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

Under the effect of secondary flow and centrifugal forces,the characteristics of fluid flow and heat transfer in helically coiled tube is different from that in straight tube.Parameters of flow and heat transfer in helically coiled tube are non-uniformly distributed.The phenomenon above is more obvious when the flow is boiling flow and heat transfer.The change of characteristics of flow and heat transfer will affect the safe and reliability of helically coiled tube.So it is of great importance to study the non-uniform distribution of fluid flow and nucleate boiling heat transfer and its effect in helically coiled tube.Physical model of single helically coiled tube is established based on SMART steam generator.Nucleate boiling heat transfer in helically coiled tube is numerically analyzed to reveal the distribution of flow field and temperature in helically coiled tube.The effect of tube shape and operating condition on fluid flow and heat transfer is analyzed.The reason and influence factors of parameters non-uniform distribution is revealed.The two fluid control equations,combined with RSM turbulence model,Non-equilibrium Subcooled Boiling model and interphase mass,momentum,energy exchange model,is suitable for simulating the boiling heat transfer in the helical tube.The boiling heat transfer characteristics and the non-uniform wall temperature distribution are in excellent agreement with existing experimental data.Under simulated conditions,the heat transfer coefficient of the fluid inside the helically coiled tube increases sharply to the maximum value and then declines slightly due to the increase of the wall temperature.The heat transfer coefficient of single phase zone and boiling starting position is greatly affected by the fluid disturbance in the pipe and increases with the increase of the curvature and the inlet Re obviously.The heat transfer coefficient of nucleate boiling region is greatly affected by the nucleate boiling intensity and change with the heat flux distinctly.The pressure drop of the fluid includes three parts: the friction pressure drop,the acceleration pressure drop and the gravity pressure drop,in which the friction pressure drop and gravity pressure drop account for the main parts.And the proportion of the two increases with the development of boiling heat transfer.The pressure drop of the fluid is influenced greatly by the lift angle,pipe diameter and mass flow rate,while the curvature and the heat flux have little effect on it.Besides,the second flow velocity in the helically coiled tube is less than 0.1 times of the main velocity,and is affected by the curvature and the mass flow rate obviously.The second flow in liquid phase can carry the bubble and the vapor block from the region with high vapor to the he region with low vapor,it is contributed to uniform the vapor liquid distribution.The influence of operating condition and shape on wall temperature are discussed in detail.The results show that the liquid gathers at the outside and the bottom of the cross section during nucleate boiling heat transfer,resulting in a lower wall temperature in these region.However,the movement of the vapor phase to the top and the inside wall results in higher localized wall temperature.The maximum temperature difference throughout the cross section is approximately 10 K.Furthermore,the distribution of vapor volume fraction becomes uniform as the secondary flow enhances.Due to the vapor-liquid phase distribution,the wall temperature of the cross section shows a sharp rise before decreasing.Under different experimental conditions,the position of the maximum wall temperature changes from 90° to 180° and the maximum deviation is more than 60°.