This workflow is now obsolete, please use the Arnold Volume object node to render VDB files.
To render a vdb file it is recommended to use the Arnold VDB Volume node, which is a premade network for rendering VDB Volumes. The UI combines parameters from the Volume Container, Volume Collector and Volume VDB Nodes.
This can be created at scene level by Tab > Arnold > Arnold VDB Volume.
Select the VDB Volume tab and in the File Name put the path to the VDB file. In the Grid parameter select the volume element you want to view choosing from density (smoke), heat (flames) or temperature. It is recommended to choose density here, see below.
Navigate down through the arnold_vdb node, the shopnet and arnold_vopnet to see the shading network creating the volumes. By default you will see that it is setup for density (smoke) only as the vdb file is connected to the Scattering and Attenuation of the Volume Collector.
Create another Volume OpenVDB node and add the VDB file. In Grid this time select heat.
The information coming from the heat field needs to be coloured. Create a Ramp RGB node and connect the Volume OpenVDB to the input. Choose colors like below to map the range.
Connect the Ramp RGB to the emission input of the Volume Collector node.
Rendering again should now also show the flames.