Numerical Study On The Heat Transfer Enhancement Of Composite Porous Media

Almost all production activities in contemporary society are inseparable from energy.As the basis for the rapid development and stable operation of human society,energy has become an important cornerstone for the survival of human civilization.Natural gas is one of the three major energy sources in China.In the process of mining and using natural gas,often because of technical problems,a large amount of lowconcentration methane is emitted into the atmosphere because it is difficult to burn at normal temperature,resulting in waste of resources and increased greenhouse effect.Low emission clean catalytic combustion technology is an effective method to solve this problem.As one of the important researches in the energy field,catalytic combustion has an extensive research foundation.Among them,the catalytic combustion in structured catalysts has been a research hotspot in recent years.The process involves a multi-scale space from micro-nano-scale pores to macro-scale channels,coupling pore boundary catalytic reactions,pore radiation,and macro-scale multi-components In complex processes such as heat and mass transfer,the internal multi-scale and multi-field coupled heat and mass transfer mechanism and coordination mechanism are very complex.Therefore,the heat and mass transfer characteristics and evaluation indexes during the flow-enhanced heat transfer of porous media are obtained,the specific foam metal structure supporting the catalyst framework and its flow heat transfer characteristics are studied,and the multi-field coupling mechanism of flow,heat transfer and radiation at high temperatures is exploredIn order to reduce the reaction requirements and energy consumption of catalytic combustion and improve its energy utilization rate,it is of great significance.In order to provide part of the theoretical support for the interaction and coupling principles of different scales in the study of multi-scale heat and mass transfer in the porous medium flow channel,this paper is divided into two steps.First,the circular regular channels are filled with foam metal direct numerical study of air flow heat transfer in porous media,using Nusselt number and drag coefficient to calculate comprehensive heat transfer efficiency,and using different evaluation criteria to evaluate its heat transfer enhancement effect.The effects of the combination of the thickness and porosity of single-pore structure porous media and composite porous media on the overall heat transfer efficiency are discussed.The results show that the integrated heat transfer efficiency of equal flow rate is deteriorated under different filling conditions.When the composite porous medium with a large porosity in the inner layer is filled,the integrated heat transfer efficiency of equal pump work and constant pressure drop increases with the near wall layer.The increase in thickness will decrease in the middle,which is lower than the comprehensive heat transfer efficiency corresponding to the porous media filled with two single pore structures.When the composite porous medium with a small porosity in the inner flow layer is filled,its comprehensive heat transfer efficiency generally changes monotonously with the thickness of the near-wall layer.Then build a three-dimensional body-centered cubic foam metal structure with coatings,and through geometric calculations,calculate the geometric parameters such as the window diameter,the volume ratio of each part,and the specific surface area,construct the grid by ICEM,and use FLUENT software to calculate the heat transfer characteristics of the coated foam metal unit cell structure when considering radiation are calculated.It was found that the increase in the thickness of the coating had little effect on the radiation heat transfer coefficient,but the absence of a catalyst coating would significantly reduce the radiation heat transfer effect.After that,the coupling of flow heat transfer and radiation in the coated foam metal was simulated.The results show that the presence of radiation will affect the temperature distribution,and the temperature of the wall surface under the heat flow calculation model will be greatly reduced.The increase in the thickness of the catalyst coating will also make the temperature distribution more even,but the increase in the temperature of the lower surface is not obvious.In addition,the higher the temperature,the greater the radiation heat transfer,which is more conducive to improving the temperature uniformity of the porous media framework and reducing the maximum temperature.The heat transfer at the gas-solid interface in the foam metal unit was simulated and analyzed,and it was found that the radiative heat transfer had little effect on the calculation results of the heat transfer Nusselt number on the gas-solid surface,but it would reduce the wall temperature difference and the maximum wall temperature.The increase of the unit cell side length ratio l / dc will deteriorate the flow heat transfer effect,and the same structure unit cell model,the inlet temperature rise and the coating thickness decrease will also reduce the Nusselt number.