Numerical Investigation on the Condensation Heat Transfer of FC72 in the Presence of Air

Condensation of FC72 in the presence of air has been numerically investigated. The influence of concentration of inlet non-condensable gas on the condensation heat transfer has been examined. It is found that when air is introduced at the inlet, condensation heat transfer deteriorates, more so in the case of condensation under natural convection. The diffusion layer, in which the gradient of non-condensable gas concentration is very high, can clearly ne observed adjacent to the interface. The formation of the diffusion layer provides a driving force for the vapor to reach the interface for continuous condensation. However, it is the presence of this layer that causes a very low vapor partial pressure near the interface, which dramatically decreases the condensation temperature at the interface. This will cause a much smaller temperature difference across the condensate film, which will eventually result in a poorer condensation rate compared with the pure vapor case. It is concluded that effective disturbance or even breakdown of this diffusion layer is the key to enhance condensation heat transfer if the presence of air in the system is inevitable.