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Thomas Jasinski | February 2006

Liquid Cooling ? Water Cooling


liquid cooling ≠ water cooling 

with the rise in power density of advanced electronic components, the use of liquids to carry the heat from the source to the ambient has become a topic of great interest. however, liquid cooling invariably conjures the image of a leaky water faucet. the prospect of having water all over the electronic components is, indeed, a scary thought. what is usually overlooked, however, is the fact that water-although the best coolant-is not the only option. in fact, if you have a liquid cold plate that has a sufficiently low thermal resistivity, you can remove hundreds of watts using a dielectric fluid without ever worrying about potential leaks reeking havoc with the electronics.

a cooling loop operates between two temperature limits. the highest temperature is at the junction and the lowest temperature is the ambient temperature. so, as long as the total resistance of the loop is below the allowed r”max, the maximum junction temperature won’t be exceeded:

(1)


where δttotal is the temperature difference between the junction and the ambient and  is the heat flux on the surface of the component. for example, if the junction temperature must be kept below 100 °c in a 40 °c environment, then the whole loop will have a 60 °c temperature budget. this temperature budget must be divided among the various components of the loop:

(2)


where

δti: the temperature rise due to the internal structure of the device including the internal heat spreader (if present) and thermal interface materials between the die and the heat spreader and between the heat spreader and the cold plate.

δtcp: the temperature rise in the cold plate

δthx: the temperature rise in the liquid-to-air heat exchanger

(we have neglected any temperature change across the pump and the piping because these temperature changes are typically order of magnitude smaller than the three temperature differences listed above.)

the internal resistance of the package is independent of the type of the fluid used in the loop. therefore, replacing water with a different fluid can only impact the thermal performances of the liquid-air heat exchanger and the cold plate. (of course, a fluid with a higher viscosity than water will require a more powerful pump to handle the increased pressure drop.) since the performance of the liquid-air heat exchanger is typically limited by the air side resistance, its performance is also fairly insensitive to the choice of fluid. hence we are mainly concerned with the effect of the fluid properties on the cold plate thermal resistance.

to illustrate the heat removal capacities of different fluids, we will assume that the total temperature budget is divided evenly among the three main components.  that is, the internal structure, the fluid-air heat exchanger, and the cold plate each consume 20 °c, or one-third of the total 60 °c temperature budget.  what is the maximum heat flux that could be sustained with different fluids?

mikros technologies has developed a simple way of calculating the performance of its ncp cold plates for different fluids. according to this methodology, if you know the performance for water, you will be able to obtain both pressure drop and thermal resistivity of a given ncp cold plate for any fluid of known properties.

the total thermal resistivity of a cold plate can be viewed as the sum of two parts: the core resistivity and the flow resistivity. the core resistivity depends on the internal configuration of the micro-channels and how they interact with the coolant. the flow resistivity represents the portion of the approach temperature difference associated with temperature rise of the coolant as it flows through the cold plate.

(3)

(4)

where the subscript  refers to a particular fluid of interest. 

since the flow resistivity can be made suitably small by increasing the flow rate, the performance of the cold plate is only limited by its core resistivity.  by examining the heat transfer in the micro-channel matrix in the limit of infinite flow rate (uniform fluid temperature), one finds that the core resistivity is inversely proportional to the square root of the fluid thermal conductivity.   hence, the core resistivity for fluid  can be expressed in terms of the resistivity for water as:

(5)

where  and   are the thermal conductivities of water and the fluid of interest.

the core resistivity of mikros ncp cold plates operating with water is in the range from  0.02 to 0.04 oc/(w/cm2).  equation (5) indicates that even if we use a fluid such as pao (with the thermal conductivity of about 23% of the thermal conductivity of water), the core resistance of the cold plate will only be a factor of 2 larger, or 0.04-0.08 oc/(w/cm2). for our assumed temperature budget of 20 oc, a mikros ncp cold plate operating with pao could handle a heat flux in the range from 250 to 500 w/cm2.  hence, if it will help you sleep at night, consider using a dielectric fluid to cool your high heat flux electronics.

for a more detailed discussion of the performance of the mikros ncp with different fluids, please visit http://www.mikrostechnologies.com

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