Experimental Research And Numerical Simulation Of Heat Transfer Enhancement For Wastewater Heat Exchanger In Wastewater Source Heat Pump

Heat pump technology can improve the quality of low-grade energy sources and excavate the low-temperature waste heat stored in the medium,which can effectively save energy.The wastewater temperature changes slightly throughout the year,and wastewater has stable thermal characteristics and a rich amount of water.Using heat pump to extract energy from wastewater which not only recycles heat energy from urban emissions,but also reduces the consumption of traditional energy sources and meets the development requirements of China.It is a new type of wastewater treatment strategy that is beneficial to global sustainable development.The wastewater heat exchanger is the key part to the entire system.It can extract the wastewater thermal energy.The heat pump technology upgrades the thermal energy position.Wastewater heat exchangers not only need to have high heat transfer efficiency,but also need to have the ability to prevent heat exchanger fouling.In the traditional heat exchangers,the fouling phenomenon is not only easy to occur in the heat exchange tubes,it will also cause additional problems of reducing heat exchange efficiency.Therefore,it is necessary to improve the structure of the wastewater heat exchangers to promote the market-oriented development of the wastewater source heat pump.This subject proposes an improvement to the structure of the wastewater heat exchanger.Based on the original heat exchange tube,the rotating single helix and the rotating double helix are respectively inserted.The rotation of the helix directly acts on the fluid near the wall,enhancing the disturbance of the fluid in the tube,breaking the original movement law of the fluid in the tube.The fluid changes from unmixed laminar flow to spiral flow,and the fluid particles are mixed with each other.The heat exchange efficiency is improved,and the flushing effect of the fluid on the tube wall is enhanced.There is a strong descaling effect.This paper first analyzes the mechanism of heat transfer enhancement and concludes that the rotating helix can effectively enhance heat transfer.Because it is closer to the tube wall,and it can destroy the boundary layer that hinders heat transfer,and enhance the mixing of fluids inside and outside the helix.In addition,the angle between the temperature field and the velocity field is effectively reduced.The heat transfer effect is improved under thecondition that the fluid properties,flow velocity,and tube diameter are constant.In order to further study the effect of heat transfer enhancement of inserting rotating helix,a single heat exchange tube experimental platform was set up,to measure the heat transfer effect of two kinds of enhanced heat exchange tubes and conventional heat exchange tube.At the same time,FLUENT simulation software was used for numerical simulation,and the simulation results were compared with experimental results.In addition to analyzing the enhanced heat transfer effect of the enhanced heat exchange tube,the subject has analyzed the heat transfer characteristics of the helix wire from two aspects: pitch and speed.The results show:(1)The flow direction of the fluid near the wall of the inserting rotating helix heat exchange tubes is significantly changed,the tangential motion of the fluid is more severe,the center fluid and the fluid near the tube wall are mixed with each other,and the fluid has a regular spiral flow;(2)The enhanced heat exchange tube designed in this paper has the temperature distribution characteristic that is the flow boundary layer in the tube has a large temperature gradient,and the temperature changing area expands toward the center of the heat transfer tube,and the isothermal area of the main flow area decreases;(3)Under the same conditions,the heat exchange efficiency of the inserting rotating single helix heat exchange tube and the inserting rotating double helix heat exchange tube are better than the heat exchange efficiency of the traditional heat exchange tube;For the influence of the helical pitch and rotating speed,the change tendency of the two is same;Under the same rotating speed or the same helical pitch condition,the heat transfer coefficients of the two heat exchange tubes are similar;At the same time,the heat transfer coefficient of the enhanced heat transfer tube increases with the increase of the rotating speed,and decreases with the increase of the helical pitch.(4)In practical projects,a single helix with an economical speed of 10 rad/s and a pitch of 10 mm can be used to enhance the heat transfer effect.