Refrigerant-side Heat Transfer Coefficient Research Articles

Numerical study on the thermal-hydraulic characteristic of an automotive parallel-flow condenser under non-uniform airflow

The performance of an automotive parallel-flow condenser was numerically investigated considering both airside and refrigerant-side flow mal-distributions, the thermal-hydraulic characteristic of the condenser under the air-side blockage of each tube pass was first studied, then the influence of the form and degree of uneven airflow that may be revealed in vehicles on the performance of the condenser was analyzed. At last, using the simulation model combined with the measurements of the vehicle in the climatic wind tunnel, the thermal-hydraulic features of the condenser as well as the performance of the A/C system under various front-end status and vehicle conditions was evaluated. It was found that the performance degradation of the condenser resulting from the air-side blockage of the first pass exceeds that caused by the obstruction of all the other three passes together. According to the performance deterioration sequence of the condenser from high to low, the order of the four possible non-uniform airflow patterns in vehicles is low on the top and high at the bottom (LTHB), low in the middle (LM), high in the middle (HM), and high on the top and low at the bottom (HTLB), and the last two forms could even enhance the performance of the condenser under certain working conditions. The rule of cooling airflow optimization is to arrange large/small air velocity on the area with intrinsically large/small refrigerant side heat transfer coefficient of the condenser. The possible direction of cooling airflow design in vehicles is inferred.

Modeling and Optimization of a Micro-Channel Gas Cooler for a Transcritical CO2 Mobile Air-Conditioning System

This study focuses on developing and optimizing of a microchannel gas cooler model for evaluating the performance of a transcritical CO2 mobile air-conditioning system. A simulation model is developed with the aid of MATLAB R2022a. A segment-by-segment modeling approach is utilized by applying the effectiveness-NTU method. State-of-the-art heat transfer and pressure drop correlations are used to obtain air and refrigerant side heat transfer coefficients and friction factors. The developed model is validated through a wide range of available experimental data and is able to predict a gas cooler capacity and pressure drop within an acceptable range of accuracy. The average errors for a gas cooler capacity and pressure drop are 3.79% and 10.24%, respectively. Furthermore, a parametric optimization method is applied to obtain optimal microchannel heat exchanger dimensions, including the number of tubes, microchannel ports, and passes. Different combinations were selected within the practical range to obtain optimal dimensions while keeping the total core volume constant. The simultaneous effect of the number of tubes, the number of ports in each tube, and the number of passes is determined. The objective of the current optimization technique is to minimize the pressure drop for the specific design capacity under different operating conditions without changing the overall volume of the gas cooler. The average pressure drop reduction for the optimal geometry as compared with the baseline geometry under all operating conditions is about 15%. The results from this study can be used to select an optimal geometric design for the required design capacity with a minimal pressure drop without the need for expensive prototype development and testing.

Mathematical Model of Air Dryer Heat Pump Exchangers

This paper presents a mathematical model of heat pump exchangers and their thermal interaction with a fan for an air dryer. The calculation algorithm developed for the finned heat exchangers is based on the ε-NTU method, allowing the determination of air side and refrigerant side heat transfer coefficients, evaporator and condenser heat capacity and air parameters at the dehumidifier outlet with known exchanger geometries, initial air parameters and mass flow rate. The model was verified on an industrial dehumidifier test bench. This enabled the heat transfer coefficients for the exchanger to be calculated as a function of the speed and, therefore, the power of the fan’s drive motor. An increase in fan performance on the one hand results in an increase in the heat transfer rate, but, on the other hand, it causes an increase in the total energy consumption of the motor. Thus, while it causes an increase in drying capacity, it also causes an increase in the energy consumption of the dehumidifier. In order to optimise the unit in terms of energy consumption, it is therefore necessary to determine a function that relates the amount of heat exchanged to the efficiency of the fan.

A mathematical modelling of frost growth dynamics on a surface of fin-and-tube air cooler

A mathematical model was developed to predict the performance characteristics of fin-and-tube air coolers with plane fins, taking into account the temperature and humidity conditions, non-uniform thickness of the frost layer along the heat exchanger core, flow-pressure characteristics of the fans and the refrigerant superheat zone. The model was verified on the experimental data of air cooler performance in frosting conditions obtained by the authors in air temperature range from minus 6 to minus 20 ° C, and experimental data from independent researchers. The presented model predicts the cooling capacity and the overall heat transfer coefficient in the evaporator with error not exceeded 15%. It was found that during dry operating conditions of the air cooler, the refrigerant boiling temperature descent by 10 °C leads to a decrease in the overall heat transfer coefficient by 35%. The main reason of that is double decrease of the refrigerant mass velocity and the refrigerant-side heat transfer coefficient by 75%. The process of frost grow leads to an increase in overall thermal resistance in the studied air cooler by 30%, more than double increase in the aerodynamic resistance and decrease in cooling capacity by 15%.

