Effect Of Fluid Pulsation And Wall Vibration On Heat Transfer

Both pulsating flow and wall vibration can affect heat transfer by making thefluid fluctuate. Lots researches have been done on convection heat transfer withpulsating flow, but the results is not consistent even conflicting;wall vibration is aninevitable problem in engineering application. In the present dissertation, the effectsof pulsating flow and wall vibration on convective heat transfer were investigated,and the mechanism on whether the pulsating flow and wall vibration can enhanceheat transfer or not was analyzed by field synergy principle.Theoretical studies on convection heat transfer in channel between two parallelplates and circular tube with laminar pulsating flow under constant heat flux wereconducted by complex analysis and variable separation method, and their analyticalsolutions of temperature distribution were obtained respectively;numericalsimulations on convective heat transfer with constant heat flux and constant walltemperature were also performed. The theoretical and numerical results show thatfluid pulsation can not enhance the convection heat transfer in channel between twoparallel plates and circular tube from the period-averaged viewpoint, which onlymakes the transient velocity, temperature and the Nusselt number fluctuate around thevalues of steady convection at same Reynolds numbers.Numerical simulations on the convection heat transfer in a periodically baffledtube with laminar pulsating flow show that fluid pulsation changes the votex between thebaffles, which results in heat transfer enhancement, meanwhile, there exists an optimaldimensionless angular frequency, ω *, which increases with the Reynolds number Remincrement.The numerical simulation on the convection heat transfer of laminar air flowacross a cylinder with sinusoidal vibration shows that the cylinder vibration can enhancethe convection heat transfer remarkably. For the simulated cases, the Nusselt number can be 9times than that without surface vibration, and there exists critical vibrating parameters whichdetermine whether the susface vibration can enhance the convection heat transfer or not.The relations between period-averaged Nusselt numbers and flow andtemperature fields were derived for laminar pulsating convection heat transfer inchannel between two parallel plates, circular tube and for fluid flow across a cylinderwith sinusoidal vibration, which indicate that the field synergy principle is also validfor unsteady convection. Same as the steady case, better synergy between the velocityand temperature gradient fields corresponds to more enhanced heat transfer. Theanalyses on the above periodical unsteady convection heat transfer by field synergyprinciple show: the reason of fluid pulsation can not enhance the convection heattransfer in a channel between two parallel plates and a circular tube is that no velocitycomponent along the direction perpendicular to the wall surface exists, flow pulsationonly makes the velocity fluctuate along flow direction, and the period-averaged fieldsynergy number is equal to the steady one. On the contrary, for convection heattransfer in a periodically baffled tube or around a vibrated cylinder, there existvelocity component normal to the wall surface, both the flow pulsation and thecylinder vibration make the fluid oscillate perpendicular to the wall, which improvesthe synergy between the velocity and temperature gradient fields, therefore, the heattransfer can be enhanced by flow pulsation or cylinder vibration. Therefore, it can beconcluded that the fluid should be oscillated normal to the wall surface when usingfluid oscillation to enhance convection heat transfer.