Abstract—Multi-modal film boiling simulations are carried out on adaptive octree grids. The liquid-vapor interface is captured using the volume-of-fluid framework adjusted to account for exchanges of mass, momentum, and energy across the interface. Surface tension effects are included using a volumetric source term in the momentum equations. The phase change calculations are conducted based on the exact location and orientation of the interface; however, the source terms are calculated using the mixture variables to be consistent with the one field formulation used to represent the entire fluid domain. The numerical model on octree representation of the computational grid is first verified using test cases including advection tests in severely deforming velocity fields, gravity-based instabilities and bubble growth in uniformly superheated liquid under zero gravity. The model is then used to simulate both single and multi-modal film boiling simulations. The octree grid is dynamically adapted in order to maintain the highest grid resolution on the instability fronts using markers of interface location, volume fraction, and thermal gradients. The method thus provides an efficient platform to simulate fluid instabilities with or without phase change in the presence of body forces like gravity or shear layer instabilities. Keywords—Boiling flows, dynamic octree grids, heat transfer, interface capturing, phase change. NOMENCLATURE Letters A Amplitude (m) f A Area side fraction         cell inter V A Area of the interface to cell volume ratio (1/m) c Mixture (mass-averaged) constant pressure specific heat (J/kg/K) g Gravitational acceleration (m/s 2 ) h Enthalpy (J/ kg mol) h fg Enthalpy of phase change (J/ kg mol) k Thermal conductivity (W/m 2 K) l s Length scale L Latent heat of vaporization (J/ kg) m    Mass flux per unit area across the interface (kg/s/m 2 ) N Number of Fourier terms n Unit normal vector to interface x n Number of modes of the perturbation P Volume averaged mixture pressure (N/ m 2 ) q  Heat flux vector (W/m 2 ) r Random number energy S Energy source term associated with phase change (W/m 3 ) pc mom S , Momentum source term associated with phase change (N/m 3 ) M. W. Akhtar was with the University of Houston, Houston, TX 77204 USA (e-mail: mwakhtar3@uh.edu). st mom S , Momentum source term associated with surface tension (N/m 3 ) T Mass averaged mixture temperature (K) ∆T Superheat (K) t Time (s) t s time scale u Mixture fluid velocity (m/s) u s velocity scale W Domain width (m) y Interface height z Interface height Greek Letters α Volume fraction ε Perturbation constant λ Wavelength (m) μ Dynamic viscosity ν Kinematic viscosity ρ Volume averaged mixture density (kg/m 3 ) σ Surface tension (N/m)  Volume averaged mixture shear stress (N/m 2 ) Subscripts c Critical value cell Value of the computational cell d2 Value for the two-dimensional problem d3 Value for the three-dimensional problem f Value at cell face l Value for the liquid phase mom Value associated with momentum pc Value associated with phase change sat Value for saturation condition st Value associated with surface tension v Value for vapor phase w Value at the wall I. INTRODUCTION OMPUTATION of boiling flows is one of the most challenging problems in computational fluid dynamics not only because two distinct phases must be modeled with correct accounting of surface tension and interfacial discontinuities, but a wide range of scales must be resolved both spatially and temporally. The problem is further complicated by the mass exchange between the phases and the need to solve at least one additional equation in order to capture/track the liquid-vapor interface. Also, the interface temperature condition can be a function of the heat flux at the interface, which requires additional modeling effort to set the right temperature. Given the spatial resolution and grid quality required to simulate such problems, a uniform grid is highly desirable and almost required in order to maintain the accuracy of the Multi-Modal Film Boiling Simulations on Adaptive Octree Grids M. Wasy Akhtar C World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:12, No:6, 2018 642 International Scholarly and Scientific Research & Innovation 12(6) 2018 scholar.waset.org/1307-6892/10009159 International Science Index, Mechanical and Mechatronics Engineering Vol:12, No:6, 2018 waset.org/Publication/10009159