Abstract
In this work, speed of sound in 2 phase mixture has been explored using CFD-DEM (Computational Fluid Dynamcis - Discrete Element Modelling). In this method volume averaged Navier Stokes, continuity and energy equations are solved for fluid. Particles are simulated as individual entities; their behaviour is captured by Newton's laws of motion and classical contact mechanics. Particle-fluid interaction is captured using drag laws given in literature. The speed of sound in a medium depends on physical properties. It has been found experimentally that speed of sound drops significantly in 2 phase mixture of fluidised particles because of its increased density relative to gas while maintaining its compressibility. Due to the high rate of heat transfer within 2 phase medium as given in Roy et al. (1990), it has been assumed that the fluidised gas-particle medium is isothermal. The similar phenomenon has been tried to be captured using CFD-DEM numerical simulation. The disturbance is introduced and fundamental frequency in the medium is noted to measure the speed of sound for e.g. organ pipe. It has been found that speed of sound is in agreement with the relationship given in Roy et al. (1990). Their assumption that the system is isothermal also appears to be valid.
Highlights
Kundt et al (1868) studied sound waves by sprinkling ‘Lycopodium seeds’ into an oscillating column of air within a tube to identify the nodes of the standing wave
It was found that the sound waves diminished in the presence of particles and that the sound velocity changed compared to its theoretical value in air
Roy et al (1990) studied the speed of sound in a gas-fluidised bed by cross-correlating pressures measured at different points in the bed and by measuring the frequency of standing wave after a disturbance had been introduced
Summary
Kundt et al (1868) studied sound waves by sprinkling ‘Lycopodium seeds’ into an oscillating column of air within a tube to identify the nodes of the standing wave. Where us the speed of sound is, p is the pressure and ρ is the bulk density For this to apply to a two phase mixture of gas and particles a number of assumptions need to be made as given by Roy et al (1990). By assuming that Nu = 2 (lowest possible value), particle diameter d = 70 μm, specific heat capacity cp = 1045 J/KgK, density of gas ρg = 1.28 Kg/m3, and the voidage (spaces between solid particle filled by gas ratio) ε = 0.4, τs is equal to 8.4 * 10Ϫ6s This indicates that the time taken for heat transfer process between solid and gas in a two phase medium is very small e.g. fluidised bed. Results are compared with given theoretical relationship and conclusion is drawn
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