Numerical modelling of alternative refrigerants to HCFC‐22 through capillary tubes

In this paper, a numerical model is presented for predicting capillary tube performance using new alternative refrigerants to HCFC‐22. The model has been established after the fluid flow conservation equations written for a homogeneous refrigerant fluid flow under saturated, sub‐cooled and two‐phase conditions. Numerical results showed that the proposed model in question fairly simulated our experimental data and fairly predicted the capillary tube behaviour under different conditions. The results also indicated that a system using R‐407C would experience smaller pressure drop compared to R‐410A and R‐410B. Copyright © 2000 John Wiley & Sons, Ltd.     

Two‐phase flow boiling characteristics of alternatives to R‐22 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two‐phase flow boiling of alternative zeotropic refrigerant mixtures to R‐22, on air/refrigerant horizontal enhanced surface tubing is presented. The new alternatives considered in this study are: R‐507, R‐404A, R‐408A, R‐407C, and R‐410A. The experimental data showed that R‐22 has the highest heat transfer rate compared to the other blends in the range investigated. Furthermore, it was also quite evident from these data that R‐410A has the highest pressure among the blends under investigation for Reynolds number greater than 3.5×10⁴. However, for Reynolds number less than 3.6×10⁴, it appears from the data that R‐22 has the highest pressure drop compared to other refrigerants under investigation. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical analysis of adiabatic flow of refrigerant through a spiral capillary tube

In the present work, a homogenous model including the metastable liquid region has been developed for the adiabatic flow of refrigerant through the spiral capillary tube. In order to develop the model, both liquid region and two phase region have been discretized into infinitesimal segments to take into account the effect of varying radius of curvature of spiral tube on the friction factor. The effect of the pitch of spiral on the mass flow rate of refrigerant and capillary tube length has been investigated. A comparison of flow characteristics of refrigerant R22 and its alternatives, i.e., R407C and R410A has been made at different operating conditions at the inlet of the capillary tube and it has been found that the flow characteristics of R22 and R407C are almost similar for a given condenser pressure and degree of subcooling at the inlet of capillary tube.     

Boiling characteristics of alternatives to R‐502 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two‐phase flow boiling of alternative zeotropic refrigerant mixtures to R‐502, on air/refrigerant horizontal enhanced surface tubing is presented. The new alternatives considered in this study are: R‐507, R‐404A, R‐407B, and R‐408A. It was evident from the experimental data that R‐502 has the lowest heat transfer rate compared to the other blends in the range investigated. Furthermore, it was also quite evident from these data that R‐407B has the highest pressure among the blends under investigation. However, at a Reynolds number less than 3.6×10⁴, it appears from the data that R‐404A has the lowest pressure drop compared to other refrigerants under investigation. Copyright © 2000 John Wiley & Sons, Ltd.     

Prediction of capillary tubes with alternative refrigerants to CFC‐502

A numerical model is presented in this paper, for predicting capillary tube performance using new alternative refrigerants to CFC‐502. The model has been established after the fluid flow conservation equations written for a homogeneous azeotropic refrigerant fluid flow under saturated, sub‐cooled and two‐phase conditions. The study was limited to the following azeotropic mixtures; R‐507, R‐404A, and quaternary mixture (R32/R125/R134a/R143a). Numerical results showed that the proposed model in question fairly simulated our experimental data and fairly predicted the capillary tube behaviour under different conditions. The results also indicated that a system using R‐507 would experience smaller pressure drop across the capillary compared to the other alternatives under question. Copyright © 2001 John Wiley & Sons, Ltd.     

