Heat Transfer Characteristics In Rectangular Enclosures For Low-Prandtl-Number Fluids

Fluid flow is one of the most common phenomena in nature.The phenomena and laws of flow have great research significance in the heat dissipation of electronic devices,large industrial machinery and energy transmission,so they have received extensive focus.Generally,buoyancy-driven convection differs from the extemal-agent-driven counterpart in that flow momenta and the energy transport are intimately coupled.Although much related research has already been conducted for high Prandtl numbers(Pr’s),some basic unexplored issues for low Pr’s remain.In this study,characteristics of fluid flows and heat transfer for low Pr’s in enclosures have been investigated.Numerical simulations are conducted using the finite-element-based commercial package,COMSOL,with the focus on the effect rendered by varying geometric aspect ratios,and the conclusions are as follows:(1)Regarding the characteristics of heat transfer,a sudden change of average Nusselt number will appear in low aspect ratio under high Rayleigh number.The average Nusselt number(Nuavg)of one aspect ratio will be obviously higher than other surrounding points.In addition,as the aspect ratio goes up to a certain extent,average Nusselt number(Nuavg)will not change,which simply presents continuous oscillation within a small range.By comparison,Nusselt number will increase with the increase of aspect ratio under the condition of moderate and low Rayleigh number.(2)As for the stability of the flow,we find that with the increase of aspect ratio,the flow state will go through five stages:firstly,a steady-state flow will appear,which will next become a periodic flow,then there will be a second steady-state flow,and it will become a second periodic flow.Finally,the flow will transform into a non-periodic flow.However,under the conditions of medium and low Rayleigh number.low-Prandtl-number fluids has been in a steady flow state.The study investigates the heat transfer characteristics of low-Prandtl-number fluids in natural convection and finds out the aspect ratio of transitional point,which provides new ideas for the design in thermal insulation and heat dissipation of industrial devices.