A numerical and experimental study of turbulent flow through the evaporator coil in an air-conditioning unit

Computational fluid dynamics (CFD) and heat transfer has matured to the stage where state-of-the-art commercial codes can address real engineering problems. However, the advances in finite volume numerical capability which permit this, such as full three-dimensionality, generalised coordinate systems and multiblocking, require convincing validation procedures to demonstrate quantitative engineering potential. In this paper a two-dimensional incompressible turbulent flow in an industrial packaged air-conditioning system is investigated numerically, with specific reference to the air flow through the tube banks of the evaporator coil. The system modelled comprises an outer casing containing the coil which has three rows of 20-mm diameter tubes in staggered formation, each row having tubes, and a condensate tray with complex geometry. The analysis is based on the Reynolds-averaged Navier-Stokes equations in conjunction with the standard k-ε turbulence model. The finite-volume method (FVM) is used to solve the governing partial differential equations on a non-staggered grid. In order to treat the detailed fin tube geometry, a novel method called ‘masking’ has been developed to construct body-fitted curvilinear grids quickly and efficiently. In the computation, several differencing schemes of various orders are outlined and their accuracy are examined.In a parallel experimental investigation conducted on the actual air-conditioning unit, both the mean velocity profiles and turbulence properties of the flow were obtained from triple hot-wire anemometry measurements. These were then used to compare with the computational results and validate the mathematical models developed. It is found that the predictions were generally in good agreement with the measurements, especially for the higher-order differencing scheme. An immediate practical effect of the study was that it revealed the flow disturbance resulting from the condensate tray. This has been suspected but its magnitude not anticipated. The consequent amendment of the design of this component in the commercial unit led to an improved performance of the evaporator.