Venturi Vaccum Pump

[memepe]

A simple vaccum can be made using compressed air though an orifice with an inlet placed just downstream of the orifice.

How do you calculate the pressure and flow through this venturi?

 

[SteamJetPE]

I spent a few years doing these calculations. I will post back tonight with the equation. Any gas flow textbook would have it.

 

[LarryC]

Dear SteamJetPE -
It seems to me that this mechanism is known as an "ejector", basically a "driving" flowstream aimed right through the throat of a venturi to induce flow due to the low static pressure in the throat (as sensed from the "upstream" side). If this is what I understand memepe to mean, I am interested in this too, but in my case the driving flowstream is high-temperature, high-velocity (i.e. near-Mach) combustion product gas. Of particular importance to my application are calculating (and maximizing) the induced airflow at the "upstream" end (obviously, the venturi intake must be significantly larger than the throat area) and the resultant total massflow and velocity downstream at the venturi exit. Static pressure at the venturi intake would be 14.7 PSIabs. Any help you can offer would be appreciated (perhaps the equations you provide to memepe will be enough -- just wanted to get my problem dragged into the discussion). - LarryC

 

[jdsewell]

i don't know if this will help. i encountered it several months ago and i haven't had a chance to learn its ins and outs. it is a venturi java script calculator. there are other applications within it that may be of use.

http://www.lmnoeng.com/venturi.htm

 

[LarryC]

Thanks, jdsewell -

The little Java program is very nice for basic venturi calculation, but won't do the job in this case. Here's a more complete explanation (please bear with me!) of what I need (and memepe as well, if I really understood his original question):

Assume a smooth venturi of circular cross-section with intake diameter D and throat diameter d (just as in the Java script). Now, assume a straight nozzle concentric with the venturi centerline positioned so that its exit face is set to fire axially into the venturi throat across a gap G (the exact dimension of G is hard to determine for a smooth venturi -- we'll say it's measured from the exit face of the nozzle to the first station of diameter d in the venturi). Intuitively, G might be something like 1.0 to 1.75 times d. The straight nozzle will be called the 'driving tube', and the flow through it and across the gap into the venturi throat will be called the 'driving flowstream' or just 'driving flow'. In memepe's application, the driving flowstream is cold and dense (compressed air) whereas in my application it's hot and less dense (cumbustor exhaust gas). The basic idea is that the driving flow into the venturi throat will be 'seen' as a very low static pressure to the surrounding air in the venturi intake, so the result will be pulling a volume of air into the driving flowstream at and upstream from the throat, where complete mixing will occur. The output from the throat on the downstream side will be some sort of 'weighted average' of temperatures, compositions, densities, etc. of the driving flowstream and the intake air.

A couple of things should be noted which make this basically different from normal venturi theory: 1) ALL the motive energy for flow through the throat is supplied by the driving flowstream; 2) The device as a whole does not exhibit isentropic flow, because of the energy added to the system at the exit face of the driving tube (we CAN assume isentropic flow in the throat and downstream, due to complete mixing in the throat).

Now, we don't know much else about memepe's application; but, in my case, the idea is to draw as much standard air as possible from stationary air upstream into the open intake and then into the driving flowstream in the throat, in order to maximize the mass flow and resulting momentum coming out of the throat. In my case, D will be much larger than d (probably 6 to 8 times d). Note that the open area calculated for the intake must be reduced by the OUTER cross-sectional area of the driving tube (because of its position within the intake). So, for my problem, the design data would be, approximately:
Intake area: (PI x (D/2)^2) - (PI x (d/2)^2)
(for a very thin-walled driving tube)
Throat area: (PI x (d/2)^2)
Driving tube exit area: (PI x (d/2)^2)
Nozzle-to-throat gap G: 1.5 x d
Air at intake: Standard air at 14.7 PSIabs
Driving flow velocity: 2300 ft/sec
Driving flow temperature: 2200 deg Fabs
Driving flow density: .0281 lb/cuft
Driving flow static pressure: 14.7 PSIabs

Simplifying assumptions: Assume that there will be no 'fanning out' of the driving flowstream from the exit face of the driving tube into the venturi throat. Assume there will be no significant loss of velocity and temperature of the driving flowstream in crossing the gap into the throat (this is a ludicrous assumption, considering the mixing taking place in the gap, but right off-hand I don't know how to do any better!).

