UFLOW and Helmholtz

Moderator: Mike Everman

Post Reply
PyroJoe
Posts: 1743
Joined: Wed Aug 29, 2007 5:44 pm
Antipspambot question: 125
Location: Texas

UFLOW and Helmholtz

Post by PyroJoe » Fri Apr 16, 2010 4:54 pm

Have been looking at the way pressure disipates in some configurations, and noticed there tends to be an absent negative Helmholtz pressure swing in UFLOW. Anyone else notice this?

Is this a basic assumption that should be made when using the software, that Helmholtz pressure swings are not accounted?
It makes sense in that the software is considered 1D and mostly about flow in pipes.

If there is some question, model a basic Helmholtz volume and look for what should be the negative pressure swing.
Thanks in advance, Joe

PyroJoe
Posts: 1743
Joined: Wed Aug 29, 2007 5:44 pm
Antipspambot question: 125
Location: Texas

Re: UFLOW and Helmholtz

Post by PyroJoe » Fri Apr 16, 2010 9:06 pm


PyroJoe
Posts: 1743
Joined: Wed Aug 29, 2007 5:44 pm
Antipspambot question: 125
Location: Texas

Re: UFLOW and Helmholtz

Post by PyroJoe » Fri Apr 16, 2010 9:53 pm

Here is something I implemented in many of the recent engines.

If we look at the 2 combustors below each has a equal volume, yet looking at the diameters at various sections and start assigning frequency, then the difference becomes obvious. There is probably a better term, but I call this "frequency shifting". Basically, each combustor is throwing similar volumes one is just throwing it faster. I could be missing something, but the frequency in these engine does seem higher for there size. If true, this shift could would be like overclocking a combustor at 138%
Attachments
freqshift.JPG

metiz
Posts: 1517
Joined: Fri Apr 21, 2006 6:34 pm
Antipspambot question: 125
Location: Netherlands

Re: UFLOW and Helmholtz

Post by metiz » Fri Apr 16, 2010 11:03 pm

Heh, "hyperphysics". "Rocketscience" pales in comparisson.

How do you determine the frequency for each diameter?
I don't realy understand your reasoning for the 138% "overclock". Can you elaborate?
Quantify the world.

PyroJoe
Posts: 1743
Joined: Wed Aug 29, 2007 5:44 pm
Antipspambot question: 125
Location: Texas

Re: UFLOW and Helmholtz

Post by PyroJoe » Sat Apr 17, 2010 6:47 am

How do you determine the frequency for each diameter?
There is a chart on the site, I'll try to find it in the next day or two.
Another method is by using the PJ calculator, and plotting thrust in .25 kg increments. Also with Forrests PJ design tool. The 2 programs slightly diverge as the size increases but give a reasonable ball park figure.
I don't realy understand your reasoning for the 138% "overclock". Can you elaborate?
Frequency doesn't have to be related to volume in a linear fashion. Shape/diameters of the volume can affect frequency.

PyroJoe
Posts: 1743
Joined: Wed Aug 29, 2007 5:44 pm
Antipspambot question: 125
Location: Texas

Re: UFLOW and Helmholtz

Post by PyroJoe » Mon Apr 19, 2010 1:38 pm

This chart is from essential reading thread, it plots combustion time to diameter:


http://www.pulse-jets.com/download/saedoc%20840422.pdf
Attachments
SAEdoc18[1].JPG

hinote
Posts: 1241
Joined: Sat Oct 04, 2003 1:54 am
Antipspambot question: 0
Location: Central California

Re: UFLOW and Helmholtz

Post by hinote » Mon Apr 19, 2010 4:37 pm

PyroJoe wrote:This chart is from essential reading thread, it plots combustion time to diameter:
I'm glad you've found that chart and have observed its importance.

You'll find that it constrains how short a pulsejet can be relative to its diameter. The length of the pulsejet is the primary driver of frequency and the 1/frequency time window has to accomodate several important events of which combustion is a biggy. If you make it shorter the events will begin to overlap or cancel each other out partially, creating reductions in performance.

The generally accepted view is that ignition of the fresh fuel/air is initiated at the walls of the comb. chamber; the burn rate then has to allow ignition to occur from the wall to the centerline during the allotted time period.

The end result is that a ratio between length and diameter is established that is about 20 or so (as I recall). This number applies for front intake types BTW; I'm not prepared to make any statements about Chinese, side-port, etc.

BTW food for thought: Does the above scenario mean that multiple combustion chambers will allow for a shorter engine if they have the same volume as a single CC?

Bill

PyroJoe
Posts: 1743
Joined: Wed Aug 29, 2007 5:44 pm
Antipspambot question: 125
Location: Texas

Re: UFLOW and Helmholtz

Post by PyroJoe » Mon Apr 19, 2010 6:21 pm

Have been using a 14 ratio for most of what I do. (using the largest diameter of the CC)
17.5 if the recuperator is added.

If I average the small and large diameters of my CC I get about 16.
20 if the recuperator is added.


Without loading the intake with the recuperator I estimate there is about 25-40% of the tail pressure wave lost out the intake. With the recuperator, the leading edge of the tail pressure wave is entering the tail expansion about the same time the intake pressure wave is exiting the end of the recuperator. The Escopette is one of the few engines that use both frequency shift, and a loaded intake. It lacks Helmholtz volume for my taste, so I add a smidgen.


If loading the intake can generate enough flow in the tail pipe, the tail expansion may be eliminated, giving a ratio of about 6.5 to 7 from a folded engine(about the length of a valved engine). Wild speculation, but is on my "to try" list. :wink:

Post Reply