Design And Analysis Of Enhanced Heat Transfer Structure For Fluid Flow Modulation

As climate change coming with global warming, the attitude to energy exploration has been greatly changed. People have to consider the effect of energy using on environment whenever they decide to get the resource or use energy. Thus, new techniques have to be developed to meet this demand. In this paper, the authoer provide a new idea to develop a new generation of heat transfer enhancement technique. The idea focuses on the temperature field and velocity field modulation for single phase flow instead of on boundary layer that traditional techniques foucs. It also provides a new way to change the flow pattern for two-phase flow. Compared with the traditional enhancement technique, this technique can achieve a high heat transfer performance and a limited flow resistance increasing. Main research results are as follows:(1) Manufacture process of porous structure and microstructure characterizationTwo types of porous material were used to two-phase pattern modulation. One is porous metal and the other is porous wire-net. The metal porous structure was made by solid state sintering. The quality of metal porous structure was associated with the mould, powder characteristic, powder filling quantity, mould placement and sintering process. In this work, a new type of mould made ceramics material was used. Excellent quality was achieved by using this sort of mould. It provides a new method to sintering process. The metal porous structure made of branch shape powder with a diameter of 100-125μm showed a best combination property of permeability and capillary. The manufacture method of the wire-net cylinder was recommended using resistance welding.(2) Design and testing on pipe insertsThe pipe inserts has a great heat transfer performance, the highest Nu number was 2.7 times that of smooth tube, the PEC value was 2.23–2.7。The geometry of the small pipe inserts makes it possible for water to flow easily through the inside and outside of the pipe. This leads to a mixing of water with different temperatures and velocities. Mixing increases the temperature gradient of the thermal boundary layer and causes uniformity in fluid temperature. Therefore, a high heat transfer performance was achieved.(3)Analysis on the two-phase flow movement with porous issue insertsA visual experimental setup was used to survey the movement of the air bubble going through the porous wall. The result showed that the porous structure can separate the air and water, which forces the air go through the annular region and keep the bubble close to the tube wall all the time. However, the water can flow into the center region through the porous wall. Moreover, an equation was developed to judge whether the bubble can go through the porous wall. The results showed that it associated with the bore diameter, flow velocity and material wettability.(4)Testing on the heat transfer performance of the porous material in the two-phase flowTesting has been performed on the condensation of the porous issue. The result showed that the sample made branch shape powder with a diameter of 100-125μm has the best heat transfer performance. The heat transfer coefficient was 4.48 times that of smooth tube. The heat transfer coefficient increases first with the inlet vapor quantity increase, then decreases with the increase in inlet vapor quantity. It is largely because the flow pattern has been changing. When the flow pattern was bubble flow, slug flow or plug flow, the porous structure made it into the annular flow. However, when the flow pattern is annular flow, the efficiency begins to go bad.