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- Thread starter 5umbzyyr
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Sumbzyyr,
It is generally considered to be SHOUTING when you type in all caps.
Superheated steam is just a gas. At normal velocities it is very compressible in a bend or anywhere else. As the velocities approach Mach 1.0 it becomes much less compressible.
David
It is generally considered to be SHOUTING when you type in all caps.
Superheated steam is just a gas. At normal velocities it is very compressible in a bend or anywhere else. As the velocities approach Mach 1.0 it becomes much less compressible.
David
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Sumbzyyr,
If you are wanting to calculate the pressure drop through the bend, the general rule of thumb is that if the pressure drop through the pipe is less than 15% of the inlet pressure in absolute terms, a gas can be treated as an incompressible fluid. You should use the properties at the average between the inlet and outlet pressures. This will mean that you have to do a few iterations.
The errors arising from this assumption will generally be less than the errors in the properties of the pipe or the physical properties of the gas.
If you are not trying to calculate a pressure drop then you will have to give more details of what you want to calculate before you can expect helpful comments.
regards
Harvey
If you are wanting to calculate the pressure drop through the bend, the general rule of thumb is that if the pressure drop through the pipe is less than 15% of the inlet pressure in absolute terms, a gas can be treated as an incompressible fluid. You should use the properties at the average between the inlet and outlet pressures. This will mean that you have to do a few iterations.
The errors arising from this assumption will generally be less than the errors in the properties of the pipe or the physical properties of the gas.
If you are not trying to calculate a pressure drop then you will have to give more details of what you want to calculate before you can expect helpful comments.
regards
Harvey
sailoday28
Mechanical
zdas04 (Mechanical)As the velocities approach Mach 1.0 it becomes much less compressible.
I hope the above was a typo.
Regards
I hope the above was a typo.
Regards
Sailoday28,
God, I hope so too. The alternative is that I've lost my mind.
Sonic flow is very dense and about as incompressible as water.
I've got a graph in one of my books from grad school (Modern Compressible Flow With Historical Perspective, Second Edition by John D. Anderson, Jr. Published by McGraw-Hill, 1990, p269) that shows that gas becomes less compressible as it speeds up. At M=0.6 it has about the same ability to be compressed as water, above that the "compressiblity" value is pretty meaningless for practical applications. This change in compressibliity and density is the reason that airplane fuel use begins increasing expotentially as a plane goes above 0.6 M.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
God, I hope so too. The alternative is that I've lost my mind.
Sonic flow is very dense and about as incompressible as water.
I've got a graph in one of my books from grad school (Modern Compressible Flow With Historical Perspective, Second Edition by John D. Anderson, Jr. Published by McGraw-Hill, 1990, p269) that shows that gas becomes less compressible as it speeds up. At M=0.6 it has about the same ability to be compressed as water, above that the "compressiblity" value is pretty meaningless for practical applications. This change in compressibliity and density is the reason that airplane fuel use begins increasing expotentially as a plane goes above 0.6 M.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
sailoday28
Mechanical
zdas04 (Mechanical)I noticed that on a recent previous thread you also stated that sonic flows can be considered incompressible.
On some transient flows some analyses have considered that mixing of two different gases at different temperatatures does not occur. In those cases, Mach no. was not even considered.
Consider a gas at low pressure and high temp so that PV=RT is a reasonable assumption. Low pressure enough so that the flow will choke. At M=1, PV=RT and the gas is still highly compressible.
Regards
On some transient flows some analyses have considered that mixing of two different gases at different temperatatures does not occur. In those cases, Mach no. was not even considered.
Consider a gas at low pressure and high temp so that PV=RT is a reasonable assumption. Low pressure enough so that the flow will choke. At M=1, PV=RT and the gas is still highly compressible.
Regards
Mixing is a very complex phenomena. If you blow high velocity gas into a static volume, mixing begins when the introduced stream slows to Laminar velocities. I've seen flow visualization experiments where two sonic streams going the same direction with a small included angle actually bounce off each other.
