TABLE OF CONTENTS The atmosphere is a mixture of gases of which oxygen and nitrogen are the main constituents. It also contains small amounts of water vapor and other gases, including carbon dioxide, hydrogen, helium, and the rare inert gases argon, krypton, neon, etc. The properties of the constituents varies little across the range of altitudes involved in conventional aerodynamics, and the atmosphere may be regarded as a homogeneous gas of uniform composition.
Consider a small parcel of air that may be moved up or down in the atmospheric pressure field. If this process is adiabatic, i.e., no heat is transferred to the parcel by either conduction or radiation, it will experience a change in temperature as a result of the change in pressure.
The pressure gradient in a gravity field is given by
For an isentropic process
The air density may be expressed through the perfect gas law as
Substitute Equation 9.1.3 in Equation 9.1.2 and differentiate with respect to altitude to find
By combining the Equations 9.1.1, 9.1.3, and 9.1.4, the temperature gradient may be expressed as
The gravitational acceleration, g, is a function of both latitude and altitude. However, changes in g are so small that the influence on the performance of an air-cooled heat exchanger or cooling tower is negligible.
For dry air with ? = c p/ c v = 1.4 and R = 287.08 J/kg K, find g = 9.8 m/s 2
This temperature gradient is known as...
