3D printed plastic burner experiments (photo heavy)

[billybodgeit]

Just thought I would contribute to this talk of wasp waist burners , as I have gleamed some ideas from this site. 

Amal injectors use a simple copper cone for their taper long venturi, fairly easy to replicate either from sheet or pipe, or turn if you have access to a lathe. A morse taper reamer or pin reamer may even be close enough. If you go onto their site you will find a pdf with basic dimensions and pics. Approx taper of 1: 12. Their 3/4" has a venturi of approx 1/2", the 3/4 simply refers to the size of pipe thread. 

Currently working on a couple of all steel, water cooled narbs, powered by Amal injectors. Results so far are very promising, in terms of heat output and economy, no pop back and the ability to turn right down. Still prototyping at the moment, so time will tell.  

I had considered making my own version of their injector, as I have the necessary machine tools, but for the money just decided to buy them. It would be possible with a bit ingenuity to make a simpler copy with basic tools and pipe fittings. Their jets are very finely sized, so the 3D printer nozzles may be the way to go as a cheap alternative.

Cheers Billy

 

[D.Rotblatt]

So all this new discussion has given me some things to think about.  In all my experiments with wasp waisted burners, I kept the maximum output at 3/4" since I was trying to make a 3/4" burner.  This reduced the intake down to 1/2" depending on the length of the out-take funnel.  In looking at the pic from Frosty on the previous page, I realized that I was painting myself into a corner, and what I could to do was create a burner with a 3/4" intake, and allow the funnel to enlarge to 1".  Now here's the question....is this then a 1" burner, or a 3/4" burner?  Either way, the smaller wasps worked well, just had a much smaller flame (Due to restricted intake).  So I'm making this (printing up now) with the intake that I have that works best (rev3.4.1):

Then...All the discussion about the rabbit cage version by G-Son, I wondered what would happen if I added something like that to my 3.4.1 burner.  These are printing up as well (a 16 hour print...):

Higher fins, same intake, top with hole to allow for choke, 3/4" output pipe.  I'm also printing one with a 3" wide intake so the fins are further from the center.  I'll let you know when I test it!

DanR

 

[G-son]

32 minutes ago, D.Rotblatt said:

Now here's the question....is this then a 1" burner, or a 3/4" burner? 

Very excited to see those results!

About sizes… 1" burners and 3/4" burners have nothing to do with 1 or 3/4 inches, other than the tubing they're made of is graded by a system that once upon a time had that size, before it was changed to thinner wall thus increasing the internal diameter. I'd say scrap the incorrect grading and call it what it actually is. As it has (at least) two diameters of interest, it would make sense to call it a 0.75/1.00" burner, if those are the actual diameters of the waist and exit. You can even take one step further and grade them in metric, reducing the risk of confusion. Out with the old, in with the new.  

In the end, the diameters don't really matter. The important thing is how much heat they put out, i.e. how big forge they can handle, if they are to be used in a forge. 

[Mikey98118]

I believe the consensus of opinion is that burner sizes are rated by the inside diameter of their mixing tubes, or the narrowest point of constriction in a wasp-waist (venturi) burner.

Because all the early home-made burner designs were built from pipe fittings, pipe sizing came to be used as burner size references. It probably is time to change that; good look trying

[Another FrankenBurner]

Question for Frosty, do you happen to know what that final pipe size is in the forge you posted?  

Thanks for the information Billy.  Are your water cooled NARBs something you are keeping close to the vest or are you willing to entertain the curious?

4 hours ago, D.Rotblatt said:

is this then a 1" burner, or a 3/4" burner?

As Mikey stated, your mix tube is mostly 1 inch pipe.  I suspect this burner will need a larger jet, probably a bit larger.  

Full length tapered mix tubes can not be categorized into the same size categories.  The commercial burners I deal with are all rated in output capacity(btu/hr).  Not so easy to determine in your garage.  If you are using mig tips, you could compare to pipe burners which use the same tip at similar pressures.  

I am playing with similar.  The small tapered tube we cast is an adapter for half inch heads to a 3/4" mix tube.  I am able to go to the 045 tip in this burner.  I also have a straight 3/4" head(No tapered outlet from throat or spiral vanes) which induces similar air using the same jet.  I am now trying to figure out if there is much of a difference in performance(possibly in different applications), how so, and hopefully why.

I look forward to your results.

