Experimental And Numerical Study On Enhanced Heat Transfer Characteristics Of The Tube With Twined Coil Insert

Heat exchanger is one of the indispensable equipment in the field of heating,ventilating and air conditioning,chemical petroleum,electric machinery,which can realize the heat conversion of a variety of media,and has a very important use value.Enhancing heat transfer in tube is a good way to increase heat transfer performance of heat exchanger,especially for high viscosity fluid in tube,which can realize economic operation of equipment and achieve the effect of energy saving and emission reduction.This kind of twined coil insert has the advantages of easy fixing,optimized working environment and flexible operation.After the twined coil is inserted into the tube,the fluid flowing through the porous body around the twined coil is forced to make irregular lateral and longitudinal movements,which induces turbulence in advance,so that the radial temperature distribution is uniform and flat,and the heat transfer in the tube is significantly enhanced.Heat transfer has been greatly improved compared to twisted tapes and spiral coils,but resistance has also increased a lot.Heat exchanger has a very important use value in industrial production and chemical application.Strengthening the heat transfer in tube can achieve more economic in industry and chemical industry,and achieve the effect of energy saving and emission reduction.Based on the passive heat transfer enhancement technology,the flow and heat transfer characteristics of the tube with twined coil insert under laminar,transitional and turbulent flow conditions were studied experimentally.The flow structure,temperature distribution,vorticity and secondary flow intensity were analyzed by numerical calculation.The mechanism of heat transfer enhancement by the tube with twined coil insert was studied by numerical calculation,and the mechanism of heat transfer enhancement by the tube with twined coil insert was analyzed.The characteristics of flow and heat transfer in the heat transfer tube with with twined coil were studied in detail.The main conclusions are as follows:(1)The effects of different spiral coil number on the heat transfer and resistance characteristics of the tube with twined coil insert were studied experimentally.Under the same working conditions,the heat transfer performance of the tube with twined coil insert is much better than that of the tube without twined coil insert.The results show that the heat transfer performance of the tube twined coil insert is good.(2)Experimental study on heat transfer and flow characteristics of twined coil with different number of spiral coils in tube bundle heat exchanger was carried out.It is found that the heat transfer enhancement factors JF1 and JF2 first increase and then decrease with the increase of Re.(3)Experimental study on heat transfer and flow characteristics of twined coil with different number of spiral coils in tube bank fin heat exchanger was carried out.It is found that the heat transfer enhancement factors JF1 and JF2 decrease with the increase of Re.(4)The heat transfer enhancement mechanism of the tube with twined coil insert was studied by numerical method.The mechanism of heat transfer enhancement can be described as follows:when the fluid flows in the tube,under the guidance of the twined coil,the fluid begins to rotate,the centrifugal force generated by the tangential velocity increases,and the fluid in the central region of the tube moves to the near wall region,and the fluid in the near wall region moves to the central region of the tube.In this process,longitudinal vortices are generated in the tube,the secondary flow intensity is greatly improved,and the fluid mixing at different temperatures is enhanced,so the heat transfer performance in the tube is improved.(5)When Re is 400-1800,the heat transfer coefficient Nu/Nu_p is about 12.0-38.2,and the friction coefficient f/f_p is about 7.7-18.1.At the same mass flow rate,the maximum heat transfer enhancement factor JF is 2.16 when Re=1000.(6)Numerical study is used to study the average heat transfer characteristics and resistance characteristics of twined coil with different structural parameters,including the diameter of the central skeleton,the diameter of the coil,the diameter of the spiral coil and the number of coils.It is concluded that the heat transfer coefficient in the tube can be greatly improved by the tube with twined coil insert compared with the tube without twined coil insert in the case of little increase in resistance.(7)The heat transfer enhancement the tube with twined coil insert is optimized and evaluated under the working conditions of twined coil structure parameters and fluid velocity,and the correlation between heat transfer characteristics and resistance performance is fitted,which provides reliable basic data for the design and application of the tube with twined coil insert.The relationship between heat transfer characteristics and absolute vorticity in three directions is analyzed,and the correlation between heat transfer coefficient and absolute vorticity in three directions is obtained.