Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference

Transcription

1 NASA / CPm Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference ; L 2Z Proceedings of a conference sponsored by the NASA Glenn Research Center and held at the Sheraton City Centre, Cleveland, Ohio August 9-11, 2000 National Aeronautics and Space Administration Glenn Research Center December 2000

2 STABILITY AND HEAT TRANSFER CHARACTERISITCS OF CONDENSATE FLUID LAYERS IN REDUCED GRAVITY James C. Hermanson, Andreas N. Alexandrou and William W. Durgin Mechanical Engineering Depm'tment Worcester Polytechnic Institute, Worcester, MA Peder C. Pedersen Electrical and Computer Engineering Department Worcester Polytechnic Institute, Worcester, MA Jeffrey S. Allen National Center for Microgravity Research Cleveland, OH ABSTRACT The focus of this ground-based program is the study of film condensation phenomena under variable, reduced-gravity conditions. Experimental tests, combined with numerical modeling, are employed to gain an improved understanding of the fundamental fluid physics responsible for condensate film growth, film instability and the resulting interracial motion under variable gravity, and the conesponding implications for heat transfer. There has been relatively little research on the mechanisms operative at the film interface between condensed liquid and its vapor under reduced gravity conditions. Of particular interest are the stability characteristics of the condensate layer, and how they differ from those of films of comparable scale in the absence of condensation. A schematic diagram of the principal fluid phenomena expected to be operative during a typical condensation experiment when the acceleration (at a normal or reduced gravity level) is directed away from the condensing surface is shown in the figure below. In the presence of a body force Variable Gravity Vector _ I Cold Surface Saturated sation Condensation \ Surface Rate \ Tension Force (Stabilizing) Vapor e directed away from the plate, the condensate fluid layer will be unstable. This density-driven motion will be opposed by the surface tension, which will attempt to stabilize the fluid interface. In addition, the relatively higher thermal resistance associated with the thick fluid film at the crest will lead to a locally retarded condensation rate compared with that in the thin film regions in the fluid troughs. The opposite sign of the temperature gradient across a condensing film versus that in the more commonly studied heated fluid layer will likely have significant implications for thermocapillary effects. In the case of a condensing layer, thermocapillary forces are expected to have a stabilizing influence on the film surface. Consideration of the combined effects of body NASA/CP

3 force and thermocapillary forces also suggests that unique convective patterns may arise in the presence of condensation under low-gravity conditions. The experimental effort will involve three distinct experimental arrangements using identical test cells. First, lalx)ratory tests will be conducted with a steady, stable condensing film on an upwards-facing, cooled surface to validate the experimental hardware and diagnostics. In the second series of experiments, the test cell will be inverted, using the resulting body force to generate fluid instabilities in the presence of condensation. Finally, in a third series of tests, the test cell will be mounted on a rotating platform to be flown on board a parabolic-trajectory aircraft to yield levels of de-stabilizing artificial gravity ranging fi'om residual levels to approximately 0.1 g. This will allow the study of fluid phenomena in fihn condensation, such as thermocapillary effects, that are not detectable under normal gravity conditions. The dynamic response of the film layer a mechanical perturbation will also be examined. For these experiments, three distinct fluid environments will be created to reveal particular features of the fluid physics associated with condensation in reduced gravity conditions. The fil'st fluid condition is a condensing film in the presence of saturated vapor. Second, a non-condensing, cooled film sun'ounded by a non-condensing gas will be studied. In this case, thermocapillary effects will potentially be present owing to the cooling of the film, but there will be no effects of mass addition and film growth. Third, an isothermal, non-condensing film with mass addition will allow the study of the effects of film growth on layer stability in the absence of thermocapillary effects. The thermal instrumentation Will consist of standard thermocouples and heat transfer gauges. Direct photography and shadowgraph imaging will be employed to determine the features of the resulting instabilities, such as the wavelengths, surface deformation, and droplet formation and departure rates. Laser particle tracking will be employed to reveal the convective motion of the liquid film. Finally, the film thickness and growth rate (hence the condensation rate) will be determined using ultrasound gauging. The applicability of this technique has been recently demonstrated at WPI [1]. The technique can measure fluid film thicknesses as small as 50 #m, using a single transducer at a fi'equency of 10 MHz, with a lateral resolution of as little as 1 nma. Through multiplexing, multiple sites will =be monitored sequentially to provide information on the spatial uniforn-fity of the condensate film and the wavelength and amplitude of surface disturbances. Detailed numerical calculations will be performed for each configuration to model the fi!m layer growth due to condensation, the deveiopment of interfaciai disturbances, and to predict the heat transfer rate. The numerical modeling will be an extension of previous work at WPI on solidification and droplet dynamics [2]. The liquid-vapor interface will be followed exactly using the Inverse Finite Element Method, which has been shown to have the advantages and accuracy in both two-dimensional and three dimensional problems [2]. According to this approach; flae d[scretization is based on an inverse approach where the isotherms are a priori fixed and the non-linear problem is solved for the location of the discrete nodes. This method allows a "complete" solution to this free-and-moving type of problem. [1] Pedersen, P.C., Cakareski, Z., and Hermanson, J.C., "Ultrasonic Monitoring of Film Condensation," Ultrasonics VoI. 38, , [2] Alexandrou, A., "An Inverse Finite Element Method for Directly Formulated Free and Moving Boundary Problems", International Journal of Numerical Methods in Engineering Vol 28, , NASA/CP_

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10 ROLE OF THERMOCAPILLARY FORCES IN CONDENSING (COOLED) VS. HEATED FILMS Cold Ambient Warm - Low Cool- High Surface Tension Surface Tension HEATED SURFACE Thermocapillary forces for the (commonly studied) fluid layer heated from below can serve to drive convective motion. Warm Ambient Cold- Surface High Tension Warmer Surface - Low Tension V COOLED SURFACE Thermocapillary forces for a fluid layer COoled from below (current investigation) oppose fluid motion and are a stabilizing influence on the film. NASA/CP

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13 MEASURED THICKNESS OF CONDENSING FLUID FILMS Downward facing surface (-1 g) t15 r " "-...! "- "" --_;... [ : _' ',." : /_.,_ i 1 i i,, = 105 ' q ' ' "t 1" r''... "'''''... "''''''1 =L= "... " =''''iv t 4. q = i I t00... :-... I... :... f..., " - " :- I q _. i b 4 - i J e _lo... r... J... J... -p-..'l.2 J i!. T- II IIf,.! I "_- I I, l bo 0tJ TIrl_ 1$} Film thickness growth water vapor in air, Water bath temperature 75 C; cooled surface temperature 25 C. 160 ]40, E "=L -_ ]00 :,,': I I I I I I,,,, *...IFI...i...:..._ -:...!... r--')i... i... t... i...! '--:- L t3 E :... ":... o % i lo _ so Time Changes in film thickness for Water bath temperature 85 (s)! : ; i. 40 5(3 60 water vapor in air. C; cooled surface temperature 20 C. The periodic peaks were due to a fluid instability, such as droplet release or droplet motion across the measurement location. NASA/CP_

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