Studies Of Fundamental Heat Transfer Characteristics For Various Prandtl-number Fluids

Fluid flow and heat transfer widely exist in nature and possess extensive applications in engineering,which have intensively promoted wide-ranging attention for academic investigation.Since fluid flow and heat transfer generally conjugate and affect each other with confining,leading to the mechanism of heat transfer characteristics becomes complicated.Consequently,there are many fundamental convective heat transfer problems that still have not yet been clearly understood.In this thesis,we base on the fundamental theory of fluid dynamics and heat transfer,and using COMSOL software,to numerically investigate several fundamental problems in heat transfer.Accordingly,several attractive and anomalous heat transfer characteristics are found,and our primary research results are shown below:(1)For high Prandtl-number fluids(0.707 ? Pr ? 104),two distinctive heat-transfer characteristics under the influence of the aspect ratio(AR = L/H)for buoyancy-driven recirculating flows in rectangular enclosures(with left hot,right cold,top/bottom insulated walls)at various Rayleigh number(Ra)are investigated.Under high Ra,as the enclosure widens,the average Nusselt number(Nu)exhibits mildly zigzag behaviors,namely,decreasing first,reaching a local minimum,then rising,reaching a local maximum,and finally decreasing again.Under low Ra,as the enclosure widens,the Nu exhibits normally decreasing behaviors.The phase diagram of Nu dependent on Ra and AR are presented.The mechanism of zigzag behaviors is associated with the competition between heat convection and heat conduction.In addition,we have obtained that the Nu is independent with Prandtl number when Pr ? 7.07.(2)As a sequel of the high-Pr investigation,for low Prandtl-number fluids(Pr?0.025),heat-transfer characteristics under the influence of AR for buoyancy-driven recirculating flows in rectangular enclosures behave entirely different from those high-Pr fluids.At Ra = 106,as the enclosure widens,time-averaged and hot-wall-spatially-averaged Nusselt number(Nuavg)exhibits stair-like behaviors,namely,decreasing first,and then cascading downward.The mechanism of stair-like behaviors is associated with vortexes collapsing as well as the competition between heat convection and heat conduction.In addition,it can conclude that the movement trajectory of the local heat transfer(Nu(y))peak,whose periodicity and number of cyclic orbits could be used as an important criterion to evaluate the stability of the system.(3)Instantaneous heat transfer coefficients(h)for flows of various fluids(mercury,air,water,and motor oil)over plates inclined at various angles have been fundamentally examined.We have discovered that instantaneous heat transfer rates(q)versus h features three distinctive regimes,which constitute a humped behavior,namely,increasing regime(?),maxima regime(?),and decreasing regime(?).In regime ?,as h increases,instantaneous heat transfer rates(q)decrease,defying our physical intuition.Regime ? exhibits maxima in q-versus-h curves.In Regime ?,q increases as h increases.Finally,we explain the mechanism behind these three distinctive regimes.(4)Generally,when we heat a system up,the system mean temperature(Tm)will increase monotonically,and then reach new equilibrium state.However,in rare occasions,anomalous cooling behavior would take place when we heat a system up.The present study aims to investigate recirculating enclosure system under specified conditions,in which,during a certain time interval,although we transiently heat a system,the system reversely cools down.The reversely decreasing temperature is associated with the competition of vortexes interaction.An anisotropic heat transfer layer is created between two competing vortexes' interface,which divides the system into two significant temperature regions.In our studies,we not only have discovered interesting fundamental phenomena,but also have identified mechanisms that govern these phenomena.These results can serve as a foundation for further application-oriented studies of fluid flows and heat transfer.