compound numbers

[Charles]

Mass flow vs. volume flow, can't we all just get along? If you want to use CFM instead of lbs/min, just multiply lbs/min by 12.5 and you get CFM at STP. Then you can easily bounce back and forth.

As long as you're at or near STP.... fine.

However, your second stage compressor doesn't operate anywhere near STP. Rendering your mass flow compressor map useless, and prompting people to ask questions (as in this thread) about how to flow around the second stage once the first stage is up and running.

Which was my point all along. You can't very well make intelligent decisions if your conception is wrong to begin with. It seems however, that some people are more interested in just slopping through it, half-assed, rather than caring enough to at least try to get it right.

Since I still find all this perfectly relevant to the thread, I can't help but ask a few things in hopes of clicking on a light bulb in one of you.

Why is a 6BT labeled as a 5.9L engine? And furthermore, if volume must be corrected for atmospheric conditions constantly... does this mean a 6BT is constantly changing displacement with changes in weather? I would hope you could all quickly answer no to that rhetorical question.

However, if... as was stated above, you merely converted from the 5.9L to lbs, based on the weight of atmosphere of standard composition at STP, would we then have a more stable unit of mass to describe our engine? Such as is the case with compressors labeled as such?

For instance, a 5.9L engine would become a 0.0168lb engine.... at STP and average atmosphere composition.

Change the temperature.... oops, now the weight changed so the mass number is wrong, still 5.9L though. Chang the pressure, whoops, mass is off yet again.... still 5.9L to the T. Change the air composition.... whoops, mass is yet again wrong.... hang on, yep, still exactly 5.9L though. Hell, run the engine underwater.... mass is WAAAAY off.... but wait a sec.... still exactly 5.9L.....

WEIRD.

It's almost like the crazy guy that keeps saying CFM flow isn't dependent on ambient conditions and that mass flow is may be on to something...

I can't see why you would rate engine displacement in mass flow any more than I can see rating a compressor wheel in mass flow. It changes with the weather people. Which is precisely why that mass flow compressor map is garbage for looking at the second stage compressor. For the second stage, we've had one HELLUVA high pressure front move in, lol.

 

[Goinbyyabig]

Charles, first off I'd like to thank you for bringing my understanding of compound turbocharging to the next level. You know when you think you understand a concept and someone turns it around at a different angle and it becomes far more clear and intelligible; I appreciate your efforts. The only thing I have to add is: using CFM may confuse some because the charger's inlet, be it primary or secondary, is not aware of atmospheric conditions or previous compression by another charger. The physical "size" of the air being moved through a compressor is independant of the pressure and temp. of the inlet air, where the lb./min. numbers seem to take this into account. I'm certainly not qualified to argue the point with you, and I think I follow your point of using CFM instead of mass, but I think the turbo engineers write the maps to determine how many oxygen molecules are being delivered by a compressor, regardless of temp. or pressure, relating to the amount of "fuel" or oxygen available to be burned. Therefore, they attach a HP rating to a specific turbo. I agree that these maps as written become useless with compounds, as there is no atmospheric condition similar to 20 lbs provided by the primary, reinforcing your argument. However, I feel that the mass flow maps are simpler and better suited to single stage turbo apps. where they were originally intended, because I feel one pound of air, regardless of it's "size" and temperature, is able to support roughly 10 horsepower. Please feel free to contradict me, I'm still very much learning, Rob.

 

[Diesel Freak]

Mass flow vs. volume flow, can't we all just get along? If you want to use CFM instead of lbs/min, just multiply lbs/min by 12.5 and you get CFM at STP. Then you can easily bounce back and forth.

agreed, it really does not matter all that much (FYI, Holset maps use a 13.6 correction factor @ STP). I went down this road about 5 years ago and once all the math is done, some very basic and elegant relationships develope...the first of which is the square root of 2.

Re-inventing the wheel is great in acadamia, but as with all engineering...the math just gets you "close"...the rest is reliant on experience (or intuition). That is when you make an EWAG and thery becomes practice...and that is when the real fun begins!!! :woohoo:

 

[CSM diesel]

Here is my best shot at a non-bias, simple explanation of why mass flow is shown on the charts... take it for what you will. I have used both mass flow and volumetric flow to size pumps and pipes in several professional applications. Some require both.

