【作者】 张连山；

【导师】 邓先和；

【作者基本信息】 华南理工大学 ， 化学工程， 2014， 博士

【摘要】 本文对缩放通道内的自然对流及沸腾的流动和换热情况进行了研究，通过实验和数值模拟相结合的方式，研究了水和质量浓度为60%的糖液在内插旋流片缩放管内的自然对流和沸腾换热，并对缩放肋面对自然对流和沸腾的影响进行了研究。通过实验研究了水和糖液在内插旋流片的垂直缩放管及垂直光滑管内的自然对流流动及换热情况，重点研究了旋流片个数及旋转角度对流动及传热的影响，得到了不同旋流片个数及旋转角度下自然对流传热系数随管内传热温差的变化曲线。水和糖液在内插旋流片的光滑管和缩放管内的传热系数均远大于光滑管空管和缩放管空管，在实验条件范围内，旋流片个数的增加会提高自然对流的传热系数，但是提高的幅度会逐渐变小，同等旋流片个数下水和糖液的传热系数的差别较大，旋流片旋转角度的增加有利于水和糖液传热系数的提高。在实验研究的基础之上，对水和糖液在内插旋流片的垂直管内的自然对流进行了数值模拟。通过温度场与速度场的分析发现，介质在光滑管内流动时中心区域流体很难被加热，近壁区的热阻非常大，而内插旋流片后的缩放管，中心区域流体被充分加热，中心区域的水和糖液的轴向温度梯度很明显，介质流经旋流片后，中心区域流速升高明显，而且水的要比糖液大。通过对介质在内插旋流片缩放管内的流阻与传热性能分析，得到了阻力系数及传热系数随旋流片个数的变化曲线，并对不同旋流片个数下的综合强化传热性能进行了评价。对管内的轴向换热能力进行分析，发现介质在内插旋流片缩放管内的局部换热能力远大于光滑管。对缩放管和旋流片的复合强化传热机理进行了分析，周期性的缩放肋面和旋流片的自旋流作用使得介质的切向流速大大增加，有效的减薄了边界层，是传热得到强化的主要原因，同时分析了管内流体流动的速度场和温度梯度场的协同情况，发现介质在内插旋流片缩放管内的场协同情况优于光滑管，尤其是在刚流经旋流片的下游区域。通过实验对质量浓度为60%的糖液在内插旋流片的垂直管内的沸腾换热情况进行了研究，得到了不同旋流片个数和旋转角度下的糖液蒸发的沸腾曲线，发现管内插入旋流片后糖液的沸腾传热系数明显升高，并且旋流片个数的增加有助于沸腾传热系数的提高，以实验数据为基础得到了内插旋流片缩放管内糖液沸腾传热系数关联式。在实验研究基础之上，对用内插旋流片缩放管改造后的糖厂多效蒸发系统进行了节能优化，通过建立适当的计算模型，得到优化后的各效参数值，在总传热温差和总传热面积不变的情况下，实现了生蒸汽消耗量的减少，节能效果显著。采用数值模拟的方法对缩放通道内的流动沸腾现象进行了研究，考虑了流体热物性及相间传质传热的影响，研究发现缩放通道内的传热系数、流动压降及气相体积分率均高于光滑通道内的，通过非稳态模拟得到了缩放通道内流体气相体积分率随时间变化的分布图，研究了流速对流动沸腾传热的影响，通过分析得到缩放通道强化沸腾换热的主要原因是收缩肋面附近气泡脱离速度较快、运动较剧烈。通过数值模拟的方法研究了二维缩放方腔及缩放通道内的自然对流现象，重点考察了缩放肋面的肋高和缩放比对流体自然对流流动与换热的影响。研究给出了不同瑞利数下缩放方腔内的温度和速度矢量分布，在研究范围内瑞利数、肋高及缩放比的增加均能提高方腔内努赛尔数，在模拟数据基础之上得到了方腔内努赛尔数随瑞利数、肋高及缩放比变化的关联式。对缩放通道内的自然对流进行了分析，努赛尔数随肋高的增加呈现先增加后减小的趋势，阻力系数随肋高的增加而增加，努赛尔数随缩放比的增加而增加然后趋于恒定，缩放比的增加会减小阻力系数但是幅度不大。通过温度及速度分布、轴向换热能力的分析、场协同分析，解释了缩放肋面强化自然对流换热的原因。更多还原

