Photos from my research on fluid impingement

Whether you’re designing rockets or more obscure technology like lye smelt shattering Kraft recovery boilers – but mostly rockets! 🚀 – everybody, at some point in their lives, needs to work with impinging-jet fluid injectors. Obviously.

Fundamental studies on impinging identical liquid-liquid jets abound. Essentially, when two liquid jets meet, they spread out into a pretty leaf-shaped sheet. Formulas exist to estimate the size of that sheet, depending on the amount and velocity of the incoming liquid. Likewise, there’s a gazillion of papers on how liquid jets break up when they meet a crossflow (say, a wind tunnel). But there are no publications on how breakup happens in the in-between cases, when the jets differ in substance, size, or pressure, at various angles.

Here are the results of some of those investigations: photos and drawings I produced at Prof. Nasser Ashgriz’s lab in Toronto. I’ll keep the post totally non-technical: no formulas, no references, pretty stuff only. Ping me for more.

You’re looking at two water jets of 1.37 and 0.51 mm diameter, respectively, impinging at different flow rate combinations.

The same conditions, photographed from the side:

Here’s the basic setup: the jets are coming out of clipped hypodermic needles mounted on sleds, mounted on a rig I designed/machined – because it’s such a small setup, good alignment requires very fine adjustments:

You also need a supply panel of air and water regulators and lab stands, in addition to handful of strobe lights and remote flashes and halide lamps and backdrops. And duct tape. Lots of it.

Now check out the same pair of needles, but with pressurized air shooting out of the smaller one, instead of water:

Side view:

There are a few interesting things to note here. First, the impinging sheets don’t form a nice, flat leaf-like shape. That’s to be expected, because they’re not identical. Instead, the leaf curves around (and even towards!) the thinner, faster jet. Depending on the circumstances, the sheet breaks up right away (due to Kevin-Helmholtz waves) or it stays stable long enough for surface tension to pull it back into a single jet, which breaks up downstream (due to Rayleigh instabilities).

It’s quite fun to capture the sheet breakup, because it differs from what’s shown in typical jet impingement papers. Here’s a photo taken from underneath, at a point where the sheet is still intact but too thin to pull itself back together:

I made a drawing to better show the effect:

Check out photos 8 and 9 above for a frontal view. Photo 5 is also interesting: it shows a hole in the sheet. This happens sometimes, but using a high-speed camera we found that it’s a purely intermittent effect:

Another cool thing to do: align two unlike water jets at a very obtuse angle and fiddle around until a reverse water bell forms – that is, a curved sheet opening in the opposite direction:

I have my conjectures about how this can be reconciled with the math used to model these effects, but I never was able to work it out.

Finally, a bonus photo: using an extremely thin needle (pulled by hand from a glass capillary), you can introduce bubbles into a water jet simply by blowing onto it from the side:

The fascinating thing: all of these effects are completely invisible to the naked eye. In real life, this stuff goes pfffsshhhhh, everything gets wet, and little fun is had. But frozen in time with an expensive macro lens, all of it is somehow marvelously beautiful.

All photos under CC-SA; get in touch for high-res versions.