Thick-wall Effect In A Dense Gas-particle Coaxial Jet

In order to solve the serious wear of a coaxial nozzle in the pulverized coal gasification process which shortening nozzle life and endangering the safe production,granular flow characteristics in the dense gas-particle coaxial jet have been further investigated.A particle entrainment phenomenon at the coaxial nozzle exit has been regarded as the main reason of the nozzle wear.In addition,the influences of the nozzle wall thickness on the particle dispersion in the far-field coaxial jet have been analyzed.The main contents and results are summarized as follow:1.Using high-speed camera with high magnification zoom lens,local granular flow characteristics at a coaxial nozzle exit are observed.The experiment results demonstrate due to the thick wall of an inner nozzle,the strong annular gas recirculation induced by the thick-wall effect causes the particle entrainment and the nozzle wear.The relation of the particle entrainment with annular gas velocity,wall thickness,particle mass flow rate and size is revealed by analyzing the experimental images.The further studies show the critical annular gas velocity of the particle entrainment increases with particle mass flow rate and the empirical correlations of the particle entrainment length and the reverse dispersion angle are proposed.Moreover,the simulation results reveal the flow characteristics of gas backflow and entrained particle trajectory for high annular gas velocity.Based on the gas-solid flow theory and the modified drag model,the motion model of entrained particles is established and the entrained particle velocity is obtained,which provides the theoretical analysis for solving the nozzle wear.2.The influences of the thick-wall effect on the particle dispersion in the dense gas-particle coaxial jet have experimentally investigated.The experiment results show the thick-wall effect increases the vorticity thickness of the annular gas stream and weakens the shear effects on the granular stream.Hence,an increase of the wall thickness suppresses the particle dispersion.Additionally,the effects of the wall thickness on the wavelength,amplitude and dispersion angle are analyzed.The K-H instability analysis combining with the modification of the vorticity thickness of the annular gas stream reveals that the wavelength increases linearly with the modified vorticity thickness.The relation between the amplitude and the modified vorticity thickness is obtained as well.3.The dense gas-particle coaxial jet for different cross angles is studied.Due to a"squeeze effect" of the cross jet,a contraction of a granular stream from a central channel in the near flied is observed.The minimum diameter and contraction length of the granular stream in the near field decrease with increases of the annular gas velocity and the cross angle.The two mathematical models of the two features with cross angle,annular gas velocity and particle mass flow rate are proposed,respectively.An angle factor is induced to research the influences of the cross angle on the particle dispersion length and modify the relation of the particle dispersion length.The experimental results reveal the dispersion angle is highly non-monotonic with the cross angle and the annular gas velocity and the influences of the cross angle depends on radial squeezing and axial shearing instability.In addition,in the coaxial cross jet,the particle entrainment induced by the thick effect are observed to be closer to the nozzle wall,which intensifies the nozzle wear,and the particle entrainment length is also modified by the angle factor.