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For evaluating the heat transfer correlations from literature in designing helical coil evaporators for low-temperature organic Rankine cycles (ORC), an inverse evaporator design methodology is used. This is done by taking four already performed measurements with different working fluid mass flow rates (0.21–0.23 kg/s) and saturation pressures (1.93–3.05 MPa, reduced pressures of 0.51–0.82) at the helical coil evaporator (66 m long) in an already operational experimental solar/thermal ORC system as real-case references for the evaporator inverse design's boundary conditions. R-404A is considered as the working fluid. The heating water inlet temperature is changed between 353 K and 373 K. The total length is inversely calculated via 15 helical coil two-phase heat transfer correlations for each case. Their end designs are compared with the actual length. Results show that the correlations can be used interchangeably for designing helical coil evaporators for low-temperature ORCs since the heat transfer resistance is dominant on the shell-side. For evaluating the sensitivity of these results, a secondary analysis was made by means of changing the shell-side correlation's accuracy from its initial value of 10% to more accurate 7%. Predictions with several correlations design shorter heat exchangers at reduced pressures less than 0.7. [ABSTRACT FROM AUTHOR]

The present work deals with the development of thermodynamic model of low temperature basic Organic Rankine Cycle (ORC) system and a chevron plate heat exchanger evaporator sub-model using Engineering Equation Solver (EES). Work output is evaluated using the ORC thermodynamic model, while the evaporator sub-model calculates the total surface area of the heat exchanger. Using these mathematical models, the effect of evaporation pressure, expander inlet temperature and pinch point temperature difference (PPTD) on the network output and evaporator cost are studied. In addition to this, the effect of plate spacing and plate width of chevron plate heat exchanger on pressure drop and evaporator cost are analyzed in detail. Finally, thermodynamic and geometric optimization is carried out using genetic algorithm to identify the optimum parameters at which the network output is maximized and pressure drop in the evaporator is minimized. Sensitivity analysis showed that optimum evaporator pressure existed at which network output is maximum. Thermodynamic optimization showed that work output was maximum (5.03 kW) at evaporator pressure of 5.77 bar. No improvement in the work output was seen with increase in PPTD and expander inlet temperature. Increase in plate width and plate spacing led to increase in evaporator cost and decrease in pressure drop. [ABSTRACT FROM AUTHOR]

Highlights • A new Constructal law-based design is presented for cross-flow heat exchangers. • Compared to cross-flow design, the thermal performance is increased as much as 17%. • Compared to cross-flow design, the cost is decreased as much as 14%. • The method is computationally very efficient and can be used in preliminary design. Abstract With recent advancements in advanced manufacturing techniques, it is now possible to fabricate complex geometries that take advantage of well known principles of heat transfer. Therefore, unconventional configurations to enhance effectiveness beyond conventional designs can now be considered for practical application. Thermal performance and overall cost of a new design of heat exchangers in counter cross-flow configurations are studied using a simplified but accurate computational method. The new heat exchanger design was introduced and studied previously for a cross-flow configuration by Sabau et al. (2016, 2018). This new design concept uses multi-scale configurations with successive plenums for the working fluid. At the smallest scale the tubes are sized to be equal to the hydraulic entrance length of the inside fluid, in accord with constructal design. Results indicate that compared to the earlier cross-flow configuration, the counter cross-flow arrangement improves the thermal performance of the heat exchanger by as much as 17% and lowers the total cost by as much as 14%. [ABSTRACT FROM AUTHOR]

This paper presents new graphical tools, which can quickly identify all thermal exergy losses occurring in process design particularly in condensers/evaporators of low-temperature processes. Although the Omega -H diagram is a powerful tool for getting insights about exergy loss in a heat exchanger network (HEN), the calculation of the enclosed area is not straightforward due to non-linearity of the curves. The Omega Composite Curves (OCCs) and Omega Grand Composite Curve (OGCC) developed in this research are new graphical tools that can be applied to any process, including sub-ambient processes. These curves are linear, and all enclosed areas have a rectangular shape. So, all thermal exergy losses can readily be calculated and also necessary modifications to enhance the efficiency of refrigeration systems, either in new design or retrofit study can graphically be suggested. To demonstrate the capability of the new graphical tools, two ammonia refrigeration cycles (one single-stage and one three-stage) have been designed to fulfil the cooling demand of a sub-ambient process and achieve minimum shaft work requirements. The exergy loss associated with each unit in the refrigeration cycle is directly calculated from the OGCC diagram. This is a convenient tool for calculation of exergy losses and can be used to compute the enclosed areas between the OGCC and the condenser/evaporator horizontal lines. The enclosed areas show that exergy loss for 3-stage compression is much less than that in the 1-stage cycle. Also, for the throttling valve, the exergy loss in the 3-stage refrigeration is considerably less than that in the 1-stage cycle. The new linear curves can easily be plotted and implemented to show the share of inefficiencies occurring in different unit operations. [ABSTRACT FROM AUTHOR]

