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difference between total pressure and static press

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Old   May 13, 2005, 01:17
Default difference between total pressure and static press
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kiran
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what is the total pressure variation and temperature variation in compressible reacting convergent divergent nozzle flow.static pressure dropped continuosly in divergent portion,but what is the effect in total pressure. pls answer me.

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Old   May 14, 2005, 08:59
Default Re: difference between total pressure and static p
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Alexey
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Total pressure p0 (also so colled stagnation pressure) is the pressure after adiabatic (isoentropic) stagnation:

P0 = P*(1 + 0.5*(gamma-1)*M**2)**(gamma/(gamma-1))

where P is the static (true) pressure, gamma = Cp/Cv is the specific heat ratio, M is the Mach number (M = v/c, v - velocity, c - speed of sound).

So if v=0 then P = P0.

Because if your flow is isoentropic (inviscid and without heat exchange)then p0 is always strictly CONST! Including in Laval nozzle at any regime (subsonic or supersonic).
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Old   May 14, 2005, 23:44
Default Re: difference between total pressure and static p
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kiran
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thank u Alexey. i am modelling reacting flows in thrust chamber.the total pressure is dropped much in throat and divergent portion.the mixture i am using here is hydrogen-oxygen and methane-air.what r the reasons behind the pressure drop? what is the gamma for hydrogen=oxygen,methane-air mixtures. waiting for ur reply.
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Old   May 17, 2005, 02:56
Default Re: difference between total pressure and static p
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Alexey
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Hi kiran.

The fact that total pressure drops in your case is natural because these chemical reactions occur with heat release. As you know from thermodynamics:

dS = dQ/T,

where S - entropy and Q - heat, T - absolute temperature.

It follows that if dQ > 0 (heat release) then entropy increases also and hence total pressure drops. Details you can find in any textbook of thermodynamics or hydrodynamics.

Heat ratio (gamma) for any N components mixture you can get by formula:

gamma = Cp/Cv,

where

Cp = SUMM(f_i*Cp_i(T)), for i=1..N, Cv = SUMM(f_i*Cv_i(T)), for i=1..N

where f_i - mass fraction of i-component, Cp_i(T),Cv_i(T) - specefic heats at constant prtessure and constant volume [ J/(kg*K)]; thay are functions of temperature (see any handbook).

Alexey.

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Old   May 17, 2005, 07:47
Default Re: difference between total pressure and static p
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kiran
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thank u Alexy. i will refer these things.

kiran
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Old   June 8, 2005, 09:23
Default Re: difference between total pressure and static p
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jhihkl n ik
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