Effect of periodic reverse flow on the heat transfer performance of microchannel evaporators

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.

Investigations into air and refrigerant side heat transfer coefficients of finned-tube CO2 gas coolers

Gas coolers are heat rejection heat exchangers in vapour compression refrigeration systems that use carbon dioxide (CO2) as refrigerant. The design of gas coolers has a significant influence on the performance of CO2 refrigeration systems as it determines to a large extent the gas cooler/condenser pressure and the power consumption of the system. This paper investigates local refrigerant and air heat transfer coefficients in plain fin-and-tube gas cooler coils using Computational Fluid Dynamics (CFD) modelling. The aims were to provide insights into the variation of the local heat transfer rates in the coil and determine the influence of a) design enhancements such as the use of slit fins and b) to develop correlations for overall refrigerant and air heat transfer coefficients to be used in CO2 refrigeration component and system modelling. The results from the model which was validated against experimental measurements showed that a horizontal slit on the fin between the first and second row of tubes can lead to an increase in the heat rejection rate of the gas cooler by between 6% and 8%. This in turn can lead to smaller heat exchanger heat transfer area for a given heat rejection capacity or lower high side pressure and higher efficiency for the refrigeration system. The results and heat transfer correlations developed are a valuable resource for researchers and manufacturers of CO2 and other heat exchanger coils that experience a wide variation in refrigerant temperature during the gas cooling process.

Experimental study on single-phase heat transfer and pressure drop of refrigerants in a plate heat exchanger with metal-foam-filled channels

Metal-foam-filled channels are proposed to increase the heat transfer area between hot and cold flows in plate heat exchangers. This experimental study focuses on the single-phase heat transfer mechanism of R245fa refrigerant in a metal-foam-filled plate heat exchanger instead of more commonly used air or water in single-phase experiments. The refrigerant-side heat-transfer coefficient and pressure-drop data are reported. Different metal foams with various pore densities of values of 20, 30, and 60pore per inch (PPI) were examined. Cases were studied with uniform PPI and with two different PPI along the flow direction. A complete heat exchanger test section with an overall volume of 0.001m3 that is able to generate 1–2kW heat duty in this range of experimental conditions is manufactured. The result shows that inserting 60-PPI metal foam increases the refrigerant-side heat transfer coefficient by up to 5.1 times compared to the plate heat exchanger without the metal foam insert. As expected, the pressure drop penalty is huge. The 60-PPI metal foam had the greatest pressure drop, which was 5.7 times that of an empty-channel heat exchanger.

Nucleate boiling of low GWP refrigerants on highly enhanced tube surface

This paper presents the experimental investigation of the heat transfer performance of alternative low GWP (Global Warming Potential) refrigerants during nucleate boiling. The fluids used in the study are R-1234ze, R-1233zd(E), and R-450A. The first two HFOs (hydro-fluoro-olefins) have a GWP index less than or equal to 1 while the zeotropic blend R-450a has a GWP index of 547. This study compared R-1234ze and R-450A to their targeted replacement, R-134a, and compared R-1233zd(E) to R-123 using a highly enhanced tube surface. Tests were conducted at a heat flux range of 10–110kW/m2 and at a saturation temperature of 4.44°C. Results show that the performance of R-1234ze is very similar to that of R-134a while R-450a shows performance degradation of 28% compared to R-134a. For the R-123 replacement, R-1233zd(E) demonstrates a noticeable 19% performance increase. The results section includes a power-law model to predict the refrigerant-side heat transfer coefficient for all five fluids. Meanwhile, the heat flux used in the model covers a range of 10–60kW/m2, a typical range of interest for refrigerant evaporator design.

Evaporator performance enhancement by pulsation width modulation (PWM)

In this work, the effects of pulsation width on heat transfer coefficient and pressure drop in an air-to-refrigerant evaporator is studied experimentally. The pulsation width can be controlled to a minimum period of 2 s. A general model is also developed to predict heat transfer performance and pressure drop. The overall heat transfer coefficient, refrigerant side heat transfer coefficient, and pressure drop are measured and compared to a baseline without pulsating flow. The results show that the overall and refrigerant-side heat transfer coefficients have improved about 27% and 123%, respectively, with pulsating flow. The refrigerant mass flux and pulsation period have an important effect on the heat transfer enhancement. The average pressure drop of the pulsating flow differs little from that of continuous flow, but the temporal pressure drop of pulsating flow is quite different and varies with mass flux and pulsation period.