Graphical abstract This paper presents an experimental investigation of heat transfer performances including heat transfer coefficient along flowing direction, log-mean temperature difference and differential pressure of pure R123 and four volume concentrations 20 nm Al 2 O 3 -R123 nanorefrigerants with various volume concentrations 0.03%, 0.13%, 0.18%, 0.23% flowing inside a double-tube counter flow heat exchanger of an organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. A comparison has been made between pure R123 and four kinds of nanorefrigerants to find out the influences of different nanoparticles volume concentrations on local heat transfer coefficient and variation tendency (straight lines) along flowing direction in evaporator. By using polynomial fitting where polynomial order is 3, the polynomial fitting curves (dash lines) and the optimum of volume concentration has been acquired, which indicates the loading carrying capacity in each operation condition. Highlights • Effect of Al 2 O 3 nanoparticles on R123 heat transfer performances. • Variation tendency along flowing direction in evaporator of ORC. • Polynomial fitting curve. • Comparison of differential pressure and log-mean temperature difference of R123 and nanorefrigerants. Abstract This paper presents an experimental investigation of variation tendency of heat transfer coefficient, log-mean temperature difference and differential pressure of pure R123 and 20 nm Al 2 O 3 -R123 nanorefrigerants with four various volume concentrations, 0.03%, 0.13%, 0.18%, 0.23% flowing inside the evaporator of organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. Heat source temperatures are in the range of 50–90 °C at an interval of 10 °C, and heat source flow rates are 0.7 m3/h, 1.3 m3/h and 1.8 m3/h. Results show an increment of heat transfer coefficient along flowing direction for both pure R123 and four Al 2 O 3 -R123 nanorefrigerants with the increment of heat source temperature and flow rate, and that of four nanorefrigerants are higher than that of pure R123. There is no optimum value of heat transfer coefficient when operation condition is changed for the loading carrying capacity increasing with the increasing intensity of operation condition. Meanwhile, suspending nanoparticles and increasing heat source temperature can change the variation tendency of heat transfer coefficient along flowing direction except pure R123 and 0.18 vol% nanorefrigerant. In addition, 0.13 vol% as a whole is the optimum volume concentration for both log-mean temperature difference and differential pressure. [ABSTRACT FROM AUTHOR]

Highlights • A numerically efficient but accurate model for frosting evaporators is developed. • The enthalpy-based reformulation and linearization method are employed • Simulation results on flat plate and finned-tube heat exchangers are provided. • It is 8 to 20 times faster than reference models but withcomparable accuracy. Abstract Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy. [ABSTRACT FROM AUTHOR]

Highlights • A method for the determination of defrosting start time is proposed. • Frost volume and blocking ratio are calculated from experimental data. • Frost volume calculation method is verified by image processing of the evaporator. • This method accurately determines the timing of deterioration of the system. Abstract Naturally, frost forms at fin-tube evaporator surface when refrigerant below 0°C flows inside the heat exchanger. Frost tends to block the evaporator, hinders efficient heat exchange and degrades the performance of the system. The determination of the start time for defrosting is one of the essential issues that most of air heat pumps are confronted with because it is directly connected to energy consumption. Many types of sensors can be installed for frost detection but they are usually not cost-effective or related logic is not so physically presented. In this study, a novel frost volume calculation method is suggested in order to determine the start time for defrosting in an air-to-air heat pump system. The detailed explanation of the method is presented and verified with experimental data. With this method, the start timing of defrosting can be appropriately judged. At the time of defrosting, the heating capacity decreased from 16.2 to 39.3% and the COP reduced from 12.3 to 26.4% compared with maximum values. The blocking ratio from image processing at the time was between 0.83 and 0.97. [ABSTRACT FROM AUTHOR]

The performance and thermo-sustainability indicators (TSI) of non-hybrid organic Rankine cycles (ORCs) are considered at varying evaporator pressure (EVP) and heat source temperature (HST). The ORCs structures include the ORC-basic, ORC-internal heat exchanger (ORC-IHE), ORC-turbine bleeding and ORC-turbine bleeding/regeneration. The TSI considered exergy efficiency, waste exergy ratio (WER), exergetic sustainability index (ESI) and environmental effect factor (EEF). The calculated overall exergy efficiency varies from 35.35 to 47.23 for evaporator pressure between 2 and 3 MPa and 49.02 to 52.71 for HST between 500 and 650 K. Additionally, WER, ESI and EEF were found to vary from 0.253 to 0.278, 1.15 to 1.87 and 0.547 to 0.82, respectively while ESI decreases for all variants of HST. The results indicate high ESI and low WER in ORC-IHE and the ORC-turbine bleeding /regeneration for all variations in EVP and HST. [ABSTRACT FROM AUTHOR]

Thermal distillation systems are widely used in the evaporation concentration industry, and the steam is the heating medium of these systems. However, due to the low density of the steam, the volume of equipment and piping is large. Especially in the mechanical vapor recompression (MVR) systems, high impeller exit line speed often occurs in steam compressors, which will cause compressor blades failure easily. In order to solve this problem, an experimental system called a vapor compression heat pump combined with double‐stage forced‐circulation evaporators (VCHP‐FCE) was developed. The system incorporates the refrigerant, R‐22, which offers an energy recycling benefit. R22 is used as the heat resource and has a higher energy‐carrying ability per unit volume than steam. The advantages relative to traditional thermal distillation system include a smaller device size, compact structure and no need for steam resource. Experimental facilities were established to analyze the thermal parameters and performance of the proposed system. The preheating temperature, which was correlated with the refrigerant's evaporation temperature, was the decisive factor affecting the system. A positive correlation was identified between the system's water production rate and the preheating temperature. Based on the first and second laws of thermodynamics, we analyzed the energy balance and the exergy loss of the system. The proportion of energy loss in each unit and the complete system exergy efficiency as a function of the environmental temperature were determined, which together indicated directions for system improvement. A vapor compression heat pump combined with double‐stage forced‐circulation evaporators (VCHP‐FCE), was developed. The system incorporates the most commonly used refrigerant, R‐22, which offers an energy recycling benefit. This refrigerant is used as the heat resource and has a higher energy‐carrying ability per unit volume than steam. [ABSTRACT FROM AUTHOR]

