Local experimental heat transfer of single-phase pulsating laminar flow in a square mini-channel

Disturbing a single-phase laminar internal convective flow with a particular pulsating flow frequency alters the thermal and hydrodynamic boundary layer, thus affecting the inter-particle momentum and energy exchange. Due to this externally imposed flow disturbance, augmentation in the heat transfer may be expected. Obviously, parameters like pulsating flow frequency vis-à-vis viscous time scales and the imposed pulsating amplitude will play an important role. Conclusions from reported literature on this and related problems are rather incoherent. Lack of experimental data, especially in micro-/mini internal convective flow situations, with imposed flow pulsations, motivates this study. Non-intrusive infra-red thermography has been utilized to scrutinize heat transfer augmentation during single-phase laminar pulsating flow in a square mini-channel of cross-section 3 mm × 3 mm, electrically heated from one side by a thin SS strip heater (70 μm, negligible thermal inertia); all the other three sides of the channel are insulated. The study is done at different pulsating flow frequencies of 0.05 Hz, 1.00 Hz and 3.00 Hz (Wo = 0.8, 3.4 and 5.9, respectively). These values are chosen because pulsatile velocity profiles exhibit different characteristics for Wo > 1 (annular effect, i.e., peak velocity near the channel walls) and Wo < 1 (conventional parabolic profile). Local streamwise heat transfer coefficient has been determined using the time averaged spatial IR thermograms of the heater surface and the local fluid temperature, linearly interpolated from measured value of inlet and outlet bulk mean mixing temperature. 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 with same time-averaged flow Reynolds number. The change is either marginal or highly limited, primarily occurring in the developing length of the channel. Thus, the results suggest that heat transfer enhancement due to periodic pulsating flow is questionable, and at best, rather limited.