Applied Energy, Vol.217, 126-142, 2018
On the fundamental heat and mass transfer analysis of the counter-flow dew point evaporative cooler
The performance of the dew point evaporative cooling (DPEC) is dominated by its convective heat and mass transfer mechanism. Existing mathematical models are mainly developed for the thermodynamic analysis of DPEC under various operating and geometric conditions. The convective heat and mass transfer coefficients are estimated using the Nusselt number and Sherwood number at constant surface conditions. However, as the channel surface is subjected to a naturally-formed boundary condition, the cooler's actual heat and mass transfer performance remains unclear and has never been investigated. Therefore, we propose an experimental and numerical study, to examine at the fundamental level, the convective heat and mass transfer process of the DPEC. The temperature and humidity distributions of a counter-flow dew point evaporative cooler are measured under different test conditions. The magnitude of the convective heat and mass transfer coefficients are determined using the log mean temperature/humidity difference method. Concurrently, a 2-D mathematical model has been formulated to simulate the heat and mass transfer performance of the cooler. The model agrees well with the acquired experimental data with a maximum discrepancy of +/- 7.0%. The product air temperature, convective heat and mass transfer coefficients and the Nusselt number and Sherwood number, are further examined under different conditions. Key findings emerged from this study reveal that Re-D, r, H/L, delta/H and pi are the dominant factors related to the heat and mass transfer performance. The average convective heat and mass transfer coefficients are found to be 26.8-29.9 W/(m(2)center dot K) and 0.025-0.027 m/s. The corresponding (Nu) over bar (D,d), (Nu) over bar (D,w) and (Sh) over bar (D,w) span 8.67-9.95, 8.68-9.21 and 8.17-8.67, respectively, under varying dimensionless numbers/groups.