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Experimental Study of Entrainment Ratio of the Near Exit Region of High-Speed Jets

[+] Author Affiliations
Farhad Saffaraval, Stephen A. Solovitz

Washington State University Vancouver, Vancouver, WA

Paper No. IMECE2011-63661, pp. 89-96; 8 pages
doi:10.1115/IMECE2011-63661
From:
  • ASME 2011 International Mechanical Engineering Congress and Exposition
  • Volume 6: Fluids and Thermal Systems; Advances for Process Industries, Parts A and B
  • Denver, Colorado, USA, November 11–17, 2011
  • Conference Sponsors: ASME
  • ISBN: 978-0-7918-5492-1
  • Copyright © 2011 by ASME

abstract

The growth of volcanic plumes is highly dependent on entrainment ratio, which is a ratio of the radial mass inflow to a jet to its axial mass flow. This fraction is relatively constant in many conditions, and its magnitude is a factor in whether a volcanic column either becomes buoyant and rises or collapses. Under fully-developed, self-similar conditions, the value of this fraction is approximately 0.065 for neutrally buoyant-jets and 0.09 for buoyant jets. However, volcanic eruptions generally involve much higher pressures than these conditions, resulting in overpressured exit flows and supersonic speeds downstream, which likely impact entrainment. A small-scale laboratory model of a high-speed jet was developed, and particle image velocimetry was used to study the velocity fields both along the jet centerline and in the ambient atmosphere. The experiments consider exit speeds ranging from low subsonic levels up to overpressure ratios, K = pexit /patm , of approximately three. The resulting instantaneous fields were then integrated to determine the entrainment ratio. At these higher pressure conditions, the entrainment was significantly reduced downstream of the exit, with the mass flow more than 40% lower. This may have significant impact on analytical models of volcanic plume development, as the expected plume growth may be overpredicted.

Copyright © 2011 by ASME
Topics: Jets

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