Turbulence in transcritical CO2 jets

Paul J Seebald, Purdue University

Abstract

The objective of this thesis is to provide both qualitative and quantitative representation of turbulence in transcritical carbon dioxide jets. Transcritical jets in this case involve a thermodynamically supercritical fluid injected into an environment that is below the critical point of the injectant. Injection of supercritical fluids is necessary to study due to thermal transfer to fuel in high Mach engines. Length scales are a significant measure of turbulence, and experimental results are still needed to provide a reference for computational efforts. The complexity of this flow creates a unique situation to study multiple effects, such as swirl, varying density, buoyancy, and ultimately, turbulence. Time-resolved velocity data from laser Doppler velocimetry (LDA) measurements of transcritical jets were obtained. The slot correlation technique was used to calculate autocorrelation functions, spectra, and length scales from the velocity fluctuations. Integral scalar length scales were calculated from intensity fluctuations in shadowgraph images of transcritical jets. Radial length scales were obtained relative to several independent variables: two different Reynolds numbers (1.3 × 105, 2.2 × 105), three different density ratios (2.2, 2.5, and 2.8), four different swirl numbers (Sn = 0.0, 0.25, 0.50, and 1.0), and the axial distances of x/d = 10, 15, and 20. The zero-swirl jets showed similar behavior to variable-density jets and mixing layers. Turbulence was shown to progress faster in swirling flows, through faster mean velocity decay, significantly lower calculated dissipation values, as well as both velocity and scalar length scales. The core lengths for swirling jets strongly decreased from the zero-swirl jet to the swirling jets. Relationships for axial mean velocity and dissipation on the centerline of the jet were estimated for swirling flows. The range of density ratios was too small to notice significant effects, but effects of a density ratio greater than one were apparent through high density magnitude regions that inhibited transverse turbulent length scales from developing. The velocity length scales did not exhibit a clear analogue for the scalar integral length scales. Guidelines for transcritical injector design, as well as suggestions for experimental and numerical future work were outlined.

Degree

Ph.D.

Advisors

Sojka, Purdue University.

Subject Area

Mechanical engineering

COinS