Nicoló Scapin | Searcharxiv

arXiv2022

Evaporating Rayleigh-Bénard convection: prediction of interface temperature and global heat transfer modulation

We propose an analytical model to estimate the interface temperature $Θ_Γ$ and the Nusselt number $Nu$ for an evaporating two-layer Rayleigh-Bénard configuration in statistically stationary conditions. The model is based on three assumptions: (i) the Oberbeck-Boussinesq approximation can be applied to the liquid phase, while the gas thermophysical properties are generic functions of thermodynamic pressure, local temperature, and vapour composition, (ii) the Grossmann-Lohse theory for thermal convection can be applied to the liquid and gas layers separately, (iii) the vapour content in the gas can be taken as the mean value at the gas-liquid interface. We validate this setting using direct numerical simulations (DNS) in a parameter space composed of the Rayleigh number ($10^6\leq Ra\leq 10^8$) and the temperature differential ($0.05\leq\varepsilon\leq 0.20$), which modulates the variation of state variables in the gas layer. To better disentangle the variable property effects on $Θ_Γ$ and $Nu$, simulations are performed in two conditions. First, we consider the case of uniform gas properties except for the gas density and gas-liquid diffusion coefficient. Second, we include the variation of specific heat capacity, dynamic viscosity, and thermal conductivity using realistic equations of state. Irrespective of the employed setting, the proposed model agrees very well with the numerical simulations over the entire range of $Ra-\varepsilon$ investigated.

physics.flu-dyn↗

arXiv2021

A pressure-based diffuse interface method for low-Mach multiphase flows with mass transfer

This study presents a novel pressure-based methodology for the efficient numerical solution of a four-equation two-phase diffuse interface model. The proposed methodology has the potential to simulate low-Mach flows with mass transfer. In contrast to the classical conservative four-equation model formulation, the adopted set of equations features volume fraction, temperature, velocity and pressure as the primary variables. The model includes the effects of viscosity, surface tension, thermal conductivity and gravity, and has the ability to incorporate complex equations of state. Additionally, a Gibbs free energy relaxation procedure is used to model mass transfer. A key characteristic of the proposed methodology is the use of high performance and scalable solvers for the solution of the Helmholtz equation for the pressure, which drastically reduces the computational cost compared to analogous density-based approaches. We demonstrate the capabilities of the methodology to simulate flows with large density and viscosity ratios through extended verification against a range of different test cases. Finally, the potential of the methodology to tackle challenging phase change flows is demonstrated with the simulation of three-dimensional nucleate boiling.

physics.flu-dyn↗