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An experimental test protocol for simulating the air-side fouling of heat exchangers, as well as metrics to characterize the extent of fouling undergone by the heat exchanger and its performance in clean and fouled conditions, were proposed in a companion paper. In this study, the air-side fouling of a finned microchannel heat exchanger is experimentally investigated according to the proposed protocol. Key test parameters influencing heat exchanger fouling are identified based on studies in the literature, and their impact is experimentally investigated. The effectiveness of in situ cleaning methods is also experimentally evaluated. Transient measurement data and photographs taken during the fouling process reveal the nature of fouling, while steady-state data quantify the degradation in heat exchanger performance due to fouling. This two-part study defines a generalized experimental approach to enable characterization and comparison of heat exchanger surfaces on a standardized basis, and provides detailed experimental data for modeling heat exchanger fouling that includes all necessary information to allow for model validation. [ABSTRACT FROM AUTHOR]

Air-side fouling of heat exchangers has received much attention in the literature through both experimental and modeling studies. However, no standards currently exist that define a test procedure to experimentally foul heat exchangers and evaluate their performance in a fouled condition. This is evidenced by the variation in test procedures, fouling agents, metrics, and operating conditions reported. Based on a detailed review of past experiments, a standardized, repeatable test procedure is proposed that allows cross-comparison of data acquired in different laboratory facilities. The procedure generates time-resolved information on foulant buildup on the finned heat exchanger and the consequent impact on heat exchanger performance. Various fouling agents used in the literature and fouling deposits observed on field-installed heat exchangers are compared, and a representative fouling agent is recommended. A method is proposed to quantify heat exchanger fouling and metrics identified to quantify heat exchanger performance before and after fouling. This is the first of two companion papers on an experimental investigation of air-side fouling of heat exchangers. The companion paper presents experimental data obtained by implementation of the proposed procedure to investigate fouling of a finned microchannel heat exchanger. [ABSTRACT FROM AUTHOR]

Highlights • A comprehensive review of the geometry modeling of single-screw machines is proposed. • The main geometric parameters and constraints have been discussed. • Limitations of existing methodologies have been highlighted. • An 11 kW single-screw expander has been considered to carry out the calculations. • An example of 3D rotor design is provided as an electronic annex. Abstract Single-screw type of machines has been widely used in the vapor compression industry for decades. More recently, such machines have been proposed as volumetric expanders in organic Rankine cycle (ORC) for low-grade waste heat recovery applications because of a number of advantages compared to other volumetric machines, e.g., twin-screw, scroll, rotary, reciprocating, such as balanced loading of the main screw, long working life, high volumetric efficiency, low leakages, low noise, low vibration, and a simplified configuration. In this paper, a thorough literature review is proposed to cover the available single-screw geometry models, which are mainly applied to single-screw compressors (SSC). Thus, the geometry model is extended to single-screw expanders (SSE). The validity of the analytic and numerical solutions for the calculation of the volume curves is discussed. The geometry model is applied to a 11 kW single-screw expander and both the profile generation and geometry calculations are validated. [ABSTRACT FROM AUTHOR]

Adhesive joints are widely applied and studied for various industrial applications. The interest in adhesive joints has expanded to include heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems having a significant number of joints employed for manufacturing. This study investigates an analytical modeling approach for predicting joint stress and strain distribution under static loading with thermal strain. A review of modeling techniques identified the need to develop a joint analytical model under loading conditions representative of HVAC&R applications. The details of the model, governing equations, assumptions, boundary conditions, and solution techniques are first reported. The model is validated via comparison to existing results before performing parametric studies to provide insights on the influences of thermal expansion and inner tube pressure on possible failure. It is found that the joint overlap length plays an important role in stress distribution, while the adhesive thickness has less impact. Overall, the results indicate that static loading failure is not likely a concern for joints in HVAC&R systems, but the thermal strain and stress induced by temperature fluctuations must be carefully considered. This modeling effort establishes a framework that can be used to generate criteria and instructions on designing adhesive joints across different HVAC&R [ABSTRACT FROM AUTHOR]

