I am trying to run a thermal simulation of concentric cylinders where the inside cylinder is heated. A tube surrounds the heated cylinder with a small air gap in between (they are attached further down the cylinder with a small contact area, keeping the two concentric). The outside of the tube is at room temperature with natural convection. When I run the simulation with only conduction at the contacting surfaces, the air gap is considered insulation, so the inside cylinder does not heat up the tube at all where they are not contacting. In reality, this would not be the case. I would think there needs to be convection through the air gap, but to add a convective thermal load I have to specify the temperature of the air gap. This temperature is unknown because it depends on how much the inner cylinder heats up the air gap, versus the convection of the external air cooling down the outer tube. I seem to be able to specify radiation between the inside cylinder and outside tube, but this doesn't seem to have much of an affect- the thermal model shows the entire inside cylinder at it's original temperature (around 150 degrees C) and the entire outside cylinder as room temperature. Is there a better way to simulate the heat transfer through the air gap?