Numerical Simulation On The Flow Of Helically Coiled Tube And Heat Transfer Characteristics In Nucleate Boiling

Due to the advantages of high heat transfer characteristics and compact structure,the helically coiled tube steam generator has always been used in the wide field of heat and mass transfer.The flow inside the helically coiled tube is affected by gravity,centrifugal force and Coriolis force under the influence of the special geometrical bending structure.Causing the secondary flow and vapor-liquid separation occur in the tube which is different from the straight tube.Under the boiling conditions,the increasing vapor volume fraction in the tube makes the flow field more complicated.Meanwhile the frictional pressure drop in the helically coiled tube is much larger than that of the straight tube,which often causes a large flow loss.Therefore,it is very necessary to study the flow and heat transfer characteristics in helically coiled tube.In this paper,a single helically coiled tube physical model is established according to the helically tube bundle based on the experimental study of SMART steam generator.The flow and heat transfer characteristics of the single-phase forced convection and the nucleate boiling in the tube are studied.The influence of geometric parameters and operating parameters on heat transfer,pressure drop and secondary flow is obtained by analyzing the axial and radial distributions of temperature,velocity and pressure.The pressure drop characteristic curve of vapor-liquid two-phase flow under non-phase-change condition is obtained by adiabatic simulation of vapor-liquid mixed flow.Meanwhile the influence of the adiabatic secondary flow phenomenon on the two-phase distribution is found,and the variation of vapor-liquid distribution and pressure field under different quality,curvature and torsion rate is obtained.The research shows that the secondary flow phenomenon in the helically coiled tube can be accurately captured by using the Euler two-fluid model and the RSM turbulence model.The non-equilibrium subcooled boiling model can be used to accurately simulate the flow and heat transfer characteristics in nucleate boiling.In the single-phase flow region,as the Reynolds number increases,the friction factor decreases and the Nusselt number increases.The velocity field and the temperature field exhibit a C-shaped distribution,and the dean vortex structure shifts clockwise along the main flow direction.The pressure drop characteristic curve of the adiabatic vapor-liquid mixed flow has a maximum point;the secondary flow weakens the degree of vapor-liquid separation;the increase of the curvature causes the pressure non-uniformity to decrease and the secondary flow intensity increases.The increase of the torsion increases the pressure non-uniformity and the secondary flow intensity.The increasing quality does not have a significant effect on the vortex structure,and the vapor-liquid separation is extended.In the nucleate boiling zone,the heat transfer coefficient increases rapidly with the increase of quality and then decreases slowly.The maximum wall temperature is offset counterclockwise from 90° to 180°.The distribution of temperature field and velocity field is obviously different.The velocity field is shifted to the upper outside and the temperature field is shifted to the upper inside.In the beginning of the boiling flow,the secondary flow is an inner-outer symmetric vortex structure.As the quality increases,the vortex structure first shifts counterclockwise but the upper vortex structure is not completely formed,and then the vortex structure is affected by the tube structure to shift clockwise.The increase of the torsion,the heat flux,the inlet Reynolds number and the operating pressure will increase the surface heat transfer coefficient.The increase of the curvature will reduce the flow velocity and the surface heat transfer coefficient.The increase of the curvature,the inlet Reynolds number and the decrease of the operating pressure make the frictional pressure drop and the total pressure drop increase significantly.The increase of the torsion weakens the uniform of the circumferential wall temperature distribution,and the increase of the curvature has the opposite effect.The operating pressure reduces the vapor-liquid separation effect.