【作者】 周吉成；

【导师】 朱冬生；

【作者基本信息】 华南理工大学 ， 化学工程， 2013， 博士

【摘要】 螺旋椭形管是在普通圆管基础上进行压扁、扭曲而成型的，已有的研究表明螺旋椭形管管内流体对流换热性能强于普通圆形换热管，尽管流体阻力有所增加，但其综合性能依然更优。螺旋椭形管管束空间也形成了类似管内的螺旋形流道，其换热器壳体由于管束的自支撑结构不设折流板，流体在管束空间内呈纵向流动。针对目前相关研究的缺乏，本文通过实验与模拟的方法，探讨螺旋椭形管管束结构对于管外液相/气液两相的对流换热性能的强化机理以及对流动特性的影响，主要内容及结论如下：1.从三场协同的角度对于螺旋椭形管管束空间内流体流动及换热特性进行分析，得到螺旋椭形管管束结构有利于增强速度矢量与温度梯度、压力梯度的场协同程度，即在较小的压降下，尽可能提高管外流体的对流换热系数。并且管型尺寸如长短轴比值以及扭程长度的改变对于三场协同程度的影响有差异性。2.通过实验对比了弓形折流板油冷却器与螺旋椭形管油冷却器的管束空间内流体流动及换热特性，得到两种油冷却器管外流体对流换热系数，结果表明在相同的冷、热流体入口温度以及冷流体流量的条件下，弓形折流板油冷却器壳程流体的对流换热系数比螺旋椭形管油冷却器的高17%-26%，但在同样的实验条件下，螺旋椭形管油冷却器壳程具有更好的流阻性能，其压降仅为弓形折流板油冷却器壳程压降的40%左右。从综合性能来看，螺旋椭形管油冷却器是弓形折流板油冷却器的1.87-2.15倍，并且管外流体流量越低，螺旋椭形管油冷却器的换热流阻性能比越好。最后通过热力学性能评价指标指出了两台油冷却器的优化方向。3.根据对螺旋椭形管管外液相流动及换热特性的数值模拟，比较了在相同的流速范围内，分别改变换热管长短轴比值以及扭程长度对于流体换热与流动特性影响的大小，结果表明在相同的流速范围内，通过增大长短轴比值或者缩短扭程长度可以使得管外流体对流换热效果增强，但同时会增大流动阻力。并且上述两种参数的改变到了一定值时，对于换热性能的作用有限。分析管束空间内流体流动状态发现，管壁附近流体的流动在充分发展段后并非沿壁面均匀分布，而是趋向于管壁表面螺旋线最低点处，即管间间隔最大处。并绕着管壁呈螺旋式流动。螺旋椭形管的管壁能够改变管外流动边界层的形态，增加其在主流方向以外的扰动，从一定程度上增强了对流换热效果。因管壁形状变化产生的流体二次流集中分布在两管之间间距最大处。并且靠近管壁的地方二次流越明显。管束间的螺旋流道使流体在垂直于主流方向上产生了二次流，从而加强了流体径向混合，增加流体湍动程度，强化了对流换热。同时在远离壁面的主流区，是产生二次流最小的区域，此区域对流换热速率远大于近壁区，并且纵向流的流阻也大大的减小。4.通过对螺旋椭形管管束空间内气液两相流动特性的准三维研究，分析了不同直径的汽泡，以及螺旋椭形管螺旋表面对于气液两相流动特性的影响；对螺旋椭形管满液式蒸发器的池沸腾换热性能进行了实验研究，并将其应用于螺杆式冷水机组中，与普通满液式蒸发器在相同工况下进行测试对比。得到在相同的管内雷诺数范围内，螺旋椭形管满液式蒸发器的管外沸腾换热系数可以达到普通满液式蒸发器的1.27-1.31倍。在热负荷相同的条件下，螺旋椭形管满液式蒸发器的换热效果无疑更好。其总的换热系数为普通满液式蒸发器的1.15倍左右。5.运用数值模拟的方法，分析螺旋椭形管管壁外自由上升汽泡的运动特性以及对周围液相的影响。结果表明：（1）汽泡对液相的扰动作用随着上升速度的增快，而逐渐变强，使得液相的湍动程度不断增强，受影响的范围也逐渐变大。（2）汽泡在自由上升以及碰到管壁后，整个过程并非竖直向上运动，而是有横向位移，这是由于流场不均以及螺旋椭形管交变曲面的影响而造成的，并且管壁对于汽泡上升轨迹的影响更明显。（3）汽泡的横向位移使得上升过程中增加了与管壁的接触时间，破坏了管壁附近流动边界层，增强了近壁处流体的对流换热效果。（4）随着扭程的增大，管壁对于汽泡水平方向速度分量的改变越大，尽管增大了近壁面流体扰动，但另一方面汽泡与管壁的接触时间也变得越短。更多还原

