Reduction Of Reattachment Length Research Articles

Effect of Synthetic Jet on Flow Field over Different Shaped Ribs and Its Impact on Heat Transfer

The present work reports the manipulation of the flow field over different shapes of surface-mounted obstacles using a synthetic jet. A round synthetic jet is placed upstream and downstream of surface-mounted ribs on a flat surface. Particle image velocimetry and hotwire anemometry are used to study the flow field characteristics and features. Heat transfer rate was obtained using transient liquid crystal thermography technique. Experiments are carried out at Reynolds number of 32,000 in a low-speed wind tunnel. Flow field features behind ribs are analyzed using time-averaged velocity, streamline, vorticity Reynolds stress and turbulent intensity. The spanwise average Nusselt number distribution for the downstream rib location is compared for two optimum locations of the synthetic jet relative to the rib. The recirculation bubble length formed downstream of a rib varies with the frequency of the synthetic jet, its location, and the shape of the rib. A drastic reduction in reattachment length is observed at 50 and 80 Hz frequencies for square rib. The highest heat transfer rate is obtained for the square rib at 50 Hz and 4 voltage peak-to-peak excitation amplitude for the upstream location of the synthetic jet relative to the rib.

Upward bubbly flows in a square pipe with a sudden expansion: Bubble dispersion and reattachment length

We investigate the bubble dynamics and subsequent changes in the liquid-phase flow characteristics of upward bubbly flows in a square pipe with a sudden expansion (expansion ratio of 4.0). The experiments are conducted under three liquid-phase Reynolds numbers of 0 (stationary), 420 (laminar) and 6000 (turbulent). The inlet volume void fraction ranges about 0−1.0%, and we use a high-speed two-phase particle image velocimetry and shadowgraphy to measure two phases simultaneously. It is observed that after the expansion, smaller bubbles tend to migrate toward the wall and larger bubbles rise in a core region, which is more encouraged by the steeper velocity gradient with increasing the Reynolds number. This bubble distribution is further analyzed by estimating the interfacial forces acting on rising bubbles. Affected by this bubble distribution, the enhanced turbulence in the inlet flow energizes the separating shear layer, resulting in the reduction of reattachment length behind the edge. Finally, we suggest an empirical relation for a two-phase flow reattachment length in terms of Reynolds number and mean void fraction, which explains the contribution of added dispersed phase on the mixing enhancement in the backstep flows.

Experimental investigation of flow structure due to truncated prismatic rib turbulators using particle image velocimetry

An experimental investigation of detailed flow field measurement in a rectangular duct with truncated prismatic ribs mounted on the bottom wall has been carried out using 2-dimensional particle image velocimetry (2-D PIV) technique. The truncated prismatic ribs have been manufactured by tapering the square rib from both the sides up to the center to provide rib height at the ends (h) of 0–8 mm with an increment of 2 mm. The PIV measurements have been performed in vertical as well as in horizontal streamwise planes for different Reynolds numbers and a fixed rib pitch to height ratio of 12. In addition, a number of PIV measurements in vertical symmetry plane have also been performed while covering the streamwise region of 46≤x/e≤120 to observe the nature of the flow in the test section and the flow periodicity. Overall, the emphasis is to investigate the flow structures and their dynamics in the inter-rib region in terms of time-averaged streamlines, mean velocities, vorticity fields, instantaneous velocity vector and corresponding vorticity fields, and fluctuation statistics in both the measurement planes. The close-up inspection based on critical point theory shows the significant evidence of salient critical points and limit cycles such as saddle point, stable and unstable node, bifurcation line and stable and unstable focus in both just upstream and downstream rib regions. The critical flow patterns around the rib confirm three-dimensionality and unsteady nature of the flow. The instantaneous flow characteristics reveal the presence of large-scale coherent vortices in the separated shear layer region, the existence of periodical processes such as intermittent ejection of the recirculation zone into the main flow and motion of the reattachment point. Significant reduction in the size of primary recirculation bubble and thereby reduction in reattachment length is observed for truncated prismatic ribs as compared to square rib in spanwise direction.

PIV investigation of flow behind surface mounted permeable ribs

The flow behind surface mounted permeable rib geometries, i.e. solid, slit, split-slit and inclined split-slit ribs have been studied using flow visualization and PIV (2-C and 3-C) technique in streamwise and cross-stream measurement planes. The objective behind this study is to understand the flow structures responsible for heat transfer/mixing enhancement with simultaneous pressure penalty reduction by permeable rib geometries. The Reynolds number based on the rib height has been set equal to 5,538 and the open area ratio of permeable ribs is equal to 20%. The permeable rib geometries have shorter reattachment length in comparison to the solid rib. The maximum 41% reduction in reattachment length is observed for the inclined split-slit rib. The splitter mounted inside the slit leads to two corner vortices behind it. The corner vortices drag the flow from the primary recirculation bubble region towards the rib resulting in drop of the reattachment length. Two horseshoe vortices are present in the flow through the slit at both sides of the splitter due to the upstream flow separation. The slit inclination moves these horseshoe vortices closer to the bottom wall. A film like flow through the slit is present near the downstream corner of the inclined split-slit rib. The spanwise velocity gradient due to the splitter leads to vorticity and turbulence enhancement by vortex stretching. The inclination of the slit and the use of a splitter inside the slit are two important design parameters responsible in generation of near-wall longitudinal vortices. The flow field behind permeable ribs is dominated by vortical structures with definable critical flow patterns, i.e. node, saddle and foci. These predominant swirling flow motions contribute to the mixing enhancement behind permeable rib geometries.

Numerical studies of heat transfer enhancements in laminar separated flows

Numerical studies of unsteady heat transfer enhancements in grooved channel and sharp 180° bend flows of especial relevance to electronic systems are made. Prior to these, to find the most efficient numerical approaches, performances of various pressure correction, convective and temporal schemes are studied. To do this, the spatial development of an instability wave in a plane channel flow is modelled. Grooved channel flow predictions suggest a commonly used periodic flow assumption (for modelling rows of similar electronic components) may not always be valid over a significant system extent. For the bend flow, heat transfer enhancements due to passive and active control are considered. For the former, a thin tripping fin is attached at several inlet channel locations. If the fin is sufficiently high and far from the sharp bend edge, the flow becomes unsteady. Then, large-scale vortices emanating from the sharp edge cause a reduction in re-attachment length and a dramatic heat transfer enhancement. Active flow control is also studied for several inlet amplitudes and forcing frequencies. Like the passive fin, this active approach is effective at increasing heat transfer.

Control of turbulent separated flow over a backward-facing step by local forcing

An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ Re H ⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A 0 ⩽ 0.07) and forcing frequency (0 ⩽ St H ⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (x r )min/x x0 was obtained at St θ ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.