Organic Rankine cycle with liquid-flooded expansion and internal regeneration (ORCLFE) has been demonstrated leading to improvements in system efficiency with respect to conventional ORC with internal regeneration in the heat input temperature range from 80 ∘ C to 200 ∘ C. From a previous study [1], it was concluded that the selection of the positive displacement expander, the working fluid and its interaction with the flooding medium, and the two separate heat inputs heavily impacted the exergetic conversion efficiency of the cycle, but they were not analyzed in depth. In this paper, a more detailed thermodynamic cycle model has been developed to include a physical-based expander model to evaluate the impact of built-in volume ratio, flooding ratio, mechanical losses and the solubility of the working fluid into the lubricant oil. As waste heat recovery from 100 ∘ C to 150 ∘ C is the main application of the ORCLFE under investigation, low-GWP refrigerants such as R1234ze(Z), R1233zd(E) and R1336mzz(Z) have been considered as potential replacements of R245fa. A single-screw expander, which has been previously characterized, has been employed to optimize the ORCLFE at three heat source temperatures (100 ∘ C, 120 ∘ C and 150 ∘ C) for each refrigerant, in order to obtain more realistic performance predictions. [ABSTRACT FROM AUTHOR]

While the United States is no longer the global leader in primary energy consumption, it still represents a significant percentage of global consumption. Residential buildings account for 22% of U.S. national primary energy consumption. Research efforts improving efficiency of existing U.S. homes will have opportunity to greatly impact national energy consumption by addressing a significant energy savings potential. Driving towards net-zero energy buildings provides an ideal scenario where homes produce all energy demands on-site. A joint research project, the ReNEWW House, captured an approach of renovating a home to net-zero energy. An efficient and advanced, hybrid air-hydronic HVAC system was installed. During the first heating season, excessive heat pump short-cycling occurred from improper buffer tank controls by default manufacturer settings. Modifications to the temperature control strategy recorded lower energy use through an improved heating system effectiveness. Results were collected over two heating seasons from in-field measurements. During the first heating season, the heat pump used 5440 kWh of electricity at annual heating degree day (HDD) of 6080 °F-day. The second season measured 3920 kWh of electricity at annual HDD of 4800 °F-day. The effectiveness for the heating season increased from 0.89 kWh/°F-day for the first season to 0.82 kWh/°F-day for the second. [ABSTRACT FROM AUTHOR]

Selecting appropriate working fluids is important for improving the technology readiness level of electrochemical heat pumps (EHP), which is a promising emerging technology for space conditioning and thermal management applications. The major contribution of this paper is to develop and evaluate key metrics to identify electrochemically active working fluids based on heat pump cycle requirements. The key metrics include the possibility of phase change after a chemical reaction, reversible cell voltage and power consumption, cooling and heating capacity, and environmental and safety aspects. Such metrics are applied to conduct fluid screening to assess their suitability. Isopropanol/acetone showed the highest potential to deliver efficient cooling and heating when applied to an EHP system; the use of ethanol/acetaldehyde and methanol/formaldehyde would easily enable electrochemical fluid condensation over a wide range of operating temperatures. Although the approach is demonstrated for selected organic fluids, the screening framework can be readily employed for other chemical compounds expanding the design space of electrochemical heat pumps. • Developed a tool for thermodynamic evaluation of electro-active working fluids. • Evaluation metrics include thermophysical, reactions, GWP, and safety properties. • In cooling mode, system efficiency is improved by 15%–25% using isopropanol/acetone. • Ethanol/acetaldehyde system can operate down to −27.4 °C in heating mode. • Flammability and toxicity of the fluids should be addressed using secondary loops. [ABSTRACT FROM AUTHOR]

Highly constrained air flow pathways as experienced in tightly packaged air conditioning systems result in air flow maldistribution problems in the evaporators. The interleaved circuitry method, where the refrigerant from a circuit with high air flow is routed to a circuit with low air flow and vice-versa, has been investigated to passively reduce the air maldistribution effect. Air velocity measurements have been conducted in psychrometric chambers and the measurement locations have been defined by the log-Tchebycheff rule. The velocity profile was obtained by Lagrange Interpolation method as percentage values. The performance of the interleaved circuitry method was compared to the baseline circuitry at different operating conditions. The results show that the interleaved circuitry method uniforms the superheat of the individual circuits and improves the cooling capacity and COP by up to 16.6% and 12.4%, respectively. Furthermore, the tuned model predicted the evaporator cooling capacity within a mean absolute error of approximately ±10%. [ABSTRACT FROM AUTHOR]

The annual saving rates of primary energy and operating cost of NH 3 and CO 2 heat pumps for seven food processes in four characteristic locations with different energy prices in the U.S. are analyzed. The modeled food processes include beer brewing, food pasteurization, fluid milk processing, cheese processing, vegetable and fruit canning, juice canning, and poultry slaughtering. The investigation focuses on the effects of the characteristics of the thermal demands, the location of the proposed systems, and the energy price structure on the savings of primary energy and the operating cost. The results indicate that the characteristics of the thermal demands are the most crucial factor. The energy price structure significantly impacts the operating cost-saving rate. The analysis shows that the annual savings of primary energy can achieve 32% in food pasteurization, 44% in beer brewing, and 35% in fluid milk processing. [ABSTRACT FROM AUTHOR]

