CE319F: Elementary Mechanics of Fluids FLUID PROPERTIES Chapter 2 CE319F: Elementary Mechanics of Fluids

Elasticity (Compressibility) If pressure acting on mass of fluid increases: fluid contracts If pressure acting on mass of fluid decreases: fluid expands Elasticity relates to amount of deformation for a given change in pressure Ev = bulk modulus of elasticity How does second part of equation come about? Small dV/V = large modulus of elasticity

Example Based on the definition of Ev and the equation of state, derive an equation for the modulus of elasticity of an ideal gas.

Vapor Pressure (Pvp) Vapor pressure of a pure liquid = equilibrium partial pressure of the gas molecules of that species above a flat surface of the pure liquid Concept on board Very strong function of temperature (Pvp up as T up) Very important property of liquids When Pvp exceeds total air pressure applied at surface, liquid will boil. Pressure at which a liquid will boil for a given temperature At 10 oC, vapor pressure of water = 0.012 atm = 1200 Pa If reduce pressure to this value - get boiling of water Formation of cavitation bubbles Harm to pipes, pumps, turbines, propellers If Pvp > 1 atm compound = gas If Pvp < 1 atm compound = liquid or solid

Vapor Pressure (Pvp) Water Vapor pressure of water (and other liquids) is a strong function of temperature.

Vapor Pressure (Pvp) - continued Pvp,H2O = Pexp(13.3185a – 1.9760a2 – 0.6445a3 – 0.1299a4) P = 101,325 Pa a = 1 – (373.15/T) T = oK valid to +/- 0.1% accuracy for T in range of -50 to 140 oC Equation for relative humidity of air = percentage to which air is “saturated” with water vapor. How does RH affect drying of building materials, and why? Implications? How does RH affect dust mites? Implications?

300 mm Dust Mites

Example (HW!): Relative Humidity The relative humidity of air in a room is 80% at 25 oC. What is the concentration of water vapor in air on a volume % basis? If the air contacts a cold surface, water may condense (see effects on next slide). What temperature is required to cause water condensation?

Example: Relative Humidity Continued