novice calculations

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novice calculations

Post  gurv on Wed Aug 25, 2010 11:23 am

I'm new to this forum ... thanks to Make Magazine. Prior to scrapping all of my other projects and starting a water jet one I had a couple questions that I thought I'd throw out to those with hands on experience ... is there a formula or some standard for calculating the size of a chamber/ nozzle? If I start with some flow rate and/or pressure and a expectation of X diameter of an output stream, where can I expect to start on the length and diameter of a nozzle? The bigger question ... what is the minimum length and diameter to achieve laminar flow? I am "assuming" that there is some known minimum. Like if I want a 6mm sized stream, and I am using similar sized drinking straws in the nozzle, whats the minimum diameter of the nozzle? using smaller stirrer straws? Am I even on the right track with this or just completely not understanding this whole thing yet?

Thanks.

Andrew

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Re: novice calculations

Post  John on Wed Aug 25, 2010 12:00 pm

Andrew,

Welcome to the forum. Yes, there are some calculations that you can do to get the laminar stream size. If you are interested in those looks for posts about the reynolds number. There have been a few of them that have been very comprehensive. Otherwise you can just use what we have experimented with as a guide.

Most of the user here use 6" to 8" jets, and most use .5" for the exit orifice. There is no exact rule as to what the max/min should be. Here is what I have noticed. The larger your jet size, the more laminar it will be. As a guide I use a 8" jet and a .5" exit orifice. I'm able to get about a 5' (h)x 14' (w). If that is the type of arc you are planning on going with then you might consider using the ratio of exit/jet as a guide. That makes the ratio of exit orifice to jet size 1/16. So your jet size should probably be around 96mm.

The thing about length is different. Length is only needed to make sure that your water achieve a "fully developed flow" (see the
post). The gist of it is that if the straw is long enough then the water will reach the fully develop parabola. Once the straw ends then all the parabola from all the straws mix together to make a uniform profile across the internal diameter of the jet. When it exits the knife edge cuts it and it remains still somewhat uniform across the profile.
So what I am saying is that the smaller the diameter of the straw the shorter the straw needs to be in order to make it reach the fully developed flow.

Did I answer all of your questions?
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