3.1.1. Effect of the sound frequency
To investigate the effects of the sound frequency on the minimum fluidization velocity of lignite, experiments were carried out with the sound frequency varying from 1 kHz to 4 kHz and sound pressure level fixed at 140 dB. Fig. 2 showed that the plot of pressure drops versus gas velocity for the lignite particles. According to Fig. 2, the minimum fluidization velocity was determined at different values of sound frequency. The results are given in Fig. 3. The minimum value of mini- mum fluidization velocity was obtained at optimum sound frequency of 1.5 kHz. It can be seen from Fig. 3 that the minimum fluidization velocity of lignite decreased with increasing sound frequency, and further increased