Abstract
This paper presents a review and analysis of large-scale air convection tests and the establishment of intrinsic permeability in coarse open-graded materials. Natural air convection can make a significant contribution to heat transfer during cooling periods. In seasonally freezing environments this can result in excessive frost penetration and subsequent frost-related problems. Intrinsic permeability largely defines the onset of convective heat transfer in granular materials. Conventional methods for measuring intrinsic permeability cannot be applied to very coarse materials. Large-scale laboratory experiments on natural air convection can serve as an alternative method for determining this crucial parameter. This paper gives an overview of four different experimental test setups for measuring natural air convection, all differing in physical shape, boundary conditions and heat flux/temperature measurement devices. Comparison between these is difficult because the air convection pattern can differ and in some cases the shape and number of convection cells cannot be validated. Most of the studies available in the literature use theoretical equations to approximate intrinsic permeability. A method based on the analytical Nu-Ra number relationship is employed to establish the values of intrinsic permeability. Tests that provide enough data to enable the use of the Nu-Ra relationship are very limited. The overall results show a reasonable correlation between experiment-based intrinsic permeability and theoretical approximation. However, several issues must be addressed: first, differences may exist between the intrinsic permeability of natural and of crushed materials due to the shape effect. Second, the method used is in theory valid only for two-dimensional air convection within a square enclosure heated from below. Yet the results show that this method could be extended to other conditions with a certain degree of confidence. Third, a good estimate of intrinsic permeability is possible only with accurate experimental measurement.
Highlights
Thermal conduction is the major heat transfer mode in conventional road and railway construction materials [1,2]
Natural air convection has been a topic of interest for road and railway engineers for the last few decades because it is a temperature gradient-dependent heat transfer mode
This paper presents an overview of the establishment of the intrinsic permeability of coarse open-graded materials based on laboratory experiments
Summary
Thermal conduction is the major heat transfer mode in conventional road and railway construction materials [1,2]. When coarse open-graded materials are used, other heat transfer types may contribute significantly to the overall heat transfer rate. Natural air convection has been a topic of interest for road and railway engineers for the last few decades because it is a temperature gradient-dependent heat transfer mode (upward heat flow). It considerably increases the overall heat transfer extraction rate only during the cooling periods. This in turn can yield favorable effects only in permafrost conditions, contributing to the ground cooling rate and maintaining. The same process in a seasonally freezing seasonally freezing environment can result in excessivedepth frost-penetration [4],heave leading to frost environment can result in excessive frost-penetration [4], leadingdepth to frost problems
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