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Combustion measurements using laser diagnostics are vital for experimentally revealing both qualitative and quantitative flame characteristics. This paper focuses on the application of the quantitative OH laser-induced fluorescence (OH-LIF) method for high-pressure and high-temperature flames. Under high-pressure conditions, the intense OH chemiluminescence drastically decreases the signal to noise ratio (S/N). However, OH(2,0) band excitation is an alternative method to the typical OH(0,0) or OH(1,0) band excitation LIF under high-pressures. OH(2,0) band LIF sufficiently maintains the S/N under high-pressure because the OH fluorescence can be detected away from the peak OH chemiluminescence wavelength. In addition, the bi-directional LIF method is known as a quantitative OH concentration measurement that can ignore the quenching factor. Therefore, this study proposed a quantitative OH concentration measurement by combining the OH(2,0) band LIF and bidirectional LIF methods for high-pressure and high-temperature flames. First, an H2/air premixed flame created by a McKenna burner under atmospheric pressure was used to validate the quantitative OH(2,0) band bidirectional LIF method. Furthermore, a CH4/O2/N2 oxygen-enriched premixed flame created by a multi-hole calibration burner was used to verify the method under high-pressure (~0.5 MPa) and high temperature (~2900 K) conditions. A comparison of the experimental and numerical results revealed that the calculated OH concentrations were within the experimental uncertainty for the McKenna burner configuration. The experimental OH concentration results under high-pressure conditions deviated approximately 10%–20%, but they were also in good correlation with the calculation. In addition, the qualitative correlations with equivalence ratio and pressure variation were also acceptable. Although further consideration of the method is desirable for high-pressure turbulent flames, the results in this study showed that the OH(2,0) band bi-directional LIF method is feasible for measuring OH concentration in high-pressure and high-temperature flames.

This study focuses on the application of OH planar laser-induced fluorescence (OH-PLIF) in high-pressure rocket combustion conditions, up to 7.0 MPa. The signal to noise ratio of PLIF degrades in high-pressure combustion owing to effects such as line broadening and interference from intense chemiluminescence. The OH(2,0) band excitation method was applied to obtain the OH(2,1) fluorescence emitted near 290 nm and filter out the intense OH(0,0) band chemiluminescence emitted near 308 nm. The gaseous H2/O2 (GH2/GO2) jet diffusion flame was formed using a recessed coaxial shear injector. The GH2/GO2 injection Reynolds number, Re (ReH2/ReO2 ≈ 2320/22800–4660/45600), was varied to examine the variation of the flame structure and reaction zone thickness under each pressure condition Pc, and Re injection condition. In addition, the variation of the experimentally derived full width at half maximum (FWHM) of the radial OH distribution, δOH, with the Damköehler number, Da, was compared with that of the simulated FWHM of the OH mole fraction, δOH-SIM. The OH distribution was clearly observed in the instantaneous PLIF image while eliminating the intense OH chemiluminescence even in the highest pressure condition of 7.0 MPa, which is a pressure higher than any of the previous OH-PLIF studies conducted on rocket combustion. The flame structure showed the typical characteristics of a turbulent jet diffusion flame and depended on Re rather than on the chamber pressure Pc. The variation of δOH with Da corresponded qualitatively with δOH-SIM and showed the characteristics of flame stretch in the vicinity of the injector.

