Wednesday, September 18, 2013

Laminar Flow Nozzle

A few years ago, Hackaday, my favorite website, posted a laminar water jet project, and it inspired me to create my own.  My goal is to create nighttime light shows with computer control of the water color by driving red, green, and blue lights at different intensities throughout a musical sequence.  So far, I have built up the blue and green solution, and this video is the outcome of the testing.  More details below on the construction.

The laminar nozzle is built with roughly 800 drinking straws all placed in parallel inside a 4" PVC pipe.  The water input is a 3/4" garden hose adapter, perpendicular to the flow of the output nozzle.  The construction consists of some cleaning sponges near the water inlet, followed by the 800 straws, a couple inch gap between the end of the straws, and finally the end cap.  The exit nozzle is a critical component.  The sharper the edge on the water exit nozzle, the better the quality of the laminar flow leaving the nozzle.  To create mine, I purchased a brass pipe end cap from Home Depot, drilled a hole in it, and tapered the rest of the exit material so the water would not touch it.  I then sanded using 600ish grit sand paper to create, literally, a sharp inner edge.
To get the colored light to shine down the laminar flow, I fed three fiber optic cables up the center of the PVC pipe, all the way to about one inch before the exit nozzle.  

To create the light, I used 2 separate high power LEDs.  I used one blue and one green PT-54 Phlatlight LEDs which I drove at around 5Watts.  The pictures below show how I mounted the LEDs to individual black CPU heatsinks, and held them in place with a piece of aluminum, similar to U channel, but with 2 extra strips down the center.  I cut out custom notches to keep pressure on the LEDs for good connection to the heat sinks below.  I also drilled a hole through the U channel to hold  fiber optic cable. 

The cable I used is 3.0mm PMMA cable.  If you haven't used fiber optic cable before, you need to create clean cuts and polish both ends to get a good amount of light to pass through.  I cut the cables and found some online guides for how to polish the ends.  You basically use increasing grades of sand paper to sand out any defects at the end of the cable.  I had to go to a special wood shop to get sand paper that was 3000, 5000, and 8000 grit to get the fiber optic ends square and smooth.  I wasn't sure how much the sanding would help, so I used a light intensity meter (lumens/lux) and was surprised there was about 4x improvement in number of lumens that made it through the cable after they were polished!

The interface between the LED and fiber optic cable was nothing special. I pushed the fiber directly onto the surface of the LED, and that was it.

To drive the LEDs, I needed a high power, constant current source, so I wired up a current source using an LM388, some power resistors, and heatsinks.  For the simple experiment shown in the video, I used push buttons to activate the current source for each LED.  With this simple setup, I can create 3 colors: blue, green, and green+blue.  The long term goal is to drive each LED power source with a PWM signal with duty cycle control via an MCU so I can adjust the perceived intensity of each color individually.

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