restrictor head loss and flowrate

[ahmadreza]

Hye everyone.
Although I've searched a lot about the restrictor standards and design,I coudn't find so many usefull things eccept fomulas, even in ISTM standards.I think we have toe ways.1:if we assume the venturi to be a critical one so the wider the throat cross_section ,the higher the flowrate and the biggest is gain from the rules(20 mm diameter).2: We cosider the maximum flowrate and then find the dimension whereby we need some information about our fuel_air mixture and strock dimensions.Witch one did you choose?
thanks.

[ahmadreza]

Hye everyone.
Although I've searched a lot about the restrictor standards and design,I coudn't find so many usefull things eccept fomulas, even in ISTM standards.I think we have toe ways.1:if we assume the venturi to be a critical one so the wider the throat cross_section ,the higher the flowrate and the biggest is gain from the rules(20 mm diameter).2: We cosider the maximum flowrate and then find the dimension whereby we need some information about our fuel_air mixture and strock dimensions.Witch one did you choose?
thanks.

[CornellGixxer]

I might be wrong, but are you asking for the proper method to size your restrictor? Since the max flow rate will increase with throat cross section... and engine power increases with air flow... why would you chose anything other than the maximum for your chosen fuel? Anything other than the maximum restrictor diameter would just handicap you unnecessarily.

[ahmadreza]

Hie CornellGixxer:
What you have mentioned is not so clear.
Would you please explain your mean.
you said that the higher the flowrate,the higher the engine power. I saw that jay has achived 340m/s speed from his restrictor.
What is the effection of supersonic flow exits from the deffusing part.Do you think that the state of exiting flow affects the engine power.(which one is better?subsonic or supersonic)
And if you assume your venturi be critically designed so from the formulas we reach the outlet preusser in range of 50-53 kpsi.
assuming the inlet states(101.3 kpsi and 298.2k)
and so the presuure loss is about 47 kpsi.
But, the matter is that once I saw that up to 20 kpsi is logical.
what do you think?
thank you Cornell.

[T]

I believe he said the following:
1. He thought you were asking how to choose the size for your restrictor (the cross section that matters).

2. He replied by saying that the rules allow you only two choices, which depend on the fuel you want to use. And that to choose anything less than what the rules say is the maximum, would be foolish.

[BeaverGuy]

Ahmadreza,

I think you might have some terminology and unit mixups. A pressure of 101.3 kpsi is around 7 times atmospheric pressure so I asume you mean 101.3 kPa. Also I think you are confusing head and pressure loss with pressure differential. You can have a pressure difference without there being any losses. You may also be confusing mass flow (grams/second) with velocity (meters/second). I imagine this is just a language barrier though, but it can definitely make things more difficult.

It is clear that you understand what is going on with the restrictor. And now you are trying to understand how to design the restrictor. However, I would suggest understanding the design process for the restictor first. If you have allready done this I apologize, but here is what I would recommend.

To design your restrictor you should think about what it is intended to do and what you want the engine to do. Then look at Bernoulli's equation and understand how the terms in it apply to the situation of the restrictor.

[Jersey Tom]

You will not achieve supersonic flow at the outlet of the diverging end of your restrictor.

[CornellGixxer]

Whatever your engine choice is or will be, at a certain RPM you will achieve a sufficient flow rate to "choke" the restrictor (i.e. the flow is moving at the speed of sound through the throat and you have achieved max flow through your restrictor). No matter what your design looks like, its efficiency or its size when it is choked the flow velocity will be 340 m/s, the speed of sound.

The specifics of pressure drops across the restictor will depend heavily on engine choice, NA/turbo, fuel type, etc. Have you ironed out these issues yet?? Try googling "converging diverging nozzle" or "choked flow" or dig up a compressible fluids book... that should answer most of your general questions.

Hope that helps.

[Garlic]

It is not true that no matter what your engine choice, you will see choked flow.

In fact, I'd wager that no NA engines are seeing choked flow. If anyone can show me data that they are even approaching .95+ Mach in thier restrictor, I will give you a donut.

[Jersey Tom]

I'd put money that the restrictors choke, even NA. You'll have a normal shock wave and transition back to subsonic flow shortly thereafter, but the throat should choke.

[CornellGixxer]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Garlic:
It is not true that no matter what your engine choice, you will see choked flow.

