新型相分离冷凝器相变传热强化机理研究
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摘要
该研究针对有机工质朗肯循环系统中的关键部件冷凝器,创新性地提出了流型调控的新概念,通过采用相分离概念,在冷凝管内插入柱状金属丝网,在毛细力的作用下,液体被自动吸入到金属网内,而气相在管内壁和金属网之间的环形间隙内流动,达到了流型调控的目的。该研究通过无相变空气-水两相流型调控实验和数值研究发现:(1)水平光滑管内的分层流,经过新型结构管后,液体被全部吸入金属丝网而导出,气体则全部保留在了环形间隙内;(2)水平光滑管内的间隙流,经过新型结构管后,部分液体被吸入金属丝网而导出,气泡则在环形间隙内转变为马鞍形,从而大大增大了气相与管内壁的接触面积,形成大面积薄液膜传热,同时气相被加速,强化了对流传热;(3)垂直光滑管内的间歇流,经过新型结构管后,由于丝网表面张力作用,气弹进入环隙区域,形成拉长的环形气弹,气弹与管内壁的接触面积变大,液膜厚度大幅减薄,同时气弹上升速度大幅提高,引起环隙区域和核心区域内的流体通过网孔面进行强烈的质量与动量的交换并在核心区域产生自维持脉动流,以上因素均有利用提高设备换热性能。该研究通过光管与内插丝网管的R123水平管内冷凝对比实验以及光管与内插铜粉烧结管的R245fa倾斜内冷凝对比实验研究,得出:不锈钢丝网管和铜粉烧结管均通过流型调控强化了管内冷凝,实验工况范围内,最大传热强化比达两倍以上,综合评价因子达到1.7。
For enhancing the heat transfer performance of the condenser in the organic Rankine cycle system, a flow pattern modulation concept was proposed by our group. A mesh cylinder is suspended in the heat transfer tube. Based on the experimental and numerical researches on the air-water two-phase flow pattern modulation process, we can obtain the following conclusions.(1) For the stratified flow in the horizontal tube, all liquid can be captured by the mesh cylinder to form the"gas-floating-liquid" mode. Gas is kept in the annular region.(2) For the intermittent flow pattern in the horizontal tube,the slug bubbles are prevented from entering the mesh cylinder thus they flow in the annular region to form the saddle bubbles.The condensation can be significantly enhanced by the following two mechanisms: the significantly increased contact area between the tube inner wall and the vapor phase; the increased moving speed of the saddle bubble in the annular region.(3) For the intermittent flow pattern in the vertical tube, the slug bubble cannot completely breakthrough the mesh pores,forcing the bubble flowing in the annular region to form the elongated-ring-slug-bubble. It was found that the modulated liquid film thickness can be decreased largely; the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above factors are benefit for the performance improvement of the heat transfer facilities. R123 condensation performance in a horizontal tube compared with that in a tube with a mesh cylinder insert and R245 fa condensation performance in a horizontal tube compared with that in a tube with sintered copper powder cylinder insert were investigated experimentally. Conclusions were drawn that condensation heat transfer can be augmented considerably by flow pattern modulation in a tube inserted by empty cylinder made up of mesh screen or sintered copper powder. The heat transfer can be enhanced more than two times of that in a conventional smooth tube for the entire experimental conditions, where the performance factors are about 1.7.
For enhancing the heat transfer performance of the condenser in the organic Rankine cycle system, a flow pattern modulation concept was proposed by our group. A mesh cylinder is suspended in the heat transfer tube. Based on the experimental and numerical researches on the air-water two-phase flow pattern modulation process, we can obtain the following conclusions.(1) For the stratified flow in the horizontal tube, all liquid can be captured by the mesh cylinder to form the"gas-floating-liquid" mode. Gas is kept in the annular region.(2) For the intermittent flow pattern in the horizontal tube,the slug bubbles are prevented from entering the mesh cylinder thus they flow in the annular region to form the saddle bubbles.The condensation can be significantly enhanced by the following two mechanisms: the significantly increased contact area between the tube inner wall and the vapor phase; the increased moving speed of the saddle bubble in the annular region.(3) For the intermittent flow pattern in the vertical tube, the slug bubble cannot completely breakthrough the mesh pores,forcing the bubble flowing in the annular region to form the elongated-ring-slug-bubble. It was found that the modulated liquid film thickness can be decreased largely; the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above factors are benefit for the performance improvement of the heat transfer facilities. R123 condensation performance in a horizontal tube compared with that in a tube with a mesh cylinder insert and R245 fa condensation performance in a horizontal tube compared with that in a tube with sintered copper powder cylinder insert were investigated experimentally. Conclusions were drawn that condensation heat transfer can be augmented considerably by flow pattern modulation in a tube inserted by empty cylinder made up of mesh screen or sintered copper powder. The heat transfer can be enhanced more than two times of that in a conventional smooth tube for the entire experimental conditions, where the performance factors are about 1.7.
引文