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.

In this work, we found experimentally that there exist fairly strong fluid flows in AC electrowetting, which can be utilized as a means to mix the fluids in EWOD-based micro-devices. We visualized the internal flow. There may exist two distinct flow-generation mechanisms; one is the droplet oscillation, and the other is the electrohydrodynamic flow. The flow pattern is significantly dependent on the applied AC frequency. At low frequencies (represented here by 1 kHz), the center of the vortices is located somewhat randomly and the flow directs upward near the symmetric axis. At high frequencies (represented by 128 kHz), however, a pair of vortices having quite a regular structure is clearly visible and the flow directs downward near the symmetric axis. The flow patterns are strongly dependent on the position of the point electrode. The droplet surface undergoes a periodic oscillation (visualized by a high-speed camera) with a frequency exactly twice the frequency of applied electrical signal. The oscillating interface can generate a steady streaming. However the numerical results show that there exists no electric field at low AC frequencies. On the contrary, there exists quite a strong electric field inside the droplet at high frequencies. It means the electrohydrodynamic flow cannot be generated at the low frequency region, and the droplet oscillation might cause the flow generation at low frequencies. We also demonstrated the flow can be beneficially utilized as a mixing method.