Pitot pressure measurements in a supersonic steam jet

• Pitot pressure measurements in supersonic wet steam jet flows have been obtained.
• Analysis of measurements yields the spreading rate of the free shear layers.
• Results agree with correlations for planar compressible mixing layer spreading.
• Prospects for entrainment-based modelling of steam ejectors are good.

Ejectors produce a pumping effect based on high speed jet entrainment but data on the development of mixing in steam ejectors with condensation effects is not available. In the present work, the mixing region generated downstream of an axisymmetric supersonic steam ejector nozzle is surveyed using a pitot pressure probe. Steam was delivered to the nozzle at a stagnation temperature of 142 °C and at a stagnation pressure of either 270 or 300 kPa. The nozzle had an exit diameter of 13.6 mm and it produced a flow of steam at approximately 1150 m/s, resulting in high compressibility mixing conditions; the convective Mach number was approximately 1.5. Pitot pressure profiles at positions 1, 25, 50, and 70 mm downstream of the nozzle exit were obtained within a steam vapour-filled test section at a pressure of approximately 3 kPa. The data demonstrates that the jet was slightly over-expanded at these conditions with non-uniformities in the pitot pressure data apparent at the 25 mm station. The free shear layer growth rate deduced from the pitot pressure measurements was δ′≈0.06±0.02δ′≈0.06±0.02 and this value agrees with established empirical results for compressible, planar mixing layers. There are reasonable prospects for the development of reliable engineering models for secondary stream entrainment in ejector applications based on existing correlations for compressible mixing.