An efficient optical pressure measurement in compressible flows: Laser-induced iodine fluorescence

The understanding of complex phenomena, the experimental validation of calculation codes, and the construction of data bases in fluid mechanics require the development of non-intrusive experimental techniques. The laser-induced fluorescence of a gaseous molecule seeded into a gas flow can be related to the flowfield thermodynamic parameters, such as pressure and temperature. An experimental method is described, that allows the removal of the temperature dependence of the fluorescence signal. A narrow-bandwidth single-line laser is tuned to the center of an absorption line, whose temperature dependence of the Boltzmann fraction can be neglected. The experimental set-up requires a single-line dye laser and a high resolution spectral analysis device.The accuracy of the method, checked in a static vessel, appears to be better than 5%. The method has been successfully tested with a supersonic jet issuing from an underexpanded nozzle.The experimental results have been compared to those of an Euler calculation. A mean difference of 14% has been observed, but a major part of this can be attributed to the difference between inviscid and real gas calculation.List of Symbols A₂₁ spontaneous emission Einstein coefficient - B_v rotational energy of the J level for the v vibrational level - B_e first order approximation of B_v - c velocity of light - C_opt optical constant - E(v) vibrational energy of the v level - f₁ Boltzmann fraction - FCF_i Franck-Condon factor of the ith line - f_v vibrational fraction - tf_r rotational fraction - g efficient spectral power density - h Planck constant - I₂X iodine molecule in the X state - I₂B iodine molecule in the B state - [I₂X] molecular concentration of I₂X - [I₂B] molecular concentration of I₂B - I_i relative intensity of the ith absorption line - J rotational level of the fundamental state - J rotational level of the excited state - k Boltzmann constant - k_c collisional broadening coefficient - m iodine molecular mass - n₀ initial concentration of iodine in the absorbing state - P pressure - P_laser laser power - P_l(v) laser power spectral density - P_sl2 iodine vapor pressure - Q quenching rate - Q_v vibrational partition function - T temperature - v vibrational level of the fundamental state - v vibrational level of the excited state - V_c collection volume - X_l2 iodine molar fraction - wave length - v_D Doppler linewidth, at half maximum - v_c collisional linewidth at half maximum - Vl laser frequency - (v) Dirac function - v_i spectral location of a line, referenced to the laser frequency