High ash content and low calorific value of Indian coals are unfeasible for mining economically from very deep regions of earth. Underground coal gasification (UCG) is a promising clean coal technology which can exploit these coal seams to generate syngas by in-situ combustion and gasification. Cavity formation and the composition of product gas play a major role in deciding the economic viability of the process. Experimental observation of cavity geometry on time scale level is very difficult because of its in-situ nature. In the present study, we use camphor as the material for sublimation studies to predict the cavity geometry growth with time by experimentally and theoretically. Preheated air is passed through a bore hole of camphor block and the resulting cavity geometry is recorded for various air flow rates as a function of time. Computational fluid dynamics (CFD) simulations predict results which are in agreement with the measured shapes.