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• First and second law analyses of newly oval-CGHEs are introduced. • Oval CGHEs exhibit superior heat transfer with a trading-off in irreversibility. • The irreversibility distribution pinpoints the irreversibility of the studied CGHEs. • The results show reduced irreversibility at the oval-CGHEs sidewalls. • Sensitivity analysis highlights key factors influencing the CGHE's irreversibility. Optimizing the heat transfer and thermodynamic efficiency of Ground Heat Exchangers (GHE) is crucial for maximizing the energy efficiency of Ground-coupled Heat Pump (GCHP) systems utilized in building heating and cooling applications. This study pioneers an effective approach of both the first and second laws of thermodynamics of the newly oval-shaped coaxial GHEs (oval-CGHEs), assessing heat transfer, thermohydraulic performance, and local entropy generation rate to explore the optimum design and operational conditions for heat exchange maximization and entropy generation minimization. Utilizing a three-dimensional numerical model validated by experimental results, this study examines the trade-offs between heat transfer improvements and entropy generation rate intensification of the oval-CGHEs in comparison to the typical circular-shaped CGHE (circle-CGHE) further than exploring the effect of key operational and design parameters and performing sensitivity analysis. The results reveal that oval-CGHEs exhibit higher irreversibility as a penalty for their superior heat transfer capabilities compared to traditional circular-shaped CGHEs (circle-CGHE), which can be mitigated by optimizing the position of the inner tube. Moreover, the study underscores the criticality of maintaining a balance between enhanced heat transfer and the associated rise in irreversibility, particularly concerning the factors of mass flow rates and installation depths. [ABSTRACT FROM AUTHOR]

• A thermo-hydraulic modeling of the plate-fin heat exchanger was performed. • Variations of physical properties for streams were considered in the model. • Investigation of longitudinal conduction heat transfer through walls was performed. • Phase change for natural gas occurs in the last segment of the heat exchanger. • Geometrical parameters mainly affect the high-pressure mixed refrigerant stream. Accurate modeling of a multi-stream plate-fin heat exchanger as a vital component in various industries is crucial in dominating its design. This study considered the main heat exchanger of a single mixed refrigerant process for modeling and simulation. First, the process was simulated with an efficient specific energy consumption. Then, a robust model was developed for the thermo-hydraulic performance of the heat exchanger. The model accuracy is enhanced by considering essential features such as multiple streams, multi-component flows, variation in physical properties, phase change phenomena, and longitudinal heat conduction. The temperature and pressure drop distribution for all streams in the multi-component two-phase parallel flow heat exchanger length are evaluated. The effect of geometrical parameters of fins is investigated on the heat exchanger thermal performance. Phase change occurs at the end of the heat exchanger due to the maximum temperature driving force between hot and cold streams. The results show a good agreement between the results of the proposed model and the commercial software, as well as numerical and experimental case studies. Then, the process simulation was linked to the PFHE model to simultaneously consider the effect of PFHE geometrical parameters on the simulation results. Also, the sensitivity analysis was performed to investigate the effect of operating conditions on Specific Energy Consumption. It can be seen that methane mole% has the most significant impact on the thermal efficiency of the heat exchanger. [Display omitted] [ABSTRACT FROM AUTHOR]

• Introduces ultra-compact TPMS heat exchangers with record efficiency up to 16.5 W/(cm³·K). • Successfully 3D prints complex TPMS structures, enabling new design possibilities. • Develops experimental correlations in heat transfer and pressure drop. • Achieves up to 200 % improvement in PEC over traditional heat exchangers. With the rapid growth in energy efficiency demands, compact heat exchangers are gaining increasing attention. This study proposes a novel heat exchanger design based on Triply Periodic Minimal Surface (TPMS) structures, leveraging their unique three-dimensional porous structure to enhance heat transfer efficiency and compactness. This study employs computational fluid dynamics (CFD) simulations to analyze the thermal performance of three TPMS structures (Gyroid, Diamond, and Fischer-Koch S) heat exchangers. Two optimized structures are selected for additive manufacturing based on the simulation results. The manufactured heat exchangers are inspected using CT scans, and a dual-fluid experimental performance testing system is constructed to assess the potential application of the two heat exchangers in thermal management systems. Using the least squares method, correlations for heat transfer and pressure drop are separately fitted for the two TPMS heat exchangers. Dimensionless Performance Evaluation Coefficient (PEC) values are utilized to analyze the comprehensive performance of the two TPMS heat exchangers. The heat transfer rate per unit volume has increased by approximately 50 times compared to traditional heat exchangers reported in the literature, which reaches an impressive 717.7 W/cm3 and 16.5 W/(cm³·K). This study will provide design guidance for enhancing the compactness and efficiency of TPMS heat exchangers. [ABSTRACT FROM AUTHOR]

