Continuing the experiment with forced air, I added some of those fans. Same thermocouple arrangement as before, heatsink sitting on the bench with the fins horizontal and the fan blowing across the bench. This obviously leaves a bit to be desired as far as air flow control goes, but it’s close enough to get a general idea of what’s going on.
With the fan in the air flow straightener, exit about 1 diameter (4 inches, 100 mm) away from the heatsink, flow perpendicular to the heatsink body, 24 W to the 6 Ω resistor:
- R = 27.7 °C
- Bot = 23.4 °C
- HS = 66 °F = 18.9 °C
- Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
- Thermal resistance heatsink to ambient: ΘHA = 0.16 °C/W
That’s more like it!
With the bare fan sitting on the bench, exit about 1 diameter (4 inches, 100 mm) away from the heatsink, flow perpendicular to the heatsink body, 24 W to the 6 Ω resistor:
- R = 26.7 °C
- Bot = 22.4 °C
- HS = 64 °F = 17.8 °C
- Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
- Thermal resistance heatsink to ambient: ΘHA = 0.12 °C/W
The bare fan actually does a better job than the flow straightener. Just from the general feel of the breeze, I think the fan’s air flow entrains a bunch of ambient air and slams it across the entire heatsink, rather than hitting just the central area.
Encouraged by that, I doubled the power to 50 W: 2.6 A in the 6 Ω resistor = 41 W and 3 A (the limit of my bench supplies) in the 1 Ω resistor = 9 W. Because the heatsink is now getting energy from two sources, the heatsink temperatures won’t be directly comparable to the previous ones.
With the bare fan in the same position as before, 50 W dissipation:
- R = 36.8 °C
- Bot = 29.4 °C
- HS = 72 °F = 22.2 °C
- Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
- Thermal resistance heatsink to ambient: ΘHA = 0.14 °C/W
The heatsink will be 7 °C above ambient at 50 W and the resistors 4.5 °C at 25 W each above that. The resistors will be 11.5 °C = 21 °F over ambient.
Again, the average of the Bot and HS temperatures might be more meaningful.
The heatsink has fins on both side, but so far I’ve been using only one fan. Putting another bare fan on the opposite side, also 1 diameter away, so the heatsink gets ambient air from sides, thusly:
That produces even better results:
- R = 33.9 °C
- Bot = 24.9 °C
- HS = 66 °F = 18.9 °C
- Thermal resistance resistor to heatsink: ΘRB = 0.22 °C/W
- Thermal resistance heatsink to ambient: ΘHA = 0.067 °C/W
The 6 Ω resistor is dissipating 41 W, rather than the 24 W I plan to use: figuring the resistor-to-heatsink thermal coefficient at 0.2 °C/W seems OK. The heatsink-to-ambient coefficient is breathtakingly good: cooling both sides seems like it ought to cut the thermal resistance in half and it does! Call it 0.1 °C/W.
So, bottom line: with two fans and 50 W, the heatsink will be 5 °C over ambient. Dissipating 25 W in each resistor will raise them 5 °C over the heatsink and 10 °C = 18 °F above ambient.
With the box ambient at 140 °F and two fans per heatsink, the resistors should tick along under 160 °F. That’s plenty toasty, but only slightly above my rule of thumb: If you can’t hold your thumb on it, it’s too damn hot. And, heck, we’re building a heater here, right?
On the other paw: six fans?
In reality, that layer of thermal goop between the case and heatsink determines much of the resistor temperature. One fan per heatsink should be entirely adequate. I should try this with one fan blowing parallel to the fins, with the notion of putting a fan directly upstream of each heatsink or between each pair of heatsinks.
The raw data:
The Smell of Molten Projects in the Morning 
Yeah, I’d try blowing parallel to the fins. I’d expect the heatsink to perform quite well this way, and act as its own flow straightener (not that it matters).
From what I’ve read, the absolute best way is with the air stream hitting dead center on the heatsink as shown, with blowing-from-one-end second best. Probably has something to do with cooler air hitting both sides of the heatsink, rather than heated air flowing across it.
Not, as you say, as if it matters in the least…
Didn’t know that. I suppose the coldest air hits the heat source directly, then warms as it flows out over the heatsink. I strongly suspect this varies with the heatsink design too. With some designs, I think blowing from the end would work a lot better. This one, for instance:
Now, that’s arty!
Looks like they have some furious computational fluid dynamics simulation goin’ on there…
I’ve seen some stuff with adding entrainment venturis — primarily in pulsejet design — where they were finding that they could increase the amount of mass being moved by 40% by putting a truncated cone or bicone a little behind the output of a high-speed airstream, so I’m not surprised to hear that the open fan works as well or better than the linear-airflow fan.