The effects of flow manipulation on the heat transfer performance of a laminar flow system were investigated. The combination of series of fins and oscillating flow was used due to its inherent advantage of triggering very complex transient interaction of eddies and flow deflection within the system. The investigations were performed with fluids providing Prandtl numbers Pr > 10. Steady Reynolds number in the range of 50 < Re < 1200 were studied. The duct with a hydraulic diameter Dh = 15 mm contains series of non-conducting fins. All geometrical parameters remain constant. Low frequency oscillation f < 100 Hz was used in order to obtain oscillation effects mainly dominated by oscillation velocity, and to avoid attenuation of the oscillation amplitude A in the device. Based on the experimental data, a correlation equation was developed. The energy dissipation as a result of applied oscillation was also determined by phase resolved measurements of the pressure difference and liquid displacement. The heat transfer coefficients were found to be dynamically controlled by the oscillations. The results show efficient heat transfer within the system due to the applied oscillation especially at low flow rates. At higher flow rates, the effect of the flow oscillation on the heat transfer performance of the system diminishes. With oscillating finned flow, the influence of the geometrical parameter Dh/L is not significant due to enhanced fluid mixing and repeated thermal boundary layer rearrangement as a result of the flow oscillation. The predictions of the correlation are reasonable. The results of the CFD show that for the fin spacing to be significant on the effectiveness of the finned system, the oscillating flow velocity must be higher than the mean flow velocity. Enhanced heat transfer performance is possible with increasing fin height but theoretically, this yields high pressure drop and increased pumping power. The calculated power input due to oscillation is comparatively low and decreases towards increasing net flow rates where the pulsating flow has a diminishing effect and the system approaches non-pulsating flow behaviour.
Comparison of different pressure waveforms for heat transfer performance of oscillating flow in a circular cylinder
