Research On The Gas-liquid Flow Regimes And Transition Mechanisms In Helically Coiled Tubes

Flow regime is fundamental to the investigation on the gas-liquid two-phase flow.Most studies of flow regimes focus on straight channels,but helically coiled tubes receive rare attention.In view of centrifugal force,two-phase flow in helically coiled tubes becomes more complicated.Meanwhile,flow regimes and their transitions demonstrate specific characteristics.For above reasons,the experimental and theoretical investigations on the gas-liquid two-phase flow regimes and their transitions for helically coiled tubes have been carried out.Firstly,the experiments of flow regimes were performed in an air-water facilty,with three sets of the helically coiled tubes with coil diameter of 0.2,0.4 and 0.8 m.The conductivity probe,as the main measurement,was used to measure bubble chord length,liquid slug length and void fraction.High-speed photography provided a record of flow structure in the process of flow regime transitions.Experimental results identified a total of six flow regimes,viz.bubbly,plug,slug,wavy,annular and wispy annular flow.Each regime under the centrifugal force demonstrated specific features distinguished from straight tubes.For example,bubbles in the bubbly flow were densely distributed at the inner and upper part of the tubes.The liquid film in the annular flow showed highly asymmetrical characteristics,and the phenomenon of film inversion was found.To avoid subjectivity and improve efficiency,a series of quantitative criteria for flow regime transitions were set up on the basis of bubble chord length,liquid slug length and void fraction.Thus,the boundaries of flow regimes were obtained by the conductivity probe,and the gas-liquid flow regime maps for helically coiled tubes were put forward.Compared with the inclined tubes,the region of bubbly flow remarkably shrinks,but the region of wavy flow increases.Furthermore,as the coil diameter reduces,the transition from bubbly to plug flow is advanced,and wispy annular flow appears at a lower gas superficial velocity.Finanlly,theoretical research was developed for flow regime transitions,including bubbly to plug flow,slug to wavy flow,slug to wispy annular flow,wispy annular to annular flow and wavy to annular flow.Physical models of the flow regime transitions were established,and transition boundaries were theoretically derived by combining the physical models and experimental data.Thus,unified prediction models incorporating helically coiled tubes,horizontal tubes and inclined tubes were developed and gave a good agreement with the experimental data.Furthermore,the influence of geometrical parameters(coil diameter,tube diameter and inclined angle)and fluid properties(gas density,liquid density and surface tension)on flow regime transitions has been discussed.The research on the flow regimes for helically coiled tubes creates the foundation for subsequent studies on thermal-hydraulic performance,and provides the technical support for associated equipments in design and operation.