Numerical investigation of aspect ratio influences on Rayleigh-Benard convection of Bingham fluids in vertical cylindrical annuli

Purpose This paper aims to conduct numerical simulations to investigate steady-state laminar Rayleigh-Benard convection of yield stress fluids obeying Bingham model in rectangular cross-sectional cylindrical annular enclosures. In this investigation, axisymmetric simulations have been carried out for nominal Rayleigh number range Ra = 10(3) to 10(5), aspect ratio range AR = 0.25 to 4 (i.e. AR = H/L where H is the enclosure height and L is the difference between outer and inner radii) and normalised inner radius range r(i)/L = 0 to 16 (where r(i) is internal cylinder radius) for a nominal representative Prandtl number Pr = 500. Both constant wall temperature (CWT) and constant wall heat flux (CWHF) boundary conditions have been considered for differentially heated horizontal walls to analyse the effects of wall boundary condition. Design/methodology/approach The bi-viscosity Bingham model is used to mimic Bingham fluids for Rayleigh-Benard convection of Bingham fluids in vertical cylindrical annuli. The conservation equations of mass, momentum and energy have been solved in a coupled manner using the finite volume method where a second-order central differencing scheme is used for the diffusive terms and a second-order up-wind scheme is used for the convective terms. The well-known semi-implicit method for pressure-linked equations algorithm is used for the coupling of the pressure and velocity. Findings It is found that the convective transport strengthens (weakens) with an increase in Ra (AR) for both Newtonian (i.e. Bn = 0) and Bingham fluids, regardless of the boundary conditions. Moreover, the strength of convection is stronger in the CWT configuration than that is for CWHF boundary condition due to higher temperature difference between horizontal walls for both Newtonian (i.e. Bn = 0) and Bingham fluids. The mean Nusselt number Nucy does not show a monotonic increase with increasing Ra for AR = 1 and r(i)/L = 4 because of the change in flow pattern (i.e. number of convection rolls/cells) in the CWT boundary condition, whereas a monotonic increase of Nucy with increasing Ra is obtained for the CWHF configuration. In addition, Nucy increases with increasing r(i)/L and asymptotically approaches the corresponding value obtained for rectangular enclosures (r(i)/L -> infinity) for both CWT and CWHF boundary conditions for large values of r(i)/L. It is also found that both the flow pattern and the mean Nusselt number Nucy are dependent on the initial conditions for Bingham fluid cases, as hysteresis is evident for AR = 1 for both CWT and CWHF boundary conditions. Originality value Finally, the numerical findings have been used to propose a correlation for Nucy in the range of 0.25 <= r(i)/L <= 16, 0.25 <= AR <= 2 and 5 x 10(4) <= Ra <= 10(5) for the CWHF configuration.