Boiling flow simulations on adaptive octree grids

International Journal of Multiphase Flow, Volume 53, July 2013, Pages 88-99.
Mohammad W. Akhtar, Stanley J. Kleis

Department of Mechanical Engineering, University of Houston, Houston, TX 77204, United States.

 

Abstract

 

Boiling flow simulations are conducted on adaptive octree grids. A phase change model consistent with the mixture formulation, in conjunction with the Volume-of-Fluid (VOF) model, is used to track the liquid–vapor interface. Test cases including Rayleigh Taylor instability and bubble growth in a uniform superheat are conducted to validate the phase change model on adaptive grids. The validated model is then used to conduct film boiling simulations on both two-dimensional and three-dimensional adaptive grids. The average wall Nusselt number agrees well with the widely accepted correlations of Berenson (1961) and Klimenko (1981) and Klimenko and Shelepen (1982) for film boiling on a horizontal surface. For the test cases presented, the efficiency of the adaptive technique, as measured by the adaptive mesh refinement (AMR) efficiency, is mostly in the range of 50–80%. Although this efficiency is a function of the nature and dimensionality of the problem, this range of efficiency is comparable to those obtained in the simulations of primary jet atomization conducted by Fuster et al. (2009). This work opens the prospect of conducting more realistic (three-dimensional) multi-modal boiling flow simulations, and problems of similar complexity, in an efficient manner.

 

 

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Additional Information

An accurate and efficient technique that can simulate complex flows including boiling has been developed on adaptive octree grids.  An improved mass-conservative interface capturing technique with a sharp interface phase change model is implemented in the commercial code FluentTM with the help of user defined functions. A suite of test cases is used to validate the model, which is then used to conduct film boiling on both two dimensional and three dimensional octree grids. The adaptive mesh refinement (AMR) efficiency is seen to be in the range of 50-80%, which is comparable to simulations of primary jet atomization conducted by other researchers.

 

A test case to study film boiling with a bi-modal initial perturbation is shown in Figure (a). A heavier fluid of density 1.225 kg/m3 lies above a lighter fluid of density 0.256 kg/m3. The relevant properties/ parameters are listed in the Table below. Figure (b) shows the contours of mixture velocity (left half) and mixture temperature (right half) at 2.0 s. The interface location is shown by the solid black line. A linear temperature gradient is imposed as an initial condition with saturation temperature at the domain top boundary (625 K) and a superheated wall (640 K) at the bottom wall. The initial perturbed interface leads to a hydrostatic pressure difference between the crest and troughs of the interface. This pressure difference along with vapor generation at the interface (due to heat flux supplied by the superheated wall) leads to the formation of a mushroom shaped bubble accompanied by a thin vapor film sustained by vapor generation.

 

Figure legend:

 

Film boiling: (a) Contours of mixture temperature at 0 s (bi- modal perturbation), (b) Contours of mixture velocity (m/s, left half) and mixture temperature (K, right half) at  2.0 s. The fluid properties used are listed in Table below. Interface position is shown by solid black line.

 

Boiling flow simulations on adaptive octree grids

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