Local Experimental Heat Transfer of Single-Phase Pulsating Flow in a Square Mini-Channel

Abstract

Disturbing the flow with a particular pulsating frequency alters the thermal and hydrodynamic boundary layer thus affecting the inter-particle momentum and energy exchange. Due to this enhanced mixing, augmentation in the heat transfer is expected. Obviously, the parameters like pulsating frequency vis-à-vis viscous time scales and the imposed pulsating amplitude will play an important role in the enhancement of the heat transfer. Several numerical heat transfer and fluid flow studies on pulsating flows have been reported in the literature but the conclusions are not coherent. Lack of experimental study in hydrodynamics as well as in heat transfer of laminar pulsating flows attracts to revisit this problem especially, in mini-channels. Technological developments in measurement and instrumentation have enabled to experimentally investigate the thermo-hydrodynamic study of laminar pulsating flows in mini-channels as an augmentation technique for heat transfer. In this work, we have undertaken the experimental measurements of heat transfer of single-phase laminar pulsating flow in square mini-channel of cross-section 3 mm × 3 mm. The study is done at two different pulsating frequency 0.05Hz and 1Hz (Womersley number, Wo = 0.8 and 3.4 respectively). These two values are chosen because velocity profile exhibits different characteristic for Wo > 1 (annular effect, i.e., peak velocity near the wall) and Wo < 1 (conventional parabolic profile). The heat transfer study has been done in a square channel of made on polycarbonate sheet with one side heating. Heater (made of SS, 70 microns thin strip, negligible thermal inertia) itself is one of the walls of the square channel making constant heat flux thermal condition and its instantaneous temperature is measured by using pre-calibrated InfraRed camera. Fluid bulk mean temperature has been determined by energy balance and one K-type thermocouple is also placed in the fluid at the outlet cross-section. By using these temporal data, space averaged instantaneous Nusselt number has been obtained. It is observed that for measured frequency range, the overall enhancement in the heat transfer is not attractive for laminar pulsating flow in comparison to steady flow of same time-averaged flow Reynolds number. It is found that the change in species transport is either marginal or highly limited and is primarily occurring in the developing length of the channel/ plate. Enhancement of species transport due to such periodic pulsatile internal flows, over and above the non-pulsatile regular flow conditions, is questionable, and at best, rather limited.