Abstract
Most prior studies on the insulation effect of off-wall insulation have neglected the natural convection of the air lining; this assumption is inconsistent with reality. In this study, the effect of natural air convection on the heat transfer mechanism in off-wall insulation lining was studied via experiments and numerical simulation. The thermal insulation effect of the air lining was evident. The temperature field distribution of the surrounding rock is affected by the natural convection in the air lining and is thus clearly related to the azimuth. The temperature disturbance of the surrounding rock in the direction of θ = 0° is the minimum, whereas that in the direction of θ = 180° is the maximum. The most significant factors affecting the thermal insulation effect of the off-wall insulation lining are the thermal conductivity of the rock and thermal resistance of the insulation layer, followed by thickness of the air lining and temperature difference between the original rock temperature and the air flow. The relationships between the average unstable heat transfer coefficient and various parameters were obtained via fitting. The fitting formula can be used in engineering practice to calculate the heat transfer from the surrounding rock to the tunnel airflow under various conditions.
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