• Heat exchangers in which phase changes occur require simulation models that realistically capture the resulting temperature profiles and heat-transfer rates. • Two industrially important process examples are studied: a steam generator heat exchanger and the two heat exchangers in a compression refrigeration system. • The reality of the process is compared to the approximate simulation implementation. • Effective control structures are developed and tested. The heat exchanger is probably second only to the pipe as the most widely used piece of process equipment. A typical chemical plant has hundreds of heat exchangers that transfer heat from a hot source into a cold sink. The analysis of heat transfer is straight-forward when both the hot and the cold streams do not undergo any change in phase during the heating and cooling. However, the analysis gets more complex when there is vaporization or condensation of either of the streams. Now latent heats are involved and temperatures do not change continuously with axial location. Vaporizers, reboilers, evaporators and condensers are common examples of heat transfer with phase changes. The purpose of this paper is to demonstrate how realistic simulation models can be developed that capture the steadystate and dynamic heat-transfer differential temperature driving forces, vapor-liquid fractions and heat-transfer rates for these important and widely applied exchangers. [ABSTRACT FROM AUTHOR]

Understanding the bubble regimes is a fundamental step toward conducting heat transfer enhancement. The non-invasive measurement of mixing inside a direct-contact heat transfer process, using a direct video imaging technology, provides powerful opportunities for characterising the visual observations of the phenomena and quantifying the process complexities previously. Experimental bubble shape feature parameters were obtained by means of the photographic recording technique for a direct-contact evaporator. Four design factors with three levels respectively were analysed for the mixing system that involves the exchange of heat between two immiscible fluids (continuous and dispersed phases). Using the Ripley’s K function, new results are presented for two-phase flow mixing which can distinguish differences in the mixing behavior of dispersed phase. In all cases considered, quantitative comparisons of the evolution curves representing different experimental conditions were conducted with reported experimental data. Following the local mixing curve, the current results can also be processed to provide the mixing time found to be in good agreement with available data. The relationship between shape feature parameters of bubbles and volumetric heat transfer coefficient was found to be highly independent on experimental design parameters. [ABSTRACT FROM AUTHOR]

This paper presents a derivation of design guidelines for plate heat exchangers used for evaporation of zeotropic mixtures in heat pumps. A mapping of combined heat exchanger and cycle calculations for different combinations of geometrical parameters and working fluids allowed estimating the trade-off between heat transfer area and pressure drops on the thermodynamic and economic performance indicators of the cycle. Compressor running costs constituted the largest cost share, and increased due to a steep decrease of the heat pump coefficient of performance at high refrigerant pressure drops. It was found that the pressure drop limit leading to infeasible designs was dependent on the working fluid, thereby making it impossible to define a guideline based on maximum allowable pressure drops. It was found that economically feasible designs could be obtained by correlating the vapour Reynolds number and the Bond number at the evaporator inlet as Re V − 0.42 Bd 0.26 ≈ 0.040 . The use of the proposed guideline was illustrated for the mixture Propane/Iso-Pentane (0.5/0.5), leading to evaporator designs with net present values deviating maximum −4.4% from the best value found in the mapping. The presented methodology can be applied in different scenarios to develop similar guidelines, thereby decreasing the cost of combined cycle and component optimizations. [ABSTRACT FROM AUTHOR]

Falling film, shell-tube type evaporators are commonly used heat exchangers for the production of fruit juice concentrate. The main problem in the design of the exchanger is a reliable estimation of wall heat transfer coefficients for all effects in real operating conditions. Most literature sources for the overall heat transfer coefficients are based on laboratory measurements, where the tubes are usually short, no fouling exists and the flow rate is carefully adjusted. This paper shows the heat transfer estimated in real industrial operating conditions, compared to literature sources. Paper is based on the author's own experience in designing and launching several evaporators for juice concentrate production into operation. As a summary, the design heat transfer coefficients are provided with relation to sugar content in juice concentrate. [ABSTRACT FROM AUTHOR]

This study is developed to evaluate the possibility of R1234yf to be a drop-in replacement for a pre-designed system with R134a, including the characterization of control system in a vapor compression system. The control algorithms are required for regulating the refrigerant flow rate into the evaporator and their control parameters depend intrinsically on refrigerant fluid. This paper presents a comparison of the effects of R134a and R1234yf refrigerants on the transient response of an evaporator operating with water as secondary fluid (counter-flow). The dynamic heat exchanger distributed model combines principles of thermodynamics, heat and mass transfer with empirical correlations, which are proposed in the literature, in order to compute the void fraction, pressure drop and heat transfer in two-phase flow. Experimental tests are carried out on a fixed refrigeration system for cooling capacity less than 3 kW, with constant condensation conditions, variable evaporating temperature and compressor speed to validate the model. The results demonstrated that a good agreement between simulation results and experimental data was achieved. The evaporation temperature data were within ± 1   °C error band. Furthermore, comparative simulations between the refrigerants R-134a and R-1234yf exhibited a similar dynamic behavior during a step at the inlet mass flow rate, and the gain and time constant for superheat control were not considerably modified. Therefore, the refrigerant changing does not demand many changes in the expansion device control strategy. [ABSTRACT FROM AUTHOR]

