Study On The Influence Of The Turns Of Spring Coil On Heat Transfer Of The Tube With Twined Coil Inserts

The heat exchanger has an irreplaceable role in various fields.For oil-to-air heat exchangers,oil is a poor heat transfer medium.When the cooled medium is single-phase,the total heat transfer coefficient of the heat exchanger is much lower than the heat transfer coefficient of the air-side surface.The heat transfer performance of the heat exchanger is limited by the heat transfer coefficient of the inner surface of the tube.Therefore,it is necessary to improve the total heat transfer coefficient of the heat exchanger by enhancing the convective heat transfer in the tube.Inserting a turbulent element into a tube is a commonly used method for enhancing heat transfer in a tube.There are various forms of enhanced heat transfer inserts,including spiral twisted tapes,spiral coils,and twined coil,etc.In order to improve the performance of the tube-fin heat exchanger,a new type of twined coil insert was proposed.Experimental and numerical studies on the enhanced heat transfer characteristics of this type of twined coil insert inserts were carried out.In the experimental study,we ensure that other parameters remain unchanged,and only change the turns of spring coil.Twined coil with N=42,N=84,N=126 were selected for experiments,which are denoted as twined coil I,twined coil II,and twined coil III.Experimental research was conducted to study the effect of twined coil with different turns of spring coil on the enhanced heat transfer in the tube.The experimental research range is Re=1450-4750,compared with the non-inserted twined coil tube,the enhanced heat transfer factor of the twined coil III is the highest.The enhanced heat transfer rate of twined coil I is1.59-4.46 times,the enhanced heat transfer rate of twined coil II is 1.97-6.66 times,and the corresponding enhanced heat transfer factor of twined coil III is 2.14-7.71.twined coil III has the highest heat transfer enhancement,which indicates the greater turns of spring coil,the higher the heat transfer coefficient in the tube,but the larger the pressure loss.In the whole experimental research range,the resistance increase factor of twined coil III is also the highest.The resistance increase of twined coil I is 2.15-5.29 times,the resistance increase of twined coil II is 4.7-8.81 times,and the corresponding resistance increase of twined coil III is6.42-12.83 times.Therefore,for variable the turns of spring coil,both heat transfer and resistance are mutually restricted.The experimental research range is Re=1450-4750,the enhanced heat transfer factor JF decreases with the increase of Re,the JF factor is larger at low Reynolds number,and the JF corresponding to the twined coil III is the largest at the same Reynolds number.This shows that the twined coil III has the best heat transfer enhancement effect under laminar flow.When Re is greater than 3500,the corresponding JF values of the three twined coil are less than 1,which indicates that the twined coil insert has a general effect of enhancing heat transfer at high Reynolds numbers.When the Reynolds number is large,the heat transfer enhancement effect of the twined coil insert is less affected by the turns of spring coil.The thesis has conducted a numerical study on the enhanced heat transfer characteristics after inserting twined coil I,twined coil II,twined coil III into the tube.The range of numerical research is Re=700-4600,and the numerical values and experimental results of Nuo and fo corresponding to the three types of twined coil are compared.The comparison results show that the Nuo obtained by experiments and simulations gradually increase with the increase of Re number,and the deviation between the experimental results and numerical results of twined coil I is very limited.The deviation under laminar flow is smaller than that under turbulent flow,and all deviations are concentrated within 5%;the deviation under turbulent flow is large,and the maximum deviation occurs at the maximum Re number,which is about19.5%.The f_o obtained by experiments and simulations decreases gradually with the increase of Re number,and the deviation between the experimental results and numerical results is very limited too.When the Re number is small,the deviation is small,and as the Re number increases,the deviation gradually increases.The deviation under laminar flow is smaller than that under turbulent flow,and all deviation are concentrated within 8%;the deviation under turbulent flow is also large,and the maximum deviation occurs at the maximum Re number,which is about 19%.The comparison results show that the numerical calculation results are reliable,which can accurately reflect the flow conditions and heat transfer characteristics after the twined coil is inserted into the tube.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,making the fluid in the tube fully mixed,which induces turbulence in advance.As a result,the radial temperature distribution is is uniform and flat,and the heat transfer in the tube is significantly enhanced.At the same time,the stainless steel wire ring and the inner wall surface of the tube are in staggered point contact with each other periodically in the axial direction and the circumferential direction.These contact points make the inner wall surface rough,and the rough surface plays a important role in enhancing heat transfer.Compared with twisted tapes and spiral coils,the enhanced heat transfer of the twined coil has been greatly improved,but the resistance has also increased a lot.Both experimental and simulation results show that the twined coil insert has a very good heat transfer enhancement capability.