Laboratory study on thermal performance of natural air convection in porous media

Jianfeng Chen, Lukas Arenson, David C. Sego

In the proceedings of: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: T2-D

ABSTRACT: study on thermal performance of natural air convection in porous media Jianfeng Chen Golder Associates Ltd., Burnaby, BC, Canada Lukas Arenson BGC Engineering Inc., Vancouver, BC, Canada David C. Sego Department of Civil and Environmental Engineering University of Alberta, Edmonton, AB, Canada ABSTRACT This study focussed on a detailed understanding of the characteristics of natural air convection in porous media. Heat transfer experiments were carried out using a well-insulated cylindrical tank filled with Styrofoam chips. Convection and conduction were caused by controlling the boundary temperatures at the top and bottom of the tank, and a number of cross-sectional conductive and convective isotherms were generated from collected temperature data. Convective patterns were also obtained from tests by centrally localized heating from the bottom or cooling from the top of the tank. Air flow velocities were measured at the center of the tank. RÉSUMÉ Cette étude s'est concentrée sur un arrangement détaillé des caractéristiques de la convection naturelle d'air dans des milieux poreux. Les expériences de transfert thermique ont été effectuées en utilisant un réservoir cylindrique rempli de morceaux de mousse de polystyrène. La convection et la conduction ont été provoquées par les températures au dessus et au bas du réservoir. Un nombre d'isothermes conductrices et convectrices ont été produites des données rassemblées de la température. Des modèles convecteurs ont été obtenus à partir des essais par le chauffage centralement localisé du fond ou le refroidissement à partir du dessus du réservoir. Pareillement des vitesses de circulation d'air ont été mesurées au centre du réservoir. 1 INTRODUCTION Convective air cooling in Arctic climates has been observed for decades and had been applied in cold regions geotechnical engineering design and practice for waste rock piles and road embankments. Natural air convection in porous media, extensively studied in mechanical and thermodynamical engineering (Nield and Bejan, 1999), is gradually becoming an important consideration in geotechnical engineering mainly because of the potential for high heat transfer rates. Protection of permafrost from thaw in cold regions is a critical design consideration for various civil facilities. Convective air movement in highly permeable material is more efficient at cooling the ground in winter, whereas in summer, thermal conduction with lower heat transfer rates dominates and therefore protects the ground from thawing. As such, an air convection embankment (ACE) constructed from poorly graded coarse material was called Goering and Kumar (1996), or thermal semi-conductorCheng et al. (2007). Harris and Pedersen (1998) performed a temperature monitoring study at the Plateau Mountain in Alberta and found that the mean annual ground temperatures in the blocky materials of that area were 4 to 7C cooler than nearby mineral soils because of the air convective cooling effect. Practical application of ACE were reported by Cheng et al. (2008) and Xu and Goering (2008). A number of experimental and numerical studies on the cooling effect of air convection in highly permeable embankments have been reported by Goering (1998; 2002; 2003), Goering et al. (2000), Yu et al. (2004), Sun et al. (2005), Ma et al. (2008), Jørgensen et al. (2008), and Wu et al. (2008). Arenson and Sego (2007) presented a numerical simulation showing that convection of cold winter air in a coarse mine waste rock cover for a tailings pond can be used to accelerate the freezing and potential stabilization of the tailings. In addition, Pham et al. (2008) conducted field temperature monitoring study and numerical modelling on mine waste rock piles located in the Northwest Territories, and showed that temperatures of the pile cores lowered by up to 5 °C in nine years due to cooling effects of natural convection. Both conductive and convective heat transfer mechanisms must be fully understood when working on cold regions or permafrost engineering if poorly-graded materials are being used for construction (e.g., railway ballast embankments, rock fill dams, waste rock piles and tailings covers after mine closure). In the present study, a laboratory investigation of convective air heat transfer in a porous medium was carried out by using a cylindrical test tank. A variety of isotherms were obtained by measuring cross-sectional temperatures. Initial results of this laboratory study were published by Arenson et al. (2007). This paper presents succeeding test results based on adjusted experimental configurations. 2 TEST APPARATUS AND PROCEDURES An insulated, cylindrical plastic tank filled with Styrofoam chips as a surrogate for porous media, was selected. The 54

RÉSUMÉ: ory study on thermal performance of natural air convection in porous media Jianfeng Chen Golder Associates Ltd., Burnaby, BC, Canada Lukas Arenson BGC Engineering Inc., Vancouver, BC, Canada David C. Sego Department of Civil and Environmental Engineering

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