Passive Air-Conditioning: Flow Through Rock-Lined Tunnels Tempers Air

The insulating properties of rock surrounding underground openings have been used to advantage in creating environments that have stable temperatures and are characterized by the mean annual climatic condition at a particular geographic location. However, significant flow rates of air at surface temperatures into subsurface space will cause much of the insulated advantage of the tempered environment to be lost unless the air is first passively conditioned by passage through a rock-lined tunnel. Changes in temperature and enthalpy as air passes along a length of rock-lined tunnel will be affected by many parameters, the more important of which include: • • the airway length, • • the amount of free water present on airway surfaces, • • the thermal conductivity and thermal capacity of the rock, • • the temperature of the rock, and • • the air velocity through the airway. To achieve efficient tempering of outside air that exhibits extremes of climate, the influence of these conditions and the interrelationships of changes over time must be considered when designing an airway. An in-situ study to examine these conditions over time was undertaken at the University of Missouri-Rolla Experimental Mine. Air was passed through a 185-m length of passageway in dolomite. Continuous measurement of air/rock for 2.5 days gave data that can be used to ascertain appropriate design conditions for a particular application. To examine seasonal effects, these tests were conducted during the summer, fall, and winter months. Thermodynamic conditions at the air/rock interface are important in ventilation engineering of deep underground mines. Airflow heat transfer studies are of particular significance in hot mines in which air refrigeration is necessary. From various studies, theoretical relationships have been developed for predicting heat flow across the interface. Results of this study are compared with the theoretical relationships from which conclusions are draw