Microgravity Heat Transfer in Flow Boiling

Haruhiko Ohta    Department of Aeronautics and Astronautics, Kyushu University, Fukuoka, Japan

1 Introduction

Recent increases in spacecraft size and power requirements for advanced satellites and other orbiting platforms have increased the demands for more effective thermal management and thermal control systems. Thermal systems utilizing boiling and two-phase flow are effective means for the development of high-performance, reliable and safe heat transport systems for future space missions. Boiling heat transfer offers high heat transfer rates associated with the transport of latent heat of vaporization and has the potential to significantly reduce the required size and weight of heat exchangers. The latent heat transport in two-phase flow reduces the flow rate of liquid circulated in the loop for the same amount of heat transport and, in turn, reduces the pump power requirement. Furthermore, two-phase fluids allow for precise adjustment of the fluid's temperature responding to the thermal load by simply pressurizing the system using an accumulator.

Despite their acknowledged importance, boiling and two-phase flow systems have not yet been fully implemented in new spacecraft except for small-scale heat pipes and a thermal transport loop planned in the Russian module of the International Space Station. This is partially attributed to the lack of a reliable database for the operation of such systems in microgravity. In addition, the uncertainty in the critical heat flux (CHF) conditions discourages space system designers from introducing such systems. Single-phase liquid cooling systems are favored despite the large mass penalty. But even with single-phase systems, boiling and two-phase flow would inevitably occur as a result of, for example, accidental increase in the heat generation rate, or a sudden system depressurization caused by valve operation.

It is safe to say that, to date, there is no cohesive database for microgravity boiling and two-phase flow (reduced gravity is referred to as microgravity or μg here). There is also a prevailing misconception that few differences actually exist between normal and microgravity heat transfer coefficients in flow boiling in the existence of bulk flow. But this is not true, as is shown in the following section when bulk flow is not so large. In addition to the clarification of phenomena in microgravity, the establishment of a coherent database for microgravity flow boiling and two-phase flow provides fundamental information for the development of large-scale two-phase thermal management systems for possible implementation in future spacecraft and earth orbiting satellites.

Research on microgravity boiling has a history of more than 40 years with a short pause in the 1970s and has been advanced with the development of various microgravity facilities and with increased experimental opportunities, especially in the last 15 years. Most boiling experiments in microgravity, however, have been conducted for pool boiling, while the data on flow boiling experiments are very limited except those for isothermal two-phase flow concerning the gravity-dependent flow pattern change and pressure drop. This is partially due to the practical difficulties in adapting the flow boiling apparatus with its various components to the microgravity facilities such as drop towers, aircraft, ballistic rockets and space shuttles with limited capacities in both integration volume and power supply.

Misawa and Anghaie [1] introduced two different test sections for boiling experiments, i.e. a transparent square channel of pyrex glass with a coating of transparent heating films for flow pattern observation and a copper tube with a nichrome coil on the outer surface for the pressure drop measurements. Drop experiments were conducted for Fron113 flowing in vertical test sections. It was clarified that the slip ratio under microgravity is less than unity and the pressure drop is larger than the values predicted by the homogeneous model because of the increased contribution of acceleration resulting from the increase of void fraction. Kawaji et al. [2] investigated on board KC-135 aircraft the behavior of two-phase flow and heat transfer during the quenching of a preheated quartz tube. The tube, heated externally by a spiral nichrome tape, was initially empty and Fron113 was pumped into it. In microgravity, a thicker vapor film is formed on the tube wall making the rewetting of the wall more difficult and resulting in the reduction of the heat transfer rate. They observed flow patterns for flow boiling of subcooled Fron113 and saturated LN2 both on the ground and in microgravity, and reported marked differences in the shapes of liquid droplets in the dispersed flow region [3]. Saito et al. [4], using Caravelle aircraft, performed flow boiling experiments for water under subcooled and saturated conditions in a horizontal transparent duct with a concentric heater rod. In microgravity, generated bubbles move along the heating rod without detachment and grow and coalesce to become large bubbles, while the local heat transfer coefficients along the periphery of the heater rod, however, are quite insensitive to gravity levels. Lui et al. [5] presented experimental results on subcooled flow boiling in a horizontal tube, where the heat transfer coefficients due to nucleate boiling in microgravity increase up to 20% from those in normal gravity if subcooling is low. Rite and Rezkallah [6,7] investigated heat transfer in bubbly to annular flow regimes of air–water two-phase flow. The method is useful for the investigation of heat transfer mechanisms for two-phase forced convection under various flow rate combinations of both phases, involving those not easily realized by the single-component system, if the differences between the single-component and binary systems in the interaction of liquid and vapor phases are taken into consideration.

To improve the approach for the clarification of phenomena in microgravity, the present author developed the observation technique, i.e. transparent heated tubes and transparent heating surfaces employed in the flow boiling using round tubes and narrow channels, respectively. In the experiments for flow boiling in a tube, the effect of gravity on the heat transfer was clarified by making reference to the observed liquid–vapor behaviors in a wide quality range covering the bubble to the annular flow regime. Gravity effects on heat transfer due to two-phase forced convection in the annular flow regime were analytically investigated to clarify the mechanisms relating to the gravity-dependent behaviors of annular liquid film. Acquisition of CHF data was attempted in microgravity and one of the major dryout mechanisms was investigated based on the temperature fluctuations obtained at heat fluxes just lower and higher than the critical value. For flow boiling in a narrow gap, a transparent flat heating surface was developed and integrated in a narrow channel, and some heat transfer characteristics inherent in microgravity conditions were clarified.

2 Development of Transparent Heated Tube

2.1 Structure of Transparent Heated Tube

The heated tube is made from a pyrex tube of I.D. 8 mm with a wall thickness of 1 mm to minimize the heat capacity for the effective use of short microgravity duration. The heated length is varied from 17 mm to 260 mm depending on the purposes of individual experiments. The heater is made of a thin gold film and the heating is conducted by the application of DC electric current directly thorough it. The film has a thickness of the order of 0.01 μm and it is transparent to allow the observation of liquid–vapor behavior through the glass tube wall. At the same time the film is utilized as a resistance thermometer to evaluate directly the inner wall temperature averaged over the entire heated length. The gold film is coated uniformly along the heated length by the pulse magnetron spattering technique and therefore...