Flow boiling in straight heated tubes under microgravity conditions

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

Two-Phase Pipe Flow in Microgravity with and without Phase Change: recent progress and future prospects

Gas-liquid and liquid-vapor pipe flows in microgravity have been studied for more than forty years because of theirpotential applications in space industries for thermal control of satellites, propellant supply for launchers, and wastewater treatment for space missions. Also, microgravity experiments provide unique conditions for highlighting and modeling capillary and inertia effects in the dynamics of two-phase flows. This paper discusses the results of flow pattern characterization, void fraction measurements, wall and interfacial shear stresses, and heat transfer coefficients. The main results are compared with ground experiments and classical correlations and models from the literature. Recent results from flow boiling in pipes are also discussed and perspectives on future studies are presented

Flow Boiling in straight heated tube under microgravity conditions

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 flux and void fraction in thermohydraulic systems. Previous state of the art and data can be found in the papers of Colin et al. [1], Ohta [2], and Celata and Zummo [3]. 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

Flow boiling in tube under normal gravity and microgravity conditions

Forced convective boiling experiments of HFE-7000 were conducted in earth gravity and under microgravity conditions. The experiment mainly consists in the study of a two-phase flow through a 6 mm diameter sapphire tube uniformly heated by an ITO coating. The parameters of the hydraulic system are set by the conditioning system and measurements of pressure drops, void fraction and wall temperatures are provided. High-speed movies of the flow were also taken. The data were collected in normal gravity and during a series of parabolic trajectories flown onboard an airplane. Flow visualisations, temperature and pressure measurements are analysed to obtain flow pattern, heat transfer and wall friction data

Two-phase pipe flow in microgravity without and with phase change: Recent progress and future prospects

Gas-liquid and liquid vapor-pipe flows in microgravity have been studied for other forty years. The studies were motivated by potential applications for space industries with thermal control of satellites, propellant supply for launchers, waste water treatment for space exploration missions…Beside the applications, microgravity experiments provide unique conditions for highlighting and modeling capillary and inertia effects in the dynamics of two-phase flows. Several results were obtained on the flow pattern characterization, the measurements of void fraction, wall and interfacial shear stress and heat transfer coefficients. A summary of the main results is presented by comparison with ground experiments. Recent results on flow boiling in pipe are also discussed and perspectives for future studies are given

Two-phase pipe flow in microgravity without and with phase change: Recent progress and future prospects

Gas-liquid and liquid vapor-pipe flows in microgravity have been studied for other forty years. The studies were motivated by potential applications for space industries with thermal control of satellites, propellant supply for launchers, waste water treatment for space exploration missions…Beside the applications, microgravity experiments provide unique conditions for highlighting and modeling capillary and inertia effects in the dynamics of two-phase flows. Several results were obtained on the flow pattern characterization, the measurements of void fraction, wall and interfacial shear stress and heat transfer coefficients. A summary of the main results is presented by comparison with ground experiments. Recent results on flow boiling in pipe are also discussed and perspectives for future studies are given

Flow boiling in tube in microgravity

Forced convective boiling experiments of HFE-7000 are conducted in earth gravity and under microgravity conditions. The experiment mainly consists in the study of a two-phase flow through a 6 mm diameter sapphire tube uniformly heated by an ITO coating. The parameters of the hydraulic system are set by the conditioning system and measurements of pressure drops, void fraction and wall and fluid temperatures are provided. High-speed movies of the flow are also taken. The data are collected in normal gravity and during a series of parabolic trajectories flown onboard an airplane. Flow visualizations, temperature, void fraction and pressure drop measurements are analyzed to obtain flow pattern, liquid film thickness in annular flow, wall and interfacial shear stresses and heat transfer coefficient

New Proposal of Epiphytic Bromeliaceae Functional Groups to Include Nebulophytes and Shallow Tanks

The Bromeliaceae family has been used as a model to study adaptive radiation due to its terrestrial, epilithic, and epiphytic habits with wide morpho-physiological variation. Functional groups described by Pittendrigh in 1948 have been an integral part of ecophysiological studies. In the current study, we revisited the functional groups of epiphytic bromeliads using a 204 species trait database sampled throughout the Americas. Our objective was to define epiphytic functional groups within bromeliads based on unsupervised classification, including species from the dry to the wet end of the Neotropics. We performed a hierarchical cluster analysis with 16 functional traits and a discriminant analysis, to test for the separation between these groups. Herbarium records were used to map species distributions and to analyze the climate and ecosystems inhabited. The clustering supported five groups, C3 tank and CAM tank bromeliads with deep tanks, while the atmospheric group (according to Pittendrigh) was divided into nebulophytes, bromeliads with shallow tanks, and bromeliads with pseudobulbs. The two former groups showed distinct traits related to resource (water) acquisition, such as fog (nebulophytes) and dew (shallow tanks). We discuss how the functional traits relate to the ecosystems inhabited and the relevance of acknowledging the new functional groups

Experimental study and modelling of flow boiling in tube in normal and microgravity conditions

Forced convective boiling experiments are conducted in a tube in vertical upward flow under earth and under microgravity conditions. The experimental set-up was designed to obtain precise measurements of quality, cross-sectional averaged void fraction, wall shear stress and heat transfer coefficient. The measurement techniques are described and an evaluation of the uncertainties is provided. The test section is a 6mm diameter transparent sapphire tube, uniformly heated by an ITO coating. High-speed video pictures of the two-phase flow are taken through the transparent heated tube. The observed flow patterns are bubbly flow, slug flow and annular flow. Void fraction measurements give access to the mean vapor velocity in bubbly and slug flows and the averaged film thickness in annular flow. The simultaneous measurements of void fraction and pressure drop enable the wall and interfacial shear stresses to be determined. The wall shear stress is compared to classical Lockhart and Martinelli type models. New data on the interfacial shear stress are provided. Heat transfer coefficients are determined along the test section for a wide range of mass fluxes and qualities corresponding to nucleated boiling and convective boiling regimes. The results are then confronted to existing models and the role of buoyancy is highlighted

Meta-analysis of prececal digestibility of phosphorus in growing broilers: effects of dietary phosphorus, calcium and phytase supply

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