EXECUTIVE SUMMARY
This project’s objective is to bring unique experimental insight to the detailed interactions between a gas and dispersed particles. By informing recent theories for those interactions, this work will benefit a wide array of industrial processes involving gas-solid suspensions.
The research is made possible by our development of an axisymmetric Couette cell producing shearing flows of gas and agitated solids in the absence of gravitational accelerations (Fig. 1). The facility will permit gas-particle interactions to be studied over a range of conditions where the suspension is steady and fully-developed.
Unlike Earth-bound flows where the gas velocity must be set to a value large enough to defeat the weight of particles, the duration and quality of microgravity on the Space Station will permit us to achieve suspensions where the agitation of the particles and the gas flow can be controlled independently by adjusting the gas pressure gradient along the flow and the relative motion of the boundaries.
We will carry out two series of experiments in space, due to take place in 2007. In the first series, which we call “viscous dissipation experiments,” we will characterize the viscous dissipation of the energy of the particle velocity fluctuations, when there is no relative mean velocity between gas and solids. To do so, we will reduce the boundary speed in successive tests until the inertia of the solid particles becomes small enough for the particle motion to be affected by viscous forces in the gas. By evacuating the cell partially, we will also investigate the role of the molecular mean free path in dissipating the particle agitation.
In a second series of tests, which we call “viscous drag experiments,” we will impose a gas pressure gradient on the shearing cell sketched in Fig. 1. The gradient will induce a relative velocity between the two phases, while the shearing will set the solids agitation independently. These Viscous Drag Experiments will be unique in exploring a regime where particle velocity fluctuations are determined by a mechanism other than interactions with the gas. In this regime, we will measure the dependence of the drag coefficient on the solid volume fraction and agitation of the solid particles. Partially evacuation will also allow us to test the effects of particle Reynolds number on the drag coefficient.
In June 2000, this project passed the crucial “Science Concept Review,” where a panel of scientists evaluated the feasibility and importance of our investigation. This significant milestone strengthened NASA’s commitment toward our experiments.
In this final year of the IFPRI grant, we tested the prototype shear cell on the KC-135 microgravity aircraft and on the ground. On the aircraft, we demonstrated the accuracy of our capacitance probe system to record solid volume fractions; we obtained a large data base of digital images with metal and ceramic spheres that will be used to develop further the computer vision software; and we gained confidence in the ability to design the experimental system. The tests on the ground allowed us to demonstrate the measurement of the mean volume flow rate using an isokinetic section of the channel, and their data verified the accuracy of our gas-solid theory. This year, we also continued to support NASA’s development of the granular flow module that will run our experiment on the Space Station. Because the term of this project is longer than the three years of the IFPRI grant, we have not yet completed all experiments, which will await launch to the International Space Station.
However, we have already achieved the following:
• We have specified the conditions of all tests (ARR 30-06).
• We have developed theories to predict their outcome based upon best available knowledge of drag coefficients and constitutive relations (ARR 30-06 and ARR 30-07).
• We have developed computer vision software to measure the velocity of the solids (ARR 30-06); a new capacitance probe system for recording solid volume fractions (ARR 30-07); a new isokinetic technique to evaluate the mean gas flow rates (ARR 30-07); and we have proposed a tracer technique to measure the velocity of the gas (ARR 30-06).
• We have manufactured a prototype of the cell, which we tested this year on the KC-135 microgravity aircraft and in the laboratory, thus demonstrating feasibility of the experiments.
This Final Report summarizes our progress to date, with some detail on this year’s activities, and it includes all papers written on this research during the grant period in the Appendix.