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

bobo5195 · 10-24-2005, 03:32 PM

Been thinking about his overnight and here is my executive summary. Using the paper previously mentioned and my uni notes.

First LMTD is not useful in the case of WC radiators this is because it takes no account of the load. As my uni notes say you have to use a geometry correction factor to analyse rads and such and need to know all 4 temps this is not useful. I remain unconvinced based on model arguments that LMTD has anything to do with actual rad performance. A rad is essentially 2D. Heat transfer occurs very quickly and depth is less than the length of a fan facing side. I am comfortable in saying that air basically gets heated and expelled from the rad. I believe work done earlier for the PA160 (which I’m badly remembering) shows that depth is not so useful. LMTD is also a pain to measure as it includes an outlet air temperature that is hard to quantify.

Any measure of rad performance should be a measure of the efficiency of the geometry to move heat to the air. This is obviously C/W or W/C from scaling arguments (heat in / DT). You could equally move to say based on my arguments above that it’s a function of frontal area so C/W per m^2. I am not happy with this as absolute performance is what we are after. C/W hence forth known as SD (aka W/C) is obviously a function of specific energy output of the system (in Watts or kack handed American units) and dT. This whole argument is about which dT is it. To my mind there are only two things that matter to that rad and that’s the input conditions. The output does not determine the performance. Hence SD = Q / (Tiw – Tia), which is exactly the same as in the paper.

Hence the argument of Bill that C = water in temp – ain in temp. Stews argument of usefulness is a good one but I am not sure it exactly quantifies the performance as well. If you know Q its is obvious to back calculate the results, but the form looks nasty as shown in image 1. I am half way to convincing myself that C/W for stews case is not independent of Q.

The problem with SD is it tells you nothing off the outputs without a little maths but it does make sense to your average joe on the street from a derivation point of view. Personally I prefer efficiency as it’s a nice quantitative number that tells you what you need to know and is equivalent to SD. It can also be calculated without going into the merky depths of what the cpu core power is. This is useful not only as a test but also because your never sure what heat a CPU is dumping to the WB. You will also notice that eff is proportional to the flow rate which is useful when considering small changes in flow rate and how they will impact your loop (from better tube routing etc.)

As Les pointed out you have changes in properties to worry about. I personally think that these changes are small in the case of material properties over temperature. Adding 1c to water isn’t going to change its properties. Air is slightly different as it has a larger rise and more likelihood of change but given the errors in measurement associated with measuring it I am happy to discount humidity and air Cp. There is obviously some change from fan geometry (and shroud) and fan speed so these should always be included but the eff formula allows small changes in fan speed to be modelled.

Eqs to flow once I’ve scanned them.

10-24-2005, 03:32 PM	#52
bobo5195 Cooling Savant Join Date: Aug 2005 Location: uk Posts: 400	Been thinking about his overnight and here is my executive summary. Using the paper previously mentioned and my uni notes. First LMTD is not useful in the case of WC radiators this is because it takes no account of the load. As my uni notes say you have to use a geometry correction factor to analyse rads and such and need to know all 4 temps this is not useful. I remain unconvinced based on model arguments that LMTD has anything to do with actual rad performance. A rad is essentially 2D. Heat transfer occurs very quickly and depth is less than the length of a fan facing side. I am comfortable in saying that air basically gets heated and expelled from the rad. I believe work done earlier for the PA160 (which I’m badly remembering) shows that depth is not so useful. LMTD is also a pain to measure as it includes an outlet air temperature that is hard to quantify. Any measure of rad performance should be a measure of the efficiency of the geometry to move heat to the air. This is obviously C/W or W/C from scaling arguments (heat in / DT). You could equally move to say based on my arguments above that it’s a function of frontal area so C/W per m^2. I am not happy with this as absolute performance is what we are after. C/W hence forth known as SD (aka W/C) is obviously a function of specific energy output of the system (in Watts or kack handed American units) and dT. This whole argument is about which dT is it. To my mind there are only two things that matter to that rad and that’s the input conditions. The output does not determine the performance. Hence SD = Q / (Tiw – Tia), which is exactly the same as in the paper. Hence the argument of Bill that C = water in temp – ain in temp. Stews argument of usefulness is a good one but I am not sure it exactly quantifies the performance as well. If you know Q its is obvious to back calculate the results, but the form looks nasty as shown in image 1. I am half way to convincing myself that C/W for stews case is not independent of Q. The problem with SD is it tells you nothing off the outputs without a little maths but it does make sense to your average joe on the street from a derivation point of view. Personally I prefer efficiency as it’s a nice quantitative number that tells you what you need to know and is equivalent to SD. It can also be calculated without going into the merky depths of what the cpu core power is. This is useful not only as a test but also because your never sure what heat a CPU is dumping to the WB. You will also notice that eff is proportional to the flow rate which is useful when considering small changes in flow rate and how they will impact your loop (from better tube routing etc.) As Les pointed out you have changes in properties to worry about. I personally think that these changes are small in the case of material properties over temperature. Adding 1c to water isn’t going to change its properties. Air is slightly different as it has a larger rise and more likelihood of change but given the errors in measurement associated with measuring it I am happy to discount humidity and air Cp. There is obviously some change from fan geometry (and shroud) and fan speed so these should always be included but the eff formula allows small changes in fan speed to be modelled. Eqs to flow once I’ve scanned them.