Unformatted text preview: lve if Red is known and f is wanted. There are many alternate approximations in the literature from which f can be computed explicitly from Red 1/4
4000 Red 105
H. Blasius (1911) 0.316 Red
f
(6.55)
2 1.8 log Red
Ref. 9
6.9 Blasius, a student of Prandtl, presented his formula in the first correlation ever made
of pipe friction versus Reynolds number. Although his formula has a limited range, it
illustrates what was happening to Hagen’s 1839 pressuredrop data. For a horizontal
pipe, from Eq. (6.55),
1/4
L V2
L V2
p
f
0.316
hf
d 2g
d 2g
g
Vd
or p 0.158 L 3/4 1/4 d 5/4 7/4 V (6.56) at low turbulent Reynolds numbers. This explains why Hagen’s data for pressure drop
begin to increase as the 1.75 power of the velocity, in Fig. 6.4. Note that p varies
only slightly with viscosity, which is characteristic of turbulent flow. Introducing Q
1
2
4 d V into Eq. (6.56), we obtain the alternate form
p 0.241L 3/4 1/4 d 4.75 Q1.75 (6.57)  v v For a given flow rate Q, the turbulent pressure drop decreases with diameter even mo...
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This note was uploaded on 10/27/2009 for the course MAE 101a taught by Professor Sakar during the Spring '08 term at UCSD.
 Spring '08
 Sakar

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