Physical mechanism of the convective heat flux increasing in case of mixed boundary conditions in Rayleigh-Bénard convection

• Published in: International journal of heat and mass transfer Vol. 185; p. 122411
• Main Authors: Sukhanovskii, A., Vasiliev, A.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.04.2022; Elsevier BV
• Subjects: Adiabatic flow; Boundary conditions; Heat flux; Heat transfer; Heat transmission; Homogeneity; LES; Mixed boundary conditions; OpenFoam; Plates; Rayleigh-Benard convection; Thermal boundary layer; OpenFoam; 44.25.+f; Mixed boundary conditions; 47.27.te; 47.27.ep; Thermal boundary layer; 47.20.bp; LES; 44.20+b; Heat flux
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2021.122411

•The structure of thermal boundary layer in case of mixed boundary conditions is examined.•It is shown that thickness of thermal boundary layer strongly depends on the heater size.•The mechanism of thermal boundary layer formation is proposed.•The crucial factor of the ratio of the heater size and thickness of thermal boundary layer is shown. A series of numerical simulations of Rayleigh-Bénard convection in a cubic cavity are conducted in order to examine the structure of the thermal boundary layer in case of mixed boundary conditions. The main goal of the study is the physical mechanism which provides increasing of heat flux with spatial frequency of conducting-adiabatic pattern. Different spatial configuration of conducting plates, including the fractal one, are considered for Rayleigh numbers from Ra=107 to Ra=2.0×109. We have shown that the temperature boundary layer in case of mixed boundary conditions at the bottom is strongly non-uniform. This non-homogeneity is a result of several factors such as conducting-adiabatic pattern, large-scale circulation and small-scale motions over conducting plates. The thickness of the thermal boundary layer strongly depends on the size of the conducting plates and can be substantially smaller than for a classical Rayleigh-Bénard convection. This effect increases the heat flux with decreasing the size of hot plates, which corresponds to the increasing of spatial frequency of conducting-adiabatic pattern.