Determination of Density and Concentration from Fluorescent Images of a Gas Flow

A fluorescence image analysis procedure to determine the distribution of species concentration and density in a gas flow is proposed. The fluorescent emission is due to the excitation of atoms/molecules of a gas that is intercepted by an electron sheet. The intensity of the fluorescent light is proportional to the local number density of the gas. When the gas flow is a mixture of different species, this proportionality can be used to extract the contribution associated with the species from the spectral superposition acquired by a digital camera. In particular, the fact is exploited such that the ratio between a pair of color intensities takes different values for different gases and that different linear superpositions of different color intensities yield a ratio that varies with the species concentration. This leads to a method that simultaneously reveals species concentrations and mass density of the mixture. For the proper working of a continuous electron gun in a gas, the procedure can be applied to gas flow where the pressure is below the thresholds of 200∼300 Pa and the number density is no greater than 1023 m−3. To maintain the constancy of the emission coefficients, the temperature variation in the flow should be inside the range 75–900 K (above the temperature where the probability to meet disequilibrium phenomena due to rarefaction is low, below the temperature where visible thermal emission is present). The overall accuracy of the measurement method is approximately 10%. The uncertainty can vary locally in the range from 5 to 15% for the concentration and from 5 to 20% for the density depending on the local signal-to-noise ratio. The procedure is applied to two under-expanded sonic jets discharged into a different gas ambient—Helium into Argon and Argon into Helium—to measure the concentration and density distribution along the jet axis and across it. A comparison with experimental and numerical results obtained by other authors when observing under-expanded jets at different Mach numbers is made with the density distribution along the axis of the jet. This density distribution appears to be self-similar.