Numerical Investigation On Flow And Heat Transfer Performance In Metal Foams At High Flow Rate

In the aviation industry,with the increasing speed of flight,the aerodynamic heating makes the intake air volume of the engine decrease,which hinders the further improvement of engine power.Therefore,lowering the inlet temperature and increasing the intake air volume is of great significance for achieving supersonic fly and high maneuverability of the flight.The open-cell metal foam has a lot of advantages,such as high porosity,large disturbance to the gas flow,high specific surface area and good mechanical property,so applying the metallic foam into a compact heat exchanger can help reduce the pressure resistance,increase the heat exchange flux,make the heat exchanger compact and light.It can help obtain a higher heat exchange efficiency with a less pressure loss,which is an effective method to achieve the engine inlet precooling.However,in the present research towards flow and heat transfer on metallic foam,most of the study are aiming at a relative low speed range,study of properties on metallic foam under a high speed range is in shortage,so there is no guidance for designing a metallic foam heat exchanger.In order to solve the problem above,computational fluid dynamics method(CFD)is used to study the property of typical Kelvin structure and the real porous structure.A speed range of 4 to 90 m/s is chosen to analyze the impact trend of the main parameters,and improvement directions for designing a porous medium heat exchanger are given.Details are as follow:A model called sphere-subtraction Kelvin structure is researched for its flow and heat transfer properties.The results shows that increasing the pore density and reducing the porosity can improve the volume heat transfer coefficient(VHTC)of the structure,while the weight of heat exchanger will be increased greatly when the porosity was increased,so it is a good choice to increase the pore density to improve the VHTC.Though the sphere-subtraction Kelvin structure has a good property on heat transfer,a large pressure drop on throat makes the comprehensive factors poor,so if the pressure drop is not the main request,the spheresubtraction Kelvin structure is a good choice to enhance the heat transfer coefficient of heat exchanger.In order to decrease the pressure drop of sphere-subtraction Kelvin structure,another model called cylinder-joint Kelvin structure is studied.The results show that the cylinder-joint Kelvin structure has a lower pressure drop,while it has a worse heat transfer coefficient,too.When judged on the comprehensive factors,the cylinder-joint Kelvin structure is better than the sphere-subtraction Kelvin structure benefiting by a lower pressure drop.So when pressure drop is an important index for the heat exchanger,the cylinder-joint Kelvin structure is a better choice to improve the heat transfer coefficient.Industrial CT scanning technology is used in the present research to get the 3D structure of real porous media.The CFD results shows that the internal flow field of the structure is intricate and the vortex is diverse.In the analysis of the flow field,region and type of vortex are studied.Heat transfer coefficient,pressure drop and comprehensive factors are compared.It shows that the VHTC of real porous structure is lower than the two other structures in the speed range calculate at present,while the real porous structure has a better VHTC under a high speed range,but it cannot be excluded that the porous structure has a higher VHTC at a higher speed range.When judged on the comprehensive factors,it shows that the sphere-subtraction Kelvin structure has a better VHTC while it has a larger pressure drop,the cylinder-joint Kelvin structure has a beat comprehensive factors in a specific speed range,the real porous structure has a better performance under a larger speed range,which may be more suitable for a higher speed range.At last,in the present study,influence of porosity and pore density on heat transfer and flow of metal foam are pointed,through the comparison of the streamlines,improvement scheme are given to improve the comprehensive factors of the three structures,which have instructive meanings for the design and optimization of the metal foam heat exchangers.