Heat-mass transfer model and irreversible heat transfer process analysis of high-humidity exhaust

Abstract

In engineering, the efficient extraction of low-grade energy from mine exhaust is a key issue. Based on the heat-mass transfer between high-humidity exhaust and low-temperature spray in the heat recovery diffuser tower, a theoretical model of heat-mass transfer about the number of mass transfer units (NTUm) and Lewis number (Le) is constructed and solved, the direct contact heat-mass transfer test is carried out. Applying the entransy dissipation theory, the irreversible process of heat-mass transfer is clarified, and the relationship between Le and the thermal resistance of entransy dissipation is revealed. The results shows that the heat-mass transfer process between high-humidity exhaust and low-temperature spray is embodied as two processes, which are dehumidification cooling and quasi-isohumidity cooling. When NTUm is greater than 0.1, the high-humidity exhaust is dehumidification cooled. After heat exchange with low-temperature spray, the air temperature can be reduced by up to 6.3 ℃ and the moisture content can be reduced by up to 3.12 g/kg. In this process, Le deviates from 1, and Le is proportional to the thermal resistance of entransy dissipation. When Le approaches to 1, the thermal resistance of entransy dissipation approaches to 0, and the optimal heat transfer effect can be achieved. On the other hand, when NTUm is less than 0.1, the high-humidity exhaust is quasi-isohumidity cooled. The high-humidity exhaust enthalpy reduces and cools to saturation, air state along the saturation line changes until the heat transfer complete, and the exhaust outlet temperature is close to the dew point temperature of the exhaust inlet. It is worth noting that the thermal resistance of entransy dissipation in the quasi-isohumidity cooling process is much greater than that in the dehumidification cooling process. Dehumidification and cooling of high-humidity exhaust is more conducive to heat-mass transfer. When designing the spray diffusion tower heat recovery device, in order to make the high-humidity exhaust in the heat exchange unit for dehumidification cooling, the flow rate should not be greater than 4 m/s, and the water-air ratio is not less than 0.2.

Keywords

• high-humidity exhaust
• heat-mass transfer
• the number of mass transfer units
• lewis number
• thermal resistance of entransy dissipation