The ideal gas law was developed for steady-state conditions. Onset of mixing is very unsteady. I've never seen anyone propose that it applies to high-velocity flow (certainly not above 0.6 M) and it would take some compelling evidence to convince me that it does apply.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
The ideal gas law was developed for steady-state conditions. Onset of mixing is very unsteady. I've never seen anyone propose that it applies to high-velocity flow (certainly not above 0.6 M) and it would take some compelling evidence to convince me that it does apply.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
zdas04,
If a gas becomes less compressible as Mach 1 is approached, then the ideal gas law would not apply in that region, right?
If this is so, why has almost every engineer, scientist, and researcher at every university, government agency, and business in the entire world used the ideal gas law in deriving compressible flow equations since time began? I'm sure they could have found an equation of state that was velocity dependent if the reference you quote is true. Were they ALL wrong?
This is definitely a surprising revelation to me. Thanks for the reference, I intend to check it out.
Good luck,
Latexman
If a gas becomes less compressible as Mach 1 is approached, then the ideal gas law would not apply in that region, right?
If this is so, why has almost every engineer, scientist, and researcher at every university, government agency, and business in the entire world used the ideal gas law in deriving compressible flow equations since time began? I'm sure they could have found an equation of state that was velocity dependent if the reference you quote is true. Were they ALL wrong?
This is definitely a surprising revelation to me. Thanks for the reference, I intend to check it out.
Good luck,
Latexman
I just looked through a half dozen references and not a single one even implies that the ideal gas law applies to the transition from one state to the next. There is no "time" term, i.e., none of them say that:
[PV/(nT)]dt=R.
The flow equations that I work with every day (for turbulent flow) all started with Bernoulli's equation (and then added empirical terms to include the "friction" that Bernoulli assumed was absent), not the ideal gas law. I have three different derivations of Bernoulli's equation and none of them includes the ideal gas law.
My undergraduate Chemistry text starts the description of the ideal gas law with "for a closed system at rest ..." and goes on to discuss changing the system volume followed by closing the system again to apply the equation to the second state.
I understand applying the ideal gas law to re-state a flow at a different pressure base. This involves pretending that a daily (or hourly) flow rate is at rest and applying the arithmetic. For virtually all commercial compressible-flows you are well below 0.6 M and the calculation is valid. Sometimes it is above 0.6 M and none of the commercial arithmetic is valid, but as long as everyone believes it there isn't a problem.
Your statement that
Flow influenced density is not a new or foreign concept in the world of Engineering, Research, or Science. As CBS commercials said a couple of summers ago "If you've never seen it it is new to you", but that doesn't mean that it is new.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
[PV/(nT)]dt=R.
The flow equations that I work with every day (for turbulent flow) all started with Bernoulli's equation (and then added empirical terms to include the "friction" that Bernoulli assumed was absent), not the ideal gas law. I have three different derivations of Bernoulli's equation and none of them includes the ideal gas law.
My undergraduate Chemistry text starts the description of the ideal gas law with "for a closed system at rest ..." and goes on to discuss changing the system volume followed by closing the system again to apply the equation to the second state.
I understand applying the ideal gas law to re-state a flow at a different pressure base. This involves pretending that a daily (or hourly) flow rate is at rest and applying the arithmetic. For virtually all commercial compressible-flows you are well below 0.6 M and the calculation is valid. Sometimes it is above 0.6 M and none of the commercial arithmetic is valid, but as long as everyone believes it there isn't a problem.
Your statement that
is just wrong. I don't know how else to say it. I've spent a lot of time in my career trying to understand where the various compressible-flow equations came from and what assumptions went into them. Most state a range of Reynolds Numbers that are very consistent with minimizing the impact of "flow influenced density" (i.e., they state a valid range that includes a velocity well below 0.6 M).
