## Convective heat transfer from heatsink calculations?

## Convective heat transfer from heatsink calculations?

(OP)

Good day guys,

I am designing a cooling system for computer with large number of small processor. Simply, There will be 24 boards working together.

Each board have 60 heat sinks and generates 1600 W heat. So, we have 26.67 W heat, under each heat sink. This will be a liquid immersed cooling system. I have selected fluid. I calculated required speed of fluid to take away this heat for single heat sink, but numbers coming up here are impractical. I feel, am using wrong method.

I have these properties of fluid, density, specific heat, viscosity at 40 celsius, thermal expansion coefficient.

Heat sink is made up of aluminium and heat generated beneath the heat sink.

I need to find fluid velocity over heat sink.

Can anyone help on calculating this problem? You can also help me to setup the problem in ansys, if it can solve the problem with given information.

I am designing a cooling system for computer with large number of small processor. Simply, There will be 24 boards working together.

Each board have 60 heat sinks and generates 1600 W heat. So, we have 26.67 W heat, under each heat sink. This will be a liquid immersed cooling system. I have selected fluid. I calculated required speed of fluid to take away this heat for single heat sink, but numbers coming up here are impractical. I feel, am using wrong method.

I have these properties of fluid, density, specific heat, viscosity at 40 celsius, thermal expansion coefficient.

Heat sink is made up of aluminium and heat generated beneath the heat sink.

I need to find fluid velocity over heat sink.

Can anyone help on calculating this problem? You can also help me to setup the problem in ansys, if it can solve the problem with given information.

## RE: Convective heat transfer from heatsink calculations?

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Convective heat transfer from heatsink calculations?

Where is your 1600W from?

IF your total heat is 1600W you will need at least 2400W of cooling.

You should just need to circulate the oil, but your fluid will need to be cooler than your aim temp.

I built a single board/single CPU machine that was in an oil tank.

That was about 300W from the CPU and another 100W from power regulators and other chips.

I used a very thin synthetic oil, very thin. I had about 900W of cooling and was aiming for 10C.

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, Plymouth Tube

## RE: Convective heat transfer from heatsink calculations?

Each source is 27W, so 60*27W=1620W per board.

You're probably limited to about 150C junction temperature, and maybe have 125C case/sink temperature. The fluid needs to be flowing vigorously, so probably not cooler than about 25C, resulting in about 100C delta temperature. Assuming a sink surface area of 2 square inches, you need a fluid convection coefficient of about 250 W/m^2-K. You may find that water is probably the only fluid that comes close.

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Convective heat transfer from heatsink calculations?

Each board is generating 1600 W and each board has 60 processor sets. Each set has individual heat sink. Means each heatsink get 26.67 W heat. and total number of boards are 24. So total heat is 38.xx kW. Each heatsink has surface area of 6345 mm^2 area. This is insane, but is manufactured item. Now I am using mineral oil as water will fry that board. Theoraticaly I will need to calculate

heat transfer co-efficient (h)for this condition and then velocity of fluid. Calculatinghis the problem. I am confused what method equations I should use. I tried to calculate with this methodhttps://archive.org/stream/SolutionManualFundament...It's from Fundamental of Heat and Mass Transfer 6h Edition, By Incropera, chapter 7, problem 7.30.

I used values for Mineral oil.

## RE: Convective heat transfer from heatsink calculations?

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Convective heat transfer from heatsink calculations?

Now, in given example, I follow the method, but answer, I am getting is, 1305.309 m/s. It;s m/s. more than mach 3.

It must be wrong. I need help to solve this. I tried to do ansys analysis, but I am not that good in ansys.

## RE: Convective heat transfer from heatsink calculations?

On problem that you have is that the heat sinks were designed for air cooling, and you want to use a fluid, so the dimensions of the heat sinks are wrong. Have you looked at using very cold air to cool these? 40kW of cooling is the same either cooling air or a liquid. So supply 12T of refrigerated air at some suitably low temperature (chillers get less efficient if you go too cold, so there is a practical limit) and let the heat sinks do what they were designed to do.

You could look at high purity water.

X-ray tubes are water cooled, and they have 50kV across them.

It takes effort, the system must be sealed, the water to start with must have very high resistivity (1Mohm-cm or 0.1 microS/cm) and it gets recirculated through ion exchange resin to keep it low conductivity. This takes great care, but it works very well, and if you keep the water cold (5C) you will get great cooling.

In these multi processor system there are two approaches. One is to use Cu cored boards and mount them into cooled racks, obviously this is highly specialized. The more practical method is to cool plates that are them mounted directly to the processors (just good heat transfer compound, no heat sinks used). A large Cu plates with passages for the coolant (actually a serpentine of Cu tube soldered to a Cu plate) is used to cool the processors. These systems are usually also sealed and dehumidified so that they can run colder coolant without the risk of condensation and the board are conformal coated. This lets you run high flows through each cooling loop, and each plate is only handling 1.6kW in heat load. You control the flow in each plate based on the coolant outlet temp

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, Plymouth Tube

## RE: Convective heat transfer from heatsink calculations?

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

## RE: Convective heat transfer from heatsink calculations?

@IRstuff I calculated that for mineral oil and with temperature difference of 15 degree celsius. Can you please explain how you calculated?

## RE: Convective heat transfer from heatsink calculations?

And most liquids (besides water) actually have fairly low heat capacity and thermal conductivity (of course better than a gas, but harder to move fast).

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, Plymouth Tube

## RE: Convective heat transfer from heatsink calculations?

## RE: Convective heat transfer from heatsink calculations?

http://files.engineering.com/getfile.aspx?folder=6...

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg