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 F_{0} 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•10^{6} • 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(1-1/4^{2}) » 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. |