【Abstract】 The flow and heat transfer of natural convection and boiling in the converging anddiverging channel were researched in this paper. On the combination of experiment andnumerical simulation, the heat transfer of natural convection and boiling for water and60%sucrose solution in vertical converging-diverging (CD) tubes with regularly spaced twistedtapes (RSTT) were investigated. The effects of converging-diverging surfaces on naturalconvection and boiling were studied simultaneously.The flow and heat transfer of natural convection for water and60%sucrose solution invertical converging-diverging tube and smooth tube with regularly spaced twisted tapes werestudied experimentally. The effects of number of twisted tapes and twisted angle on flow andheat transfer were mainly presented. The variation of natural convection heat transfercoefficient with temperature difference in tube side was obtained under different twisted angleand number of twisted angle. The heat transfer coefficient for water and sucrose solution invertical smooth tube and CD tube with RSTT are much bigger than that in barely smooth andCD tube. Within the scope of the experimental conditions, the natural convection heat transfercoefficient increases with the number of twisted tapes, but the degree of increase will becomegradually smaller. The heat transfer coefficient for sucrose solution is different from that forwater under same number of twisted tapes. The twisted tapes with higher degree of twistedangle are more benefit for heat transfer performance.Based on the experimental research, the natural convection for water and sucrose solutionin vertical smooth and CD tubes with RSTT was investigated numerically. Under the analysisof temperature field and velocity field, it is found that the fluid in the central region of smoothtube is difficult to be heated because of the high heat resistance in near-wall region. When theRSTT were inserted into CD tube, the fluid in the central region was heated sufficiently. Thevelocity increases obviously when the fluid flows through the RSTT, and there is obviousaxial temperature gradient for water and sucrose solution in central region. Through theanalysis of flow resistance and heat transfer performance for CD tube with RSTT, thevariation of resistance coefficient and heat transfer coefficient with number of twisted tapeswas obtained, and the performance of comprehensive heat transfer enhancement wasevaluated. The heat transfer performance along axial direction was researched. The resultsindicated that the local heat transfer coefficient along axial direction in a CD tube with RSTTwas bigger than that in a smooth tube. It is found that the tangential velocity increasessignificantly for the compound effects of periodically converging-diverging surfaces and twisted tapes. The heat transfer boundary layer becomes thinner because of the highertangential velocity, and then the heat transfer is enhanced. The synergy of temperaturegradient field and velocity field for fluid in tube side was investigated. Because of thecompound effects, the synergy in a CD tube with RSTT is better than that in a smooth tube,especially in the region where the fluid flows through the twisted tapes.The boiling heat transfer of60%sucrose solution in a vertical tube with RSTT wasinvestigated experimentally. The curves of boiling heat transfer for evaporation of sucrosesolution were obtained under different number of twisted tapes and different twisted angle. Itis found that the twisted tapes is benefit for the boiling heat transfer, and the boiling heattransfer coefficient increases as the number of twisted tapes. Moreover, the empirical formulafor sucrose solution in a CD tube with RSTT was obtained by the fitting of the experimentaldata. Based on the experimental study, the optimization of energy conservation formultiple-effect evaporation system was conducted. The evaporator of CD tubes with RSTThas higher boiling heat transfer coefficient. Therefore, the consumption of fresh streamreduced under the same total heat transfer temperature and same total heat transfer area. Theparameters for each effect were obtained after optimization by the appropriate numericalmodel.The flow boiling in a converging-diverging channel was investigated numerically. Theeffects of physical thermal properties on heat and mass transfer of the two phases wereconsidered. It is found that heat transfer coefficient, flow pressure drop and gas-phase volumefraction in a CD channel is bigger than that in a smooth channel. The distribution of gas-phasevolume fraction along the heated time was obtained by unsteady simulation. The effects ofvelocity on flow boiling were studied. Because of the more vigorous exercise and biggerbubble departure frequency of the bubbles near the converging surfaces, the boiling heattransfer in a CD channel is enhanced significantly.The natural convection in a CD square enclosure and a CD channel was researchednumerically. The effects of rib height and pitch ratio for CD surfaces on the naturalconvection and heat transfer were particularly studied. The distribution of temperature filedand velocity field in the CD square enclosure was presented under different Rayleigh number.Within the scope of the experimental conditions, the Nusselt number increases as the Rayleighnumber, rib height and pitch ratio. The empirical correlation for Nusselt number in theenclosure was obtained based on simulated data. For the natural convection in a CD channel,the Nusselt number increases as rib height and then decrease gradually. The resistance coefficient increases as rib height and rarely decreases as pitch ratio. Because of theenlargement of pitch ratio, the Nusselt number increases and then remains unchanged. Thereasons of heat transfer enhancement for natural convection in converging-diverging surfaceswere revealed by analysis of temperature field and velocity field, axial heat transferperformance and filed synergy.更多还原