Evaporation performance of a new plate‐type heat exchanger with winding tubes on plate surfaces (estimation of performance of the heat exchanger)

A method for evaluating and predicting the performance of a newly developed plate‐type heat exchanger as an evaporator for water‐refrigerant systems such as chillers has been developed. The main component of the developed heat exchanger consists of plates packed together in a casing with winding tubes connected to both sides of the plates. Refrigerant flows inside the tubes, and water flows in the space between the plates. A herringbone‐like pattern is formed in this space by the cross sections of the winding tubes. The newly developed method estimates evaporation performance of the developed heat exchanger using new empirical correlations. There are correlations for heat transfer and pressure drop in winding‐tube banks on the water side, and correlation for the pressure drop on the refrigerant side. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 245–257, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20009     

Effect of refrigerant charge on the performance of air conditioning systems

An air conditioning system operates in an optimal condition if the system is fully charged with a specified amount of refrigerant. Poor field maintenance or refrigerant leakage causes low level of charge resulting in a lower thermal performance and higher operating cost. An experimental investigation was conducted to study the effect of low charge level of R‐22 on the performance of a 3‐ton residential air conditioning system. The experimental results show that if a system is undercharged to 90 per cent then the effect is small: a 3.5 per cent reduction in cooling capacity and a 2 per cent increase in the coefficient of performance (COP). However, the system performance suffers serious degradation if the level of charge drops below 80 per cent. An ice layer formed on the outer surface of the cooling coil impedes the heat transfer between the warm air and cold refrigerant vapour. An economic analysis shows that the cost of properly charging an under‐charged system which is at an 85 per cent charge level, can pay for itself in savings in a short period of 3–4 months. Copyright © 2001 John Wiley & Sons, Ltd.     

Prediction of the capacity of a heat exchanger by a thermal network method: Modified effective specific heat model considering the influence of static pressure distributions of refrigerants

In this paper, we proposed a method to predict the capacity of a heat exchanger by considering static pressure distributions of refrigerants. The thermal network method was modified by adding an equivalent heat generation term into a heat balance equation that was connected with calculated 1D static pressure distributions of refrigerants. An experiment was performed with a heat exchanger having two rows and two passes to verify the accuracy of the proposed method. The result showed the error of the predicted capacity to be less than 1% for an evaporator and less than 2% for a condenser. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(5): 376–390, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10039     

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     

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.     

Thermal performance analysis of fin-and-tube heat exchanger circuit in supercritical hydrogen refrigeration cycle system

A circuit arrangement model for air-to-refrigerant fin-and-tube heat exchanger with supercritical hydrogen as the refrigerant is developed. The mass, momentum and energy balance equations in the circuit arrangement model are solved by the effectiveness-NTC method. The effects of gravity force, flow state and inhibit heat conduction on the heat transfer in the supercritical hydrogen refrigerant heat exchanger are investigated. Results show that the flow arrangement placed at a location where the fluid flows from the high place to the low one can achieve a better heat heat transfer performance. The form of counter-flow has a better heat transfer performance compared to that of parallel-flow. The heat transfer performance in the six-in-three-out arrangement is the better compared to that of three-in-three-out arrangement. Thus, the structure of the six-in-three-out with counter-flow from high to low one is recommend to achieve a better heat transfer performance in the supercritical hydrogen refrigerant heat exchanger.     

Measurements of thermal conductivity and thermal diffusivity of alternative refrigerants in liquid phase with a transient short‐hot‐wire method

A transient short‐hot‐wire method has been proposed to simultaneously measure the thermal conductivity and thermal diffusivity of liquids. The method has been applied to the refrigerant HCFC‐22, alternative refrigerants HFC‐32, HFC‐125, HFC‐134a and refrigerant mixtures HFC‐32/125 [35/65, 50/50 (HFC‐410A), 68/32 wt%], HFC‐32/125/134a [23/25/52 (HFC‐407C) wt%]. From the present study, it is shown that the measured results agree well with the literature values on thermal conductivity and those on thermal diffusivity obtained from NIST's thermophysical property database, REFPROP Ver. 6.0. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 540–552, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20032     

PERFORMANCE ANALYSIS OF VAPOUR COMPRESSION REFRIGERATION WITH R22/R124/R152a REFRIGERANT

In this paper, the performance of vapour compression refrigeration with R22/R124/R152a as the refrigerant is presented. A mathematical model of the system characteristics is developed and the simulated results agreed quite well with those of the experiments. The model was also used to analyse the most suitable composition of the refrigerant. It was found that the required composition was obtained when the mass fraction of R22 was in the range 0.2–0.4. © 1997 by John Wiley & Sons, Ltd.     

太阳能双喷射式制冷系统性能计算分析

建立了双喷射式制冷系统的物理模型和数学模型,计算了太阳能双喷射式制冷系统中气体喷射器和气-液喷射器的性能参数、系统性能参数随制冷剂和工况的变化。结果表明,在给定的发生温度、蒸发温度、冷凝温度范围内,气体喷射器的喷射系数和系统COP均随发生温度和蒸发温度的升高而增大,随冷凝温度的增大而减小。气-液喷射器的喷射系数则随发生温度的升高而减小,除水外,均随冷凝温度的升高而减小。     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理,对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定,采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力,采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中,单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%,实验压力为一个标准大气压,热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入,使制冷剂R141b的核态池沸腾换热得到强化;当ω=0.2%时,换热系数提高比例出现峰值,为57.7%。伴随ω的增加,石墨烯纳米制冷剂的热导率增大、表面张力减小,沸腾表面润湿性增强且微腔数先增后减,综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。