Research in Engineering and Aviation
New Developments in Surface Heat Transfer Augmentation Technologies as Applied to Internal Flow Environments
Author(s): P. M. Ligrani
Journal: Invited Plenary Keynote Paper, ISTP-23, The 23rd International Symposium on Transport Phenomena, The University of Auckland, Auckland, New Zealand, November 19-22, 2012.
To provide an overview of the current state-of-the-art of heat transfer augmentation schemes employed for internal cooling of turbine blades and components, results from an extensive literature review are presented with data from internal cooling channels, both with and without rotation. According to this survey, a very small number of existing investigations consider the use of combination devices for internal passage heat transfer augmentation. Examples are rib turbulators, pin fins and dimples together, a combination of pin fins and dimples, and rib turbulators and pin fins in combination. The results of such studies are compared with data obtained prior to 2003 without rotation influences, as summarized by Ligrani et al. . Those data  are comprised of heat transfer augmentation results for internal cooling channels, with rib turbulators, pin fins, dimpled surfaces, surfaces with protrusions, swirl chambers, or surface roughness. This comparison reveals that all of the new data, obtained since 2003, collect within the distribution of data obtained from investigations conducted prior to 2003, without rotation influences . The same conclusion in regard to data distributions is also reached in regard to globally-averaged thermal performance parameters, Nu Nuo/(f/fo)1/3 and Nu Nuo/(f/fo), as they vary with friction factor ratio f/fo. These comparisons, made on the basis of such judgment criteria, lead to the conclusion that improvements in our ability to provide better overall, spatially-averaged thermal protection have been minimal since 2003.
When rotation is present, existing investigations provide little evidence of overall increases or decreases in overall thermal performance characteristics with rotation, at any value of rotation number, buoyancy parameter, density ratio, or Reynolds number. In addition, overall thermal performance in smooth channels with rotation is affected more by Reynolds number than rotation number. Comparisons between existing rotating channel experimental data and the Ligrani et al.  results show that rotation has little effect on overall thermal performance as a function of friction factor. This is largely because of the competing effects of rotation on the pressure (or trailing) sides and the suction (or leading) sides of internal channel flows. Differences in local Nusselt number ratios for pressure sides and suction sides are generally a result of increasing rotation numbers, and the rotation induced secondary flows caused by Coriolis vortices. Also considered are effects of buoyancy parameter, inlet density ratio, channel geometry, and wall heating arrangements.