Mass flow illustrates how much of a substance (gas, liquid, multi-phase) is in flux over a certain x-sectional area per time. M(dot) = rho*V*A

That is why they list the maps as such. And many many other pumps are mapped this way as well. We want to know how much of the working fluid we are moving around. Not just the volume the working fluid takes up, the actual mass of the fluid it moves in moles, lbs, slugs, kg's, etc.

This information is necessary for more complex processes than the matter at hand. Heat transfer, chemical process, complex combustion, etc. These process mentioned all need mass flow numbers to continue down the path of design and analysis.

Simply cramming a set mechanical volume full of air is not one of those processes.

Our discussion in this thread is taking many liberties and disregard for the ideal gas law and compressible flow side of things.

But as it has been said. Volumetric flow is just plain easier. The average joe can plot pressure ratios on a chart converted to volumetric flow with no concern or regard for fluid density, expansion, friction, mach, and specific heats.

Just pressures ratios.... That is all we care about anyways, we all have a boost gauge, very few of us have an air temp/density gauge...

So to say again, the mass flow chart is indeed not garbage, just too harry to deal with without proper instrumentation and documentation.

I hope this helps those who are struggling with why things are presented in mass flow rather than volumetric flow.

 

[Charles]

So the consensus is that we should correct all of our mass flow maps by multiplying the X-axis values on our first stage compressor map by the ratio between the density of the current ambient conditions compared to those at STP and then by multiplying the X-axis values on our second stage compressor map by that ratio and then by the density ratio across the first stage compressor itself instead of converting all maps to volume flow?

That is a real question, not a rhetorical one.

Because I keep getting the "keep it simple" notion thrown in my face, yet I can't seem to see how volume flow conception isn't far simpler than all that.


On Edit:

I hadn't yet seen CSM's post before I posted this.

 

[CSM diesel]

So the consensus is that we should correct all of our mass flow maps by multiplying the X-axis values on our first stage compressor map by the ratio between the density of the current ambient conditions compared to those at STP and then by multiplying the X-axis values on our second stage compressor map by that ratio and then by the density ratio across the first stage compressor itself instead of converting all maps to volume flow?

Just convert them to volume flow. Unless you know the mach number, then you can find the local mass flow.

I keep getting the notion of "keep it simple" thrown in my face and I can't seem to make it clear enough that I feel volume flow conception to be far simpler.

It is. I just said that...

 

[Charles]

Just convert them to volume flow. Unless you know the mach number, then you can find the local mass flow.



It is. I just said that...

I hadn't seen your post when I made my last.

 

[CSM diesel]

Are you good then? I can go into the compressible flow side of it, but it is not fun. It deals with local speeds of sound, mach, stagnation ratios, and isentropic relationships.

 

[Diesel Freak]

BINGO!!! It's based in thermo. I have yet to do an analysis on a system where volumeteric flow is used for anything more than to determine a pressure drop through pipe... I have always used mass flow rate for any energy conversion...which is EXACTLY what happens inside a turbocharger. To me it's just easier not to have to account for a change in density throughout the process. Just hold mass flow constant...

I just think of an engine as a "black box" it consumes air and fuel it rejects heat and produces work...which is exactly why I mentioned Brake Specific Fuel Consumption...it directly relates how efficient the engine is at turning that fuel into usefull work.

Methods vary, some are better suited to the application. However, none are better than the other especially when the result is to be discussed with a group of non-engineers a.k.a. this website. Either taylor the discussion to the audience, or loose their interest.

 

[Charles]

I am still a bit confused, especially with the reference to pumps above. This largely due to the fact that almost all pump specifications I have ever run across list pump flow in terms of volume. Usually in the form of CC/rev, Gallons per minute, Gallons per hour, Liters per minute, Liters per hour, so on and so forth. Heck, as I stated above, the very engines we are sizing these turbochargers to are labeled based on displacement. Hence the 5.9, 6.6, 7.3 and so on.

In fact, the only time I see reference to mass flow in terms of a pump is usually having to do with fuel pumps often labeled in terms of lbs/hr.


I'll try to make my point as concise as possible:

To restate it, I don't understand why compressor maps are displayed in mass flow as opposed to volume flow.

And first off, the measures used to describe a device should be based on the operation of the device, not on where you plan to use it. In my mind, whether or not you plan to base your calculations on mass flow or volume flow is irrelevant. What is relevant is only which of the two the device actually cares about. And to date, I have yet to see any reason to accept the fact that a compressor wheel cares more about the weight of incoming media, vs the volume of incoming media.

