified as an eco-friendly candidate with excellent heat transfer characteristics that has the potential for replacing detrimental refrigerants in future scientific and industrial cooling applications. Inspired by the positive experiences with several mid-scale refrigeration systems, the ATLAS and CMS experiments decided to use CO2 as working fluid for the thermal management of their future Particle Tracker detectors requiring cooling capacities in the order of 0.5 MW. Pressure drop and heat transfer properties are key factors for designing thermodynamic cycles properly. Motivated by the lack of experimental data of flow boiling CO2 in vertical directions, a comprehensive research program has been launched to investigate the two-phase flow behaviour of CO2, in particular in vertical up- and downward direction. As a first step, a test facility to study the two-phase flow characteristics has been built. The setup allows measurements of the significant two-phase flow parameters in horizontal, vertical up- and downward direction in macro-scale pipes with inner diameters of 8 mm. The test conditions of the experiments cover saturation temperatures from -25 °C to +5 °C and the mass velocity ranges from 100 kg m^-2 s^-1 to 450 kg m^-2 s^-1. The performance of pressure drop models has been analyzed with data sets of 512 measurements in horizontal and 295 data points for each vertical up- and downflow direction respectively. 18 frictional pressure drop models have been compared to the horizontal data set and 21 void fraction correlations have been combined with the frictional pressure drop models accounting for the static head, resulting in 378 factorial combinations to anticipate the pressure losses in vertical directions. The best models have been identified for the entire data sets for all flow directions. Moreover, a subdivided analysis, split up according to the flow regimes, has been carried out to highlight the most accurate prediction methods with regard to the flow regimes. The heat transfer properties of flow boiling CO2 have been investigated in vertical upward direction with diabatic measurements of 5 kW m^-2 and 11 kW m^-2 respectively. The dryout phenomena is observed to be influenced by the heat flux, the mass velocity and the saturation temperature. To avoid operation at reduced heat transfer coefficients, a correlation to predict the dryout inception is suggested. The comparison of the data set to established prediction methods reveals, that the heat transfer coefficients of vertical upflow are generally underpredicted. As a consequence, an enhancement factor for vertical upflow of two-phase CO2 is suggested for two prediction models.

With the future decarbonization of the electricity production, gas turbines will be an important tool for the stabilization of the electricity grid by meeting the peak of energy demand using zero-carbon fuels like synthetic gas and hydrogen. The combustion temperature in gas turbines has increased steadily over the years in a quest to increase the thermal efficiency of the machine, so that in modern gas turbines active cooling of blades and vanes is required to ensure safe operation. The need for cooling is exacerbated by the prospect of using hydrogen as fuel, since it has higher stoichiometric combustion temperature, and the combustion products include water vapor, which increases the load on parts exposed to the hot flow. For future turbines, therefore, the thermal management is of the utmost importance; this includes both the cooling system itself - internal cooling and film cooling - as well as protective coatings on the exposed surface of the parts. Additive manufacturing of metallic parts is already a reality for production of static turbine parts, and in the future rotating blades could also be produced by this method. This paradigm shift opens the design space of turbine cooling systems by reducing the manufacturing constraints of the parts, so that radically new cooling concepts can be implemented. For internal cooling, jet impingement achieves the highest local heat transfer and is therefore used in the most highly stressed regions of the turbine parts. The crossflow caused by the spent air of the jets, however, limits the number of jets that can be used in a channel while maintaining high cooling performances, especially in narrow channels with maximum crossflow configuration. The main objective of this work is the experimental investigation of a new impingement channel design that reduces the crossflow by stacking two consecutive shorter channels connected via a plenum. In total, 34 large scale models have been investigated at engine-relevant Reynolds numbers using a new transient liquid crystal technique. Experimental data include high resolution heat transfer coefficients on the target plates as well as pressure data at several locations, which allows to determine the pressure drop of each part of the channel. In a first, exploratory phase, the effect of the variation of the number of jets and of the length of the transition zone has been studied. Moreover, bypass holes that connect directly the two channels and allow for an initial crossflow in the second channel have been investigated. A low heat transfer region has been identified in the transition zone; therefore, several heat transfer enhancement devices have been installed in this zone. Ribs, pins, and cross-section reductions allow to increase the heat transfer coefficient levels. Combinations of these devices provide higher heat transfer enhancement with a marginal increase of the pressure drop. An analytical approach has been used to estimate the metal temperatures of a part cooled by the sequential channels in comparison to a conventional narrow impingement channel at the same massflow, with the new concept achieving lower temperature when using high performance heat transfer enhancement designs. With the coolant mass flow reduction allowed by the new design, an improvement of the thermal efficiency of 0.7% has been estimated when considering the thermodynamic cycle of a modern gas turbine.

