The present study of the effect of roughness density on the mean flow turbulence parameters is motivated by the need for new generation of boundary conditions for multiphase computational multiphase fluid dynamics (CMFD) models applied to boiling flows. Effect of roughness element density on the turbulent flow in a channel is quantified through direct numerical simulations (DNSs). The Navier-Stokes equations are solved using finite element method and bubbles are approximated as rigid near-hemispherical obstacles at the wall. Six different cases were analysed including channel flow with smooth wall and channel flow with rough wall for five different bubble nucleation site densities. Friction factor and the law of the wall was calculated and compared with the previously published results. Existing correlations for nucleating bubble site density dependency on a wall heat flux were used to obtain a relation between the heat flux and the friction factor, leading to the law of the wall dependency on the heat flux. This separate effect study provides new guidelines on how the heat flux in subcooled boiling regime affects the turbulence behaviour near the wall and guides the computational fluid dynamics model development for boiling two-phase flows. IntroductionDirect numerical simulation (DNS) has become an important tool to help develop new models for computational multiphase fluid dynamics (CMFD).[1] The detailed understanding of the boiling process in turbulent flow is one of the most interesting challenge problems that are faced today. This work attempts to separate and quantify the effect of non-detached bubbles on the liquid flow turbulence during nucleate boiling. We do not consider energy equation, and we simplify the bubble distribution, size and shape to more effectively develop new hydrodynamic boundary conditions for CMFD.In the past decade, DNS and interface tracking has been extensively used to study bubble-turbulence interaction. Ilic et al. quantified the turbulent kinetic energy (TKE) balance in bubble-induced turbulence [2] and evaluated the energy spectra in bubble-driven liquid flows using DNS.[3] Note that in this scenario the bubble-induced turbulence was not superimposed on the existing single-phase turbulence. Turbulent bubbly channel flows were extensively studied by under low Reynolds number conditions. Two-phase numerical simulations of heat transport in bubbling turbulence has been recently studied by Lakkaraju et al.[7,8] To specifically quantify flow parameters and the effect of bubbles at different stages of boiling, a system of separate effect studies have been conducted by Bolotnov et al.[9,10]
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