Food processing has significant simultaneous requirements of cooling, warm water and hot water. In order to reduce energy consumption and greenhouse gases emission, one type of NH 3 heat pump and two types of transcritical CO 2 heat pumps are proposed. These natural refrigerant heat pumps can supply not only cooling, but also warm water and hot water simultaneously. The characteristics and performance of the heat pumps are analyzed and simulated. Annual primary energy saving and annual operation cost saving are predicted for California, Wisconsin, New York, and Florida. Research results show that the maximum possible value of annual primary energy-saving rates using the CO 2 heat pumps ranges from 56% to 65%, and using the NH 3 heat pump is approximately 44%; the maximum possible value of annual operation cost saving rates using the CO 2 heat pumps ranges from 50% to 66%, and using the NH 3 heat pump is from 20% to 47%. [ABSTRACT FROM AUTHOR]

This paper introduces the differences of in-system testing of a dual-port vapor injected compressor as compared to test stand testing and presents the performance results. Single-port vapor injected compression has been studied previously, typically using scroll compressors. Single-port vapor injection employs one port with a single intermediate injection pressure, while dual-port vapor injection uses two ports at different angular locations, leading to two intermediate pressure levels for the injected vapor. The main differences between in-system testing and test stand testing can be found in suction and injection superheat values, discharge temperature, and range of injection flow rates. Dimensionless PI-type mappings are introduced that can be used for system simulation. [ABSTRACT FROM AUTHOR]

In conventional vapor compression systems, electronic expansion valves (EXVs) are used for refrigerant flow control. In addition, the EXV control signal can be used along with a model and measurements of boundary conditions to provide a virtual sensor for refrigerant mass flow rate as outlined by Kim and Braun (2012). For typical single-phase inlet conditions, models for characterizing the relationship between the inlet and outlet conditions, the opening, and the mass flow rate have been extensively studied, e.g. by Park et al. (2007), and semi-empirical as well as empirical correlations have been developed. However, two-phase inlet conditions can exist at low refrigerant charge, or for certain applications such as when balancing valves are used in a hybrid control scheme as proposed by Kim et al. (2008). This paper presents a continuous correlation that can be used for both two-phase and subcooled valve inlet conditions. [ABSTRACT FROM AUTHOR]

In this work, a comprehensive framework and geometric solutions are presented for the most common type of scroll machine, the symmetric constant-wall-thickness scroll compressor based on the involute of a circle. This analytic treatment comprises solutions for the volume, derivative of volume, centroid, normalized force and moment parameters, and leakage areas for all chambers in the machine. The representations for the centroid and area are based on Green's Theorem analysis which allows for general solutions that can then also be applied to other machines. Analytic solutions are also presented for the discharge geometry corresponding to arc-line-arc, perfect-meshing-profile, one-arc, and two-arc discharge geometries. Each of the derived parameters is validated against numerical solutions based on polygons. Source code in the Python programming language is provided for the analytic derivations as well as the implementation of each of the analytic outputs. [ABSTRACT FROM AUTHOR]

Ray was born in St. Louis, Missouri on November 30, 1923 and migrated to West Lafayette, Indiana following high school graduation in 1941 to attend Purdue University and study Mechanical Engineering. Raymond Cohen, founding editor of HVAC & R Research, the predecessor of Science and Technology for the Built Environment passed away on September 21, 2020 at his home in Valparaiso, Indiana at the age of 96. Ray was extremely active within ASHRAE during and well beyond his career at Purdue, receiving numerous awards including ASHRAE Fellow, the E.K. Campbell Award, the Louise & Bill Holladay Distinguished Fellow Award, and the Distinguished 50-Year Member Award. [Extracted from the article]

Abstract: A comprehensive model of a linear compressor for electronics cooling was previously presented by Bradshaw et al. (2011) then enhanced and used for a sensitivity analysis of the leakage gap, eccentricity, and piston geometry by Bradshaw et al. (2013). The current work utilizes the previously developed model to explore the energy recovery characteristics of a linear compressor as compared to those of a reciprocating compressor. The impact of dead (clearance) volume on both a linear and reciprocating compressor is analyzed. In contrast to a reciprocating compressor the overall isentropic efficiency of the linear compressor remains relatively unaffected by an increase in dead volume up to a certain point. This behavior is attributed to the ability of the linear compressor to recapture the energy of the compressed gas during the expansion process. This characteristic behavior allows a linear compressor to be used for efficient capacity control from roughly 35–100%. [Copyright &y& Elsevier]

