Numerical Study On Heat Transfer Enhancement Of Helical Baffles Heat Exchanger And Optimization Of Baffles Structure

Shell and tube heat exchanger is widely used in many industrial processes. As a general equipment, it has great significance to study heat transfer enhancement of shell and tube heat exchanger, which could promote the maximum utilization of energy and resource, and achieve the purpose of saving energy. The helical baffles heat exchanger could effectively solve the defects of segmental baffles heat exchanger, such as stagnant zones of flow, higher flow resistance, propensity of inducing fouling and vibration et al., which has been widely concerned. In this paper, medium of shell side is water, which is liquid and has constant physical properties. Under different structures of tube-bundle supports and helical baffles, flow and heat transfer performance of heat exchangers are investigated through numerical research. Under background of heat transfer enhancement technology, mechanism of heat transfer enhancement of helical baffles heat exchanger is summarized. In order to enhance heat transfer of heat exchanger, a variety of helical baffles with optimizion structures are proposed. The main contents and results are as follows:(1) The numerical model of heat exchanger is determined through comparative analysis and experimental verification. In the study of heat transfer enhancement, turbulence model uses Realizable k-ε equation; coupling of pressure and velocity uses SIMPLE algorithm; discrete of momentum, energy, turbulent kinetic energy and turbulent dissipation rate use second order upwind; boundary conditions are velocity inlet, pressure outlet, constant wall temperature.(2) Pressure drop, heat transfer coefficient and comprehensive performance of heat exchangers under different tube-bundle supports are quantitatively compared. Heat transfer coefficient and comprehensive performance of supporting rods and supporting plates heat exchangers are both obviously smaller than that of segmental baffles and quarter helical baffles heat exchangers when Re of shell side is in the range of 2200 to 6200. Compared with segmental baffle heat exchanger, the flow of shell side is more uniform for quarter helical baffles heat exchanger, and shell side pressure loss in fully developed of flow is shown as linear loss. The distributions of velocity, temperature, pressure in shell side of segmental baffles heat exchanger in fully developed of flow present obvious discontinuity, and shell side pressure loss in fully developed of flow is shown as linear loss and partial loss.(3) Shell side performance of heat exchangers under different structures of helical baffles is compared and analyzed. Shell side heat transfer coefficient and comprehensive performance of trisection helical baffles heat exchanger are obviously lower than that of continuous helical baffles and quarter helical baffles heat exchangers. Compared with continuous helical baffles heat exchanger, quarter helical baffles heat exchanger shell side flow is more complex, the whole flow of shell side is helical flow, but the local flow of shell side is also doped with obvious sideling flow. The synergy of velocity field and temperature field of continuous helical baffles heat exchanger is better than that of quarter baffles heat exchanger and the performance is better than quarter helical baffles heat exchanger.(4) The influence of blocking plates and length of blocking plates on performance of quarter helical baffles heat exchanger is investigated. Heat transfer performance and comprehensive performance of shell side could be effectively increased after adding blocking plates. Heat transfer coefficient and pressure drop of shell side increase with the increase of blocking plates length, when helix angle is 15o and length of blocking plates is 7r/9, heat transfer coefficient is basically the same compared with length of blocking plates being r, and the comprehensive performance is best. When length of blocking plates is 7r/9, comparing to non-blocking plates heat exchanger, heat transfer coefficient, pressure drop and comprehensive performance increase by 8.82%~9.61%, 8.60%~9.02% and 3.44%~5.33%, respectively.(5) Overlap baffles structure could significantly improve the performance of helical baffles heat exchanger and comprehensive performance of heat exchanger is best when overlap size is 1/4.(6) Folded plate structure could significantly improve the performance of helical baffles heat exchanger. Comprehensive performance of heat exchanger is best when folding size is 1/5. Compared with general helical baffles heat exchanger, the heat transfer coefficient, pressure drop and comprehensive performance of 1/5 folded plate helical baffles heat exchanger could increase by 11.67%~15.78%, 29.57%~30.70% and 1.86%~5.45%, respectively.(7) Variable angles helical baffles structure is proposed. When helical angle increases to an extent, through reduction of the helical angle in the first helical cycle of shell inlet, heat transfer of heat exchanger could be effectively enhanced, and comprehensive performance of heat exchanger is increased.