Study And Comparison Analysis On Heat Transfer Enhancement Of Wastewater Heat Exchanger In Wastewater Source Heat Pump System

There is a large potential in the heat loss from the urban wastewater.The temperature of wastewater is 10 to 16 °C in winter,which is higher than air temperature.The temperature is20 to 26 °C in summer,which is lower than air temperature.According to the characteristic parameters of the wastewater,ground surface water,and groundwater,wastewater source heat pump(WWSHP)exchanges heat with wastewater through the wastewater heat exchanger.By reusing the wastewater,the WWSHP could tackle the global water problems and realize the sustainable development,recover the heat of wastewater.Wastewater heat exchanger needs reliable anti-clogging,anti-pollution and anti-corrosion ability.When designing a wastewater heat exchanger,otherwise than heat transfer coefficient,corrosion-resistant and anti-fouling properties should also be considered as well.The simple structures of traditional heat tube and the heat transfer tube with rough internal surface have low heat transfer efficiency,and can not play the role of descaling or anti-fouling After long-term's use,fouling deposit progressively on the pipe and reduce the heat transfer efficiency.In order to improve the heat transfer efficiency of the heat exchanger and solve the problem of descaling and anti-scaling,this paper improves the wastewater heat exchanger on the basis of previous researches through inserting flexible single-screw,flexible twin screws and flexible screw slice into the heat exchange tube,respectively.The turbulence of the wastewater enhanced and therewith the heat transfer coefficient of heat exchanger increased.Furthermore,driven by the screw or blade,the fluid wipes the surface to prevent scaling inside the tube,which have the effect of anti-fouling.In order to select the optimal heat exchange tube,this paper used the numerical simulation to simulate the flow and heat transfer process of the three models.Pitch and rotational speed are the key factors to compare the heat transfer performance of the models.In addition to comparing the heat transfer properties of the same model at different pitch and speed,the heat transfer performance of different models at the same pitch and same speed were also compared,it found that the heat transfer efficiency of the three heat exchange tube increased with the increase of the rotating speed of the screw rod and decrease of the screwthread pitch,but the heat transfer efficiency of single-screw and twin screws tube almost are same when they have the corresponding pitch or the same speed,however,the heat transfer efficiency of rotating screw slice tube is much more than the other two kinds of heat exchange tube.In addition,this paper has separately analyzed the width to strengthen rotating screw slice tube heat transfer,it found that the heat transfer efficiency of this heat exchange tube increased with the increase of the width.The average exit temperature of rotating screw slice tube who's width is 2 mm is same as rotating single-screw tube when their pitch and rotational speed is same,so we find that the rotating screw slice tube who's width is 2 mm is equal to rotating single-screw tube when we combining the dimensions of the two models.With the change of pitch,rotational speed and width,the resistance of tube will change,so we need different motor power.The simulation results show that the insufficient for power device,this topic choose pitch of 50 mm,rotating speed of 10 rad/s single spiral pipe as the optimal heat exchange tube;For larger powerful device,this topic selection width of 12 mm,pitch to 10 mm,rotational speed of 40 rad/s rotating spiral tube for optimal heat exchange tube.Therefore,the actual project should be based on the actual situation to determine the size of the speed?pitch and width.