Flow boiling in straight heated tubes under microgravity conditions

Abstract

Boiling two-phase flow can transfer large heat fluxes with small driving temperature differences, which is of great interest for the design of high-performance thermal management systems applied to space platforms and on-board electronics cooling in particular. However, such systems are designed using ground-based empirical correlations, which may not be reliable under microgravity conditions. Therefore, several two-phase flow (gas-liquid flow and boiling flow) experiments have been conducted in the past forty years and enabled to gather data about flow patterns, pressure drops, and heat transfers including critical heat fluxes and void fractions in thermohydraulic systems. Previous state of the art data can be found in the papers of Colin et al. (1996), Ohta (2003), and Celata and Zummo (2009). However, there is still a lack of reliable data on heat transfer in flow boiling in microgravity. Therefore, the purpose of our study is to clarify gravity effects on heat transfer characteristics and provide a fundamental description of boiling heat transfer for space applications. Hence, a two-phase flow loop for the study of flow boiling has been built at the IMFT in order to perform experiments in vertical flow in normal gravity and under microgravity conditions during parabolic flights in the aircraft. The test section is a 1mm thick sapphire tube of 6mm internal diameter with an ITO coating on its outer surface. The coating is heated by Joule effect and its temperature is measured in four locations by Pt100 sensors. High-speed movies of the flow are taken with a PCO 1200HS camera. The pressure drop is measured along the test section with two differential pressure transducers Valydine P305D. The mean void fraction upstream and downstream the test section is measured by capacitance probes developed and carefully calibrated at the IMFT. The refrigerant HFE-7000, which was chosen for safety reasons in the aircraft and because of its low saturation temperature at atmospheric pressure (34°C), circulates with mass fluxes G up to 1000 kg/s/m². A wide range of flow boiling regimes are studied, from subcooled flow boiling to saturated flow boiling with vapour mass qualities up to 0.7. The wall heat flux density ranges from 0 to 45 000 W/m². In subcooled boiling, bubbly flow is mainly observed. For saturated conditions the flow patterns are slug and annular flows depending on the quality value (Figure 1). Preliminary data were collected during a first flight campaign and on ground. The wall local heat transfer coefficients are deduced from the wall heat flux density and the local wall temperature measurements. Heat losses are characterized. Joint measurements of pressure drop and mean void fraction along the test section allow to access to the wall shear stress. Preliminary results suggest that gravity has no noticeable effect on heat transfers for mass fluxes G superior to 400 kg/s/m², which implies that lower mass fluxes should be investigated. Results obtained under normal and microgravity conditions are compared to existing models in order to obtain reliable and precise closure laws for boiling heat transfer in microgravity.