Abstract: A comprehensive model of a linear compressor for electronics cooling was previously presented by Bradshaw et al. (2011). The current study expands upon this work by first developing methods for predicting the resonant frequency of a linear compressor and for controlling its piston stroke. Key parameters governing compressor performance – leakage gap, eccentricity, and piston geometry – are explored using a sensitivity analysis. It is demonstrated that for optimum performance, the leakage gap and frictional parameters should be minimized. In addition, the ratio of piston stroke to diameter should not exceed a value of one to minimize friction and leakage losses, but should be large enough to preclude the need for an oversized motor. An improved linear compressor design is proposed for an electronics cooling application, with a predicted cooling capacity of 200 W a cylindrical compressor package size of diameter 50.3 mm and length 102 mm. [Copyright &y& Elsevier]

Abstract: A comprehensive simulation model of a novel rotating spool compressor is presented. The spool compressor provides a new rotary compression mechanism with easily manufactured components. A detailed analytical geometry model of the spool compressor is presented, which includes the geometry of the vane. This geometry model is included in an overall comprehensive compressor model that includes sub-models for friction, leakage, and heat transfer. The results of the comprehensive model were validated using experimental data from a prototype compressor. The prototype compressor has an overall displacement of 23.9 cm3, and was operated using R410A as the working fluid. The model predicts the volumetric efficiency, discharge temperature, and shaft power of the prototype compressor to within 3.13% MAE, 16.5 K and −13.2 W average deviation, respectively. The trends and spread in the data indicate that additional effort should be focused on the operation of the active sealing elements within the compressor. [Copyright &y& Elsevier]

Abstract: An empirical frictional correction factor to the isentropic nozzle model has been developed for application to refrigerant leakage modeling in scroll, rotary and other similar compressors and expanders. This correction factor is derived by calculating the leakage mass flow rate with a compressible, variable area, real gas properties model and referencing the results to an isentropic nozzle model. The ratio of flows is correlated to the Reynolds number, a characteristic length and the leakage gap width. A representative selection of fluids and geometries are employed. For all the correlations, at least 93% of the points are predicted within an absolute error band of 20%. [Copyright &y& Elsevier]

Abstract: In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. The compressor was tested with oil injection mass flow fractions of up to 45%. At an evaporating temperature of −10 °C and condensing temperature of 30 °C, the overall isentropic efficiency was up to 70% at the highest oil injection rate. Overall, over the testing envelope investigated, there are no significantly negative effects experienced for the compressor and the compressor isentropic efficiency and refrigerant mass flow rate improve monotonically as the oil injection rate is increased. [Copyright &y& Elsevier]

Abstract: In a scroll-type compressor, compression is achieved through relative contact between two spiral curves. Since the scroll invention by Leon Creux (1905), multiple methods have been developed for calculating scroll geometry. What can generally be considered the most classical method, is defining each scroll curve as the involute of a circle. Gravesen and Henriksen (2001) introduced a new method to calculate scroll geometry by deriving each scroll curve from the radius of curvature parameterized with involute angle. This allows a wide range of involute geometries to be considered not included in the classical method. In this paper, Gravesen's method is extended to the tip region to include all tip geometries involved in a two arc configuration resulting in a more comprehensive scroll geometry definition. Lastly, with parametric representation of all scroll geometry, the pocket volume can be easily solved using a derived control volume approach. [Copyright &y& Elsevier]

Abstract: In two companion papers, simulation models for the working processes of liquid-flooded scroll compressors and expanders have been developed and validated against experimental data. In this study, analytic models are presented for the modes of irreversibility generation in the liquid-flooded scroll compressor including built-in volume ratio maladjustment, pressure drop and leakage. A thermodynamic model is used to derive the ideal volume ratio for a liquid-flooded compressor, which is higher than that of dry compression. An optimum set of built-in volume ratio and scroll base circle radius is found which maximizes the overall isentropic efficiency by minimizing the leakage and pressure drop irreversibilities. The irreversibility generation models are used to optimize a scroll compressor for the Liquid-Flooded Ericsson cycle application. The model predicts scroll compressor overall isentropic efficiency of over 80% (based on the shaft power) at an oil mass fraction of 88%. [Copyright &y& Elsevier]

