Miniaturization of electronic equipment has forced researchers to devise more effective methods for dissipating the generated heat in these devices.In this study,two methods,including porous media inserting and adding...Miniaturization of electronic equipment has forced researchers to devise more effective methods for dissipating the generated heat in these devices.In this study,two methods,including porous media inserting and adding nanoparticles to the base fluid,are used to improve heat transfer in an annulus heated on both walls.To study porous media insert,porous ribs are used on the outer and inner walls independently.The results show that when porous ribs are placed on the outer wall,although the heat transfer enhances,the pressure drop increment is so considerable that performance number (the ratio of heat transfer enhancement pressure increment,PN) is less than unity for all porous rib heights and porous media permeabilities that are studied.On the other hand,the PN of cases where porous ribs were placed on the inner wall depends on the Darcy number (Da).For example,for ribs with Da=0.1 and Da=0.0001,the maximum performance number,PN=4,occurs at the porous ribs height to hydraulic diameter ratios H/Dh=1 and H/Dh=0.25.Under these conditions,heat transfer is enhanced by two orders of magnitude.It is found that adding 5% nanoparticles to the base fluid in the two aforementioned cases improves the Nusselt number and PN by 10%–40%.展开更多
Problems that may accompany downscaling in internal flows are discussed. Non-dimensional similarity parameters （like Re, M, etc.） depend on parameters that scale down fully （e.g., length）, or that are specified by...Problems that may accompany downscaling in internal flows are discussed. Non-dimensional similarity parameters （like Re, M, etc.） depend on parameters that scale down fully （e.g., length）, or that are specified by design （velocities, cascade geometry）, or that are scale invariant （like thermophysical and transport properties of fluids）.This fact has to be taken into acount in the similarity considerations. With decreasing Reynolds number the surface forces become of growing importance, however, if not going down directly to microflows the main problems are connected with the three-dimensional flow structure affecting the skin friction, and with the fact that some of the flow phenomena （e.g. vortical structures） cannot fully develop.展开更多
文摘Miniaturization of electronic equipment has forced researchers to devise more effective methods for dissipating the generated heat in these devices.In this study,two methods,including porous media inserting and adding nanoparticles to the base fluid,are used to improve heat transfer in an annulus heated on both walls.To study porous media insert,porous ribs are used on the outer and inner walls independently.The results show that when porous ribs are placed on the outer wall,although the heat transfer enhances,the pressure drop increment is so considerable that performance number (the ratio of heat transfer enhancement pressure increment,PN) is less than unity for all porous rib heights and porous media permeabilities that are studied.On the other hand,the PN of cases where porous ribs were placed on the inner wall depends on the Darcy number (Da).For example,for ribs with Da=0.1 and Da=0.0001,the maximum performance number,PN=4,occurs at the porous ribs height to hydraulic diameter ratios H/Dh=1 and H/Dh=0.25.Under these conditions,heat transfer is enhanced by two orders of magnitude.It is found that adding 5% nanoparticles to the base fluid in the two aforementioned cases improves the Nusselt number and PN by 10%–40%.
文摘Problems that may accompany downscaling in internal flows are discussed. Non-dimensional similarity parameters （like Re, M, etc.） depend on parameters that scale down fully （e.g., length）, or that are specified by design （velocities, cascade geometry）, or that are scale invariant （like thermophysical and transport properties of fluids）.This fact has to be taken into acount in the similarity considerations. With decreasing Reynolds number the surface forces become of growing importance, however, if not going down directly to microflows the main problems are connected with the three-dimensional flow structure affecting the skin friction, and with the fact that some of the flow phenomena （e.g. vortical structures） cannot fully develop.
