Advanced Flow Visualisation Techniques in Cold Gas Subscale Nozzles, a Comparison

Tests were conducted at DLR-Lampoldshausens cold gas subscale test stand P6.2 to visualise the separation in strongly over expanding nozzles. Several techniques were tested or developed like: Pressure Sensitive Paint, Backflow Frosting and Infrared Thermography. Pressure sensitive paint reacts with oxygen. The intensity of the emitted light depends on the partial pressure of the surrounding oxygen. As an operating gas nitrogen was used to prevent the reaction of the paint with oxygen in the attached flow region. This results in a light zone representing the attached flow and a dark zone representing the backflow of ambient air into the separated region. A clear visible contrast line visualises the physical separation of the flow. Backflow Frosting uses the behaviour of nearly saturated air at its due point. The saturated back flowing air passes the cooled nozzle wall downstream the separation line and hoarfrost forms out determining the physical separation. Infrared Thermography shows the deviation of the recovery wall temperature from its expected decreasing trend and therefore the start of the separation. All methods in combination give an impression of the separation zone. The visualisation results and the belonging pressure measurements are compared with an analytical model of the separation zone.