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

Surface Roughness Effects on Impingement Jet Array Surface Heat Transfer

**Surface Roughness Effects on Impingement Jet Array Surface Heat Transfer**

Overall, the focus of the effort is fundamental understanding of thermal transport, and heat transfer phenomena, as altered by impingement array jets as they impact upon different target surface roughness arrangements. Applications are varied, and include electronic cooling, electronic component chip cooling, heat exchangers, utility gas turbine engines employed for power generation, micro-fluidic devices which are employed within electronic components, as well as a variety of other heat transfer augmentation devices.

A variety of different surface roughness configurations are considered, along with a smooth target plate, which is employed to provide baseline comparison data. Included are surface arrays of small rectangular roughness ^{1-2}, small triangle roughness ^{3-4}, small cylinder roughness ^{5-6}, as well as surface arrays of small roughness, employed in conjunction with large pin roughness ^{1-6}. Considered are a variety of different roughness shapes, different roughness configurations, different roughness heights, and different values of *Rej*, impingement jet Reynolds number.

Figure 1. SPATIALLY-AVERAGED NUSSELT NUMBER RATIOS FOR DIFFERENT ROUGHNESS HEIGHTS AND DIFFERENT ROUGHNESS CONFIGURATIONS, *Rej*=5000, from Buzzard et al. ^{1,2}.

An example of spatially-averaged surface Nusselt number ratios are given in Figure 1, from Buzzard et al. ^{1,2}. Here, the baseline Nusselt numbers used for normalization of these results are obtained with impingement cooling applied to smooth target surfaces at the same experimental conditions. Note that no other baseline condition is appropriate for normalization of these impingement cooling results. The turbulent data in Figure 1 are given for rectangle small roughness, both with and without large pin roughness. Here, the jet Reynolds number is 5000. Within this figure, spatially-averaged Nusselt number ratios generally increase at each *x/D* location, as small roughness height increases (from 0.222D to 0.333D to 0.444D, where D is impingement hole diameter), and as large pins are added to the configurations. Such behavior results from increased local mixing and local turbulent transport, as roughness elements become higher and/or more numerous. Also important is the increase in wetted surface area, relative to a smooth target surface, which results from the addition of the different roughness elements. Note that the highest spatially-averaged Nusselt number ratios are present for a surface array of small triangle roughness with 0.444D height, with large pin roughness. With this arrangement spatially-averaged Nusselt number ratios are as high as 2.2 ^{1,2}.

**REFERENCES**

^{1} Influences of Target Surface Roughness on Impingement Jet Array Heat Transfer, Part 1: Effects of Roughness Pattern, Roughness Height, and Reynolds Number, (W. C. Buzzard, Z. Ren, P. M. Ligrani, C. Nakamata, and S. Ueguchi), Paper Number GT2016-56354, ASME TURBO EXPO 2016: Turbomachinery Technical Conference and Exposition, Seoul, South Korea, June 13-17, 2016.

^{2} Influences of Target Surface Small-Scale Rectangle Roughness on Impingement Jet Array Heat Transfer, (W. C. Buzzard, Z. Ren, P. M. Ligrani, C. Nakamata, and S. Ueguchi), __International Journal of Heat and Mass Transfer__, Vol. 110, pp. 805-816, July 2017.

^{3} Influences of Target Surface Roughness on Impingement Jet Array Heat Transfer, Part 2: Effects of Roughness Shape, and Reynolds Number, (W. C. Buzzard, Z. Ren, P. M. Ligrani, C. Nakamata, and S. Ueguchi), Paper Number GT2016-56355, ASME TURBO EXPO 2016: Turbomachinery Technical Conference and Exposition, Seoul, South Korea, June 13-17, 2016.

^{4} Impingement Jet Array Heat Transfer: Target Surface Roughness Shape, Reynolds Number Effects, (Z. Ren, W. C. Buzzard, P. M. Ligrani, C. Nakamata, and S. Ueguchi), __AIAA Journal of Thermophysics and Heat Transfer__, Vol. 31, No. 2, pp. 346-357, April 2017.

^{5} Influences of Target Surface Cylindrical Roughness on Impingement Jet Array Heat Transfer: Effects of Roughness Height, Roughness Shape, and Reynolds Number, (Z. Ren, W. C. Buzzard, and P. M. Ligrani), Paper Number IMECE2016-67655, IMECE 2016: ASME International Mechanical Engineering Congress and Exposition, Phoenix, Arizona, USA, November 11-17, 2016.

^{6} Impingement Jet Array Heat Transfer With Small-Scale Cylinder Target Surface Roughness Arrays, (P. M. Ligrani, Z. Ren, and W. C. Buzzard), __International Journal of Heat and Mass Transfer__, Vol. 107, pp. 895-905, April 2017.