I understand we can create Fluid Subdomains, but considering the fluids need to be seperated with a solid region, I can't immediately imagine where this could be of value? I'm sure I'm missing out on some obvious application areas but I'm coming up empty.
Kevin,
Heat exchangers typically always transfer heat from fluid to another. It could be water to water, water to air, refrigerant to air, refrigerant to water, ad infinitum. There is a tutorial on heat transfer that illustrates using fluid sub-domain to model heat exchange in a heat exchanger.