Heat exchangers are common components of many industrial, residential, and automotive systems. In automotive, heat exchangers are employed in engine cooling and air conditioning to transfer the undesired heat from the engine or the passenger compartment as a condenser, evaporator, inter-cooler, heater core, radiator, and oil cooler. Since, the thermal performance of heat exchangers influences the underhood automotive thermal management system, it is essential to characterize their dynamic response in general and specifically, when a sudden change to their operating parameters occur. Experimental measurements provide the scope for in depth understanding of the dynamic behavior of heat exchangers with respect to various parameters. This study investigates a liquid to gas cross flow meso heat exchanger subjected to transient conditions. Step variations in the hot fluid inlet temperature in the absence of mass flow rate disturbances was considered for 5 different levels ranging from 1.5 to 3.0, while other operating parameters are kept constant. The effect of step change on the thermal performance of a heat exchanger is demonstrated by both fluids outlet temperatures, heat transfer rate, effectiveness, and heat balance error. Results obtained show a faster response of the cold fluid outlet temperature compared to the hot fluid, however, an adverse effect is noticed for the heat transfer rate. Higher step changes in temperature lead to higher heat transfer rate, as expected; though, the hot side exhibited more heat transfer and higher effectiveness than the cold side. The hot fluid heat transfer rate and effectiveness display a non-linear increase reaching a peak, after which, it drops down at a slower rate compared to the cold fluid steady increase. A significant effect on normalized exit temperatures, heat transfer, and effectiveness at low step changes while it diminishes at higher step changes. The key element of this work is the use of meso heat exchanger to examine the transient conditions effect on its performance. This work covers a wide range of temperature step changes and aims to enrich the limited experimental database of dynamic response of heat exchangers. The outcome of this work is of interest to designers for thermal testing, characterization, and performance enhancement of compact heat exchangers. [ABSTRACT FROM AUTHOR]

Performance of a frost-free refrigerator may be deteriorated by frost accumulation on its evaporator surface. The objective of this study is to investigate the feasibility of using an enthalpy exchanger (EE) to reduce the frost accumulation on evaporator surface in frost-free refrigerator. The influence of EE on air circulation and the change process of air humidity is analyzed, and it is indicated that the application of EE is capable to decrease the amount of water vapor entering the evaporator and consequently reduce the frost accumulation on evaporator surface. The design process of EE in frost-free refrigerator is illustrated, and an optimal type of EE is presented. An experiment on a frost-free refrigerator shows that the application of EE decreases the frost accumulation by 18.8%. In addition, effects of EE on other main performances of frost-free refrigerator besides the frost accumulation are evaluated. [ABSTRACT FROM AUTHOR]

In this work, the experimental examination was carried out using a mixture of R134a and LPG refrigerant (consisting of R134a and LPG in a proportion of 28:72 by weight) as a replacement for R134a in a vapor compression refrigeration system. Exergy and energy tests were carried out at different evaporator and condenser temperatures with controlled environmental conditions. The results showed that the exergy destruction in the compressor, condenser, evaporator, and a capillary tube of the R134a / LPG refrigeration system was found lower by approximately 11.13-3.41%, 2.24-3.43%, 12.02-13.47% and 1.54-5.61% respectively. The compressor exhibits the highest level of destruction, accompanied by a condenser, an evaporator and a capillary tube in refrigeration systems. The refrigeration capacity, COP and power consumption of the compressor of the R134a /LPG refrigeration system were detected higher and lower compared to the R134a refrigeration system by about 7.04-11.41%, 15.1-17.82%, and 3.83-8.08% respectively. Also, the miscibility of R134a and LPG blend with mineral oil discovered good. The R134a and LPG refrigerant mixture proposed in this study perform superior to R134a from component-wise exergy and energy analyses under similar experimental conditions. [ABSTRACT FROM AUTHOR]

Bubble column evaporator (BCE) systems have been studied and developed for many applications, such as thermal desalination, sterilization, evaporative cooling and controlled precipitation. The heat supplied from warm/hot dry bubbles is to vaporize the water in various salt solutions until the solution temperature reaches steady state, which was derived into the energy balance of the BCE. The energy balance and utilization involved in each BCE process form the fundamental theory of these applications. More importantly, it opened a new field for the thermodynamics study in the form of heat and vapor transfer in the bubbles. In this paper, the originally derived energy balance was reviewed on the basis of its physics in the BCE process and compared with new proposed energy balance equations in terms of obtained the enthalpy of vaporization (ΔHvap) values of salt solutions from BCE experiments. Based on the analysis of derivation and ΔHvap values comparison, it is demonstrated that the original balance equation has high accuracy and precision, within 2% over 19-55 °C using improved systems. Also, the experimental and theoretical techniques used for determining ΔHvap values of salt solutions were reviewed for the operation conditions and their accuracies compared to the literature data. The BCE method, as one of the most simple and accurate techniques, offers a novel way to determine ΔHvap values of salt solutions based on its energy balance equation, which had error less than 3%. The thermal energy required to heat the inlet gas, the energy used for water evaporation in the BCE and the energy conserved from water vapor condensation were estimated in an overall energy balance analysis. The good agreement observed between input and potential vapor condensation energy illustrates the efficiency of the BCE system. Typical energy consumption levels for thermal desalination for producing pure water using the BCE process was also analyzed for different inlet air temperatures, and indicated the better energy efficiency, of 7.55 kW·h per m3 of pure water, compared to traditional thermal desalination techniques. [ABSTRACT FROM AUTHOR]

The heat‐transfer characteristics of polymer hollow fiber heat exchanger were investigated by analyzing the heat‐transfer coefficient (HTC) and the heat‐transfer resistance (HTR) distributions of both the lumen side and the shell side. The influences of the fiber wall thickness and the polymer thermal conductivity on the heat‐transfer performance were studied numerically based on the experimental validated simulation model. It is found that the original overall HTC value is below 1032 W/m2·K and the HTR is focus on the fiber wall. However, if enhancing the polymer thermal conductivity to be higher than 1.0 W/m·K and/or lowering the fiber wall thickness to be less than 0.1 mm, the overall HTC could be improved to over 2000 W/m2·K, which indicates that the fiber wall HTR is no longer the limiting factor of the polymer hollow fiber heat exchanger applications. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1783–1792, 2018 [ABSTRACT FROM AUTHOR]