I cannot make this any more clear than a second stage compressor does for me. The mass flow is Drastically altered above and beyond what the map accounted for, usually a few Times what the map lists, yet the volume flow will be the same as what was recorded on the flowbench to begin with.

Now if the wheel honestly moves ___ CFM at ___rpm and a __PR, then why do we kid ourselves into contorting it into mass flow?

It seems clear (at least to me) that the wheel is actually displacing ___ Cubic Feet of air per rev at any given rpm and pressure ratio independent of the density of that air on its inlet.

If I hold a centrifugal compressor shaft spinning at 100,000 rpm with a PR of 2.0:1 and I change nothing except to increase the inlet pressure 2 fold, do I expect that CFM flow has changed? I expect not. Do either of you believe it does? If so why?

Secondly, do I expect that mass flow has changed? I certainly hope so.

These are the reasons that I will not just accept these maps being labeled in terms of mass flow and corrected up and down, left and right when volume flow appears to be the true constant. The true measure that is actually directly linked to the compressor's operation. Mass flow is indirectly linked to compressor operation in that for any given compressor operating scenario the mass flow is highly dependent on the environment the compressor is run in. Volume flow is not. Or at least none of you has shown that it is.

 

[Diesel Freak]

Dude, it really does not matter how the axes are labeled shoot it could be "slugs per fortnight" for all I care...regardless of what "you" think of how the map should read, most just use mass flow...and it really is easy to use..so much so, it becomes almost seccond nature. You can use Vdot if you want...so could I...but I prefer Mdot especially for a thermodymanic analysis.

ok I really gotta ask, what is your background and education?????

 

[Charles]

Dude, it really does not matter how the axes are labeled shoot it could be "slugs per fortnight" for all I care...regardless of what "you" think of how the map should read, most just use mass flow...and it really is easy to use..so much so, it becomes almost seccond nature. You can use Vdot if you want...so could I...but I prefer Mdot especially for a thermodymanic analysis.

ok I really gotta ask, what is your background and education?????

I'm just wondering why a volume based device is labeled in terms of mass flow. Nobody has chosen to answer that. I don't care what calculations you like to do, the device is what it is. Volume flow is directly what the wheel is doing, and mass flow is indirectly related and dependent on ambient conditions.

I wouldn't expect a compressor map labeled in units of mass flow anymore than for a gearset to be labeled in terms of mph. I would expect the gearset to be labeled with a ratio. Even though, much like mass flow for a compressor, you could very well label it in terms of mph as long as you stated the input rpm, and tire diameter just as you must state temperature and pressure for a mass flow comp map. But that doesn't make labeling a Ring and pinion as an 85mph set any less retarded does it? The fact that you might choose to use mph for your gear calculations, such as 20mph first gear, 45mph second, 89mph third and 130mph 4th has little to do with whether or not labeling a gear set in terms of mph makes any sense. That is why I don't choose to use mph for ring and pinion sets, nor mass flow for compressor wheels. 3.73 works much better for describing my R&P, as does 1900CFM for describing my first stage max flow.


And I have no background on any of this. I'm just a guy looking at a wheel trying to figure out what media density has to do with it's flow and coming up empty.

 

[CSM diesel]

I'm just wondering why a volume based device is labeled in terms of mass flow. Nobody has chosen to answer that. I don't care what calculations you like to do, the device is what it is. Volume flow is directly what the wheel is doing, and mass flow is indirectly related and dependent on ambient conditions.

And I have no background on any of this. I'm just a guy looking at a wheel trying to figure out what media density has to do with it's flow and coming up empty.

It is a velocity based device. Pumps impart velocity to the medium using pistons, spinning blades, rotors, lobes, etc. How we turn that velocity into flow is based on areas.

I have given you the reason as why the pump is labeled in mass flow numbers. The fluid parameters that require mass flow are not necessary to know for the matter at hand. It is just a common convention to have the charts in mass flow as the same style of charts are used in many industries and many areas of design and analysis.

:Cheer:

 

[Big Blue24]

I'm just wondering ...


And I have no background ...I'm just a guy looking at a wheel trying to figure out what media density has to do with it's flow and coming up empty.

Another name for a turbocharger is a cetrifugal supercharger. A centrifugal supercharger creates airflow "boost" by accelerating air molecules away from the center of the compressor wheel.

Media density is a huge factor in turbocharging because the "denser the media" the more energy it takes to accelerate it outward into the compressor cover.

