Frederick R. Best

Gravity dependent flow regime mapping

Accurate prediction of the regions of two-phase flow regimes is crucial for system design and operation because these regimes directly affect the thermal hydraulic characteristics of the evaporating fluid. However, as is commonly the case, the gravitational field significantly affects the formation of these regimes. For these reasons, the present investigators have developed a three dimensional flow regime map with superficial vapor and liquid velocities as well as gravity as the axes. Using this new display approach, the effect of varying gravity on flow regime transitions is readily apparent.

Comparison of methods for measurement of cooling tower drift

Abstract An international comparison of methods for measurement of cooling tower drift has been performed at the Massachusetts Institute of Technology. Participants from Belgium, the United States and the Federal Republic of Germany participated in measurements of a spectrum of test environments, which span the range of cases which would typically be encountered in operating cooling towers. The environments differed according to droplet mass flux, droplet size distribution and gas speed. A wind tunnel was built to provide the various test environments, and a special optical drift measurement system was built to permit simultaneous monitoring of the environment sampled in the tests. Cases tested included both mechanical and natural draft cooling tower environments. Among the types of instruments tested are the pulsed laser light scattering system (PILLS), sensitive paper and other sensitive surface droplet impaction systems, isokinetic drift mass flux measurement systems and photographic systems. The results indicate that the instruments tested vary widely in their capabilities, with droplet sizing instruments being more effective in low load, small droplet size spectrum situations, and isokinetic mass and chemical assay techniques being most accurate in high load, large droplet distribution cases. Instruments relying upon thermodynamic state measurements in most cases agreed mutually within an order of magnitude. Their major source of error is believed to arise in the measurement of the gas stream relative humidity. This quantity is necessary for inferring the drift mass flux from the measurement provided by such instruments, which is the mixture saturation deficit or excess. For these tests the relative humidity was typically ⩽ 98%.

Definition of two-phase flow behaviors for spacecraft design

Two‐phase flow, thermal management systems are currently being considered as an alternative to conventional, single phase systems for future space missions because of their potential to reduce overall system mass, size, and pumping power requirements. Knowledge of flow regime transitons, heat transfer characteristics, and pressure drop correlations is necessary to design and develop two‐phase systems. This work is concerned with microgravity, two‐phase flow pressure drop experiments.The data are those of a recent experiment (Hill et al. 1990) funded by the U.S. Air Force and conducted by Foster‐Miller in conjuction with Texas A&M University. A boiling and condensing experiment was built in which R‐12 was used as the working fluid. A Forster‐Miller two phase pump was used to circulate a freon mixture and allow separate measurements of the vapor and liquid flow streams. The experimental package was flown five times aboard the NASA KC‐135 aircraft which simulates zero‐g conditions by its parabolic flight tra...

Definition of condensation two phase flow behaviors for spacecraft design

This paper presents an analysis of the condensation heat transfer data from 1‐G and zero‐G testing of a two‐phase flow experiment employing R‐12 (dichlorodifluoromethane) as the working fluid. The data is compared with condensation models. Condensation heat transfer coefficients from the working fluid to the inner wall of the condenser tube, calculated from the data by using an energy balance across the condenser cooling water, are presented for both zero‐ and 1‐G conditions. The measured zero‐G heat transfer is compared with Pohner’s film model and Best’s heat transfer coefficient. A comparison is made between the zero‐G and 1‐G heat transfer, and a discussion of the results follows.

Gas-liquid annular flow under microgravity conditions : a temporal linear stability study

A temporal linear stability study was performed for a gas—liquid annular flow configuration under microgravity conditions. Data used to validate the modeling includes that generated by Texas A&M as well as all the other known data in two-phase flow under reduced gravity conditions. Following a discussion of theoretical considerations on the growth rates of different instabilities, it is shown that given the fluid properties, pipe diameter and phasic flow rates, one can predict with a high level of confidence the flow regime in the pipe. Acceptable confidence levels (∼80%) are achieved when one differentiates between slug, slug—annular, and annular flow. Higher confidence levels (∼90%) are found when one differentiates between slug and annular flow by merging the annular and slug—annular categories.

Comprehensive Gas Ejector Model

A comprehensive one-dimensional analytical model has been developed for gas ejector design and analysis. Unlike existing models, no assumptions have been made to simplify the momentum conservation equation for the ejector mixing chamber (that is, constant-pressure and constant-area models). Instead, the new model solves the momentum equation, which results in improved accuracy and versatility over existing models. Previous models can be derived from the new model as particular cases. These derivations provide new understanding of the relationships between the constant-pressure and constant-area one-dimensional ejector analytical models. The new model extends to the problems left unsolved by existing models and is efficient in analyzing off-design operating conditions, such as the shock that occurs in the primary stream. From the new model, the limitations on ejector design and operation are also recognized.

