CFD, Correlation, Heat Exchanger, Heat Transfer, Pressure Drop
Air-to-refrigerant heat exchangers are a key component in air-conditioning and heat pump systems. A great deal of effort is spent on the design and optimization of these heat exchangers. One path towards improving their performance is the transition to smaller hydraulic diameter flow channels. This is evident by the recent introduction of microchannel heat exchangers in the stationary HVAC market. Systematic analyses demonstrates a great potential for improvement in terms of size, weight, refrigerant charge and heat transfer performance by employing small diameters in tube-fin heat exchangers. In particular, tube diameters below 5mm need to be investigated. It is known that as tube size reduces, at some point, fins are no longer required and the heat exchanger then comprises of just bare tubes. The in-tube refrigerant flow characteristics are well understood for small diameter tubes and accurate heat transfer and pressure drop correlations are available in the literature. On the air side, however, most of what is available in the literature has no or very limited applicability to small tube diameter heat exchangers. In these situations numerical methods such as CFD are commonly employed in the performance evaluation of tube and fin surfaces. Although CFD has been a powerful and reliable tool it is still computationally expensive if used for evaluating a large number of parameterized geometries. This work presents new CFD-based correlations for finned and finless tube heat exchangers for tube diameter ranging from 2mm to 5mm. The methodology implemented in this work consists of analyzing air-side heat transfer and pressure drop characteristics by using a method called Parallel Parameterized CFD (PPCFD). PPCFD allows for fast, automated parametric CFD analyses of various geometries with topology change and reduces the engineering time significantly. The CFD models are verified using uncertainty analysis methods and by comparing the predictions against available experimental data. The validation study shows that CFD can predict the air side performance within 10% as compared to experimental data and can reproduce the performance trends very well. The verified CFD model are then used to generated air-side performance data for a wide range of geometrical parameters such tube diameters, spacing and fin density and operating parameters such as tube wall temperature and inlet air state. The resulting data is reduced into correlations that can be easily implemented in various heat exchanger analyses tools. As new experimental data becomes available, the correlations will be updated. In the meanwhile, researchers and engineers can use these correlations for evaluating the performance of small tube diameter heat exchangers.