Natural convection of viscoplastic fluids in an enclosure with partially heated bottom wall

In this paper, heat transfer characteristics and flow patterns of viscoplastic fluid due to natural convection in a two-dimensional heated enclosure have been investigated both computationally and experimentally. The enclosure is heated partially at the bottom wall at its centre. The side walls are isothermally cooled. The rheology of the enclosed fluid is represented by Bingham model. In addition, an experimental investigation on the viscoplastic test fluids, prepared from Carbopol Ultrez 20 polymer, has also been carried out. The proposed computational model has been validated with our own experimental results. The results in terms of flow and thermal characteristics are presented for different values of Bingham number (Bn) varying between 0 and 30, Rayleigh number (Ra) ranging 10(4) to 10(6), Prandtl number (Pr) from 1 to 100 and heated lengths of the bottom wall (epsilon). The critical value of yield stress of enclosed fluid for which convection circulation diminishes and conduction becomes the sole mode of heat transfer in the enclosure has also been evaluated. The effect of constant heat flux boundary condition at the heated portion has been compared with that of constant temperature (isothermal) boundary conditions. It has been found that convection is weak in constant heat flux case compared to the isothermal condition case.