Numerical study on subcooled flow boiling in a serpentine tube using VOF multiphase model

Abstract: The importance of surfare condition on nucleate boiling has long been recognized. It has also been known that only cavities of a narrow size range can be active nucleation sites. In order to define the size range of active cavities as a function of wall temperature or heat flux, a model is proposed. The model pictures a bubble nucleus at a site enveloped by a warm liquid. The nucleus will begin to grow into a bubble only when the surrounding liquid is sufficiently superheated. The time required for the liquid to attain this superheat is called the waiting period. The transfer of heat from the superheated liquid into the bubble is considered to be a transient conduction process. A cavity is considered effective only if the waiting period is finite. This criterion gives the limiting sizes of effective cavities. The equations show that maximum and minimum sizes of effective cavities are functions of subcooling, pressure of the system, physical properties, and the thickness of the superheated liquid layer. Comparison of theoretical prediction with experimental data from several sources was made. The fluids considered were ether, pentane, and water, with water under various degrees of subcooling. The theory did predict the incipience of boiling and size range of cavities successfully.

Abstract: The partitioning of the heat flux supplied at the wall is one of the key issues that needs to be resolved if one is to model subcooled flow boiling accurately. The first step in studying wall heat flux partitioning is to account for the various heat transfer mechanisms involved and to know the location at which the onset of nucleate boiling (ONB) occurs. Active nucleation site density data is required to account for the energy carried away by the bubbles departing from the wall. Subcooled flow boiling experiments were conducted using a flat plate copper surface and a nine-rod (zircalloy-4) bundle. The location of ONB during the experiments was determined from visual observations as well as from the thermocouple output. From the data obtained it is found that the heat flux and wall superheat required for inception are dependent on flow rate, liquid subcooling, and contact angle. The existing correlations for ONB underpredict the wall superheat at ONB in most cases. A correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature. Experimental data are within630 percent of that predicted from the correlation. Active nucleation site density was determined by manually counting the individual sites in pictures obtained using a CCD camera. Correlations for nucleation site density, which are independent of flow rate and liquid subcooling, but dependent on contact angle have been developed for two ranges of wall superheat—one below 15°C and another above 15°C. @DOI: 10.1115/1.1471522#

Abstract: A numerical simulation, using the VOF multiphase flow model, and the corresponding experiments were conducted to investigate the boiling flow of R141B in a horizontal coiled tube. The numerical predictions of phase evolution were in a good agreement with the experimental observations, and the two phase flow in the tube bends was much more complicated due to the influence of liquid–vapor interaction with the interface evolution. The associated heat transfer was also considered. It was found that the temperature profile in the two phase flow was significantly affected by the phase distribution and higher temperature always appears in the vapor region.

Abstract: A model has been developed allowing to simulate the flow boiling process of a hydrocarbon feedstock. For the calculation of the different horizontal two-phase flow regimes that evolve during flow boiling, use is made of the Volume Of Fluid (VOF) model that uses a Piecewise Linear Interface Calculation (PLIC) method to reconstruct the interface between both phases in each computational cell. In-house developed codes calculate the mass and energy transfer phenomena occurring during this flow boiling process. As such, an existing CFD code is completed with a newly developed complete evaporation model. The developed model is used to simulate the flow boiling process of a hydrocarbon feedstock in the tubes of a convection section heat exchanger of a steam cracker. The simulation results show a succession of horizontal two-phase flow regimes in agreement with the literature.

Abstract: Numerical simulations were conducted to investigate the refrigerant flow boiling in a horizontal serpentine round tube with the Eulerian multiphase flow model and a phase-change model for the mass transfer. Correspondingly, an experimental investigation was conducted to provide validation and data for the simulations. The liquid/vapor phase distributions show stratification in horizontal tubes, indicating the buoyancy force caused by gravity acceleration is dominant, especially when the vapor void fraction is sufficiently high. The adiabatic bend sections served to redistribute the vapor phase, which was induced by the centrifugal force and re-condensation of the vapor (due to thermal non-equilibrium of two phases). The phase distributions in the bend sections showed the competitive influence of buoyancy force and centrifugal force at different operating conditions. In all cases, the numerical simulations appear reasonably consistent with the experimental observations. In particular, the simulation very well explains the bend effects on flow reconstruction and thermal non-equilibrium release observed in the experiments.