Appendix A1: The equivalent ideal diffuser
| The conversion of kinetic energy into pressure in the diffuser is accompanied by considerable losses which must be accounted for. |
Fig 4 Equivalent ideal diffuser
| The equivalent ideal diffuser is one in which the outlet section F0 is calculated so that the sum of the kinetic energy loss is equal to the kinetic energy loss at section Fs plus diffuser friction. |
it is
|
Equation 10 |
and, in the ideal diffuser, where h =1
Hence
Therefore
| It is most difficult with
the present state of knowledge existing in this
particular problem, to give definite values for the
diffuser efficiency h , especially because it is not known what are
the inlet conditions on which that efficiency depends so
much. Besides the whole question of the behavior of a
diffuser under pulsating conditions, the following points
should be considered to favor the efficiency of kinetic
into pressure energy. • Higher Reynolds number, of the order of 1•106 • Very high turbulence resulting from the previous mixing process. • The application of boundary layer suction. • Smoothness of the surfaces. Against good efficiency, it can be said: • The limited length available. • The surface roughness obtaining in working conditions, especially in oil-burning engines. • Some unavoidable inaccuracy of the whole draughting apparatus; it is a boilersmith construction operating in a hostile environment. As a tentative figure, it is proposed to set h =0.8 up to 0.85 under favourable circumstances (short smooth accurate diffusers). With an area ratio of 4, the corresponding fraction of the kinetic energy converted is 0.80(1-1/42) » 0.75, which is very high. • Very high turbulence resulting from the previous mixing process. • The application of boundary layer suction. • Smoothness of the surfaces. |
