【作者】 崔文智；

【导师】 辛明道；

【作者基本信息】 重庆大学 ， 工程热物理， 2003， 博士

【摘要】 螺旋管换热器因其传热性能好，结构紧凑，制造简单而被广泛应用于各种工业行业中。螺旋管强化传热的根本原因在于弯曲通道的特殊几何结构可使流体受离心力作用在流道的横截面上形成二次流，二次流与主流的叠加使流体在弯管内形成螺旋运动，从而使换热增强。迄今为止，螺旋管内流动沸腾传热的研究对象大多是蒸汽-水，重点集中在临界热流密度（CHF）等问题，对制冷剂在螺旋管内流动沸腾传热的研究不多；在直管中被广泛采用的强化流动沸腾传热的微肋表面还没有被应用于螺旋管；螺旋管内流动沸腾流型分区与转变的研究还不充分。本文首次将三维微肋表面强化传热技术应用于螺旋管，加工形成新型三维微肋螺旋管换热元件，以绿色环保制冷剂R134a为换热流体进行了新型三维微肋螺旋管内流动沸腾传热与流动实验。用可视化方法实现了对流动沸腾流型的观察与拍摄，首次得到了R134a在螺旋管内流动沸腾的两相流型图及主要流型之间的转变判据；实验研究了三维微肋螺旋管流动沸腾的传热和阻力性能，分析了微肋螺旋管对流动沸腾传热的强化性能及其机理，得到了基于不同流型的流动沸腾传热系数和两相摩擦阻力计算关联式。1. 首次观察和拍摄了光滑螺旋管和三维微肋螺旋管内R134a流动沸腾的两相流型。两种螺旋管的流型均可以用泡状流，塞状流，分层流，分层－波状流，间歇流以及环状流等来划分。与管内绝热两相流相比，流动沸腾泡状流型的形成机制不同，即使在分层流、间歇流时也可以观察到连续液相中夹杂大量汽泡与主流一同运动。并且，由于汽泡在加热壁面的不断生成，长大与脱离，造成汽－液相界面的剧烈波动和液滴的溅射，两相界面呈现出极不规则的状态。由于螺旋管离心力的作用削弱了重力造成的两相分层效应，使螺旋管内更容易形成环状流。经修正的Mandhane流型图可以正确反映螺旋管内两相流型的分区及其转变。但螺旋管分层－波状流和环状流的区域都比光滑直管Mandhane流型图的划分区域有所扩大；Taitel-Dukler流型图表明光滑螺旋管和三维微肋螺旋管分层－波状流与间歇流、环状流间的过渡曲线基本一致，可表示为；光滑螺旋管和微肋螺旋管间歇流与环状流之间的转变曲线分别为Xtt＝0.4和Xtt=0.7；G-x流型图显示并非在所有质量流速条件下，螺旋管内都会依次出现上述的各种流型；当质量流速G<100 kg/(m2s)时，光滑螺旋管和三维微肋螺旋管内都只有分层－波状流存在；在一定条件下，分层－波状流也可以直接转变为环状流。2. 实验测量了三维微肋螺旋管内流动沸腾的阻力性能，其结果表明：在实验范围<WP=6>内，三维微肋螺旋管流动沸腾的阻力随质量流速和干度的增加而增大，随饱和压力的增加而减小；与光滑直管的阻力特性相比较，三维微肋螺旋管的两相流动阻力增加了26%－130%。用Lockhart-Martinelli方法计算了螺旋管流动沸腾的摩擦压降，整理出液相摩擦因子与Martinelli参数Xtt之间的关系图线；将实验数据按主要流型分类回归，首次获得了基于分层流和环状流的具有较高精度的两相摩擦压降因子的计算关联式，它们与实验数据的平均绝对误差分别为13.4%和19.3%；3. 在质量流速G=65－380 kW/(m2s)，热流密度q＝2.0－21.8 kW/m2，质量干度x＝0.05－0.95范围内，分别对光滑螺旋管和三维微肋螺旋管的流动沸腾传热性能进行了实验研究。两种螺旋管的局部流动沸腾传热系数的基本变化规律都是随质量流速和热流密度的增加而增大；在低干度区，局部传热系数也随干度增加而增大，但在高干度区，若质量流速较低，热流密度较高时，管壁会出现部分蒸干，螺旋管的局部传热系数随干度增加反而下降。将光滑螺旋管的实验数据与水平光滑直管流动沸腾传热系数关联式的计算值进行比较，发现光滑螺旋管平均传热系数比光滑水平直管高出6.0%－15.0%；并且由于液相所受离心力作用强于汽相，导致低干度区螺旋管的传热强化效果比高干度区更为显著。三维微肋螺旋管的传热实验数据与与光滑螺旋管的比较表明前者对流动沸腾传热具有明显的强化效果，其平均传热系数比光滑螺旋管提高了50%到170%，且提高的幅度随着质量流速和热流密度的增加而有所减小；与光滑直管相比，三维微肋螺旋管的平均传热系数增加了65%~200%。螺旋管内流动沸腾传热机制是核沸腾和对流传热两种贡献的叠加。三维微肋管强化管内流动沸腾传热的机理在于其特殊的表面结构一方面增加了传热面积，更重要的是，与光滑表面相比，三维微肋表面使沸腾核化点的数量大大增加，降低了起始沸腾所需的过热度，汽泡在受热表面上更容易生成和长大，从而强化了核沸腾传热；另外微肋的存在亦能增强对液膜的扰动，从而也能强化对流传热过程；4. 对实验数据进行回归，分别得到了光滑螺旋管和三维微肋螺旋管内流动沸腾环状流区传热系数的实验关联式，它们的计算值与实验值之间的平均绝对误差分别为13.8%和9.1%；得到了螺旋管内流动沸腾分层流区传热系数计算的经验关系式，它们的计算值与实验值之间的平均绝对误差分别为13.4%和14.1%。更多还原

