Another newby! - General flow bench questions...

[5.0joe]

Doug~

One more thing, looking at your example for your plenum rule of thumb...just wanted to double check: converting 15L to cubic inches, it becomes 915.36 cubic inches, which would be a cube with 9.7 inch sides. Correct me if I am wrong. I figured using a Chevy 350 c.i. cylinder head with a design factor of 3.0, this puts it at 1050 c.i. Which would in turn lead to a 10.2 inch cube. I dunno, maybe I am confused. That seems a bit small just from what I have seen in the research I have done.

Thanks.

[Tony]

Welcome to the Forum Joe,

There are no hard and fast design rules for a settling plenum, except that it can be too small ! So just make it as large as is practical. Several cubic feet at least.

The first decision might be, does the measurement orifice have to flow in only one direction, or both directions ? In other words do you plan to reverse the flow through the orifice ? If so, then you will need a settling chamber on either side of the measurement orifice, preferably in a symmetrical arrangement.

This basically means a big box with a dividing bulkhead to house the measurement orifice. Actual dimensions are probably best determined by the sizes of the sheets of material available, and how best to cut them for minimum wastage.

If the bench only has to flow in one direction, then obviously a settling volume is only required on the up stream side of the measurement orifice.

[5.0joe]

Tony-

You brought up something I was recently thinking about. Say I use a powerful shop vac(s) for the power source and they have a reversable motor for both suction and blowing through the test piece (intake and exhuast). Now suppose I use a 2' x 2' x 2' cube expansion chamber (MDF or wood) with a 45-degree diagonal divider that separates the top chamber from the bottom. Centered on the 45-degree divided is the orifice plate of choice (2.5 inch more than likely). Would this be okay for flow in two directions? I think where I get confused is with the sharp edged orifice concept; how does the reverse flow act when it hits the edge from the opposite side and how does this affect calculations?

[SWR]

[SWR]

[Tony]

Joe,

The shop vac ides would work o/k if it is powerful enough, and has both a blow and a suck output port into which the hose plugs. Maybe two shop vacs ? I really don't know. But a bare vacuum motor will always perform much better than when it is mounted inside a restrictive shop vac casing. A long convoluted hose is not ideal either.

Better if the divider is not sloping, because a slope may cause the air to slide into the orifice from one preferred side.

Curiously, most of the air enters an orifice radially and falls over the edge into the hole. Very little air flows into an orifice from straight ahead. Some wool tuft testing will quickly demonstrate this effect.

The best place for an orifice is right in the middle of a rectangular wall, with enough surrounding volume in all directions to slow down the up stream velocity as much as possible.

Making a very thin sharp edged orifice that flows in both directions is difficult to fit onto a thick bulkhead. It really needs to be transferred to the up stream side so it is flush when reversing the flow. but some compromise may be possible. The pressure drop across this bulkhead should always be quite low, so it need not be massive.

[115-1172523331]

Hi Joe, The problem with mixing units! I should have said a 24 "cm" cube (the cube root of 15000 cc). Oops! Good catch!

Doug

[106-1194218389]

[5.0joe]

SWR-

So it is not a necessity to be able to flow in both directions for exhaust port testing? I have read a lot of mixed thoughts on this on different sites. I have already ordered the radius flow guide for the intake ports, so that is covered. But I have read that sucking through the exhaust port will cause the air to shear as it hits the sharp edge, as would the intake port without the radius guide. Now, my concern is, in proving this project out to my design advisor, I have to be close to professionally tested values (+/- 10%, corrected to 28 in. H20) for both intake and exhaust ports. So, if I can simply add like a 6" long piece of exhaust tubing to the port with a matching bolt pattern and I can still suck air through and get similar readings to what a Superflow will get (even if it is blowing), then I am all for that idea. If not, maybe I can put two orifice holes on one disc with opposite facings edges, then have a rotating mechanism to control the disk for intake or exhaust testing. To the next line of business, I did a lot of research tonight on proper sizing of a settling chamber. It seems that a good universal size is 24"x24"x18" deep, with the divider and orifice plate(s) in the middle. Is it appropriate or okay to go any smaller than this to conserve space, or does everyone recommend that size? I will work on a revised sketch of the proposed system with the settling chamber incorporated and have it up soon. In the mean time, I'm gonna keep on cramming my brain with the wealth of flow bench knowledge on here.