Desired calculation results:
1. Equivalent low static pressure 'seen' in the throat
by the surrounding standard air;
2. Standard air mass flow rate and velocity at the
intake (determined from result 1, intake area and
pressure and density of standard air at intake;
3. Velocity, density, static pressure and absolute
temperature of the fully-mixed gas in the venturi
throat.

To me, the problem seems clearly non-trivial, even with the simplifying assumptions given. If I understand memepe's original question, results 1 and 2 would satisfy his requirements; in my case, result 3 would be of equal importance.

Thanks! - Larry Cottrill

 

[jdsewell]

if i'm not mistaken you are talking about a lockwood/hiller thrust augmentor?

 

[SteamJetPE]

Larry and Memepe,

I know what you're both after. The first part of Memepe's question is about simple motive nozzle design, and any gas dynamics text book should have the solution. Unfortunately once you start talking about the relationship between the motive nozzle and a venturi (entrained capacity, suction pressure, critical/noncritical flow, shock waves, geometry, discharge conditions), you open up to the world of ejector design. And please realize that ejector design is laced with proprietary empirical correction factors that have been closely held by ejector manufacturers for generations. This is how people in the industry put food on the table. I hope that was tactful. I'm not trying to sell you anything, but I can't just post what amounts to crown jewels on a forum. Fax me a means of getting in touch with you at 908-575-1573. I'll point you in the right direction.

 

[LarryC]

Dear jdsewell -

The thrust augmentor is one application, and technically very similar to what I have in mind. The main difference is that in the thrust augmentor (at the exhaust nozzle of a jet engine) the venturi entrance is only slightly larger than the driving flowstream (i.e. the jet exhaust), presumably because you don't want the augmentor to be the dominant source of engine drag once the thing is moving forward (at which time the "ram" effect into the venturi changes everything about how it's actually working, anyway!). In my case, I need a much larger intake than the width of the driving flowstream, because I need to maximize the output momentum and re-oxygenation of the flowstream under purely static operating conditions. It's interesting how often the ejector-type thrust augmentor is referred to as though devised by Lockwood/Hiller -- though it does seem to be an important part of the their engine design, it actually appears in earlier patents (from at least back to the early 1950s, I think). Thanks!

Dear SteamJetPE -

Thanks! I certainly have no interest in stepping on anyone's toes, depriving design professionals' families of needed sustenance, or committing theft of hard-won intellectual "private stock". Ignorant as I am, I naturally assumed that there were well-known standard design methods that would lead me to a reasonable approximate solution for my particular application. I really don't need exactitude -- I need to be able to tell whether my particular design is in the ball park of delivering the kind of mass flow and oxygen content needed to support the downstream process. I am willing to experiment (and hence, cook out my own empirical data -- HA!), but am out of money for the moment, so it would be nice to have a simple approximate method that I could cook down into a nifty Java applet, as I recently did for basic nozzle flow analysis. I will FAX you a request, with gratitude.

Thanks again to you both for your responses to my long-winded description!
Larry Cottrill

p.s. Has anybody noticed that the word vacuum is mis-spelled in the heading of this thread? Might be tough to search on! (Maybe it's bad etiquette to point out such faux pas, so nobody said anything? Anyway, I had no idea how it could be changed, once put in place.)

 

[jdsewell]

larryc,
you are absolutely right about the previous designs. you're description of your lay out imediately reminded my of the lockwood/hiller concept. after steamjetpe's last post, a few key words popped out that prompted me to do a search on "motive nozzle" and "ejector" designs. these have been around a very long time so the theories are solid. i encountered a few cfd analysis pertaining to this working theory. i am currently cooking up my own cfd setup. the design i'm wanting to lay out utilizes this application which is what peaked my interest in this thread.

