Heat transfer in a train/tunnel annulus

Airflows in the annular space around a train in a tunnel are approximated by two boundary layer flows for which matching solutions are obtained at an intermediate radius. The resulting solution is used to obtain a relationship between the rate of flow along the annulus and the pressure difference between the front and rear of a train. When solved simultaneously with further relationships describing flows in the annulus and the open tunnel, this enables unsteady Bows to be predicted throughout a train journey. Boundary layer relationships provide convective heat transfer coefficients at the train and tunnel surfaces. They are used as boundary conditions for transient heat conduction analyses in the tunnel lining and in the train body. It is found that heat transfers into the tunnel wall are limited more strongly by conduction than by convection. In contrast, convection is found to be the limiting condition at train surfaces. The implications of these results are discussed in the context of analyses of overall airflows in tunnels.