| Thermal convection on rough surfaces is a common occurrence and is widespread in nature and industry.For example,the heat exchange between the urban surface and the atmosphere,and the heat exchange between the seabed and the seawater are all under the rough surface.The Rayleigh-Bénard Convection(RBC)system is a classical fluid mechanics model abstracted from complex natural convection phenomena as the experimental theoretical model of this paper.Surface roughness is usually a method of enhancing heat transfer,but in some cases,rough surfaces do not necessarily enhance heat transfer,and sometimes even reduce heat transfer,this paper will analyze this phenomenon experimentally.In the experiment,the ratio of length(L),width(W),and height(H)of the RBC system is 4:1:4,Pr=6.2,and the range of Ra Number is 7.47?10~8~4.48?10~9.In the experiment,four rough surfaces with different roughness heights(h=0.5mm,h=1mm,h=2mm,h=4mm)were measured to analyze the influence of rough surface on heat transfer intensity,and the microscopic mechanism was understood by measuring the temperature profile.ExperimenTal measurements show that the rough surface does have a weakening effect on heat transfer at a lower roughness height,and a non-scaling law relationship between the Nu Number and the Ra number.The factors affecting the heat transfer intensity are the magnitude of the Ra Number,the ratio of the roughness height to the thickness of the temperature boundary layer.The thickness of the boundary layer of the rough surface with lower roughness height is basically the same as the thickness of the boundary layer of the smooth surface with the corresponding Nu Number,thickening the thermal boundary at the same Ra number.The shape of the temperature profile is similar,except that the mixing zone is slightly different.the temperature pulsation is in the boundary layer zone as same as a smooth surface,but also in the mixing zone.The temperature probability density is gaussian in the boundary layer region,exponentially distributed in the mixing region,and positively offset.in the central region,it is also a power exponential distribution,but the forward offset is reduced,and in the opposite direction,the shape of the exponential distribution appears in the area. |
