ABSTRACT
A numerical and experimental investigation of convective heat transfer within railway embankment ballast material has been carried out. The study reveals that natural convection of the ballast pore air can have a large impact on heat transfer rates during periods when unstable pore-air density gradients exist within the material. This may lead to enhanced frost penetration and frost heave in foundation soils beneath the embankment and have an adverse impact on the performance of high-speed rail lines during winter. Experimental measurements were obtained using a test cell with dimensions of 1 × 1 × 0.75 m (length, width, and height). When operated in a stable mode, with heat input at the top and heat removal from the bottom, the vertical temperature profile was linear and measurement of the downward heat flux allowed us to calculate the thermal conductivity of the material. However, when heat was supplied at the bottom, we observed an increase in the apparent thermal conductivity of the material due to the presence of pore-air convection. In addition to the experimental studies, a numerical analysis of the pore-air convection and heat transfer for both the top and bottom heating cases has been carried out. Results were obtained with a three-dimensional finite difference model that solves the steady state energy and fluid flow equations. Coupling between the pore-air flow and the temperature field was accomplished via the Bousinesq approximation. For the top-heating case, the model results indicated that pore-air flow within the test cell was negligible, and heat transfer was dominated by downward conduction. For the bottom-heating case the pattern of pore-air circulation produced by the model consisted of a four-lobed structure which was consistent with experimental observations. In this case the model results show that upward heat transfer was enhanced significantly by the presence of pore-air convection.
