Comparative numerical analysis and optimization in downhole combustion chamber of thermal spallation drilling

Thermal spallation drilling is a non-contact and efficient technology, which is suitable for hard rock formations. It uses downhole combustion to generate high temperature media to heat the bottom rock, which leads to the spallation of the rock. However, no investigation has been carried out in combustion of thermal spallation drilling. In this paper, three combustion models (eddy dissipation model, eddy dissipation concept model and non-premixed model) are applied and compared to simulate the reaction in the combustion chamber. Through modifying Magnussen constants, the eddy dissipation model is optimized and validated. Results show that under high velocity conditions, combustion models based on infinite fast reaction mechanism may not be accurate. An additional reaction step in eddy dissipation model may significantly improve the accuracy. Besides, there is a linear relationship between the average outlet temperature and Magnussen constant in eddy dissipation model. Moreover, through modifying the Magnussen constants, the accuracy of the results can be significantly improved. The optimized eddy dissipation model with two reaction steps not only has the advantages of small computational cost, but also may reflect the actual situation of the combustion reaction. Results in this paper could provide guidance in the design of combustion chamber in thermal spallation drilling.