Theoretical Analysis And Experimental Study Of Shell-and-Tube Heat Exchanger With Continuous Helical Baffles And Overlapped Helical Baffles

The heat exchanger is the universal process equipment which is applied in various industrial sectors, such as chemical industry, petroleum, light industry, metallurgy, energy and power, etc. Shell-and-tube heat exchanger (STHX) has been the most comprehensive heat exchanger due to its simple construction, low manufacturing cost, high reliability, and excellent adaptability, etc. In view of the important role in industrial areas of STHX, to increase its efficiency by improving traditional shell-side structures becomes a crucial way for energy saving and emission reduction. On the basis of this way, shell-and-tube heat exchanger with helical baffles (STHXHB) is invented as novel energy efficient heat transfer equipment.This paper adopts theoretical analysis, numerical simulation and experimental study to research structure design and performance of STHX with continuous helical baffles and STHX with overlapped helical baffles. Researches and main results are as follows:The structural characteristics of STHX with continuous helical baffles and STHX with overlapped helical baffles are analyzed. The relationship among helix angle, overlap size and helix pitch is derived, and also the calculation method of geometric parameters of baffles is obtained. The mathematic model of shell-side laminar continuous helical flow is established based on certain simplifications and assumptions. The characteristics of velocity components of axial, tangential and radial are analyzed. The study of forces between fluid and baffles is made. Based on above researches, the model of shell-side actual flow is established. The shell-side actual helical flow can be divided into several independent flow channels, such as helical mainstream flow channel, leakage flow channel and bypass flow channel. The formation causes, characteristics and impacts on heat exchanger of each flow channel are studied respectively.The physical models of computation domain of STHX with continuous helical baffles and STHX with overlapped helical baffles are established based on certain reasonable simplifications, and boundary conditions are properly defined. RNG k-ε turbulence model is applied to numerical simulate shell-side and tube-side flow and heat transfer of heat exchanger. New calculation method of shell-side Reynolds number is put forward which can better reflect the state of shell-side flow fluid.The effects of helix angle on distributions of physical quantities and heat transfer and resistance characteristics are similar in STHX with continuous helical baffles and in STHX with overlapped helical baffles.Shell-side helical channel consists of inlet region, helical flow region and outlet region, and the variations of each region vary with helix angle. Axial velocity decreases along radial direction, and the larger helix angle is, the more uniform distribution is. Both shell-side heat transfer coefficient and pressure drop decrease with the increase of helix angle, whereas heat transfer coefficient per unit pressure drop increases with the increase of helix angle at certain mass flow rate. Entropy generation number decreases with the increase of helix angle, and the larger helix angle is, the lower optimum operation shell-side Reynolds number is.Overlap size is the peculiar structural parameter of overlapped helical baffles. When baffles are connected in an overlapped manner, the variation trends of axial velocity are different before and after the overlap point, and it is parabolic distribution which first decreases and then increases. Both shell-side heat transfer coefficient and pressure drop increase with the increase of overlap size, whereas heat transfer coefficient per unit pressure drop decreases with the increase of overlap size at certain mass flow rate. Both entropy generation number and optimum operation shell-side Reynolds number increase with the increase of overlap size.Comparing the simulation results between STHX with continuous helical baffles and STHX with overlapped helical baffles, it is found that the leakages from triangle zone and overlapped zone between two adjacent baffles cause shell-side flow deviates from continuous helical flow, which increases flow back mixing along radial direction, and exacerbates the non-uniform of radial distribution of axial velocity. There exists transition shell-side Reynolds number which is related with helix angle, and the relative size of irreversible loss in STHX with continuous helical baffles and in STHX with overlapped helical baffles is different before and after the transition shell-side Reynolds number.A novel STHX with overlapped helical baffles is proposed based on thorough researches on STHX with quarter sector helical baffles, which consists of six sixth sector helical baffles one helix pitch. Simulation results show that the axial peripheral overlapped structure between two adjacent baffles can effectively reduce the leakage from triangle zone, which makes radial distribution of axial velocity more uniform. Shell-side heat transfer coefficient is higher, shell-side pressure drop is lower and entropy generation number is lower, so irreversible loss is also lower in STHX with sixth sector helical baffles compared with in STHX with quarter sector helical baffles. The novel overlapped structure makes both the synergy between velocity field and pressure gradient field and the synergy between velocity field and temperature gradient field better in shell-side from point of field synergy principle analysis.Shell-side Nusselt number correlations and shell-side resistance factor correlations of STHX with continuous helical baffles, STHX with quarter sector helical baffles and STHX with sixth sector helical baffles are fitted, which can provide references for the heat exchanger design calculations.STHX with sixth sector helical baffles is experimental studied by setting up experimental apparatus. Results show that STHX with sixth sector helical baffles has better shell-side comprehensive performance than STHX with segmental baffles. The reliability of numerical method is verified by comparing experimental values and numerical values.