The assumption made earlier that a turbo is a fan that moves "x" cfm and it doesn't care what it's moving is incorrect. The more dense the incoming airflow, the harder the turbo has to work to increase the pressure ratio.

Charles, do you know what the choke line of a compressor map really means? It's very close to the point where some of the air molecules are being accelerated faster than the speed of sound. We all know that when an object travels faster than the speed of sound it creates powerful disruptive sound waves. These "sonic waves" create instability in the flow of the compressor and efficiency drops very very quickly.

When efficiency drops, you have essentially reached the point of maximum mass flow for the compressor. Increase in pressure ratio, or "boost", or shaft rpm beyond the choke line results in increased discharge air temperature with almost zero increase in mass flow.


I think everyone will agree that twin turbo setup's work, and they work well. I think we can all agree that most turbo maps are plotted in lbs/min and not CFM. Why? .... Why does it matter? Why does America use standard measurements and the rest of the world use metric? Lbs/min is what we have so lets not waste hours posting about why CFM is better.

This thread was supposed to help derive and equation to size out twin turbos. It doesn't matter if our created formula uses CFM or Lbs/min, all that matters is someone come up with a creative way to size out twin turbo setups.

 

[Charles]

It is a velocity based device. Pumps impart velocity to the medium using pistons, spinning blades, rotors, lobes, etc. How we turn that velocity into flow is based on areas.

I have given you the reason as why the pump is labeled in mass flow numbers. The fluid parameters that require mass flow are not necessary to know for the matter at hand. It is just a common convention to have the charts in mass flow as the same style of charts are used in many industries and many areas of design and analysis.

:Cheer:

Again...

Your reason has nothing to do with the device itself. Saying that we should measure the operation of a device based on what units are easiest because they are most commonly used elsewhere doesn't work for me. I tried to express that with my 85mph gearset example.

 

[Charles]

This portion alone strikes straight to the heart of my point.

Charles, do you know what the choke line of a compressor map really means? It's very close to the point where some of the air molecules are being accelerated faster than the speed of sound. We all know that when an object travels faster than the speed of sound it creates powerful disruptive sound waves. These "sonic waves" create instability in the flow of the compressor and efficiency drops very very quickly.

When efficiency drops, you have essentially reached the point of maximum mass flow for the compressor. Increase in pressure ratio, or "boost", or shaft rpm beyond the choke line results in increased discharge air temperature with almost zero increase in mass flow.

You just explained why I should accept mass flow as the true measure of compressor function, and in doing so, completely validated my point that it is in fact not. I'll show you how.

You explain the choke point wonderfully. The velocity of the incoming media is approaching that of the speed of sound, obviously sonic disruptions would accompany that point and disturb the operation of the compressor and reduce efficiency to the point where diminishing returns have basically flatlined the compressor's ability to handle any increase to mass flow rate.

Alright. I follow fine.

This is precisely what the mass flow compressor map has told me all along. It is rather simple to locate the point of maximum flow for any given PR as the points all along the choke line.

And here's the fun part. If this is true, and the limit is mass flow because any increase in mass flow only worsens the issue, we are now right back to my point that crushes mass flow yet again. And it seems simple to me, yet it keeps getting missed, or I'm missing the reasoning from you all. But if this point of maximum mass flow is determining this choke point, how is it that this point occurs at ~70lbs/min on my second stage compressor map, yet at full song my first stage is pulling in over 120lbs/min?

In other words, if it really was mass flow that determined this choke point, how do you account for the fact that the second stage charger is perfectly happy, with calm shaft speeds, good outlet temps and long life while moving roughly TWICE the amount of mass flow that you so eloquently stated to have defined the choke point?

I think it should be OBVIOUS that the mass flow was NOT determining that choke point, but in fact the VOLUME of air was. And that volume of air only indirectly coincided with a certain mass flow value based on the specific ambient conditions present during the test.

This seems so obvious it's like I'm fighting regurgitated textbook lines instead of conceptualization here.


On Edit:

I think I should add that it's not as if I cannot see that accelerating a media of increased mass per unit time does not require an increase in applied power. I have been thinking along these lines all along, yet all along it still doesn't mean anything about the compressor, but that more power must be generated by the TURBINE to turn the compressor. A separate discussion.

 

[Charles]

To follow up, and take your point even farther...

