They are interesting!

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Viv
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They are interesting!

Post by Viv » Thu Dec 11, 2003 12:35 pm

Well I have to admit I was little dismissive of the whole pressure jet idea in the beginning, "just a burner" I thought:-)

Oh well if you are going to get it wrong you may as well do it in style:-)

Since Luc posted his questions and got me interested i have to admit they are just as complicated as you could wish for (and then some)

I just never knew they had a stationary flame front, how cool is that for a party trick:-)

Viv
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Post by Mike Everman » Fri Dec 12, 2003 5:46 pm

Yeah, I think it was a pressure jet featured in a Popular Mechanics when I was a kid that turned me on to jets. Then there was a BD-5J with two of them in it. I remember distinctly dreaming of building one. Do you know what's the typical SFC for these?
Mike
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Post by Viv » Fri Dec 12, 2003 6:44 pm

Here is a link to about the only chart I know off.

http://www.rqriley.com/gluharef.html

It cant be that good by its very nature but there may be ways to improve it.

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Post by Mike Everman » Sun Dec 14, 2003 8:20 am

thanks, Viv, now I know what I want for Xmas. I'll pay $75 to see how to do all the sheet metal!
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Post by paul skinner » Mon Dec 15, 2003 7:05 pm

The set from United Nuclear looks interesting, and a hell of a lot cheaper than the Riley set...


http://www.unitednuclear.com/jetplans.htm

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SFC??

Post by decook1110 » Tue Dec 16, 2003 7:03 am

The SFC on these bigger engines is likely exagerated. If you look at the data for the different engines you will see a distinct jump from sfc's around 4 to 1.3 and even .75 . I played around with the idea of experimenting with these for many years and couldn't get one to do a well instrumented test on. Anyway I did do a simplified math exercise on what the efficiency could be and the best if could be under perfect conditions would be in the 3.5 range, most likely it will be in the 4's like the other engines. The analysis is in the old forum, I'm still waiting for someone to seriously critique the analysis or do a well instrumented test to verify the sfc of one of these.

Dave

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Re: SFC??

Post by Viv » Tue Dec 16, 2003 10:37 am

decook1110 wrote:The SFC on these bigger engines is likely exagerated. If you look at the data for the different engines you will see a distinct jump from sfc's around 4 to 1.3 and even .75 . I played around with the idea of experimenting with these for many years and couldn't get one to do a well instrumented test on. Anyway I did do a simplified math exercise on what the efficiency could be and the best if could be under perfect conditions would be in the 3.5 range, most likely it will be in the 4's like the other engines. The analysis is in the old forum, I'm still waiting for someone to seriously critique the analysis or do a well instrumented test to verify the sfc of one of these.

Dave
Oh sorry I must have missed that, can you remember what thread it was in so I can look it up?

I am finding the varibles in this motor and its possibilitys very interesting but like you I ether want to get my hands on one or get the time to knock one up.

I was thinking of making a test rig one day just to model the inlet stack and how it works.

But time is the problem as allways:-)

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SFC calc repost

Post by decook1110 » Tue Dec 16, 2003 11:18 pm

I cut it out of the original post, it took a while to find..

Its actually a pretty simple calculation, surprises me that it took 5 years of daydreaming about using the engine to do a simple analysis like this.

To make this easier Im going to define the variables I'm using, this isn't matlab....

ft (feet)
a (speed of sound in ft/sec)
r (degrees Rankine)
lbf (pounds force)
lbm (pounds mass)
R (Gas constant for air in (ft lbf)/(lbm-r) 53.352
mdot (mass flow in lbm/sec)
k (specific heat ratio) 1.4
Gc (gravitational constant (32 lbm ft /lbf sec^2) )
a/f (air fuel ratio) 14

Gc makes the units work out in the English system (which the English no longer use, Thanks) system. Seeing how this is international, we'll probably get a metric calculation to double check it. All right I was too lazy to convert it!

