That’s why they are called standard values. The values presented on the slide are simply average values used by engineers to design machines. And while it is hotter in some seasons than others, pressure and temperature change day to day, hour to hour, sometimes even minute to minute (during severe weather). We are all aware that pressure and temperature (and density) of the air depend on your location on the earth and the season of the year. 715 Joules per gram per degree Kelvin or. 286 Joules per gram per degree Kelvin or 53.5 foot-pounds per pound per degree Rankine. Viscosity: 1.73 time 10^-5 Newton-second per square meter or 3.62 times 10^-7 pound-second per square foot. Temperature: 15 degrees Celsius or 59 degrees FarenheitĪbsolute Temperature: 288 degrees Kelvin or 519 degrees Rankine Pressure: 101.3 kilo Newtons per square meter or 14.7 pounds per square inch 814 cubic meters per kilogram or 422 cubic feet per slug Typical values of the density, pressure, and temperature of air at sea level static conditions for a standard day are:ĭensity: 1.229 kilogram per cubic meter or. The ratio of these coefficients is denoted by the greek letter gamma and appears in many thermodynamic equations. Since the amount depends on the process used to raise the temperature, there a specific heat (cv) coefficient for a constant volume process, and a different valued coefficient for a constant pressure process (cp). The specific heat of a gas is a measure of the amount of energy necessary to raise the temperature of the gas by a single degree. The state of a gas can be changed by external processes, and the reaction of the gas can be predicted using the laws of thermodynamics. Studies of the zeroth and first laws introduce the idea of the heat capacity of a substance. Including the value of the molecular weight, we can define a particular gas constant (R) for air.
There is a universal gas constant which relates these variables and the molecular weight of any gas.
The density (specific volume), pressure, and temperature of a gas are related to each other through the equation of state. Either variable can be used to define the state of the gas, since they are reciprocals. When a gas is moving, it is more convenient to use the density (r) of a gas, which is the mass divided by the volume the gas occupies. When working with a static (unmoving) gas, it is convenient to use specific volume (v), which is the volume divided by the mass. Since the mass and volume are directly related, we can express both the mass and volume by a single variable. For a given pressure and temperature, the volume depends directly on the amount of gas. The sum of the mass of all the molecules is equal to the mass of the gas.Ī gas occupies some volume in three dimensional space. The temperature (T) of a gas is a measure of the kinetic energy of the gas. A gas is composed^M of a large number of molecules which are in constant motion. This “sticky” property of the gas is called the viscosity (mu) and it plays a large role in aerodynamic drag. A gas can also exert a tangential (shearing) force on a surface, which acts like friction between solid surfaces. The pressure (p) of a gas equals the perpendicular (normal) force exerted by the gas divided by the surface area on which the force is exerted. The values and relations of the properties define the state of the gas. We usually model air as a uniform (no variation or fluctuation) gas with properties that are averaged from all the individual components.Īny gas has certain properties that we can detect with our senses. Home > Beginners Guide to Aeronautics Properties of Air – Text VersionĪir is a mixture of gases, 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components.
0 kommentar(er)
