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myv65 · 11-12-2002, 07:38 PM

OK, I *do* have the background and I'll try to give this one a final shot.

Ask yourself a question. When I stand in cold air or jump into cold water, do I feel colder if the air/water is still or blowing/flowing past me? The issue in question is one of convection and the medium, whether air, water, or any other fluid/gas, follows the same rules.

The confusion in radiators seems to stem from the fact that there are three main heat flows that must occur. Note that all three *must* transfer the same amount of energy, as energy can't simply continue to build up indefinitely. OK, so the first is convection from fluid to tube. The second is conduction from tube to fins. The third is convection from fins to air.

Look at these items individually. If you want to get heat from a liquid to a solid, do you want high fluid velocity or slow? The answer is high. In getting the heat from one area of a solid to another, do you want high conductivity or low? The answer is high (aluminum falls behind copper). In getting heat from a solid back to air, do you want high velocity air or low? The answer is again high.

Lots of confusion seems to come from the fact that the air has so much lower density (hence lower convective properties) that people seem to think that keeping the fluid around longer gives the air a greater chance. Fact is that the only thing tying all these things together is that a uniform amount of heat must get through all the barriers. The air really has no idea how fast the fluid is travelling through the tubes. Likewise the fluid has no idea how fast the air is travelling. All the water knows is that it requires a certain delta-T from it to the tube based upon flow rate and heat load. Higher flow rate equates to lower delta-T.

What further muddies the water (sorry for the pun) is that there are at least two other things that can occur here. The first is internal flow enhancements within the tubing. These break up boundary layers to enhance convection. There is evidence in BillA's testing to show that these indeed may have a "bump" in heat transfer versus flow velocity. Note that as a whole, heat transfer goes up with flow, but some radiators have a bump in the curve that I can't personally explain definitively. Second is that it takes energy to drive flow through a radiator. This means that a radiator must get rid of both the thermal energy from the fluid as well as the energy dissipated in the form of a pressure drop over the radiator. This latter term is extremely small for most cases; however, if you start putting massively powerful pumps onto systems that only have a CPU heating the water, then the pump energy can become a dominant contributor. In these rare, rare cases, you can see an increase in CPU temperature in spite of increasing flow. By this time, the pump is probably dumping 200 watts into a system heated by a 75 watt CPU.

Helpful at all?

11-12-2002, 07:38 PM	#43
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	OK, I do have the background and I'll try to give this one a final shot. Ask yourself a question. When I stand in cold air or jump into cold water, do I feel colder if the air/water is still or blowing/flowing past me? The issue in question is one of convection and the medium, whether air, water, or any other fluid/gas, follows the same rules. The confusion in radiators seems to stem from the fact that there are three main heat flows that must occur. Note that all three must transfer the same amount of energy, as energy can't simply continue to build up indefinitely. OK, so the first is convection from fluid to tube. The second is conduction from tube to fins. The third is convection from fins to air. Look at these items individually. If you want to get heat from a liquid to a solid, do you want high fluid velocity or slow? The answer is high. In getting the heat from one area of a solid to another, do you want high conductivity or low? The answer is high (aluminum falls behind copper). In getting heat from a solid back to air, do you want high velocity air or low? The answer is again high. Lots of confusion seems to come from the fact that the air has so much lower density (hence lower convective properties) that people seem to think that keeping the fluid around longer gives the air a greater chance. Fact is that the only thing tying all these things together is that a uniform amount of heat must get through all the barriers. The air really has no idea how fast the fluid is travelling through the tubes. Likewise the fluid has no idea how fast the air is travelling. All the water knows is that it requires a certain delta-T from it to the tube based upon flow rate and heat load. Higher flow rate equates to lower delta-T. What further muddies the water (sorry for the pun) is that there are at least two other things that can occur here. The first is internal flow enhancements within the tubing. These break up boundary layers to enhance convection. There is evidence in BillA's testing to show that these indeed may have a "bump" in heat transfer versus flow velocity. Note that as a whole, heat transfer goes up with flow, but some radiators have a bump in the curve that I can't personally explain definitively. Second is that it takes energy to drive flow through a radiator. This means that a radiator must get rid of both the thermal energy from the fluid as well as the energy dissipated in the form of a pressure drop over the radiator. This latter term is extremely small for most cases; however, if you start putting massively powerful pumps onto systems that only have a CPU heating the water, then the pump energy can become a dominant contributor. In these rare, rare cases, you can see an increase in CPU temperature in spite of increasing flow. By this time, the pump is probably dumping 200 watts into a system heated by a 75 watt CPU. Helpful at all?