Numerical simulation of multi-pass parallel flow condensers with liquid-vapor separation

Installation of a refrigeration and air conditioning system to regulate temperatures indoor tends to consume huge amounts of electricity, especially in summer season. For that reason, design and optimization of refrigeration and air conditioning plants are therefore important in reduction of fossil fuel consumption and CO₂ emission. Research has shown that the air-to-refrigerant condenser is one of the key components in all these plants and has thus been widely investigated. In fact, numerical simulations have been widely used to obtain a high efficiency and low cost design. Such developments have contributed to the replacement of a novel multi-pass parallel flow condenser with liquid-vapor separation (MPFC-LS), which is cheaper and more efficient. Attempts to further optimize the design of this novel condenser have over the years been presented, where mixed success has been reported. Flow distribution is important in design of heat exchangers. Various methods have been developed to determine the flow distribution for the air-to-refrigerant heat exchangers. Nonetheless, further research is still necessitated.

Presently, the requirement for more detailed characteristics or more effective factors of the performance of MPFCs-LS is very high credit to current global push for less carbon emission. Consequently, improved models are needed. To address this, a team of researchers from the School of Engineering and Materials Science at Queen Mary University of London: Dr. Nan Hua, Dr. Huan Xi and led by Professor Hua Sheng Wang, in collaboration with Dr. Rong Ji Xu at the Beijing University of Civil Engineering and Architecture and Professor Ying Chen at the Guangdong University of Technology developed a new distributed-parameter model for MPFCs-LS. The team based their model on that presented by Hua et al. Their work is currently published in International Journal of Heat and Mass Transfer.

Generally, refrigerant flow in a condenser appears in single-phase vapor, two-phase and single-phase liquid. The fluid properties, heat and mass transfer characteristics are significantly different. Thus, to enable high accuracy, the team used a ‘segment self-subdivision’ approach to track the location of phase change in a segment. After identification of the flow regimes, several empirical correlations of heat transfer and frictional pressure drop of flow condensation were compared. Finally, the model proposed for the MPFCs-LS was verified with experimental data obtained in a wide range of working conditions.

The authors reported that the locations of onset and completion of condensation in the refrigerant flow side were correctly traced and the flow distribution in the refrigerant side was successfully determined by genetic algorithm. Interestingly, the flow patterns of condensation were identified to use relevant empirical correlations for heat transfer and pressure drop.

In summary, the study demonstrated the development of a distributed-parameter model and numerical methods to simulate the performance of MPFCs-LS. Remarkably, the predictions of the model agreed well with experimental data with the root-mean-square deviations of heat transfer capacity and pressure drop being within 7.5% and 20.6%, respectively. In a statement to Advances in Engineering, Professor Hua Sheng Wang remarked that the model and the numerical methods they demonstrated would provide a useful tool for design and performance simulation and optimization of these new advanced condensers.