Visualization of low-density gas-dynamic objects in full-scale processes modelling on small experimental plants

Abstract

The developed and tested visualisation method of supersonic under-expanded gas jets expanding from sonic and supersonic nozzles (or nozzle blocks) into a rarefied medium (deep vacuum) with the background gas pressure range from 3 mPa to 1 Pa was presented. It was based on the use of a well-focused electron beam whose energy was on the order of 10 keV for glow initiation in a local volume of stationary gas jet. The radiation of the excited gas was read by the recorder (modern scanner) and converted into a digital code. The local gas density was determined using special calibration procedures and software. The flow field was scanned by a special device when moving the nozzle pre-chamber by a stepper motor with a coordinate mechanism relative to a stationary electron beam crossing the gas object perpendicular to its axis. Using optical lenses, luminescence from a localised jet volume was focused to the input aperture of a scanner. High-quality focusing of the electron beam along the investigated section of the gas flow and the dimensions of the scanning unit, exceeding the investigated size of the jet, provide registration of a section image of a gas flow along or across the flow axis with sufficient resolution. The advantages and practical perspectives of the method were reviewed based on the results of the visualisation of complex supersonic jets escaping from a block of several supersonic nozzles. The reliability of the information obtained is provided by control measurements of the local gas density distribution along the axis of the supersonic nitrogen flow expanding from the sonic nozzle in comparison with the available theoretical and experimental data.