Research Of Flow Characteristic And Heat Transfer In Platelet Microchannels

As the characteristic length of the flow field reaches micro level, some factors thatcan be neglected in macro level will take the dominant parts, which may result in somestrange phenomenon. In this thesis, the flow characteristic and heat transfer inmicrochannels which equivalent diameter less than0.6mm are studied. These studiesare focused on the stability of microchannels flow and the computation of thermalresistance in microchannels.The thesis researches the flow effects of some factors, such as aspect ratio androughness of microchannels. With low Reynolds Number, the flow effects of aspectratio agree with conventional behaviors observed in macro scale channels. Theroughness surface is generated by using Gaussian distributed roughness heights andasperities spaces, the effects of the peak roughness height, H, asperities spacing in the Xdirection, Sx, standard deviation of the Gaussian distribution,,on the Poiseuillenumber, Po are investigated. The results show that when(H/D<3%) the Po number canstill be predicted by the conventional flow theory, The flow over roughness featureswith high relative roughness induces recirculation and flow separation, which plays animportant role in Po number, the Po number increases with Re and deviates from theprediction up to10.9%(H/D=6%).Considering the effects of velocity slip and apparent viscosity on the stability ofmicrochannels flow, apparent viscosity is employed in N-S equations while velocity isadopted. It is found that the profile of mean velocity closing to wall appears concaveand those results in reducing the critical Reynolds number and easing to lose thestability of flow when considering the apparent viscosity. Conversely, the velocity slipmakes mean flow fat and stabilizes the flow. It also shows that the effects of velocityslip are reduced sharply when the model parameters (k and) of apparent viscositybecome large.The flow characteristics in small rectangle channels with the height ranging from0.2to0.8mm were studied both experimentally and numerically. The Reynolds numberranging from approximately100to3000.The experiment data showed that in laminarregion, the characteristics of flow agree with conventional behaviors in macro scalechannels without considering the effects of micro scales. The smaller equivalentdiameter becomes the smaller critical Reynolds number gets. The experimental resultsshow that transition from laminar to turbulent flow occurred at critical Reynoldsnumbers about1300in the smaller channels of the dimensions considered here. Theroughness should play a great part in advancing the critical Reynolds. In turbulenceregime the apparent friction coefficient got from experiment are larger than numericalresults.Finally, the thermal resistances of flow and heat transfer in microchannels were analyzed using thermal resistance network model that is compared with the CFD results.The thermal resistances of single-channel about different structure parameters werecalculated assuming flow rate is constant, and then got the effects of different structureparameters on thermal resistance. The heat transfer of different structure channels werecalculated by numerical methods assuming pump power or flow rate is constant, andoptimum heat transfer structures were given.