Richard K. Cohn

Cryogenic shear layers: experiments and phenomenological modeling of the initial growth rate under subcritical and supercritical conditions

A jet of a cryogenic fluid, typically liquid N2, is injected into a chamber whose ambient pressure is varied to values exceeding the critical pressure of the injectant. The structure of the jet and the shear layer between the jet and the ambient have been examined. Results from visualization, jet initial growth rate, fractal analysis, and Raman scattering measurements indicate that the behavior of the injected fluid changes from liquid spray-like to gaseous jet-like behavior as pressure increased. This is attributed to the reduction of the surface tension and enthalpy of vaporization as the critical pressure of the injectant is approached. The initial divergence angle indicating the growth rate of the jet is measured at the jet exit. These values are then compared with those measured from a large number of other mixing layer flows, including atomized liquid sprays, turbulent incompressible gaseous jets, supersonic jets, and incompressible but variable density jets covering over four orders of magnitude in the gas-to-liquid density ratio, the first time such a plot has been reported over this large a range of density ratios. At and above the critical pressure of the injected fluid, the jet initial growth rate measurements agrees well with the theory and measurements of incompressible, variable density, gaseous mixing layers. This is the first time a quantitative parameter has been used to demonstrate that the similarity between the two flows extends beyond mere qualitative physical appearance. The initial growth rate using Raman scattering is also in reasonably good agreement with our measurements using shadowgraphy if twice the FWHM of the normalized intensity radial profiles are used. Finally, an equation based on a proposed physical mechanism combined with the characteristic gasification time (τg) and interfacial bulge formation/separation time (τb) is proposed, θ=0.27[τb/(τb+τg)+(ρg/ρl)0.5], that shows good agreement with the measured initial growth rate data. It is found that the transition point from sub- (liquid-jet like) to supercritical (gas-jet like) behavior occurs when the time scale ratio (τb/(τb+τg)) is approximately equal to 0.5.

Simultaneous whole-field measurements of velocity and concentration fields using a combination of MTV and LIF

A new technique for the simultaneous measurement of velocity and concentration fields is described. We describe here applications in liquid-phase flows, but the methodology can be extended to gas-phase flows with appropriate tracers. In this single-laser, two-tracer approach, molecular tagging velocimetry (MTV) based on the use of a phosphorescent compound is combined with laser induced fluorescence (LIF) using fluorescein as a tracer. Results show that one can design experiments with minimal cross-talk between the LIF and MTV signals. Applications of the simultaneous MTV-LIF technique are demonstrated by performing simultaneous flow visualization and vorticity measurements in a low Reynolds number forced wake and simultaneous velocity-concentration measurements in a turbulent mixing layer. Preliminary data on the mean and RMS fluctuation of velocity and concentration are presented, together with the correlation between velocity and concentration fluctuations.

Raman Scattering Measurement in the Initial Region of Sub- and Supercritical Jets

Abstract : A high-pressure chamber is used to investigate and further enhance our knowledge and physical understanding on effects of thermodynamical subcritical-to-supercritical transition of ambient condition on cryogenic liquid injection using two-dimensional scattering. Pure liquid N2 is injected into N2. The injector is a 508-micron diameter straight hole having a long length-to-diameter ratio of 100. The optical setup uses a pulsed Nd:Yag laser frequency-doubled to 532 nm. Difficulties arise with optical breakdown of the N2 molecules in drops and ligaments by local focusing of the laser beam dominating the Raman signal particularly at sub- and near-critical regions. The severity of this problem is reduced by stretching the laser pulse width using a double-loop design with mirrors and beam splitters. Careful and painstaking alignment is needed to take advantage of this pulse-stretcher design. Two-dimensional images are taken near the injector and results interpreted in terms of density plots. At subcritical ambient conditions a small number of images are needed for averaging and strong Raman signal is obtained.

Effect of boundary conditions on axial flow in a concentrated vortex core

The character of axial flow in the core of a concentrated line vortex is studied using flow visualization. Results show that axial flow can be initiated when the no‐slip boundary condition is imposed on the vortex core over a spatial extent comparable to the core diameter. It is further demonstrated that no‐slip is not a necessary condition to generate the axial flow; a region of spanwise shear (i.e., a slip boundary) is sufficient. Two distinct axial flow velocity profiles are observed; they are either jetlike or spatially undulated corresponding to a maximum axial speed occurring at the core center or outer edge, respectively.