[D.Rotblatt]

6 hours ago, Mikey98118 said:

I believe the consensus of opinion is that burner sizes are rated by the inside diameter of their mixing tubes, or the narrowest point of constriction in a wasp-waist (venturi) burner.

Because all the early home-made burner designs were built from pipe fittings, pipe sizing came to be used as burner size references.

As many things so, the nomenclature works great until you step outside the box a little .  I was amazed that just because of this I was limiting myself to a preset tube size...just following the herd.... Says something about the power that giving something a name has on our psyche.

I guess I was just trying to keep things modular.

3 hours ago, Another FrankenBurner said:

I am playing with similar.  The small tapered tube we cast is an adapter for half inch heads to a 3/4" mix tube.  I am able to go to the 045 tip in this burner. 

That's what I'm using on my new burners.  If you are getting that with a 1/2" opening and getting a neutral flame, I might have to give one a try...Hmmmm.....back to the Inventor for some designing!

7 hours ago, G-son said:

Very excited to see those results!

I'll post when I get them!  Thanks for your input!

DanR

[Another FrankenBurner]

23 hours ago, D.Rotblatt said:

I was amazed that just because of this I was limiting myself to a preset tube size

Yes, but it's fun.  It's kind of the blacksmith burner way.  Trying to squeeze as much down the pipe as possible.  More btu's without staying within those confines is easy.  

We are reinventing the wheel back to the venturi burner but it isn't a fair fight.  Venturi burners are almost the only NA burners I see commercially.  I have been experimenting, trying to increase my understanding of the inner happenings.  I originally thought the outlet taper was about reduced downstream drag and vortex geometry but the more important thing happening is the increase in static pressure of the FAM at the delivery point.  

Now I am trying to determine how changing the inlet reducer shape, throat diameter, and outlet taper length changes the static pressure, what the limits are(and why), and what this does to the flame.  As Frosty stated, it's an endless journey.  

I took a handful of pictures of some of the commercial burners I have seen recently .  These are all low pressure, relatively large orifices, and low temperature application.  

This guy is one of 20 burners which were in a large HVAC unit.  

 

This one is out of a large griddle top.  The orifice is behind the wall on the left size which feeds into the venturi under the cross bar.  

Here is a shot of that venturi, after I cut the burner apart.

 

This big cast guy is for a pizza oven.  IIRC he is 170,000 btu/hr.  He is a blown burner.

 

I thought this one was neat.  It is the only one that I have seen like it. It was on an old condemned HVAC.

On the lower left you see the gas valve which is plumbed right into the blower housing.  The orifice discharges into the blower suction and the blower mixes the FAM as it is delivered.  

[Frosty]

The size of NA burners is the ID of the throat, the narrowest point in the flow path, what we're currently calling the waist. It's a good descriptive term, probably better than throat certainly more intuitive so it should do for the discussion. Wasp waist is a FAR better term than "Venturi" There is NO venturi in a NA burner none, zip, zero. We aren't forcing air through a restriction and using the resulting low pressure to suck the fuel in. NA burners operate in as close to opposite a manner as it gets.

AFB: I may have misunderstood what you meant I do that a lot on a good day.  More vortex doesn't mean or even imply lower induction unless energy is being absorbed to produce the vortex. All the energy in a vortex made by a low pressure zone, hurricane, burner or bathtub drain is in the Brownian motion in the fluid and is directed by the Coriolis effect. Yes, the Coriolis effect is energy introduced into the system and slows the Planet's rotation or perhaps the source is the rotation of the galaxy. It's so insignificant to Earth as to be . . . It only applies as a source of energy introduced into the system under discussion. Vortex.

However, forcing a vortex means exerting or redirecting force directly, be it static force, vanes or active force rotating van blades to make it rotate.  Static vanes exert frictional loss to redirect a flow. Rotating fan blades introduce force to the system and probably or might not decrease combustion air induction.

How a tapered mixing tube increases induction is by causing the F A flow to fill an increased volume. With the restricting surfaces (inside of the cone) withdrawing from contact with the flow it can't in any reasonable way consume energy from the flow. Simple physics applies directly here. 

Regardless that the primary mover (propane jet) is not rotating and pretending the induced flow isn't rotating as it passes the throat, it WILL begin rotating in the mixing tube, even a straight pipe. However an expanding cone amplifies the rotation just as it amplifies the low pressure caused by the primary induces air.

