TM-5-855-4
e. The equivalent radius r3, being twice the ratio of the tunnel cross-section S to its perimeter P, is by
definition the tunnel hydraulic radius. The Biot number B is based on the tunnel wall surfs
based on a time constant equal to the reciprocal of the angular velocity. Functions G1 and G2 of Z and B
are shown in figure 3-10.
f. The amplitude or maximum air temperature departure at point L is the product of the two first
factors in equation 3-23 and occurs when the time is equal to the phase lag GG2 divided by the angular
velocity.
g. If a tunnel or shaft is used only intermittently as an airway, the equations in this section do not
apply. However, it is believed that these equations will give conservative values for heat exchange in that
full utilization of the heat capacity of the surrounding rock is not realized for the intermittent operation.
I
3-16
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