Study On Fluid Flow And Heat Transfer Outside The Twisted Elliptical Tube Bundles

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.