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A study of scalar mixing in gas phase turbulent jets using high repetition rate imaging

Papageorge, Michael
http://rave.ohiolink.edu/etdc/view?acc_num=osu1482094751398442

Abstract Details

2017, Doctor of Philosophy, Ohio State University, Mechanical Engineering.
In this dissertation, high-speed mixture fraction (a conserved flow scalar) and velocity measurements were performed to understand the linked spatio-temporal dynamics of scalar mixing processes in gas-phase turbulent jets. The current research focused on four over-arching topics: (i) design and construction of the high energy pulse burst laser system (HEPBLS), (ii) experimental verification of statistical convergence theory for time-series measurements in turbulent flows and the development of a new "multi-burst" data processing methodology for "short duration" time-series measurements, (iii) development and application of high-speed (10 kHz) two-dimensional mixture fraction imaging for spatio-temporal statistical analysis of scalar mixing, and (iv) development and application of simultaneous high-speed velocity and mixture fraction measurements to understand the space-time coupling between velocity and scalar fluctuations. The high-speed measurements presented in this dissertation were facilitated through the design and construction of a new high-energy pulse burst laser system (HEPBLS). The design target of the HEPBLS was ultra-high pulse energy output at high repetition rates for turbulence and combustion imaging diagnostics. Several modifications were made to the original pulse burst concept leading to ultra-high laser pulse energies (> 1 Joule/pulse at 532 nm and 10 kHz) over long burst durations (> 20 ms). The high laser pulse energies enable high-fidelity two-dimensional mixture fraction measurements at 10's of kHz using planar Rayleigh scattering and two-line mixture fraction imaging using spontaneous Raman scattering. While the primary imaging diagnostics used the second-harmonic (532 nm) output from the HEPBLS, it is noted that high-energy ultra-violet (266, 355 nm) output was demonstrated as well. The result is a flexible system capable of facilitating a wide range of laser diagnostic techniques at 10's of kHz that have not been available previously. Convergence of turbulent flow statistics from finite-record length time-series measurements was examined via theory and experiment. Analytical solutions of the convergence of statistical moments and correlation functions were developed and experimentally verified for the first time, providing a practitioner a method for accurately estimating the uncertainty of a measurement for a given record length. In addition, a new "multi-burst" data processing method was proposed (and experimentally validated) based on combined ensemble and time-series statistics specifically targeted for shorter-duration time-series measurements characteristic of data acquired using the HEPBLS. A subtle, but important, result was that the primary factor governing statistical convergence was the total amount of data and not the exact manner (i.e., record length or number of individual time series) in which it was collected. In this manner, a large number of short-duration time-series measurements can be acquired and achieve high statistical convergence. Scalar mixing dynamics was first examined using high-speed (10 kHz) planar Rayleigh scattering imaging in a series of turbulent propane jets. In this manner, quantitative two-dimensional measurements of the mixture fraction field were collected for jets with Reynolds numbers of Red=10000, 20000, and 30000 with high signal-to-noise ratios (60 < SNR < 200). The integral length and time scales were calculated via correlation functions across the full range of Reynolds numbers and as function of axial and radial position. The radial dependence of the integral scales was shown to be strongly affected by the increasing intermittency of the turbulence with increasing radial location. Without accounting for local intermittency effects, the integral time scales were overestimated by as much as a factor of three. The results also showed that Taylor's hypothesis, a common Galilean transformation between space and time, properly predicts the functional relationship between the integral length and time scales, but does not allow for a quantitative transformation. A recently proposed "elliptical" model for transformation of correlation functions between space and time was found to be more accurate. The current work demonstrates the accuracy of the elliptical model in turbulent free shear flows for the first time. Subsequently, the model was used to help understand the relationship between the scalar fluctuations and turbulent velocity field. Results showed that the decorrelation of scalar fluctuations is governed by both convection and turbulent velocity fluctuations. Simultaneous 10 kHz mixture fraction and velocity measurements were performed using two-line spontaneous Raman scattering and particle imaging velocimetry (PIV). Of particular note from a diagnostic standpoint, is that the presence of the PIV seed particles was found to have a negligible influence on the accuracy of the scalar measurements. A qualitative analysis of observed scalar and velocity features show that the majority of the time the axial velocity component and the mixture fraction are highly correlated, but there are distinct periods in which the two fields appear to be relatively uncorrelated or even anti-correlated. Quantitative analysis of statistical metrics including autocorrelation functions, two-point temporal correlation functions, and temporal scalar-velocity cross correlation were performed. The results show that scalar fluctuations and axial velocity fluctuations are highly correlated with a peak correlation coefficient of 0.6 that occurred at zero time lag indicating that the velocity and scalar fluctuations are in phase. Overall, it is concluded that scalar mixing within gas-phase jets is predominantly a passive process and largely controlled by the local axial velocity fluctuations.
Jeffrey Sutton (Advisor)
Mohammad Samimy (Committee Member)
Datta Gaitonde (Committee Member)
Gregory James (Committee Member)
352 p.
Mechanical Engineering
Turbulence; Turbulent Jet; Round Jet;

Recommended Citations

Citations

  • Papageorge, M. (2017). A study of scalar mixing in gas phase turbulent jets using high repetition rate imaging [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1482094751398442

    APA Style (7th edition)

  • Papageorge, Michael. A study of scalar mixing in gas phase turbulent jets using high repetition rate imaging. 2017. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1482094751398442.

    MLA Style (8th edition)

  • Papageorge, Michael. "A study of scalar mixing in gas phase turbulent jets using high repetition rate imaging." Doctoral dissertation, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1482094751398442

    Chicago Manual of Style (17th edition)

osu1482094751398442
289
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This open access ETD is published by The Ohio State University and OhioLINK.