Water Mass Flow and Air Relative Humidity on Performance of Air - Source Heat Pump Water Heater

The water mass flow and air relative humidity on the performance of air-source heat pump water heater was discussed in this paper. The results show that the energy efficiency of heat pump water heater is improved by correct the water mass flow and discharge pressure can be reduced, the value of the condenser temperature difference between the import and export of water also can be reduced. Increasing the air relative humidity, the more condensated water on evaporator surface effective, the greater the wetting area. In addition, the heat transfer of latent heat can be increased by the growth of air side and refrigerant side heat transfer coefficient.

An Experimental Investigation of the Reynolds Analogy and its Modifications Applied to Annular Condensation Laminar Flow of R134a in a Vertical Tube

The Reynolds analogy and its modifications are applied to forced convection laminar in-tube condensation to predict the heat transfer coefficient of R134a by means of well-known two-phase friction factors and agreed void fraction models and correlations explained in the authors’ previous works. The vertical test section is a 0.5 m long countercurrent flow double tube heat exchanger with refrigerant flowing in the inner smooth copper tube (8.1 mm i.d.) and cooling water flowing in the annulus (26 mm i.d.). The test runs are performed at average saturated condensing temperatures of 40 °C (Pr = 0.92) and 50 °C (Pr = 0.97). The heat fluxes are between 10.16 and 66.61 kW m−2 while the mass fluxes are between 260 and 515 kg m−2 s−1 for the vertical test sections. The Reynolds’ model is modified by various two-phase flow models and correlations to account for the partial condensation inside the tube. The refrigerant side heat transfer coefficients are determined within ±30% using the two-phase friction factors of Wallis, Moeck, Fore et al., while all the proposed friction factor correlations for the Reynolds analogy, Prandtl and Taylor analogy, and Colburn analogy predict the experimental friction factor within a ±20% deviation band. The importance of altering the Prandtl number to the Reynolds analogy (Pr = 1) is also shown in the paper.

Experimental investigation of novel heat exchanger for low temperature lift heat pump

In this paper, the thermal and hydraulic performance of a novel low temperature lift heat exchanger (LTLHX) was experimentally investigated. The novel LTLHX was developed to improve the performance of the conventional plate type heat exchanger (PHX) under low temperature lift operating conditions. The optimized novel LTLHX was fabricated and investigated experimentally with various operating conditions. The heat transfer coefficient correlation and friction factor correlation of the water-side in the novel LTLHX were formulated from the experimental data. The overall heat transfer coefficient of the novel LTLHX with ammonia ranged from 1300 to 2000 W K−1 m−2, and the pressure drop per length of the water-side ranged from 4 to 10 kPa m−1. The refrigerant-side heat transfer coefficient ranged from 2900 to 5000 W K−1 m−2, and water-side heat transfer coefficient ranged from 3900 to 5100 W K−1 m−2. The overall heat transfer performance of the novel LTLHX was more than double that of the PHX at the same operating conditions. Moreover, the water-side pressure drop of the novel heat exchanger was drastically lower than that of the PHX. It was due to the balanced thermal and hydraulic performance of the novel heat exchanger.

소프트 아이스크림 제조기 증발기의 전열 특성

Soft icecream is made by scraping an ice formed on the inside of the cylindrical evaporator, where R-404A is evaporating in the annulus. The heat transfer characteristics of the refrigerant evaporation and those during icecream formation were experimentally investigated. Results show that the refrigerant-side heat transfer coefficients are highly dependent on the location in the evaporator due to the complex annulus configuration. The heat transfer coefficient at the inlet is generally lower than those of other locations. The average heat transfer coefficient increases as heat flux increases or saturation temperature decreases. A correlation is developed to predict the refrigerant-side heat transfer coefficient. The icecream-side heat transfer coefficient oscillates continuously due to the periodic removal of ice formed on the surface. The average heat transfer coefficient during icecream formation is approximately 280 W/m 2 K, and that during single-phase cooling increased from 150 W/㎡K to 250 W/m 2 K.

Mini-channel evaporator/heat pipe assembly for a chip cooling vapor compression refrigeration system

We investigate a novel evaporator design for a small-scale refrigeration system whose function is to assist the existing heat pipe technology currently used in chip cooling of portable computers. A heat transfer model for the evaporator/heat pipe assembly was devised specifically for sizing the evaporator in order to keep the chip surface temperature below a certain value. A prototype was tested with R-600a at saturation temperatures of 45 and 55 °C, mass flow rates between 0.5 and 1.5 kg h −1 and heat transfer rates between 30 and 60 W. The experimental results demonstrated that the average refrigerant-side heat transfer coefficient is more sensitive to a change in the refrigerant mass flux than to changes in the saturation temperature and heat transfer rate. The agreement between the calculated heat transfer coefficient and the data was within ±10% for the conditions evaluated.

