Experiments in Fluids

, Volume 21, Issue 6, pp 401–409

An experimental study of supersonic microjets

  • S. D. Scroggs
  • G. S. Settles


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.

List of symbols


boundary layer displacement thickness, mm


diameter of nozzle exit, mm


length of nozzle diverging section, mm


inviscid core length, mm


supersonic region length, mm


convective Mach number


exit Mach number


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


exit pressure of the supersonic jet


exit pressure ratio (Pb/Pe)


impingement pressure (pitot pressure)


stagnation pressure supplied to nozzle


overall pressure ratio (P0/Pb,)


radial dimension (cylindrical coordinate system), mm


radius of throat, mm


Reynolds number, based on nozzle exit diameter


exit velocity, m/s


axial dimension (cylindrical coordinate system), mm


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Copyright information

© Springer-Verlag 1996

Authors and Affiliations

  • S. D. Scroggs
    • 1
  • G. S. Settles
    • 1
  1. 1.Gas Dynamics Lab Mechanical Engineering DepartmentThe Pennsylvania State UniversityUniversity ParkUSA

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