Combined combustion and heat-flux measurements on a supersonic jet-in-crossflow configuration using luminescent paint

Experiments are conducted on a planar ramp in high-enthalpy flow, with gaseous hydrogen injected through a porthole at the ramp surface. The ramp is covered in a thin layer of temperature-sensitive luminescent paint, originally intended to allow surface heat-flux measurements; however, anomalous transient features are identified in high-speed recorded images of the paint emission, which are postulated as originating from excitation by OH* chemiluminescence produced during hydrogen combustion. By assuming the OH*-induced emission to occur on much shorter timescales than surface temperature changes, it is possible to extract this pseudo-OH* signal, allowing time-resolved combustion features to be investigated. Two cases are examined, the first exhibiting marginal combustion and the second with more robust ignition. For the first, the weak OH* excitation allows concurrent temperature and heat-flux distributions to be derived. In both cases, the time-averaged combustion distribution shows intense ignition in the immediate vicinity of the injection hole, with extended fingers of weaker combustion extending downstream; the latter are assumed to be associated with the main counter-rotating vortex pair produced by the injection. By examining shorter timescales, however, it is revealed that these combustion fingers are not steady features but rather the time-integrated result of more localized ignition streaks convecting downstream. Overall, the presence of combustion is associated with higher surface heat fluxes. The pseudo-OH* visualization method is a possible alternative to conventional OH* chemiluminescence for wall-bounded combusting flows, allowing a conventional visible-wavelength camera to be employed in place of a UV-sensitive imaging system.