In fact, I'd wager that no NA engines are seeing choked flow. If anyone can show me data that they are even approaching .95+ Mach in thier restrictor, I will give you a donut. </div></BLOCKQUOTE>

Exactly what are you basing that on?? If no NA motor ever chokes its restrictor then the restrictor is not really a restrictor at all.

I'm not familiar with the theoretical or experimental maximum mass flow rates through a gas restrictor but for ethanol NA choke should happen around 10k RPM. If the 72 g/s figures Ive seen tossed around here for gas restrictors are accurate then you're talking more like 12k RPM. These are both back of the envelope style (for a 600cc) but the basic point is if you dont choke you dont have a restrictor.

[Jersey Tom]

72 g/s is about right for ideal, isentropic flow through the restrictor.

[Garlic]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by CornellGixxer:
If no NA motor ever chokes its restrictor then the restrictor is not really a restrictor at all.
</div></BLOCKQUOTE>

It's not true that if you don't choke you don't have a restrictor. If you say that you are either just not thinking straight at the moment or have no idea how fluid flow works (I'm hoping the former)

A restrictor increases pumping losses. There is a reduced rate of return as you increase the pressure drop on the engine side. When you get zero return for increased pressure drop on the engine side, that is the choking point. Up until that point, obviously, the restrictor has an effect.

Anyway I am certain teams are getting into ranges where the restrictor affects performance. However, I am not buying that 'any' engine will choke the restrictor. In fact, I think very few NA are. Unless you measure mass flow and it stops dead with increasing RPM, then you aren't choking the restrictor.

You bet money, well I bet donuts. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Jersey Tom]

It doesn't take much of a pressure ratio to choke that 20mm restrictor. If you go to FSAE East I may have to hit you up for some donuts.

[Garlic]

Hey, I'm not afraid to part ways with donuts if proven wrong. http://fsae.com/groupee_common/emoticons/icon_wink.gif Somebody show some mass flow data.

[Marshall Grice]

wouldn't a torque plot show you the same data? I would say that a linearly decreasing(with rpm) torque curve would indicate choked flow as your VE gets worse as rpms go up with equal mass flow. Obviously it won't be perfectly linear as the frictional losses increase with rpm.

[Garlic]

No, too many variables go into the torque curve. It is not definitive.

[BeaverGuy]

The torque plot won't correlate at all. A power plot, however, correlates fairly well but still won't tell you because there are other vaiables.

The problem with most mass flow measurement is that it is averaged. This is due to inertial responses with vane or turbine flow meters and electrical resistance with hotwire or hotfilm anemometers. I doubt any team has access to the equipment to measure the airflow through the restrictor instaneously. I have airflow data that says we peaked at 68 g/s through the restrictor at 11600 RPM, measured with a turbine flow meter. That flow however corresponds to a mach number of only 0.67 at STP.

The maximum airflow through the 20mm restrictor is around 75 g/s and around 68 g/s for a 19mm restrictor.

[Kirk Feldkamp]

I've found that fuel flow and lambda (1/[air fuel ratio]) will get you pretty damn close. I tend to believe a fuel flow meter more than an airflow meter. The calibration and accuracy of the fuel flow meter is a heck of a lot easier to deal with in my humble opinion.

-Kirk

[Garlic]

I would beleive that. Mass flow is tricky to get right. Most OEMs use some kind of massive damper and get pure steady state to calibrate. FSAEers could probably get the job done with something like a 55 gallon drum.

[ahmadreza]

Thanks guys.
In the design process of our venturi I came to conclusion that designing it without considering the manifold pressure is in vain,
because that is a theoritical design and if downstream conditions aren't satisfied it will never become chock.Dependency on the engine may be the same that everyone has pointed out.What did you assume for manifold pressure?
the other thing that dosen't matter seems to be pressure drop(when it's chocked),because the pressure in outlet plate as the flow leaves the venturi equals to the downstream pressure .what do you think?
bye.

[BeaverGuy]

I would suggest that you design your manifold to meet your restrictor manufacture requirements. If you are having a restrictor made from Alluminum it can only be so long.

Ideally we would have atmospheric pressure in the plenum. It isn't always going to happen but if you taper your restrictor such that it has a smooth transisiton into you plenum and opens to a large area in your plenum you will recover as much pressure as possible. Look at Guelph's and Penn State's intakes for examples, basically the same idea with two different executions though both integrated the restrictor into the plenum.