Researches On Heat Transfer And Flow Characteristics Of Overlapped Helical Baffled Heat Exchangers

Heat exchanger is the heat-transfer equipment which is widely used in various kinds of industrial production process. At present, helical baffled heat exchanger is the high-technology product among heat exchangers. It possesses the advantages of enhancing heat transfer, reducing flow resistance and fouling, weakening vibration and prolonging the service life. It is particularly suitable for high viscosity fluid and become the essential equipment in the field of petroleum, chemical industry, nuclear engineering and so on. Because of the monopoly of the key technology by occident, our country should strengthen research and application of helical baffled heat exchangers urgently.Based on overlapped helical baffled heat exchangers (OHBHX) as the research object, this paper adopts theoretical analysis, numerical simulation and experimental study to research heat transfer and flow characteristics in shell side. Researches and main results are as follows:Due to the analysis of geometric structures of OHBHX, the general calculation expressions of main geometric parameters are obtained. Taguchi method was applied to investigate the influence of five geometric parameters on the comprehensive performance of OHBHX, which are helix angle, overlap size, diameter of tube, central distance of tube and tube layout, and an optimal parametric combination is got. It is found that, when the volume of shell side is determined, the key to improve comprehensive performance of OHBHX is supplying the larger flow space in order to reduce the shell-side flow resistance, and the triangle leakage zone formed by the overlapped baffles has significant effect on the comprehensive performance.Based on the theoretical study of the continuous helical laminar flow mathematical model, the characteristics of flow field and stress of helical baffled heat exchangers are analyzed in cylindrical coordinates. The conclusion is drawn that triangle leakage zone is the main structure which results in fluid back mixing and short flow in shell side.The shell-side actual flow of overlapped helical baffled heat exchanger is studied on the basis of stream analysis method. A new method of describing shell-side characteristic velocity is proposed. The superiority of reflecting the actual flow situation is validated, and it is important to improve the accuracy of the Reynolds number and analyze the flow pattern.The visual experiment of gas-liquid two-phase flow is constructed. High-speed video technology is applied to capture the flow patterns of OHBHX in multi angle and full direction. The bubble motion characteristics are analyzed based on the videos and confirmed the influence of triangle leakage zone on the shell-side helical flow and pressure drop. By tracking bubble trajectories, the helical flow stream, the triangle leakage flow stream, the helical bypass stream between shell inner wall and tube bundles and the leakage between shell inner wall and baffles are captured and described, which is verified some results conclude by the theoretical analysis. Gas-liquid flow regime of OHBHX is analyzed by the methods of gray histogram analysis, classical flow pattern map comparison, and graph of pressure fluctuations observation, and the flow pattern map of the OHBHX is given, which provides reference to study the shell-side flow pattern of OHBHX in the future.Numerical simulation is carried out to analyze heat transfer and flow characteristics of OHBHX with different helix angles. The results show that, with the increase of helix angle, the area of triangle leakage zone increases, and the amount of back mixing on the backwind of baffles decreases; the tangential velocity decreases, the axial velocity increases, and the helical flow feature weakens; the convective heat transfer coefficient and the shell-side pressure drop decrease, and the comprehensive performance improves.Numerical simulation is carried out to analyze heat transfer and flow characteristics of OHBHX with different helix angles. The results show that, with the increase of helix angle, the area of triangle leakage zone increases, and the amount of back mixing on the backwind of baffles decreases; the tangential velocity decreases, the axial velocity increases, and the helical flow feature weakens; the convective heat transfer coefficient and the shell-side pressure drop decrease, and the comprehensive performance improves.The characteristics of triangle leakage zone are high velocity, low flow resistance and heat transfer enhancement. A novel OHBHX with oval tubes is designed on the basis of the above three features. In the novel OHBHX, baffles are cut annularly, and the fluid flows through the triangle leakage zone in longitudinal flow, which reduces the shell-side pressure drop further. The round tubes in the triangle leakage zone are replaced by oval tubes, which can enhance heat transfer and guide the flow direction. It is proved that the comprehensive performance of the novel OHBHX is better than the traditional OHBHX based on the numerical simulation. By analyzing the influence of the major-and-minor-diameter ratio of oval tubes and incline anlges of tube layout, the optimal parametric combination is obtained. Compared with the tradional OHBHX, the JF factor of the novel optimal parametric combination increases by more than 14%, the minimum entropy generation decrease by more than 25%.