【作者】 李晓伟；

【导师】 过增元；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2008， 博士

【摘要】 能源问题是当今世界面临的首要问题,而近年来我国的能源问题尤为突出。强化传热技术可以通过提高换热器的热量传递性能而提高能源利用效率,从而达到节约能源的目的。本文首先分析了低雷诺数及充分发展湍流的换热强化机理,然后在此基础上提出和研究了一种用于板式换热器的新型强化换热板片,并对管内二维粗糙元和微肋管的湍流换热强化进行了深入的研究。低雷诺数和充分发展湍流换热强化分析指出,通道层流换热横截面上的温降比较均匀,因此要在整个截面内增加横向速度来强化换热,大尺度纵向涡是一种很好的同功耗换热强化方法(由纵向涡强化后的流动是低雷诺数湍流流动);充分发展湍流(Re>10000)的温降集中在壁面附近,因此要在壁面设置粗糙元增加壁面湍流度来强化换热。当粗糙元高度小于粘性底层厚度时几乎不强化换热也不增加阻力,而大于5倍粘性底层厚度后,强化换热不再增加但阻力迅速增加,粗糙元为2~3倍粘性底层厚度时同功耗条件下换热强化最佳。提出了用于板式换热器的新型不连续交叉肋板片。数值分析和流动显示实验表明,不连续交叉肋板片间产生了包括前纵向涡、后纵向涡及主纵向涡等一系列纵向涡,揭示了其强化换热的物理机制。不连续交叉肋板片比目前常用的人字形板片同功耗换热强化25%以上。用数值计算和流动显示实验方法对肋参数对流动和换热的影响进行了分析,给出了不连续交叉肋板片的最佳结构参数。用二维粗糙元管进一步实验研究了充分发展湍流换热强化机理。二维粗糙元管与光滑圆管的努谢尔数比随雷诺数的增加存在极值。粗糙元和粘性底层高度定量分析表明,粗糙元为2~3倍粘性底层厚度时同功耗换热强化最佳。微肋管的数值和实验研究表明,微肋管存在强化换热临界雷诺数,把微肋管的换热分为强化区和非强化区。对于普朗特数很大的流体,因为导热底层厚度的减小,使其强化换热临界雷诺数减小。在非强化区内,努谢尔数对普朗特数的依赖关系约为0.3次方,而在强化区内约为0.56次方。螺旋微肋在Re>30000后通过增强肋表面的湍流度强化换热,直微肋则不能增加湍流度,从而也不能强化换热(Re=10000~90000)。更多还原

【Abstract】 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.更多还原