• Higher fluid flow rate or flow velocity mitigates fouling resistance and particulate deposition. • Impurities accumulate in the region of low velocities. • Higher foulant material concentration induce higher fouling rate. • Physical water treatment methods impart prominent role in fouling retardation. • Fouling rate and particles deposition depicts significant dependency on material surfaces. Mitigation of fouling and particulate deposition in heat exchangers is the foremost concern of this critical review study. Formation of vindictive scale, sludge and fouling layers on heat transfer surfaces have severe influence on thermo-hydraulic performance and overall efficiency of heat exchangers. Fouling on gas or liquid side prevent heat transfer from bulk fluid to the solid surfaces resulting lower thermal performance of heat exchangers. Furthermore, fouling layers on heat transfer surfaces induce blockage to the fluid flow and hence increase pumping power by amplifying pressure drop. The use of various system operating variables in controlling fouling and particulate deposition is comprehensively discussed. The influence of flow rate or velocity, temperature, concentration, material surfaces and other miscellaneous factors on reduction of fouling and particle deposition is extensively investigated. It was concluded that higher flow velocity induces strong shear force across heat transfer surface which in results eradicate deposits and reduces the fouling resistance at the expense of higher pressure drop. The enhancement in fouling concentration leads to the higher fouling resistance and materials with lower thermal conductivity yield inferior fouling resistance. To conclude, this review study will be exceedingly useful for designers to design heat exchangers with higher overall efficiency under the influence of fouling. [ABSTRACT FROM AUTHOR]

Highlights • The mini-scale laminar flow which is asynchronous and intermittent is presented for a micro swing engine recuperator. • Axial wall heat conduction has become more significant in this flow type than the steady state. • The temperature fluctuation in solid wall for intermittent pulsed flow will further reduce heat transfer efficiency. • Heat transfer in the flow is dually affected by wall thickness ratio and flow frequency, whose mechanism is proved. Abstract A numerical investigation into conjugate heat transfer for a double-tube heat exchanger in mini-scale was conducted, with an asynchronous intermittent laminar flow imposed at the inlets. As a basic component of the micro recuperator for a micro swing engine, a mini double-tube exchanger under the same flow conditions was chosen as the physical model with consideration of variable fluid properties. Results indicate that, for the asynchronous intermittent pulsed flow, the axial wall heat conduction is much more serious and the periodic fluctuation of the solid wall temperature further worsens the heat transfer efficiency. Two dimensionless numbers R ∗ and C ¯ ∗ are proposed to evaluate the two adverse effects. Then, two key parameters involved in the design of the micro recuperator for the micro swing engine are studied, namely, wall thickness ratio δ / D c and flow frequency. For certain flow frequencies, as the value of δ / D c increases, the axial heat conduction is enhanced while the wall temperature fluctuation is reduced. For a given δ / D c , the flow frequency dually affects the heat exchanger efficiency and the optimum frequency exists. The optimal value increases with the wall thickness ratio. These results lay a good foundation of the micro recuperator design for the micro swing engine. [ABSTRACT FROM AUTHOR]