In this study, an arrangement of Kalina cycle is proposed in which it is intended to implement weak solution of Kalina cycle system 11 (KCS 11) for more power production. In this cycle, which is a version of KCS11 (KCS111), the weak solution of the traditional separator is heated in hot section of the evaporator. The stream that now includes vapor is separated in high-temperature separator that brings about an extra potential to produce work. In order to set pinch temperature in each of the heat exchangers included in the cycles an iterative method is used. The two cycles are compared for the same conditions as the base case, which is followed by a comprehensive sensitivity analysis. The consequences of streams’ curves in the heat exchangers are considered to present analytical justification for the improvements. Comparing the cycles at the base condition, it is observed that the presented cycle improves the exergy efficiency by nearly 18% while more than 17% improvement at the optimum ammonia mass fraction is achievable. Results show that, the proposed cycle produces larger mass flow rate of vapor passing through the turbines and is more efficient than KCS 11 for varying ammonia mass fractions. Results indicate that the trend of thermal efficiency versus ammonia mass fraction is descending for the both cycles. [ABSTRACT FROM AUTHOR]

In this paper, a novel geometry is proposed for evaporators that are used in Supercritical Organic Rankine Cycles. The proposed geometry consists of successive plenums at several length-scale levels, creating a multi-scale heat exchanger (HX). The channels at the lowest length-scale levels were considered to have their length determined by the thermal entrance-length. Numerical simulations based on turbulent flow correlations for supercritical R134a and water were used to evaluate the performance of heat exchangers. Using the data on pumping power and area of heat exchange, the total present cost was evaluated using a cost model for shell-and-tube heat exchangers. With respect to the shell-and-tube baseline case, the cost per heat load and total costs of new HXs is lowered by approximately 20–26% and 15–30%, respectively. This reduction in present costs of the new HXs were found to be attributed to higher operational costs for the shell-and-tube HXs, as evidenced by the higher pumping power, as well their capital investment costs. The cost savings in the new HX designs compared to those of the shell-and-tube HXs, at similar heat load performance, indicate that the new HX architectures proposed in this paper are valid alternatives to traditional HX designs. [ABSTRACT FROM AUTHOR]

The multi-functional variable refrigerant flow system enables simultaneous heating and cooling of the zones it serves by allowing the heat exchangers of both indoor units and outdoor units to operate as evaporators or condensers. To realize automatic and smooth switching between different modes, a mode switching strategy is proposed for a multi-functional variable refrigerant flow system. Whether to turn on or off an indoor unit is determined by the zone temperature and a preset hysteresis band about the temperature set-point. Based on a thermodynamic analysis, a decision variable for determining outdoor unit-heat exchangers mode switching is proposed as the outdoor unit-heat exchangers airside temperature differential normalized by the dimensionless outdoor unit fan speed. Two bumpless transfer strategies are applied to produce smooth mode switching. To evaluate the proposed strategy, a simulation study is performed with a Modelica-based dynamic simulation model of a four-zone multi-functional variable refrigerant flow system. Simulation results validate the effectiveness of the proposed strategy and the performance of the bumpless transfer strategies. [ABSTRACT FROM AUTHOR]

The present study focuses on the energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations. More specifically, three regenerative ORCs are examined. The first includes an open preheater, in which the bleed stream is mixed with the working fluid exiting the pump of the cycle (O-ORC). The other two configurations include a closed preheater. In the second configuration, the bleed stream is throttled and conveyed to the condenser (CB-ORC), while in the third one, it is repressurized via a secondary pump and recirculates into the evaporator of the cycle (CF-ORC). The systems are optimized for different working fluids, and their energetic efficiencies are estimated and compared to that of a standard ORC (S-ORC). In all cases, the inclusion of a recuperator has also been investigated. In principle, recuperative and regenerative ORCs are mostly suitable for dry fluids, while the critical temperature can also have a positive influence on the performance improvement. Furthermore, it is estimated that while the recuperative S-ORC has a higher efficiency than the non-recuperative regenerative cycles, recuperative O-ORC and CF-ORC exhibit a relative efficiency gain ranging from 4.98% to 8.05% and 6.22% to 9.29%, respectively. The highest efficiency improvement achieved by the CB-ORC, however, is minimal. [ABSTRACT FROM AUTHOR]

In this study, the use of R1234ze(E) refrigerant in an R134a-based mobile air-conditioning (MAC) system was experimentally investigated. Additionally, the effect of internal heat exchanger (IHX) on the performance of MAC was determined using R1234ze(E). Under the same conditions, we have tested and compared two different cases that use R1234ze(E) with and without an IHX by taking the performance of R134a as a reference. The inlet airflow temperatures of the evaporator and condenser were kept at 27 °C and 35 °C during the experiments, respectively. Performance parameters were determined using energy and exergy analysis. Compressor power was reduced by an average of 19 %, and the COP of R1234ze(E) were increased by an average of 4 % after activating the IHX. Moreover, it was determined that the value of exergy destruction of the system with R1234ze(E) was decreased by an average of 50 % when compared to R134a. [ABSTRACT FROM AUTHOR]

This study presents an experimental and theoretical evaluation of frost formation on the moist air side of a peripheral finned-tube (PFT) heat exchanger. Previous studies of this compact geometry correlated the friction and heat transfer parameters above the dew-point temperature of the air (no frost or condensate formation). Here, for the first time, the thermal-hydraulic performance of a PFT heat exchanger is analyzed under frosting. A PFT heat exchanger prototype was evaluated experimentally in a closed-loop wind tunnel calorimeter to determine the influence of the tube wall temperature, air velocity and psychrometric properties (temperature and relative humidity) on the heat transfer rate, air-side pressure drop and frost buildup on the surface. The thermal and hydrodynamic behavior of the enhanced air-side surface was analyzed using a distributed heat exchanger model in which mass, momentum and energy balances are applied to one-dimensional control volumes in the air flow direction. The model treats the air flow path as a porous medium in which the porosity, equivalent particle diameter and thermal properties vary as a function of time due to the frost accumulation. Good agreements (within 20% average error) between the model predictions and the experiments for the air-side pressure drop and heat transfer rate have been found. The enthalpy effectiveness was found to drop from 0.68 for dry to 0.50 under severe frosting, which suggests that the PFT heat exchanger continues to be effective under frost. [ABSTRACT FROM AUTHOR]