Abstract: A rigorous mathematical approach is developed for optimization of sustainable single-effect water/Lithium Bromide (LiBr) absorption cooling cycles. The multi-objective formulation accounts for minimization of the chiller area as well as the environmental impact associated with the operation of the absorption cycle. The environmental impact is quantified based on the global warming potential and the Eco-indicator 99, both of which follow principles of life cycle assessment. The design task is formulated as a bi-criterion non-linear programming problem, the solution of which is defined by a set of Pareto points that represent the optimal compromise between the total area of the chiller and global warming potential. These Pareto sets are obtained via the epsilon constraint method. A set of design alternatives are provided for the absorption cycles rather than a single design; the best design can be chosen from this set based on the major constraints and benefits in a given application. The proposed approach is illustrated design of a typical absorption cooling cycle. [Copyright &y& Elsevier]

Abstract: This paper presents a study of an ejector expansion CO2 air conditioner system. A comprehensive analytical model for 3-ton air-to-air controllable ejector expansion transcritical CO2 air conditioners was developed and validated. The ejector component efficiencies were estimated using empirical correlations, instead of being simply assumed to be fixed values as done in other literature studies. Parametric studies of the ejector expansion CO2 systems show that the coefficient of performance (COP) and the cooling capacity reach maximum levels when the motive nozzle throat diameter becomes 2.8mm; the maximum cooling COP and cooling capacity occurred for a mixing section constant-area diameter of between 4.1 and 4.2mm; COP and cooling capacity are affected by the outdoor air temperature. At particular ejector geometries and an outdoor temperature of 37.8°C, the transcritical CO2 system using an ejector as the expansion device outperformed a conventional expansion-valve transcritical CO2 system in COP and cooling capacity by approximately 30.7% and 32.1%, respectively. A careful selection of ejector geometry, such as motive nozzle throat diameter and mixing section constant-area diameter, is critical in order to improve the performance of an ejector expansion CO2 air conditioning system. [Copyright &y& Elsevier]

Abstract: This paper presents a comprehensive study on a controllable two-phase flow ejector component efficiencies and ejector performances in transcritical CO2 refrigeration cycles. A two-phase flow ejector expansion model was developed and experimentally validated. A method of determining the efficiencies of ejector motive nozzle, suction nozzle, and mixing section based on the measured performance data external to the ejector was developed. It was found that motive nozzle efficiency decreases as ejector throat area decreases, and that suction nozzle efficiency is affected by outdoor air temperature. Empirical correlations of ejector component efficiencies were reduced. Parametric modeling study results show that the operation of an ejector reaches its optimum performance at ejector throat diameter approximately of 2.3 mm, mixing section constant-area diameter approximately of 3.5 mm, and diffuser diameter ratio a little bit larger than 2. Higher efficiencies of motive nozzle and suction nozzle as well as higher motive nozzle inlet pressure result in better ejector performances in certain ranges of motive nozzle throat diameter and mixing section constant-area diameter. [Copyright &y& Elsevier]

Abstract: Vapor compression refrigeration technology has seen great improvement over the last several decades in terms of cycle efficiency through a concerted effort of manufacturers, regulators, and research engineers. As the standard vapor compression systems approach practical limits, cycle modifications should be investigated to increase system efficiency and capacity. One possible means of increasing cycle efficiency is to flood the compressor with a large quantity of oil to achieve a quasi-isothermal compression process, in addition to using a regenerator to increase refrigerant subcooling. In theory, compressor flooding and regeneration can provide a significant increase in system efficiency over the standard vapor compression system. The effectiveness of compressor flooding and regeneration increases as the temperature lift of the system increases. Therefore, this technology is particularly well suited towards lower evaporating temperatures and high ambient temperatures as seen in supermarket refrigeration applications. While predicted increases in cycle efficiency are over 40% for supermarket refrigeration applications, this technology is still very beneficial for typical air-conditioning applications, for which improvements in cycle efficiency greater than 5% are predicted. It has to be noted though that the beneficial effects of compressor flooding can only be realized if a regenerator is used to exchange heat between the refrigerant vapor exiting the evaporator and the liquid exiting the condenser. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive model of a miniature-scale linear compressor for electronics cooling is presented. Linear compressors are appealing for refrigeration applications in electronics cooling. A small number of moving components translate to less theoretical frictional losses and the possibility that this technology could scale to smaller physical sizes better than conventional compressors. The model developed here incorporates all of the major components of the linear compressor including dynamics associated with the piston motion. The results of the compressor model were validated using experimental data from a prototype linear compressor. The prototype compressor has an overall displacement of approximately 3 cm3, an average stroke of 0.6 cm. The prototype compressor was custom built for this work and utilizes custom parts with the exception of the mechanical springs and the linear motor. The model results showed good agreement when validated against the experimental results. The piston stroke is predicted within 1.3% MAE. The volumetric and overall isentropic efficiencies are predicted within 24% and 31%, MAE respectively. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an analytical approach for modeling the transient dynamic forces in a diaphragm compressor which operates under the action of an electrostatically actuated diaphragm. An experimentally validated, quasi-static model for a diaphragm compressor for electronics cooling was previously developed in which dynamic effects were neglected. In the new model, the dynamic forces induced due to the finite time necessary for deflection of the diaphragm are taken into consideration using the segmentation approach developed earlier. Results from the analytical model compare favorably with those from a detailed numerical simulation as well as with experimental measurements available in the literature. The analytical dynamic model is applied to two different pumping devices to illustrate the effects of the dynamic forces on the overall performance of the device. The effect of pumping frequency of the device on the operating voltage is also explored. [Copyright &y& Elsevier]