• A four-domain numerical model of the tube-in-tube heat exchanger is developed and modified. • A heat exchanger experimental setup from room temperature to liquid hydrogen temperature is built. • The effect of mass flow rate and pressure ratio on the heat exchanger efficiency is studied. • Heat transfer coefficient and the heat exchanger efficiency are more accurately estimated by modified model. With the advantages of no moving parts at low temperature and a flexible and simple structure, the precooled Joule–Thomson cryocooler working at liquid-hydrogen temperature has the potential to be used in the liquid-hydrogen zero-boil-off storage system. The recuperative heat exchanger is one of the key components of Joule–Thomson cryocoolers, and it is usually a tube-in-tube heat exchanger (T-THX). In this study, a numerical model of a T-THX working from room temperature to liquid-hydrogen temperature was developed. Based on the requirement that the heat-exchanger efficiency should be at least 97%, a T-THX was designed and tested. The performance of the T-THX with a temperature span of 280 K was obtained at pressure of 0.539–0.982 MPa and mass flow rate of 4.57–25.81 mg/s. The heat-exchanger efficiency varied from 95.83% to 97.50%, by which the feasibility of the numerical model was preliminarily verified. To improve the calculation accuracy, the T-THX model was modified. Compared with further experimental results, using the modified T-THX model, the relative deviation of the outlet temperature was within 0.3% and the deviation of the heat-exchanger efficiency was less than 0.05%. [ABSTRACT FROM AUTHOR]

• Vapour condensation in the presence of CO 2 in a horizontal tube is targeted for UUV application. • Real gas effect for high pressure conditions is considered using available fluid databases. • The vapour partial pressure can be decisive for the ability of vapour latent heat recovery. • Higher pressure is beneficial to improve the heat transfer efficiency for UUV heat exchangers. Vapour condensation in the presence of noncondensable gases in a high-pressure horizontal tube is targeted for heat exchangers of UUV system. The theoretical model of film-wise condensation, heat and mass transfer analogy, and several potential key factors are reviewed. A numerical model and experimental validation are then conducted. The results show that the numerical prediction is in good agreement with experimental results. Based on the established numerical model, the heat and mass transfer mechanism under the pressure of 10 MPa, characterised by a UUV heat exchanger is investigated. The study case shows that the fluid database can provide more accurate results than the existing real gas model since more non-ideal fluid properties are included. The vapour partial pressure can be decisive since the deviation of 4∼5% can result in a discrepancy of 47.4–51.6% in the ability of vapour latent heat recovery. High pressure conditions present a strong adverse impact on the mass diffusivity, but the effect is not sufficiently reflected in the suppression of condensation heat transfer coefficient. In a UUV heat exchanger, a reasonable high pressure in the outer loop is beneficial to increase the heat transfer performance since it can give rise to a higher bulk saturated temperature and more leeway for setting the subcooling degree, by which the two-phase flow can be sufficiently overlapped with each other to reduce the total thermal resistance. This work provides an insight into the potential application of the heat transfer mechanism of vapour condensation in the presence of noncondensable gas in a horizontal tube. [ABSTRACT FROM AUTHOR]

• The mathematical model is useful for analyzing the heat exchanger performance. • The most probable variant of heat exchanger operation is established. • There is a risk of frost accumulation within the fixed-bed regenerator. The paper describes an original mathematical model of heat and mass transfer for a fixed bed regenerator used to determine the effect of internal airflow leakage on heat exchanger efficiency. First, the build and operating principles of a fixed-bed heat exchanger were discussed. The presence of internal leakage was shown along with a proposed solution for its elimination. Then, a mathematical model based on a modified ε-NTU method for numerical simulation and analysis of coupled heat and mass transfer in a fixed-bed heat exchanger was presented. The model was supplemented with an additional detailed submodel designed to determine frost surface temperature and frost growth analysis. Initial numerical simulations of the operation of the model were performed for low outdoor air temperature conditions. Different zones of heat and mass transfer within the regenerator matrix were distinguished. The presence of accumulated frost area and its influence on the course of heat and mass transfer processes was revealed. In addition, the presence of a relative humidity limit curve associated with a mass balance deficiency conditioning the occurrence of frost accumulation was established. [ABSTRACT FROM AUTHOR]

