CrazyEngineers Archive
Old, but evergreen and popular discussions on CrazyEngineers, presented to you in read-only mode.
@zaveri • 12 May, 2013
What is the difference between static and stagnation properties involved in any thermodynamic cycle ?

for example , what is meant by static pressure and stagnation pressure ?
@kamal61435 • 12 May, 2013 • 1 like Static Pressure is the pressure exerted by the fluid in all directions at a certain point i.e stagnation point.

Stagnation Pressure is the static pressure at a stagnation point in a fluid flow i.e static pressure + dynamic pressure.To be more clear, Stagnation pressure is the pressure one would have after isentropically decelerating the fluid to a rest and properties at that point is stagnation properties.

In above fig, the total pressure indicates stagnation pressure.

They are measured using pitot tube.

Static properties are those that would be measured if one could travel with the fluid at its exact velocity.

Well, gathered from books and internet ,so if need more explanation tell me.
@Ammar Aziz • 12 May, 2013 • 1 like Suppose that a fluid is moving with very high velocity. Due to this velocity it is obvious that it possesses some kinetic energy and may be some other type of energy as well. But if through some mechanism or arrangement this fluid is suddenly brought to rest then according to law of conservation of energy all the kinetic energy of the fluid is converted to other forms of fluid energy like fluid pressure etc. At this instant all the properties of the corresponding fluid is known as stagnation properties.

On the other hand if you have some fluid with no kinetic energy then the pressure exerted by the fluid on its surrounding due its molecular structure of that fluid is known as static pressure. so it is its inherent tendency.
@zaveri • 13 May, 2013 Thanks guys. i did manage to grab something from both your replies.

now consider this case:

air is being compressed in a reciprocating compressor and this in turn is being stored in the tank (the pressure vessel). now the pressure of the air in the tank is stagnation pressure, isn't it , since it was first imparted some pressure and velocity, and then brought to a complete rest in the tank ?

am i right guys ?
@Ammar Aziz • 13 May, 2013 you are saying that air is being compressed in a reciprocating compressor.
after passing through the compressor it enters a tank or pressure vessel.
actually the basic function of compressor is to compress, not to increase the velocity. on the other hand if you say that after passing through a nozzle, the air enters a tank and eventually tank's wall restricts the velocity of air to zero or near to zero, than the pressure of the air at this state is stagnation pressure. it only exists at the instant when air is stopped by the walls of container. because it may be possible that due to elastic collision between air molecules and wall, the air again gains some velocity.
you are also right in your scenerio. but keep in mind that this state only exists for an instant in your case.
@zaveri • 13 May, 2013 Ammar Aziz

thanx

from what i have learnt, a reciprocating compressor delivers air at high pressure and low velocity, quite unlike the rotary compressor which delivers air at a lower pressure and higher velocity.

but then even in the case of the reciprocating machine, there does exist some velocity after all, since the compressed air has to move from the machine to the tank.
@Anand Tamariya • 14 May, 2013
zaveri
Thanks guys. i did manage to grab something from both your replies.

now consider this case:

air is being compressed in a reciprocating compressor and this in turn is being stored in the tank (the pressure vessel). now the pressure of the air in the tank is stagnation pressure, isn't it , since it was first imparted some pressure and velocity, and then brought to a complete rest in the tank ?

am i right guys ?
Stagnation pressure is defined only for incompressible fluid.
@zaveri • 14 May, 2013
Anand Tamariya
Stagnation pressure is defined only for incompressible fluid.
What do you mean by that ?

incompressible fluids are water, oil and other liquids.
@Anand Tamariya • 14 May, 2013
zaveri
What do you mean by that ?

incompressible fluids are water, oil and other liquids.
Meaning it doesn't apply to compressed air.
@zaveri • 14 May, 2013 Anand Tamariya

you will deal with stagnation properties while dealing with centrifugal compressors and fans.
@Ammar Aziz • 14 May, 2013 to some extent it is true that to achieve the stagnation state, the corresponding fluid must be incompressible. because if something is compressible than it is possible to have more than one stagnation states for that. for example, at two different compressions although it have zero velocity but now internal energy plays role in defining the state of the gas .....
@Anand Tamariya • 14 May, 2013
zaveri
Anand Tamariya

you will deal with stagnation properties while dealing with centrifugal compressors and fans.
A little more context would be helpful - if you can elaborate in detail.
@zaveri • 15 May, 2013 Anand Tamariya

Well we have machines like centrifugal compressor and centrifugal fans.

these are used to increase the pressure of air.

the pressurised air from these machines also have stagnation properties.

and air as you know is a compressible fluid.

then how come you say that stagnation properties do not apply to compressible fluids.
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