Mike's burners have lower vortex velocity because the only impetus is what can happen in the limited time the high velocity flow is IN the mixing tube. It rotates just not as much, there isn't any structure to encourage nor amplify the vortex.

Increasing the intake's radius must increase the velocity of the vortex, it would require barrier structures to prevent it. Just like a weight spinning on a string accelerates as you shorten the string. I can use an ice skater doing a toe spin as an example but a nut on a string is something anybody can try and see, TV or not. Yes?

Making the starting point for intake air farther from the center of the Primary increases the effects of conservation of angular momentum and the Coriolis effect geometrically. Being as the "restricted" (I'll explain as I go) volume available is greater the gap in the choke structure needs to be less. There is, as observed and described, a "sweet spot" when choking intake air. Too small a gap and the flame leans out, too large and the flame richens up and weakens. 

All I can offer for explanation is logic. The rotation is conservation of angular motion and Coriolis effect redirecting Brownian motion. This redirection requires energy which is provided by the above forces. Like any energy source it has limits and redirecting more material results in less directed motion. Providing a 2D air source results in less 3D motion to redirect while increasing velocity directly towards the primary jet. 

Were I doing the experiments to prove or disprove my above hypothesis I'd simply hot glue a flat ring around the bell reducer of a plumbing linear burner with a matching choke plate. Using compressed air for the primary, a stream of smoke at the intake and observing straight up the burner nozzle and noting differences in rotation between different intake radii.

Were I wanting greater precision than mark one eyeball, I'd place a spinner in the center of the air flow with vanes parallel to the primary so it didn't measure the primary. One black vane and however many others white. Video the rotation and count RPM. 

Frosty The Lucky.

 

[Another FrankenBurner]

That is a long post, thank you.  I will have to churn that for a day.   

I have to respectfully disagree with the burners not using the Venturi effect.  We are forcing fuel down a constricted pipe and using the resulting low pressure to draw in air.  

7 hours ago, Frosty said:

Too small a gap and the flame leans out, too large and the flame richens up and weakens. 

This is interesting and I am quite curious about it.  I am planning several experiments to understanding more about the basics.  

[Mikey98118]

On 8/11/2019 at 6:57 PM, Chris Williams said:

Malleable iron, by contrast, just takes a long (and therefore expensive) heat treatment. Again, I think it is just a resource constraint. 

I have been involved in pottery and hot glass work in the past; in both cases electric pottery kilns were involved with long expensive cool down cycles. While appreciating your point, the equipment used was the obvious problem; it was large, and its insulating efficiency was a joke...lots of room there for improvement in both aspects.

[D.Rotblatt]

On ‎9‎/‎19‎/‎2019 at 11:35 AM, Frosty said:

Increasing the intake's radius must increase the velocity of the vortex

Great post.  Still digesting it..will have to read a few more times.  One question: don't you mean here that "Increasing the intakes radius must decrease the velocity of the vortex" - when a skater pulls his/her arms and legs in they move faster, arms out they move slower.  Correct me if I'm wrong...It's not unusual

 

On ‎9‎/‎18‎/‎2019 at 9:49 PM, Another FrankenBurner said:

This big cast guy is for a pizza oven.  IIRC he is 170,000 btu/hr.  He is a blown burner

Looks like a kiln burner.  I have a few of those laying around.

DanR

[Frosty]

1 hour ago, D.Rotblatt said:

One question: don't you mean here that "Increasing the intakes radius must decrease the velocity of the vortex" - when a skater pulls his/her arms and legs in they move faster, arms out they move slower.  Correct me if I'm wrong...It's not unusual

No, that's what I meant but didn't say it well. The longer the original radius, say 36" string, the faster the nut will be moving when it's shortened the person swinging it. Same for the intake air. The farther from the center it is when drawn between the bottom and choke plates, the faster it will be moving when it reaches the eye of the vortex. 

AFB: You're confusing Bernoulli's principles with Venturi's. Venturi applied Bernoulli's to a specific aspect of the over all fluid dynamics Berjulli was describing. 

For a NA burner to be a Venturi device high velocity air flowing through the throat would have to be sucking fuel. The air flow would have to be the driving force just like the venturi in an early carburetor used the air being sucked into the engine to suck gasoline in through the jets via the Venturi effect. That's early carbs

A NA burner is using the fuel as the driving force and is indeed operating under effects described by Bernoulli but not the one Venturi claimed as his own.

Frosty The Lucky.

