Influence Of Interphase Force Model And Turning-bend On Heat Transfer Of Subcooled Boiling

The heat exchanger is the key equipment of cogeneration,refrigeration and heat pump systems.When the unit tube of heat exchange contains U-shaped bends and other curved parts,under the restriction of local curvature,there is a secondary flow effect that gradually decays along the heated tube.As a result,the existing straight tube and continuous bend tube models are unable to predict this tube heat transfer well,with an obvious deviation.The accurate prediction and design of boiling heat transfer in the tube depends to a large extent on the understanding of the bubble behavior.The flow process of subcooled boiling is complicated,involving the generation,growth,departure and coalescence of bubbles.The flow boiling that occurs in the U-shaped bends is also affected by the secondary flow effect of the bend.Therefore,it is necessary to explore the influence of the turning-bend on the two-phase heat transfer characteristics in depth.In this paper,water was used as the working fluid,the occurrence of fully developed boiling incipent point can be predicted objectively by numerical simulation,and the prediction error is controlled within 5%.Once the grid independence is achieved,the inflection point of boundary void fraction coincides with the onset point of fully developed boiling.The wall temperature varies greatly with the different wall lubrication force models.The wall temperature difference caused by other interphase force models can be ignored.Therefore,it may be inappropriate to determine the onset point of fully developed boiling based on the wall temperature.In the study of the effect of interphase force on boiling process,the effect of lift on the axial distribution of average void fraction is less than 0.05,but it has a significant effect on the radial distribution of void fraction and the determination of the onset point of fully developed boiling.The maximum error of turbulent dispersion force on the axial distribution of average void fraction is 0.2.These forces have obvious influence on the determination of the onset point of fully developed boiling;It is suggested that the most feasible combination of interphase force models,ie.Burns-et al model of turbulent dispersion force,Tomiyama model of wall lubrication force and Moraga model of lift force,be used to determine the onset point of fully developed boiling in the flow.The U-bend have a significant effect on distribution of average void fraction.The Ushaped with smaller curvature has less effect on the onset of fully developed boiling.The mean void fraction of the pipe presents the result of the combined effect of the secondary flow effect of the bend and the increase of the mean void fraction of the cross-section caused by the onset of fully developed boiling point.The critical heat flux of departure of nucleate boiling is effectively predicted by numerical simulation,and the prediction error is less than 16%.The interphase force model of departure of nucleate boiling is obviously different from that of nucleate boiling.When the tube is under process of departure of nucleate boiling,the wall temperature rises significantly in the upward flow,and there is no obvious change in the wall temperature of the downward flow.The critical heat flux of downward flow was in the combined effect of heat flux,mass flow rate,and pressure.In the bend pipe with the same heating length as the straight pipe,due to the change of the flow direction and the influence of the secondary flow in the downstream of the bend,the phenomenon of the wall temperature rising is weakened.When the bend is near the deterioration position,the deterioration is aggravated,and the critical heat flux in the bend is lower than that of the straight pipe.