I am currently working on the effects of underground coal gasification on the nearby strata. I have been trying to simulate the same using COMSOL.
I made a model consisting of a host rock (1000m x 1200m) and a rectangular cavity (7.5m x 100m) within it, signifying the cavity left after burning of coal.
1. First I made the model having no cavity and allowed it to settle on its own by gravity. The top boundary of the model was kept free and roller were placed on the remaining three boundaries. The total displacement noticed was 15.7 mm downwards.
2. Next I simulated the cavity and noticed the strata behaviour. It gave expected results showing a subsidence on the surface.
3. Now, to simulate the heating caused by burning of coal, I placed four temperature source of 1273.15 K at the boundaries of the cavity and applied room temperature conditions to the external boundaries. (See Attachment 1).
4.The temperature distribution looks fine. As the top boundary is still at room temperature.(See Attachment 2).
The problem started here as the whole model started behaving like a single body showing thermal expansion. The expansion was visible on the top boundary of the model, as the other three boundaries were simulated with rollers. The total displacement of the top surface is about 750 mm upwards. (See Attachment 3). The reason I am confused is, if the top boundary is at room temperature why is there a upheaval?