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Periodic flow reversal are commonly present in microchannel heat exchangers, but have been studied in air conditioning systems only recently. This paper presents the effect of periodic reverse flow on the heat transfer performance of a heat exchanger. Two heat exchangers with identical geometries in the heat transfer areas are employed and an artificial upstream flow resistance is added for one of them. The heat exchanger without artificial flow resistance is subject to more severe boiling instabilities and consequently generates four times more reverse vapor flow than the other one. The comparison of capacities under identical operating conditions reveals that higher intensity of reverse flow helps to improve cooling capacity by up to 13.3%. Meanwhile, numerical simulations of bubble dynamics coupled with heat transfer are carried out for both heat exchangers. Results show that in the heat exchanger with more reverse flow, the refrigerant side heat transfer coefficients are enhanced, especially in the upstream part of a channel where the flow velocity is relatively low. [ABSTRACT FROM AUTHOR]

Boiling instability and periodic flow reversal are commonly seen in microchannel heat exchangers, but have been studied in air conditioning systems only recently. The amount and frequency of reverse flow largely depends on the geometry of the channels. This paper presents results obtained in a R134a automotive air conditioning system operated under flash gas bypass mode with venting of reversed vapor (to provide better control of the flow). Results for multiple microchannel evaporators with different channel diameters and lengths are presented. The mass flow rate and frequency of reversed vapor are measured for each geometry. Meanwhile, a mechanistic model is developed to simulate the bubble dynamics inside of a single microchannel tube. This model is validated against experimental data and employed to explain the effect of channel length and diameter on flow reversal. Both experimental and simulation results show that larger channel diameter and longer channel length result in less flow reversal with a lower frequency. [ABSTRACT FROM AUTHOR]

Reducing the diameter of copper tube is beneficial to reduce the amount of metal material used in heat exchanger. The change of the diameter of the copper tube can not only reduce the volume of the heat exchanger, but also the filling amount of the refrigerant can be reduced. In this paper, the R22 fin-tube evaporator with 7mm tube diameter is improved. The diameter of the tube is reduced to 5mm, and the environment protection refrigerant R290 is used. Diameter reduction may cause the pressure drop increases, and the new branch number is 5 by calculating. Using the simulation software EVAP-COND to simulate the heat transfer performance and compare the refrigerant flow, pressure drop, copper usage with the evaporator before improved. Result show that the heat transfer effect of R290 evaporator is better than that of R22 system, and the heat transfer effect of system is the best. Under the premise of no increasing pressure drop, the heat transfer of the R290 fin-tube evaporator with a small tube diameter increased by 34.62%, refrigerant flow rate, pressure drop, copper and aluminum consumption decreased by 28.7%, 43.1%, 28.6%, 30%. [ABSTRACT FROM AUTHOR]

• A novel evaporator for R744 heat pumps with wastewater as heat source was presented. • This evaporator splits the evaporation in two levels, connected by an ejector. • This evaporator was tested and compared to one-stage heat exchangers. • Heat pump COP can improve more than 10 % compared to the benchmark evaporators. Heat pumps are pinpointed as a key technology in decarbonizing buildings' heat supply, which is currently dominated by fossil-fuel based systems. To minimize the environmental impact of heat pumps over their lifetime, selecting natural working fluids (refrigerants) is the only viable long-term choice. Independent of the refrigerant, the efficiency of a heat pump is closely linked to temperature of the applied heat source, and to this end, wastewater is a promising candidate that is abundant in all urban environments. This experimental study investigates a novel two-stage, ejector-supported, brazed plate evaporator for its use in R744 (CO 2) heat pumps with greywater as a heat source and compares it to more conventional evaporators. The work shows that splitting the evaporation process in two stages with an ejector in between allows reducing the evaporation pressure while the compressor suction pressure can be increased, in some cases significantly. Even if several possibilities for further optimization of the novel two-stage evaporator were identified, the results show heat pump efficiency improvements above 10 % compared to an equivalent heat pump with a well-performing one-stage evaporator. Heat pump systems aiming at utilizing maximum temperature difference for the greywater should use a high pressure lift ejector. [ABSTRACT FROM AUTHOR]

Evaporators for adsorption heat pumps, chillers and storage devices mostly use water as refrigerant and work at sub-atmospheric pressures. However, there are hardly any applicable performance correlations or sizing guidelines for these rather unusual operating conditions. Within this study geometric and process-related impacts on non-stationary evaporation performance of copper tube-fin heat exchangers are investigated to start filling that gap. Cyclic condensation/evaporation measurements were performed in a thin film evaporation mode and in partially flooded operation with changing refrigerant filling level to cover different applications. For thin film evaporation a thermal resistance model was developed to quantify resistance contributions and identify performance-limiting factors. The presented measurement results reveal that evaporation performance in thin film operation is crucially governed by fluid side heat transfer (UA raise by 146% within tested Reynolds numbers) and wetting conditions while fin sheet thickness plays a marginal role (8% increase of UA). In partially flooded operation performance strongly depends on filling level. Apart from some refinement potential of the thin film model, the simulations reflect evaporation dynamics and effects of influencing factors fairly well which indicates that the employed model approach could be a suitable tool for an effective optimization of evaporator geometry and process parameters. [ABSTRACT FROM AUTHOR]

