(199b) The Flowability of Fine Powders in Reduced Gravity Conditions

NASA's exploration initiative will require the ability to effectively perform critical processes outside of Earth's gravity and utilize in-situ resources. These processes include the ability to, among others; make repairs to critical systems, fabricate spare parts utilizing either in-situ or pre-staged raw materials, process waste into usable resources, and transform in-situ resources into usable products. While most of these processes currently do not exist, many of them have the potential to require the need to utilize fine particulates or powders. Therefore, the knowledge and understanding of how to transport, convey, fluidize, and de-fluidize powders in low-g extraterrestrial environments underpin the successful implementation of these enabling technologies, and hence, the successful achievement of NASA's mission.

There are several techniques employed to measure or characterize the flowability of powders and granular materials. These include measuring the angle of repose, Hausner ratio, Carr index, and several methods of shear measurement. Each method has its own parameter to measure the degree of flowability of powder. The Hausner ratio is determined by measuring the bulk material density. The Carr index is based on the compressibility of bulk material. The shear tester, originally developed by A.W. Jenike, relates the mechanical force to the flowability of powder. Recent modifications of Jenike's method include the annular shear cell, the rotational cell, and the Johanson indicizer. The angle of repose measurement can be obtained using a heaping test, a tilting bed, or a rotating drum. Utilizing a high-speed digital camera and image analysis software, the measurement of angle of repose can be very precise. Moreover, the other advantages of this method also include its simple construction and ease of set-up and operation. The present work utilizes both fluidization and a rotating drum to characterize the affect of reduced gravitation acceleration on the flowability of fine powder systems.

Typically, powders have been classified into four groups based upon the powder's behavior under fluidization. This classification scheme was first developed by Geldart in 1973 and is now universally accepted. Geldart characterized one group of powders by their extreme difficulty to fluidize by normal techniques. These powders are known as Geldart Group C powders. In general, Group C powders are any powders that exhibit cohesive behavior. The cohesive forces may be due to Van der Waals forces, capillary forces, and electrostatic forces, as well as other interparticle forces. Since these powders tend to stick together, they lift as a plug in small diameter tubes and crack and channel in larger fluidized beds.

The powders that were studied within this work are powders that may be classified as Geldart Group C powders. In Earth's gravity, cohesive powders are normally any powders with a diameter < 20 microns. The Lunar regolith has been shown to be made up of a significant mass percentage (~30%) of particles less than 75 microns in diameter. While there has obviously been no Martian soil returned to Earth for analysis, it is reasonable to expect some similarities between Martian soil and Lunar soil as they are both fine grained silty soils. The works of Molerus and Qian suggest that powders typically not considered cohesive in Earth's gravity may become cohesive in reduced gravity (even Martian or Lunar gravity). This implies that present powder technologies that are considered mature and well understood here on Earth, will behave much differently in an extraterrestrial environment.

Using a fluidized bed, we empirically mapped the shift in the Geldart Group A (non-cohesive) - Group C transition for two sets of powders due to changing gravity levels. We characterized the fluidization behavior of glass spheres with average diameters of 11, 20, 35, and 71 microns and alumina powders with average diameters of 12, 13, 18, and 45 microns. The experiment was carried out onboard NASA's KC-135 reduced gravity aircraft; providing gravity levels of 1.8g, 1.0g (lab based), 0.38g (Martian), and 0.16g (Lunar). An observable shift in the fluidization behavior was noted as a function of gravity level. The data was compared to the models of Molerus and Qian, demonstrating fair agreement.

While the fluidization approach provided empirical support of the general trend predicted by the models of Molerus and Qian, it did not provide the required resolution for validation of the models. This was primarily driven by three factors: 1) the powder diameter classification was too broad, creating a large degree of uncertainly, 2) the short time duration of the reduced gravity conditions and the relatively long time constant of the fluidized bed system resulted in transient data, and 3) the fluidization behavior classification is somewhat subjective.

Our more recent approach to understand the affect of gravitation acceleration on the flowability of powder systems utilizes a rotating drum to provide the characteristic frequency of avalanches that occur in cohesive powders. The rotating drum addresses the above shortcomings of the fluidization approach as follows: 1) the drum volume is two orders of magnitude smaller than the fluidized bed, facilitating the use of smaller volumes of powders and tighter powder diameter classifications, 2) the small size of the system also reduces the system's time constant to well within that required to achieve steady state during the reduced gravity parabola, and 3) the technique provides an objective measurement of the flowabilty of the powder system from the angle of repose measurement.

In addition to the above improvements over the fluidization approach, our rotating drum apparatus provides the ability to control the atmosphere (pressure and gas species) within the drum. This provides the ability to simulate both the gravitational acceleration and atmospheric conditions of Mars and to a lesser extent, the Moon.

The rotating drum was utilized to provide the characteristic frequency of avalanches as a function of rotational speed for powders diameters ranging from about 10?100 microns in Earth's gravity and at a pressure of 101 kPa. The data reduction technique provides a measurable parameter characterizing the behavior of ?flowable? and ?non-flowable? powders.