Converging and Diverging Compression

When an airplane exceeds the speed of soimd, we say that it breaks the sound barrier. In so doing, it generates a sonic wave or pressure wavefront. When steam and gas flow into the converging section of the jet diffuser shown in Fig. 19.1, the same thing happens. The gradually converging sides of the diffuser increase the velocity of the steam and gas as the vapor enters the diffuser throat up to and even above the speed of sound. This creates a pressure wavefront, or sonic boost. This sonic boost will multiply the pressure of the flowing steam and gas by a factor of perhaps 3 or 4. 

Note something really important at this point. If for any reason the velocity of the steam and gas falls below the speed of sound in the diffuser throat, the sonic pressure boost would entirely disappear. 

As the steam and gas leave the diffuser throat, the flow then enters the gradually diverging sides of the diffuser. The velocity of the steam and gas is reduced. The kinetic energy of the flowing stream is partially converted to pressure as the steam and gas slow down. This increase in pressure is called the velocity boost, which will multiply the pressure of the steam and gas by a factor of 2 or 3. 

While smaller than the sonic boost, the velocity boost is more reliable. Even though the velocity in the diffuser throat in Fig. 19.1 falls well below the speed of soimd, the increase in pressure in the diverging portion of the diffuser is only slightly reduced. 

The overall pressure boost of a steam jet is obtained by multiplying the sonic boost effect times the velocity boost effect. The overall boost is called the jet s compression ratio. 

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