Experimental investigation of two phase flow condensation of alternatives to HCFC-22 inside enhanced surface tubing

This paper presents an experimental study of two phase flow condensation of some alternative zeotropic refrigerant mixtures to R-22, inside air/refrigerant horizontal enhanced surface tubing. The alternatives considered in this study are; R-507, R-404A, R-407C, and R-408A as well as R-410A. It was evident from the condensation experimental data that R-408A has the highest heat transfer rate compared to the other blends under the investigated range of refrigerant mass flow rates and heat flux. However, when the thermophysical properties are factored in, the condensation data showed that R-410A has the highest heat transfer rate at Reynolds number higher than 2.35E+7 Furthermore, experimental data of two phase condensation pressure gradient data across the test section at different Reynolds numbers showed that R-410A has the highest convective pressure drop among the blends under investigation.     

Heat transfer prediction of air‐to‐refrigerant two‐phase flow boiling of alternatives to HCFC‐22 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two‐phase flow boiling of some alternative refrigerants to HCFC‐22, on air/refrigerant horizontal enhanced surface tubing, is presented. Correlations have been proposed to predict the heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of alternatives to R‐22; such as R‐507, R‐404A, R‐407C, R‐410A and R‐408A in two‐phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was found that the correlations were applicable to the entire heat and mass flux, investigated in the present study, for the proposed blends under question. The deviation between the experimental and predicted values for the heat transfer coefficient and pressure drop were less than ±20, and ±35 per cent, respectively, for the majority of data. Copyright © 2000 John Wiley & Sons, Ltd.     

Prediction of two‐phase condensation characteristics of some alternatives to R‐22 inside air/refrigerant enhanced surface tubing

The results of an experimental study on the heat transfer characteristics of two‐phase flow condensation of some azeotropic refrigerant mixtures, proposed as alternatives to R‐22, on air/refrigerant horizontal enhanced surface tubing are presented. The condensation data indicated that the heat transfer coefficient of the blend R‐408A has the highest heat transfer rate among the blends under investigation. The condensation data also showed that R‐507 and R‐404A have similar heat transfer rates to that of R‐22 when plotted against the refrigerant mass flow rate. It can also be observed that, as the mass flux increases, the heat transfer coefficient increases. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of alternatives to R‐22 such as R‐507, R‐404A, R‐407C and R‐408A, as well as R‐410A in two‐phase flow condensation inside enhanced surface tubing. In addition, proposed correlations were found to fairly predict the two‐phase flow heat transfer condensation data. Copyright © 2000 John Wiley & Sons, Ltd.     

Analysis on the adiabatic flow of R407C in capillary tube

In this paper the adiabatic flow in the capillary tube is analyzed and modeled for R407C, which is a non-azeotropic mixed refrigerant and one of the alternatives to R22. The equations of energy, continuity and pressure drop through a capillary tube are presented. A mathematical model of the sub-cooled flow region and the two-phase flow region is developed. The results of the calculation show that this numerical model is capable of providing an effective means to analyze components’ performance in optimizing and controlling a R407C air-conditioning system.     

Performance analysis of new alternatives to HCFC‐22 inside air/refrigerant‐enhanced surface tubing

In this paper, the test results of a performance analysis of new alternatives; R‐410A, R‐507, R‐407C, R‐408A and R‐404A proposed as substitutes to HCFC‐22 are presented. The test results were obtained using an air‐source heat pump with enhanced surface tubing. Performance tests were conducted according to the ARI/ASHRAE Standards. The performance data demonstrated that as an interim replacement, the R‐404A blend has a superior performance particularly at low temperatures among the proposed blends. Furthermore, the alternatives to R‐22 are characterised by high discharge pressure compared to that of R‐22. In particular, R‐410A has the highest discharge pressure among the blend studied. R‐407C has similar discharge temperature to R‐22 at temperatures lower than −8°C. Copyright © 2000 John Wiley & Sons, Ltd.     

