Flowfield Interactions in Low Aspect Ratio Pin-fin Arrays

Open Access
Author:
Ostanek, Jason
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 27, 2012
Committee Members:
  • Karen Ann Thole, Dissertation Advisor
  • Dr Eric Paterson, Committee Member
  • Savas Yavuzkurt, Committee Member
  • Cengiz Camci, Committee Member
Keywords:
  • heat transfer
  • turbulence
  • heat exchangers
  • pin-fins
Abstract:
Pin-fin heat exchangers are frequently used in many important applications ranging from gas turbine airfoils to computer processors. While many studies have investigated pin-fin heat transfer, few studies have directly measured the flowfield in pin-fin arrays. The present work compares time-dependent flowfield measurements with time-mean heat transfer measurements to determine the relative influence of certain flowfield features on array heat transfer. For single pin-fin rows, increasing Reynolds number decreased the wake closure position downstream of the pin, which corresponded with the position of maximum heat transfer. For multiple pin-fin rows, the wake closure position in the first row was influenced by streamwise spacing. For low Reynolds numbers, heat transfer in the initial rows was dominated by the horseshoe vortex. For high Reynolds number, heat transfer in the initial rows was influenced by the horseshoe vortex and generation of turbulence in the wake, close to the pin-fin. Despite localized differences in heat transfer when varying streamwise spacing, the row-averaged heat transfer in the initial rows of the array, was independent of streamwise spacing. In downstream rows, however, decreasing streamwise spacing (from 3.03 diameters to 2.16 diameters) was found to increase heat transfer because there was less streamwise distance for the decay of turbulence prior to the flow encountering the next row of pin-fins. Decreasing spanwise spacing (from 3 diameters to 2 diameters) resulted in increased heat transfer for both low and high Reynolds numbers in multiple row pin-fin arrays. In the initial rows, close spanwise pin spacing resulted in a significant portion of the channel walls being disturbed by the horseshoe vortex that wrapped around the pins. In downstream rows, however, the vortex breakdown to turbulent flows resulted in increased heat transfer for close spacings. In addition, decreasing spanwise spacing (from 3 diameters to 2 diameters) resulted in a thinner wall layer resulting in increased heat transfer compared to wider spanwise spacings. For a given percentage decrease in either streamwise or spanwise spacing, a similar percentage increase in heat transfer was observed. Improvements to pin-fin arrays may be possible through modifications to the pin-fin shape, the endwall shape, or to the pin-fin spacing. In the present work, a non-uniform array was considered having variable streamwise pin-fin spacing. The non-uniform array performed as well as a closely spaced array at high Reynolds number. The important result was that improvements were realized through non-conventional spacing schemes. Further modifications that generate turbulence and prevent the streamwise decay of turbulence would be beneficial for array heat transfer based on the results presented in this work.