How to show the boundary/outline of a curved region?

I am visualizing CFD results from a nozzle simulated in OpenFOAM. My colormap shows high temperatures as a white-ish color, as seen below.

Here’s what it looks like with a different colormap:

How can I render a boundary around the entire computational domain so that the shape of the nozzle is visible on the left?

Does “Feature Edges” give the desired appearance (perhaps loading the dataset twice, once with the temperature field and once with Feature Edges)?