The Mechanism Of Thermal Stresses In Rock Breaking Of High Pressure Jet

The high pressure jet is widely used in rock breaking fields,such as mining and well drilling.In recent years,supercritical carbon dioxide jet has been widely studied because its better performance in rock breaking than that of water jet.The experimental results shows that jet temperature is a key factor affecting rock breaking of supercritical carbon dioxide jet.But the sysmatic mechanism of jet temperature in rock breaking has not been explained clealy so far.Besides,current theories of high pressure jet in rock breaking are not able to explain the reasons of the optimum standoff distance of water jet rock breaking and why the supercritical carbon dioxide jet has better rock breaking performance than that of water jet.It’s of great significance to study the mechanism of the above problems comprehensively,which can not only perfect the current rock breaking theories,but also lay the foundation for the efficient use of jet energy in rock breaking.In this paper,at the aim of studying the thermal stresses effect on rock breaking stresses,the physical and mathematical models of rock breaking stresses under jets are established.The method of mutual authentication between simulation results and existing experimental results is adopted.The flow fields and rock stresses of supercritical carbon dioxide jet,nitrogen jet and water jet are studied and the evaluation criterion of rock breaking stresses is established on the bases of current jet rock breaking theories.The relation between rock breaking stresses and fluid properties,jet parameters and rock properties is obtained.The mechanism of thermal stresses on rock breaking stresses and the optimum standoff distance of rock breaking at different working conditions are analyzed.The comparative study between the simulation results and indoor experimental datas shows that both the supercritical carbon dioxide jet and water jet have heating effect on the rock.The reason for the former is the manual increase of jet temperature and that for the latter is the viscous heating effect.Studies show that thermal stress is the main reason that supercritical carbon dioxide jet has better rock breaking performance than that of water jet.The increase of jet temperature or jet pressure difference of supercritical carbon dioxide leads to the increment of the rock breaking stresses and damage radius.And with the increase of elastic modulus,Poisson’s ratio or thermal expansion coefficient,both the rock breaking stresses and damage radius increase.Thermal stress is also a key factor affecting the optimum standoff distance of rock breaking under water jet.As the water jet standoff distance increases,the maximum rock breaking stress firstly increases and then decreases.Considering the dynamic process of rock breaking,the optimum standoff distance is a comprehensive result of jet time,rock strength and jet pressure difference.And the comparison with current experiments of water jet erosion of alloy shows that the effect of thermal stresses has the same effect on the erosion tendency of rock and metal.The extension study is conducted in drilling conditions.At the formation temperature,the supercritical carbon dioxide jet has bigger temperature decrease than nitrogen jet,and the water jet has a heating effect on the rock.Compared to water jet and nitrogen jet,supercritical carbon dioxide jet has a bigger efficient standoff distance of rock breaking,because it has a lower turbulent viscosity and turbulent dissipation rate at the same time.The tensile stresses and shearing stresses of supercritical carbon dioxide jet are bigger than those of water jet and nitrogen jet,and the comparative advantage increases when the jet time is longer and the rock tensile strength is lower.To solve the problem of computational divergence near the critical point,an explicit fitting equation of heat capacity is given and its fitting error is within 1%.The viscosity of drilling fluid on rock breaking of water jet is also studied.When the viscosity of water exceeds 0.01 Pa.s,thermal stresses begin to dominate the rock breaking process and the efficient standoff distance of jet rock breaking increases.At the same time,the maximum tensile stress decreases along with the standoff distance increase and thus no optimum standoff distance exists.The results show that increasing viscosity has better effect than increasing pressure in improving rock breaking ability of water jet under the condition of limited pump pressure.In this paper,the simulation method and the evaluation criterion of rock breaking stresses considering thermal effect are developed.The mechanism of thermal stresses in rock breaking at different jet operating conditons is analyzed.New method and foundamental theories are developed for the development of jet rock breaking technique.