Towards "numerical experimentation": the development of a full scale CFD model of a roof-top air conditioning evaporator to predict the two phase conjugate heat and mass transfer.

Recently, Moukalled et al. (2011) conducted an extensive CFD based numerical study for a 10 ton roof-top AC packaged unit to investigate the hydrodynamic and thermal fields on the air flow side separately and considering the condensation of water vapor at the evaporator coil surface. This step was considered a recent work and was not adopted by previous researchers. This paper extends the previous work of Moukalled et al. (2011) by considering only the evaporator compartment and simultaneously integrating the refrigerant cycle with the air flow; thus, the evaporator coil temperature and heat fluxes are obtained with the solution prediction. For this purpose, a full scale CFD model was developed to predict the multi-phase conjugate heat and mass transfer. Exact modeling of the evaporator accessories (blower, pulley and motor) were also considered; thus eliminating the use of drag/artificial models. A conformal mesh was generated at the common interface regions separating fluid and solid zones to account for the continuity of heat fluxes at these interfaces. A total of 37 million control volumes were generated to model the whole evaporator compartment. The mixture model with multi-species transport was adopted to account for the condensation (coil surface) and evaporation (refrigerant circuits) occurring phenomena. The realizable 𝑘−𝜀 model was implemented to model high velocity fluctuations and recirculation zones. High turbulence regions were predicted at different locations in the evaporator compartment. The base case model was simulated on a cluster of 64 CPUs. Predictions were then compared with available experimental data for validation purposes. A good confirmation was achieved. The current study targets air conditioning manufacturers through giving them a great opportunity towards “numerical experimentation” to get the facts before manufacturing.