 

[LarryC]

Dear jdsewell,

The last post from SteamJetPE leads me to think that the ejector principle must be highly under-utilized. The next question I would ask is whether this is due to a) lots of inherent physical problems (e.g. no matter what you do, it's still too low in overall mechanical efficiency to be useful for much) or b) simply that good design methods are so empirical that they just aren't widely known or available. At this point, I strongly suspect the latter.

Where thrust augmentors always seem to appear is in conjunction with pulsejets (in which I am also very interested). The reason is probably that the pulsejet provides the amusing combination of dismally low thermal efficiency with very low machine weight -- so, anything simple that doesn't add much machine mass and boosts net thrust is viewed as a welcome addition. The main flaw in pulsejet design is the rapid degradation of efficiency with increasing forward speed, a fault that (I theorize) would not be helped much by an augmentor, especially since the additional drag would increase with airspeed. With pulsejets, there's just no perfect solution. But, they are simple suckers, and at least will run statically (by which I mean at zero forward velocity), which their vastly more efficient ramjet cousins will not.

LarryC

 

[jdsewell]

larryc,
Without calculations and not knowing enough right now to make any calculations, my theory is to add more fuel to the mixing chamber (a term i have learned from my motive nozzle research, in this particular application it would simply be the venturi entrance) and achieve auto-ignition as in a ram jet application.

 

[LarryC]

Dear jdsewell,

We've gotten into (almost) more of an aerospace discussion now, although what we're talking about is certainly still a valid and interesting mechanical problem. I'd like to get back to memepe for a moment, since he (or she) asked the original question and I'm not sure has ever gotten a satisfactory reply. The real expert here was SteamJetPE, who apparently has had a career (or some portion thereof) in designing these things; I think the most important facts we learned from him were that design methods are to a large extent proprietary, and that empirical data is an important part of the methods. So, no matter what we do, we're going to have to experiment some to achieve our design goal. Now, I'll throw in my two cents' worth to get us started, based purely on "thought experiments", not real-world experimental processes: Intuitively, I would suggest that a) the driving flowstream should fill the receiving venturi throat, so that flow velocity is as nearly constant as possible throughout; b) the gap between the driving tube exit and the throat is the most critical dimension to be determined, but it must be fairly long (maybe, at least the throat diameter?) so that good induction and mixing will occur at and within the driving flowstream boundary; c) the higher the driving flowstream velocity the better, because it means you've done a good job of reducing fluid static pressure, and because it means greater downstream momentum imparted to the gas molecules picked up at the boundary of the driving flowstream; d) the larger the venturi intake the better, up to a point. memepe, please accept that for what it's worth (possibly, about two cents). One other point: this feels like the kind of thing that doesn't scale especially well, so I'm guessing that optimizing a small model probably doesn't guarantee optimum results when you double or quadruple the dimensions. Again, this is purely intuitive, not experientially based.

Now, getting back to your idea, which (I think) amounts to a thrust augmentor with added fuel injection. Theoretically, this should work -- remember, though, that adding energy via combustion is not in itself enough to improve thrust performance. You have to provide something that will communicate forward momentum back to the body of the device. Presumably, the dowstream skirt of the venturi will perform this function, but to do so, you'll have to make sure that it's a sufficiently long cone to take up the expansion of the exhaust gas without the exhaust stream separating from the cone wall. In other words, adding fuel for combustion increases the static pressure in the flowing gas, and you want to effectively convert that static pressure into rearward momentum of the gas stream (and corresponding reaction momentum into the cone and thereby through bracing into the rest of the device). This is likely to take a pretty long cone, because of the already high rearward velocity of the gases.

Your mention of ramjet design suggests a slightly different approach (though maybe this is closer to what you really had in mind) -- provide a ramjet type diffuser leading into an expansion chamber, with flameholders, etc. followed by a nozzle for re-acceleration. I have seen (now expired) patent drawings of a whole chain of these things "boosting" air input into each other, (supposedly) to gain efficiency as you go along. Such multi-stage "combustor chains" are probably excessive, but providing a single-stage "ramjet afterburner" would seem reasonably efficient and certainly doable. For efficient combustion, you would have to achieve oxygenation equivalent to at least a 30lb/lb air/fuel ratio, taking into account the added fuel and air and the unconsumed oxygen still available in the driving exhaust stream (as utilized in conventional built-in jet afterburners). Obviously, "some experimentation required".

