Resistance on heat sink fins???
Resistance on heat sink fins???
(OP)
Dear all,
I am working on a liquid cooling system and have to deal with resistance on base,fins and fluid flow. I found the traditionally used equation for fin resistance doesnt have any dependance on fin materials thermal conductivity. Isit that if i use a plastic fin which has a very low thermal conductivity will not affect the overall heat transfer? I see only the huge dependance of materials thermal conductivity on base resistance. i tried to make a analytical tool with excel and found out that the resistance of fin is not changing much with change in the fin material. But in a broader look we can say that if a fin matreial is plastic there wont be a good conduction of heat to coolant. Please provide your guidance on this..
thanks in advance
Arun
I am working on a liquid cooling system and have to deal with resistance on base,fins and fluid flow. I found the traditionally used equation for fin resistance doesnt have any dependance on fin materials thermal conductivity. Isit that if i use a plastic fin which has a very low thermal conductivity will not affect the overall heat transfer? I see only the huge dependance of materials thermal conductivity on base resistance. i tried to make a analytical tool with excel and found out that the resistance of fin is not changing much with change in the fin material. But in a broader look we can say that if a fin matreial is plastic there wont be a good conduction of heat to coolant. Please provide your guidance on this..
thanks in advance
Arun





RE: Resistance on heat sink fins???
The "standard" equations are assuming that the thermal conductivity of the material used to make the fin is small compared to the convective transfer coefficient.
TTFN
RE: Resistance on heat sink fins???
However in a forced flow liquid cooling situation this may be totally untrue.
In this situation the resistance of the fin material and the resistance of the convection area are both going to have a large impact.
How accurate are you looking for here. You could always use a FMEA tool to give you an answer but the biggest problem is going to be getting the fluid velocity over the fins and therefore the convective heat rate (which is dependent on the conduction because of the surface temp
).
If you are looking for a quick dirty way to get an order of magnitude estimate look at the system and try and evaluate the thermal resistance of each side individually and then balance them for the final qdot.
RE: Resistance on heat sink fins???
I take my hat off to the man if he's still alive.
gwolf.
RE: Resistance on heat sink fins???
RE: Resistance on heat sink fins???
For a 3 in wide, 0.1 in thick, and 0.5 in long fin, I get 65 K/W using your plastic, and 0.17 K/W for your copper value. How is that only a little difference?
TTFN
RE: Resistance on heat sink fins???
http://www.genium.com/drafting/ht.shtml
RE: Resistance on heat sink fins???
This is to say if you have a convection coeff of 1w/m^2K and a conduction coeff of 500W/m^2K then changing the conduction coeff by even as much as 10% will make little difference.
Think in terms of thermal resistance. If the conduction makes up only 1% of the total then doubling it is only a 1% change in the overall system. This is what the basic calcs are based on.
Now you have forced flow of a liquid over the fins. With this setup your convection coeff could be as high or higher than your conduction coeff. Because of this you need to figure out what your convection coeff is as it is largely independent of fin material.
Start by figuring out the resistance on 1 rectangular channel (between 2 fins) on the fluid flow. Then use this to find the total flow with your given pump. once you have that you can extimate the average flow rate within the channnel (averaged by surface area of the fins not area of the fluid flow) and use this to calculate the convection coeff. I assume you will not be hot enough to take radiation into account (but keep it in mind).
Also remember that as a rule of thumb you will not need fins that are more than 10 times as long as they are wide.
In this situation you may only need 3 times....
If you find your flow velocities to be slow you may need to take natural convection into account.
If you give me a better description of the problem we can help out a little more.
RE: Resistance on heat sink fins???
With the exception of the fin tip area, the major assumption in fin analysis is no temperature gradient in the direction that is normal or perpendicular to the convective surface.
RE: Resistance on heat sink fins???
Thanks drkillroy
RE: Resistance on heat sink fins???