Clean and energy-efficient turbomachinery are playing an increasingly important role in the decarbonization of heating systems, waste heat recovery and transportation, as the use of small-scale heat pumping and fuel cells spreads. Such machines typically employ aerodynamic bearings because of their olfreeness and very large lifespan. In particular, grooved bearings enjoy a high stiffness and repeatable performance under similar manufacturing conditions and are particularly well suited for small-scale rotors. However, such gas bearings are usually modeled under the assumption that the lubricant follows the ideal gas law, which may not be valid in conditions met in heat pumping (high pressure condensable gases) and ambient air compression (condensation of moisture). In addition, a strict separation between grooved and compliant bearings exist without justification and optimization methods used for the design of such elements ignore the concept of robustness. This thesis explores these shortcomings. First, the classical modeling theory for grooved bearings, the Narrow-Groove Theory (NGT), is experimentally validated in dynamic conditions. A test rig is built, consisting of a 16mm rotor rotating at 100 krpm on two herringbone grooved journal bearings (HGJBs) driven by an impulse turbine. The bearing bushings are excited in two orthogonal directions using piezo-electric shakers, allowing the deduction of the frequency-dependent force coefficients. The measurements are in good qualitative agreement with the theory, although stiffness and damping coefficients were observed 23 and 29% lower than the predicted values, respectively. The influence of real-gas effects and humid air on the static and dynamic performance of plain bearings and HGJBs is theoretically and numerically investigated on a wide range of operating conditions. Real-gas effects were found to negatively affect the load capacity and either have a positive or negative influence on the stability depending on the operating conditions. Humid air effects have a negligible yet negative influence on the static performance and can decrease the critical mass of HGJBs by 25%. The effects of pure-fluid condensation are assessed theoretically using a 1D slider bearing. Further, an experimental setup consisting of a 20mm three-pad Rayleigh step journal bearing operating at 30 krpm in pressurized R245fa is built to experimentally validate the proposed model by measuring and comparing the pressure in the thin gas film. Theoretically- predicted characteristic features of a condensing gas film are observed experimentally, suggesting sustained operation of a gas lubricated bearing with local condensation. The path toward hybrid foil and grooved bearings is explored, as a tentative to combine the best of the two worlds. A model based on the simple foundation approach is pro- posed and the improvement potential of spiral grooves applied on foil thrust bearings is numerically assessed using multi-objective optimization of the grooved pattern. Results suggest that the load capacity can be improved while reducing the drag torque. However, the ultimate load capacity does not systematically benefit from the presence of spiral grooves. Finally, a novel design procedure maximizing the robustness of aerodynamic bearings against manufacturing deviation is proposed and applied to HGJBs.

