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Consider the flatwalled diffuser in Fig. 6.26a, where section 1 is the inlet and section 2 the exit. Application of Bernoulli’s equation (6.112) to this diffuser predicts that or V2
1 p2 Cp,frictionless p01 1 1
2 p1 1
2 V2
2 V2
V1 p02 2 (6.114) Meanwhile, steady onedimensional continuity would require that
Q V1A1 V2A2 (6.115) Combining (6.114) and (6.115), we can write the performance in terms of the area ratio AR A2/A1, which is a basic parameter in diffuser design:
Cp,frictionless 1 (AR) 2 (6.116)  v v A typical design would have AR 5 1, for which Eq. (6.116) predicts Cp 0.96, or
nearly full recovery. But, in fact, measured values of Cp for this area ratio [14] are only
as high as 0.86 and can be as low as 0.24.  eText Main Menu  Textbook Table of Contents  Study Guide 6.9 Experimental Duct Flows: Diffuser Performance 383
Thin
boundary
layers Low
velocity,
high
pressure (a)
Backflow Thick
boundary
layers High
velocity,
low
pressure Fig. 6.27 Diffuser performance:
(a) ideal pattern...
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 Spring '08
 Sakar

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