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An experimental study of supersonic microjets

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Abstract

Miniature axisymmetric supersonic nozzles were produced with exit Mach numbers ranging from 1.0 to 2.8 by forming Pyrex® capillary tubing of 0.6 and 1.2 mm inside diameter into converging-diverging channels. The nozzle contours were measured and were found to compare favorably to ideal solutions given by the axisymmetric method of characteristics. In addition, the surfaces of these nozzles were quite smooth, providing featureless flows at perfect expansion. Schlieren visualization and pitot pressure measurements of the resulting microjets were compared to the literature available for jets produced by larger-scale nozzles. A postponed transition to turbulence is noted in these microjets due to their low Reynolds number. The pitot pressure on centerline is nearly uniform at perfect expansion over core lengths up to 12 nozzle exit diameters. Supersonic microjet nozzles thus provide a more effective small-scale high-pressure gas delivery device than do sonic nozzles of comparable scale at equivalent mass flow rates. Supersonic microjets may therefore have several industrial applications.

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Abbreviations

δ* :

boundary layer displacement thickness, mm

d :

diameter of nozzle exit, mm

L :

length of nozzle diverging section, mm

L c :

inviscid core length, mm

L s :

supersonic region length, mm

M c :

convective Mach number

M e :

exit Mach number

P b :

backpressure at nozzle exit, (equal to ambient pressure in this experiment)

P e :

exit pressure of the supersonic jet

P be :

exit pressure ratio (P b /P e )

P p :

impingement pressure (pitot pressure)

P 0 :

stagnation pressure supplied to nozzle

P n :

overall pressure ratio (P 0/P b ,)

r :

radial dimension (cylindrical coordinate system), mm

r 0 :

radius of throat, mm

Re d :

Reynolds number, based on nozzle exit diameter

V e :

exit velocity, m/s

x :

axial dimension (cylindrical coordinate system), mm

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Authors and Affiliations

  1. Gas Dynamics Lab Mechanical Engineering Department, The Pennsylvania State University, 16802, University Park, PA, USA

    S. D. Scroggs & G. S. Settles

Authors
  1. S. D. Scroggs
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  2. G. S. Settles
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Additional information

This research was sponsored by National Science Foundation Grant DMI 9400119, as part of a study of the assist-gas dynamics of laser cutting.

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Scroggs, S.D., Settles, G.S. An experimental study of supersonic microjets. Experiments in Fluids 21, 401–409 (1996). https://doi.org/10.1007/BF00189042

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  • DOI: https://doi.org/10.1007/BF00189042

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