Flow influenced density is not a new or foreign concept in the world of Engineering, Research, or Science. As CBS commercials said a couple of summers ago "If you've never seen it it is new to you", but that doesn't mean that it is new.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
sailoday28
Mechanical
Pv=RT,VDW, etc, are equations of state. Kinetic energy, potential energy are taken into consideration in other conservation equations.
Solution of unsteady flow equations, equations at M>1 can use an equation of state.
Regards
Solution of unsteady flow equations, equations at M>1 can use an equation of state.
Regards
zdas04,
I have also had some experience with compressible flow, and I truly cannot understand what you mean by "flow influenced density" and “As the velocities approach Mach 1.0 it becomes much less compressible.” by what you have written so far.
Most compressible flow equations are derived by writing the physical equations for a control volume extending between a stagnation section (velocity = 0) and any other section in the channel, depending on what is being examined. The physical equations used most often are the 1st and 2nd Laws of Thermodynamics, the Continuity Equation, an Equation of State (EoS), conservation of momentum, and the definition of the Mach number. All the compressible flow texts I’ve seen go through the obligatory derivations assuming a “perfect gas”. A perfect gas is an “ideal gas” (equation of state is PV = nRT) whose Cv is constant. For “real gases”, the weaker assumption of the two is that Cv is constant. This led to the “semi-perfect gas”, which is an ideal gas with Cv = f(T). Beyond this, compressible flow equations of real gases also require an EoS more sophisticated than the ideal gas law to relate it’s PVT behavior. I have probably seen dozens of EoS’s over the years, but I don’t believe I have ever seen one that uses velocity of the fluid as one of the variables. I just looked through my 5th edition of The Properties of Gases and Liquids and can confirm it does not contain an EoS that is dependent on fluid velocity.
I have not looked for the reference you quoted yet, but I did Google "flow influenced density" and did not get any hits at all. Can you suggest another phrase or something else to search for, for an explanation of this phenomenon?
Good luck,
Latexman
I have also had some experience with compressible flow, and I truly cannot understand what you mean by "flow influenced density" and “As the velocities approach Mach 1.0 it becomes much less compressible.” by what you have written so far.
Most compressible flow equations are derived by writing the physical equations for a control volume extending between a stagnation section (velocity = 0) and any other section in the channel, depending on what is being examined. The physical equations used most often are the 1st and 2nd Laws of Thermodynamics, the Continuity Equation, an Equation of State (EoS), conservation of momentum, and the definition of the Mach number. All the compressible flow texts I’ve seen go through the obligatory derivations assuming a “perfect gas”. A perfect gas is an “ideal gas” (equation of state is PV = nRT) whose Cv is constant. For “real gases”, the weaker assumption of the two is that Cv is constant. This led to the “semi-perfect gas”, which is an ideal gas with Cv = f(T). Beyond this, compressible flow equations of real gases also require an EoS more sophisticated than the ideal gas law to relate it’s PVT behavior. I have probably seen dozens of EoS’s over the years, but I don’t believe I have ever seen one that uses velocity of the fluid as one of the variables. I just looked through my 5th edition of The Properties of Gases and Liquids and can confirm it does not contain an EoS that is dependent on fluid velocity.
I have not looked for the reference you quoted yet, but I did Google "flow influenced density" and did not get any hits at all. Can you suggest another phrase or something else to search for, for an explanation of this phenomenon?
Good luck,
Latexman
GregLocock
Automotive
Well, a google search for 'incompressible flow' might yield, oh 934000 hits. Haven't had time to check them all out.
So, politely, your fluids/thermo course at uni never mentioned the above phrase? Why not?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
So, politely, your fluids/thermo course at uni never mentioned the above phrase? Why not?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
GregLocock
Automotive
Ah, got it.
Yes, there is a fair degree of confusion in this thread. Usually in aero the low speed regime is described as 'incompressible flow' (density varies slowly with respect to geometry), and as M approaches 1 so-called compressibility effects become increasingly important.
