Experimental Investigations For Transitions Of Heat Transfer And Temperature Structure In Horizontal Convection

Horizontal convection(HC)has been widely studied as a classic thermal convection in order to understand some geophysical flows and related engineering process.Recently,some progresses are made in theoretical analysis,experiments and numerical simulations,however the flow properties in HC are still unsolved with respect to heat tranfer,flow structure and transition,especially for the theoretical interpretation of the boundary layer strcucture and its transition.Besides,how to evaluate the difference of heat transport efficiency in experiments,numerical simulations and theories,and how to understand the dynamics of thermal dissipation,both of them still remain illusive.Based on above-mentioned situations,we have established a HC system and take some advanced measurement technique in order to study the heat transport,the thermal boundry layer,the dynamics of thermal dissipation and their transitions.All of these study could provide some insights to understand the HC transport properties in nature and engineering applications.At first,three different length(L=0.5,1.0 and 2.0 m)rectangular HC samples are establised,and have the same aspect ratio L:W:H=10:1:1(L,W and H is,respectively the length,width and height of the sample).Both heating(Th)and cooling(Tc)copper plates are placed at the two ends of the bottom to supply and remove heat of the system and futhermore to drive the HC large-scale circulation.Through changing the applied temperature difference Δ?Th-Tc and the sample length L,we can span the experimental Rayleigh number Ra varing in the range 2×1010?Ra?1.3×1013;By applying water as the working fluid,the Prandtl number Pr is controlled in the range 3.9?Pr?6.5.To obtain accurate input heat flux density,we have applied high-precision heat flux sensors and designed several automated temperature-controlled boxs.The measured normalized heat tranfer Nu indicates:There exsit large deviations of the Nu data between the present experiment and the direct numerical simulation,and the Nu deviation also occur among different HC samples at a same Ra.The Nu devations orginate from nonOberbeck-Boussinesq effect in the experiment,therefore we proposed a quantitative model to predict the deviation.By using the Nu correction of the model,the experimental results is well aggreement to the DNS data in the overlapped Ra range,and the Nu deviation among different HC samples approximately vanishes.The corrected Nu data shows a scaling Nu～Ra0.227Pr0.027 in the stuied Ra-range of 2×1010?Ra?1013.The Nu saling is between low-Ra and high-Ra scaling regime prediced by the SGL model,which gives a laminar boundry-layer dominated regime Nu～Ral/5Pr1/10 at low Ra and a bulk-turbulence dominated regime Nu～Ra1/4pr0 at high Ra.The measured results reveal the heat transport of the HC flow is under a transition regime.At such a transition regime,we measured temperature profiles in the thermal boundary layers above the heating and cooling plates.The measurements revealed that above the cooling plate,the mean temperature profiles have a universal form,being independent of the studied Ra-range and the horizontal position.The universal curve devaites from the PBP boundary layer quation,which results from the horizonal velocity ditribution of the large-scale flow.The profiles agree well with solutions to a laminar BL equation,which is derived under assumption that the horizontal velocity achieves its maximum near the plate and vanishes in the bulk.Above the heating plate,the mean temperature field has a double-layer structure.With Ra increasing,the double-layer structure becomes unsteady,the temperature fluctuations within thermal boundry layer indicates a series of several transtions from the laminar state to the chaotic state.The observed transitions are coincident with the transition regime of the heat transport in horizontal convection.Futhermore,experimental measurements of local thermal dissipation rate εθ by the designed temperature gradient probe have been conducted above the heating and cooling plates.For all studied Ra numbers,the HC system is in a state,where,the mean temperature gradient dominate the total thermal dissipation,and present different vertical dissipation structures above the two thermal plates.There exists a complicated two-layer structure above heating plate,and its shape changes with varying Ra.By a laminar BL scale-analysis combined with the SGL model,the normalized bulk temperature X affect the Ra-dependence of the local thermal dissipation with the expression χ-2εθ～Ra2/5.These results reveal that thermal dissipation in the present HC is dominated by lamimar structures,albeit the heat transport of the system is under a transition regime.In summary,this thesis fouses on heat transport,thermal boundry layer,the dynamics of thermal dissipation in a laboratory-scale HC system.The present work step into an unexplored direction of the heat transfer deviation under the NOB effect,firstly derive the thermal boundary layer equation.This research not only improves the understanding in HC,but also gives new insights to general thermal convection systems.