Experimental And Numerical Study On Heat Transfer Enhancement Of Finned Tube Air-liquid Heat Exchanger

With the rapid development of the economy,people's focus shifts gradually to environmental protection and the realization of a harmonious and sustainable development of human society advocating low-carbon operation and enhanced resources utilization.Except vigorously promoting energy conservation,scientific utilization of energy and efficient utilization of fossil fuels,studies on accelerating the development and utilization of renewable energy such as solar energy and wind energy are also needed in order to solve a series of problems such as a sharp increase in energy demand and insufficient supply.However,wind energy,solar energy and other renewable energy have characteristics of intermittent and volatility,and the impact of the grid is very large,resulting in serious waste of wind and solar energy.In order to solve this problem,scientists propose using energy storage for sustainable energy regulation,but traditional pumped-storage and compressed-air energy storage cannot work on a large-scale because they are limited by the geographical conditions.To solve this problem,scientists from Chinese Academy of Sciences propose a supercritical compressed air energy storage system,which can be applied on a large scale without geographical constraints.Supercritical compressed air energy storage system will produce a great deal of compression heat during the operation.The supercritical compressed air energy storage system mainly compresses the air to a supercritical state and stores it in a cryogenic storage tank after cooling and liquefaction.During the compression process,a large amount of heat is generated due to the increase of the air pressure.In order to improve the efficiency of the system,the system use the inter stage cooling in the compression process.Inter stage cooling is stored the compression heat in a good heat storage medium molten salt by finned tube air-liquid heat exchanger.Therefore,the heat exchanger acts as a core part of the heat storage part.Its performance will affect the efficiency of the entire system.Based on the national"973"project--"High Temperature Storage Material and Thermal Storage Unit Heat Transfer Enhancement Mechanism"?Project Number:NO.2015CB251303?.In the experimental part,convection heat transfer experiments were performed on high-temperature air-molten salt?Hitec?,water-air,and ethylene glycol solution-air in four different fin pitch finned tube air-liquid heat exchangers.Changing the flow velocity of the media inside and outside the tube to compare the heat exchange and flow conditions under different working conditions;numerical simulation part established a three-dimensional model of the finned tube air-liquid heat exchanger,focused on the effect of changes in operating conditions such as pressure and flow rate on heat transfer performance and resistance characteristics with the high temperature air-molten salt.It offers certain guidance on the efficient recovery and utilization of the compressed heat in the supercritical compressed air system.The main contents and conclusions of this paper are as follows:?1?The fined heat exchanger of high temperature air-molten salt was researched in this paper,builded the high temperature air-molten salt heat exchange system.It mainly includes the molten salt systems,hot air system,heat conduction oil cooling system,cooling water circulation system and test system.The experiment is guided by heat balance principle experiment.The experiments were carried out by means of the principle of heat balance.The molten salt absorbs heat from the air,and then releases heat with the heat conducting oil to make the system achieve thermal equilibrium state.?2?The experiment is divided into four steps.The first step,reverses flow rate of molten salt from the cooling system of heat conducting oil system,then determine the relationship between molten salt pump frequency and the flow rate of molten salt to got a linear relationship q_ms=0.01456f_p-0.2318 with data processing;The second step,carrys out the experiment of air-molten salt heat transfer,then the effects of air and molten salt velocity and fin pitch on the heat transfer and flow are obtained;Next,convection heat transfer experiments of water-air and ethylene glycol-air were conducted.The effects of changes in the flow rate of water,glycol,air and fin space on the heat transfer and flow characteristics were obtained.The fourth step,summarizing the heat transfer criteria correlation in each experimental part.?3?The three-dimensional calculation model of the finned tube air-liquid heat exchanger is established,and perform numerical simulation study through FLUENT software.In the modeling,considering the kinetic viscosity of molten salt inside changes a lot with temperature,the coupling calculation of convective heat transfer between the molten salt in the tube and the outside air is carried out.Therefore,the error caused by the constant heat flux or constant wall temperature in the inner wall of the pipe is avoided.The results show that:1)With the flow of air,the wake flow area in the front row is smaller than the wake flow area in the rear row,and the heat transfer in the rear row is poor.With the increase of the flow velocity,the range of the low-temperature zone behind the tube row gradually decreases,the area of the dead zone caused by the back flow gradually decreases,and the heat exchange is enhanced.2)As the air pressure increases,the average temperature of the fin surface increases,the isotherm is relatively regular,close to the annular,and the temperature isoline distribution becomes more and more dense,the temperature gradient gradually becomes larger.The heat is strengthened and the heat transfer coefficient gradually increases.3)When the air pressure is 0.1 MPa,the heat transfer coefficient and the flow resistance coefficient change almost linearly with Re number on air side.When the pressure condition is 0.63 MPa,both of them show a nonlinear trend.