Prof John Davidson
|Qualifications/honours||MA, Engineering, University of Cambridge, 1950
PhD, Engineering, University of Cambridge, 1953
ScD, Chemical Engineering, University of Cambridge, 1968
FRS, FREng, FIChemE, MIMechE
Particle mechanics in a rotating calciner (jointly with David Scott)
Particulate material is fed into a slightly sloping cylinder rotating slowly about its nearly horizontal axis. The particles are fed into the high end of the cylinder; gravity carries them down to the bottom end. The arrangement is widely used for roasting particulate material: hot gas moves countercurrent to the particles, in the free space in the cylinder above the rolling bed.
A current project is to measure heat transfer rates between the gas and the rolling bed of solids: (i) there is direct heat transfer from the gas to the solids (ii) but there is also 'regenerative' heat transfer from the gas to the cylinder, which by its rotation conveys heat to the underside of the rolling bed. The relative importance of (i) and (ii) is being measured on a rig with thermocouples in the gas, the rolling bed of solids, the cylinder wall and its insulation.
It is planned also to study unsteady state motion concerned with filling and emptying, especially the velocity of the front of the particle bed as the kiln fills and the velocity of the rear of the bed as the kiln empties.
Another project planned is to study segregation of different particles, e.g. sand and glass ballotini in a rotating cylinder with continuous feed: previous work in the laboratory showed that in a rotary kiln, the two types of particles separate into 'bands', so that the exit flow is successively glass ballotini, followed by almost pure sand and then ballotini again.
Fluidised bed gasification of sewage sludge (jointly with John Dennis and Allan Hayhurst)
The project is concerned with gasification of sewage sludge, a major problem for UK water treatment, as landfill costs increase. Burn-out studies have shown how volatiles from a sludge pellet are released and decomposed in a hot fluidised bed of sand. An important factor is the buoyancy of the pellet, because it is desirable for the volatiles to be released before the pellet rises to the surface of the bed. A method of suppressing the rise rate of a pellet has been discovered by using a slugging bed.
Clean coal combustion (jointly with John Dennis, Allan Hayhurst and workers at Imperial College)
The objective is to underpin the design of a power plant using coal, to give zero emission of CO2. One option is to use CaO to absorb CO2 from a gasifier, thus facilitating the sequestion of the CO2. The effect of cycling from CaO to CaCO3 followed by calcination to release CO2 is being studied, including work at high pressure.
Another option being studied is 'chemical looping' to supply oxygen to a coal combustor: a metal oxide e.g. Fe2O3 is fed in solid form and is converted to Fe3O4 in the combustor or gasifier. The Fe3O4 is then oxidised by air in a separate reactor. Thus the flue gas from the combustor or gasifier contains no nitrogen, avoiding the usual problem of separating CO2 from nitrogen.
Use of ultra-fast magnetic resonance imaging (MRI) to study fluidisation (jointly with Lynn Gladden, John Dennis and Allan Hayhurst)
Studies have shown that MRI is a powerful tool for studying particle flow patterns and mixing within fluidised beds. For example bubble and slug velocities and coalescence of bubbles and slugs can be observed in detail. Likewise MRI gives detailed information about air jets in an air fluidised bed of poppy seeds, these giving a good image. Likewise mixing patterns can be observed when a batch of poppy seeds is added to a fluidised bed of sugar particles.
Breakup of oil droplets in aqueous liquid (jointly with Malcolm Mackley)
The formation of very fine edible oil droplets in water is important for the soft drinks industry. This project, in collaboration with Quest (The Netherlands), is concerned with breaking up the oil droplets by passing the suspension through a nozzle with a very high pressure (50 - 100 bar). Extensive experiments, using the Multi-Pass-Rheometer, show fair agreement between the measured droplet size distributions and the predictions using Computational Fluid Dynamics to predict the pattern of strain rates.
Scott, D.M. & Davidson, J.F. Dynamics of Particles in a Rotary Kiln, in Granular Materials, Fundamentals and Applications Edited S.J. Antony, W. Hoyle & Yulong Ding published The Royal Society of Chemistry Chapter 12 pages 319 - 335 (2004). ISBN0-85404-586-4.
Rees, A.C., Davidson, J.F., Dennis, J.S. & Hayhurst, A.N. The rise of a buoyant sphere in a gas-fluidised bed. Chemical Engineering Science 60, 1143 - 1153 (2005).
Fennell, P.S., Davidson, J.F., Dennis, J.S., Gladden, L.F., Hayhurst, A.N., Mantle, M.D., Müller, C.R., Rees, A.C., Scott, S.A. & Sederman, A.J. A study of the mixing of solids in gas-fluidized beds, using ultra-fast MRI. Chemical Engineering Science 60, 2085 - 2088 (2005).
Scott, S.A., Dennis, J.S., Davidson, J.F. & Hayhurst, A.N. An algorithm for determining the kinetics of devolatilisation of complex solid fuels from thermogravimetric experiments Chemical Engineering Science 61, 2339 - 2348 (2006).
Scott, S.A., Dennis, J.S., Davidson, J.F. & Hayhurst, A.N. Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge Fuel 85, 1248 - 1253 (2006).