CFD simulations of air flow
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In Notes on CandyFab and heater design (1.0), Windell lays out the goals of the CandyFab project and what he sees as a (maybe "the") design limitation standing in the way, namely, the design of the heater. He put out a call for ideas. Ultimately, I'd like to see it be able to melt a single grain at a time. Also it seems there is interest in melting more than sugar. So any heater design should be scalable and not be simply focused on sugar. I posted that I thought a second flow of air, not as hot, not as fast, around the first one might solve several problems.
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I used the aquarium pump information of 1 L/min. and the fact that Windell said it was throttled by about half. So 0.5 L/min. equals 0.509 in^3/sec. Windell stated that they had to use a 1/8" dia. nozzle to keep from blowing sugar everywhere. That's an area of 0.0123 in^2. This gives a velocity of 41.4 inches per sec. (2.35 MPH). This is an ideal number, not that it's the goal but that it doesn't account for the expansion of the hot air or the viscous drag induced by the tubing, etc. I assume that a 1/16" dia. nozzle is closer to ideal so I use that in nearly all the simulations, but since the estimated 1/8" nozzle velocity seemed to work okay for them so far, this is the nozzle velocity used throughout. These simulations were run to satisfy vague "what ifs," so there is no added heat and the media surface is assumed to be smooth and non-porous.
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We begin with the original setup (illustrated to the left): 1/8" nozzle, 41 inches per second, and 1/2" from the media.
The images from the simulations shown below are available in much higher resolution; click through to get to medium resolution. (From there, full resolution is available if you click "all sizes," above the picture.)
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So what do we see? These are obviously two-dimensional side views of the air exiting the nozzle and blowing into the media. These are not thermodynamic images. Heat is not involved here. These give us velocity and direction vectors at different mesh points. And they show us pretty much what we already know, that the air strikes the media and forms a stagnation area of low velocity (in blue), high pressure air directly beneath the nozzle. This is what deforms the puddle and wedges the surrounding air away. The nozzle flow (in orange) draws air from around the nozzle and ultimately creates a region of flow (in red) that is actually faster than the nozzle exit velocity.
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The same setup, but only 1/4" (6.4 mm) from the media.
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Again, the same setup, but 1" (25.4 mm) from the media.
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This creates a ceiling to the media's floor. The big picture looks strange but the reality is that both the size and color of the arrows indicate velocity. No outside air is drawn in. The close up shows that the vortex acts like the outer nozzle flow from before. This could be a more clean (cleaner) solution to the blow away problem. A plate around the nozzle tip will shield the hot nozzle from gathering and then re-depositing the black sugar drops. It could be removable for cleaning. The downside is that one loses separate control of the outer nozzle air for speed building or tuning. And unless the plate is made of pyrex, the melt can't be seen, and face it, that's the real fun in this whole endeavor.
Enough with simulation-- what about when air really meets sugar? I wanted to see what velocity sugar was stable at. So I used a three speed table fan and a tray of sugar. I turned the fan on and brought the tray closer until it was right up against the cage of the fan, and the sugar didn't move. This surprised me, because I half expected to be cleaning sugar from all over the place. I bump the fan up through all three speeds and the sugar stayed. I moved the tray around and started to get some movement near the lip due to vortices. I turned off the fan and blew the sugar around myself and it behaved normally. So the question was how fast was the air from the fan going? I shot some video of the fan as I released tiny cotton balls (the fluffy tips from some cotton swabs) in front of the fan. By putting a ruler in the video for scale I could now figure out roughly the velocity. Although it feels faster, the fan at its highest setting puts out about a bit over 55 inches per second (3.125 MPH). I don't have an actual speed limit for when the sugar moves but I have something in the ballpark of when it doesn't. That's useful when looking at the simulations.
Comments and other ideas are welcome.
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