Bolaji O. Olayiwola

Experimental Investigation of the Effects of Fluid Properties and Geometry on Forced Convection in Finned Ducts With Flow Pulsation

An experimental study was conducted on the effects of flow pulsation on the convective heat transfer coefficients in a flat channel with series of regular spaced fins. Glycerol-water mixtures with dynamic viscosities in the range of 0.001-0.01 kg/ms were used as working fluids. The device contains fins fixed to the insulated wall opposite to the flat and smooth heat transfer surface to avoid any heat transfer enhancement by conduction of the fins. Pulsation amplitude x o = 0.37 mm and pulsation frequencies f in the range of 10 Hz<f<47 Hz were applied, and a steady-flow Reynolds number in the laminar range of 10 <Re <1100 was studied. The heat transfer coefficient was found to increase with increasing Prandtl number Pr at a constant oscillation Reynolds number Re osc . The effect of the d h /L ratio was found to be insignificant for the system with series of fins and flow pulsation due to proper fluid mixing in contrast to a steady finned flow. A decrease in heat transfer intensification was obtained at very low and high flow rates. The heat transfer was concluded to be dynamically controlled by the oscillation.

Efficient Heat Transfer in a Laminar Flow System by Hydrodynamic Manipulation

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.Copyright

Computational and PIV Analysis of the Fluid Flow in a Channel With an Oscillating Finned Surface

Experimental and CFD studies were performed to investigate the kinematics of flow resulting from oscillation of a finned surface in a duct. The experiments were performed with working fluid with a kinematic viscosity of 1.8×10−6 m2 /s. A steady flow Reynolds number in the laminar range of 0 < Re < 400 was studied. The oscillation Reynolds number Reosc was between the range of 50 and 1000. Oscillation amplitude range of 0.2 mm < A < 1.0 mm together with oscillation frequency in the range of 5 Hz < f < 90 Hz were employed. The acquired images were analysed using the particle image velocimetry (PIV) software. Three experimental conditions were studied, i.e. oscillating finned surface in a fluid at rest, steady finned flow and oscillating finned flow. CFD simulations were performed using the software suit CFX11 from ANSYS GmbH, Germany. The simulation results were compared with the PIV measurements using the time averaged velocity. The results of the visualization reveal periodic recirculation eddies around the fins which enhances the fluid mixing. The flow patterns and the crossflow effects depend on the geometries of the fins and the oscillation parameters. CFD results allow for performance predictions of different geometries and flow conditions. Enhanced heat transfer was obtained at moderate flow rates when applied in cooling system. Triangular finned geometry gives better performance.Copyright

Convective Heat Transfer Intensification in Laminar Duct Flow

An experimental investigation was carried out to study the influence of pulsation and special surface geometry on the convective heat transfer in laminar flow. The experiments were performed using a glycerol-water mixture of 23 wt% glycerol. Ethanol was used as a coolant. The amplitude of pulsation was between 0.37 and 0.91 mm and the frequency range was 26.7 to 42.7 Hz. The mean flow Reynolds number range was between 50 and 1143. All the geometrical parameters of the channel such as the relative fin spacing and relative fin thickness were constant. The enhancement factor E, i.e. the ratio of heat transfer coefficient due to pulsation compared to steady flow diminishes at low Pe. A maximum E was observed in medium ranges of Pe and small Pe. The amount of heat transferred from the working fluid also depends on κ value. So far, a maximum heat transfer enhancement of E = 2.5 at κ = 3 and Pe = 2750 was obtained. The enhancement factor also increases with increasing pulsation amplitude.Copyright

CFD Simulations of Flow and Heat Transfer in a Zigzag Channel With Flow Pulsation

Heat transfer enhancement by pulsating flow in a zigzag channel has been numerically studied using a commercial CFD software for the ranges of laminar flow 0 41.32. The effect of superposition of oscillation is not significant using a zigzag channel with inclination angle α = 60°. When the oscillation amplitude is increased up to 4 mm at Reynolds number Re = 107, frequency f = 2.17 Hz and inclination angle α = 45°, the heat transfer enhancement E of about 3.3 is obtained.Copyright

Applied Pulsed Flow for Single-Phase Convective Heat Transfer Enhancement in a Laminar Flow Cooling System

Experimental and CFD studies were performed to investigate the enhancement of convective heat transfer in a laminar cooling system using flow pulsation in a flat channel with series of regular spaced fins. Glycerol-water mixtures with dynamic viscosities in the range of 0.001 kg/ms–0.01 kg/ms were used. A steady flow Reynolds number in the laminar range of 10 < Re < 1200 was studied. The amplitudes of the applied pulsations are in the range of 0.25 < A < 0.55 mm and the frequency range is 10 < f < 60 Hz. Two different cooling devices with active length L = 450 mm and 900 mm were investigated. CFD simulations were performed on a parallel-computer (Linux-cluster) using the software suit CFX11 from ANSYS GmbH, Germany. The rate of cooling was found to be significant at moderate low net flow rates. In general, no significant heat transfer enhancement at very low and high flow rates was obtained in compliance with the experimental data. The heat transfer coefficient was found to increase with increasing Prandtl number Pr at constant oscillation Reynolds number Reosc whereas the ratio of the hydraulic diameter to the length of the channel dh /L has insignificant effect on the heat transfer coefficient. This is due to enhanced fluid mixing. CFD results allow for performance predictions of different geometries and flow conditions.Copyright