Effects of Nonuniform Outflow and Buoyancy on Drag Coefficient Acting on a Spherical Particle

Abstract

Pyrolysis gas jets out from the surface of a solid fuel particle when heated. This study experimentally observes the occurrence of gas jets from heated solid fuel particles. Results reveal a local gas jet occurs from the particle's surface when its temperature reaches the point at which a pyrolysis reaction occurs. To investigate the influence of the gas jet on particle motion, a numerical simulation of the uniform flow around a spherical particle with a nonuniform outflow or high surface temperature is conducted, and the drag force acting on the spherical particle is estimated. In the numerical study, the magnitude of the outflow velocity, direction of outflow, and Rayleigh number, i.e., particle surface temperature, are altered, and outflow velocities and the Rayleigh number are set based on the experiment. The drag coefficient is found to decrease when an outflow occurs in the direction against the mainstream; this drag coefficient at a higher Rayleigh number is slightly higher than that at a Rayleigh number of zero.

Drag force for a burning particle

“…In previous studies about outflow particles [13][14][15]17], the Stefan flow tends to weaken the drag force. In these studies, the Stefan flow has little influence on the pressure but attenuates the friction.…”

“…This nonzero normal velocity changes the structure of the particle boundary layer. In some previous studies of particles with outflow [13,14,17,26] and porous particles [11,12,40,41], the effect of wall-normal velocity is also mentioned. As shown in Fig.…”

“…In many studies on the drag force of particles with outflow [13,14], Restefan is used as a variable to describe the change in the drag force. However, when the gaseous reaction is introduced into the system, the validity of using only this dimensionless number to predict the drag force should be verified.…”

“…A particle with outflow has previously been studied as a simplification of an evaporating droplet or a solid fuel particle with a Stefan flow [13]. The outflow velocity condition was implemented at the surface without considering the effect of the species.…”

“…A number of studies [13][14][15] show that the outflow tends to reduce the drag force. For this problem, a blowing correlation [16] was introduced to quantify the effect of pyrolysis when calculating the particle response time.…”