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Visualisation of coal desintegration and investigation of rapid changes of gas pressure, temperature |
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Visualisation of coal desintegration and investigation of rapid changes of gas pressure, temperature and strain during gas-geomechanical phenomena Authors: M. Gawor, J. Rysz
The paper reviews the results of experimental tests involving the disintegration of coal briquettes saturated with gas. Pressure measurements were taken on the briquette side, strain and temperature were measured inside the briquette. At the same time photos were taken of the briquette surface while it disintegrated. Application of a fast CCD camera operating in the Frame-Transfer mode allowed for recording of the emerging cracks and briquette slices and for correlating of thus obtained image with the changes of thermodynamic parameters (gas pressure, briquette deformations, temperature). The earlier experiments revealed that the strain gauge is broken before the crack, presented in the photo, is actually formed. It can be explained by the fact that because of friction forces acting upon the side of the briquette, the disintegration process there is slower than inside the briquette. The briquette decompresses, which is registered by the strain gauges, long before it disintegrates. At the time interval of about 2 ms all strain gauges would record the increasing deformation which indicates the briquette is decompressing before it disintegrates. At the moment of strain gauge failure, the magnitude of strain amounts to 13%. The decompression wave moves much faster than the destruction wave, reaching 80 m/s while the rate of the briquette disintegration ranges from 5 to 10 m/s. The rapid decompression wave inside the briquette facilitates gas filtration towards the free front section and "prepares" the conditions necessary for crack formation. The motion of briquette slices formed during the destruction process is accelerated. The maximal calculated velocity of the slice motion near the briquette front is 23 m/s. Near the bottom the slices move more slowly, reaching 8 m/s. The rate of crack formation is estimated to be about 20 m/s. The temperature changes pattern indicates that briquette cooling is an adiabatic process. At the instant the briquette breaks, the temperature changes rapidly as the result of the change of the medium in which the thermometer is placed. Most significant temperature changes occur near the briquette front where the destruction proceeds at a higher rate than near the bottom. |
2011