Study On The Influences Of Coil Tube Vibration On Heat Convection Outside The Tube Under Low Flow Velocity

Heat exchanger is a general device that has been widely used in industrial production.Every improvement of its performance always means huge economic and social benefits.Fluid-induced vibration in heat exchanger is a difficult problem which has not been solved for a long time.In traditional heat exchanger design,the element damage which caused by fluid induced vibration is usually solved by adding rigid constraint.However,the effect of vibration on heat transfer enhancement was demonstrated experimentally as early as the 1960s,which had attracted wide attention.With the introduction and utilization of fluid-induced vibration elements such as floating coil tubes which could enhance the heat transfer,vibration can be effectively utilized in heat exchangers reasonably.In this thesis,the force of the coil tube in a certain fluid domain is analyzed,and the effects of fluid damping and structural damping on the coil tube vibration are studied.The vibration equation of the coil tube under the influences of flow field damping and structural damping are derived theoretically.According to the additional mass coefficient,the effects of flow field damping on the coil vibration are expressed to describe the wet modal vibration characteristics of the coil tube.The numerical simulation analysis of the coil tube is carried out,and the intrinsic modal characteristics of the coil element are obtained through the vibration modal hammering experiment,which verifies the correctness of the numerical simulation results.Furthermore,the effects of the flow field environment on the wet mode frequency of the coil element are analyzed by numerical simulation,and the effects of the coil tube wet modal characteristics on the heat transfer performance are studied experimentally.When the coil vibrates in the flow field,the presence of the additional mass causes the wet modal frequency of the coil tube decrease to a certain extent compared with its natural frequency.The viscosity of the flow field has great influences on the wet modal frequency of the coil tube.The heat transfer performance of coil tube is worse with the increasing added mass of flow field.For analyzing the effects of fluid-induced vibration on the heat transfer,this thesis simulated vibration coils with different curvature radii in the pulsating flow field by programming a user defined function(UDF)to define the pulsating flow at the inlet of the flow field.When the coil is fixed in the form of cantilever,the outside-plane vibrationm of the coil is more likely to happen,and the vibration amplitude goes increases then decreases with the coil central angle increasing,and when the coil central angle is 180°,the structure has the maximum amplitude.When the coil is restrained by springs at both sides,the probability of outside-plane vibration and inside-plane vibration is equal,and the larger core angle is more likely to cause the coil vibration.The relative velocity in the external boundary layer also changes periodically due to the vibration of the coil.In this thesis,the influences of the angle between the direction of vibration and the direction of flow on the heat transfer intensity of convection are analyzed and verified through the theoretical derivation of the degree of synergy between relative velocity and temperature gradient under the condition of vibration.Based on finite element division of coils,the local vibration characteristics of coils are defined by UDF programming.The element vibration characteristics including frequency,amplitude,vibration direction are correspond to different vibration conditions along the coilside.Unidirectional heat fluid-solid coupling was carried out through ANSYS to simulate the influences of the different element vibration characteristics on the convection heat transfer coefficient outside the pipe when the flow field passed through the surface of the vibrating element in the way of transverse sweep flow and axial flow.When the velocity of flow is close to the maximum velocity of vibration,the heat transfer enhancement effect is better caused by inside-plane vibration.When the flow velocity is much higher than the vibration velocity,outside-plane vibration strengthens the effect of heat transfer is better,but the strengthening effect will decrease with the increasing of the flow velocity.To explore the connection between coil vibration and its heat transfer performance,this thesis demonstrates flow damping and structural damping affect coil vibration,and further influences coil heat transfer performance through theoretical analysis,numerical simulation and part of experimental data.It is concluded that the scheme of enhancing coil heat transfer under different damping conditions:the outside-plane vibration can promote heat transfer more effectively when the velocity is large,and the outside-plane vibration of the structure is more easily caused by the cantilever than the elastic constraints at both sides.The inside-plane vibration is also an important factor to promote heat transfer under low fluid velocity,and the elastic constraints at both sides of the structure are more likely to induce the inside-plane vibration.The study of this thesis provides guidance for industrial production and design optimization of heat exchange coil.