ESTIMATION OF THE DEPOSITION OF FINE SOLID PARTICLES IN A LAMINAR FLAT-PLATE BOUNDARY LAYER

1. Sprays: modelling versus experimentation UK-Israel Workshop Queens Hotel, Brighton, UK July 16-18, 2007 ESTIMATION OF THE DEPOSITION OF FINE SOLID PARTICLES IN A LAMINAR FLAT-PLATE BOUNDARY LAYER M. Hussainov, A. Kartushinsky, Ü. Rudi , I. Shcheglov, S. Tisler Tallinn University of Technology, Faculty of Science, Laboratory of Multiphase Media Physics, Akadeemia tee 23A, Tallinn, 12618, Estonia Adhesive properties of pair “particle-surface” Prediction of particle deposition on a flat plate Motivation Objectives Measuring test rig • extend of knowledge on the deposition processes at • the two-phase laminar boundary layer • lack of reliable methods of assessment of the • particles deposition • investigation of influence of the two-phase • flow conditions on the particles deposition • examine of the adhesive properties of pair • “particle-surface” • elaboration of the method for the prediction of • the total amount of deposited particles at flat • plate surface FA is the adhesive force Fel is the elastic force stemming from the elastic collision between the surface and the particle Ffr is the frictional force Fad is the aerodynamic drag force Fgr is the gravitational force 1 – fixed supporting plane; 2 – DC electric motor; 3 – sleeve for installing the prototype surface on the shaft of the electric motor; 4 – stainless steel hollow conic prototype; 5 – infrared transducer for the rotational speed of the prototype; 6 – special cardboard head for the removal of dust from the model surface; 7, 8 – traverse positioning system; 9 – extra metallic slab; 10 – film specimen for collecting particles removed from the prototype surface DEPOSITION PARTICLE FLUX Influence of flow conditions on particles deposition Probability of entrainment of particle by the surface Particles adhesion number Deposition particle flux Mps is the mass of the particles detached from the surface in the given cross-section by the centrifugal force mps is the mass of the particles initially deposited at the given section of the surface Experimental set-up DM is the mass of particles deposited on the plate surface during the given period of time, t S is the area of the stripe on the plate surface from which the particles were removed Probability of adhesion of particle to the surface Probability of lift-off of particle from the surface CW and VsW are calculated by the mathematical model of the laminar flat-plate boundary layer by Hussainov et al. (Int. J. Multiphase Flow, 1995). Results 1 – main channel; 2 – channel of cocurrent pure air flux; 3 – dust evacuation channel; 4 – outlet; 5 – blower; 6, 17 – particle screw feeder; 7 – air flow governor; 8 – thermocontroller; 9 – flowmeter; 10 – pressure converter; 11 – He-Ne laser; 12 – transmitting optics; 13 – receiving optics;14 – registering, processing and controlling system; 15 – confuser; 16 – set of grates; 18 – model The adhesion number vs the detaching centrifugal force for different particles sizes (log-normal distribution). a) b) Deposition fluxes of particles dp=23 m for various flow velocities: 1 – U∞ =1.5 m/s, 2 – U∞ =3 m/s. Experimental conditions Calculated and experimental deposition fluxes for the flow velocity: a) U∞ =1.5 m/s; b) U∞ =3 m/s. Conclusions A method for the assessment of the deposition of fine solid particles in a vertical two-phase laminar flat-plate boundary layer was investigated. The method is based on a probabilistic approach to the particles deposition/adhesion process and takes into account both the hydrodynamics of the flow past the flat plate and the adhesive characteristics of particles and the plate surface. The experimental results and analysis revealed a log-normal dependency of the adhesion number for the fine solid particle systems that were investigated. The analysis takes under consideration the macroscopic characteristics and dynamics of the deposition and lift-off processes. Some discrepancy between the probabilistic distribution of the deposition, as obtained by the experiments and as calculated by the proposed calculation method, was observed. This may be attributed to the microphysics and environmental condition of the adhesion process, such as the influence of the electrostatic charges of the particles and the surface, chemical bonding and local humidity. However, the method developed yields robust results and may be applied with relative confidence for the calculation of the amount of fine solid particles deposited from the two-phase flows in laminar boundary layers. Measuring test rig for the deposited particles 1 – plate; 2 – fixed bottom; 3, 6 – holders; 4 – cardboard head; 5 – telescopic bar; 7 – film specimen; 8 – slab References Acknowledgements Deposition fluxes of various particles for the flow velocity U∞ =1.5 m/s: 1 – dp=12 m, 2 – dp=23 m. Hussainov, M., Kartushinsky, A., Mulgi, A., Rudi, Ü., Tisler, S. Experimental and theoretical study of the distribution of mass concentration of solid particles in the two-phase laminar boundary layer on a flat plate. Int. J. Multiphase Flow, Vol. 21, 6, 1141-1161 (1995). The authors are grateful to the Estonian Science Foundation (Project Ref. SF0812526s03) and the International Science Foundation (Grant No. LG 6000) for financial support of the work on this project.