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Two-phase flow instability and dryout in parallel channels in natural circulation

Description: The unique feature of parallel channel flows is that the pressure drop or driving head for the flow is maintained constant across any given channel by the flow in all the others, or by having a large downcomer or bypass in a natural circulation loop. This boundary condition is common in all heat exchangers, reactor cores and boilers, it is well known that the two-phase flow in parallel channels can exhibit both so-called static and dynamic instability. This leads to the question of the separability of the flow and pressure drop boundary conditions in the study of stability and dryout. For the areas of practical interest, the flow can be considered as incompressible. The dynamic instability is characterized by density (kinematic) or continuity waves, and the static instability by inertial (pressure drop) or manometric escalations. The static has been considered to be the zero-frequency or lowest mode of the dynamic case. We briefly review the status of the existing literature on both parallel channel static and dynamic instability, and the latest developments in theory and experiment. The difference between the two derivations lies in the retention of the time-dependent terms in the conservation equations. The effects and impact of design options are also discussed. Since dryout in parallel systems follows instability, it has been traditional to determine the dryout power for a parallel channel by testing a single channel with a given (inlet) flow boundary condition without particular regard for the pressure drop. Thus all modern dryout correlations are based on constant or fixed flow tests, a so-called hard inlet, and subchannel and multiple bundle effects are corrected for separately. We review the thinking that lead to this approach, and suggest that for all multiple channel and natural circulation systems close attention should be paid to the actual (untested) pressure ...
Date: June 1, 1993
Creator: Duffey, Romney B.; Rohatgi, U. S. & Hughes, E. Daniel
Partner: UNT Libraries Government Documents Department

Liquid jet impingement cooling with diamond substrates for extremely high heat flux applications

Description: The combination of impinging jets and diamond substrates may provide an effective solution to a class of extremely high heat flux problems in which very localized heat loads must be removed. Some potential applications include the cooling of high-heat-load components in synchrotron x-ray, fusion, and semiconductor laser systems. Impinging liquid jets are a very effective vehicle for removing high heat fluxes. The liquid supply arrangement is relatively simple, and low thermal resistances can be routinely achieved. A jet`s cooling ability is a strong function of the size of the cooled area relative to the jet diameter. For relatively large area targets, the critical heat fluxes can approach 20 W/mm{sup 2}. In this situation, burnout usually originates at the outer edge of the cooled region as increasing heat flux inhibits the liquid supply. Limitations from liquid supply are minimized when heating is restricted to the jet stagnation zone. The high stagnation pressure and high velocity gradients appear to suppress critical flux phenomena, and fluxes of up to 400 W/mm{sup 2} have been reached without evidence of burnout. Instead, the restrictions on heat flux are closely related to properties of the cooled target. Target properties become an issue owing to the large temperatures and large temperature gradients that accompany heat fluxes over 100 W/mm{sup 2}. These conditions necessitate a target with both high thermal conductivity to prevent excessive temperatures and good mechanical properties to prevent mechanical failures. Recent developments in synthetic diamond technology present a possible solution to some of the solid-side constraints on heat flux. Polycrystalline diamond foils can now be produced by chemical vapor deposition in reasonable quantity and at reasonable cost. Synthetic single crystal diamonds as large as 1 cm{sup 2} are also available.
Date: September 1, 1993
Creator: Lienhard, J. H. V & Khounsary, A. M.
Partner: UNT Libraries Government Documents Department

Freeze-out and the failure of Richtmyer`s prescription

Description: In the standard Richtmyer-Meshkov instability, perturbations at a shocked interface grow after the passage of a shock. Freeze-out refers to the phenomenon whereby the perturbations do not grow, i.e., freeze-out, after the passage of a shock. This is fairly straightforward, at least theoretically (no experiments have been done so far) in a doubly shocked system. The first shock induces a growth which can be completely neutralized by a second shock, provided that the direction and the strength and timing of the second shock are properly chosen. This type of double-shock freeze-out occurs in compressible as well as incompressible fluids, and is easy to understand. Somewhat more subtle is single-shock freeze-out; in the pursuit of this phenomenon, the author found that in certain cases Richtmyer`s prescription fails to give the correct growth rate.
Date: April 23, 1993
Creator: Mikaelian, K. O.
Partner: UNT Libraries Government Documents Department

Retained gas sampler visualization guide

Description: In a series of experiments performed in Phase II of the retained gas sampler visualization task, the effect of sampler tip geometry on waste sampling process has been investigated. From flow visualizations, which were captured on video, it is clear that disturbances on the surrounding fluid and the fluid entering the sampler were reduced as the tip changed from a flat to a sharper truncated cone shape. It has been shown, throughout this report, that deformation and disturbance of the waste is dominated by shape of the sampler tip, which moves the stagnation point, and not by viscosity of the fluid or sampling rate.
Date: September 1, 1994
Creator: Shekarriz, A.
Partner: UNT Libraries Government Documents Department

Stabilizing S.P.H. with conservative smoothing

Description: There is an instability in certain S.P.H. (Smoothed Particle Hydrodynamics method) material dynamics computations. Evidence from analyses and experiments suggests that the instabilities in S.P.H. are not removable with artificial viscosities. However, the analysis shows that a type of conservative smoothing does remove the instability. Also, numerical experiments, on certain test problems, show that SPHCS, and S.P.H. code with conservative smoothing, compares well in accuracy with computations based on the von Neumann-Richtmyer method.
Date: August 1, 1994
Creator: Wen, Y.; Hicks, D. L. & Swegle, J. W.
Partner: UNT Libraries Government Documents Department

Analysis of a High Velocity Oxygen-Fuel (HVOF) thermal spray torch. Part 2, Computational results

Description: The fluid dynamics inside and outside a High Velocity Oxygen-Fuel (HVOF) torch are analyzed using computational fluid dynamic (CFD) techniques. The thermal spray device analyzed is similar to a Metco Diamond Jet torch with powder injection. The spray nozzle is axisymmetric with powder injected on the centerline, premixed fuel and oxygen fed from an annulus, and air cooling injected along the interior surface of the aircap choked flow conditions occur at the exit of the aircap and a supersonic, under-expanded jet develops externally. The details of the CFD simulation are given in a companion paper. This paper describes the general gas dynamic features of HVOF spraying and then gives a detailed discussion of the computational predictions of the present analysis. The gas velocity, temperature, pressure and Mach number distributions are presented for various locations inside and outside the torch. Characteristics of the metal spray particle velocity, temperature, Mach number, trajectory, and phase state (solid or liquid) are also presented and discussed. Extensive numerical flow visualization is provided to show flow features such as mixing layers, shock waves, and expansion waves.
Date: December 31, 1993
Creator: Oberkampf, W. L. & Talpallikar, M.
Partner: UNT Libraries Government Documents Department

Microchannel flow boiling mechanisms leading to burnout

Description: The boiling mechanisms for microchannel flow are investigated when the channel cross-section in height to width is large (of order 10/1), near its single-phase optimum. A separated flow model was developed which allowed for saturated boiling near the heated base and single-phase flow elsewhere within the channel cross-section. In these high aspect ratio heat sinks, the role of subcooled boiling was found to be insignificant relative to that of saturated boiling, the latter allowing for a doubling of the applied heat load from single-phase operation before burnout was experienced. As the exit mass quality of the saturated region approached one for increasing heat flux, both the model and the experimental case indicated a burnout condition had also been approached. The model underpredicted the measured base temperature, which has been generally noted for saturated boiling in annular two-phase flow.
Date: March 1, 1994
Creator: Landram, C. S.
Partner: UNT Libraries Government Documents Department