The paper seeks to focus on obtaining the transient torque required to rotate the inner cylinder in open ended vertical concentric annuli for a fluid of Pr = 0.7 in the laminar natural convection flow regime over a wide range of the controlling parameter Gr2/Ta. The inner wall is heated and subjected to an impulsive rotation while the outer one is stationary and maintained adiabatic.
The governing transient boundary‐layer equations are numerically solved using an iterative linearized finite‐difference scheme.
The transient induced flow rate and absorbed heat for different annulus heights are presented. High rotational speed (i.e. low values of Gr2/Ta) increases the flow rate and heat absorbed in short annuli. However, for considerably tall annuli, Gr2/Ta has slight effect on the flow and heat absorbed. The steady‐state time is tangibly influenced by Gr2/Ta in considerably short annuli and very slightly affected for considerably tall annuli.
The investigated problem can simulate the start‐up period of naturally cooled small vertical electric motors.
The paper presents results not available in the literature for the effect of Gr2/Ta on the developing velocities, pressure, flow‐rate induced, absorbed heat by fluid and required torque in vertical concentric annuli with impulsively rotated inner walls under the transient free‐convection heat transfer mode.