Abstract: This paper explores the feasibility of using electrostatically actuated diaphragm compressors in a miniature-scale refrigeration system for electronics cooling. A previously developed experimentally validated analytical model for the diaphragm compressor is used in conjunction with an optimization approach to determine the required dimensions for the compressor. The analysis reveals that the pressure rise and volume flow rate required for the electronics cooling application are not achieved using a single compressor because of material property limitations. A three-dimensional array of compressors is proposed instead with which the cooling requirements and the size restrictions for electronics cooling applications may be simultaneously satisfied. [Copyright &y& Elsevier]

Abstract: Recent reviews of flow boiling heat transfer in small tubes and channels have highlighted the need for predictive correlations that are applicable over a wide range of parameters and across different studies. A composite correlation is developed in the present work which includes nucleate boiling and convective heat transfer terms while accounting for the effect of bubble confinement in small channels. The correlation is developed from a database of 3899 data points from 14 studies in the literature covering 12 different wetting and non-wetting fluids, hydraulic diameters ranging from 0.16 to 2.92mm, and confinement numbers from 0.3 to 4.0. The mass fluxes included in the database range from 20 to 3000kgm−2 s−1, the heat fluxes from 0.4 to 115Wcm−2, the vapor qualities from 0 to 1, and the saturation temperatures from −194 to 97°C. While some of the data sets show opposing trends with respect to some parameters, a mean absolute error of less than 30% is achieved with the proposed correlation. [Copyright &y& Elsevier]

Abstract: Flow boiling heat transfer with the refrigerants R-134a and R-245fa in copper microchannel cold plate evaporators is investigated. Arrays of microchannels of hydraulic diameter 1.09 and 0.54mm are considered. The aspect ratio of the rectangular cross section of the channels in both test sections is 2.5. The heat transfer coefficient is measured as a function of local thermodynamic vapor quality in the range −0.2 to 0.9, at saturation temperatures ranging from 8 to 30°C, mass flux from 20 to 350kgm−2 s−1, and heat flux from 0 to 22Wcm−2. The heat transfer coefficient is found to vary significantly with heat flux and vapor quality, but only slightly with saturation pressure and mass flux for the range of values investigated. It was found that nucleate boiling dominates the heat transfer. In addition to discussing measurement results, several flow boiling heat transfer correlations are also assessed for applicability to the present experiments. [Copyright &y& Elsevier]

Abstract: Air-source heat pumps for residential heating are more widely used than geothermal heat pumps mainly due to their lower installation costs. Major disadvantages of air-source heat pumps are the decrease of heat output and coefficient of performance (COP), and increase in discharge temperature, towards low outdoor temperatures. In this paper an air-source two-stage heat pump using R410A as the refrigerant was simulated, designed, constructed, and tested at ambient temperatures as low as −30°C and supply temperatures of up to 50°C in air and water heating mode. In addition, the system is able to provide sufficient air conditioning in cooling mode (approximately 50–60% of the design heat transfer rate in heating mode). A short summary of an extended literature review is shown as well as a theoretical screening study of the three most promising cycles. Furthermore, the design and test results of a breadboard system and a comparison with commercially available heat pumps are presented. Second law efficiencies of up to 45% could be achieved, resulting in a coefficient of performance (COP) of 2.1 at −30°C ambient temperature. [Copyright &y& Elsevier]