Thanks All!

[Tony]

Several options Joe.

Suck air out of the exhaust port using the flow bench in suck mode. This is a perfectly valid method, but it makes probing the port with a velocity probe next to impossible.

Build two test holes into your bench top, one hole sucks, the other blows, but flow through the flow bench never need to be reversed. This constructionally is probably the simplest, my own bench works this way.

Build a complex system of flow reversal around the blower and measuring orifice, so the measuring orifice always flows in the same direction.

Some combination of the above to suit your own tastes.

[5.0joe]

Tony~

Let me try to clarify your suggestions, and you let me know if I am going in the right direction. You said I can suck air out of the exhaust port...do you mean out of or through? Meaning I would have to reorient the cylinder head so that the header side is mated with an adapter plate, or do you mean I can leave the head in the same orientation as intake testing and suck air through and get reliable numbers that way? Sorry, I am confused. BTW, this is an orifice plate flow bench.

Your second suggestion, which seems appealing: the settling chamber is divided into two equal parts...cut matching holes on the top of each side of the chamber. I will have an adapter plate come out of that hole and then also a PVC flange coming out the sidewall for external shop-vac(s) hookup. So i would need to also have this vacuum hookup flange for each chamber (right side / left side), then switch the vacuum hose between those flanges for exhaust / intake testing, and reverse the flow on the shop-vac(s) accordingly, as well as switch the inclined manometer hookups? Wow, that was a lot of words! Did I get that right for the most part though? So if I have two holes for the adapter plate and two for the vacuum hookups, they will need to have flow valves to close the 2 that aren't being operated during the particular test. Correct me if I am off on my assumptions, please. Also, one more thing...this setup will avoid having to reverse the orifice plate (great) since it will switch the side that the flow comes from, but how does this affect the U-tube manometer in place for test pressure. I guess it is just a bit confusing to think of the manometers in reverse.

Thanks.

[Tony]

First idea. You mount your cylinder head on a bore adapter as usual, but the underside of this is fully open to atmosphere.

You then draw air out of the exhaust port through a short length of simulated header tube into the suction test hole in the top of your bench.

Second suggestion. Airflow is in through the "suck" test hole, straight into a settling chamber. The side wall contains the orifice plate, and air flows through this into the suction side of the blower compartment.

The blower then pushes the air up out through a "blow" test hole. You need two test pressure manometers (one positive one negative) and one flow manometer. No need to change anything around at all, just move the whole cylinder head across from one test hole to the other.

Here are some pictures of a bench that works this way:

[SWR]

[bruce]

I'm going to add something into this discussion and take it for a shameless plug for my bench plans if you want but . . . there are now 2 benches up and running built from my plans, both are on water gages using my plates and both on startup with no tweaking flowed within .5-1% of the pass-around-plates.

What I am trying to point out is this; the plenum/orifice plate divider style bench has been a proven design with little to no problems. Not just on my design but others who have built benches this way, that's why I went that way with my plans. It works and it works quite well!

Orifices in a pipe have been discussed on here in-depth and it has been proven it is not a solid design for flowbenches. It probably can be made to work, but if your goal is to build a bench and get testing then go with the proven design that works for a lot of people that have come and gone on this forum over the years. There is an extensive amount of info on this forum to allow you to do just that, I know I've made the plans from the info found here on the forum and they work.

Just have to look at myself . . . I'm a converted Pitot bench guy!!

Also don't take me wrong I'm not trying to discourage "out-side-the-box" thinking but keep your goals in mind with what you want to accomplish and the time you have to accomplish it in. Keep it simple and get testing . . .

PS ($50 donation to the forum for a set of plans and you can be building next week)

[Tony]

Bruce's plans are an excellent way to go about building a first flow bench.

But.....

For Joe this is a student project. If his Professor ever discovers that Joe has just simply followed a purchased commercial plan for his class project it might furrow the good professors brow, and may well cast a very long dark shadow over Joe and his end of year assessment. That would be a most unfortunate outcome.

Better for Joe to come up with something truly original but based on sound principles and with a bit of friendly guidance from the Forum.

But hey,
Joe could get your plans and be "inspired" to create something not exactly the same. There are a huge number of very clever design features in Bruce's design well worth thinking about.