【Abstract】 The twisted elliptical tube (TET) is acquired by flattening and twisting the ordinarycircular tube. According to the existing literatures, the TET has stronger convective heattransfer performance and overall performance than the ordinary circular tube while the TEThas larger flow resistance. The space of TET bundle can be considered to consist of severalspiral flow channels. And in the shell side of a TET heat transfer, the tubes get aself-supporting structure without the conventional segmental baffles. Thus the shell-side fluidflows spirally and longitudinally in the TET bundle space. For the lackness of relevantresearches, the present work analyzes the heat transfer and fluid flow characteristics of liquidphase/gas-liquid two-phase flow in the space of TET bundle based on the experimental andnumerical methods. The main contents and conclusions are as follows:1. The field synergy principle is used to analyze fluid heat transfer and flowperformances in the space of the TET bundle. It is found that the structure of TET bundleimproves the synergy degree between velocity vector and temperature gradient or pressuregradient. In other words, it can obtain a better heat transfer enhancement performance with abit of increase of pressure drop. Moreover, the major axis to minor axis ration of oval crosssection and the twisted pitch of the TET have different influences on improving the synergydegree between velocity vector and temperature gradient or pressure gradient, respectively.2. An experimental comparison is conducted to study the heat transfer and flowresistance performances of a conventional oil cooler with segmental baffles and a TET oilcooler, respectively. Results show that under the same test conditions, the shell-sideconvective heat transfer coefficient of conventional oil cooler is17%-26%higher than that ofTET oil cooler. However, the TET oil cooler has a40%less pressure drop in the shell sidethan that of conventional oil cooler. And the overall performance of TET oil cooler is muchbetter especially with a low inlet velocity. 3. Using a numerical simulation to change the major axis to minor axis ratios and twistedpitches, and analyze the impacts on the heat transfer and fluid flow characteristics within asame range of flow rates. Results show that the heat transfer can be enhanced by increasingthe major axis to minor axis ratio and decreasing the twisted pitch. But when the twoparameters are at certain values, their impacts on heat transfer will become unapparent. Thecontours of secondary flow of fluid outside the TET bundle provide the intensities of radialmixing of fluid. It is found that the secondary flow concentrate in the place where the intervalof two tubes is the largest. This is due to the spiral curved wall of the TET. The geometricconstruction of TET bundle induces the secondary flow of fluid in the space of TET bundle.And this effect leads to a radial mixing of fluid and breaks up the boundary layer. Meanwhilethe increasing of fluid flow is minor.4. A quasi3D video and a system test are conducted to study the heat transfer and fluidflow characteristics of the TET bundle in pool boiling. It is argued that the radial crosssections with different angles have different influences on bubbles of several diameters. ATET flooded evaporator is manufactured and installed to a water-cooled screw chiller. Theresults of system test show that the shell-side heat transfer coefficients of the TET floodedevaporator is1.27-1.31times that of a conventional flooded evaporator within a sametube-side Reynolds numbers. When the heat transfer capacities are same, the test results showthat the overall heat transfer coefficient of the TET flooded evaporator is about1.15times thatof the conventional flooded evaporator.5. A numerical simulation method is used to analyze the impacts of a rising bubble nearthe TET wall to the surrounding liquid phase. The results show that:(1) The rising of bubbleleads to a turbulence of liquid phase. And the influenced region will gradually become extent.(2) The motion path of a bubble is not going straight up either contacting the TET wall or not.Firstly it is due to the ununiformity of flow field around the TET bundle. And once the bubblecontact the spiral curved wall, its directions of body forces become varing.(3) The lateralmovement of bubble on the TET wall increases the time of contacting and destroys theboundary layer.(4) With the increase of twisting pitches, the horizontal velocity component of bubble rising become larger. Although this can increase the turbulent degree of near-wall fluid,the time of contact between bubble and the TET wall decreases.更多还原