| Gas (vapor) liquid two phase flow widely exist in many fields of nature and industry, which is consistently associated with safety of people’s life and manufacture. Actual results of fluid flow and heat transfer are effected by the inherent physicochemical properties of fluid and flow field environment. The former is often known or measurable but the flow field environment usually change initiatively or passively. Commonly, it is difficult to control and predict the passive change of flow field environment. Resulting in the real value of fluid flow and heat transfer different from the prediction. Due to the ever-present flow field environment influence and even determine the internal fluid flow and heat transfer characteristics, so its research importance is self-evidentIn this paper, on the basis of considering the influences of different flow field, utilize the advantages of numerical methods. Using the computing platform Fluent, which is based on the theory of computational fluid dyanmics (CFD). Choose VOF multiphase flow model and RNG k-ε turbulent flow model to calculate the behavior of phase interface between Gas (vapor) and liquid. User defined function (UDF) is used to add source item of vapor and liquid to simulate the heat and mass transfer between vapor and liquid at phase interface during heat transfer. Second order upwind format is used for difference scheme, PRESTO format is used for pressure discrimination, and PISO format is used for pressure-velocity coupling.To verify the Accuracy of numerical model and user defined function, multiple approaches for verification are used in this paper. Experimental measured datas, empirical remedy equation and theoretical derivation formula are choosed as validation criteria. Comparison between numerical results and existing data results obtained in same flow field shows a good agreement. Indicating the numerical model can accurately simulate fluid flow and heat transfer under certain flow field environment. According to different case, use air-water, vapor-water and R134a vapor-liquid as working material. Influences of gravity flow field, pressure flow field and heat flow field on characteristics of gas (vapor) liquid two phase flow and heat transfer are systematically and comprehensively analized.1. The gravity flow field. Numerical model of gas liquid two phase flow in tube under reduced gravity flow field is built. Influence of reduced gravity on kinetic characteristic of gas (vapor) liquid two phase flow is studied. Due to raised influence of inertia force, viscous force and surface tension under reduced gravity, these three forces come to the major factor for formation and transform of flow pattern. This result in the shape of flow pattern under reduced gravity is different from normal gravity. Similar with normal gravity flow field, raise of gas (vapor) and liquid flow velocity can increase pressure drop, however, the smaller the gravity flow field the more intense this phenomenon becomes. Prediction model for two phase flow pressure drop for normal gravity such as homogeneous model, Friedel model and Chisholm model can not be used for predicting pressure drop under reduced gravity. Zoning flow according to liquid Re number again, modified Chisholm relation can accurately predict pressure drop for reduced gravity. For churn flow which contain wave of phase interface, its wave amplitude increase with reduce of gravity, also comes with enlonged wave period and improved stability of phase interface.2. The pressure flow field. According to characteristics of high pressure environment, numerical model for vapor liquid two phase flow in vertical tube under high pressure flow field is built. Flow pattern map for high pressure is different form Flow pattern map by Hewitt and Roberts. There is no mist flow under high pressure, area of slug flow zone is compressed, and area of bubble flow is enlarged which almost takes up the area of bubble flow, slug flow and partial area of mist flow for normal gravity. Research on interfacial wave shows that, when the pressure of flow field becomes higher, the amplitude of interfacial wave becomes larger, wave period prolonged and instability of phase interface improved. For churn flow, pressure of flow field has little influence on distribution of velocity profile in centrel of tube. Comparison with normal gravity shows that increase of flow field pressure can weaken randomness and disorder degree of velocity profile in local close to tube wall.Coordinate is established with axis of dimensional vapor and liquid flow velocity. For countercurrent flow between vapor and liquid in vertical tube under high pressure, distribution of flooding points is quadratic and independent from flow filed pressure. By the least square method, predict model for flooding points is given. Complete carry up, flow reverse and deflooding with hysteresis are linear distribution. On the basis of normal pressure flow field, influence of flow field pressure on vapor liquid countercurrent flow is discussed.3. Thermal flow field. By adding vapor, liquid mass and energy source item, numerical model for vapor liquid two phase flow and heat transfer in vertical tube for high pressure is established. When bared heated surface rewetted by falling liquid film, wetting front is formed from the contact angle at the beginning of falling film. Wetting front will separate from heated surface and no longer take part in heat transfer when superheat of hot wall exceeds a certain value. For research on movement characteristics of wetting fronts, critical separated length and its appeared location, critical separated hight and critical duration for sepearation are defined in this paper. Set heat flux of hot wall and liquid film Re number as Variables to investigate the distribution of the characteristic parameter defined above.For flow and heat transfer characteristics of falling film during flow, time and location where nucleate boiling happened, process of saturated bubbles formed during initial nucleate boiling and its influence on surface wave of liquid film, and heat transfer state of falling film are discussed. Variation trend of surface heat transfer coefficient for falling film under different cases are same, that is gradually decrease from the initial value until reach to a certain value. According to calculation range in this paper, surface heat transfer coefficients are same for same inlet mass flux but independent to wall heat flux. For constant inlet liquid mass flux, the higher the wall heat flux, the greater the gradient of initial surface heat transfer coefficient, the smaller the final stable value. However, in this process, time costs for development are same for same inlet mass flux. |
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