The ISG (Idle Stop and Go) systems are commonly used in modern automobiles because they are economical and environmental friendly technology. However, when a vehicle stops, the air-conditioning system stops, resulting in thermal discomfort to passengers in the cabin. This paper examines a cold storage heat-exchanger (CSH) integrated with an evaporator. The position of the cold storage parts inside a heat exchanger was analyzed through numerical simulations using FLUENT to create an adequate design for a CSH. The CSH performance was then examined with various airflow volumes and optimized experimentally in terms of the refrigerant flow circuit and fin density in the heat exchanger. Next, an experiment on the coldness release performance of the CSH was conducted in the air-conditioning system. The cold storage system with optimized CSH experiment resulted in lower air discharge temperatures (3.5 °C ~ 4.9 °C) than current air-conditioning systems, and delayed the warm-up by approximately 155 seconds to reach 18 °C temperature of air discharge. For this study, the CSH is an effective solution for the ISG-applied vehicles with less investment by transforming current air-conditioners' structures more effectively. [ABSTRACT FROM AUTHOR]

Due to the lack of knowledge about water behavior in conditions that might occur in sorption chillers using water as refrigerant, designing compact heat exchangers remains mainly empirical. The objective of this paper is to gain further understanding of phenomena occurring in compact plate-type evaporator in order to be able to design them properly. In that goal, experiments were conducted under various operating conditions during adsorption cycles. Although interactions take place between the adsorber and the evaporator due to coupling phenomena, the present study focuses only on the evaporator. The vaporization of water occurring inside the channel of a smooth plate-type evaporator during these cycles is observed. Two main distinct phenomena observed along the time of the adsorption cycle are identified, described and discussed. Time-averaged cooling capacity and instantaneous cooling capacity for given operating conditions are plotted. A parallel with result obtained with the same experimental apparatus but in absorption configuration (mimicked by a thermosiphon loop) is done. Model developed in absorption configuration is tested. It is shown that, except during the beginning of the cycle, for the major part of running conditions tested, this model is able to well predict the instantaneous cooling capacity obtained during the adsorption cycle as long as the variation of the height of liquid level and the equilibrium pressure is known. The evolution of the cooling capacity measured in adsorption configuration with dimensionless number developed in absorption configuration is plotted for comparison. These dimensionless numbers were developed in order to optimize the design of compact evaporator used in sorption chillers. It is shown that the same trend could be observed for the two configurations. [ABSTRACT FROM AUTHOR]

The cost of heat exchangers account for a large proportion of total investment in organic Rankine cycle (ORC). In this paper, plate heat exchanger (P), shell-and-tube heat exchanger (S) and finned-tube heat exchanger (F) are used as evaporator and condenser of four subcritical ORC configurations: ORC-PP, ORC-SS, ORC-FP and ORC-FS. The thermo-economic models are built and a thermo-economic evaluation and comparison of four ORC configurations is presented in order to recover the low-temperature waste heat. The optimal evaporating pressure, pinch point temperature difference, net power output and dynamic payback period corresponding to the minimum electricity production cost (EPC) are obtained for different ORC configurations under different heat source temperatures. Results show that the EPCs of ORC-PP and ORC-SS are apparently higher than that of ORC-FP and ORC-FS. Among them, ORC-FS is the most cost-effective configuration. The optimal pinch point temperature difference in evaporator has a decreasing trend with the increase of critical temperature of working fluid for ORC-FS and ORC-FP, while the optimal pinch point temperature difference in condenser keeps nearly constant. [ABSTRACT FROM AUTHOR]

This study explores the implementation of two large micro-channel heat exchangers as evaporators in a refrigeration cycle intended for thermal management onboard future space vehicles. Examined are thermal performances of the two heat exchangers, thermodynamic performance of the refrigeration cycle, and system instabilities. Because of quality differences, different two-phase flow patterns and dryout effects are encountered in the two heat exchangers. A trade-off exists between optimum performances of the evaporators and compressor, with former favoring outlet quality values below unity to guard against dryout, and the compressor favoring a quality equal to or exceeding unity to preclude internal damage to the compressor from entrained droplets. Thermodynamic analysis of the refrigeration cycle shows exergy destruction is minimum when the avionics HX’s outlet quality is close to unity. The system is shown to be susceptible to instability resulting from interactions between compressible volumes both upstream and downstream of the condenser, and inertia of liquid in the condenser. This instability is manifest by pressure oscillations that are sensitive to both flow rate and evaporator heat input. [ABSTRACT FROM AUTHOR]

A shell and tube heat exchanger is used as a condenser and an evaporator in this theoretical study. Parametric performance analyses for various actual refrigerants were performed using well-known correlations in open sources. Condensation and evaporation were occurred in the shell side while the water was flowing in the tube side of heat exchanger. Heat transfer rate from tube side was kept constant for condenser and evaporator design. Condensing temperatures were varied from 35 to 60 °C whereas evaporating temperatures were ranging from −15 to 10 °C for the refrigerants of R12, R22, R134a, R32, R507A, R404A, R502, R407C, R152A, R410A and R1234ZE. Variation of convective heat transfer coefficients of refrigerants, total heat transfer coefficients with Reynolds numbers and saturation temperatures were given as validation process considering not only fouling resistance and omission of it but also staggered (triangular) and line (square) arrangements. The minimum tube lengths and necessary pumping powers were calculated and given as case studies for the investigated refrigerants considering validation criteria. It was understood that refrigerant type, fouling resistance and arrangement type are one of the crucial issues regarding the determination of heat exchanger's size and energy consumption. Consequently, R32 and R152a were found to require the shortest tube length and lowest pumping power in the condenser, whereas R507 and R407C have the same advantages in the evaporator. Their heat transfer coefficients were also determined larger than others as expectedly. [ABSTRACT FROM AUTHOR]

