Looking for opinions. Better to suck hot air or push cool air?

**bluTDI09** · 03-30-2010, 07:00 PM

Originally Posted by OldSpark

Excellent Blu!
But you didn't say that if the air goes too fast, the heatsink does not have time to cool down.... LOL!

I couldn't resist.... Many car buffs claim that is the reason cars overheat when high-revving or after removing the thermostat (removing thermostats on some engines makes them run cooler; others hotter).
IE - the water/coolant isn't in the radiator long enough to cool down, or it travels too fast to get rid of its heat! (Yet surprisingly it's in the engine long or slow enough to pick it up!)

I'd try to explain the non-time dependent relationship and the difference between heat transfer and temperature - eg, if travelling three times as fast it may only fall by half the normal temp drop, but it does that 3x faster - ie, more heat transferred per unit time.
And lots of other analogies....

For those that don't know why....
If fluid moves very fast, it can jetstream or streamline (pick a narrower path through the radiator etc).
Also if too fast, the driving blade (waterpump) can cavitate and hence reduce flow.
Removing the thermostat reduces restriction (back-pressure) and causes increased flow rate - unless cavitation occurs. (And increase flow may jetstream.)

Hence hi-revving engines may need to reduce waterpump speeds.
Removing thermostats depends on normal versus cavitation speeds etc.
Jetstreaming depends on the layout (water paths).

And on a related note - cavitation reduces input power. EG - a fan blockage causes the fan to speed up and its current drop.
That's the opposite to what many failed alarm systems seem to think!

Blu - is my novice explanation(s) ok?
And I assume this is valid for low speeds - too high and friction causes heating, but we are talking speeds way below that (ie, subsonic?)?

I thank you for your q formula which is devoid of time & speed (other than q being a "rate", but that's not "speed").
I must use it next time....

Without starting a debate about the complexities of fluid flow (of which I certainly do not claim to be an expert), I will say that things rarely behave as simply as to be directly applicable to fundamental equations such as the one I gave above. That is just the simplest way to express convection heat transfer. All the complexity of the geometry is accounted for in the convection coefficient. The "speed" of the fluid is one of the things that is included in it.

You are right that fluid flow rates can change the heat transfer properties through boundary layer effects and flow properties such as turbulence. This is the primary reason that heat exchangers have an optimum flow rate. It is not so much that the fluid doesn't have "time" to absorb the heat. Consider a closed loop system where 50% of the length is inside the heat generating part (engine) and 50% is in the heat dissipating part (radiator). No matter what the velocity of the fluid (neglecting flow properties and such mentioned above), every fluid particle is still spending 50% of its time getting hotter and 50% of its time getting cooler. Increasing the velocity just means that they do more "laps." What increased flow will do is minimize the gradient in the loop, meaning the hottest point where the fluid leaves the engine and the coolest point where the fluid leaves the radiator will be closer to the same temperature. It is also important to keep in mind that there is also a difference between a system such as a car radiator system where the coolant is recirculated and where the cooling fluid is expelled directly into the ambient.

Within the scope of this problem (80-120mm fan spinning at less than 2000 rpm and heat generation from computer components), all that is probably completely irrelevant. If the goal is to cool computer components with a fan system, higher velocity with cooler air will always be better for heat transfer. That is not to say that an exhaust fan might not help the system as well though.

**Sonicxtacy02** · 03-31-2010, 08:32 AM

Originally Posted by bluTDI09

Without starting a debate about the complexities of fluid flow (of which I certainly do not claim to be an expert), I will say that things rarely behave as simply as to be directly applicable to fundamental equations such as the one I gave above. That is just the simplest way to express convection heat transfer. All the complexity of the geometry is accounted for in the convection coefficient. The "speed" of the fluid is one of the things that is included in it.

You are right that fluid flow rates can change the heat transfer properties through boundary layer effects and flow properties such as turbulence. This is the primary reason that heat exchangers have an optimum flow rate. It is not so much that the fluid doesn't have "time" to absorb the heat. Consider a closed loop system where 50% of the length is inside the heat generating part (engine) and 50% is in the heat dissipating part (radiator). No matter what the velocity of the fluid (neglecting flow properties and such mentioned above), every fluid particle is still spending 50% of its time getting hotter and 50% of its time getting cooler. Increasing the velocity just means that they do more "laps." What increased flow will do is minimize the gradient in the loop, meaning the hottest point where the fluid leaves the engine and the coolest point where the fluid leaves the radiator will be closer to the same temperature. It is also important to keep in mind that there is also a difference between a system such as a car radiator system where the coolant is recirculated and where the cooling fluid is expelled directly into the ambient.