I'm not sure what k is for propane, but the mixture is 14 parts air and propane was close, I used airs value at 1.4


Thrust = mdot x velocity

maximum velocity at the tailpipe can only be mach one as its not a converging diverging duct. Speed of sound is dependent on temperature of exhaust gasses. Propane flame temp is 2045K or 3681r .

a = (k * R * flame temp * Gc) ^.5

(1.4*53.352*3681*32.2) ^ .5 = 2975 ft/sec

This is the speed of sound of air at 2045K or 3681r.

mdot = (sfc)(thrust)(A/f + 1) / ((sec/hour)(lbm/slug))

(.78*130*15)/(3600*32.2)=.01312 slugs/sec

.78 is the sfc quoted in the literature for the round intake g8-2-130r

first equation turned around V = thrust/mdot

130/.01312 = 9907 ft/sec

V/a = mach number

9907/2975 = mach 3.33

my conclusion is that the sfc was overstated by at least a factor of 3.33 . This is giving the benefit of the doubt that the gases leave at mach one which would be perfect and also not counting the radiant heat loss of energy which does go down as the engines get larger (surface area/volume ratio) but nowhere near justifying an sfc drop from 4.2 to .79 . I assumed the air fuel ratio as 14, as that is stoichiometric for propane, as you deviate from that a/f ratio the flame temp should drop because of the extra mass that needs to be heated, either fuel or air. A spreadsheet would indicate the performance drop with deviation from 14. I'm also not sure if that flame temperature includes the heat loss encountered by vaporizing the fuel from liquid to a gas, maybe somebody else can talk to that.

I just looked through my notes and k for propane is 1.126 at 300k, much lower with air, I don't know wether it rises with temp or not, but it is 1/15 of the mixture and lowering the value of k from 1.4 will only reduce the speed of sound and increase the needed mach number to get the thrust.

Let me know what you think, I could be way out there... If I'm wrong I'd really like to know, because other than a greatly exagerated SFC, it is a cool little engine.

Dave

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SFC

Post by milisavljevic » Wed Dec 17, 2003 12:06 am

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Post by Mike Kirney » Wed Dec 17, 2003 4:07 am

Perhaps effective combustion mass temperature could be calculated using a calculation involving heat of combustion, mass flow/second, and a heat absorbtion value for air.

propane: 530.57 kg-cal/gm-mol @ 25 deg C, 1 atm.
air: 0.24 cal/g-K; 191.47 g/sec

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Post by Mike Kirney » Wed Dec 17, 2003 4:34 am

Fuel: 530.57 kcal/g-mol x 44.1 g/mol = 23,398.137 kcal/mol
Air: 14 x 44.1 = 617 g /mole of propane

23, 398, 137 cal / 617 g = 37922.4 cal/g air

Okay, I have no idea what I'm doing.

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Post by milisavljevic » Wed Dec 17, 2003 1:34 pm

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Post by Mike Kirney » Wed Dec 17, 2003 3:45 pm

I don't know how to put all those numbers together. I thought it would be straightforward but it isn't really. I tried a couple of things but nothing gave me reasonable values for tenperature (in K). Some of the units baffle me. What is a kg-cal? I presumed it was the same as a kilocalorie. G-mol means "gram molar" but what the hell does that mean? I got my figures from my CRC Handbook.

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Re: SFC

Post by Viv » Mon Jan 26, 2004 12:40 am

milisavljevic wrote:Hello Dave,

Someone else (Craig Wall? or someone who was debating him) performed a similar set of calculations on the old forum; that calculation resulted in an TSFC of 2.5 lb/lbf/hr as the minimum attainable. (I will be supremely embarrassed to discover that this "someone else" was you! ;-)

Setting the duct geometry aside, we can be confident that the effective exhaust velocity is subsonic, and that the effective speed of sound is not defined by the flame temperature. The effective gas temperature in the so-called "propulsive ducts" (i.e., pressure jets, (subsonic combustion) ram jets, pulsejets... ) is much lower than the flame temperature of the fuel.

For example, the highest effective temperature that I have found in any pulsejet is about 730 K, and this engine was able to operate on either propane or gasoline. I am willing to concede that the effective temperature is much higher in the Gluhareff pressure jet; my point is to not to argue, but to agree with you -- the TSFC for these engines was greatly exaggerated.

What you appear to have left out of your calculations is the additional mass flow rate of entrained air through the pressure jet. The combustion process may be occuring at near-stoichiometric conditions; however, these engines (and pulsejets) aspirate additional air, which augments the total thrust. This internal augmentation will "pull down" the required Mach number to a high subsonic value, if we assume a reasonable TSFC.

I would expect the "real world" TSFC performance of these engines to bottom out around 2.0-2.2 lbs/lbf/hr; however, I am no pressure jet expert. Where is Craig Wall?

Best regards,
M.
Sorry just rereading the thread, were is this extra air coming from? any extra in the intake stacks will throw the mixture out as all of the air entrained is mixed with the propane from the nozzle.

Pressure jets do not pulsate (but they do resonate) so no air is drawn back up the tail pipe.

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