Numerical Simulation And Theoretical Study On Boiling Heat Transfer Of The Kettle-type Reboiler Shell-side

The heat exchanger and its related technologies are very important for the national energy of the industry.The heat transfer performance of the heat exchanger directly affects the energy utilization.As an energy transfer equipment,reboiler is widely used in chemical,petroleum,energy and other industrial fields.The main principle for the reboiler is boiling heat transfer.Boiling heat transfer is a kind of convection heat transfer which is brought up by the generation of bubbles.Compared to heat transfer with no phase-change,the heat transfer efficiency of boiling heat is more efficient.Therefore,the research on boiling heat transfer has become a wider trend.But the studies on shell-side boiling heat are slim and mostly concentrated in the single-phase flow research.The reason is that immature multiphase flow theory and relatively complex heat exchange which make it difficult to conduct simulation studies of the reboilers.And especially when coming down to the phase transformation,the multiphase flow simulation is becoming more difficult.The history,development and mechanism of boiling heat transfer are introduced.At the same time,the gas-liquid two-phase flow pattern and numerical simulation model are discussed.Wall Boiling Model of Eulerian model and the RPI model in Fluent is described in detail,which provide the necessary theoretical basis for the following numerical simulation.Boiling heat transfer of the fluid around a single tube in shell-side is simulated using the Wall Boiling Model of Euler Model in Fluent.The vapor volume fraction along the axial direction is analyzed as well as the changing of heat transfer coefficient with different heat flux,different inlet velocity and temperature and different surface structure.Finally,the two and three tubes shell-side models are established to simulate for the changing of heat transfer coefficient and vapor volume fraction with different tubes.The fluid model of the kettle-type reboiler shell-side is established.Using the Realizable k-ε turbulence model and the multiphase flow model,the scattergrams of vapor volume fraction,temperature and density are obtained.Based on these results,the characteristics of convective heat transfer in the shell-side are described.The results show that with the continuous involvement of heat,vapor flow direction is vertical upward,and finally left the vaporization space.At the same time,the convective heat transfer coefficient of the reboiler shell-side is obtained.The reboiler is simulated by EDR program of Aspen Plus software,and the result of the Fluent model is verified.The general boiling heat transfer relationship is formulated,and the boiling heat transfer coefficient of shell reboiler is theoretically calculated.The results show that the Modified Mostinski formula is the most accurate.It provides a theoretical basis for the calculation of shell-side boiling heat transfer coefficient of reboiler.