Two-phase gas-liquid flows with heat transfer occur in a variety of process equipment such as petroleum production facilities (simultaneous flow of oil and gas in pipelines and in oil wells), condensers and reboilers, gas-liquid reactors (used in the production of pharmaceuticals and specialty chemicals), trickle-bed contactors (used in hydrodesulfurization, hydrogenation and hydrocracking processes in refineries), wetted wall absorbers, falling film reactors, power systems and core cooling of nuclear power plants during emergency operation. In addition to these normal gravity commercial applications, two-phase flows also occur in a variety of space operations such as active thermal control systems, power cycles, propulsion devices and storage and transfer of cryogenic fluids. In order to design these systems with greater efficiency and minimum cost, a fundamental understanding of the transport processes occurring in normal and microgravity two-phase flows is needed.

For conditions of technological interest, there are few major types of flow regimes observed for gas-liquid flows in pipes. Characteristics of these flow patterns and the conditions under which those flow patterns exist depends on the orientation of the pipe with respect to gravity. At low gas flow rates, a "bubbly flow" pattern, in which small gas bubbles are uniformly distributed in the liquid, is obtained. Increasing the gas flow rate leads to 'slug flow'. This flow pattern is characterized by large bullet shaped gas bubbles separated by liquid slugs. At even higher gas flow rates, a highly agitated "churn flow" is observed. Increasing the gas flow rate further leads to the annular flow regime in which the liquid moves along the pipe wall in a thin, wavy film and the gas flows in the core region. The above description applies only to two-phase flows in vertical pipes. In horizontal pipes, churn flow does not exist. At low gas flow rates, smooth and wavy stratified flows exist in such pipes. Annular flow regime is obtained in many practical occasions where two-phase gas-liquid flows occur in normal gravity conditions.

Figure 1. Flow patterns observed in microgravity two-phase flows.

In the absence of gravity, there exist only three major flow patterns; bubbly, slug and annular (Fig. 1). In microgravity, annular flows are obtained for a wider range of gas and liquid flow rates. Also, it is the preferred flow pattern for the operation of two phase systems in space, as the slug flow violates microgravity environment. Only at very low gas and liquid flow rates, "bubbly flow" is obtained. Experiments have shown that in the annular flow regime, the waves appearing on the liquid film have a profound influence on the transfer of heat and momentum (or pressure drop) in these systems. Neglecting the wavy nature of the film can seriously underestimate the transfer rates.

The main goal of our work is to understand through a combined program of experimental, and modeling studies, two-phase gas-liquid flows in pipes under normal as well as microgravity conditions. The two main objectives of our work are: (1) collection of experimental data on pressure drop and heat transfer on developing and fully developed two-phase flows in microgravity and (2) modeling studies on the scale-up of two-phase flows to predict flow regimes and transitions at zero-gravity and validation with data.

We plan to collect the experimental data on gas-liquid two-phase flows by using the aircraft KC-135 (of NASA-JSC) and DC-9 (of NASA-Lewis Research Center) which in parabolic trajectories give about 20 seconds of microgravity. The data will be collected using the air-water flow loop as well as the more recent refrigerant (R134a) flow loop. The data will be analyzed and organized using mathematical models. These experiments will include detailed measurements of the liquid film in two-phase annular flows, and these measurements will be used to validate the mathematical models being developed. Normal gravity measurements under comparable conditions will be conducted at the two-phase flow laboratory at the University of Houston. The techniques needed to make detailed measurements will be developed at the University of Houston and then incorporated to the test rigs at NASA-JSC or at NASA Lewis Research Center.

Figure 2. Flow pattern transition map for microgravity two-phase flow. For Su <10e6 (key: o bubbly flow, + slug flow, x annular flow)

Prediction of the flow pattern at given conditions for a two-phase flow system without prior experimental measurements is very difficult. We have developed a flow pattern transition map for microgravity two-phase flows using dimensionless parameters. Our results suggest that flow pattern transition in microgravity can be predicted using the maps shown in Figs. 2 and 3. These were developed using experimental data collected by many researchers using different tube sizes and different working fluids. These maps suggest the importance of a dimensionless group (Suratman number, Su) in determining the transitions between the flow patterns. In the absence of gravity, we can use gas and liquid Reynolds numbers and Suratman number to predict the flow pattern in microgravity.

The definitions of these dimensionless numbers are as follows:

These numbers take into account the effects of surface tension, inertial and viscous forces acting on microgravity two phase flows.

Figure 3. Flow pattern transition map for microgravity two-phase flow. For Su > 10e6 (key: o bubbly flow, + slug flow, x annular flow)

We are currently working with researchers at NASA JSC and Lewis Research Center, Cleveland, OH, on the design of a long duration shuttle experiment to collect scientific and technological data on two-phase gas-liquid flows in microgravity. It is hoped that this experiment will provide data that will lead to a greater understanding of how gas-liquid and vapor-liquid flows behave in microgravity. The data collected in this experiment may be used by engineers to design better thermal control devices (heating and cooling systems) for the international space station, which will be more efficient and require less energy and space than the single phase devices used at present.

Acknowledgements
This work is supported by grants from the NASA-Lewis Research Center (NAG3-1840) and the UH-JSC post-doctoral fellowship program.

Left. Subash Jayawardena, a native of Sri Lanka, came to the University of Houston as a NASA Post-Doctoral Fellow after three years at the NASA Lewis Research Center in Cleveland, OH. His studies focus on turbulent gas core in vertual annular gas liquid flows.
Middle.Luan Nguyen, currently pursuing a doctoral degree in chemical engineering, arrived in the U.S. from Vietnam.
Right. Dr. Vermuri Balakotaiah (l.) with post-doctoral fellow Subash Jayarwadena (r.) in their laboratory at the University of Houston.