Effect of vacuum environment on air side heat and mass transfer characters of plain fin-tube heat exchangers

Abstract

Air-side heat and mass transfer performance of plain fin-tube heat exchangers at different vacuum degree have been experimentally investigated. In this experiment, influence of air inlet humidity on air-side heat and mass transfer under vacuum environment is studied by controlling vacuum degree and air inlet humidity. Results show that: variation trend of air-side sensible heat transfer coefficient (ha), heat transfer j-factor (jh), mass transfer coefficient (hm) and mass transfer j-factor (jm) are not all time remain the same. Variation trend of ha, jh are the same with increase of vacuum degree when air inlet humidity changes, ha is gradually decreased and jh is gradually increased. Variation trend of hm is opposite with the increase of vacuum degree when the air inlet humidity changes. Under constant air inlet relative humidity conditions, hm is gradually increased. Under constant air inlet moisture content conditions, hm is gradually decreased. However, variation trend of jm is the same with increase of vacuum degree when air inlet humidity changes, both are increased. But slope of jm is greater under constant air inlet relative humidity conditions. Overall heat transfer performance is greatly affected by air inlet humidity changes. Under constant air inlet relative humidity conditions, when air inlet relative humidity increases from 50% to 70% and then to 90%, proportion of latent heat transfer increases from 30.98%∼53.99% to 36.53%∼56.95% and then increases to 44.19%∼64.04% with vacuum degree increases. As proportion of latent heat transfer increases, air-side total heat transfer performance is not obviously reduced. Under constant air inlet moisture content conditions, when air inlet moisture content increases from 15 to 20 g/kg, proportion of latent heat transfer is basically unchanged with increase of vacuum degree, at about 36% and 37%, respectively. So, total heat transfer performance on air side decreases obviously. In order to model air-side heat and mass transfer performance of the complex structure fin-tube heat exchangers, a simplified model is investigated under vacuum environment. Results show that influence of pressure changes on heat and mass transfer based on theory is basically consistent with the results of regression analysis based on experimental results. This study can provide guiding suggestions for the design of heat exchangers under vacuum environment.