Study On Different Freezing Tests And Enhanced Heat Transfer Mechanism For Local Hard-to-freeze Strata In Shaft Freezing Projects

Considering local difficult-to-freeze strata (DFS) in shaft construction engineering, twointensified freezing schemes are provided in the paper: an optimization freezing suggestion offlowing different negative temperature solutions in a freezing pipe, and a scheme of reamingborehole filled with heat conduction material for local DFS. To verify the effect of intensifiedfreezing and the adjustment ability of the freezing pipe, a single freezing pipe test is conducted in amodel test box usingtwo sets of circulatory systems to provide calcium chloride solution of differentnegative temperatures for two sections of the freezing pipe. Meanwhile, artificial freezing tests,single freezing pipe is covered with heat conduction materials, are conducted. Test results show thatthe freezing pipe of dual circulation system can realize freezing of two different temperatures, andsuch two schemes can adjust the thickness of frozen wall and average temperature effectively andrealize intensified freezing.Based on coupling between fluid and solid, heat transfer mechanismbetween Calcium chloride solution and freezing pipe is studied, and the temperature curves offreezing pipe's outer wall can be achieved by the calculation. It is proved that heat boundary layerwould be thinner when the flow velocity of Calcium chloride solution is higher, and thus the stratawould be intensified freezing. Based on FVM principle, calculation models of cold energyconsumption about no seepage strata and seepage strata are established and numetrical analysis aredone to prove the good effect of two intensified freezing schemes. The results indicate thatintensified freezing methods can intensify the freezing of strata for the same freezing time. The twoschemes can also decrease freezing time, reduce the average temperature and reduce the amount offrozen wall which should be excavated in the process of construction, though the cold energyconsumption is not increasing. For seepage strata, intensified freezing schemes can realize the frozenwall to be closed earlier, and thus cold energy consumption is optimized finally. Caclulation about aproject shows that such two intensified freezing schemes can intensify freezing coal stratumeffectively.