Abstract: Flow boiling of refrigerant HFC-134a in a multi-microchannel copper cold plate evaporator is investigated. The heat transfer coefficient is measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. The test piece contains 17 parallel, rectangular microchannels (0.762mm wide) of hydraulic diameter 1.09mm and aspect ratio 2.5. The design of the test facility is validated by a robust energy balance as well as a comparison of single-phase heat transfer coefficients with results from the literature. Results are presented for four different mass fluxes of 20.3, 40.5, 60.8, and 81.0kgm−2 s−1, which correspond to refrigerant mass flow rates of 0.5–2.0gs−1, and at three different pressures 400, 550 and 750kPa corresponding to saturation temperatures of 8.9, 18.7, and 29°C. The wall heat flux varies from 0 to 20W/cm2 in the experiments. The heat transfer coefficient is found to vary significantly with refrigerant inlet quality and mass flow rate, but only slightly with saturation pressure for the range of values investigated. The peak heat transfer coefficient is observed for a vapor quality of approximately 20%. [Copyright &y& Elsevier]

Recent studies relating to saturated flow boiling in minichannels and microchannels are reviewed, with a focus on the functional dependence of the heat transfer characteristics on thermodynamic vapor quality. While there is general agreement in the literature that the heat transfer coefficient increases with increases in heat flux, conflicting trends have been reported on the influence of vapor quality. A compilation and analysis of the results from recent investigations in the literature on flow boiling in small channels is presented. Further, correlations for flow boiling proposed in these studies are quantitatively assessed by comparing them against 10 independent data sets from the published literature covering a range of hydraulic diameters from 0.16 to 2.01 mm. Recommended topics for future research in this field are also identified. [Supplemental materials are available for this article. Go to the publisher's online edition of Nanoscale and Microscale Thermophysical Engineering for the following free supplemental resources: an appendix containing heat transfer correlations and a database collected from the literature for flow boiling heat transfer in small channels.] [ABSTRACT FROM AUTHOR]

Abstract: A novel refrigeration compressor with an integrated method of capacity modulation for use in domestic refrigerators/freezers is proposed and analyzed here. The compressor is called bowtie compressor due to its two sector-shaped, opposing compression chambers forming a bowtie. The bowtie compressor modulates the cooling capacity by changing the piston stroke without changes of the clearance volume for better thermodynamic efficiency. The new compressor includes a unique off-center-line mechanism so that the piston stroke can be varied without creating an extra clearance volume. To investigate the feasibility of the proposed compressor, a simulation model has been developed. A detailed description of the bowtie compressor and its simulation model are presented in this paper. In addition, parametric studies have been carried out to see how the proposed compressor performance can be improved. [Copyright &y& Elsevier]

This paper contains a review of the latest research activities toward transcritical CO2 cycle technology as a replacement for the fluorocarbon-based vapor-compression technology for heat pumping, air-conditioning, and refrigeration applications. In addition, the potential of using the transcritical CO2 cycle technology for particular applications and the barriers that must be overcome before its acceptance are reviewed. Advances in theory and recent experimental data are also included. [ABSTRACT FROM AUTHOR]

A miniature-scale refrigeration system suitable for electronics cooling applications was developed and experimentally investigated. A detailed review of the literature on refrigeration systems and system simulation models for application to electronics cooling is also provided. Experimental results obtained with the prototype system demonstrate its feasibility for use in cooling compact electronic devices. The cooling capacity of the system investigated varied from 121 to 268 W, with a COP of 2.8 to 4.7, at pressure ratios of 1.9 to 3.2. The effectiveness of the condenser ranged from 52% to 77%, while a thermal resistance of 0.60 and 0.77 °C-cm²/W was achieved at the evaporator. The evaporator-heat spreader thermal resistance is defined as the ratio of the temperature difference between the chip surface and the refrigerant evaporator to the evaporator heat transfer rate. The overall system thermal resistance, defined as the ratio of the temperature difference between the chip surface and the condenser air inlet, is of 0.04 to 0.8 °C-cm²/IW. An overall second-law efficiency ranging from 33% and 52% was obtained, using a commercially available small-scale compressor. The measured overall isentropic efficiency was between 25% and 60%. [ABSTRACT FROM AUTHOR]

This paper intends to present a comprehensive summary of the various studies that were conducted with respect to the lubricant influence on refrigerant/oil mixture properties, condensation, and pressure drop, and tries to identify some general relationships. In addition, the research methods and correlations presented on each of these subjects are summarized. Finally, technical recommendations regarding the lubricant influence in each of these aspects are provided in this paper. [ABSTRACT FROM AUTHOR]

Abstract: An ejector expansion transcritical CO2 refrigeration cycle is proposed to improve the COP of the basic transcritical CO2 cycle by reducing the expansion process losses. A constant pressure mixing model for the ejector was established to perform the thermodynamic analysis of the ejector expansion transcritical CO2 cycle. The effect of the entrainment ratio and the pressure drop in the receiving section of the ejector on the relative performance of the ejector expansion transcritical CO2 cycle was investigated for typical air conditioning operation conditions. The effect of different operating conditions on the relative performance of the ejector expansion transcritical CO2 cycle was also investigated using assumed values for the entrainment ratio and pressure drop in the receiving section of the ejector. It was found that the COP of the ejector expansion transcritical CO2 cycle can be improved by more than 16% over the basic transcritical CO2 cycle for typical air conditioning operation conditions. [Copyright &y& Elsevier]

