Investigation of the influence of gravitational body force effects on critical heat flux for flow boiling with subcooled and two-phase inlet
Space agencies worldwide are being confronted with the challenges of more distant manned space missions, which will demand greater energy efficiency and reduced weight and volume. One method being considered to reduce the weight and volume is to replace present single-phase Thermal Control Systems (TCSs) with ones that rely on flow boiling and condensation. This transition will require a thorough understanding of the influence of reduced gravity on flow boiling and condensation, and the development of predictive tools for both. The primary purpose of the present study is to investigate the impact gravitational body force effects have on flow boiling heat transfer performance and critical heat flux (CHF). Two flow boiling investigations will be presented, where experimentation was conducted both on-ground and in microgravity conditions. The terrestrial-based study explores the mechanism of flow boiling CHF for FC-72 in a rectangular channel fitted along one side with a heated wall. The flow is supplied as a two-phase mixture and the channel is tested at different orientations relative to Earth's gravity. High-speed video imaging is used to identify the complex flow boiling CHF trigger mechanism for different orientations, mass velocities and inlet qualities. It is shown that orientation has a significant influence on CHF for low mass velocities and small inlet qualities, with the orientations surrounding horizontal flow with downward-facing heated wall causing stratification of the vapor towards the heated wall and yielding very small CHF values. High mass velocities cause appreciable diminution in the influence of orientation on CHF, which is evidenced by similar flow patterns and CHF trigger mechanism regardless of orientation. The Interfacial Lift-off Model is shown to predict the influence of orientation on CHF with good accuracy. Overall, this study points to the effectiveness of high mass velocities at combating buoyancy effects and helping produce CHF values insensitive to orientation. It is also shown that the influence of orientation can be negated by simultaneously satisfying three separate criteria: overcoming the influence of gravity perpendicular to the heated wall, overcoming the influence of gravity parallel to the heated wall, and ensuring that the heated wall is sufficiently long to endure liquid contact. These criteria are combined to determine the minimum mass velocity required to negate gravity effects in both terrestrial and space applications. Exceeding this minimum is of paramount importance to space systems since it enables the implementation of the vast body of published CHF data, correlations and models developed from terrestrial studies for design of thermal management systems for space applications. This study also investigates the interfacial phenomena preceding the occurrence of CHF for flow boiling with a finite inlet vapor void. Temporal records of the heated wall temperatures are used to track the complex transient response of the heated wall, and identify differences between temperature excursions associated with momentary localized dryout and those with true CHF. It is shown that the flow enters the channel fully separated, with a liquid layer sheathing all four channel walls surrounding a central vapor core. At high heat fluxes, a wavy vapor layer begins to form beneath the liquid layer adjacent to the heated wall, and cooling is provided mostly though wetting fronts associated with the wave troughs in accordance with the Interfacial Lift-off Model. However, depending on mass velocity, inlet quality and flow orientation, conditions may arise that cause breakup of the heated wall liquid layer into ligaments that are entrained in the vapor core. This phenomenon causes localized dryout and wall temperature excursions at heat fluxes well below CHF, but the wall is able to recover from these excursions by a combination of reattachment of ligaments with the heated wall and lateral heat conduction within the wall itself. Recommendations are made concerning construction of the heated wall and CHF detection in pursuit of reliable CHF data. (Abstract shortened by UMI.)
Mudawar, Purdue University.
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