Compact heat exchangers modeling: Condensation

A model for the analysis of compact heat exchangers working as either evaporators or condensers is presented. This paper will focus exclusively on condensation modeling. The model is based on cell discretization of the heat exchanger in such a way that cells are analyzed following the path imposed by the refrigerant flowing through the tubes. It has been implemented in a robust code developed for assisting with the design of compact heat exchangers and refrigeration systems. These heat exchangers consist of serpentine fins that are brazed to multi-port tubes with internal microchannels. This paper also investigates a number of correlations used for the calculation of the refrigerant side heat transfer coefficient. They are evaluated comparing the predicted data with the experimental data. The working fluids used in the experiments are R134a and R410A, and the secondary fluid is air. The experimental facility is briefly described and some conclusions are finally drawn.

Effect of Coolant Temperature on the Condensation Heat Transfer in Air-Conditioning and Refrigeration Applications

This article directly investigates the effect of a cooling medium's coolant temperature on the condensation of the refrigerant R-134a. The study presents an experimental investigation into condensation heat transfer, vapor quality, and pressure drop of R-134a flowing through a commercial annular helicoidal pipe under the severe climatic conditions of a Kuwait summer. The quality of the refrigerant is calculated using the temperature and pressure obtained from the experiment. Measurements were performed for refrigerant mass fluxes ranging from 50 to 650 kg/m2s, with a cooling water flow Reynolds number range of 950 to 15,000 at a fixed gas saturation temperature of 42°C and cooling wall temperatures of 5°C, 10°C, and 20°C. The data shows that with an increase of refrigerant mass flux, the overall condensation heat transfer coefficients of R-134a increased, and the pressure drops also increased. However, with the increase of mass flux of cooling water, the refrigerant-side heat transfer coefficients decreased. Using low mass flux in a helicoidal tube improves the heat transfer coefficient. Furthermore, selecting low wall temperature for the cooling medium gives a higher refrigerant-side heat transfer coefficient.

Heat transfer and pressure drop during HFC refrigerant vaporisation inside a brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during HFC refrigerant 134a, 410A and 236fa vaporisation inside a small brazed plate heat exchanger: the effects of heat flux, refrigerant mass flux, saturation temperature, outlet conditions and fluid properties are investigated. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow. HFC-410A shows heat transfer coefficients 40–50% higher than HFC-134a and 50–60% higher than HFC-236fa and frictional pressure drops 40–50% lower than HFC-134a and 50–60% lower than HFC-236fa. The experimental heat transfer coefficients are compared with two well-known equations for nucleate boiling [M.G. Cooper, Heat flows rates in saturated pool boiling – a wide ranging examination using reduced properties, Advanced Heat Transfer, Academic Press, Orlando, Florida, 1984, pp. 157–239; D. Gorenflo, Pool boiling, in: E.U. Schlünder (Ed.), VDI Heat Atlas, Dusseldorf, Germany, 1993, Ha1-25] and a correlation for frictional pressure drop is proposed.

HFC-410A vaporisation inside a commercial brazed plate heat exchanger

This paper experimentally investigates HFC-410A vaporisation inside a commercial brazed plate heat exchanger: the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are evaluated. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux. The experimental heat transfer coefficients are compared with two well-known equations [Cooper, M.G., Heat flows rates in saturated pool boiling – a wide ranging examination using reduced properties, in: J.P. Hartnett, T.F. Irvine Jr. (Eds.), Advanced in Heat Transfer, Academic Press, Orlando, FL, 1984, pp. 157–239; D. Gorenflo, D. Pool boiling, in: E.U. Schlünder (Ed.), VDI Heat Atlas, Dusseldorf, Germany, 1993 (Ha1-25).] for nucleate boiling and a correlation for frictional pressure drop is proposed.

Refrigerant R134a vaporisation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant R134a vaporisation inside a small brazed plate heat exchanger (BPHE): the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are investigated. The BPHE tested consists of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and corrugation amplitude of 2 mm. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity both to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow. The experimental heat transfer coefficients are also compared with two well-known correlations for nucleate pool boiling and a correlation for frictional pressure drop is proposed.

Numerical analysis on a microchannel evaporator designed for CO 2 air-conditioning systems

A microchannel heat exchanger was numerically analyzed using the finite volume method. The air and refrigerant-side heat transfer coefficients and pressure drops were calculated using the existing correlations that were developed for microchannel heat exchangers. To verify the present model, performance tests of the microchannel heat exchanger were conducted at various test conditions with R134a. The present model yielded a good correlation with the measured heat transfer rate, demonstrating a mean deviation of 6.8%. The performance of the microchannel evaporator for CO 2 systems can be improved by varying the refrigerant flow rate to each slab and changing fin space to increase the two-phase region in the microchannel. Based on the comparison of the performance of the microchannel heat exchanger with that of the fin-tube heat exchanger designed for CO 2 systems, it was proposed that the arrangement of the slabs and inlet air velocity in the microchannel heat exchanger need to be optimized by considering heat exchanger size, air outlet conditions and required capacity.