Pro/Forums - View Single Post - Swapped an Eheim 1048 for a Eheim 1250, Interesting results!

myv65 · 09-10-2002, 10:00 PM

This one is specifically for Ben, but for all to consider.

It is a myth that radiators have a sweet spot. Perhaps "myth" is too strong a term. Maybe misunderstanding is more accurate.

When you're considering fluid flow through a radiator you are concerned with convection from a fluid to a solid. Sound familiar? It's the same concern with a water block. And just like a water block, higher velocity will always improve heat transfer.

Take a gander at the equation for convection again. Q = h * A * delta-T. Both "h" and "delta-T" are continuous functions that vary throughout the radiator. Nonethless, "h" is largely a function of velocity with higher velocity increasing "h". For a fixed "Q" and "A", this means higher velocity equates with lower delta-T.

So why the misunderstanding? Just like with waterblocks, pushing more flow through a radiator requires more pump power. Power tends to go up as flow^3. When flow and power are both low, it doesn't take a whole lot of power to increase flow rate "significantly". As a result you get better results from a flow increase. Once you get to a certain point, it requires a greater increase in power to generate more flow than the improved "h" can compensate.

There's a double-whammy here, too. As "h" gets better, delta-T drops toward zero. You start getting delta-T too small and it requires massively more flow to make a dent toward yet lower delta-T values.

So practically, yeah, if you get beyond a certain flow (which varies by radiator) then you'll see a drop in "performance" versus increased flow. This drop is largely imaginary, as the radiator is really dissipating more and more energy as the flowrate goes up. The only way to truly judge this is to decrease heat input from a CPU (or simulator) as the power to the pump goes up such that total power input remains constant. Even this is a crapshoot, however, as pump efficiency varies according to flow rate.

Anyway, more flow will always make a radiator more effective. It just won't always make your fluid temperatures better.

Should also note that water flow is only a part of the overall picture. Airflow is every bit if not more important.

09-10-2002, 10:00 PM	#50
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	This one is specifically for Ben, but for all to consider. It is a myth that radiators have a sweet spot. Perhaps "myth" is too strong a term. Maybe misunderstanding is more accurate. When you're considering fluid flow through a radiator you are concerned with convection from a fluid to a solid. Sound familiar? It's the same concern with a water block. And just like a water block, higher velocity will always improve heat transfer. Take a gander at the equation for convection again. Q = h * A * delta-T. Both "h" and "delta-T" are continuous functions that vary throughout the radiator. Nonethless, "h" is largely a function of velocity with higher velocity increasing "h". For a fixed "Q" and "A", this means higher velocity equates with lower delta-T. So why the misunderstanding? Just like with waterblocks, pushing more flow through a radiator requires more pump power. Power tends to go up as flow^3. When flow and power are both low, it doesn't take a whole lot of power to increase flow rate "significantly". As a result you get better results from a flow increase. Once you get to a certain point, it requires a greater increase in power to generate more flow than the improved "h" can compensate. There's a double-whammy here, too. As "h" gets better, delta-T drops toward zero. You start getting delta-T too small and it requires massively more flow to make a dent toward yet lower delta-T values. So practically, yeah, if you get beyond a certain flow (which varies by radiator) then you'll see a drop in "performance" versus increased flow. This drop is largely imaginary, as the radiator is really dissipating more and more energy as the flowrate goes up. The only way to truly judge this is to decrease heat input from a CPU (or simulator) as the power to the pump goes up such that total power input remains constant. Even this is a crapshoot, however, as pump efficiency varies according to flow rate. Anyway, more flow will always make a radiator more effective. It just won't always make your fluid temperatures better. Should also note that water flow is only a part of the overall picture. Airflow is every bit if not more important.