| In the existing waste heat recovery in drying industry,the coupled heat and moisture effect of the moist air condenser is usually not considered,but the state of moist air at the outlet of condenser is given directly.In addition,there are few studies on condensation of air with high temperature and high humidity.To fill this gap,by studying the heat and mass transfer mechanism of moist air condensation,this dissertation,using the MATLAB software,establishes a more accurate model of moist air condenser for nonlinear heat and moisture coupling calculation,based on enthalpy difference analysis and moisture evolution coefficient method.This thesis mainly analyzes the relationship between the heat transfer performance of the moist air condenser and the parameters of the moist air at the inlet,when the cold fluids are dry air and organic working fluid R134 a respectively.Furthermore,this dissertation also applies the model to analyze the effects of changes in parameters of the moist air inlet on the performance of the whole drying system.In the case where cold fluid is dry air,the results are listed as follow.If the moisture content is the same,when the temperature of moist air at the inlet increases,the temperature of moist air outlet gradually increases;the mass flow rate of condensate water gradually decreases;heat exchange decreases at first and increases later,meaning that there exists a minimum heat exchange.When relative humidity of the moist air at the inlet increases,the temperature of the moist air at the outlet gradually increases;the heat exchange of the condenser increases and this increasing trend accelerates after condensation occurs;the mass flow rate of condensate water the gradually increases.When the mass flow rate of moist air increases,the temperature of the moist air at the outlet increases while the increasing rate gradually slows down with the increase in the mass flow rate;the mass flow rate of condensate water increases at first and decreases late,meaning that there is an optimal value for the mass flow rate of moist air that maximizes mass flow rate of condensate water;the moisture content at the wet air outlet increases gradually;the heat exchange increases at first and decreases later,meaning that there is an optimal value for the mass flow rate of moist air that maximizes the heat transfer.Within the range of this study,when the relative humidity is 60%,the optimal mass flow rate of moist water is 0.02 kg/s while the maximum mass flow rate of condensate water is 0.48 g/s;when relative humidity is 70%,the optimal mass flow rate is 0.02 kg/s while the maximum mass flow rate of condensate water is 0.61 g/s;when the relative humidity is 80%,the optimal mass flow rate is 0.03 kg/s while the maximum mass flow rate of condensate water is 0.74 g/s.In the case where cold fluid is organic working fluid R134 a,the results are listed as follow.If the moisture content is the same,when the temperature of moist air at the inlet increases,the mass flow rate of condensate water gradually decreases to zero;heat exchange decreases at first and increases later.If the temperature of moist air at the inlet is constant,when moisture content decreases,the mass flow rate of condensate water decreases;heat exchange decreases;after achieving the minimum heat exchange,the changes in moisture content have no effects on heat exchange.When the mass flow rate of moist air increases,the temperature of the moist air at the outlet increases while the increasing rate gradually slows down with the increase in the mass flow rate;the mass flow rate of condensate water increases at first and decreases late,meaning that there is an optimal value for the mass flow rate of moist air that maximizes mass flow rate of condensate water;the higher the moisture content,the larger the mass flow rate;heat exchange increases at first and decreases later,meaning that there exists a maximum heat exchange.When the evaporation temperature of R134 a raises,the mass flow rate of condensate water decreases;the heat exchange decreases.When the evaporation temperature rises to a certain value,no condensation occurs and the decreasing rate of heat exchange decreases slowly.In the coupling of the drying system model and the wet air condenser model,the change rule of drying system performance with the temperature of the wet air inlet(i.e.,the outlet of the drying chamber),the dew point temperature and the evaporation temperature of the organic working fluid is predicted.The results are listed as follow.When the moisture content is the same,the system performance is better with lower temperature of the moist air at the inlet.When the inlet temperature is the same,the higher the relative humidity of wet air is,the better the system performance will be.Within the range of research,when the evaporation temperature is at a lower or higher level,the mass flow rate of condensate water and heat transfer in system are larger.The network of ORC system increases with the increase of evaporation temperature,therefore,the system can achieve better comprehensive performance when the evaporation temperature is higher. |
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