Influence of magnetic field on two‐phase flow convective boiling of some refrigerant mixtures

In this paper, an experimental study on the influence of magnetohydrodynamic (MHD) on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of MHD on the heat transfer characteristics such as average heat transfer coefficients, and pressure drops of R‐507, R‐404A, R‐410A, and R‐407C in two‐phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was also evident that the proposed correlations for predicting the heat transfer characteristics were applicable to the entire heat and mass flux, investigated in the present study. The deviation between the experimental and predicted value using new and improved correlations for the heat transfer coefficient and pressure drop were less than ±20%, for the majority of data. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance study on the evaporation and pressure drop of low‐temperature refrigerant blends in porous media

An experimental investigation on the performance of different low‐temperature refrigerant blends is presented in this work. Five different low‐temperature refrigerant blends are put on display to replace the R22 refrigerant, which has a high ozone depletion potential. These five blends are R404A, R407C, R410A, R417A, and R422A. Different performance studies have been performed on these alternative refrigerants to replace R22. A comparative experimental performance study is performed during the evaporation of these refrigerant blends in porous media. A porous metallic heat transfer medium is used with different porosities (40%, 43%, and 45%) in the evaporator during the test experiments. The evaporator superheat and the condenser subcool are maintained constant throughout the experiments at 8°C (±0.5°C) and 6°C (±0.5°C), respectively. The condensing temperature is kept constant at 38.5°C, and the mean evaporating temperatures were selected to be from ?33 to ?18°C. The effect of the above‐mentioned given operating conditions on the compressor discharge temperature, evaporation pressure drop, evaporation capacity, and coefficient of performance of these five low‐temperature refrigerant blends has been analyzed for different porosities. This experimental study showed that the refrigerant R422A can give a similar or greater performance to R22 and R404A with a global warming effect and zero ozone depleting potential.     

Two phase flow heat transfer of binary mixtures inside enhanced surface tubing

The primary heat transfer parameters such as coefficient of heat transfer and pressure drop observed during condensation of binary azeotropic refrigerant mixtures R-410a (R125/R32: 50/50), and R-507 (R125/R143a: 50/50) are presented in this paper.Experiments showed that for Reynolds numbers higher than 4.2 E06, R-410a appears to have greater heat transfer rates more than the other blends under investigation. Furthermore, it is quite evident from this data that R-507 has the highest pressure drop among the refrigerants under investigation.     

Performance evaluation of heat exchanger using alternative refrigerant R407C

R22 (HCFC22) has been widely used as the refrigerant in air conditioners. According to the Montreal protocol for ozone layer protection, the total production of HCFCs has been capped since the beginning of 1996. Zeotropic refrigerant mixture R407C and nearly azeotropic refrigerant mixture R410A have been selected as alternatives to R22. We examined refrigerant passages in heat exchangers used in heat pump‐type room air conditioners using zeotropic refrigerant R407C through simulation, and obtained the following conclusions. In an indoor heat exchanger, a counter flow configuration when operating as a condenser has higher temperature efficiency. When an outdoor heat exchanger operates as an evaporator, a configuration that suppresses the temperature glide by partially reducing the refrigerant passage not only produces high efficiency, but also reduces the frost formation on fins. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 626–638, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10064     

Experimental investigation of condensation heat transfer and pressure drop of R22, R410A and R407C in mini-tubes

Condensation heat transfer and pressure drop of R22, R410A and R407C were investigated experimentally in two single round stainless steel tubes with inner diameter of 1.088 mm and 1.289 mm. Condensation heat transfer coefficients and two phase pressure drop were measured at the saturation temperatures of 30 °C and 40 °C. The mass flux varies from 300 to 600 kg/m² s and the vapor quality 0.1–0.9. The effects of mass flux and vapor quality were investigated and the results indicate that condensation heat transfer coefficients increase with mass flux and vapor quality, increasing faster in the high vapor quality region. The experimental data was compared with the correlations based on experimental data from large diameter tubes (d_h > 3 mm), such as the Shah and Akers correlations et al. Almost all the correlations overestimated the present experimental data, but Wang correlation and Yan and Lin correlation which were developed based on the experimental data from mini-tubes predicted present data reasonably well. Condensation heat transfer coefficients and two phase pressure drop of R22 and R407C are equivalent but both higher than those of R410A. As a substitute for R22, R410A has more advantages than R407C in view of the characteristics of condensation heat transfer and pressure drop.     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.     

