| With the development of China’s transportation construction technologies,many tunnels are being built under complex geological conditions.When tunneling progresses into the geothermal areas,the high-temperature environment threatens the quality of engineering structures and the health of workers,reduces the work efficiency of staff,or impacts the normal operation of mechanical and electronic equipment.Therefore,it is significant important to understand the heat transfer characteristics of geothermal construction tunnels for cost-effective heat-hazard-prevention measures.In practice,tunnel excavation contributes to the formation of a"dead-end tunnel",which is unfavorable for inside air circulation.When the blasting excavation method is adopted,duct ventilation is the most commonly used measure for tunnel construction ventilation.However,it is clear from the previous literatures that the heat transfer characteristics or laws of construction tunnels under duct ventilation conditions are not clear enough,and there is less research on the heat-hazard-prevention methods based on classification evaluation of heat hazard for construction tunnels in geothermal areas.This dissertation carried out an in-depth study on the heat transfer characteristics,the temperature distribution laws of surrounding rock,and heat-hazard prevention of construction ventilation tunnels using the methods of theoretical analysis,numerical simulation and case tests,and the main research contents and results are as follows:(1)This work conducts numerical simulations and parameter analyses to investigate the performance of tunnel-air convective heat transfer(CHT)forced by duct ventilation in a dead-end tunnel concerned in tunnel space cooling inside a geothermal construction tunnel.In this work,the distribution curve of Nusselt(Nu)at tunnel wall is investigated.According to the performances of convective heat transfer at tunnel wall,the inside region of a construction tunnel ventilated by duct is divided into two sub-regions,namely the CHT augmentation region(near the duct outlet)and thermally fully developed region(away from the duct outlet).The three-dimensional Computational Fluid Dynamics is based on to simulate the actual tunnel-air CHT in the CHT augmentation region,and the influences of parameters considering different(ⅰ)shapes of tunnel cross-section,(ⅱ)outlet airflow Reynolds number(Re),and(ⅲ)ventilation duct position and(ⅳ)diameter on Nussel number(Nu)longitudinal distribution are respectively identified.The results show that a duct with a smaller diameter and position at the tunnel side is optimal to the CHT augmentation,and hence should be considered in the duct ventilation design for tunnel space cooling in a geothermal construction tunnel.(2)The relationship between air Nusselt number Nu and air flow Reynolds number Re in the CHT augmentation region is studied using numerical methods,and a semi-empirical formulation of Nu-Re relationship is proposed.According to the numerical results,the Nusselt number(Nu)at tunnel wall is less affected by the change of longitudinal coordinates in the region of thermally fully developed region(away from the duct nozzle)in a dead-end tunnel under duct ventilation conditions,and its value can be predicted by Gnielinski’s empirical formula approximately.While,the Nu value in the CHT augmentation region can be estimated by the semi-empirical equation of Nu-Re relationship.(3)A new formula calculation method is proposed to solve the instantaneous radial temperature in the surrounding rock region based on Green Function Method.Namely,the temperature field caused by the heat flux intensity_hB through the convection boundary is approximately regarded as the temperature field caused by the point heat sources with intensity g’(7)r’,t’(8)in the infinite domain,with g’(7)r’,t’(8)obeyingg’(7)r’,t’(8)(28)2B_h.According to the analysis of a field case,it is shown that the calculation results of this new method basically agree with the results of numerical simulation and field test.(4)The influences of various condition parameters on the radial temperature distribution of surrounding rock were analyzed by using the formula calculation method proposed in this paper in combination with numerical method.The results show that:(ⅰ).under the ventilation conditions,the surrounding rock near tunnel wall obtains a greater temperature reduction than the surrounding rock far from tunnel wall;(ⅱ).prolonging ventilation time increases the temperature reduction of the surrounding rock and also expands the spatial cooling range of the surrounding rock;(ⅲ).increasing the thermal conductivity of the surrounding rock does not significantly improve the cooling effect on the surrounding rock near the tunnel wall,however,in the deeper part of the surrounding rock,the larger the thermal conductivity of the surrounding rock,the more obvious the effect of ventilation;(ⅳ).strengthening ventilation to enhance CHT at the wall surface can reduce significantly the temperature of the surrounding rock near tunnel wall,while have less influence on the temperature distribution of the surrounding rock in the far field.(5)A new formula calculation method is proposed to approximately predict the longitudinal temperature distribution of the surrounding rock.A calculation case shows that the maximum deviation of results of the new formula calculation method from that of the numerical method is within 4%.In this paper,the influences of different parameters on the longitudinal distribution trend of internal temperature of surrounding rock under non-uniform ventilation conditions is analyzed by using both the proposed formula calculation method and numerical method.The results show that:(ⅰ).the change of ventilation time mainly affects the longitudinal distribution amplitude rather than the longitudinal distribution trends of internal temperature of surrounding rock;(ⅱ).the unevenness trend of longitudinal distribution of internal temperature of surrounding rock weakens with the increase of depth of surrounding rock;(ⅲ).the unevenness trend of longitudinal distribution of internal temperature of surrounding rock weakens with the decrease of thermal conductivity of surrounding rock.(6)Based on the understanding of the thermal characteristics of geothermal construction tunnel,a method of heat hazard prevention based on fuzzy comprehensive evaluation of heat hazard is proposed.Firstly,considering the factors of geothermal,construction conditions,subjective and objective construction worker,the fuzzy comprehensive evaluations of heat hazard for different stages of tunnel excavation are carried out.Then,the corresponding heat hazard prevention measures are formulated according to the heat hazard evaluation results.Finally,the heat hazard evaluation and heat hazard prevention measures are constantly updated and iterative throughout the construction period.While ensuring the cooling and prevention of heat hazard,the cost of heat hazard prevention measures is reduced,as well as the consumption of energy and resources,that makes the whole process of geothermal tunnel construction safe and economical.The research results of this paper were applied to a case tunnel,and the calculation and analysis results verify the research results of this paper.The method and analysis results presented in this paper may provide references for the prediction of the surrounding rock temperature field,heat estimation,ventilation design,and heat hazard prevention of a construction tunnel in geothermal area,and hence have certain practical value. |
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