TRANSITION TO PERIODIC UNSTEADY AND EFFECTS OF PRANDTL NUMBERS ON THE FLOW ACROSS A CONFINED HEATED TRAPEZOIDAL BLUFF BODY

Abstract

Effects of wall confinements (β) and of Prandtl numbers (Pr) on the two-dimensional laminar flow around a heated tapered trapezoidal bluff body are investigated numerically at low Reynolds numbers (Re) for 0.71≤Pr≤100 and 12.5%≤β≤50%. The critical Reynolds number (i.e. the switch from a steady to a time-periodic flow) is calculated and it exists between Re = (36 and 37), (60 and 61) and (91 and 92) for β of 12.5%, 25% and 50%, respectively. The Nu (average) increases with the increasing value of Re and/or Pr and/or β. The variation of Nu (local) on each surface of the cylinder and the isotherm contours are presented to clarify the role of Prandtl number and β on transfer of heat from the trapezoidal bluff body. The maximum heat transfer enhancement with respect to a square bluff body on the basis of equal projected area is found to be approximately 31% and 45% for β = 12.5% and 25%, and for Pr=0.71 and Re=1. Finally, the Colburn heat transfer factor correlations are obtained for the preceding range of settings. Keywords: Trapezoidal bluff body; Blockage ratio; Critical Reynolds number; Prandtl number; Nusselt number; the Colburn heat transfer factor and Heat transfer enhancement