Mixed convection heat transfer in a two-dimensional annular cavity using an off-lattice Boltzmann method

Abstract

In the present work, a characteristic-based off-lattice Boltzmann method is used to study mixed convection heat transfer in a two-dimensional concentric annular cavity. First, the numerical method is verified using two benchmark cases: the Taylor–Couette flow and mixed convection in a concentric annular cavity. Thereafter, the effects of strength and the direction of rotation of the cylinders on the flow and heat transfer characteristics are analyzed. To this end, four different configurations of the cylinders, namely, the counter-rotating-cylinders, the co-rotating-cylinders, outer-rotating-cylinder with a stationary inner-cylinder, and inner-rotating-cylinder with a stationary outer-cylinder are considered. Further, a range of Rayleigh numbers, Ra=104,105 and 106 and a range of Reynolds numbers, Re=0 to 104 are considered for a Prandtl number of, Pr=0.71. It is observed that irrespective of the Reynolds number of the flow, the forced convection always results in a lower heat transfer rate. At a lower Rayleigh number of Ra=104, heat transfer in the natural convection is always higher than the mixed convection heat transfer for all the four rotation configurations considered. Both the co-rotating-cylinders and inner-cylinder-rotating configurations always result in a lower heat transfer rate than the natural convection heat transfer rate. However, up to certain Reynolds numbers, the counter-rotating-cylinders and outer-rotating-cylinder configurations result in a higher heat transfer rate for Ra=105 and 106. Beyond this Reynolds number, the heat transfer rate begins to decrease with an increase in the Reynolds number. Compared to other configurations, the co-rotating-cylinders configuration behaves similar to forced convection at lower Reynolds numbers. The profiles of the overall Nusselt number as a function of the Richardson number are studied for all four configurations, and the map thus obtained could be divided into three different zones. The distinguishing features of each of these zones are explained.