基于对流降温效应的青藏铁路碎石护坡层厚度研究

Study on Thickness of Qinghai-Tibet Railway Fractured-rock Revetment Layer on the Basis of Convection Cooling Effect

导出适用于变渗透率多孔介质的无量纲控制方程,并针对青藏铁路工程建设,分析了碎石护坡路堤冬季自然对流降温效应的产生和演化过程。通过引入平均Nusselt数来评价碎石护坡路堤的冬季自然对流降温能力,具体分析了平均Nusselt数随碎石护坡层厚度、路面温度振幅的变化关系。数值计算表明,平均Nusselt数并非随路面温度振幅增加而单调增加,而是存在一种初期的下降-恢复效应,且将随碎石护坡层厚度的增加而迅速减弱。根据平均Nusselt数随路面温度振幅变化关系中的下降-恢复效应,可以确定路堤碎石护坡层的最小厚度。

The non-dimensional governing equations for natural convection in variable permeability porous media are derived. Triggering and evolution of winter-time natural convection cooling effect in fractured-rock revetment embankments of the Qinghai-Tibet Railway are investigated by means of an air-flow function. In order to evaluate the effectiveness of the cooling effect of winter-time natural convection in railway embankment with fractured-rock revetment, the mean Nusselt number representing heat transfer through unit length at the base of embankment is defined. The dependences of the average Nusselt number on the thickness of fractured-rock layer and temperature amplitude of embankment surface are analyzed using a numerical representation of the non-dimensional governing equations for heat convection. The analysis shows that the mean Nusselt number in fractured-rock revetment embankments decreases early and then increases with increasing of the temperature amplitude on the surface instead of increasing monotonically. This decrease-increase effect during increasing of the mean Nusselt number will rapidly weaken with increasing of the temperature amplitude. The approach to evaluation of the critical thickness of fractured-rock layer in revetment embankments is developed on the basis of the decrease-increase effect. Validity of estimating the fractured-rock revetment layer thickness in the railway embankment is demonstrated both numerically and experimentally. Therefore, this evaluation approach in the present work is available for construction of the Qinghai-Tibet Railway.