Research Papers

Heat Transfer and Pressure Drop Measurements for a Square Channel With 45 deg Round-Edged Ribs at High Reynolds Numbers

[+] Author and Article Information
Akhilesh P. Rallabandi, Nawaf Alkhamis

 Texas A&M University, College Station, TX 77840-3123

Je-Chin Han

 Texas A&M University, College Station, TX 77840-3123jc-han@tamu.edu

J. Turbomach 133(3), 031019 (Nov 18, 2010) (10 pages) doi:10.1115/1.4001243 History: Received September 28, 2009; Revised November 09, 2009; Published November 18, 2010; Online November 18, 2010

Experiments to determine heat transfer coefficients and friction factors are conducted on a stationary 45 deg parallel rib-roughened square channel, which simulates a turbine blade internal coolant passage. Copper plates fitted with silicone heaters and thermocouples are used to measure regionally averaged heat transfer coefficients. Reynolds numbers studied range from 30,000 to 400,000. The ribs studied have rounded (filleted) edges to account for manufacturing limitations of actual engine blades. The rib height (e) to hydraulic diameter (D) ratio (e/D) ranges from 0.1 to 0.2, while spacing (p) to height ratio (p/e) ranges from 5 to 10. Results indicate an increase in the heat transfer due to the ribs at the cost of a higher friction factor, especially at higher Reynolds numbers. Round-edged ribs experience a similar heat transfer coefficient and a lower friction factor compared with sharp-edged ribs, especially at higher values of the rib height. Correlations predicting Nu and f as a function of e/D, p/e, and Re are presented. Also presented are correlations for the heat transfer and friction roughness parameters (G and R, respectively).

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

(a) Experimental setup; (b) detailed view of the instrumented test section

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Figure 2

(a) 3D view of the rib with rounded edge attached to the copper plate; (b) cross section of the rib along the plane normal to the rib

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Figure 3

Measured Nusselt number enhancement ratios for various (indicated) spacing and height values. For the smooth case (i) the Nusselt number ratios attenuate to unity with increasing x/D. Shaded regions in (c) and (j) indicate the regions considered to compute for the average Nusselt Numbers (NuR)

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Figure 4

(a) Linear regression used to determine the friction factor; (b) measured smooth friction factor compared with the standard correlation; and (c) variation in the friction factor with Re for various rough cases

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Figure 5

One dimensional resistance model for the plate-rib system indicating convective and contact resistances

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Figure 6

Effects of the (a) rib spacing and (b) rib height on the flow field

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Figure 7

Variation in the averaged Nu/Nuo with Re for various cases

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Figure 8

Proposed Nu power law correlation for round-edged ribs (for range 0.095<e/D<0.19 and 5<p/e<10)

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Figure 9

Effect of the rib spacing on the Nusselt numbers based on the total area (including ribbed and smooth surface areas)

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Figure 10

Proposed friction factor power law correlation for round-edged ribs (range: 0.095<e/D<0.19 and 5<p/e<10)

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Figure 11

(a) Comparison of the measured round-edged rib Nusselt Numbers with values predicted for identical sharp-edged ribs. Agreement within 10% indicates that round corners do not pose any heat transfer penalty for range considered; (b) comparison of the measured friction factors for round-edged ribs with correlation for sharp-edged ribs. Correlation is found to overpredict the friction factor, indicating that round-edged ribs incur a lower pressure loss than corresponding sharp-edged ribs

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Figure 12

Speculative flow-field comparisons around sharp and rounded edge ribs, indicating regions with high and low heat transfer and pressure losses

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Figure 13

(a) Heat transfer enhancement plotted against the friction factor penalty (as compared with the smooth channel) for various cases; (b) measured TP plotted as a function of the Reynolds number (higher Reynolds numbers offer lower thermal performance)

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Figure 14

(a) Measured heat transfer roughness parameter (G) plotted against the roughness Reynolds number (e+). Indicated correlations refer to the e/D=0.188 case. Sharp-edged rib correlation does not agree with round-edged rib data; (b) measured friction roughness parameter (R) plotted against the roughness Reynolds number (e+)

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Figure 15

Heat transfer roughness parameters determined in the current work, benchmarked against sharp-rib results presented by Han (25-26)



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