• Air cycle machine efficiency increased by using superhydrophobic heat exchangers (SHPHX). • Heat transfer rate and pressure drop is measured and compared to an uncoated counterpart. • Droplet jumping, droplet removal, and condensation are visually analyzed. • SHPHX show increased frost cycle times and can eliminate frost formation completely. • SHPHX show reduced defrost-to-cycle time ratios and lower water retention. Commercial aircraft operate over a wide range of atmospheric conditions, with temperatures exceeding 40 °C during takeoff and dropping below -50 °C at cruising altitudes near 10 km. An aircraft environmental control system (ECS) works to keep the crew and passengers comfortable by supplying them with conditioned air. The refrigeration unit of these aircraft, known as the air cycle machine (ACM), sees extreme conditions, ranging from engine bleed air exceeding 100 °C at the ACM compressor inlet to subzero temperatures at the ACM turbine outlet. These conditions can cause the ACM's condenser to frost, reducing the efficiency of the ECS. Engine bleed air can be used to defrost the condenser at the price of further reducing efficiency and cabin comfort. A larger condenser can alleviate frosting but will increase the cost and weight of the aircraft. An alternative is to apply a superhydrophobic coating to the condenser to provide increased resistance to frost growth with a minimal impact on nominal heat exchanger efficiency. In this work, we modified the surface wettability of an aluminum heat exchanger and tested it in a wind tunnel under a wide range of hot-side temperatures to study both frosting and defrosting performance. The superhydrophobic heat exchanger showed considerable improvement in system efficiency, sometimes completely eliminating frost growth and lowering the cycle normalized defrost time by up to 50%. A superhydrophobic coating on the ACM condenser of a turbine-powered aircraft has the potential to increase overall efficiency of the ECS and improve the energy efficiency of aircraft. [Display omitted] [ABSTRACT FROM AUTHOR]

• The present work performs a hybrid numerical-experimental analysis to obtain the thermal and hydraulic performances of the perforated plate-finned heat exchangers. • An experimental apparatus was devised for validation of the numerical data. • The PEC parameter of the heat exchanger was enhanced with increasing the size of the fin perforations (either circular or diamond shape). • The optimization metrics, heat transfers per unit mass and specific heat transfer of the fin surface, enhanced when the fins are perforated. Design of fin profiles is a critical step in searching for more efficient heat exchangers. The present work performs a hybrid numerical-experimental analysis to obtain the thermal and hydraulic performances of the plate-finned heat exchangers. The analysis comprises a four-tube heat exchanger with in-line arrangement, being tested three fin patterns, i.e., rectangular with no insert (standard) and rectangular with diamond and circular perforations. The proposed methodology combines numerical simulation of the conjugate heat transfer problem (convection in the fluid/diffusion in the solid) with wind tunnel experimental data as boundary conditions. The ANSYS-Fluent® software is adopted to solve the three-dimensional average turbulent flow of air over the finned tubes within the framework of RANS model. The validation procedure compares the present results against empirical correlations for the Nusselt number available in the literature. Finally, a parametric study is addressed evaluating the influences of both the size and the shape of fin perforations upon the fluid dynamics and heat transfer behaviour. The main results indicate that larger sizes of the fin perforations (either circular or diamond shape) will enhance the comprehensive Performance Evaluation Criterion – the PEC parameter – and mass/volume thermal efficiency of the heat exchanger. [ABSTRACT FROM AUTHOR]

Abstract: The concept of heat exchanger efficiency eliminates the need for charts, or complicated performance expressions, providing a convenient approach for solving heat exchanger rating, and sizing problems, as well as network of heat exchangers. The efficiency of all heat exchangers is determined from a single algebraic expression. This paper is comprehensive, and streamlined presentation of the approach. It also provides a new expression for solving sizing problems, provides a closed form expression for determining the minimum number of heat exchangers needed when one cannot meet the design specifications, presents a new methodology for analyzing network of heat exchangers connected in series, and provides closed form expressions for determining the size and the rate of heat transfer in individual heat exchangers of a network. [Copyright &y& Elsevier]

