Use of single air extraction and injection to thermodynamically balance the combined heat and mass transfer process

Abstract

In order to thermodynamically balance the combined heat and mass transfer process at the conditions of unsaturated, saturated and supersaturated state of humid air, measures like single air injection and extraction are implemented to address the distribution of driving forces. From the microscopic point of view, integrated mathematical models for simultaneous heat and mass transfer process are built, coupled with zero, single air extraction and single air injection methods. Off-design analysis for the balance process of driving forces including the heat transfer temperature difference and mass transfer pressure difference are accomplished. The research results exhibit that single air extraction is the preferable measure to lower the total entropy generation in general operating regions for simultaneous heat and mass transfer process (the liquid gas ratio less than 2.9), compared with single air injection. It is found that a minimum value for normalized entropy generation is appeared for all the cases, as the heat capacity rate ratio is unity, and a specific extraction ratio is obtained to minimize the entropy generation with fixed liquid-gas ratio. In addition, it is observed that higher inlet relative humidity will decrease the total normalized entropy generation, while an optimal value is obtained as 0.0064 at inlet relative humidity of 0.85. Meanwhile, inlet relative humidity represents a positive effect on system energy efficiency. It is also confirmed that as the number of extractions is constant and other measures are limited, adjusting the extraction height is an available additional option for further optimizing the thermodynamic performance.