Monday, 21 January 2013

Shadow Sizing

I have mentioned before that I am playing catch up with this blog; the initial experiments with the Shear Coaxial Injector took place over several months between 2010 and 2011. My original intention was to analyse the results of these experiments and post them here. However, I have decided on a much more interesting and hopefully illuminating course of action.

I am going to repeat the Shear Coaxial experiments, but this time I am going to attempt to set up a rudimentary particle sizing arrangement. At the start of the project I wrote to Dr Matt Stickland at the Mechanical Engineering Department of Strathclyde University, asking for advice on particle sizing. He very kindly wrote back and advised that the best method is Laser Doppler Interferometry - equipment costing hundreds of thousands of pounds. That aside, he also outlined a method that seemed to be within the grasp of the amateur workshop. This technique is called Shadow Sizing.

The basic premise is to take a backlit photograph of the spray with a very short depth of focus. The back lighting comes from a speedlight flash unit that is well diffused by a sheet of frosted glass or perspex. If the speed of the flash and the camera shutter are fast enough, the result should be a photograph of the shadow of the spray with the droplets frozen in mid flight. Software can then be used to measure the size of the droplets with reference to a known dimension in the frame, for example part of the injector structure.

I have all the items needed to carry out this test either on order or in place. The camera in use will be a Nikon D70s DSLR. I also have a piece of software that can be pressed into service to perform the droplet measurements.

So hopefully in the next few weeks you will see an analysis of the Shear Coaxial Injector based on the theory outlined so far, as well as some droplet sizes to compare with the empirical predictive relations.

Once I have covered the Shear Coaxial Injector, I shall go on to explain the measures taken to improve its performance, including the introduction of swirl. I can hear your hearts beating faster at the very thought.

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