Study On Flow Pattern Evolution And Heat Transfer Characteristics Of Boiling Two-phase Flow In The Spiral Tube

The spiral tube is the main element of the heat exchanger and its special structure has great influence on the heat transfer characteristics and pressure drop in the tube.Under boiling condition,the evolution mechanism of two-phase flow pattern in spiral tube is complex,and the flow and heat transfer characteristics of different flow patterns are quite different.Based on this,numerical simulation was adopted in this paper to study the flow pattern evolution and heat transfer characteristics of boiling two-phase flow in spiral tubes.The main contents and conclusions are as follows:(1)A boiling two-phase flow model in a spiral tube was established.The Realizable k-ε turbulence model and VOF multiphase flow model were used to caculate the boiling process of water in a spiral tube.The main characteristics of each flow pattern in a spiral tube were analyzed.The results show that there are six conventional flow patterns: bubble flow,plug flow,slug flow,wavy flow,annular flow and mist flow,and there is also a sandwich flow,which is newly defined in this paper.(2)The effects of structural parameters and boundary conditions on the transition boundary of boiling two-phase flow are analyzed.The results show that the transition boundary of bubble-plug flow,plug-slug flow and slug-wavy flow can move backward with the increase of winding diameter.The spiral pitch mainly affects the uniformity of gas-liquid distribution.The increase of the spiral pitch leads to the advance of the bubble-plug flow transition boundary and the lag of wavy-mist flow transition boundary.With the increase of heat flux,the gas holdup of the section increases,and the conversion position of each flow pattern is advanced.At the same time,the intense gas-liquid mixing improves the uniformity of gas-liquid distribution.With the increase of tube diameter,the position of flow pattern transformation in spiral tube moves backward.With the increase of inlet velocity,the position of each flow pattern in the spiral tube will move backward,and the change of inlet velocity will significantly affect the fluctuation of phase interface in the horizontal direction and the distribution of gas and liquid in the vertical direction.(3)The transformation mechanism of bubble-plug flow,plug-slug flow,slug-wavy flow and annular flow were analyzed and discussed respectively.The results show that:The process of bubble-plug flow transformation and plug-slug flow transformation is mainly affected by bubbling polymerization and burst.Centrifugal force and buoyancy force promote bubbling polymerization,while turbulence force and boiling disturbance accelerate bubbling burst.Moreover,the effect of gas plug coiling should be considered in the process of plug-slug flow transformation.In the process of slug-wavy flow transformation,it is mainly affected by centrifugal force and buoyancy force,both of which can restrain the fluctuation,and the degree of influence depends on the relative magnitude of centrifugal force and buoyancy force.The annular flow film is the result of secondary flow and liquid surface tension.(4)The effects of different flow patterns on the heat transfer characteristics in the tube were compared and analyzed.The results show that under different flow patterns,the average surface heat transfer coefficient of cross sections follows such order:bubble flow > annular flow > slug flow > wispy flow > wavy flow.The bubble flow has the highest surface heat transfer coefficient between the wall surface and the fluid.The annular flow has the smallest circumferential wall temperature difference.