12 research outputs found
Growth of boundary-layer streaks due to free-stream turbulence
The growth of laminar boundary-layer streaks caused by free-stream turbulence
encountering a flat plate in zero-pressure-gradient conditions is investigated experimentally
in a wind tunnel and numerically by solving the unsteady boundary-region
equations. A comparative discussion amongst the most relevant theoretical frameworks,
such as the Goldstein theory, the Taylor-Stewartson theory, the optimalgrowth
theory and the Orr-Sommerfeld theory, is first presented and parallels and
complimentary aspects of the theories are pointed out to justify the use of the
Goldstein theory in our study. The statistical properties of the positive and negative
fluctuations of the laminar streaks are discussed, showing how the total time average
of the boundary-layer fluctuations masks the true character of the disturbance flow
and revealing that the maximum values and the root-mean-square of positive and
negative fluctuations grow downstream at the same rate. The downstream growth
rate of the low-frequency disturbances and the decay rate of the high-frequency
disturbances are also computed for the first time. The numerical solutions of the unsteady
boundary-region equations are compared successfully with the streak profiles
measured in the wind tunnel and with direct numerical simulation results available
in the literatur
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Turbulence structure in the viscous layer of strongly heated gas flows
For dominant forced convection with significant gas property variation, even in low Mach number flow through a circular tube, apparently the only published profile data available to guide (or test) the development of predictive turbulence models are K. R. Perkins` measurements of mean temperature structure. The work here takes the next step: the first mean velocity distributions for this situation are presented. In order to dissect the anatomy of the viscous layer in gaseous, turbulent, tube flow with strong heating, it has been probed via thermal anemometry coupled with diagnostic application of simple computational thermal fluid dynamics. Experiments for air flowing upward in a vertical circular tube were conducted for heating rates causing significant property variation. An unheated entry of fifty diameters preceded the heating. Examination emphasizes the wall region which would conventionally be expected to contain the viscous layer, if the flow were unheated. In the flow called turbulent, after being disturbed in the first few diameters by the heating profiles representing the turbulence quantities appear to recover to approximately self-preserving conditions. In the other two runs with higher heating rates, the turbulence quantities decrease after the immediate thermal entrance until they are small relative to molecular effects
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Natural convection in nonvertical wells
Convective instabilities and the shapes of the ensuing convection cells were experimentally studied for nonvertical wellbores. Steady-state temperature distributions were measured for three inclination angles over a wide range of heating rates to demonstrate the effects of drilling angle and Rayleigh number. In addition, velocities were estimated by measuring the time-of-flight of tracers formed by the Thymol blue technique. 8 refs., 6 figs
Study of Convection Heat Transfer in a Very High Temperature Reactor Flow Channel: Numerical and Experimental Results
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Three-dimensional, transient natural convection in inclined wellbores
The occurrence of natural conduction in a wellbore can affect geothermal gradient measurements and heat flow estimates. In the Hot Dry Rock geothermal concept, the wellbores are purposely inclined in the deep regions to enhance heat production. To simulate natural convection flow patterns in directionally drilled wellbores, experiments and analyses were conducted for a circular tube with length to diameter (L/D) ratio of 36 at angles of 0{degrees}, 20{degrees}, and 35{degrees} from the vertical. The tube was heated at the bottom and cooled at the top, and the insulation was adjusted so that approximately one- to two-thirds of the power dissipated was transferred through the tube wall to the surroundings. An aqueous solution of polyvinyl alcohol was employed as the working fluid in order to obtain low Rayleigh numbers corresponding to conditions in geothermal wellbores. Temperature distributions were measured for the three orientations and for several heating rates to demonstrate the effects of tube angle and Rayleigh number. Comparison with measurements showed good agreement of the predicted temperature levels for the maximum inclination and slightly poorer agreement for the other limit, a vertical tube. 50 refs., 9 figs
Nucleate boiling of low-concentration alcohol aqueous solutions for the development of thermal management systems in space
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Fundamental Thermal Fluid Physics of High Temperature Flows in Advanced Reactor Systems - Nuclear Energy Research Initiative Program Interoffice Work Order (IWO) MSF99-0254 Final Report for Period 1 August 1999 to 31 December 2002
The ultimate goal of the study is the improvement of predictive methods for safety analyses and design of advanced reactors for higher efficiency and enhanced safety and for deployable reactors for electrical power generation, process heat utilization and hydrogen generation. While key applications would be advanced gas-cooled reactors (AGCRs) using the closed Brayton cycle (CBC) for higher efficiency (such as the proposed Gas Turbine - Modular Helium Reactor (GT-MHR) of General Atomics [Neylan and Simon, 1996]), results of the proposed research should also be valuable in reactor systems with supercritical flow or superheated vapors, e.g., steam. Higher efficiency leads to lower cost/kwh and reduces life-cycle impacts of radioactive waste (by reducing waters/kwh). The outcome will also be useful for some space power and propulsion concepts and for some fusion reactor concepts as side benefits, but they are not the thrusts of the investigation. The objective of the project is to provide fundamental thermal fluid physics knowledge and measurements necessary for the development of the improved methods for the applications