High temperature flames, which can be produced via oxygen-enriched combustion, potentially have improved the combustion characteristics over air-only combustion flames due to their substantially higher flame temperatures. These conditions necessitate the use of non-intrusive optical measuring methods to measure the temperature and the chemical species in the flame. To develop an optical measurement calibration burner that can be used at high pressure and temperature conditions, a new calibration burner which employed water-cooled multi-hole nozzle was developed in this study. Premixed CH4/O2/N2 oxygen-enriched conditions were selected to investigate both the heat-resisting properties of the developed burner nozzle and the burner’s flame characteristics. OH-Planar Laser-Induced Fluorescence (OH-PLIF) measurements were conducted on the flames to observe the OH distributions. The flame temperature at 0.10 MPa was derived using a Boltzmann-plot for the OH fluorescence excitations. To verify the variation of molecular concentration with equivalence ratio for the experimental flames qualitatively, the experimentally acquired OH and CH chemiluminescence intensities were compared with the simulated partial pressure of OH* and CH*, respectively. Experimental results showed that the CH4/O2/N2 flames were stabilized on the burner nozzle in a wide range of oxygen-enrichment ratio, from 0.40 to 1.0 at atmospheric pressure. At an oxygen-enrichment ratio of 0.45, the flames were also stabilized in pressure conditions up to 0.49 MPa, while the inner nozzle temperature was lower than 400 K. The OH-PLIF images showed that the OH was distributed almost uniformly along the axial direction of the burner, and demonstrated similar characteristics to that of a flat flame. The derived maximum flame temperature at atmospheric pressure was approximately 2650 K at an oxygen-enrichment ratio of 0.80. The variation of the OH and CH chemiluminescence intensities with change of equivalence ratios corresponded roughly with the simulated partial pressures of OH* and CH* at each pressure condition.

Measurement technique that can be adapted to high temperature and pressure environment in the rocket combustion chamber has been limited to qualitative. Therefore, establishment of a quantitative measurement technique of rocket combustion is demanded. Laser Induced Fluorescence (LIF) method targeting OH radical is the most promising measurement technique on rocket combustion. In the present study, we explored the temperature measurement method by LIF with promising OH(2,0) band measurement for high-pressure environment. The experiment was performed at atmospheric pressure for the verification test. In the present study, Laser Induced Fluorescence Spectroscopy (LIFS) temperature measurement using overlapping excitation line was found to feasible by applying the theory of Planer Laser Induced Fluorescence (PLIF)-2 line method. In addition, the LIFS measurement was found to be advantageous for measurement in high-pressure environment as it allowed discrimination from chemiluminescence.

This study focuses on the measurement of the flame base in a gaseous H2/O2 coaxial jet diffusion flame under highpressure (1.0--5.0 MPa) rocket combustion conditions. OH(2,0) band excitation was utilized for OH-planar laser-induced fluorescence (OH-PLIF) measurement to obtain the flame cross-section. The injection Reynolds number was varied under each pressure to examine the flame structure variation. Numerical simulation was also conducted to visualize the injector vicinity flow field. Results showed that the OH-PLIF measurements had sufficient signal intensity for evaluation. An analysis of the OH-PLIF images revealed variations in the flame thickness and the position of the flame edge under each condition. A numerical simulation of the cold flow was also performed to aid analysis of the OH-PLIF results. [ABSTRACT FROM AUTHOR]

The purpose of this study was to develop an advanced method to measure the properties of rocket combustion using the OH(2,0) band-excited Planer Laser-induced Fluorescence (OH-PLIF) method. This diagnostic method was applied to capture images of the high-pressure H2/O2 jet diffusion flames found in typical liquefied bi-propellant rocket combustion. In addition, axisymmetric numerical simulations of H2/O2 jet flames modeling the experimental conditions were conducted to evaluate the consistency of the OH-PLIF imaging results and to predict the OH chemiluminescence intensity and flame temperature at high pressure. Experimental results show that it is possible to detect the OH(2,1) band fluorescence effectively by eliminating the interference of OH(0,0)-band chemiluminescence under high-pressure conditions of up to 2.0 MPa. The OH fluorescence signal distributed near the injector face almost corresponded to the OH molar concentration distributions simulated by numerical simulations. Moreover, the simulated pressure dependence of the local OH* peak mole concentration reasonably corresponded to that of the local peak chemiluminescence intensity of the experimental chemiluminescence images. [ABSTRACT FROM AUTHOR]