• A new optimization concept for heat exchanger network synthesis is proposed. • Functional sequential method is used in simultaneous optimization. • The new concept arranges optimization route by combining functional modules. • The optimization route varies according to the optimization requirement of different stages. Heuristic algorithms are widely used in the global optimization of heat exchanger network synthesis (HENS). However, since heuristic algorithms are often characterized by a divergent search and a combination of multi-parameters, it is difficult to weigh and balance the global and local search in the process. In existing algorithms, the global and local search weight is commonly fixed during optimization. As a result, the algorithms hardly distribute the structural and continuous variables optimization suitably, sometimes even becoming invalid. Even more so, such algorithms fail to realize functions enhancing the searching ability. Therefore, this paper proposes a new, phased, modular concept that supports functional and sub-functional combinations to timely escape the stagnant status and obtain satisfying solutions with high efficiency and high quality. The concept's main goal is to achieve flexible combining and sequencing of different functions during the optimization process. Hence, the optimization route arrangement emphasizes the functions' distribution, which differs from the sequential methods decomposing the whole problem. Each route's components are not unique, and the structure's current needs can change the modules' sequences and categories. Such coordination of modules improves the algorithm's exploration and exploitation abilities, enabling the proposed method to attain better results within less computational time and rendering. Therefore, the proposed method is more suitable in large-scale HENS owing to much local optima involved in cases. Three cases are used to demonstrate the proposed routes' validity, which shows that the optimization route arrangement successfully decreases the total annual cost and improves computational efficiency. [ABSTRACT FROM AUTHOR]

Improving heat exchanger thermal-hydraulic efficiency is of utmost importance for energy conversion and can be carried out through various techniques which are divided into active and passive methods. Advancing nanotechnology in surface modification produces fantastic advances and high potential for heat transfer enhancement in heat exchangers. Research has shown that a micro/nano surface improves heat transfer by inducing turbulence and fluid mixing in single phase heat exchangers. Modified surface in a phase change heat exchanger promotes prompt vaporized bubble release and enhances communication between nucleation sites for easier liquid transport in boiling, while a condenser engineered surface improves heat transfer by switching thin film to dropwise condensation (DWC) or jumping droplet condensation. This paper aims to provide a comprehensive view of the latest status of the application of micro/nanoscale surface modification techniques in heat exchanger for heat transfer intensification. In addition, the heat transfer enhancing mechanism and recommended future research directions are addressed for both single and phase change heat exchanger. • Comprehensive review of micro/nanoscale surface modification technique for heat transfer enhancement. • Application on single-phase and phase-change heat exchanger. • Roughening methods and nanocoating methods. • Heat transfer enhancement mechanisms. • Turbulence enhancement in single-phase heat exchanger. • Stimulate dropwise condensation by hydrophobic surface. • Advanced superhydrophobic surface for jumping droplet condensation. • Enhance boiling heat transfer by nano porous coating. • Enhanced boiling mechanism on nano engineered surface. [ABSTRACT FROM AUTHOR]

• It is easy to overlook a violation of the 2nd law in analysis of heat exchangers. • Maximum permissible degree of heat exchange between two fluids is derived. • Fluid temperatures at the pinch point can easily be obtained. • Realistic heat regeneration efficiencies can be evaluated. • Heat exchange efficiency is determined by technical and thermodynamic limitations. Heat regeneration is important in numerous heat engines and energy-intensive processes. The theoretical analysis of this non-steady-state process becomes highly complicated when working fluid properties change significantly, as in processes with the phase change of a working fluid or near its critical point. Usually, simplifying assumptions are made; for instance, in the preliminary design of regenerative type heat engines, perfect regeneration or a certain regeneration efficiency is commonly assumed. However, this can result in violation of the second law of thermodynamics and lead to erroneous conclusions with respect to the engine performance. In this paper, the maximum thermodynamically permissible degree of heat exchange between two fluids with specified amounts, initial temperatures and pressures is derived. The analysis is performed for a general-type heat exchanger, with arbitrary flow arrangements and conditions. A simple determination method for the fluid temperature at the pinch point is proposed. When the pinch point is achieved away from the ends of a heat exchanger, the fluid temperatures at these ends are determined, too. The results of the derivation are illustrated graphically to clarify their physical significance and examples of the calculation of the regeneration efficiency are provided. The derived simple equation for the maximum heat exchange enables to avoid violations of the second law in heat exchange studies and permits the maximum possible efficiency of a heat exchanger to be estimated. The equation can also be used as a preliminary criterion for selection and/or design of working fluids optimal for particular power or refrigeration (heat pump) cycles. The maximum degree of energy exchange between two fluids is shown to be determined by the maximum enthalpy difference at the same temperature; this temperature can generally deviate from the inlet temperatures, as in case of fluids with complex temperature and pressure dependant heat capacities. [ABSTRACT FROM AUTHOR]