In this study a methodology to develop a semi-empirical modelling of a classic refrigeration system is presented, which is composed by a compressor, a condenser, an expansion valve and an evaporator, and is characterized under extended operating conditions. The models are developed in a modular way, thus these can be adapted to a similar configuration related with refrigeration, air conditioning and heat pumps. The results of the modelling are compared with experimental results obtained in a refrigeration system composed by a scroll compressor, a plate heat exchanger as evaporator a louvered fin and flat tube condenser. This comparison is also used to determine some empirical parameters of the models. First the models are validated individually and then these are interconnected to develop the global model. In this approximation, the condenser imposes the condensing pressure, the evaporator the evaporating pressure and the compressor the refrigerant flow rate, among other intermediate variables. The main outputs of the model are the evaporator cooling power, the compressor power and the system COP (Coefficient of Performance). The model is validated with 76 tests developed in a steady-state, by obtaining a COP with a difference lower than 8% between the measurements and the results of the model, which is here considered as acceptable. Finally, by simulation, the model is used to determine the refrigeration system working domain. [ABSTRACT FROM AUTHOR]

Now-a-days fin-and-tube heat exchangers are employed in a wide variety of engineering applications such as modern heat exchangers, automotive radiators, automotive air conditioning evaporators and condensers. In order to improve the performance, further innovative designs, additional applications, and advanced geometries are expected. Researchers and academicians had been working on this topic for last few decades. A number of experimental and numerical studies have been conducted and various correlations were developed. The optimum design parameters with the maximum overall heat conductance and minimum pressure drop needs more focus and research. In this paper, different experimental/numerical studies performed are reviewed, grouped and summarized based on the types of heat exchangers, heat transfer and pressure drop performance, effects of geometrical parameters under different flow conditions. Overall, this paper will highlight on the existing technologies and emerging trends in designing of finned-tube heat exchangers considering different arrangements and geometric parameters under variable flow conditions which will be helpful for selecting appropriate design depending on the requirement. [ABSTRACT FROM AUTHOR]

R-410A distribution was experimentally studied for an evaporator having two row/four pass configuration. The evaporator has inlet, intermediate and top (or bottom) row-crossing headers. Tests were conducted for the mass flux from 70 to 130 kg m −2 s −1 with inlet quality 0.2 and exit superheat 5 °C. Flow distribution at the inlet header generally follows the well-known trend. For downward configuration, more liquid flows into upstream channels. For upward configuration, more liquid flows into downstream channels. At the intermediate header, flow distribution is dependent on the direction of the tubes. When tubes are heading upward, more liquid is supplied into upstream. When tubes are heading downward, on the contrary, more liquid is supplied into downstream channels. Flow distribution at the top row-crossing header is entirely different from that at the bottom row-crossing header. Thermal degradation by flow mal-distribution is much larger for top row-crossing header configuration than for bottom row-crossing header configuration. Correlations are developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number. Header pressure drops are obtained by subtracting flat tube pressure drops and other minor pressure drops from measured pressure drops. [ABSTRACT FROM AUTHOR]

Boiling heat transfer of R134a flowing in the inner tube and hot water flowing in the annulus were examined inside smooth and corrugated tube-in-tube heat exchangers in authors' previous studies experimentally. The apparatus has 0.85 m long double tube for vertical orientation and 2.5 m long double tube for horizontal orientation as its test sections whose inner ones are smooth and corrugated copper tubing having inner diameters of 0.0087 m for vertical tubes and 0.0081 m for horizontal tubes. The range of mass fluxes are 20,300 and 400 kg m − 2 s − 1 for vertical tubes, and 300, 400 and 500 kg m − 2 s − 1 for horizontal tubes. The average vapor qualities vary from 0.14 to 0.86 for vertical tubes, 0.09–0.81 for horizontal tubes, and saturation pressure interval is between 4.5 and 5.7 for both tube orientations. The average boiling heat transfer coefficient of R134a is calculated by means of the energy balance in the test section. In this existing study, the predictability of almost whole well-known empirical correlations in literature suggested for convective boiling flows in smooth and corrugated tubes is experienced by using the authors'large number of experimental database (302 data points for vertical tubes and 227 data points for horizontal tubes). Characteristics of trend lines plotted for the change of vapor quality, saturation temperature, mass flux and heat flux with experimental boiling heat transfer coefficients are examined as well in relation to various saturation temperatures and mass fluxes. Furthermore, the most successful correlations validated their predictabilities for both vertical and horizontal oriented evaporators having smooth and corrugated tubes. [ABSTRACT FROM AUTHOR]

Case Studies Editor's Note: We at the Journal of Heat Transfer Engineering (HTE) are happy to announce the introduction of a new section entitled “Case Studies.” The first case study was published in vol. 37(3–4) and the second case was published in vol. 37(7–8). This is the third case study in this series. We are asking authors to contribute their practical experiences in the form of real-world case studies. The goal is to disseminate practical information especially to young engineers who are stepping into the real world. The format of such articles will be a full-length paper or a short brief. It does not have to entail a typical research-type paper format with details about measurement accuracies, references, and so on. The article could have a brief introduction with the problem description, remedy, lessons learned, and recommendations. The review process will be simple and less involved than with typical research papers. The goal is not to discredit a person or a company but to share experiences. We all make mistakes; however, we also find solutions, and it is the solution part that is rewarding and should be shared for the betterment of the society. Zahid Ayub HTE Case Studies Editor [ABSTRACT FROM AUTHOR]