Development of a Unique, Passive, Microgravity Vortex Separator

In the microgravity environment experienced by space vehicles, liquid and gas do not naturally separate as on Earth. This behavior presents a problem for two-phase space systems, such as environment conditioning, waste water processing, and power systems. Furthermore, with recent renewed interest in space nuclear power systems, a microgravity Rankine cycle is attractive for thermal to electric energy conversion and would require a phase separation device. Responding to this need, researchers have conceived various methods of producing phase separation in low gravity environments. These separator types have included wicking, elbow, hydrophobic/hydrophilic, vortex, rotary fan separators, and combinations thereof. Each class of separator achieved acceptable performance for particular applications and most performed in some capacity for the space program. However, increased integration of multiphase systems requires a separator design adaptable to a variety of system operating conditions. To this end, researchers at Texas A&M University (TAMU) have developed a Microgravity Vortex Separator (MVS) capable of handling both a wide range of inlet conditions as well as changes in these conditions with a single, passive design. Currently, rotary separators are recognized as the most versatile microgravity separation technology. However, compared with passive designs, rotary separators suffer from higher power consumption, more complicated mechanical design, and higher maintenance requirements than passive separators. Furthermore, research completed over the past decade has shown the MVS more resistant to inlet flow variations and versatile in application. Most investigations were conducted as part of system integration experiments including, among others, propellant transfer, waste water processing, and fuel cell systems. Testing involved determination of hydrodynamic conditions relating to vortex stability, inlet quality effects, accumulation volume potential, and dynamic volume monitoring. In most cases, a 1.2 liter separator was found to accommodate system flow conditions. This size produced reliable phase separation for liquid flow rates from 1.8 to 9.8 liters per minute, for gas flow rates of 0.5 to 180 standard liters per minute, over the full range of quality, and with fluid inventory changes up to 0.35 liters. Moreover, an acoustic sensor, integrated into the wall of the separation chamber, allows liquid film thickness monitoring with an accuracy of 0.1 inches. Currently, application of the MVS is being extended to cabin air dehumidification and a Rankine power cycle system. Both of these projects will allow further development of the TAMU separator.Copyright

Microgravity two-phase flow experiment and test results

A two-phase flow system was tested in the NASA KC-135 reduced gravity test facility. Its innovative flow configuration results in an unusual ease of control and significantly reduced power and heat rejection requirements, while permitting flow regimes, pressure drops, and boiling and condensing heat transfer to be examined. Numerous design features were incorporated that minimize the impact of the KC-135 environment on system condition. Among other results, flight testing indicates no significant effect of gravity on pressure drop for the conditions examined, and that many conditions can be examined definitively only in the long-term reduced gravity afforded by a space experiment.

Experimental and analytical results of a liquid-gas separator in microgravity

The microgravity phase separator designed and fabricated at Texas A&M University relies on centripetally driven buoyancy forces to form a gas-liquid vortex within a fixed, right-circular cylinder. Two phase flow is injected tangentially along the inner wall of this cylinder. Centripetal acceleration is produced from the intrinsic momentum of the resulting rotating flow and drives the buoyancy process. Gas travels under density gradients through the rotating liquid, eventually forming a gaseous core along the centerline of the cylinder. Gas core stability, the presence of liquid in the air line, and the presence of air in the liquid line determine whether a successful core results. To predict separation failure, these three factors were examined both analytically and empirically with the goal of determining what operating circumstances would generate them. The centripetal acceleration profile was determined from angular velocity measurements taken using a paddle wheel assembly. To aid in understanding the ...

Zero‐G two phase flow regime modeling in adiabatic flow

Two‐phase flow, thermal management systems are currently being considered as an alternative to conventional, single phase systems for future space missions because of their potential to reduce overall system mass, size, and pumping power requirements. Knowledge of flow regime transitions, heat transfer characteristics, and pressure drop correlations is necessary to design and develop two‐phase systems. This work is concerned with microgravity, two‐phase flow regime analysis. The data come from a recent sets of experiments. The experiments were funded by NASA Johnson Space Center (JSC) and conducted by NASA JSC with Texas A&M University. The experiment was on loan to NASA JSC from Foster‐Miller, Inc., who constructed it with funding from the Air Force Phillips Laboratory. The experiment used R12 as the working fluid. A Foster‐Miller two phase pump was used to circulate the two phase mixture and allow separate measurements of the vapor and liquid flow streams. The experimental package was flown 19 times for...