Spatial Distribution Of The Thermal Dissipation Rate In Rayleigh-Bénard Convection With Rough Plates

Convective heat transfer phenomena are widespread in nature and various engineering applications.The Rayleigh-Bénard convection(RBC)system is a typical model of natural convection.By studying the heat transfer mechanism in the RBC system,people can find the principle of natural convective heat transfer.In this paper,experiments were carried out in an RBC system with rough boundaries of 3 mm and 6 mm high pyramids.The microscopic heat transfer mechanism of heat was studied by measuring the spatial distribution of heat dissipation rate.All experiments in this paper were performed under the condition of Pr = 5.4,and the fluid medium in the RBC system was water.By measuring the temperature profile under the conditions of Ra = 3.3×109?1.053×1010 in the RBC systems of different roughness,three regimes were identified: thermal boundary layer regime,mixing regime and central regime.It was found that the relationship between the thickness of the boundary layer and Ra under 6 mm and 3 mm roughness is ? = 6410 × Ra-0.36 and ? = 6543 × Ra-0.36,respectively.Under identical Ra,it was found that the larger the roughness,the smaller the thickness of the thermal boundary layer,leading to a larger heat transfer intensity.In the same RBC system,the larger the Ra,the smaller the boundary layer thickness ?,which shows that the heat transfer is stronger.This experiment measured a vertical profile of the heat dissipation rate.It was found that ?N,?m and ?f all decrease with increasing height in the two RBC systems of different roughness.In the mixing zone and the central zone,?f is significantly larger than ?m,and the difference between them shrinks when approaching the central zone,which indicates that the closer to the central area,the weaker the thermal plumes.At the same height,?N and ?f in the RBC system of 6 mm roughness are much larger than that in the RBC system of 3 mm roughness.This is because when the roughness increases,the thermal plum es shedding from the thermal boundary layer will be strengthened,resulting in an increase of the pulsating temperature gradient,thus the heat dissipation rate will be enhanced correspondingly.In the center of the system under 6 mm,measurements under constant ? show that the G(?T),which is the PDF of ?T with respect to ?,follows the Gaussian distribution.It shows that the mathematical model based on the smooth surface RBC system is also applicable to the rough surface RBC system.