Heat transfer and deformation characteristics of different rank coals in low-temperature cyclical freeze-thaw process
Identifying an efficient method for liquid nitrogen(LN2)fracturing in coal layers of different ranks is an urgent issue.To analyze the heat transfer and deformation damage charac-teristics during the LN2 freeze-thaw cycle in coal bodies of various ranks,this experiment used saturated and dry states of lignite,bituminous coal,and anthracite as subjects.The heat trans-fer and deformation evolution laws of coal bodies were investigated using a temperature-strain monitoring system,and the evolution of porosity before and after coal body fracturing was ana-lyzed using a nuclear magnetic resonance system.The results show that:After cyclic freeze-thaw,the porosity of each coal sample in a saturated state generally increases,while that of dry samples decreases.Regarding the temperature evolution during the freeze-thaw process,the absolute values of the rate of temperature decrease/increase for saturated samples of the three coal ranks are in the order of anthracite＞lignite＞bituminous coal;for dry samples,the rate of temperature decrease is lignite＞anthracite＞bituminous coal,and the rate of temperature increase is positively correlated with coal rank.The volume strain evolution struc-ture during the freeze-thaw process is nearly identical for both saturated and dry samples,indi-cating that a certain closed space is formed during the freezing process in both states,and the complete formation time of the closed space shortened with the increase in coal rank.By analy-zing the amplitude of strain evolution during the cyclic freeze-thaw of the coal body,it is found that for dry samples,the higher the coal rank,the more simplified the strain evolution struc-ture during the freeze-thaw process,and the smaller the deformation amplitude;for saturated coal samples,the deformation amplitude is determined by both fissure water and coal rank.This study clarifies the freeze-thaw characteristics of coal bodies under LN2 cyclic impact and has reference significance for the study of LN2 fracturing of coal rock.
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