Truck engine waste heat recovery (WHR) systems have been investigated for many years. Among them, organic Rankine cycle (ORC) systems show the highest potential, but still lack efficient small-scale expansion devices, in practice. Proposed expanders are often of the volumetric type with oil lubrication, while partial-admission axial turbines are occasionally considered. However, despite a suitable stage pressure ratio capability and a high efficiency potential, the radial-inflow turbine remains mostly unexplored for this application, due to its high rotational speed requirement. A 8 kW ORC turbo-generator is developed based on a single-stage full-admission radial-inflow turbine and a direct-driven permanent-magnet generator. In order to handle the nominal rotational speed of 100000 revolutions per minute of the expander, the single rotating part is supported on self-acting gas bearings that are lubricated with the working fluid, i.e. halocarbon R245fa. Furthermore, in view of characterizing the turbo-generator, a fully-instrumented ORC test setup is designed and built. In a first step, the gas-bearing-supported spindle is investigated experimentally. In this configuration, the electrical machine is used in motor mode to accelerate the rotor shaft up to nominal speed, and therefore to validate the mechanical design of the rotor shaft and the bearings. Furthermore, coast-down tests are performed in order to measure the friction torque acting on the rotor shaft, both in ambient air and in the halocarbon vapor at various pressures. Experimental data and state-of-the-art model predictions are in good agreement for air. That model, however, underpredicts the measured losses by up to 36% in the halocarbon. By applying a multi-flow-regime loss model for enclosed rotating cylinder and disk, the peak deviation drops to 6.5%, suggesting non-laminar flow patterns in the gas film of the thrust bearing. In a second step, the turbo-generator is investigated experimentally on the ORC test setup. The prototype is driven up to nominal speed under off-design operating conditions, reaching a peak electrical power output of 2.3 kW and a peak overall efficiency of 67%. Despite a limited pressure ratio and the low performance of the used commercial pump, a 6% thermal efficiency is achieved on the cycle. The experimental data is compared to a reduced-order model of the turbo-generator, which is found to overpredict the prototype power output by 5 to 20%. In a last step, the validated turbo-generator model is used to assess the potential of a truck engine WHR ORC system, considering a "flat-hills" driving cycle. Under conservative assumptions, the quasi-static simulation suggests that the ORC turbo-generator could increase the powertrain power by 3.7%, and even 4.5% with a regenerative cycle. In addition to increasing the recovered power, the use of a regenerator is suggested to mitigate the cycle load fluctuations and to reduce the required size of the vapor generators and the air-cooled condenser. More experiments, with a suitable power converter, will be necessary in order to proof the ORC turbo-generator prototype up to its nominal design point, to further validate the simulation tools, and to confirm the on-board waste heat recovery potential of the technology. Nevertheless, the proposed solution already proves to be competitive compared to 1-10 kW range expanders presented in the literature.

Dynamic non-contacting seals have been identified as the most appropriate sealing technology for reduced-scale and high-speed turbomachinery applications since they provide a clearance gap preventing rotor-to-seal contact during operation. Yet, any clearance, however small, between rotor and seal permits the passage to fluid flow across regions of unequal pressure as a leakage which penalize the overall system efficiency. A thorough theoretical and experimental investigation of these types of gas seal at a reduced scale, however, is missing today. Furthermore, in thermodynamic cycles, fluids often operate within the vicinity of the saturation line where the well-known ideal gas law is not valid. For reduced scale turbomachinery seals operating close to the fluid saturation line, validated models to capture the real gas effects are not available yet. The aim of this thesis is to address these shortcomings. Initially, a qualitative research approach has been used for the selection of the ideal seals for reduced-scale applications. Using the qualitative data available from the literature of large-scale seals, a weighted decision matrix has been implemented. For assessing the robustness of the selection process, the sensitivity analysis has been carried out by using different weight distributions. The top two ranked seals, namely, Hole Pattern Seal (HPS) and Pocket Damper Seal (PDS), have been identified as the most promising seals for reduced-scale turbomachinery applications due to their low leakage rates and their low influence on the rotordynamic system. In order to predict the seal performance, the commonly used bulk-flow approach has been used for the numerical modeling approach. Furthermore, for the comparison purpose, three different state-of-the-art models have been implemented. As a benchmark, the predicted results from the different models are compared with the experimental data of well-established literature. A good agreement between the different seal models and the experimental data has been observed. A dedicated hermetic test-rig for the experimental investigations on the reduced-scale seal has been designed and built. A modular test section design has been adopted to test different types and sizes of seals. Six different reduced-scale seals, both on cylindrical and conical cross-section, have been investigated experimentally at different inlet boundary conditions with R134a and air. The experimental data for different seals were compared between the predicted results from the developed models as a validation process. Compared to the-state-of the-art models, the author’s model demonstrates a superior prediction quality in terms of leakage, cavity pressure, and temperature distribution in all test cases. The Hole Pattern Seal (HPS) has been identified as the most effective seal for reduced-scale applications. The experimental data suggests a strong impact of the real gas effects on leakage. Compared to a high degree of superheat the leakage increases by up to 10% for an operation close to the saturation line. The author’s model shows better prediction quality at different degrees of superheat compared to state-of-the-art models and demonstrates its ability to capture the real gas effect.