You will have primarily 3 resistances to heat transfer in this system. The first is the contact resistance from the source to the heat sink. The second is the conductive resistance through the heat sink. The third is the convection coefficient from the heat sink to the fluid medium.
Now if you make the assumption that the resistance resulting from the convection is >> the resistance from the conduction then making changes to the conduction resistance will have little effect.
That said to directly answer your question yes there will be a higher dT for conduction as the conduction coefficient drops. However if originally you had a dT of 0.5 degrees through the copper and 50 degrees across the convective boundary then changing the dT of conduction to say 5 degrees (factor of 10) will only result in an increase of the overall dT of less than 10%. How much room you have in picking materials for the heat sink is 100% based on the whole system.
If you do not have a super clear picture of the system and can afford to use copper or aluminum do it. It is rarely a bad idea to over design.
RE: Resistance on heat sink fins???
You should calculate the fin efficiency. Let us know if you need the formulas.
The effect is small for a typical aluminum vs copper analysis with relatively thick fins. Expect a dramatically worse fin efficiency with plastic unless you have unusually low, thick fins -- the fin tips will be approx the same as the air temperature and very little convective heat transfer will occur.
ko (www.ecooling.biz)
RE: Resistance on heat sink fins???
sorry Pzas, i was out of that topic for long time. I have calculated the resistance in the channel heat sink with the following dimensions:
Fin thickness=1.07mm
Length of heatsink=30mm
width of heat sink=15mm
Base thickness=0.8mm
fin heigth =1mm
number of fins=12 with Aspect ratio of 5
here i considered a spreading heat source of width=5mm
And then calculating the total thermal resistance by adding resistance in base, resistance in fin and channel resistance.
For Copper(k=385W/mK)the total thermal resistance i found is Rth= 0.26 K/W. (Rfin=0.15,Rbase=0.06 and channel resistance is negligible.)
For Plastic(k=1W/mK)the total thermal resistance i found is Rth= 24.76K/W.(Rfin=0.47,Rbase=24.24.channel resistance is negligible.)
Okay let me consider a material with thermal resistance of 20W/mK.the total thermal resistance i found is Rth= 1.49K/W.Rfin=0.23,Rbase=1.21.(channel resistance is negligible.)
So can you see there is a huge difference of heat conduction/convection effect in copper and normal plastic.But there is only a small difference of 1K/W in material having 20W/mK and copper.
So let me say the material i stated having 20W/mK is a plastic(there are some thermally conductive plastics with huge thermal conductivity). So my question is in a right application why not plastic(k=20W/mK)can be used for heat transfer application. One more thing is as Ko99 said, there is an impact of thermal conductivity of fin material. I agree with his statement But the difference is very small when compared to the base resistance, which u can see in the above example. So in this case of small difference in fin resistance which agrees with Pzas statement of conduction(thermal conductivity) is overtaken by convection.
Awaiting your critics and views.
Thank you,
Arun
RE: Resistance on heat sink fins???
I have been a little sick so I may just be missing something here...however it appears you did not consider the convection to the air. As stated before this is very important when determining what materials you can use. For example some base line estimates allow your thermal resistance from convection to be as high as 200 K/W in still air. dependent on orientation and all that of course. I can't really solve this with the information at hand. However lets just assume that you have a convective resistance of 100K/W from your given heat sink.
If this is the case than a change from a material with K=300+ to one with k=20 results in a change in the overall thermal resistance of less than 1%! As you can see depending on the situation thermally conductive plastics may very well be functional and less expensive.
Now with forced conduction of a fluid as you stated in your original post you will likely have a convective resistance << 100K/W. Keep that in mind. If say your convective resistance was 1K/W then a change from copper to plastic could double your source's temperature.
Really the moral here is to keep your mind open when approaching the problem. You do not have to assume that copper/aluminum are your only options when creating heat sinks. It may very well be that a slightly larger sink made of a material that is inert to the environment is the better choice. Gather your data and do some good engineering.