[Mikey98118]

On 9/19/2019 at 11:35 AM, Frosty said:

Mike's burners have lower vortex velocity because the only impetus is what can happen in the limited time the high velocity flow is IN the mixing tube. It rotates just not as much, there isn't any structure to encourage nor amplify the vortex.

I agree completely. Mikey burners do induce swirl, but no vortex motion; that just isn't in the cards for a tube burner, sans restriction at any point. No restriction, maens no vortex. I had to go back to linear burners to begin playing with a vortex.

[D.Rotblatt]

10 hours ago, Frosty said:

The farther from the center it is when drawn between the bottom and choke plates, the faster it will be moving when it reaches the eye of the vortex

Ah!  So you are talking about a change in radius?  

 

11 hours ago, Frosty said:

For a NA burner to be a Venturi device high velocity air flowing through the throat would have to be sucking fuel.

If I could reword that to see if I’ve got it: in our burners, the gas jet creates a low pressure zone the air is sucked into, while in a Venturi (like an SU carb on my Spitfire) air is sucked into the intake (like by a vacuum created by a moving piston) which forces it past a necked area in the venturi creating an increase in velocity and a low pressure zone which in turn pulls fuel into the air steam.

Given that there is no actual pulling or “sucking” - everything is actually pushed by higher pressure air behind it. But we tend to think of it that way and it’s easier to describe. 

[D.Rotblatt]

11 hours ago, Frosty said:

Bernoulli's principles with Venturi's

Let's see if my application of these ideas to our burners seems reasonable.  I used to teach science in middle school, but that hardly gets into any fluid dynamics.

So Bernoulli states (via wikipedia):  "Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy." while Venturi applied that to a constricted pipe shape: "The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe."  As you stated Frosty, the Venturi effect is Bernoulli's effect applied to a specific situation; a constricted pipe.

I've found that a jet shooting gas into a plain straight  3/4" pipe without any reducer got a pretty good, stable, serviceable flame.  There is no constricted pipe around it the entrance, just open air.  This is totally the Bernoulli effect? the gas jet moves the air around it faster and this creates a low pressure area that sucks in more air.  It was actually kind of disheartening, since we are spending all this time on our reducers.  The reducers do stabilize the flame at lower pressures and do a lot to fine tune the flame.  I'm imagining that they focus the air allowing an even flow and more stability.

We are adding fins to increase the vortex effect.  As I understand this, propane is difficult to mix and the intention of this is primarily to aid in mixing.  The fins would not increase induction of air into the tube.

When we get into Wasp waisted burners we should be looking at some venturi effect coming into play as well since we are going into a pipe that the restriction lessens after the neck .  The air pulled into the pipe through restricted neck and then travels out an enlarging cone where it will slow down.  The way I see it is that this will slow any vortex, and the gas will exit the pipe at a slower velocity. It will also take more time to exit the pipe, perhaps allowing more time to mix?  I've got some 3D printed, but need to make a new test rig with a 1" pipe to see what they do.  Should get that done this week.

Lot's of variables.  Lots to play with.

DanR

[Mikey98118]

The reducer openings have everything to do with vortex motion, and with Bernoulli's principle. If you have fluid flow through a restriction in a square tube, Bernoulli's principle will still apply, but without creating a vortex; fluid motion through a round restriction, like a pipe reducer, a kitchen funnel, or bathtub drain will create a vortex.

[D.Rotblatt]

27 minutes ago, Mikey98118 said:

The reducer openings have everything to do with vortex motion, and with Bernoulli's principle.

Thanks Mikey, yes. I didn't really address rotational motion of the reducer shapes themselves.    

14 hours ago, Mikey98118 said:

Mikey burners do induce swirl, but no vortex motion

If I could pick your brain on this: What is the difference between a swirl and vortex? And if they are basically the same, is there a practical difference?

Though already probably talked about, from what I've read, the coriolis effect is practically non-existent for small systems like we are using.  It's complicated, but air/liquid flowing straight into the funnel is a higher energy system then flowing in at an angle, therefore we get angular flow or a vortex.  The direction being dependent on irregularities in the system. Mikey burners, 'T" burners, AFrankenburners create the irregularities or obstructions that enhance this angular motion, so I assume this is what you mean by 'swirl'.  Is this correct?  

DanR

[Frosty]

6 hours ago, D.Rotblatt said:

Ah!  So you are talking about a change in radius?  

