Effectiveness and physics behind Heat Transfer

[Vacras]

Hey guys,

Today I started a litle "experiment" to check how fan speed impacts the performance of my CPU cooler ("Alpenföhn Brocken" for anyone wondering).

So I started one test with my fan running at the lowest RPM possible and monitored the temperatures and one test with my fan running at max speed.

 

Spoiler

Fan running at lowest speed:

 

Fan running at highest speed:

 

You can see that I turned up the fan speed whilst stresstesting which is visible in the drop of temperature on all cores.

As you can see the impact of the fan is quite small, but still bigger than I expected it to be. A drop from ~65°C to ~60°C on the hottest core by increasing fan speed is noticable. So around 4-6 degrees C lower temperature with a fan speed that is nearly doubled. Which really isn't that much considering the jet-engine next to me at nearly 1500 RPM. Apparently heat dissipation doesn't seem to scale well with the amount of air moving though the fins, but why is that?

I am really curious about the physics behind heat transfer from my aluminium heatsink into the air. Does anybody know how exactly this works and how the amount of air moving through the fins scales with heat transfered? I can't seem to find any good source of information on german and my english skills aree not good enough to search trough english wikipedia entries. I would realy appreciate some nice and simple explenations behind the numbers I've got as results.

[Nineshadow]

In physics, the second law of thermodynamics says that heat flows naturally from an object at a higher temperature to an object at a lower temperature, and heat doesn’t flow in the opposite direction of its own accord. Heat is being transferred to the air surrounding the cooler , which is then recycled by the fans.

[Vacras]

1 minute ago, Nineshadow said:

In physics, the second law of thermodynamics says that heat flows naturally from an object at a higher temperature to an object at a lower temperature, and heat doesn’t flow in the opposite direction of its own accord. Heat is being transferred to the air surrounding the cooler , which is then recycled by the fans.

I am aware of that yeah. But the air isn't realy getting recycled, is it? It's getting sucked in at the front and the bottom of my case, pushed through my CPU heatsink and then exhausted at the back of the case. Obviously not all the air is getting refreshed and the case itself heats up, but that still doesn't explain to me how the amount of airflow impacts the amount of heat dissipated from my heatsink into the moving air.

 

[Nineshadow]

Just now, Vacras said:

I am aware of that yeah. But the air isn't realy getting recycled, is it? It's getting sucked in at the front and the bottom of my case, pushed through my CPU heatsink and then exhausted at the back of the case. Obviously not all the air is getting refreshed and the case itself heats up, but that still doesn't explain to me how the amount of airflow impacts the amount of heat dissipated from my heatsink into the moving air.

 

The air gets heated up and the fan displaces it somewhere else and replaces it with other "cooler" air. The more air you displace and the faster you do it the better.

Anyway , this is not an ideal case scenario.

[Vacras]

1 minute ago, Nineshadow said:

The air gets heated up and the fan displaces it somewhere else and replaces it with other "cooler" air. The more air you displace and the faster you do it the better.

Anyway , this is not an ideal case scenario.

Is there some sort of formula to calculate the amount of heat being dissipated (in the form of Joule) with taking the amount of ariflow into consideration? I'm just surprised on how little change in temperature is noticable even with the huge increase of air moved.

[Nineshadow]

3 minutes ago, Vacras said:

Is there some sort of formula to calculate the amount of heat being dissipated (in the form of Joule) with taking the amount of ariflow into consideration? I'm just surprised on how little change in temperature is noticable even with the huge increase of air moved.

It's because the air and the cooler are in contact for such little time.

[Vacras]

1 minute ago, Nineshadow said:

It's because the air and the cooler are in contact for such little time.

why does the amount of airflow matter anyways then? When the air isn't taking up as much heat as it possibly could during the time it is moving through the heatsink why does an increased airflow still lower the temperature? Or does the ratio of heat addmision of the air decrease with raising temperature?