Two applications of the instrument are discussed below.

Two-Phase Flow Behavior in Spacecraft Thermal Management Systems.
Vapor/liquid, two-phase flow regimes have been well characterized at normal Earth gravity and in microgravity situations using air and water, but much remains to be learned. In the past, NASA has relied upon single-phase, liquid cooling systems for spacecraft heat dissipation, but the reduced system mass possible with two-phase systems fuels the need to better understand two-phase flow behavior in space. Up to now, obtaining quantified void fraction information has dictated undertaking simulations using water and air at low gas densities. However, the larger gas densities encountered using ammonia systems leave legitimate questions unanswered about slug formation and annular flow behavior that may not be accurately addressed by the application of unqualified scaling functions.

Several prototype void fraction instruments were built at various sizes for ground testing and testing onboard NASA’s microgravity aircraft. Additional space-qualified instruments were fabricated and delivered to NASA for a large Space Shuttle experiment.

Instruments at 12.7 mm ID were installed in a two-phase flow test facility managed by Texas A&M University. Creare engineers performed extensive experiments aboard NASA’s microgravity aircraft, during 11 flights, experiencing approximately 400 parabolic arcs, to produce periods of reduced gravity. Creare obtained detailed void fraction, liquid film thickness, pressure drop, and flow rate measurements; investigated the annular flow regime, the slug flow regime, and the transition between them at over 100 microgravity test conditions; and examined and characterized the scaling of multiphase behavior through analysis of data, comparison with mechanistic analytical models, and comparison with previously obtained data. The results extend our knowledge of fluid behavior in space and should enable the confident design of multiphase cooling systems for microgravity environments.

Two-Phase Flow Behavior in Packed-Bed Reactors.
Life support systems for future lunar and planetary missions involving human crews will utilize regenerative physical/chemical technologies to minimize the quantity of make-up gas and water required. Packed-bed reactors and related devices are a common component in many anticipated configurations. Results of Space Shuttle flight experiments, as well as experiments on microgravity aircraft, have shown that many aspects of two-phase flow in packed-bed reactors remain poorly understood.

To address the need for fundamental void fraction information within packed beds under realistic conditions, Creare developed a capacitive void fraction instrument for use with packed-bed reactors operating with water. A high operating frequency was selected for this sensor configuration to mitigate the impact of water’s electrical conductivity. This capacitive-based instrument addressed the substantial limitations associated with other approaches based on optical, electrical conductivity, or ultrasound. Creare also developed a packed-bed, two-phase flow calibration facility to characterize the capacitive sensor. The facility included an X-ray densitometer (also developed by Creare) in line with the capacitive sensor to provide a measurement standard. The calibration facility and sensors were used to successfully study flow regimes in packed-bed reactors at Earth gravity over a wide range of conditions with a variety of packing materials. The instruments and test facility were delivered to NASA for future microgravity testing.