 

YES! It wasn't until you questioned the statement I realized I'd left that part out. Another example of forgetting other folk don't know what the writer is thinking, only what they're saying.

I believe Mike is referring to turbulence when he uses swirl in this context, separating it from vortex. I often use swirl and vortex to mean the same thing, I need to watch that.

And those descriptions of Bernoulli's principle (SINGULAR! ) and Venturi's are another example of why I hate wiki. Anybody can get sucked in but do you realize as an ex science teacher you just quoted social media regarding scientific principles? 

Bernoulli, tested calculated and described every phenomena of fluidics that could be observed and measured by the current instrumentation, inventing a number himself. So the wiki definition of THE(?!) Bernoulli principle is like describing weather as moving air. Oh, here's a more current analogy, one social media does every day. Describe weather as if it were climate.

The description of Venturi's principle is closer to accurate but misses the point almost entirely. Venturi was describing the lowering of pressure in the boundary layer as a fluid moves over a curved surface. PERIOD. And he only got that partly right, he was referring to it moving over a curved surface as in a wing, across a curved surface. In action a boundary layer forms when a fluid flows over, under, along, inside or outside a curved surface. This is why round chimney tiles are typically smaller than square, smoke flows better so round has a higher carrying capacity.

Bernoulli described and quantified that effect a number of times in a definitive manner but Venturi applied it to something that was in vogue at the time and had better connections so his name is remembered.

About the venturi effect being a fluid flowing through a restriction explain a Pitot tube. It's a tube facing into the air flow typically on aircraft Pic 1, that measures the difference between static air pressure and the low pressure in the boundary layer as the flow passes over the rounded surface of the outside of the tube. The greater the pressure differential between ambient and the boundary layer the higher the velocity through the air. 

There is NO venturi effect in operation in a NA burner such as we make or use. Crack a window in your car to suck your cigarette smoke out on the other hand is an excellent demonstration of one of maybe hundreds of Bernoulli's principles Venturi may have claimed along with the one I know for sure he did.

The NA burners we're using are "induction or entrainment" devices.

Frosty The Lucky.

[Mikey98118]

Technically, any swirling motion of a fluid, from the turbulence behind an airplane's wings, to a tornado, can be described as a vortex, or as turbulent flow; technically they are both. The difference between rotational motion in a pipe, and rotational motion in a tornado (or pipe reducer) or how many principles of fluid motion are engaged.

https://en.wikipedia.org/wiki/Vortex

[D.Rotblatt]

On 9/21/2019 at 4:28 PM, Frosty said:

And those descriptions of Bernoulli's principle (SINGULAR! ) and Venturi's are another example of why I hate wiki. Anybody can get sucked in but do you realize as an ex science teacher you just quoted social media regarding scientific principles? 

Yes, I quoted social media about scientific principles...sigh....you may whack me upside the head should we ever meet. 

DanR

 

[D.Rotblatt]

On 9/21/2019 at 4:28 PM, Frosty said:

There is NO venturi effect in operation in a NA burner such as we make or use.

This is a picture of a Venturi tube.  It's basically the same shape as our burner.  But the lower pressure area created by the constriction does not pull air in from the direction of the flow, but instead would pull air/gas/fuel from a tube in the side (like the tube showing pressure differential which is actually the outside pressure pushing the liquid into the constriction).  So,  when you are saying there is NO Venturi effect in operation, you are saying it does not apply to our situation, the flow of air is imparting the energy to create the low pressure zone, rather than the low pressure zone creating the flow of air.  I guess if it did pull air, we could blow in one end and it would continue to run by itself...a perpetual motion machine.

Just because you asked...With the pitot tube, there is no restriction in the tube so it doesn't have anything to do with Venturi.  But to equate it; instead of inside, the "restriction" is on the rounded outside of the tube.  Take the Venturi tube shown above, split it down the middle horizontally, and put the bottom half on the top - you now have the pitot tube.  It's basically an inside out Venturi tube.  It's not the Venturi effect since it's not a constricted tube, but it is a nice application of Bernoulli's principle (singular ).

DanR

[D.Rotblatt]

 

I got a chance to try the wasp waisted burners with 3/4" opening flaring to 1" tube.  The reducer is the same as my best reducer.  The one with fins is the same reducer with fins shown in the diagram in an earlier post.  The choke is shown in the second pic.  Didn't have the camera with me to take pics of the test but here's the results.