LarryC

 

[SteamJetPE]

LarryC,

You're starting to speak the language.

Regarding some of your points:
b) The gap you speak of is very critical. Say for a minute you properly designed an ejector and ended up with a gap for some given motive pressure, built it, and you produced some operating condition with it (entrained load, suction and discharge pressures). Move the nozzle in and you'll get a slightly higher discharge pressure, but capacity falls off. Move the nozzle out and you get a slight increase in capacity, but pay for it in decreased discharge pressure. Here's the important point: think of the original design point as being close to the top of a bubble. If you start changing geometry, you start sliding off the top of the bubble. And it doesn't take much to fall completely off.

c) Motive velocity at the nozzle exit is important, but taking a fixed geometry ejector designed for one set of conditions may not see the same (or even better) performance with a higher nozzle exit velocity. It all depends on the geometry, conditions, what you're using it for.

d) Venturi inlet diameter is also critical. Without revealing any secrets, the venturi throat diameter is normally calculated first, then the proper inlet diameters, lengths, and tangents are selected based on equations and empirical data.

Scaling up:
That's the name of the game. You get a robust combination of geometry and conditions, and it sets you up for good scaled designs in the future for both smaller and larger units. But again, testing and/or empirical data are part of any design.

 

[LarryC]

Dear SteamJetPE,

As usual, intuition (at least mine) is only so good. At least, the tradeoff between capacity and exit pressure makes perfect sense to me -- with more intake air being accelerated into the stream, more of the original stream energy is consumed (i.e. the total momentum is shared by more molecules after mixing). The effect of varying nozzle velocity (your response to point c) isn't as obvious, but might be if I had more knowledge and/or experience -- I'll gladly take your word for it. Most surprising (if I'm reading it rightly) is your Scaling up: comment -- you seem to be saying that once you achieve a really good design for a given condition, you can basically re-scale it with the expectation of more-or-less minor experimental 'tweaking' to bring it back into optimization (for a similar condition). Am I reading you correctly on this? (In my case, wide range scalability is an important concern.)

Is it possible for an experienced designer such as yourself, once the operating conditions are clearly specified, to look at a proposed design and make a judgment "at a glance" as to whether "Yes, we can make that work!" or "What a piece of junk -- get out of here!"? Such an evaluation would be deeply appreciated. (Be careful not to sell or promote here -- if you like, you could just say Yes or No in response to the FAX that I sent yesterday.)

LarryC

 

[SteamJetPE]

I'm going to move to trading faxes with you.

 

[LarryC]

Dear SteamJetPE -
That will be fine. This is in danger of becoming The Thread That Never Dies .
LarryC

 

[jdsewell]

at least its not over which cad system is the best.

 

[cleaner]

we are using a ventury pump for cleaning. the air flow is @87psi is 35CFM. how do you I calculate the vaccum in mmHg or inHg in this case?

 

[SteamJetPE]

Cleaner,

No single equation exists that would get you the result you need. The manufacturer of your venturi should have the data in the form of a test curve. SteamJetPE
Ejectors, LRVPs, Hybrids
Troubleshooting, startup, design

 

[LarryC]

SteamJetPE -

I've been trying to FAX you at 908-575-1573 (as you mentioned way back on 14 Oct) -- it rings many times but won't answer. Please verify here that this is the correct number and/or send a short FAX to LarryC at 515-557-7132 & hopefully I'll get it and send you what I have (OR email me at larry@cottrillcyclodyne.com). Also, I'm not sure you ever recvd my first FAX attempt (must have been 15 or 16 Oct) which was poor information, anyway.

Thanks!
LarryC
Cottrill Cyclodyne Corporation
We don't need to spin a turbine ...
to create a revolution