A Performance Study For A Novel Fresh Air Equipment

The supply of fresh air is an important factor to ensure the health of residents.Existing residential buildings generally adopt the way of opening windows to supply fresh air.This not only introduces new pollution sources,which causes hidden risks of secondary pollution of indoor air,but also causes the increase in energy consumption of air conditioning.In order to solve the above problem,this research studies a novel fresh air equipment which can realize heat recovery.The research considers an office with a building area of 24 m~2 in Nanjing as the application carrier and a novel fresh air equipment which mainly consists of thermoelectric modules and heat pipe heat exchangers as the research object.Its air volume is 110 m~3/h and it can realize heat recovery of the exhaust air and auxiliary coolingand heatingof the fresh air,based on the cooling and heating characteristic of thermoelectric modules and the efficient heat transfer of the heat pipe heat exchanger.Through the combination of theoretical analysis,experimental research and numerical simulation,the effects of fresh air and exhaust air inlet velocity,fresh air inlet temperature and heat pipe heat exchanger structure parameters on equipment COP and heat recovery efficiency are explored.The main research work are as follows:1.Theoretical analysis:Based on the characteristic equations of thermoelectric modules and the thermal resistance network model of heat pipes,a theoretical model was established to explore the effects of inlet velocity(0.14 m/s～0.63m/s)and fresh air inlet temperature(30℃～50℃)on the equipment performance coefficient COP and heat recovery efficiency of the device.2.Experimental research:A test rig was designed and built to investigate the effects of the fresh air inlet temperature(30℃～50℃)and the inlet velocity of fresh air and exhaust air(0.14m/s～0.63 m/s)on the heat resistance of heat exchanger,the temperature difference between air and heat source,the performance coefficient of equipment and the heat recovery efficiency of equipment.3.Numerical simulation:With circular finned tubes,fin-pitch 20 mm,fin height 13.5 mm,the space between heat pipes 29 mm as the reference model,a series of numerical models were established by CFD fluid calculation software and solved by pressure-based solver basing finite volume method.Based on them,the changes of equipment’s performance were studied when the fin-pitch is from 5 mm to 20 mm,the fin height is from 13.5 mm to 19.5 mm and the spacing between heat pipes is from 29 mm to 44 mm.Additional under the same fin height(13.5 mm)the effects of four types of fins including circular,rectangular,H-type I(slot width is 3 mm)and H-type II(slot width is 6 mm),on the performance of the device were explored.The main results are as follows:1.Theoretical analysis results show that:(1)The hot end temperature of the thermoelectric modules,the temperature difference between the hot and cold ends,and the heat recovery efficiency of the equipment are negatively correlated with the air inlet velocity,but the cooling capacity and performance of the coefficient of the thermoelectric modules are positively correlated with it.When the fresh air inlet temperature is 50°C and the air inlet velocity is increased from 0.14 m/s to 0.63 m/s,the cooling capacity and cooling coefficient of thermoelectric modules can be increased by 54.96 W and0.235,respectively,while the device’s heat recovery efficiency is reduced by 16.45%.(2)The temperature of the hot end of the thermopile and the heat recovery efficiency of the equipment are negatively correlated with the fresh air inlet temperature,and the cooling capacity and COP of the thermopile are positively correlated with it.When the air inlet velocity is 0.14 m/s and the fresh air inlet temperature is increased from 30°C to 50°C,its cooling capacity and COP can increase by 3.22 W and 0.036 respectively,but the heat recovery efficiency of the equipment decreases by 28.91%.2.The experimental results show that:(1)The simulation results of the theoretical model show the same trend as the experimental results in terms of the change of COP and heat recovery efficiency with the air inlet velocity and the fresh air inlet temperature.(2)At the hot end of the thermopile,the thermal resistance of the heat pipe heat exchanger has a negative correlation with the air inlet velocity and a positive correlation with the fresh air inlet temperature.When the fresh air inlet temperature of is 40℃,the air velocity changes from 0.14 m/s to 0.63 m/s,the thermal resistance decreases by0.025 K/W.But When the air inlet velocity is 0.52 m/s,the fresh air inlet temperature reduces from 50℃to 30℃,the thermal resistance is reduced by 0.032 K/W.(3)The temperature difference between the fresh air inlet and cold source,the fresh air outlet and the cold source,the exhaust air inlet and the heat source and the exhaust air outlet and the heat source are all positively correlated with the fresh air inlet temperature.When the fresh air inlet temperature increases from 30°C to 50°C,the temperature difference between the fresh air inlet and the cold source and the fresh air outlet and the cold source can be increased by 3.91°C and 1.58°C,respectively.But the temperature difference between the exhaust air inlet and the heat source and the exhaust air outlet and the heat source can be increased by 11.92°C and 9.90℃,respectively.3.The numerical simulation results show that:(1)Both the theoretical model and the numerical model can accurately simulate the performance of the prototype,but the numerical model has higher accuracy.Except for the heat recovery efficiency,the errors of other factors are all less than 10%.(2)Reducing the fin-pitch,increasing the fin height and the heat pipe space are conducive to the improvement of COP and heat recovery efficiency,among which the fin space has the greatest impact.If the fin-pitch reduce from 20mm to 5mm,the COP will increase 0.813,and the heat recovery efficiency will increase 0.60%;if the fin height increase from 13.5 mm to 19.5 mm,the COP will increase0.282,and the heat recovery efficiency will increase 0.21%;When the heat pipe space increase from 29mm to 44mm,the COP will increase 0.324,and the heat recovery efficiency will increase 0.20%.(3)Under the same fin height,among the four fin types of circular,rectangular,H-type I(slotted width of 3mm)and H-type II(slotted width of 6mm),the COP and heat recovery efficiency of the equipment are the highest when using the rectangular fins.Theoretical analysis,experimental research and numerical simulation are used to study the performance of the novel fresh air equipment which can realize heat recovery,and the heat recovery efficiency was used as one of the performance evaluation standards.The research results have a certain guiding significance for the further optimization of the novel fresh air equipment with heat recovery function.