Flame-Jet Induced Thermal Spallation as a Method of Rapid Drilling and Cavity Formation

Abstract

ABSTRACT The recent, favorable results of field testing of spallation drilling have directed new research into the rock mechanics and jet dynamics associated with rock spallation under high heat fluxes. Wei-bull's statistical theory of rock failure has been applied to define the mechanics of spallation of brittle materials. A reasonable spallation temperature of 500°C at the rock surface can be computed using typical physical property values for granite, neglecting any temperature dependence of the Weibull parameters because of additional microcracking upon heating. A numerical simulation of the jet dynamics and rock cutting has been formulated to model the spallation drilling process. Conservation equations for the heat, mass, and momentum of the gas jet with appropriate models of turbulent transport phenomena are used to estimate rock spallation rate and steady-state hole geometries as a function of heat flux and position of the drill. The predicted trends in drilling performance with changes in flow input are reasonable in comparison with field data but require future verification. Experiments with well-characterized granites are proposed to test both the Weibull fracturing theory and the dynamic simulation results. Specific tests involve the use of high-speed videotaping to measure spallation times under defined heat flux conditions with laser and gas-jet heating. Quarry-face drilling will also be attempted with a scaled-down jet penetrator. Results of all experiments will be presented at the conference.