In recent years, conservation of energy has become a challenging issue in air-conditioning applications. In order to overcome this issue, many researchers have recommended the use of a Parallel Flow Condenser and a low Global Warming Potential and Ozone Depletion Potential refrigerant, such as R32, in air conditioning systems. However, PFC faces the critical challenge of flow maldistribution in the tubes. This literature review mainly examines the refrigerant maldistribution problem which has been investigated by previous researchers. It was found that many of the researchers did not properly analyse the influence of flow maldistribution profiles on the performance degradation of heat exchangers. In order to have a comprehensive analysis of tube-side maldistribution in Parallel Flow microchannel heat exchangers, it is recommended that the influence of the higher statistical moments of the probability density function of the flow maldistribution profiles on performance degradation be quantified. Additionally, R32 maldistribution should be analysed and compared with R410A, which is the current commonly used refrigerant in air conditioning units. Moreover, in order to have a realistic simulation of the effect of refrigerant flow maldistribution profiles on performance degradation of heat exchangers, the effect of superheat and sub-cooling must be analysed.

The aim of this research is to study the impact of statistical moments of probability density function such as mean, standard deviation, skew of R32 flow maldistribution profile on the thermal performance of microchannel heat exchanger under superheat, and subcooling effect. A mathematical model was developed in order to analyse the influence of the statistical moments of probability density function of R32 flow maldistribution on the thermal performance of microchannel heat exchanger under superheat and sub-cooling effect. It was found that the high standard deviation and high negative skew of R32 flow maldistribution profile gave a large impact on the D of microchannel heat exchanger and can achieve up to 10%. Moreover, it was found that the heat transfer performance of microchannel heat exchanger dropped significantly when the sub-cool increases. In short, low standard deviation, high positive skew, and superheat of a flow maldistribution profile is preferred in order to minimize the performance deterioration effect. An experiment was set up to verify the mathematical model. The results from the mathematical model agreed well within 10% of the experimental data. A performance deterioration correlation related to refrigerant maldistribution under superheat and subcool was developed to provide a faster solution to design an even flow distribution heat exchanger. The proposed correlation in this research offers a quicker and simpler way to study the R32 flow maldistribution problem.

In order to improve the flow uniformity in a plate-fin heat exchanger (PFHE), a modified header structure with a plain baffle inside is proposed. Flow distribution and pressure drop in the header were studied by numerical methods. The results show that the modified header is highly efficient in improving the flow uniformity with negligible pressure drop increase. A performance effectiveness factor was introduced to predict the effects of the modified structure on the performance of the PFHE. Influences of baffle structure parameters on modified structure performances were further discussed. The modified header structure can effectively improve the performance of a PFHE.

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