Pro/Forums - View Single Post - can we agree on the basis of "C" in C/W ?

bobo5195 · 10-26-2005, 06:39 AM

Eff, SD and C/W (using input conditions are interchangeable), the equations are the same

Before I say anything going through the other method is a CHECK. It reduces the likelihood of error. Gives you two separate paths to the same answer. It tells you if your C/W gathered in the standard way is on the low or the high side. By gathering the results anyway you can tell loads of useful things. You can predict using fan PQ curves the flow through the fan and various other measures. More importantly it provides a check for Q. If your using a test bench setup you better make sure that your heating apparatus is right this does it. If your using a real loop then how on earth do you know Q to anything more accurate than 10%. Microprocessor heat for a given voltage is hardly exact. How do you know that a program loading a cpu will give X amounts of heat. The system is not well insulated, how much heat is being dissipated in the wires. Its not possible, repeat tests won’t eliminate it. Even you if got C/W exactly right and it was a perfect relationship, when it comes to someone else’s setup you can never be sure. Back pressure in the case will inhibit fan flow so they will not get the same results. As I’ve mentioned they are never go to be able to accurately guess the heat input to give them the right dT. What about radiators, because of manufacturing problems their performance is going to vary it might be within 5% of each other. These are engineering approximations, the fact that a result is given with in 10% is good enough. It tells you that this is better than this with a reasonable accuracy.

I’m saying the current approach works well enough. Its not exactly right but is good enough. The SD and eff approach is based on the 1st law of thermodynamics. Energy in = Energy out, but as you’ve already notice there are things like friction (which you’ve already calculated) which makes this approach less than absolutely correct. By the fact that SD is varying it is obvious that its not a geometric constant that is perfectly stable, independent of flow rate and heat input magnitudes. However the SD approach has been used for over half a century, engineers know its wrong but its easy to measure and is a good approximate catch all value. If people wanted more than SD they had to resort to graphs and correction factors normally.

As way of an example if you use first law to calculate gasoline engine performance you get a value about double what it actually is. Its in the right ball park and the 1st law will tell you loads of good ways to increase performance (it easily predicts that 2 strokes are better than 4 strokes, for example). An approximation using the second law is better still. After that your still not there and you basically need a myriad of gradually more accurate equations to make things right but the complexity goes up exponentially. A similar thing can happen with rads. SD is accurate enough, deriving SD then needing a page of maths to correct things is not the way to go.

As far as I can remember the 3, 2nd law errors that were identified in the book, were boundary layer effects, friction (water AND air) and losses form transferring heat over a finite temperature difference. There are others as well, flow mal-distribution, the fact that rads are thick, end effects from the sides dumping to the air. Change of thermal properties of the air (water content makes a near 5% difference in air Cp), errors in geometry, dust in the radiator, thermal properties of the air (you add 10% de-icer, things are going to change). I may post some of the results out of interest but the idea of applying and using them is a bit far fetched. For example removing heat addition from friction due to flow rate. Does trying to remove this effect actually make results more accurate? Does it tell you more about the radiator? Or does it just mean that you’ve got to recorrect for it to see how the rad actually performs.

In short SD gives you a nice approximate number it varies but its easy to understand and estimate. If don you honestly thing that Bills thermochill results are within 10% for an actual loop?

10-26-2005, 06:39 AM	#66
bobo5195 Cooling Savant Join Date: Aug 2005 Location: uk Posts: 400	Eff, SD and C/W (using input conditions are interchangeable), the equations are the same Before I say anything going through the other method is a CHECK. It reduces the likelihood of error. Gives you two separate paths to the same answer. It tells you if your C/W gathered in the standard way is on the low or the high side. By gathering the results anyway you can tell loads of useful things. You can predict using fan PQ curves the flow through the fan and various other measures. More importantly it provides a check for Q. If your using a test bench setup you better make sure that your heating apparatus is right this does it. If your using a real loop then how on earth do you know Q to anything more accurate than 10%. Microprocessor heat for a given voltage is hardly exact. How do you know that a program loading a cpu will give X amounts of heat. The system is not well insulated, how much heat is being dissipated in the wires. Its not possible, repeat tests won’t eliminate it. Even you if got C/W exactly right and it was a perfect relationship, when it comes to someone else’s setup you can never be sure. Back pressure in the case will inhibit fan flow so they will not get the same results. As I’ve mentioned they are never go to be able to accurately guess the heat input to give them the right dT. What about radiators, because of manufacturing problems their performance is going to vary it might be within 5% of each other. These are engineering approximations, the fact that a result is given with in 10% is good enough. It tells you that this is better than this with a reasonable accuracy. I’m saying the current approach works well enough. Its not exactly right but is good enough. The SD and eff approach is based on the 1st law of thermodynamics. Energy in = Energy out, but as you’ve already notice there are things like friction (which you’ve already calculated) which makes this approach less than absolutely correct. By the fact that SD is varying it is obvious that its not a geometric constant that is perfectly stable, independent of flow rate and heat input magnitudes. However the SD approach has been used for over half a century, engineers know its wrong but its easy to measure and is a good approximate catch all value. If people wanted more than SD they had to resort to graphs and correction factors normally. As way of an example if you use first law to calculate gasoline engine performance you get a value about double what it actually is. Its in the right ball park and the 1st law will tell you loads of good ways to increase performance (it easily predicts that 2 strokes are better than 4 strokes, for example). An approximation using the second law is better still. After that your still not there and you basically need a myriad of gradually more accurate equations to make things right but the complexity goes up exponentially. A similar thing can happen with rads. SD is accurate enough, deriving SD then needing a page of maths to correct things is not the way to go. As far as I can remember the 3, 2nd law errors that were identified in the book, were boundary layer effects, friction (water AND air) and losses form transferring heat over a finite temperature difference. There are others as well, flow mal-distribution, the fact that rads are thick, end effects from the sides dumping to the air. Change of thermal properties of the air (water content makes a near 5% difference in air Cp), errors in geometry, dust in the radiator, thermal properties of the air (you add 10% de-icer, things are going to change). I may post some of the results out of interest but the idea of applying and using them is a bit far fetched. For example removing heat addition from friction due to flow rate. Does trying to remove this effect actually make results more accurate? Does it tell you more about the radiator? Or does it just mean that you’ve got to recorrect for it to see how the rad actually performs. In short SD gives you a nice approximate number it varies but its easy to understand and estimate. If don you honestly thing that Bills thermochill results are within 10% for an actual loop?