This tool will take an object and turn it into a source of heating when particles pass through. You would typically use this tool if you want to heat a fluid in your scene that has been set up with another one of the tools on the Viscous Fluid shelf. For example, simulating a cool object being put onto a heated grill, causing it to melt. Another use would be for artistically controlling part of your viscous fluid. For example, if you were simulating a river of lava which cools over time and comes to a stop, you could place hot objects along the river to prevent it from cooling down too much.
Using Heat within Object
Select the object to be the heat source.
Click the Heat Within Object tool on the Viscous Fluids tab.
Select the fluid object to heat, and press Enter.
Heat Within Object vs Cool Within Object
The Heat Within Object and Cool Within Object tools are very similar. The main difference is that the heating tool has a high Increase Rate meaning that melting objects will heat up, and the cooling tool has a high Decrease Rate meaning that objects will cool off quickly.
The Increase/Decrease Rate parameters can be found on the POP Attribute from Volume node, and work similar to the way the Temperature Diffusion parameters on the Gas Temperature Update node work. They control how quickly the temperature from your heated or cooled object will affect your viscous fluid.