1) Pulls a lot more air, I had to move up from a .046" mig tip to a .056" mig.

2) Flame was stable at higher pressures, but huffed at 2.5 lbs and lower.  This is consistent with what I would expect of the flare.  The gas velocity is decreasing as the tube flares, so as the pressure is turned down the FAM burns back at higher pressures than in a straight tube.

3) On the one with fins, the flame was smaller and even slower, thus it huffed at 3.5 lbs. The fins imparted an extreme vortex to the flame causing it to feather out a bit (but not as much as AFB's version of G-Son's idea).  If the choke was in far enough in for the back hole to be open, the vortex virtually disappeared. When closed with tape it worked again.  The flow of gas wants to come from parallel to the burner tube which is consistent with my observations using smoke on open burners.

4) I have no idea if this applies here, but I was reading about Venturi (or restricted pipes like this) being used to mix fluids.  This might work to help mix the propane/air.

Conclusion: For a forge, this would be a failed direction due to the poor low pressure results.  For a furnace, this would be an excellent choice.  Is it better then a 1"  Straight burner?  Would have to try it and see.

For now, I'm taking a break and rebuilding my gun burner longer ribbon forge to try a massive burner with 336 holes @ 1/8" each.  I'll put it up in the ribbon burner section.  Got it invested and setting now!  Pic below.  My little 124 hole NA burner is next to the board with holes.  

DanR

[Another FrankenBurner]

Great stuff happening in here.  

I have done a bit of research on the Venturi effect and have found nothing except the decrease in pressure through a constriction stuff.  One page stated it as "Increase in fluid speed results in decrease in internal pressure."  One page listed airfoil lift as an example of the effect which did not fit the in a pipe narrative but it contained the same explanation and listed gas burners as one of it's other examples.  Several of the pages with examples included inspirators(burners).  Several of the pages list different jet pumps as examples of the effect.  If researched, inductors, injectors, eductors, and ejectors all lead to several pages which state that they employ the Venturi effect.  They appear to be using similar dynamics.  

Unfortunately, I can not find a good source to explain what the Venturi effect is in any more detail.  I found his writings converted to English but it is a deep dig.  

If not for the lower pressure, what is the purpose of the constricted section in the griddle burner I posted above?  

As to the pitot tube and Venturi, I don't understand.  As far as I know, the pitot tube has nothing to do with the Venturi effect.  The front port measures the total pressure which will be a higher pressure than the side ports static pressure unless there is no flow across the device.  I thought the rounded front was to reduce drag.  There are two tube versions which measure the stagnation pressure with a straight tube.  The pitot tube and the venturi meter are two contrasting methods of flow measurement.  

 

[D.Rotblatt]

4 minutes ago, Another FrankenBurner said:

If not for the lower pressure, what is the purpose of the constricted section in the griddle burner I posted above?

Mixing?  I found a discussion of the Venturi effect on the Inox-Fer Brine Mixers website (google that with the word Venturi and it will be the first hits).  They use a Venturi tube for mixing liquids - the increase in the speed seems to mix the fluids.  

Or, slowing down the speed of the gas so it doesn't blow out from the nozzles?  

13 minutes ago, Another FrankenBurner said:

As to the pitot tube and Venturi, I don't understand.  As far as I know, the pitot tube has nothing to do with the Venturi effect. 

From what I've read, it doesn't have anything to do with Venturi, since it's not a constricted tube.  It's Bernoulli all the way (of course so is a Venturi, which is a specific instance of Bernoulli's principle).  But looking at it, I noticed it is like an inside out Venturi. The air is increasing in speed to rise around the outside rounded front of the tube, thus creating a low pressure area at the widest part of the tube and static pressure inside the tube.  While in a Venturi, the air increases in speed to flow around the inside rounded area while the outside of the tube is static pressure.  Inside out Venturi!

[Another FrankenBurner]

I don't believe the curve has anything to do with the measurements of the pitot tube aside from preventing turbulence at the ports.  The center port is pointed parallel and into with the fluid flow so it is being pressurized by both the static and dynamic pressure, the total pressure.  The side ports are perpendicular to the fluid flow so they are pressurized only by the static pressure.  The difference between the two measurements is the dynamic pressure.  With these measurements, the Bernoulli equation can be solved for velocity.  In a plane, you can determine your airspeed.  In an HVAC duct, you can determine the air velocity which can be used to calculate air volume.