| Cryogenic gas-liquid two-phase flow is usually encountered in cryogenicengineering such as pipe transportation of cryogenic liquid and high heat-fluxelectronic chip cooling. Due to the inevitable heat leak along the pipe, smallbubbles form from the boiling cryogenic liquid and coalesce to bullet shapedlarge bubbles (also named Taylor bubble) with almost the same diameter ofthe pipe. These Taylor bubbles cause pipe line blockage and extrude thecryogenic liquid from the pipe line, and could lead to the destruction of pipeand valve, and also cavitation erosion of rocket engine. Thus the Taylorbubble formation and evolution in the pipe line are the importantcharacteristics in cryogenic engineering.Cryogenic gas-liquid slug flow has already formed under some thermalconditions when there is no geysering phenomenon in the tube. This is hiddendanger for the safty operation of crogenic tubes. Thus the prediction forTaylor bubble formation is very important for the safe operation of cryogenictransportation tube.The flow field around Taylor bubble, especially the flow field in bubblewake has great influence on the flow parameters of cryogenic two-phase slugflow, and then affects the interaction of two consectutive Taylor bubbles.Firstly, the flow pattern in the leading Taylor bubble’s wake determines theinteraction between two adjacent Taylor bubbles. Different flow patterns(laminar, transition or turbulent) result in different bubble coalescence speedand rate, and some will prohibit the coalescence. Then the flow field inbubble wake affects the length fraction of gas slug in a slug unit. Finally itinfluences the pressure drop and void fraction in cryogenic two-phase flow.And the turbulence intensity in Taylor bubble wake defines the velocity ofdispersed bubbles in a liquid slug, the velocity of liquid phase and the distribution of void fraction. Meanwhile, the velocity field ahead of Taylorbubble is highly important for Taylor bubble velocity. Thus the present studyis meaningful for studying on these important parameters in cryogenicengineering.In cryogenic tube, the coalescence of dispersed vapor bubbles leads tothe formation of Taylor bubbles. And the continuous coalescence of Taylorbubbles leads to the geysering phenomenon. In this continuous process, thereis a lack of the criterion of Taylor bubble formation in cryogenic tube causedby heat leak. And the research about this process has not included the effect offlow structure on the formation of Taylor bubbles. For the cryogenictwo-phase slug flow, most studies are limited to the macroscopic properties,e.g. the pressure measurement or photographic study. However, moredeveloped investigation on the flow field in cryogenic two-phase slug flow isscarce. And little literature can be found to define the wake pattern of a Taylorbubble in cryogenic fluid. The critierion for defining the wake pattern ofcryogenic Taylor bubble has not been developed. In this particular field, theresearch in cryogenic engineering has fallen behind that focusing on ordinaryfluid.The purpose of the present study is to experimently and numericallyinvestigate the formation, motion and coalescence process of Taylor bubblecaused by heak leak in cryogenic tubes. The formation position, length andvelocity distribution of Taylor bubble are studied by vasulization technique.The flow field around Taylor bubble was obtained by PIV technique. Basedon PIV results, snapshot POD was introduced to reveal the effect of flowstructure on the interaction of Taylor bubbles, and the effect of turbulence onthe formation of Taylor bubble. At last, the motion of a single Taylor bubblein stagnant liquid nitrogen and the flow structure aound it were numericallyinvestigated. The main conclusions are summarized as follows:(1) An experimental system was built to investage the characteristics andthe flow field of cryogenic gas-liquid slug flow. A correction algorithm wasdeveloped for reconstructing the flow field based on ray tracing method,which showed the maximum error was3%by the uniform grid validation. Acorrection program was built using Fortran language.(2) The characteristics of Taylor bubble in cryogenic slug flow were studied, including the length, velocity and length fraction. The coalescenceprocess of Taylor bubble was analysed under different inclination angles. Thevelocity ratio of trailing and leading bubble as a function of bubble distancewas built based on the experimental data. The existing correlations fromordinary two-phase flow are no longer suitable for liquid nitrogen, andoverestimate the present result. The present correlation agrees well with theexperimental data. Further more, the formation position, velocity and lengthdistribution in different tubes and inclination angles were obtained.(3) The flow structure around Taylor bubble was obtained by PIVtechnique based on the present experimental system. The traditional criterionraised from water based fluid experiments is no longer applicable for liquidnitrogen. According to the traditional criterion, the occurrence of turbulentwake appears when the Nfnumber is larger than1500. However, in this study,both the transitional and laminar flow patterns are observed with the Nfnumbers much larger than1500.(4) The PIV experimental results were further analyzed by POD method.The large-scale structure of Taylor bubble wake was calculated to explain thecoalescence between consecutive Taylor bubbles. The coalescence processcan be inhibited by changing the large-scale structure of Taylor bubble wake.(5) Based on the PIV and visualization experiment, the effect ofturbulence intensity, heat flux, tube diameter and inclination angle on theformation of Taylor bubble was investigated. A new dimensionless gasvelocity vqwas introduced. On this basis, a correlation for predicting theformation position of Taylor bubble caused by heat leakage in liquid nitrogentube was developed. Compared with the experimental results of present studyand reference, the relative error is less than±20%.(6) A detailed CFD research on the motion of a single Taylor bubble inliquid nitrogen was conducted using VOF method. The transition of flowpattern in Taylor bubble wake, bubble rising velocity and geometriccharacteristics of trailing edge were obtained. The comparison with theexsiting literature data showed that the present model is credible. Thetransiton of laminar, transitional and turbulent flow pattern was oobtained. Onthis basis, the transition criterion of the flow pattern in nitrogen Taylor bubblewake was built. The numerical result also shows the traditional criterion raised from water based fluid experiments is no longer applicable for liquidnitrogen. |
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