Document 93gbBb7X4VjwR9ED0Nbz1EG3q

80 CHAPTER 4 1951 Guide It is convenient in the analysis of compressible flow to introduce the velocity of propagation of pressure impulses or, more familiarly, the sonic velocity, a. For perfect gases this is given by the equation: a1 = kgp/p = kgRT Accordingly, Equation 21 may be written (22) 1(TV - TV) + or, by rearrangement, 0 (23) which permits the calculation of the ratio of pressures at entrance and exit of the steady flow device--pipe, orifice, or nozzle. From Equations 19 and 22 it follows that so that Tl /p?W = =W (25) ^y-1 (TV - TV) + at - at = 0 and (26) at 1+ k -- 2 1 TV o, at 1+ k 2 1 TV at (27) The ratio of flow velocity to sonic velocity is known as the Mach number, M = V/a This parameter is particularly useful in compressible flow analysis. In general, if M S 0.1 the flow may be considered to be incompressible. This is generally true in heating and ventilating air ducts. In terms of the Mach number at T, i + at T, (28) and i*-i pi pi (29) The quantity k_ ifc-1 (30) Fluid Flow. ` called the stagnation pressure, and gives a measure of pressure energy. For incompressible flow 7/1 = p + --pF* = p + g (31) where g =3,~IppKy* 2g (32) and is the dynamic pressure A total head tube measures stagnation pres sure directly. From Equation 29 it follows that for frictionless, adiabatic flow Pi0 = Pi0 (33) This represents another extension of the Bernoulli equation to compressible flow. Friction will cause a loss in pressure energy. Ideal Flow through Nozzle or Orifice The majority of low measuring systems depend upon a correlation between pressure drop, area, and quantity of flow. The basic formulas may be stated on the assumption that the flow is frictionless and adiabatic Designating the mam stream by station 1, and flow at some measuring restriction by station 2, the flow in pounds per second is to -- piAtVi = piAtV% (34) or, in terms of Mach number, to AtMt y/kgpipi ' According to Equation 29 1 + ^7--^ MS w l+*Mt (35) (36) from which Mt . k - 1 \ 1 - Mi'/Mtj L\ptj J (37) so that 1+Vw ifeftteJMw = At , 1 - Mt/Mt (38) If the initial velocity is sufficiently small, Mt will be negligible so that If this is computed and the'figures are plotted, the curved'line (partly