Thermal performance comparison between radially rotating ribbed parallelogram channels with and without dimples

The experimental study examines the comparative full-field Nusselt number (Nu) distribution on two opposing enhanced channel walls, including pressure drop coefficient (f), and the thermal performance factor (TPF) of the two radially rotating ribbed channels - with and without dimples. Control conditions were imposed on: Reynolds (Re), rotation number (Ro), buoyancy number (Bu) numbers for both rotating channels, radial outward flows were controlled in the ranges of: 5000 <= Re <= 15000, 0 <= Ro <= 0.3 and 0 <= Bu <= 0.23 respectively. The full-field Nu generated data were recorded and interpreted by the steady-state infrared thermo graphic principle. The isolated interdependent Re, Ro, and Bu impaction on the Nu distribution were accounted; also, the rib-wise average Nusselt number ((Nu) over bar (alpha)) at rib and mid-rib locations inclusive over the leading and trailing ribbed walls were recorded. The data for both test section walls, dimpled and non-dimpled were parametrically analysed. For the dimpled test section located opposite one and other, the ribbed wall augmented the near-wall turbulent mixing by means of shedding vortices from the effect of dimples, and the relaxation of shear strain by enlarging channel sectional area thereby causing Nu elevation with a minor f reduction from the reference result generated by the parallelogram ribbed channel at static and rotating conditions. The amplified heat transfer enhancement (HTE) accounting for a minor penalty of pressure drop from adding dimples on to the rib floor led to further TPF elevation. By comparison, the rotational influence on heat-transfer and pressure-drop considered, the performance is similar for both test channels. Due to the combined effect of Coriolis force, rotating buoyancy and the geometric features defined by the test section channels, the rib-wise average (Nu) over bar (alpha)/(Nu) over bar (alpha,0) over the leading and trailing walls respectively fall in to the ranges of: 0.71-1.42 and 1.26-1.88 at rib location, and 0.74-1.26 and 1.11-1.77 at mid-rib location for the un-dimpled channel. For the dimpled test channel the ranges were: 0.74-2.07 and 1.11-2.26 at rib location; and 0.75-2.43 and 1.18-2.4 at the mid-rib location. The f coefficients were raised to 1.34-29.8 and 1.81-17.1 times the referenced smooth channel level (f(infinity)) for the parallelogram ribbed channel with and without dimples; developing the TPF factor toward the range of: 0.979-1.575 and 2.62-4.4. A congruous physical data set consistently produced an empirical (Nu) over bar (alpha) correlation which evaluated the individual variables, and the interdependent Ro and Bu effect on (Nu) over bar (alpha) at rib and mid-rib locations for the parallelogram ribbed dimpled channel to further assist in design applications. (C) 2012 Elsevier Ltd. All rights reserved.