Experimental Shear Flow

Publication Reference: 
Author Last Name: 
Michel Louge
Report Type: 
ARR - Annual Report
Research Area: 
Powder Flow
Publication Year: 
Publication Month: 
United States

This project focuses on the fluid dynamics of vertical gas-solid risers. Its principal objective is to produce data for evaluating theories elaborated by Professors Sundaresan and Jackson at Princeton. In this report, we review Cornell activities in the area of gas-solid suspension flows in 1997.

At Cornell, we possess a unique facility with the ability to recycle - rather than discard - fluidization gases of adjustable composition to a vertical riser of 20cm diameter and 7m height. This allows us to simulate the fluid dynamics of industrial units (atmospheric and pressurized coal-burning circulating fluid beds, catalytic crackers) in a cold, atmospheric riser by matching the dimensionless parameters that govern the flow. The facility is equipped with capacitance, optical fiber and pressure instrumentation that records solid concentration profiles in the vertical and radial directions.

By matching five dimensionless parameters, experiments employing plastic and glass powders fluidized with mixtures of sulfur hexafluoride, carbon dioxide, helium and air near ambient temperature and pressure achieved hydrodynamic similarity with generic high-temperature risers of variable scale operating at pressures of 1 and 8 at-m.

We interpreted our results in the upper riser using steady, fully- developed momentum balances for the gas and solid phases. This analysis showed that, for a wide range of experiments, two parameters capture the dependence of the pressure gradients upon the ratio of the mean gas and solid mass flow rates. The first is the ratio of the mean particle slip and superficial gas velocities. The second represents spatial correlations between the radial profiles of interstitial gas velocity and voidage. Variations of the first with dimensionless parameters indicated that our “atmospheric” and “pressurized” experiments conformed to distinct viscous and inertial regimes.

In 1997, we have also established that the descending velocity of particles clusters at the wall of a riser scales exclusively with the square root of the particle diameter and the gravitational acceleration. This observation showed that the dynamics of wall clusters is chiefly determined by inter- particle contacts. Because these clusters govern heat transfer at the wall, this conclusion has important consequences for modeling.