This thesis compares experiments to the analytical and numerical investigation of a small-scale radial-inflow turbine (diameter of 15 mm). This turbine is part of the Fan-Turbine Unit (FTU). The turbine of this FTU propels the fan that recirculates the unused hydrogen and water vapor from the anode off-gas of a Solid Oxide Fuel Cell (SOFC) to its inlet. This recirculation improves the electrical efficiency of the SOFC system due to higher global fuel utilization and allows for an operation without external water supply for the steam reformer. Due to its high efficiencies, also at small-scale and the possibility for heat cogeneration, it is competitive compared with other energy generation systems. The main objective of this thesis is to improve the existing analytical and numerical models of the turbine and compare their respective results with experimental measurements. Good results are obtained with the analytical (38 Watt) and the numerical investigation (43 Watt), if the gas film bearing mechanical loss (18 Watt) is added to the experimental measurement (22 Watt). The turbine power at the design point is very low (40 Watt) compared to the bearing mechanical losses (18 Watt), so heat fluxes to the turbine impeller domain have a high impact.

Reduced-scale refrigeration compressors supported on gas-lubricated bearings have been identified as a key technology for domestic heat pump applications, in order to improve both the system efficiency and the reliability. Unfortunately, reduced-scale machines suffer from increased aerodynamic losses compared to large-scale industrial machines, which are caused by the small feature size and manufacturing tolerances. Due to the small feature size, the machine Reynolds number is low and higher frictional losses occur. The manufacturing tolerances as well as form and positioning tolerances for assembly require larger relative clearances in reduced-scale machines compared to large-scale ones, leading to increased losses caused by tip leakage. Centrifugal compressor design and performance prediction strongly depends on empirical guidelines and correlations, which are deduced experimentally and numerically from test data of large-scale machines. Hence, the question arises whether these empirical design implications and correlations are applicable for reduced-scale compressors. This PhD-thesis presents results on the investigation of the impact of low Reynolds number flow and large relative tip clearances on the compressor performance and flow patterns. The investigation is performed both experimentally and numerically (CFD and one-dimensional compressor model) on two reduced-scale centrifugal compressor designs. An experimental test facility of an up-scaled refrigeration compressor has been realized and tests in terms of impact of clearance ratio alteration as well as performance improvements by impeller design have been performed. Large relative clearance ratios of up to 20 % have been investigated. Empirical correlations accounting for the impact of geometrical scaling as well as altering the tip clearance gap are assessed and improved. These correlations are tested and validated at design and off-design conditions. Design implications on how to design a centrifugal compressor with respect to large clearance ratios have been postulated. The hub and shroud end-wall distribution as well as the shroud blade angle distribution have been identified as main design parameters for mitigating the negative effect of large relative clearance ratios. Furthermore, it has been shown that compressor optimization in terms of tip leakage induced phenomena depend on tip leakage and even more on the guidance of main and splitter blade tip leakage vortices. A mid-loaded shroud blade loading distribution offers the best tradeoff between both. Superior efficiency of such a loading configuration has been verified both numerically and experimentally. The loading distribution is suggested to have a minor impact on compressor efficiency at low relative tip clearance ratios, however, at large relative clearance an efficiency difference of up to 2 % occurs between the different loading distributions.

