Study On Heat Transfer Characteristics Of Dense Particle Flow Around Heat Exchange Tube

In the context of"carbon neutrality,carbon peaking",carbon emissions and global climate change are important issues that the energy industry has to face.However,energy is an indispensable condition for the survival and development of human society,so controlling and optimizing the utilization efficiency of energy on the basis of the existing energy structure is also an important means to solve global warming.The high-temperature solid particles produced by the current social production materials often carry a lot of waste heat due to production needs.Reasonable and efficient use of this part of waste heat will be of great significance in solving global energy problems such as emissions and improving energy efficiency.Moving bed heat exchanger(MBHE)is an efficient heat exchange equipment for high-temperature granular materials.It is widely used in the heat exchange of solar central receivers and the waste heat recovery of high-temperature granular materials such as blast furnace slag.However,MBHE is still in the development stage,and the related heat transfer characteristics of its dense-phase particle flow around heat transfer tubes still need to be studied urgently.Taking the heat transfer process of the dense particle flow around the heat exchange tube as the research object,based on the CFD-DEM method,a heat transfer model of dense particle flow around a circular tube was established.Numerical simulation of the heat transfer process of dense particle flow around a circular tube was carried out,and the heat transfer characteristics of particle flow around a circular tube were analyzed.The effects of particle physical properties(particle size,density),particle surface characteristics(particle static friction coefficient,particle rolling friction coefficient,particle collision recovery coefficient),and particle shapes(spherical,ellipsoidal,cylindrical,regular tetrahedron,regular hexahedron)on the heat transfer characteristics of dense particle flow around a circular tube were systematically explored.The criterion number equation that can predict the effective heat transfer coefficient of the particle flow around the heat exchange tube was obtained by fitting,and the main conclusions are as follows:(1)When the dense particle flow flows around the heat exchange tube,there is an obvious particle stagnation area above the heat exchange tube,and an obvious empty area below.The temperature of the particles around the heat exchange tube is significantly reduced,and the low-temperature particles are concentrated in the"empty area"and continue to move below the heat exchange tube.The heat flow of the heat exchange tube is stable at about 140.80 W,and the contact number between the particles and the heat exchange tube shows a stable state of fluctuation,and the contact number in the steady state is 55.4.The average effective heat transfer coefficient of the heat exchange tube is 238.68 W/(m~2·K).(2)The influence of particle physical properties on the heat transfer characteristics of the particle flow around the heat exchange tube was explored.Within the studied particle size range,with the increase of particle size,the degree of particle flow compaction decreases,and the maximum temperature and minimum temperature of the particles both increase relatively.The maximum temperature difference of particles decreases,the rate of decrease in average particle flow temperature decreases and the trend slows down.The heat exchange tube average heat flux decreases,the average particle-heat exchange tube contact number decreases,and the heat exchange tube average effective heat transfer coefficient of the particles decreases.The average particle flow temperature drop rate decreased from 6.74 K/s to 2.06 K/s,the heat exchange tube average heat flux decreased from 155.48 W to 128.78 W,and the heat exchange tube average effective heat transfer coefficient is reduced from 262.35 W/(m~2·K)to 217.31 W/(m~2·K).Within the studied density range,with the increase of particle density,the maximum temperature and minimum temperature of the particles both increase relatively.The maximum temperature difference of the particles decreases,the rate of decrease in the average temperature of the particle flow decreases and the trend slows down.The average heat exchange tube heat flux increases,the average particle-heat exchange tube contact number does not change significantly,and the average effective heat transfer coefficient for the heat exchange tube increases.The average temperature drop rate of the particle flow decreased from 5.95 K/s to 2.92 K/s,the average heat flux of the heat exchange tube increased from 137.00 W to 142.14 W,and the average effective heat transfer coefficient of the heat exchange tube increased from 231.18W/(m~2·K)to 239.85 W/(m~2·K).(3)The effect of particle surface characteristics on the heat transfer characteristics of the particle flow around the heat exchange tube was explored.The particle static friction coefficient has the most obvious effect on the heat transfer characteristics of the heat exchange tube.With the increase of the static friction coefficient of the particles,the"empty area"at the bottom of the heat exchange tube expands,and both the maximum and minimum temperatures of the particles decrease relatively.The maximum temperature difference of particles increases,the rate of decrease of the average temperature of particle flow increases and the trend slows down.The average heat flux of the heat exchange tube decreases,the particle-heat exchange tube average contact number decreases,and the average effective heat transfer coefficient of the heat exchange tube decreases.The average temperature drop rate of the particle flow increased from3.48 K/s to 3.65 K/s,the average heat flux of the heat exchange tube decreased from 142.22 W to 139.03 W,and the average effective heat transfer coefficient of the heat exchange tube decreased from 239.99 W/(m~2·K)to 234.59 W/(m~2·K).The particle rolling friction coefficient and elastic collision recovery coefficient have little effect on the heat transfer characteristics of the heat exchange tube.With the increase of particle rolling friction coefficient and elastic collision recovery coefficient,the particle flow average temperature drop rate,the heat exchange tube average heat flux,the particle-heat exchange tube average contact number and the average effective heat transfer coefficient of heat exchange tube did not change significantly.(4)The effect of particle shape on the heat transfer characteristics of the particle flow around the heat exchange tube was explored.In the case of the same equivalent diameter,the temperature uniformity of spherical particle flow is the best,while the temperature uniformity of tetrahedral particle flow is the worst.The average temperature drop rate is the largest for spherical particle flow,and the heat exchange tube average heat flux for regular tetrahedral particle flow is the largest.The heat exchange tube average heat flux of regular hexahedral particle flow is the smallest,and the particle-heat exchange tube average number of contacts of cylindrical particle flow is the largest.The overall temperature distribution uniformity of non-spherical particle flow and the overall heat transfer effect of particle flow are worse than spherical particle flow.(5)The particle size,particle density and particle static friction coefficient are integrated into the fitting correlation,and the criterion number equation that can predict the effective heat transfer coefficient of the particle flow around the heat exchange tube was obtained.The maximum relative error between the calculated value and the simulated value of the prediction correlation is less than 10%.