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Old 10-05-2012, 08:29 AM   #73
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Originally Posted by TerraPhantm View Post
PV=nRT (or PV=NkT for those who prefer that form) is applicable to gasses. So as the temperature increases, the pressure exerted by the gas in the ET will increase. So while the flowing water is not applicable to the ideal gas law (as it is not a gas), it still has relevance. Remember Bernoulli's equation 1/2(rho)v^2 + (rho)gh + P = constant... The "P" value will increase as a result of the pressure exerted by the gasses.

Sean, what WDE46 wrote is indeed a simplification. Since we are assuming volume and n are constant, we can write the equations for the 2 scenarios as follows --

P1*V = n * R * T1
P2*V = n * R * T2.

Rearranging, we get:
P1 / T1 = nR/V
P2 /T2 = nR/V


Obviously this only applies when n and V are constant, but I think it's pretty obvious that WDE46 understood that.
Terra, I know the algebra, but thanks for laying it out for those who may not. Technically it is a called special case of the ideal gas law when n and V are constant. It is not called a "simplified" version. If you insist on writing it for 2 different conditions, you need to have the knowledge of 3 of the 4 unknowns in the form that you write it (1 equation 4 unknowns). That means you first have to use the PV=nRT twice to get to those 3 unknowns (actually you'd know all 4 by then). How's this for simplified?

Originally Posted by TerraPhantm View Post
Sean, I disagree with with your statements about the coolant boiling around areas of higher temperature. Remember, the fluid will be flowing a lot slower around the block - there's a much larger volume for the fluid to fill. Slower fluid = higher pressure = higher boiling point.

There are only a couple ways the cooling system will actually reach 2 or 3 bar. Either A) Coolant starts to boil and the total molecules of gas increase or B) More fluid enters the expansion tank and reduces the volume the gasses can occupy (this will happen when in situations where the heater core closes, thermostat closes, or if the system is overfilled).

If coolant starts boiling, you're in trouble - the engine will overheat drastically anywhere the coolant turns into a gas. So I highly doubt the system is engineered to even allow that possibility. I must therefore conclude that the cap is to allow ventilation in the event the system is overfilled.
Good thoughts, but you need to quantify how much fluid velocity will affect it's temperature, hence its boiling point, in order to convince me. I never said my calculation accounted for everything in this system. It's not comprehensive by any means. I am well aware of a lot of other things that might affect, but I am not an automotive engineer. This is the best I can do with my current knowledge of the matter

Originally Posted by dknightd View Post
You are forgetting that the liquid in the system also expands when it is heated, so, the volume of gas is not a constant.
I am aware of a lot of other things buddy, this being just one. I mentioned about it somewhere above. My calculation is just an approximation, and I believe I took into account the most important factors, and my conclusions make sense for what they are. If you can do a better calculation, I'm all ears

Originally Posted by WDE46 View Post
Yeah, you're right aobut the steam. No need to be a dick though. I know this stuff, but I will admit I am a dynamics and kinematics expert, not a theromodynamics expert. You apparently just took Chem 101, though. Also, that equation is "simplified". It is two ideal gas equations combined into its simplified version, eliminating the common constants.
Sorry if it came that way. I have to say that I have a Ph.D. in theoretical physics. While my Chem 101 may not be as fresh as those who just took it, I know a thing or two about thermo

Let's try that again. That equation is not "simplified". You cannot combine two gas equations that "easily". If you do, you'll end up with more unknowns (4) than you have equations (1). The way you wrote it applies to systems where you have 3 of those 4 unknowns beforehand. See my reply to Terra above. I hope it's clear now.

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