RESEARCH AREAS

– Dimple Array on Surfaces of Channels

– Investigations of Confined, Millimeter-Scale, Unsteady Laminar Impinging Slot Jets

– Aerodynamic Losses and Mixing Losses from Turbine Airfoils

– Impingement Cooling

– Internal Cooling – Surface Heat Transfer Augmentation

– Miniature and Micro-Scale Pumps

– Surface Roughness

– Electronics Cooling

– Transitional Flows in Curved Channels

– Film Cooling

– Flow and Heat Transfer on and Near a Transonic Turbine Blade Tip

– Slip Phenomina in Micro-Fluidic Devices

– Buoyancy-Driven Continuous SPLITT Fractionation: A New Technique for Separation of Microspheres

– Investigations of Full-Coverage Film Cooling

– Shock Wave Boundary Layer Interactions

– Double Wall Cooling

– Elastic Turbulence

– Surface Roughness Effects on Impingement Jet Array Surface Heat Transfer

– Dean Flow Dynamics in Low-Aspect Ratio Spiral Microchannels

Investigations of Confined, Millimeter-Scale, Unsteady Laminar Impinging Slot Jets

**Visualization and Structure of Confined, Milliscale, Unsteady Impinging Slot Jets and Associated Vortices**

Flow characteristics of confined, laminar milliscale slot jets are investigated from visualizations, as they impinge upon a flat target plate, with a fully-developed velocity profile at the nozzle exit. The effects of Reynolds number *Re *and normalized nozzle-to-plate distance *H/B* are considered for a nozzle width *B* of 1.0 mm. Transition from a stable symmetric jet to an unsteady oscillating jet is observed as the Reynolds number increases (with *H/B* constant), where the Reynolds number associated with this transition decreases as the normalized nozzle-to-plate distance *H/B* increases. Instantaneous visualizations show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations, intermittent oscillating motion of the jet column, and jet flow fluctuation/flapping motion. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures, which form as secondary flows develop in fluid which, initially, is just adjacent to and within the jet column. Associated jet and vortex structural changes are described as different modes of unsteadiness are present, including characterization of jet column unsteadiness using jet column oscillation frequency, and lateral and streamwise extents of jet distortion.

Arrangement of the slot nozzle and flow channel, including coordinate system.

Flow visualization data layout and configuration with *H/B*=12, *Re*=160.

Temporal sequences of jet flow images in the stagnation point region at different *H/B* and a Reynolds number of 200 to illustrate different types of unsteadiness. (a) *Re*=200, *H/B*=8, (b) *Re* =200, *H/B*=10.

Experimental domain map showing the experimental conditions for different types of unsteady jet behavior in *H/B* and Reynolds number coordinates.

Variation and trajectories of vortex centerline paths in non-dimensional coordinates. (a) Variation with *Re* for *H/B* of 14. (b) Variation with *H/B* for *Re* of 200.

**Milliscale Confined Impinging Slot Jets: Laminar Heat Transfer Characteristics for an Isothermal Flat Plate and for a Constant-Heat-Flux Flat Plate **

Heat transfer and overall visualized flow characteristics of confined, laminar milli-scale slot jets are investigated, as they impinge upon an isothermal flat target plate, with a fully-developed profile at the nozzle exit. The effects of Reynolds number *Re *and normalized nozzle-to-plate distance ratio *H/B* are investigated for *Re*=120~200, *H/B*=2-10, and *B*=1.0 mm, with a nozzle aspect ratio of *y/B*=50. Instantaneous visualizations of overall slot jet flow structure show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures which form as secondary flows develop in fluid which, initially, is just adjacent to the jet column. For each Reynolds number considered, local stagnation region Nusselt numbers *Nu _{o}* decrease dramatically as

*H/B*increases to become greater than 7.2~13.2, as the Reynolds number is maintained constant at a value from 200 to 120, changes which occur just as continuous sinusoidal oscillation of the jet column begins to develop. The further development of continuous sinusoidal oscillating motion results in an approximate collapse of stagnation region Nusselt numbers measured at different

*Re*and

*H/B*values. When surface thermal boundary condition data are compared, the constant surface temperature data are generally higher than the constant surface heat flux data near the stagnation location, and lower at locations where

*x/B*is greater than 1-2. The constant surface temperature data also show relatively low values with only very small changes with

*x/B*, for

*x/B*values which are greater than about 5.0, which illustrate the sensitivity of Nusselt numbers for laminar boundary layer and laminar slot jet flows to thermal boundary condition, as well as the restrictions on near-wall temperature gradients which result from a constant surface temperature thermal boundary condition.

Comparison of the lateral variation of local Nusselt number from different investigations for *H/B* of 5 and 6, and for *Re* of 120 and 125.

Variation of experimentally-measured, stagnation point Nusselt numbers with *H/B* for Reynolds numbers from 120 to 200 for a nozzle width *B* of 1.0 mm, and a constant surface temperature boundary condition.

Variation of experimentally-measured, stagnation-point, local Nusselt numbers with H/B for Reynolds numbers from 120 to 200 for a nozzle width B of 1.0 mm, and a constant surface heat flux boundary condition.

Comparison between stagnation Nusselt number variations with Reynolds number (for different *H/B* values) for a nozzle slot width *B* of 1.0 mm for two situations. (i) Experimental conditions which correspond to a constant surface temperature thermal boundary condition when the Nusselt number data show a decrease with increasing *H/B* at a particular Reynolds number *Re*, near the onset of continuous sinusoidal oscillation of the jet column. (ii) Experimental conditions which correspond to the onset of unsteadiness of the jet column within the impingement channel for a constant surface heat flux thermal boundary condition.

__ASSOCIATED REFERENCES__

Confined, Milliscale Unsteady Laminar Impinging Slot Jets and Surface Nusselt Numbers (D. H. Lee, J. R. Bae, H. J. Park, J. S. Lee, and P. M. Ligrani), __International Journal____ of Heat and Mass Transfer__, Vol. 54, Nos. 11-12, pp. 2408-2418, May 2011.

Milliscale Confined Impinging Slot Jets: Laminar Heat Transfer Characteristics for an Isothermal Flat Plate (D. H. Lee, H. J. Park, and P. M. Ligrani), __International Journal of Heat and Mass Transfer__, Vol. 55, No. 9-10, pp. 2249-2260, April 2012.

Confined, Milliscale Unsteady Laminar Impinging Slot Jets: Effects of Slot Width on Surface Stagnation Point Nusselt Numbers (D. H. Lee, J. R. Bae, M. Ryu, and P. M. Ligrani), __ASME Transactions – Journal of Electronic Packaging__, Vol. 134, No. 4, pp. 041004-1 to 041004-11, December 2012.

Visualization and Structure of Confined, Milliscale, Unsteady Impinging Slot Jets and Associated Vortices (D. H. Lee, H. J. Park, and P. M. Ligrani), __Experiments in Fluids__, Vol. 54:1420, pp. 1-15, 2013.

Heat Transfer Measurements From Concave and Convex Surfaces With a Fully Developed Confined Impinging Slot Jet (S. J. Kim, Y. H. Kim, H. J. Park, D. H. Lee, and P. M. Ligrani), The 15^{th} International Heat Transfer Conference, IHTC2014, Kyoto, Japan, August 10-15, 2014.