【Abstract】 Because of the high efficiency in heat transfer and compactness in volume, helically coiled tubes are used extensively in heat exchangers, nuclear reactors, solar collectors, and the food, drug and refrigeration industries. It is well known that the secondary flow in the cross section of the helically coiled tube, due to centrifugal force and torsion force, is a significant factor affecting the flow and heat transfer. In the open literature, the experimental fluids of flow boiling heat transfer in helically coiled tubes are often water and steam. And the focuses are mainly pertinent to the behaviors of critical heat flux (CHF) or dryout in helically coiled tubes. The heat transfer performance of refrigerants flow boiling in helically coiled tubes has seldom been involved. Moreover, microfinned surface, widely used and investigated in straight tubes, has not been applied to helically coiled tubes. The flow boiling two-phase flow patterns and their transitions in helically coiled tube, another interesting and challenging field in two-phase flow study, have also not been sufficiently studied, compared to that in straight tube.In this paper, the heat transfer augmentation technology of a new type of three dimensional (3-D) microfinned surface was applied to helically coiled tube for the first time. The flow boiling heat transfer and flow patterns have been experimentally studied, in such kind of 3-D microfin helically coiled tube, using refrigerant R134a, an environment- friendly refrigerant. The flow boiling flow patterns have been recorded and filmed through flow visualization method. Several different two-phase flow pattern maps, together with the transitions between the main flow patterns, were obtained based on the experimental data. The performance of heat transfer and pressure drop have also been measured and the mechanism of heat transfer augmentation of three-dimensional microfin helically coiled tube were analyzied. Finally, several correlations of heat transfer and pressure drop were correlated based on different flow patterns.(1) Two-phase flow patters of R134a flow boiling in smooth and 3-D microfin helically coiled tubes were observed and filmed for the first time. The flow patterns in both kinds of helically coiled tubes could be divided into bubbly flow, plug flow, stratified flow, stratified-wavy flow, intermittent flow and annular flow. Compared to the flow patterns in adiabatic two-phase flow, the formation mechanism of bubbly flow in flow boiling was quite different, i.e., a lot of vapor bubbles could also be found in <WP=8>continuous liquid phase even though in stratified or intermittent flow regime. Further more, the continual formation, growth, and departure of bubbles from the heated surface made the interface of vapor-liquid strongly wavy and the droplet splash out of the two-phase interface. This caused the two-phase interface usually quite irregular geometry. On the other hand, because of the secondary flow in helically coiled tubes, the stratified effect was weakened, so that annular flow could be formed more easily. The modified Mandhane flow map can essentially descript the flow regimes and their transitions in helically coiled tubes. Taitel-Dukler flow map showed that the transition curves between stratified-wavy and intermittent or annular flow were almost the same, i.e., . The transition curves between intermittent flow and annular flow were Xtt＝0.4 and Xtt=0.7 for smooth and microfin helically coiled tubes, respectively. In the G-x flow map, it was obvious that not all the flow patterns mentioned above appeared one by one under each mass velocity. Only stratified-wavy flow existed in both kinds of tubes while the mass velocity less than 100 kg/(m2s).And in some cases, stratified-wavy flow could directly transform to annular flow.(2) The experimental data showed that flow boiling pressure drop in 3-D microfin helically coiled tube increased with the increases of both mass flux and the vapor quality, and decreased with the increase of saturated pressure. The comparison更多还原