2.2-Ideal Gas Law, fluid mech

2.2-Ideal Gas Law

2.2-Ideal Gas Law, fluid mech

[ Pobierz całość w formacie PDF ]
Ideal Gas Law
2.2 Ideal Gas Law
The
ideal gas law
relates important thermodynamic properties, and is often used to calculate density.
One form of the law is
(2.4)
where
p
is the absolute pressure, is the volume,
n
is the number of moles,
R
u
is the universal gas constant (the
same for all gases), and
T
is absolute temperature. Absolute pressure, introduced in Chapter 3, is referred to
absolute zero. The universal gas constant is 8.314 kJ/kmolK in the SI system and 1545 ftlbf/lbmol°R in the
traditional system. Eq. 2.4 can be rewritten as
where is the molecular weight of the gas. The product of the number of moles and the molecular weight is
the mass of the gas. Thus is the mass per unit volume, or density. The quotient is the gas
constant,
R
. Thus a second form of the
ideal gas law
is
(2.5)
EXAMPLE 2.1 DESITY OF AIR
Air at standard sealevel pressure (
p
= 101 kN/m
2
) has a temperature of 4°C. What is the density of
the air?
PROBLEM DEFINITION
Situation:
Air with a known temperature and pressure.
Find:
Density (kg/m
3
).
Properties:
Air, 4°C,
p
at 101 kN/m
2
; Table A.2,
R
= 287 J/kg K.
PLAN
Apply the ideal gas law, Eq. 2.5, to solve for density, ρ
SOLUTION
1 of 2
1/15/2009 12:09 AM
Ideal Gas Law
REVIEW
1. Remember: Use absolute temperatures and pressures with the ideal gas law.
2. Remember: In Eq. 2.5, use
R
from Table A.2. Do not use
R
u
.
Although no gas is ideal, Eq. 2.5 is a valid approximation for most gas flow problems. Values of
R
for a number
of gases are given in Table A.2. To determine the mass density of a gas, solve Eq. (2.5) for ρ:
Copyright ¨ 2009 John Wiley & Sons, Inc. All rights reserved.
2 of 2
1/15/2009 12:09 AM
[ Pobierz całość w formacie PDF ]

Linki

: 2006 US Army Civil Disturbance Ops 116p, Law Enforcement and Security

: 2002 US Army MP Operations 163p, Law Enforcement and Security