• A new configuration is proposed for triangular vortex generators. • The application of vortex generator for different cross sections are studied. • Different nano-fluids are employed for the new proposed configuration. • All thermal, frictional and exergetic behavior are analyzed. Vortex generators augment the thermal efficiency of heat exchangers by creating vortices, swirl flows and subsequent breaking of thermal boundary layer. Practically, the vortex generators commonly install through the central longitudinal direction of the tube. As a negative consequence, the wall-side of the tube, which is the most thermally effective area through the heat exchangers remain in the physical absence of the vortex generator. Hence, present research proposes the use of two semi longitudinal non-central vortex generators (instead of conventional single centrally configuration) as shown in graphical abstract. The best efficacious radial distance (from the central longitudinal line to the place of vortex generators) are evaluated and reported as well. Triangular-winglet type of vortex generator is selected for this 3-D validated numerical study. Interestingly, the results showed that the thermally effective location is neither the central line nor close to the wall. Additionally, application of vortex generator for the tubes with non-circular cross section (ellipsoidal and flat shape cross section with the same hydraulic diameter) is studied in this paper as well for the first time. Moreover, water, air and two different types of Nano-fluids are analyzed as the working fluid with non-central vortex generator. All thermal, frictional, exergetic and economic characteristics of the aforementioned conditions are investigated in present work. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Multi-scale coupling is used for 87.05% improved accuracy than the existing method. • The effect law of altitude on heat transfer efficiency is studied. • The changes in heat transfer in/below the stratosphere are separately discussed. • Formula for the dependency of heat exchanger efficiency on height is developed. Altitude affects the heat exchange capacity of a heat exchanger, which in turn affects the performance of an aircraft with piston engines. In order to study this mechanism of influence and the associated principles, a three-dimensional computational fluid dynamics (CFD) simulation of an intercooler is conducted based on multi-scale coupling. The simulation results have a high degree of coincidence with the test data on ground, with maximum error of no more than 10%. The simulation height is then extended to an altitude of 20 km. It is found that before the aircraft enters the stratosphere, the overall efficiency of the intercooler decreases by 1.98% on average for every 1km increase in the altitude and decreases by 3.26% when the height is above 11 km. The heat transfer capacity of the intercooler is gradually enhanced owing to the gradually increasing temperature difference while atmospheric density is relatively large. After entering the stratosphere, the external temperature stops changing; the density decreases to 1/14 of the value at the ground, and the low density becomes the dominant factor, resulting in reduced heat transfer coefficient. Finally, an empirical formula for the overall heat exchange efficiency of the intercooler as a function of the altitude is proposed. [ABSTRACT FROM AUTHOR]

