Numerical Investigation On Bubbly Flow In Flow Boiling Of Liquid Nitrogen In A Vertical Channel

As the latent heat of vaporization is small,nucleation of vapor bubbles is easily to occur on the tube surface due to heat influx from the environment.In a vertical tube,eventually these bubbles coalesce into a Taylor Bubble which fills the cross-section of the tube.As the Taylor bubble rises in the tube,it expels the liquid from the tube into the tank ahead of it and,as a result,geysering occurs.This phenomenon creates a potential destructive water hammer with surge pressure and may damage the transport tube and system equipment.Thus,it is crucial to investigate on the process of the formation of the Taylor bubbles.Before the existence of Taylor bubbles,the flow pattern is named as bubbly flow.In the zone of bubbly flow,both subcooled and saturated boiling flows exist in the channels.Therefore,the characteristics of bubbly flow in the flow boiling of liquid nitrogen should be attached great importance.In the last ten years,the two-fluid model was introduced into simulating boiling flow of liquid nitrogen in a vertical tube.The comparison of the numerical results against the experimental data shows that the two-fluid model for flow boiling of liquid nitrogen has an accuracy of±5%when predicting the wall temperature and the pressure drop.In the zone of bubbly flow,bubbles have a large variation in size.The two-fluid of CFX-4.3 becomes invalid since it is assumed that bubble diameter varies linearly depending on liquid temperature in a certain temperature range and outside this range the diameter becomes constant according to Anglart and Nylund.The present study focuses on bubbly flow boiling flow of liquid nitrogen（LN2）in a tube and its aim is to establish a mechanical model,which can make accurate predictions.In order to establish a mathematic model for bubbly flow in boiling flow of LN₂,the following studies are conducted and some achievements are obtained:Firstly,an average bubble number density（ABND）equation was formulated based on the population balance model.The known coalescence models and breakup models were summarized and suggested.Through the jobs,together with the two-fluid model for boiling flow of LN₂,an average bubble number density（ABND）model is finally proposed.Secondly,An average bubble number density（ABND）model was numerically resolved for the bubbly flow boiling of liquid nitrogen.The effects of bubble coalescence and breakup were taken into account.Some new closure correlations describing bubble nucleation and departure on the heating surface were selected as well.For the purpose of comparison,flow boiling of liquid nitrogen was also numerically simulated using a modified two-fluid model.The results show that the simulations performed by using the ABND model achieve encouraging improvement in accuracy in predicting heat flux and wall temperature of a vertical tube.Moreover,the influence of the bubble coalescence and breakup is shown to be great on predicting overall pressure beyond the transition point.Thirdly,the basic theory of the MUSIG model is introduced.MUSIG（Mutltiple-Size-Group）model supplies a frame to incorporate the population balance model and two-fluid model.This model is used to analyze flow boiling of nitrogen in a vertical annular channel.Finally,A MUSIG（Multi-Size-Group）model combined with a force balance model was formulated for the subcooled flow boiling of liquid nitrogen in a vertical annular pipe.For comparison,the MUSIG model combined with Kirichenko’s and Fritz’correlations were also tested.It is found that force balance model gives a better prediction of bubble lift-off diameter of nitrogen bubble.This model also accurately predicts the variation of bubble lift-off diameter along the flow direction.Meanwhile,numerical results show that modeling of the bubble lift-off diameter has significant effect on predicting evaporation heat transfer,void fraction and bubble size distribution.Therefore,MUSIG（Multi-Size-Group）model combined with a force balance model can help to improve the accuracy of predictions of flow parameters in the flow boiling of liquid nitrogen,accordingly help to accurately analyze the formation of slug bubble and geysering.Briefly,the present study deals with boiling flow of LN₂ in a vertical channel.Both theoretical、numerical and experimental investigations are conducted.A multi-dimensional two-fluid model for bubbly flow in boiling flow of LN₂ is proposed,which shows a good agreement with the experimental data.Meanwhile,a MUSIG model and a modified MUSIG which take account of forces acting on departure bubbles were suggested to bubbly flow in boiling flow of liquid nitrogen.The newly proposed models can extend the use of two-fluid flow.The present study not only contributes to the theoretical improvement of cryogenic two-phase flows,but also provides helpful information for designs.