边界条件对粉体混合器影响的数值模拟研究

Numerical simulation of influence of boundary condition on powder mixer

定量分析文丘里煤粉混合器风机曲线等边界条件对其性能参数的影响,有助于解决该设备的运行效果在不同工程中可重复性较差的问题。使用CFD软件Fluent,考察了风机曲线、气流温度、出口压力等边界条件对粉体混合器压力损失、落料室负压等关键参数的影响。通过实验测量并拟合得到XGB50-1050型旋涡气泵、FRA-100型罗茨风机出口在约60℃下的风机曲线方程,使用UDF将该方程导入Fluent,在每次迭代开始时,读取入口压力均值,使用拟合方程计算风机曲线上对应的质量流量,使用F_PROFILE函数输出到入口边界。模拟结果表明:随着混合器喷口截面积减小,若入口空气质量流量恒定,则混合器阻力系数呈二次曲线加速上升,喷口负压持续增大;若考虑风机曲线的影响,混合器阻力和负压增加速度均趋缓;且风机曲线的流量衰减越明显,阻力和负压增加的速度越慢。当风机曲线流量衰减较大时,若混合器喷口截面积缩小超过临界值,则负压开始减小。同时,随着喷口截面积的减小,单位压损能够产生的负压逐渐减小,压损—负压的转换效率持续下降。混合器入口压力随出口压力线性变化,当流量恒定时,入口压力增加值与出口压力增加值相同;当考虑风机曲线且入口压力较高时,由于流量衰减,混合器入口压力增加小于出口压力增加值。落料室压力随混合器出口压力呈线性变化,风机曲线对该项影响较小。当混合器出口压力超过喷口射流产生的负压后,落料室即进入正压区,因此,为了提高煤粉混合器负压转换效率,应尽量减小后续阻力。随着入口气流温度的上升,落料室压力呈线性下降,且不同喷口口径下下降趋势近似;混合器阻力呈线性上升趋势,且喷口截面积较小时,落料室负压随温度变化更加明显。

Quantitative analysis of the influence of boundary conditions such as fan curve on the performance parameters of Venturi pulverized coal mixer is helpful to solve the problem of poor repeatability of the equipment in different projects.CFD software Fluent was used to investigate the influence of boundary conditions such as fan curve,airflow temperature and outlet pressure on key parameters such as pressure loss of powder mixer and negative pressure of blanching chamber.Through the experimental measurements,the fan curve equation under 60℃of XGB50-1050 type vortex air pump,roots blower FRA-100 type export were fitted,the equation using UDF was imported into the Fluent software,at the beginning of each iteration,the average inlet pressure was read,fitting equation was used to calculate the mass flow corresponding to fan curve,F_PROFILE function was used to export to entrance boundary.The simulation results show that,with the decrease of the nozzle cross-sectional area,if the inlet air mass flow is constant,the resistance coefficient of the mixer accelerates in a quadratic curve and the negative pressure of the nozzle continues to increase;if the influence of fan curve is considered,the increase speed of both resistance and negative pressure of the mixer tend to slow down.The more obvious the flow attenuation of the fan curve,the slower the resistance and negative pressure increasing speed.When the fan curve flow attenuation is large,if the sectional area of the mixer nozzle is reduced beyond the critical value,the negative pressure begins to decrease.At the same time,with the decrease of the nozzle cross-sectional area,the negative pressure generated per unit pressure loss gradually decreases,and the conversion efficiency of pressure loss-negative pressure continues to decrease.The inlet pressure of the mixer changes linearly with the outlet pressure.When the flow rate is constant,the added value of the inlet pressure is as same as the added value of the outlet pressure.When the fan curve is considered and the inlet pressure is high,the increase in the inlet pressure of the mixer is less than the increase of outlet pressure due to the flow attenuation.The blanking chamber pressure changes linearly with the outlet pressure of the mixer,and the fan curve has little effect on it.When the outlet pressure of the mixer exceeds the negative pressure generated by the jet flow at the nozzle,the blanking chamber enters into the positive pressure area.Therefore,in order to improve the negative pressure conversion efficiency of the pulverized coal mixer,the subsequent resistance should be minimized as far as possible.As the inlet air temperature rises,the negative pressure in the blanking chamber decreases linearly,and the decreasing trend is similar for different nozzle diameters.When the resistance of the mixer increases linearly,and the nozzle cross-sectional area is small,the negative pressure of the blanking chamber changes more obviously with the temperature.