As to why you don't see plastic heat sinks out there ....you do. Why do you think the box that holds the power supply for your lab top has fins on it :). Plastic heat sinks are not very common because much of the design work today is done by individuals who do not understand the heat transfer of the entire system. One of the problems that arises when companies try to cut costs by having people without an education do their designs because their salary is lower...but lets not discuss that in this thread.
RE: Resistance on heat sink fins???
TTFN
RE: Resistance on heat sink fins???
Also, the base conduction loss looks too optimistic for a 5mm source on a 15 x 30 mm plastic base only 0.8mm thick. The heat will not get to the outer fins. If possible, consider making the base thicker or making the source larger (such as adding a cu spreader between the source and the plastic sink).
ko (www.ecooling.biz)
RE: Resistance on heat sink fins???
I believe that you are looking at the small picture here. To make the assumption that the junction temperature is already maxxed out in most "real" applications is just a leap of faith. I personally work in R&D. When I look at a junction temperature it is never defined already. When I finish a project is the junction temperature maxxed out? Hardly ever. Remember the OEM has to design for the system to be used in the hottest environment likely.
Just take a look at the computers in your office. Do you think the heat sinks on even the same processors are all the same?
Ko99
I agree with most of your statement. However without knowing the flow it is probably a bit early to choose a material.
RE: Resistance on heat sink fins???
ko (www.ecooling.biz)
RE: Resistance on heat sink fins???
In any case, for many semiconductor failure modes, a 3ÂșC increase in temperature results in about a 40% reduction in life.
TTFN
RE: Resistance on heat sink fins???
PZas,
Thank you for that motivating suggestion...i will go ahead. Takecare of your health...
Ko99,
Your suggestion of using Cu heat spreaders are already in my mind. I am also thinking to use the heat sink upside down to have the base on top to get rid of huge base resistance. In that case i can use a highly conductive think layer of heat spreader between source and the channel heat sink to reduce the overall thermall resistance.
IRstuff,
I know even a small increase in jn temp will cause a huge damage on semiconductor chip. Thats why i have specified in my previous posting that "In right application why shouldnt we use the plastic heat sinks". But if i get rid of base resistance i can get the thermal resitance of plastic heatsink(k=20W/mK) close to that of copper or aluminium.
Thank you all,
Arun
RE: Resistance on heat sink fins???
Given that you would have a required delta temperature between ambient and junction temperature and a fixed power dissipation, a metal heatsink will provide the most efficient solution for a given cooling capacity.
Any plastic solution, simply by virtue of its poorer thermal conductivity, must result in a larger heatsink to provide the same cooling capacity. Since people don't make plastic heatsinks in general, you're going to incur cost for the development of the molds or extrusion dies, special handling of non-standard parts, etc. You're going to be limited in the number of suppliers, since a metal heatsink could probably be readily copied by any heatsink supplier, but a plastic heatsink will have to go to a specialty plastics supplier and changing suppliers will result in a new learning curve.
Note also, that the dimensional stability of metals is much better than plastics and many designs have relatively tight dimensional requirements in placing the parts against a heatsink while being installed on a board that's mounted with the heatsink.
TTFN
RE: Resistance on heat sink fins???
I'm courious about reactions between the heat sink and the fluid used to transfer heat. Plastic would, I believe, be much less reactive than aluminum. Automotive applications see white powder corrosion even related to the cooling system running antifreeze. Also a jell of some kind build up on the aluminum surfaces. Can this problem happen in the application discussed here?
Thanks PZas Your post was quite helpful!
drkillroy
RE: Resistance on heat sink fins???
TTFN
RE: Resistance on heat sink fins???
What fluid with a low freezing point (near 0 degrees F.) that can conduct and carry a large quantity of heat per pound mass can be used that is not reactive with copper and aluminum (not in direct contact) within the same system? I'm mentioning copper as well to cover other potential problems.
Thank you Drkillroy
RE: Resistance on heat sink fins???
TTFN