BEstimation of velocity fields and spatial distribution in binary fluidized bed of coal-ash mixtures.
Sangram Roy (Prof. Shantanu Roy)
21st February 2018, Wednesday, 04:00‐04:30 pm (25+5 mins), ChE Seminar hall
Abstract:-
Radiation-based interrogation has become a popular tool for laboratory investigations of flow fields in multiphase reactors. These methods have been further used to validate CFD models. This work combines two novel studies. Firstly, as a domain of interest, it presents measurements of the distribution of coal and ash mixtures through noninvasive γ ray Densitometry technique. Secondly, we used Radioactive Particle Tracking (RPT) to study the velocity fields under similar operating conditions. Such an application is invaluable for developing a database in establishing pilot-scale coal gasification units. The overall objective is to establish the applicability of radiation based techniques for studying hydrodynamic features of real industrial gasifier mixtures.
In order to simulate a realistic bed composition of a gasifier, hydrodynamic characteristics were studied under various proportions of coal and ash on laboratory scale cold flow setup. Fluidization experiments were performed on a laboratory scale cylindrical fluidized bed (0.10 m ID) employing dry air as a fluidizing medium. The bed comprised of a mixture of coal and bottom ash (70% Coal + 30% Bottom Ash and 20% Coal + 80% Bottom ash). For determination of solids holdup through densitometry variation in transmission of γ-rays emitted from 137Cs through the heterogeneous medium of the coal-ash mixture. Variation in phase distribution results in variation in attenuation and thus yields a distribution of photon counts. On the other hand, in RPT, the motion of a freely moving 46Sc radiotracer was tracked. Subsequently, mean velocities were evaluated by ensemble averaging instantaneous velocities collected over a long duration.
The same was validated using CFD against chordal averaged solids holdup (coal and ash) characteristics measured using γ-ray densitometry. The other part of the study involves validation, involving the comparison of measured solids velocity profiles and CFD-predicted ones, is accomplished by implementing the Radioactive Particle Tracking (RPT) technique. As the RPT was conducted to determine the velocity of a typical solid in a mixture, it could not reveal the individual coal and ash velocities. In order to circumvent this problem, the hydrodynamic modeling through CFD was performed. As Eulerian approach was followed, it could predict the typical coal and ash velocities separately at various locations. Notably, in case of all the superficial velocities, a reasonable agreement was observed. Similar, agreement has also been noted in case of spatial distributions of solid measured through Densitometry. It could thus be established that both spatial distributions of phases measured by Densitometry and velocity measurements through RPT are versatile to reveal underlying flow physics.