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Monster Media 1996 #14
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Monster Media No. 14 (April 1996) (Monster Media, Inc.).ISO
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COMPMETH.TXT
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COMPRESSIBLE FLOW METHODS
The major difference in the pressure drop calculations
between compressible flow and incompressible flow is the
great change that can occur in the density of the gas between
the inlet and outlet conditions. The use of incompressible
flow relationships can lead to major errors if the pressure
drop in the pipe exceeds 10 % of the inlet conditions.
However, the incompressible flow methods can be used with
fair accuracy provided the average density between the inlet
and outlet conditions are used. However, this procedure is
also prone to error if the gas velocity approaches sonic
limits. It also requires considerable trial and error to
recalculate to determine the outlet conditions and to
determine the average properties for the gas.
The method used in this program is based upon exact
theoretical solutions of the flow equations, and are accurate
for high Mach numbers. The maximum flow rates are base upon
a limitation for adiabatic flow of a Mach number of 1.0 and
for Isothermal flow at it's limit of 1 / SQRT( K ).
This is the most accurate method for calculation of adiabatic
and isothermal flow, but it is too complex for normal hand
calculation procedures due to the recursive methods needed
for the solution.
The equations used are presented in Table 1.
MACH NUMBER
The maximum possible velocity in a pipe determined by the
velocity of sound in the gas. Once the velocity of sound is
reached the pressure in the pipe cannot be reduced below a
minimum value. This is also called the choke point. The
pressure downstream of the choke point remains at this
constant value until the outlet of the pipe is reached and
the remaining energy is dissipated as a shock wave.
The velocity of sound in the gas is a function of the gas
properties and the temperature of the gas. It is calculated
by the equation.
Cs = Sqrt[ k*Gc*(Z*R*T/MW) ]
Where Gc = 32.17 and R is gas constant 1545.
The Mach Number M is the velocity of the gas in the pipe
divided by Cs. For adiabatic flow the highest gas velocity
possible is achieved at a Mach number of one. A Mach number
of one at the source conditions determines the maximum
possible flow through the inlet nozzle. The Mach number
increases while the gas flows through the pipe due to the
effects of friction. Friction reduces the pressure and
increases the volumetric flow rate in cubic ft/min.
STAGNATION STATE
Flow is possible between two extremes. At one extreme, the
velocity is zero and temperature is a maximum, since all the
potential kinetic energy is converted to enthalpy. The speed
of sound is also a maximum. At the other extreme, the
velocity is a maximum and the temperature falls to zero, all
the enthalpy being converted to kinetic energy. The speed of
sound is then zero ( zero temperature state). Between these
extremes the flow may be subsonic, transonic, or supersonic.
However, Supersonic flow can only be achieved in diverging
nozzles and is not the subject of this program. Friction
flow in a pipe is limited by the Mach number constraints
discussed above.
The stagnation state is the condition where the gas is
compressed and stored in a reservoir under stagnant
conditions. The fluid is at rest and has zero linear
volocity. These conditions correspond to the information
entered at the source conditions. These conditions are
given the subscript o in the Table below.
CRITICAL STATE
The critical state condition refered to in this program is
not the thermodynamic condition of the gas at high pressures
where its properties blend with the liquid state, but the
conditions at maximum flow in the pipe.
The critical state corresponds to the conditions fixed by the
sonic velocity of the gas. These conditions are given the
subscript * in the Table.
FRICTION EFFECTS
The pipe definition is converted to velocity head; by the
equation.
Nf = f(Length/Diameter) where f is the friction factor
The maximum friction drop at a specified location in the pipe
is defined as a function of the Mach number, and the K ratio
of the gas.
The equations for this are different for isothermal and
adiabatic flow and are given in the Table. The friction
factor is essentially constant in the pipe since it is
a function of the Reynold's number and pipe roughness, and at
the high velocities encountered in gas flow the friction
factor is approachs a constant limit.
The value of Lmax derived from the equations in the table
corresponds to the maximum length where the flow reaches
critical conditions. This value is reported in the program,
based upon the inlet conditions.
The conditions at the outlet of the pipe are calculated by
subtracting the Nf of the specified pipe from the value at
the inlet of the pipe, and solving for the outlet Mach
number.
(fLmax/D) = (fLmax/D) - (fLmax/D)
M2 M1 actual
The flow conditions at the outlet of the pipe are calculated
by computing the ratio of the pressure to the critical
pressure at the inlet and outlet Mach numbers.
ie: Pout = [(P/P*)out / (P/P*)in ] * Pin
Tout = [(T/T*)out / (T/T*)in ] * Tin
Pressures are in Psia
Temperatures are in degrees R. absolute
The conditions at the inlet of the pipe are calculated by
using the ratio equations for isentropic flow on the
conditions at the source.
The derivation of these equations and methods are presented
in Volume 1 of :
Shapiro 'The Dynamics and Thermodynamics of
Compressible Fluid Flow' ,
The Ronald Press Co, New York, 1954
TABLE 1
ISENTROPIC FLOW
PROPERTY EQUATION
Mach N M = 1
Temp To/T = 1 + ((k-1)/2) * M^2
Pressure Po/P = (1 + (k-1)/2*M^2 )^((k/k-1))
Friction f*LENmax /D = 0
ISOTHERMAL FLOW
PROPERTY EQUATION
Mach N M = 1 / SQRT(k)
Temp Constant
Pressure P/P* = 1/ Sqrt(k)
Friction f*LENmax / D = (1-k*M^2)/k*M^2 + ln[k*M^2]
ADIABATIC FLOW
PROPERTY EQUATION
Mach N M = 1
Temp T/T* = (k+1)/2*(1+((k-1)*M^2) / 2 )
Pressure P/P* = (1/M)*SQRT [ (k+1)/(2*(1+((k-1)*M^2)/2]
Friction f*Lmax/D = (1-M^2)/(k*M^2) +
((k+1)/2k)* Ln [(k+1)*M^2 / 2*(1+(k-1/2)*M^2)]