Flow boiling characteristics of refrigerant mixture R22/R114 in the annulus of enhanced surface tubing

The paper presents an experimental study of flow boiling heat transfer characteristics of refrigerant mixture R22/R114 in the annuli of a horizontal enhanced surface tubing evaporator. The test section had an inner tube bore diameter of 17.3 mm, an envelope diameter of 28.6 mm and an outer smooth tube of 32.3 mm internal diameter. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW/m² and 180–290 kg/m²/s, respectively, at a pressure of 570 kPa. The enhanced surface tubing data shows a significant enhancement of the heat transfer compared with an equivalent smooth tube depending on the mixture components and their concentrations. Correlations are proposed to predict such heat transfer characteristics as the average heat transfer coefficients as well as pressure drops of R22/R114 nonazeotropic refrigerant mixture flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture composition.     

Prediction of condensation characteristics of alternatives to R-502 inside air–refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two-phase flow condensation of alternative azeotropic refrigerant mixtures to R-502, on air/refrigerant horizontal enhanced surface tubing, is presented. The condensation data indicated that the heat transfer coefficient on the blend R-408A has the highest heat transfer rate among the blends under investigation. The condensation data also showed that R-502 and R-407B have similar heat transfer rates when plotted against the refrigerant mass flow rate. It also can be observed that, as the mass flux increases, heat transfer coefficient increases. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of alternatives to R-502; such as R507, R404A, R407B and R408A in two-phase flow condensation inside enhanced surface tubing. In addition, proposed correlations were found to fairly predict the two-phase flow heat transfer condensation data.     

Energy and exergy analyses of a two‐stage vapour compression refrigeration system

In this paper exergy analysis of two‐stage vapour compression refrigeration (VCR) system has been carried out with an objective to evaluate optimum inter‐stage temperature (pressure) for refrigerants HCFC22, R410A and R717. A thermodynamic model based on the principles of mass, energy and exergy balances is developed for this purpose. The computed results illustrate the effects of evaporation and condensation temperatures, isentropic efficiencies of compressors, sub‐cooling of refrigerant and superheating of suction vapour on optimum inter‐stage saturation temperature (pressure). The optimum inter‐stage saturation temperatures (pressures) for HCFC22 and R410A are proximate to arithmetic mean of evaporation and condensation temperatures (AMT) when assuming superheating of suction vapour and non‐isentropic compression processes in low‐pressure and high‐pressure compressors. The optimum inter‐stage saturation temperatures (pressures) for HCFC22 and R410A are near to geometric mean of evaporation and condensation temperatures (GMT) when it is assumed that cycle involves the effects of sub‐cooling, superheating of suction vapour and non‐isentropic compression of the suction vapour. The optimum inter‐stage saturation temperature (pressure) for R717 is close to GMT irrespective of sub‐cooling, superheating of suction vapour and non‐isentropic compression in the cycle. The efficiency defects, computed corresponding to optimum inter‐stage temperature in condenser is higher in comparison to the other components. Finally, it is deduced that R717 is a better alternative refrigerant to HCFC22 than R410A in two‐stage VCR system. Copyright © 2009 John Wiley & Sons, Ltd.     

Boiling Heat Transfer and Pressure Drop of a Refrigerant Flowing Vertically Upward in Small Rectangular and Triangular Tubes

In the present study, experiments were performed to examine the characteristics of flow boiling heat transfer and pressure drop of a refrigerant R410A flowing vertically upward in small copper rectangular and triangular tubes with hydraulic diameters of 1.04 and 0.88 mm, respectively, for the development of a high-performance heat exchanger using small tubes or minichannels for air-conditioning systems. Their characteristics were clarified by comparing the previous experimental data of the small circular tube (1.00 mm internal diameter). In the rectangular and triangular tubes, the pressure drop was found to be slightly lower and the heat transfer was much better than in the circular tube.