Study On Heat Transfer Enhancement Characteristics Of Inserted Spiral Spring Fins

Nowadays,our country’s social economy developing rapidly,the demand for energy is increasing day by day,however,the utilization rate of energy is relatively low,energy shortage and the consequent environment pollution become the shackles of sustainable economic development and people’s life quality improvement.Many countries have realized the importance of energy conservation,have carried out in-depth research and development of various new energy-saving technologies,therefore,heat transfer enhancement technology has been developed rapidly.At present,the enhancement of heat transfer technology is mainly divided into three categories:the treatment of the internal and external surface of the heat exchange tube,the inter-tube support structure and the tube insert.Among all kinds of heat exchange tube inserts,the built-in spiral spring has attracted the attention of many scholars at home and abroad because it does not need to change the shape of the heat transfer surface,the production process is simple and the pitch of the spring can be freely changed,suitable for heat exchange tubes with different diameters.In this paper,on the basis of the traditional spiral spring heat exchange tube,the shape of the cross section of the spring is changed to a square,and the spring is contacted with the inner wall of the heat exchange tube to strengthen the heat exchange in the tube by increasing the heat exchange area and damaging the fluid boundary layer.The heat transfer model of spiral spring heat exchange tube is established,in order to ensure the accuracy and availability of the model,the independence of the heat transfer model is verified,and the simulation results are compared with others’results.On the basis of ensuring the accuracy of the heat transfer model,the numerical simulation of the heat transfer characteristics of the spring heat exchange tube was carried out by Fluent,the influence mechanism of spring parameters on heat transfer process of heat exchange tube was calculated and analyzed,and the values of each parameter when the heat transfer capacity was optimal were determined by the comprehensive heat transfer performance.The influence of the shape,equivalent diameter and pitch of the spring on the heat transfer process of the heat exchange tube is studied:The square section spring heat exchange tube has the best heat exchange performance,followed by the triangle section and finally the circular section,compared with the smooth tube,the heat transfer coefficient increases by an average of 1.4 times,1.3 times and 1.1 times,and the pressure drop increases by an average of 11 times,9 times and 10 times.When the pitch remains unchanged,the heat transfer coefficient and internal pressure drop of the spiral spring heat exchange tube increase with the increase of equivalent diameter of the spring.When the air temperature at the inlet is 310K and the outer wall of the heat exchange tube is 500K,in the range of Reynolds number 20000～40000,the heat transfer coefficient of the square section spring heat exchange tube increases by109%～164%and the pressure drop increases by 5～19 times compared with the smooth tube;when the equivalent diameter is constant,the heat transfer coefficient and internal pressure drop of the spiral spring heat exchange tube increase with the decrease of spring pitch,in the range of Reynolds number 20000～40000,the heat transfer coefficient increase by 109%～158%and the pressure drop increases by 8～16times compared with the smooth tube.With the increase of equivalent diameter and pitch,PEC first increases and then decreases.When the equivalent diameter of spring is 5～6mm and the pitch is 33.5～37mm,the comprehensive heat transfer performance of spiral spring heat exchange tube reaches the best value,and the heat transfer coefficient increases by 138%～143%compared with that of the smooth tube,while the pressure drop increases by 10～12 times.When multiple springs are inserted into the tube,the combined action of multiple springs balances the influence of single spiral spring on the eccentric flow of the fluid,so that the fluid can present a rotating flow state along the central axis,the flow rate of the fluid gradually decreases with the increase of the number of springs,and the pressure drop also gradually decreases.The velocity field gradually presents a symmetrical distribution along the central axis,the fluid flow state gradually tends to be stable,and the distribution of temperature field and pressure field in the tube gradually becomes uniform.When the air temperature at the inlet is 310K and the outer wall of the heat exchange tube is 500K,in the range of Reynolds number20000～40000,compared with the single spiral spring heat exchange tube,the outlet temperature of the four-spiral spring heat exchange tube increases by 5.77～8.14℃,the heat transfer coefficient increases by 3.94～11.45W·m^-2·K^-1,and the pressure drop decreases by 40.31～157.88Pa,but the increase and decrease gradually weaken with the increase of the number of springs.The study on the spring and blade mixed heat exchange tube shows that the heat transfer coefficient of the mixed heat exchange tube is better than that of the spring heat exchange tube and blade heat exchange tube.Within the range of Reynolds number 20000～40000,the heat transfer coefficient of the mixed heat exchange tube is62%,35%and 45%higher than that of the smooth tube,the spring heat exchange tube blade heat exchange tube,respectively.However,too many built-in components make the internal pressure drop of the mixed heat exchange tube increase sharply,and the pressure drop of the mixed heat exchange tube is 39 times,7 times and 3 times higher than that of the smooth tube,the spring heat exchange tube and the blade heat exchange tube respectively,resulting in a lower comprehensive performance of the heat exchange tube than the spring and blade heat exchange tube.Through orthogonal simulation,it is found that the comprehensive heat transfer performance increases first and then decreases with the increase of spring pitch and equivalent diameter,and increases with the increase of the number of springs inserted into the tube.The number of springs inserted in the tube has the greatest influence on the comprehensive performance of heat transfer,followed by the equivalent diameter of spring,and finally the spring pitch.The spring parameters with the best comprehensive heat transfer performance were obtained:the pitch is 37mm,the equivalent diameter is 6mm,and the number of springs inserted in the tube is 4.