Abstract: To clarify the key parameters of a heating insulation system for a highway tunnel fire pipeline that utilizes solar energy, a numerical calculation model of a "pipe in a pipe" system is established based on the Fucunchuan tunnel project. This model determines the insulation layer thickness and inlet temperature that are both economically and practically reasonable. Furthermore, a heat transfer amplification factor that comprehensively reflects the heating of the closedloop pipe is proposed through fitting calculations. The results reveal the following: (1) There is an upper limit to the effectiveness of the external insulation layer, and a reasonable thickness of about 50 mm should be maintained. Additionally, the inlet temperature of the heating pipe should be kept above 40 ℃ to effectively guarantee the antifreeze effect of the fire pipeline system. (2) The numerical calculation results of the second heat transfer process are in good agreement with the theoretical results, which validates the reliability of the numerical results. (3) Based on numerical results and considering the first heat transfer process, a heat transfer calculation formula of doubletube closedloop heating is proposed. The feasibility of this formula is validated through the Fucunchuan tunnel project, and it is suggested that when the water temperature of the fire pipeline is below 36.4 ℃, the fire pipeline will not freeze.

Key words:  , highway tunnel, solar energy, fire pipeline, pipe in pipe, heat transfer mode, theoretical calculation