What determines the speed of the media in a confined space such as is the case here, where the inlet is of finite size as is the outlet? I agree that approaching the speed of sound with any fluid flow is going to produce issues with that flow.

But does fluid density determine fluid velocity through such a device? For instance, if we hold the compressor wheel rpm fixed, and hold the Pressure ratio across the wheel fixed such that the point formed at this shaft rpm and PR occurs at a point well left of the choke line for the compressor, will it then choke if we do nothing but increase mass flow? How do we do that without changing any of the above you might ask? By simply compressing the inlet air, with say, another compressor (lol). Now we can easily produce TWICE the mass flow through that compressor without moving any closer to the choke line.

You agree? You must! Even though I just showed you precisely how to exceed the mass flow choke point you described.

So what would push it over choke since mass is obviously irrelevant? We can increase the mass infinitely without increasing the velocity at all. So what does increase the velocity of the fluid through the device? Increased VOLUME for any given PR. And the only way to do that would be to either increase the shaft rpm while holding PR constant (moving right off the map) or by decreasing PR while holding shaft rpm constant (moving down off the map).

Either way, the way to choke the compressor is with VOLUME flow. The mass flow can exceed the map values by an infinite amount, yet the volume flow cannot.

Why? Because the function of that compressor is to move VOLUMES of media based on PR across it, and current shaft speed.

If this is not true, then you guys are doing one horrible job at proving otherwise.

(I'm not saying it isn't. I'm merely asking that you respect the fact that if you can't do any better than this, please don't expect me to just throw my hands up and agree when basic logic pleads against you given the information I have to go on)

 

[CSM diesel]

What happened?

It is not my job, prerogative or burden to convince you what to do with all of this.

I am happy to continue down the path of all this and throw all the math out there. Differential equations and all, I just don't want it to fall on deaf/stubborn ears.

 

[kscumminsdriver]

I am happy to continue down the path of all this and throw all the math out there. Differential equations and all, I just don't want it to fall on deaf/stubborn ears.

Please do. I have an engineering degree so it won't fall on deaf ears.. might be outside of my license but I think I can handle the math.

 

[Charles]

What happened?

It is not my job, prerogative or burden to convince you what to do with all of this.

I am happy to continue down the path of all this and throw all the math out there. Differential equations and all, I just don't want it to fall on deaf/stubborn ears.

What on earth makes you think that in-depth mathematics dealing with the acceleration and deceleration of the incoming mass, resultant pressure formed or anything else is needed when we can't even get on the same page conceptually?

How on earth did you read my last two posts and not see a big F'ing problem with using mass flow to describe compressor performance?

Let me break it down again:

You hold the wheel speed constant, you hold the pressure ratio constant, and..... IF mass flow is the TRUE indicator of compressor function, and volume flow is the variable as keeps getting indicated, then you should record the same values as long as the variables related to COMPRESSOR operation are UNCHANGED, you should NOT see any change in the measure that is indicating compressor function. You chose mass flow. Well I'm here to tell you that with all compressor variables HELD CONSTANT, mass flow changes on a whim any time inlet temperature or pressure conditions change.

Yet..... exact same scenario, and volume flow DOES NOT CHANGE IF THE COMPRESSOR VARIABLES DO NOT CHANGE.

The gentlemen with the wonderful example of mass flow at choke made it abundantly clear that volume flow is the true measure of compressor operation in the fact that this ultimate mass flow choke point can be exceeded multiple TIMES by merely installing a separate compressor on the inlet of the one being tested.

Yet..... once again, volume flow DOES NOT CHANGE with the addition of this new compressor. It still only moves ___ CFM for any given scenario.

The correlation here is NOT between the wheel and ___lbs/min, the correlation is between the wheel and ___CFM, which happens to coincide with ___lbs/min WHEN at ___Temperature and ___Pressure ONLY. As soon as those inlet conditions change that mass flow value is INCORRECT, yet the volume flow is again... UNCHANGED!

Address that Directly if you don't mind. I'm tired of hearing about how we use mass flow because we use mass flow because we use mass flow and that's just what we use. Please. I don't want to hear about what you like to do your calculations in. My point is that the WHEEL is NOT directly related to mass flow.

None has managed to explain why it is with remotely the same level of explanation that I have given for why it is not. And I don't refer to explanation quantity, but explanation depth. Regurgitating information from an old fluids class is not sufficient. If you don't understand the conception behind something, it's nearly pointless to hurl fun facts at a discussion.