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Let's say I have two stream supplys that have separate mass flow rates, enthalpies, and velocities that flow into an engine. Upstream of the engine's inlet, the two streams mix to form one stream that flows into the engine inlet. Do the properties of the streams add together when they mix like the mass flow rate adds together?

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It depends on whether the entropy of mixing is negligible, I suppose, and whether the streams mix into a homogenous fluid. Can you be more specific about the engine, like the actual meaning of the enthalpies? If you are talking about a heat engine operating with only a slightly warmer input than output, the answer might be very different than an internal combustion engine with huge chemical energies released internally. – Ron Maimon Nov 22 '11 at 7:12
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@Greg: if it was a homework question, I would guess they are most likely asking you about entropy of mixing, which is a common course topic in thermodynamics. The flow rates will add, the velocities will combine so that the velocity times pipe cross section adds up, and the enthalpies will have a mixing entropy correction, so that mixed fluids will cool down. For homework questions, you might want to give the exact phrasing, and the topics covered in the class. – Ron Maimon Nov 23 '11 at 7:37
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If it's a homework question, you most certainly should say so. It's not that we're less willing to help with homework questions, but you may have noticed that some people just post their homework problems verbatim and hope someone here will provide the solution. We don't do that. For a question like this one which is asked in a "conceptual" manner, identifying it as homework simply means that we'll answer it a little more generally than we would otherwise. (P.S. I deleted an inappropriate comment above) – David Zaslavsky♦ Nov 23 '11 at 22:02
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I've actually been thinking: the way you're asking your questions, they're general enough that they don't really count as "homework questions." Rather, they're general conceptual physics questions that just happen to be motivated by homework problems. That's a good thing - what you're doing is exactly what we wish most people who come here looking for homework help would do. It also means that the "homework" tag doesn't need to be applied to your questions, at least not the way you've asked them. (The tag is more for people who feel the need to post their specific homework questions.) – David Zaslavsky♦ Nov 23 '11 at 22:27
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I do realize that this makes it kind of tough to decide when you need to use the "homework" tag and when you don't... when it doubt, it doesn't hurt to use it at first, and we can always take it off later. – David Zaslavsky♦ Nov 23 '11 at 22:30
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