The influence of refrigeration lubricants on pool and flow boiling of refrigerant-oil mixtures is a complex subject, and a consistent relationship cannot be identified. Instead, the influence varies greatly depending on oil concentration, operating parameters, and application. This paper intends to present a comprehensive summary of the various studies that have been conducted in this area and tries to identify some general relationships. In addition, the research methods and correlations presented on each of these subjects are summarized. Finally, technical recommendations regarding the lubricant influences on these aspects are provided in this paper. [ABSTRACT FROM AUTHOR]

A detailed model for hermetic scroll compressors was presented in a companion paper. The current paper presents validation results for this model based upon detailed experimental measurements and application of the model to study the impact of design changes. The experimental study included a range of operating conditions that were controlled using a hot gas bypass compressor load stand. The model was validated in terms of overall performance predictions (refrigerant mass flow rate, discharge temperature, and compressor power consumption) and predictions of internal measurements (refrigerant pressures within the scrolls as a function of orbiting angle, temperatures along the scroll wraps, and temperatures at various points within the shell). The model performed very well in terms of predicting overall performance and was adequate in predicting the internal measurements. The parametric modeling study identifies some areas for potential improvement. Most significantly, a relative improvement of about 5% in overall compressor efficiency can be achieved through small changes in the scroll geometry. [ABSTRACT FROM AUTHOR]

The Nusselt number variations of supercritical carbon dioxide during in-tube cooling are presented and discussed. Using data presented in this paper as well as prior publications, a new correlation to predict the heat transfer coefficient of supercritical carbon dioxide during in-tube cooling has been developed. The new correlation is presented in this paper. It is based on mean Nusselt numbers that are calculated using the thermophysical properties at the wall and the bulk temperatures, respectively. It is seen that the majority of the numerical and experimental values are within ±20% of the values predicted by the new correlation. [Copyright &y& Elsevier]

This paper assesses the application of the second-law of thermodynamics in a military Environmental Control Unit (ECU) to evaluate the exergy destruction (or irreversibility) in each component when operating at high ambient temperature. Experimental testings were conducted on three ECUs, 1.5 (5.3 kW), 3 (10.6 kW), and 5 (17.6 kW) tons of refrigeration (RT), to assess the potential contribution of each component to enhance the overall energy efficiency of the system, and to determine the feasibility of the thermodynamic model presented herein. The analysis provided for extreme high ambient conditions up to 51.7 °C (125 °F). The results yielded that the highest irreversibility was associated with compressors (32.4% to 42.5%). This is followed by the heat exchanges (19.6% to 32.9%) in the case of 1.5-RT and 3-RT units, whereas for the 5-RT unit, the highest irreversibility was associated with the evaporator followed by the one of the compressors. In the 3-RT ECU, the condenser's second-law efficiency enhanced due to an additional fan, yet the working refrigerant increased the irreversibility in the expansion device. The second-law analysis recognized the components with the highest exergy destruction and identified the direction to enhance the exergetic efficiency of any ECU operating at high-temperature climate. [ABSTRACT FROM AUTHOR]

• Design and commissioning for a novel transcritical CO 2 test stand is presented. • The system is based on multi-stage compression and two-evaporators with open economization. • Thermodynamic modeling is used to size the testing apparatus. • Both electronic expansion valve and ejector expansion methods are tested. • Operation of six different cycle architectures without turning off compressors is validated. As the adverse effects of global warming become increasingly apparent, the need for low-GWP solutions in the HVAC&R industry is its highest in decades. Alternative working fluids, including natural refrigerants such as Carbon Dioxide (CO 2), and high efficiency systems are ways to mitigate the negative environmental impact of refrigeration systems. Researchers have focused on investigating several transcritical CO 2 cycles for supermarket and transport applications. Yet, a systematic experimental comparison between multiple advanced cycle architectures is still lacking. To this end, a novel test stand capable of comparing two methods of expansion as well as open-economization at one or both evaporator pressure levels, allowing six cycle configurations that represent much of the state-of-the-art has been developed. Cycle designs, thermodynamic modeling for component sizing, and experimental validation of test stand operation and energy balances are discussed in-depth. Limitations and modifications to the test stand are discussed, as are next steps. [ABSTRACT FROM AUTHOR]