In order to increase their efficiency and power-density, turbomachines are continuously pushed to run faster, and hotter rotors. These requirements create enormous engineering challenges that affect the design of turbomachines down to the component level. Among these challenges is the choice of an adequate bearing technology. Gas lubricated foil bearings showed competency to support several high-speed turbomachinery applications. The foil bearing performance is governed by the properties of the gas film and the underlying compliant structure. A significant amount of research is dedicated to analyze the latter. However, the gas film was addressed only once in the experimental research efforts on foil bearings extending from the 1960s. This gap in the literature is due to the complexity of the foil bearing structure that hinders the placement of sensors through the bearing surface. As a consequence, the pressure profile inside the gas film of compliant foil journal bearings were never measured. The lack of such experimental data is hampering the conclusive validation of foil bearing models using pressure as the fundamental variable. The goal of this thesis is to provide pressure profile measurements within the gas film of compliant foil journal bearings at different rotational speeds. The experimental data will be a step towards the validation of foil bearing models using gas film measurements. An instrumented rotor with embedded pressure probes and a wireless telemetry is used to execute that mission. The designed rotor is capable of measuring the pressure profile at two different axial planes inside the bearing. The developed embedded pressure probes consisted of pressure transducers, and pneumatic channels to connect them to the measurement point on the surface of the rotor. Such layout required a special calibration procedure in order to account for the dynamics of the pneumatic channel that influences the pressure signal. A Siren Disk was designed and manufactured to produce periodic pressure signals with a controlled frequency and amplitude. Such signal was used to excite the pressure probes, and consequently identity their transfer functions, which are used to correct the pressure signals afterwards. As a proof of concept, the instrumented rotor was tested on externally pressurized gas journal bearings up to a speed of 37.5 krpm. The test bearings were equipped with pressure taps to measure the spatially sampled pressure profiles from the bearing side. The two measurements were compared and were in good agreement at quasi-static conditions. The bearing side measurement was considered as a reference signal (input), and once compared to the rotor side measurement (output), an in-situ calibration and system identification is performed. The pressure measurements were used to validate an externally pressurized bearing model based on the compressible Reynolds equation at different rotational speeds and supply pressures. The developed transfer function was subjected to several fitness tests before placing the instrumented rotor on foil bearings and measuring the pressure profiles at different rotational speeds. The developed transfer functions were used to correct the measured signal within the gas film of the foil bearing. Finally, the pressure profiles were compared to a foil bearing model based on the compressible Reynolds equation.

Electrically Driven Heat Pumps (EDHPs) have been identified as a key technology to reduce the energy consumption in the domestic space heating sector. However, since EDHPs require electrical power, they face issues related to network overload at peak-time, high operating costs, and increased carbon footprint. A promising alternative to address these shortcomings is the use of Thermally Driven Heat Pumps (TDHPs), which are powered by a heat source instead of electricity. TDHPs offer the possibility of running with numerous types of heat sources, even renewable ones. A promising TDHP technology is the ORC driven Heat Pump (HP-ORC). It consists in the combination of an Organic Rankine Cycle (ORC) and a Heat Pump (HP). This technology provides high flexibility in the heat source selection while offering the possibility of producing electricity. Furthermore, when combined with gas bearing supported turbomachinery, the HP-ORC technology offers an oil-free heating solution. The goal of this thesis is to identify the potential and challenges of the HP-ORC technology. Since HP-ORCs are complex systems, an integrated design and optimization procedure has been applied, aiming at objectives of performance, investment cost, and feasibility. While such integrated design procedures are attractive, they are complex and time consuming. Accurate reduced order models for the various system components are, therefore, highly beneficial for improving the design process. These models are, however, currently missing for small-scale turbomachinery, and hence are developed in a first step. These pre-design models are three orders of magnitude faster than mean-line analysis models while predicting isentropic efficiencies within a 4% error band. Moreover, the presented models provide updated design guidelines for radial turbomachinery and offer insights into the underlying phenomena that shape the efficiency contours. In a second step, the improved turbomachinery models have been used for the integrated optimization of the Compressor Turbine Unit (CTU). The results suggest that the performance trade-off is governed by the turbomachinery components. Further, the design robustness of the CTU is investigated, showing the importance of mitigating bearing manufacturing errors while having fluid leakage and turbomachinery tip clearances as small as possible. In a third step, the thermo-economic optimization of the HP-ORC is developed. For domestic heat pump applications, the optimum working fluid and heat exchanger design are retrieved. Using a hot source at 180°C, exergetic efficiencies over 50% and COPs above 1.8 are achieved, showing a 30% increase compared to the proof of concept. In addition, two configurations are compared, whether the ORC expander and HP compressor are mechanically coupled or not. Although the uncoupled HP-ORC offers more flexibility, it presents inferior thermo-economic trade-offs compared to the coupled configuration. The HP-ORC is compared to typical absorption systems, suggesting that single effect absorption heat pumps are competitive at low heat source temperatures (

