Study On The Variation Of Temperature Field And Thermal Cracking Characteristics For Rock Irradiated By Fiber Laser

The cost of traditional mechanical rock breaking is high and the efficiency is low,which leads to the problem of high-efficiency cracking of hard rock in underground engineering construction,deep oil and gas exploitation,nuclear waste disposal.New rock breaking technology is urgently needed to meet current needs.High-energy laser beams illuminate the rock to create extremely high temperature and temperature gradients inside the rock.When the rock is melted,the rock is also cracked by thermal stress.Laser rock breaking is the most easily realized and efficient method at present.Temperature evolution is the key factor of laser thermal cracking rock.This paper combines physical experiments,theoretical analysis and numerical calculations to study the variation of rock temperature field and thermal cracking characteristics under different illumination parameters,and provides some basic support for laser thermal cracking of high strength rock.(1)This paper experimentally investigates temperature field,peak stress,specific energy(SE),rate of perforation(ROP),and modified specific energy(MSE)of rocks that irradiated by continuous fiber laser under different laser power,irradiation time,irradiation distance,laser frequency,illumination angle,and auxiliary gas with the control variable method.The results show that the surface temperature of the laser irradiated rock sample satisfies the Gaussian distribution,the central temperature is high,and the edge temperature is low.Higher laser power,higher laser frequency,and smaller irradiation angle resulr in larger surface temperature,lower SE and MSE,higher ROP,and more significant rock breaking effect;the longer the irradiation time,the higher the surface temperature of the rock sample,but the secondary effect makes the SE and MSE become larger and the ROP becomes smaller.When the irradiation distance is larger,the surface temperature of the non-direct irradiation area is higher,but the heat flux density value in the phase change zone is lowered.Therefore,when the irradiation distance is small,the rock sample is obviously destroyed,SE and MSE become smaller,and ROP becomes larger;when oxygen is used,the volume of rock spalling is the largest,the MSE is the smallest,and the rock breaking effect is the best;(2)Under the same illumination parameters,the surface temperature changes of sandstone,limestone and granite are similar.The surface temperature of granite is the highest,the sandstone is the second,and the limestone is the lowest.The sandstone has the largest drilling depth and diameter,the lowest SE value and the highest ROP value;the sandstone does not peel off,the spalling volume of granite is small,and the limestone spalling volume is the largest.The thermal cracking effect of rock samples with different water content is studied.It can be found that the dry rock samples have the most cracks,the diameter and depth of hole are the largest,the SE and MSE values are the smallest,and the ROP value is the highest.The saturated water-bearing rock samples had the worst rock breaking effect.The underwater laser thermal cracking test shows that as the thickness of the water layer increases,the surface temperature of the rock sample in the irradiated area decreases significantly,and the depth of hole and ROP decrease,and SE increases,indicating that the effect of rock breaking in water is worse than that in air;(3)Establish a three-dimensional unsteady heat transfer equation for laserirradiated rock,and give the corresponding initial conditions and boundary conditions for numerical calculation by ANSYS.Firstly,the numerical experiments were verified.With the increase of irradiation time,the higher the temperature of the same position,the larger the heat-affected zone,and the larger the diameter and depth of the hole,which are basically consistent with the test results.The numerical experiments of medium-high power laser irradiated rock show that as the laser power increases,the temperature of the rock sample increases,and the diameter and depth of the hole become larger.The size of the borehole obtained by continuous illumination of 6kW for 5s is greater than that of 1kW for 20 s,indicating that the effect of medium-high power laser irradiation on rock breaking is more significant.