Numerical And Experimental Study Of The Heat Transfer Enhancement In Turbulent Channel Flow

Worldwide energy shortage is the key to the development of modern society and recently energy problem becomes especially urgent in our country China. Enhanced heat transfer techniques can raise the effectiveness of the heat exchangers and consequently save energy. Following the analysis of the enhanced heat transfer for low Reynolds number turbulent and fully turbulent flows a new type of enhanced plate for plate heat exchanger is proposed, the fully turbulent heat transfer enhancement in two dimensional roughness and micro-fin tubes are also investigated in detail.The analysis of heat transfer mechanism for the laminar duct flow shows that the radial velocity in the whole cross section is needed to enhance the heat transfer because the thermal resistance in the whole cross section of the duct must took into account. Therefore, to generate large scale longitudinal vortex is a good enhancement technique. For fully turbulent flow (Re>10000), the roughened wall via the increase of the turbulence intensity near the wall can efficiently enhance heat transfer since the thermal resistance in the wall region is dominant.The roughness has no influence on the heat transfer coefficient and flow friction when the roughness is inside the viscous sublayer. With increasing the roughness height, heat transfer is no longer enhanced, but the flow friction begins to increase rapidly, when it is over 5 times of the viscous sublayer. Analyses demonstrate that the optimal roughness for maximizing the performance of fully developed turbulence heat transfer is 2~3 times as high as the viscous sublayer.Based on the above analysis, a discontinuous cross rib plate for plate heat exchanger is proposed. Numerical analysis and flow visualization show that the front vortex, the back vortex and the main vortex are formed between the discontinuous cross rib plates. Numerical analysis and the experimental measurement also show that the heat transfer enhancement at the given pumping power for the discontinuous cross rib plate can be 25% higher than the currently popularly used chevron type plate. Based on the investigation about the influence of the rib parameters on the flow and heat transfer through the numerical simulations and the flow visualization, the optimum rib parameters are proposed.The mechanisms of the turbulent heat transfer are further investigated by experimentally measuring the heat transfer in two dimensional roughness tubes. There exists a maximum enhancement ratio of heat transfer with increasing Reynolds number for the fixed roughness height. Quantitative analysis shows that when the roughness is 2~3 times of the viscous sublayer, the enhancement ratio of heat transfer at the fixed pumping power is the highest.The numerical and experimental investigations of the micro-fin tube indicate that there exists a critical Reynolds number for heat transfer enhancement, which divides heat transfer into the enhancement and non-enhancement regions on a basis of Reynolds number. The critical Reynolds number for large Prandtl number fluid is lower than that for small Prandtl number fluid due to the height of conduction sublayer is smaller for large Prandtl number fluid. In the non-enhancement region, the Prandtl number dependence n of the Nusselt number in the form of Nu∝Prn is about 0.3, while in the enhancement region, n is about 0.56. Helical micro-fins can greatly improve the performance of the turbulence heat transfer due to the enhanced turbulence intensity near the tube surface at Re>30000. In contrast with helical micro-fins, the straight micro-fins have basically no enhancement effect on heat transfer at the Reynolds numbers from 10000 to 80000.