Hi, I'm looking for a very small DC fan or blower that is rated IP67 or IP68. I haven't been able to find anything smaller than 40 x 40 x 20mm. Any ideas?
IRstuff is correct. The thermal resistance should include the full path. I'm embarrased to have missed this. My comments on the transient calculations still apply.
Note that if there's significant solar heating it will dominate the heat flow.
ko (www.msthermal.com)
It's the total mass at the initial temperature (35F). The air mass is probably negligible as was already noted, but the mass of the entire building and it's contents should be estimated since it contributes to slowing the temperature rise.
The temperature rise of the building vs time is an...
bcng,
6915 btu/hr is the heat flow rate used to estimate the heat transfer coeff: h = heat flow rate/(deltaT x area)
FYI, the concept of a time constant is not limited to heat transfer. The most common example is an R-C circuit.
ko (www.msthermal.com)
Your heat transfer estimate is too high because it assumes delta T is fixed at 65F. It's only 65F initialy then it decreases. You need to estimate a thermal time constant, Tau = mass x Cp / h x Area
Cp = specific heat
h = convective heat transfer coefficient
ko (www.msthermal.com)
Brandon,
For turbulent air, pressure drop is proportional to velocity2
I can only speculate on your setup but if the screen is butted up against the fan, you'll have a ring of high velocity air racing past a small cross-section of the screen. If the screen is placed further away, such as...
Hi Chad, I've run into this problem before. Forced-convection environmental chambers give misleading results. Attempts to fix them with blockages, etc, often only make things worse. I suggest you consider a natural convection thermal chamber.
Another option, which I've resorted to a few...
Hi George, I assume you want to know the temperature profile of the cube. If so, I suggest you define the thermal path from the red sphere to the external environment.
The transfer from the red sphere through the blue cube appears to be simple thermal conduction:
dT at a given point = (power...
swguru,
Consider it a problem of parallel resistances.
The case-to-PWB is about 19 C/W using L/KA through the leads.
The case-to-air is ??? C/W in natural convection. Component suppliers often have test data but be aware it includes die-air plus die-pwb. The pwb will likely be a JEDEC std...
Yes, that's exactly what I see. You model everything very carefully but when you measure the actual components, they are often cooler than expected. Typically this is due to an overestimate of power or a lack of the proper software to exersize the components. Revise the model to reflect...
elogesh,
Unfortunatly, modeling electronics in CFD is not an exact science -- it's still an engineering task. The right amount of detail to add to a particular component depends on the concern you have that it may overheat or cause ther parts to overheat.
1. Some components that do not...
If I understand correctly, you would have to introduce arbitrary impedances (in addiion to the package) to create multiple points for a fan+package fan curve.
I can't think of any value for this, can you?
ko (www.ecooling.biz)
noslo21,
In theory, you measure the system resistance curve so you can predict the operating point with different fans or fan speeds.
In practice, I don't see much value in measuring this curve if the fans aren't running. I can think of two reasons.
1. You can't isolate the fans from the...
The liquid dT doesn't help you predict the air dT since, among other things, the flow is different. Also, the energy equation uses specific heat, not thermal diffusity:
Q = Mdot (Cp) dT
Using your terms
Tout2 - Tout1 = Q1 / (Mdot1 Cp1)
I'm using Mdot for mass flow to avoid confusion with...
tunalover,
Looks like you have an error in your formula:
Fluid temperature rise = Q / Cp Vdot
where Cp = specific heat
Warning - This only gives the temperature rise of the fluid. It's of little value in determining the heat sink temperature, which is usually the more important value. I'm...
Why not just use a resistor attached to a piece of copper? Watlow and Omega make flat flexible heaters -- you can exceed their power specs if you provide enough cooling.
ko (www.ecooling.biz)
It may take a lot of phone calls to Intel or AMD, but I wouldn't give up on the TTVs just yet. Maybe they have some older models available.
Maximizing power is not that important as long as you can get a good dT measurement. Thermal resistance (C/W) and impedance (C-in2/W) should be...
The resistance doesn't have to be coincident with the recirculator, you should be able to just put in series and get the same results.
ko (www.ecooling.biz)
Great info, Keith. The only thing I would add is that the power per core should be consistent for all your tests or you won't have an apples-to-apples comparison of TIMs. That's one of the nice things about TTMs. Intel TTMs are difficult to get -- they don't sell them and only supply them...