Small-scale turbomachinery is increasingly used in carbon-free energy conversion systems, such as commercial or domestic scale heat pumps, fuels cells for transportation and waste heat recovery. The usage of aerodynamic bearings allows the design of compact compressors while guaranteeing a high degree of working fluid purity. Herringbone-grooved bearings have the advantages of oil-free operation, long-lifetime, high rotational speeds with relatively low frictional losses, no sealing requirements and avoidance of auxiliary systems. For the design of robust gas-bearing supported rotors accurate models are necessary and validated experimentally. Novel models for the herringbone grooved journal bearing (HGJB) are developed and used for comparison with the simplified narrow groove theory (NGT). A novel finite groove approach (FGA) is introduced for HGJB with rotating grooves, which spatially resolves the rotating grooves and offers a computation time speed-up of order 2, compared to the transient analysis. Good agreement between the FGA and the NGT is found for groove angles beta35deg and compressibility numbers of Lambda>20, large discrepancies are reported, which are explained by increased compressibility effects, which are considered by the FGA but not by the NGT. In order to speed up computation time bearing meta-models are derived, enabling multidimensional regression by applying large scale vectorized computation. Artificial neural networks (ANN) are applied for modeling the highly non-linear multi-degrees of freedom solution parameter space of HGJBs static and linearized dynamic reaction forces. With the developed ANNs, speed-up factors of >10^5 for the static forces and >10^3 for the linearized dynamic reaction forces are achieved. A comparison between solving the partial differential equations directly and using the ANNs in combination with a rotordynamic system analysis shows a relative difference of

Gas lubricated Herringbone-Grooved Journal Bearings (HGJB) are a promising solution to support high-speed rotors in oil-free turbo-machinery due to their compactness, relatively low losses, no need for lubrication and low wear. Gas lubricated bearings, however, generally require very small clearance to diameter ratios to allow stable rotor operation. The consequences are high manufacturing cost, stringent misalignment tolerances and increased specific windage losses. Increasing the bearing clearance while maintaining the performance of the rotor-bearing-system allows to significantly reduce these shortcomings and would help to make the application of HGJBs in products less challenging. Two approaches have been investigated that allow an increase in bearing clearance while maintaining the rotordynamic stability of the system: (1) a flexible support structure for HGJB bushings and (2) enhanced groove geometries. Both approaches suggest a 50% increase in bearing clearance. A dedicated test-rig was designed to allow the experimental characterization of stiffness and damping of various O-Ring geometries and installation specifications. In a first stage, a Design of Experiments approach enabled the identification of the governing variables on the stiffness and damping of O-Rings. The measurement results of 60 representative O-Ring geometries then allowed the development of data-driven, reduced-order models for the stiffness and damping of O-Rings as a function of the excitation frequency, rubber hardness, O-Ring geometry and squeeze. A generic flexible bearing bushing support for HGJBs has been developed to address the shortcomings of the commonly used O-Rings to flexibly support the bearing bushings. The novel flexible support is based on membrane shaped disks featuring specific cut-outs, resulting in an independently tunable support stiffness. An alignment procedure has been presented that is able to perform the very challenging task of aligning two bearing bushings separated by a considerable distance. The novel flexible support has been successfully implemented in a base-line prototype and aligned with the presented alignment concept, which showed significantly improved repeatability, lower lift-off speeds and less wear compared to O-Ring supported bearing bushings. An experimental campaign was conducted to verify the theoretically predicted potential of enhanced groove geometries to increase the bearing clearance of HGJBs. Good agreement between experimentally determined speed of instability onset and prediction was found for the investigated enhanced groove patterns. By using rotors with enhanced groove geometries, it was possible to increase the speed of instability onset by more than a factor three compared to a classically grooved shaft with the same bearing clearance, which confirmed the potential of enhanced groove geometries to stabilize HGJBs.