Within the scope of this problem (80-120mm fan spinning at less than 2000 rpm and heat generation from computer components), all that is probably completely irrelevant. If the goal is to cool computer components with a fan system, higher velocity with cooler air will always be better for heat transfer. That is not to say that an exhaust fan might not help the system as well though.

this is sorta what i'm seeing with my install. I hooked up a single high-velocity exhaust fan connected to a tube to suck air out of the DIN, but in doing that i'm not really relieving any of the heat. It seems the components (motherboard, processor, PSU, AND a monitor) simply create heat faster than the exhaust can expel it. If i pull my bezel out of the dash only slightly so there's a half inch of space between bezel and dash it GREATLY reduces temps (-20°C).

In the next day or two i will connect a second fan to blow cabin air into the DIN through a similarly sized tube. I now believe that cooling the air rather than attempting to simply expel the hot air may be exactly what i need.

**surfntomm** · 03-31-2010, 02:11 PM

Originally Posted by Sonicxtacy02

this is sorta what i'm seeing with my install. I hooked up a single high-velocity exhaust fan connected to a tube to suck air out of the DIN, but in doing that i'm not really relieving any of the heat. It seems the components (motherboard, processor, PSU, AND a monitor) simply create heat faster than the exhaust can expel it. If i pull my bezel out of the dash only slightly so there's a half inch of space between bezel and dash it GREATLY reduces temps (-20°C).

In the next day or two i will connect a second fan to blow cabin air into the DIN through a similarly sized tube. I now believe that cooling the air rather than attempting to simply expel the hot air may be exactly what i need.

if you have a fan pulling air but no real inlet then that fan isnt doing all that much. Once you pull out the bezel and create and opening you have a draft. its like opening a window in your room but leaving the door closed. If you open the door a draft will result, assuming the rest of your house isnt closed. Hopefully this helps you out. If you can not create an inlet your best option is to accept the fact that the air will be much hotter and localize fans on critical components to increase the velocity and thereby increasing your heat transfer.

I do not claim to be an expert either but I do design cooling solutions for a living so I may have some valuable experience for those of you with cooling issues.

**SteveSleeves** · 08-11-2010, 06:37 PM

Originally Posted by Sonicxtacy02

I've got my core 2 duo pc successfully mounted in the double din of my rsx. Everything is working, but the combination of a mobo, dsatx, and monitor has the temps of the components way too high. I figured out a way to run hvac tubes to a pc fan which will route air... But which way should I route it? Should I suck cool air from the cabin or suck hot air from the din. Your opinion counts!

Why choose if you dont have to? Do both forced air in and out. Make them independent of each other, that way if one system fails, the other will keep working.

**OldSpark** · 08-12-2010, 08:40 PM

I think that thread was done.....
Pushing air is usually the preferred & better method because the push can be directed.
Suckers are harder to "air flow".

**Sonicxtacy02** · 08-13-2010, 08:25 AM

Originally Posted by SteveSleeves

Why choose if you dont have to? Do both forced air in and out. Make them independent of each other, that way if one system fails, the other will keep working.

it is what i ended up doing. I kept my one exhaust fan, and connected an array of 6 fans to a tube that stretches all the way in my trunk for blowing air into the pc. What i need to do now is figure a way to automatically turn these fans on when the car isnt running. The only time heat is an issue now is when the cars been sitting in the sun on 95+ degree days.

**JuniorGeezer** · 08-23-2010, 04:09 PM

I use a single 120 mm fan cooling both the CPU and the entire enclosure.

When sucking up and out, the CPU runs a few degrees hotter but the chipset and the voltage regulators run a few degrees cooler.

When blowing down and in, its vice versa.

I use the sucking up direction. Also, this way, the hot air is blown in its natural direction, up.

I feel a lot more pity with the voltage regulator than with the CPU.

**HiJackZX1** · 08-25-2010, 09:50 AM

With my install I use both methods. On one end of the case two fans draw air in, and directly on the other end are two more fans that pull the air out. Also the case is tall enough to allow clearance for the soundcard and CPU fan, other then that, no space for the air to get caught. I did this with my main PC and the temp drops were astronomical. I am using the same design with my rear PC case, but its much bigger and awkward. With that one, one large fan draws air in, then 2 draw air out. The only issue I see is that it creates extra noise.

Left side is front, with two fans drawing air in. Right side is rear with two fans pull air out.

Rear case. One fan blows air in, the two other fans draw air out.