Download Free Examining The Effects Of A Controled Turbulent Velocity Field On A Passive Scalar Book in PDF and EPUB Free Download. You can read online Examining The Effects Of A Controled Turbulent Velocity Field On A Passive Scalar and write the review.

The study of passive scalars in turbulence research is a technologically important topic. This study seeks to investigate the effect of the variation of integral length scale, and scalar injector size and separation on turbulent mixing. The study is motivated by devices such as the simple gas turbine pre-mixer where these three relevant length scales are present. Heated wakes of different sizes and separations are produced by heating various sets of thin wires in a horizontal wire grid. The horizontal wire screen is placed downstream of biplane grids of different mesh sizes. Time-resolved velocity and temperature are measured by a dual hot and cold wire probe. Assessments of the effect of the array of thin wires used as the scalar generator on the velocity and scalar fields behind the turbulence grid are presented. The evolution of the profiles of the first four statistical moments of temperature fluctuations across the wake with downstream distance is studied for various scalar injector size and separation for several integral length scales. Specifically, the downstream evolution of a single thermal wake is studied for source sizes that range from less than the Kolmogorov to larger than the integral length scale. Also included in the study the downstream evolution and interaction of two thermal wakes is also presented. For the single wake, the wake growth, sufficiently far from the generator, for all source sizes is found to be consistent with the assumption that the effective diffusivity is constant across the wake. The evolution of the effective diffusivity with downstream distance is found to be independent of the source size and identical to the evolution of the effective diffusivity of a line source in the same velocity field. The effects of changing the velocity integral scale are governed by the effect of the integral scale on the evolution of the effective turbulent diffusivity. Those effects are found to be similar in character to the effect on a line source. The mean scalar of two sources is found to be the sum of the mean of the individual sources. Higher statistical moments show more complex relationships.
"The effect of background turbulence on the scalar field of an axisymmetric turbulent jet is investigated experimentally. The present investigation builds on the work of Gaskin et al. (2004), who studied the concentration and velocity fields of a plane jet in a shallow coflow with different turbulence levels and Khorsandi et al. (2013), who studied the velocity field of an axisymmetric turbulent jet emitted into a turbulent background. Different driving algorithms for a large RJA were tested and the statistics of the turbulence generated downstream of the RJA were compared to characterize the algorithms' performance. Variations in the spatial configuration of jets operating at any given instant, as well as in the statistics of their on/off times were studied. The algorithm identified as RANDOM generated the closest approximation of zero-mean-flow homogeneous isotropic turbulence. The flow generated by the RANDOM algorithm had a relatively high turbulent Reynolds number (ReT = uTl/[nu] = 2360, where uT is a characteristic RMS velocity, l is the integral length scale of the flow, [nu] is the kinematic viscosity of the water) and the integral length scale (l = 11.6 cm) is the largest reported to date. Thus, RANDOM algorithm was used to generate the background turbulence for the investigation of scalar mixing within a turbulent jet.The effect of background turbulence on the mixing of a passive scalar within a turbulent jet at different Reynolds numbers was investigated. To this end, planar laser-induced fluorescence was employed to obtain concentration measurements of dye (disodium fluorescein, Schmidt number = 2000) within the jet. Two jet Reynolds numbers (Re=UjD/[nu], where Uj is the jet exit velocity, D is the nozzle diameter and [nu] is the kinematic viscosity of the jet fluid, water) were studied: 10600 and 5800. The resulting statistics of the scalar fields showed that the mean concentrations of jets emitted into turbulent backgrounds were lower than those of jets emitted into a quiescent background near the centerline. However, near the edges of the jet (r/x>0.15), the concentrations were higher for the jets issued into turbulent surroundings. The RMS concentrations of the jet emitted into a turbulent background significantly increased. Examination of the probability density functions of concentration revealed a higher degree of intermittency of the scalar field. The probability of low concentrations increased in the presence of background turbulence although the maximum concentrations were comparable to those of the jet emitted into a quiescent background. Flow visualizations revealed meandering of the jet issued into background turbulence, which is associated with the increased probability of lower concentrations and higher intermittency. Additionally, the widths of the jets emitted into a turbulent background were increased. For the lower jet Reynolds number, the described effects were more evident and the jet structure was destroyed by the background turbulence within the measurement region, resulting in flat radial profiles of both the mean and RMS concentrations. Comparison of the results of the scalar field with those of the hydrodynamic jet of Khorsandi et al. (2013) revealed a similar behavior of the two fields. However, the most significant difference was the larger radial extent of the profiles of mean and RMS concentrations, which resulted from the meandering of the jet and increased transport of scalar by turbulent diffusion. The flow visualizations suggest that the entrainment and mixing in the jet in a turbulent background changes with the destruction of jet structure, from jet driven entrainment to become potentially dominated by i) increased lateral advection of the jet by large scales of the background turbulence during the meandering of the jet, which is subsequently mixed by its smaller scales, and ii) turbulent diffusion that is significantly enhanced by the turbulent background." --
Due to the requirement for enhanced cooling technologies on modern gas turbine engines, advanced research and development has had to take place in field of thermal engineering. Among the gas turbine cooling technologies, impingement jet cooling is one of the most effective in terms of cooling effectiveness, manufacturability and cost. The chapters contained in this book describe research on state-of-the-art and advanced cooling technologies that have been developed, or that are being researched, with a variety of approaches from theoretical, experimental, and CFD studies. The authors of the chapters have been selected from some of the most active researchers and scientists on the subject. This is the first to book published on the topics of gas turbines and heat transfer to focus on impingement cooling alone.