Experimental Measurements of the Mean Flow Field in Wide-Angled Diffusers: A Data Bank Contribution

Due to adverse pressure gradient along the diverging walls of wide-angled diffusers, the attached flow separates from one wall and remains attached permanently to the other wall in a process called stalling. Stalled diffusers render the whole fluid flow system, in which they are part of, very inefficient. There is then an engineering need to try to understand the whole process of diffuser stall if any meaningful attempts to improve on diffuser efficiency are to be made. In this regard, this paper provides a data bank contribution for the mean flow-field in wide-angled diffusers where the complete velocity and static pressure fields, and pressure recovery data for diffusers in the fully stalled flow regime are experimentally measured. The measurements were carried out at Reynolds numbers between 1.07×105 and 2.14×105 based on inlet hydraulic diameter and centreline velocity for diffusers whose divergence angles were between 30Ôùª and 50Ôùª. Variation of Reynolds number did not significantly affect the velocity and static pressure profiles. The wall static pressure recovery was found to be more sensitive to changes in the Reynolds number. By increasing the velocity from 10 m/s to 20 m/s, the wall static pressure recovery increased by 8.31%. However, as the divergence angle was increased, a similar increase in the Reynolds number resulted in a higher percentage increase in pressure recovery. Experimental results showed that regardless of the wall to which the flow was attached, both the velocity and pressure fields were replicated with discrepancies below 2%.

Authors:

• Karanja Kibicho
• Anthony Sayers

Keywords:

• Two-dimensional
• wide-angled
• diffuser
• stall
• separated flows
• subsonic flows
• diffuser flow regimes

References:

[1] R. Fox and S. Kline, "Flow regime data and design methods for
curved subsonic diffusers," Transactions of the ASME, Journal of Basic
Engineering, vol. 84, pp. 303-312, 1962.
[2] L. Reneau, J. Johnston, and S. Kline, "Performance and design of
straight, two-dimensional diffusers," Transactions of the ASME, Journal
of Basic Engineering, vol. 89, no. 1, pp. 141-150, 1967.
[3] S. Kline, D. Abbort, and R. Fox, "Optimum design of straight-walled
diffusers," Transactions of the ASME, Journal of Basic Engineering,
vol. 81, pp. 321-331, 1959.
[4] C. Sagi and J. Johnston, "The design and performance of twodimensional,
curved diffusers: Part I - Exposition of method," Transactions
of the ASME, Journal of Basic Engineering, vol. 89, pp. 715-731,
1967.
[5] O. McMillan and J. Johnston, "Performance of low-aspect ratio diffusers
with fully developed turbulent inlet flows - Some experimetal results,"
Transactions of the ASME, Journal of Fluids Engineering, vol. 95, no. 3,
pp. 385-392, 1973.
[6] J. Norbury, "Some measurements of boundary-layer growth in a twodimensional
diffuser," Transactions of the ASME, Journal of Basic
Engineering, vol. 81, pp. 285-296, 1959.
[7] J. Johnston and C. Powars, "Some effects of inlet blockage and aspect
ratio on diffuser performance," Transactions of the ASME, Journal of
Basic Engineering, vol. 91, pp. 551-553, 1969.
[8] J. Hoffmann, "Effects of free-stream turbulence on diffuser perfromance,"
Transactions of the ASME, Journal of Fluids Engineering,
vol. 103, no. 3, pp. 385-390, 1981.
[9] S. Wolf and J. Johnston, "Effects of non-uniform inlet velocity profiles
on flow regimes and performance in two-dimensional diffusers,"
Transactions of the ASME, Journal of Basic Engineering, vol. 91, no. 3,
pp. 462-474, 1969.
[10] K. Kaiser and A. McDonald, "Effect of wake-type non-uniform inlet
velocity profiles on first appreciable stall in plane-wall diffusers,"
Transactions of the ASME, Journal of Fluids Engineering, vol. 102,
no. 3, pp. 283-289, 1980.
[11] K. Yajnik and R. Gupta, "A new probe for measurement of velocity and
flow direction in separated flows," Journal of Physics, Series E, Science
Instrumentation, vol. 6, no. 82-86, 1973.
[12] M. Gundogdu and M. Carpinlioglu, "A multi-tube pressure probe calibration
method for measurements of mean flow parameters in swirling
flows," Flow Measurement and Instrumentation, vol. 9, pp. 243-248,
1998.
[13] A. Rhagava, K. Kumar, R. Malhotra, and D. Agrawal, "A probe for the
measurement of velocity field," Transactions of the ASME, Journal of
Fluids Engineering, vol. 11, no. 1, pp. 143-146, 1979.