Let's take another, closer look at what's happening on in the stack, and get some deeper insight into where the energy comes from and where it goes.
First off, remember that we haven't said anything yet about what's making the air resonate. We're just assuming that the air is resonating. Of course, something must be making it resonate. We haven't said what that is yet, we'll return to this very shortly.
If there are no heat exchangers, the stack sits there with one end hot and one end cold. The temperature gradient of the stack is the same as the gradient of the parcel of air we're talking about. There's no work being done, no net flow of energy in or out (assuming we can neglect viscous and other losses).
In this figure, and the figures following, we're showing temperature (vertical) as a function of position (horizontal) of the parcel. The temperature of the stack is drawn as a dotted line.
Figure 5. Gradient with no work, passive stack. The parcel
temperature (round blob) is the same as the stack temperature (dotted
line), so no heat flows in or out of the parcel.
This page maintained by Wil Howitt