This paper focuses on a small-scale radial steam turbine which is part of a fan-turbine unit (FTU) realizing the anode off-gas recirculation of a Solid Oxide Fuel Cell (SOFC). Due to the small size of the turbine (15mm diameter), it is not possible to determine its power by directly measuring the torque. Therefore, the performance parameters are evaluated using thermodynamics. However, heat transfers within the FTU have a non-negligible impact on the evaluation of the turbine power and total-to-total isentropic efficiency. New temperature and pressure measurements were thus conducted to refine the experimental results and the numerical model. The analysis of the experimental and numerical results highlights the significant influence of heat transfers, and provides a range for the turbine power and total-to-total isentropic efficiency, which are on the order of 34W and 39%, respectively. Furthermore, a heat investigation was carried out on the FTU in order to localise and determine the heat losses to the environment by using a simplified model. Through investigation, the primary source of heat loss was located at the turbine outlet.

In this study the influence of tip leakage on the performance of a small-scale fan is evaluated through computational fluid dynamics. ANSYS CFX Release 18.1 is employed for the numerical simulations, the results are extracted, analyzed and compared to the existing literature on the subject. Having discussed the original problem geometry, the computational domain is modified to evaluate the impact of adding the diffuser on the results of the simulation.

Machine intelligence greatly impacts almost all domains of our societies. It is profoundly changing the field of mechanical engineering with new technical possibilities and processes. The education of future engineers also needs to adapt in terms of techniques and even skills. Using the design of electro-mechanical actuators as a common thread, this work explores the many-facets of automated design: modeling, optimization, and education, and looks for the prerequisites essential to its successful application. The journey starts by building a modular and integrated model. It focuses on the prediction of system-level specifications that yield high added-value for decision-makers and shorten the path from the model to the final product. Combined with multi-objective evolutionary algorithms (MOEAs) and visualization tools, the model forms an automated design tool that helps engineers and decision-makers to rapidly get important insights into their design task. Its potential and benefits are validated through two specific applications. The results, however, also highlight a gap between the reported performance of optimizers on common benchmark problems and the actual performance on these problems. To further develop optimizers, appropriate and realistic benchmark problems are needed. A subset of the integrated design model is used to formulate a new test suite called MODAct, composed of 20 constrained multi-objective optimization problems (CMOPs) with variable levels of complexity. In addition, numerical approaches to evaluate the constraint landscape of CMOPs are introduced and applied to identify the differences in features of MODAct against 45 benchmark problems from literature. Further, the convergence performance of three algorithms on the same problems highlights the key role of constraints and, in particular, the number of simultaneously violated constraints in MODAct problems. In a next step, existing constraint handling strategies suitable for MOEAs along with a newly proposed technique for many-constraint problems are evaluated. Their parameters are tuned for different problems. The performance of the various configurations further highlights the difference between MODAct and other benchmark problems and show the highly competitive results of the proposed constraint handling technique on realistic design problems. As the technical limits are removed, the impact of automated design on the work of future engineers should be considered. On the one hand, the development of professional skills by students working on team project in different settings has been evaluated thanks to 205 students from three classes. Explicitly addressing these skills within the project seems key to support stronger and broader learning, suggesting changes that do not require a full curriculum redesign. On the other hand, nine groups (33 students) have been asked to design an actuator using a conventional approach followed by an automated design approach. The actuators suggested by students using the automated tool outperform the designs obtained through the traditional approach. Six groups even suggest solutions cheaper, three of which are also smaller, than the product of experienced industry engineers. Students proved thus capable of leveraging the tool within a short time. The analysis of their mistakes suggests possible improvements for future tools. As these students leave university, they carry the hope to see such methods spread in industry.