It is bad terminology, the gas is compressible in both speed regimes, but at high speeds local effects cannot smoothly disperse into the surrounding fluid, and can only do so by creating thermodynamically irreversible pressure fields.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
Yes, there is a fair degree of confusion in this thread. Usually in aero the low speed regime is described as 'incompressible flow' (density varies slowly with respect to geometry), and as M approaches 1 so-called compressibility effects become increasingly important.
It is bad terminology, the gas is compressible in both speed regimes, but at high speeds local effects cannot smoothly disperse into the surrounding fluid, and can only do so by creating thermodynamically irreversible pressure fields.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
Latexman,
Interesting question. I have no idea. I also give up. It is obvious that you have not found my arguments compelling and that I've not found your arguments compelling. Since there's no money on the table, there doesn't seem to be much sense in my continuing to try.
For the sake of anyone who reads this thread in the future let me put the discussion in perspective. Most engineers I've worked with use a gas-velocity around 20 ft/s as their design point. Many companies set a maximum velocity somewhere around 100 ft/s. I've seen companies that require flow reductions as velocity approaches 150 ft/s. The speed of sound in a gas is a function of gas composition and flowing temperature--numbers around 1,500 ft/s are common. In other words a reasonable maximum allowable velocity is in the neighborhood of 0.1 M. All of the controversy in this thread is about velocities higher than 0.6 M.
David
Interesting question. I have no idea. I also give up. It is obvious that you have not found my arguments compelling and that I've not found your arguments compelling. Since there's no money on the table, there doesn't seem to be much sense in my continuing to try.
For the sake of anyone who reads this thread in the future let me put the discussion in perspective. Most engineers I've worked with use a gas-velocity around 20 ft/s as their design point. Many companies set a maximum velocity somewhere around 100 ft/s. I've seen companies that require flow reductions as velocity approaches 150 ft/s. The speed of sound in a gas is a function of gas composition and flowing temperature--numbers around 1,500 ft/s are common. In other words a reasonable maximum allowable velocity is in the neighborhood of 0.1 M. All of the controversy in this thread is about velocities higher than 0.6 M.
David
zdas04,
Ah, and I was saving my best one for last.
The Concorde cruises along at Mach 2 (back in the day it was flying). Basically, the turbo jet engine is thrusting itself along and relatively still air is engulfed and passes through the engine. Whether the conduit is passing through still air or air is flowing through the conduit, not much difference from an engineering perspective. What's the first part of the jet engine that contacts the air? The compressor! Is the air compressible or incompressible when engulfed by a compressor travelling at Mach 2? If you don't know, I'll tell you. You bet your bippie it is.
Good luck,
Latexman
Ah, and I was saving my best one for last.
The Concorde cruises along at Mach 2 (back in the day it was flying). Basically, the turbo jet engine is thrusting itself along and relatively still air is engulfed and passes through the engine. Whether the conduit is passing through still air or air is flowing through the conduit, not much difference from an engineering perspective. What's the first part of the jet engine that contacts the air? The compressor! Is the air compressible or incompressible when engulfed by a compressor travelling at Mach 2? If you don't know, I'll tell you. You bet your bippie it is.
Good luck,
Latexman
sailoday28
Mechanical
GregLocock (Automotive)FYI
For a perfect gas with constant spec heats, the ratio of stagnation density to static density is a function of gamma(Cp/Cv) and Mach no.
With gamma in the range of 1.1 to 1.4 the percent change in the above ratio is less than 5% up to Mach nos. of about 0.3.
M>0.3 represents tne general criteria for which compressibilty must be taken into account in steady flow.
Regards
For a perfect gas with constant spec heats, the ratio of stagnation density to static density is a function of gamma(Cp/Cv) and Mach no.
With gamma in the range of 1.1 to 1.4 the percent change in the above ratio is less than 5% up to Mach nos. of about 0.3.
M>0.3 represents tne general criteria for which compressibilty must be taken into account in steady flow.
Regards
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