COAL DUST GAS SUSPENSIONS WITHIN SHOCK WAVES

The experiments were conducted with a shock tube [8]. The high- and low-pressure sections were 1.5 and 5 m in length, respectively, with the cross section of the channel having dimensions of 52 ram 2. Helium at a pressure of P4 = 2.5-5.0 MPa was used as the propulsion gas, and either oxygen or air with Pl = 0.01-0.6 MPa served as the working gas, the range of Mach numbers M = 2.5-4.0 for the incident SW. The parameters of the gas behind the incident and reflected SW were determined from the tables given in [9, 10], calculated for a real gas, with consideration given to the temperature dependence of the adiabatic exponent. The pressure of the propulsion and working gases were chosen so that the pressure behind the front of the reflected SW be constant (P5 = 2.3 MPa), independent of M. The studies were carried out with dust from black coking coal in which the content of the volatile materials was V 0 = 9 and 26% of the combustible mass, and tests were also carried out on lignite with V 0 = 55%, which produced fractions with a particle size of d < 40 #m through a mesh. One of the most frequently used methods of producing gas suspensions in shock-tube experiments was employed here, namely the pulverization of the powder by the gas flow which moved behind the front of the passing SW. A coal-dust specimen of mass m = 1-20 mg in the form of a compact charge was pulverized against a flat substrate with a diameter of 10 mm, mounted at the level of the channel axis at a distance of 10 = 70-150 mm from the closed end of the tube. The dynamics of two-phase mixture formation was tracked by means of a high-speed multiframe laser shadow visualization procedure. The duration of the exposure (-30 nsec), the number of frames, and the time intervals At between the frames were established in this case by means of a laser stroboscopic light source [11], and the frames were separated spatially by means of a ZhFR-3 high-speed reflecting self-activated photorecorder. The synchronization system provided the required time sequence for the actuation of individual shock-tube elements and of the diagnostic apparatus, thus precisely coordinating light pulse generation to the instant of SW passage through a particular area. Each experiment yielded a series of 15-20 frames, reflecting the dynamics of the process over 300-400 #sec, including the stage of gas-suspension formation in the passing SW and the interaction of the reflected SW with the coal-dust gas-suspension cloud being formed. We also used a method of scanning photographically the luminescence of burning particles in the investigation of ignition and combustion of coal-dust gas suspensions in a complex with multiframe shadow laser visualization, accomplished' with the aid of a second high-speed ZhFR-1 photorecorder. Synchronization of this process with the instant of SW reflection, as well as the