Temperature measurements in hypervelocity flows using thermally-assisted laser-induced fluorescence

Sopek, Tamara (2018). Temperature measurements in hypervelocity flows using thermally-assisted laser-induced fluorescence PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2018.206

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Author Sopek, Tamara
Thesis Title Temperature measurements in hypervelocity flows using thermally-assisted laser-induced fluorescence
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2018.206
Publication date 2018-02-27
Thesis type PhD Thesis
Supervisor Tim McIntyre
Richard Morgan
Stefan Brieschenk
Language eng
Subjects 0901 Aerospace Engineering
0205 Optical Physics
Formatted abstract
Scramjet engines with their very high specific impulse have the potential to outperform rocket motors as a means of propelling hypersonic vehicles. The scope of this project is to determine temperatures in the scramjet combustor using an advanced optical diagnostic technique, namely laser-induced fluorescence. Presently, direct temperature measurements are not readily available for scramjets, due to three reasons. Firstly, commonly-used gauges for temperature measurements, such as thermocouples and heat transfer gauges would not survive or would be damaged at the extreme temperatures typically occurring in scramjets (around T = 1100 - 2500 K), and therefore these gauges would not give accurate readings. Secondly, measurements with any gauges would be intrusive, and their presence in the flow might cause large disturbances. Thirdly, these gauges can only provide discrete, point measurements. Laser-induced fluorescence measurements present an optical non-intrusive technique to resolve temperatures in the combustor. Additionally, LIF measurements allow the temperatures in the combustor to be resolved at a higher spatial resolution than regular gauges. The new contribution made by this PhD project is the application of a sophisticated optical diagnostic technique, thermally-assisted laser-induced fluorescence, to the temperature measurements in the combustor of a supersonic combustion engine. The issues with the technique and the way it should be used for application to scramjet flows were explored. This required the development of the new knowledge and understanding of the spectroscopy that underpins the technique. The project is significant because, to the author’s knowledge, this is the first time thermally-assisted LIF has been used for scramjet flows in a test facility. Currently available thermometry methods usually require averaged repeated experiments, thus causing high cost of experiments. This study represents successful single-shot scramjet temperature measurements. The project was approached theoretically (synthetic spectra), experimentally and numerically (CFD and LIF modelling).

Radiation simulation programs providing synthetic OH spectra represent the theoretical part of this study, as they employ theoretical expressions for spectral calculations. The OH radical is an intermediate species in the combustion, created in high quantities, therefore allowing direct fluorescence measurements. In this work, three numerical codes were used - LIFBASE, SPARTAN and Photaura. SPARTAN and Photaura were modified in order to include the OH molecule. The results presented here show that OH spectra produced using these programs show very good agreement. All three programs give correct and reliable representation of the physical phenomena in the wavelength range studied.

Experimental data was obtained through laser-induced fluorescence with a laser beam focused into the combustor exciting an OH molecule transition. The experiments were conducted in the T4 shock tunnel using a scramjet model. LIF measurements were performed to resolve temperatures in the combustor.

The numerical aspect presented in the project are the results of Bricalli (2015), in the form of the computational fluid dynamics (CFD) simulations of the combustion process, which provided the indication of the phenomena occurring in the flow studied. Additionally, these results presented a valuable comparison to experimental temperature distribution.

Detailed numerical modelling which simulates all radiative and collisional processes of relevance with the appropriate system of differential equations is required to accurately calculate the total molecular concentration and temperature from the observed fluorescence signal. Such numerical modelling, especially for the unsteady-state case, requires accurate knowledge on electronic quenching as well as information on vibrational and rotational energy transfer. Calculations of this type are necessary for analysis of the influence of energy transfer processes on the fluorescence signal. Thus, a detailed numerical model was developed for purpose of such analysis.

The results of all aspects of the project were compared to deduce the accuracy and reliability of the experimental results and the experimental technique. Comparison of the experimental spectra with the three synthetic spectra showed that the OH concentrations were only partially thermalised. Therefore, non-equilibrium spectral simulations had to be used. By evaluating the scramjet LIF spectra with a full spectral fit, a temperature distribution across the combustor width was obtained. The comparison of this temperature distribution with the CFD results showed a reasonable agreement between the experiments and CFD for one side of the scramjet, while the other side showed a significant discrepancy between the data. As both LIF and pressure experimental data indicated combustion in this region, while the CFD results showed no combustion occurrence, the CFD results were found to be an inadequate representation of the combustion in the used scramjet. The population distributions achieved with numerical modelling of the LIF process validated the approach used to deduce temperatures.

This study produced a novel method for temperatures measurements in the scramjet combustor. The availability of a thermometry technique that provides temperature from a single run of a test facility is especially appealing considering the significant per-shot cost of running high enthalpy facilities. The outcome of the project brings new resources which can be further used in advancing the technology of operational scramjet engines for hypersonic vehicles.
Keyword Laser-induced
Shock tunnel
Supersonic combustion
Air-breathing propulsion

Document type: Thesis
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Created: Fri, 23 Feb 2018, 11:27:53 EST by Tamara Sopek on behalf of Learning and Research Services (UQ Library)