A novel configuration of compact cross-flow helical tube (CFHT) heat exchanger for cooling applications is proposed, in view of its inherent high surface area density and tortuous flow path. The cooling potential of such a configuration is assessed against that of its straight tube counterpart i.e., cross-flow straight tube (CFST) heat exchanger for similar volumetric air flow rates under laminar regime. Cross-flow experiments are conducted on CFST and CFHT unit systems for different helix angles ranging from 46∘ ≤ φ ≤ 72∘. CFHT exhibits superior cooling potential, with a manifold increase in heat extracted per unit volume of heat exchanger, as high as 1.5-3.3 times, and heat exchanger efficiency as high as 70% (for φ = 72 ∘). The comparative assessment is extended to an array of (3 × 3) tubes in an inline arrangement, through CFD analysis, for a wider range of helix angles from 17∘ ≤ φ ≤ 72∘. Analysis reveals (i) flow intensification reflected in ~2.3 times increase in Re and (ii) 3D flow that eliminates dead zones otherwise observed behind the straight tubes. The flow features result in 1.4-2.5 times increase in Nu , 2-4 times increase in the rate of heat extracted and higher heat exchanger efficiency upto 90%. Thereby, the efficacy of CFHT heat exchanger in cooling applications is established. [ABSTRACT FROM AUTHOR]

• The 1D model is in good agreement with the CFD model. • Regenerators perform effectively at high speeds. • Heat transfer mechanisms are enhanced at high speeds. • Expressions for the heat transfer coefficient vs Reynolds number were generated. • The efficiency of the designed Vuilleumier machine increases with the speed. A one-dimensional analytical model and a three-dimensional CFD numerical model were used for the investigation of the effect that the rotational speed has on several thermodynamic quantities and the performance of a Vuilleumier machine. The machine, was designed as a combination of two opposing Stirling engines. The one-dimensional program was based on the energy balance at each control volume in order to calculate the energy flow inside the machine and then losses were added. On the other hand, the CFD model utilized fundamental conservation equations at each of the numerous computational cells which resulted to accurate calculations at every dimension. It was able to provide the heat transfer coefficients inside the heat exchangers that were in turn utilized by the analytical model. The pressure drop was computed directly at each space by the numerical model and with equations for flow losses from the bibliography by the analytical model. Pressure drop increased significantly with the speed. The effectiveness of the regenerators was evaluated by an existing analytical model and resulted to reduce drastically with the drop of speed. The effectiveness plays very important role on the efficiency of the machine. Furthermore, there appears to be a discrepancy between the heat flow in the heat exchangers and the wall-gas temperature difference at high speeds which has to be examined according to the oscillatory nature of the gas flow. Heat transfer coefficients were generated in relationship with the Reynolds number for each speed investigated, yielding less thermal resistance when the speed is high. Finally, the change of heat amounts through the four heat exchangers and change of the efficiency of the Vuilleumier machine with the speed, resulted to be similar with experimental data. [ABSTRACT FROM AUTHOR]

• Radiative fins with dynamic geometry can influence heat transfer rates in real time. • Self-irradiation decreases turn-down ratio but increases fin efficiency. • Fin efficiency is greatest for collapsed fins and smallest for extended fins. • A turn-down ratio of three or greater is possible with dynamic radiative fins. Efficient heat transfer is critical in the design and optimization of thermal control systems. Static radiative heat exchangers are often simple and reliable systems but typically cannot be adapted to environmental changes. Adaptable radiative heat exchangers can be adjusted in response to variations in the thermal environment or operating conditions and have the potential for increased efficiency and reduced cost. Dynamic control of a radiative heat exchanger is possible through geometric manipulation of a segmented, self-irradiating fin, consisting of rigid panels that are linked by thermal hinges in an accordion arrangement. In this paper, a numerical model is described to predict the temperature profile and efficiency of a radiative heat exchanger, accounting for conduction and self-irradiation. Governing equations are cast in terms of the conduction-radiation interaction parameter, surface emissivity, actuation angle, and the thermal conductance of the hinges linking the panels. Results indicate that a turn-down ratio (largest possible heat rate divided by smallest possible heat rate) of greater than three is possible for realistic panel geometries and materials. Self-irradiation decreases the turn-down ratio, and there is evidence that an optimal number of rigid panels exists for any combination of panel geometry and device temperature. The maximum efficiency occurs when the plates are in the collapsed position, but the heat rate is at a minimum in this configuration. Finally, the properties and geometry of the plates are shown to have a more significant effect on the turn-down ratio than the properties of the thermal hinges. [ABSTRACT FROM AUTHOR]