Concept explainers
A diffuser in a pipe flow is basically a slow expansion of the pipe diameter, which slows down the fluid velocity and increases the pressure (the Bernoulli effect). Water at room temperature flows with a volume flow rate of 00250 m3/s through a horizontal diffuser in which the pipe diameter increases gradually from D1= 6.00 to D2= 11.00 cm. The irreversible head loss through the diffuser is estimated to be 0.450 in. The flow is turbulent, and the kinetic energy correction factors at both the inlet and outlet of the diffuser are assumed to be 1.05.
(a) Calculate the pressure difference P2- P1in units of kPa using the energy equation.
(b) Repeat using the Bernoulli equation (ignore irreversible head losses and ignore kinetic energy correction factors-in other words, set the kinetic energy correction factors to 1). Calculate the percentage error in the result due to the Bernoulli approximation. and explain why (or why not) Bernoulli is applicable here.
(c) It may be surprising that the answer to part (a) is positive. i.e., the pressure rises downstream. How is this possible? Explain by calculating the change i n energy grade line EGL and the change in hydraulic grade line .HGL from the upstream to the downstream location. In particular, does EGL go up or down, and does HGL go up or down?
(a)
The pressure difference between outlet to inlet of diffuser.
Answer to Problem 116P
The pressure difference between outlet to inlet of diffuser is
Explanation of Solution
Given information:
The volume flow rate of diffuser is
Write the expression for area of pipe.
Here diameter of section is
Write the expression for velocity.
Here volume flow rate is
Consider inlet section as
Write the expression for energy equation in control volume.
Here, density is
Substitute
Calculation:
Substitute
Substitute
Substitute
Substitute
Refer to table "Properties of saturated water" to obtain density of water as
Substitute,
Conclusion:
The pressure difference between outlet to inlet of diffuser is
(b)
The pressure difference using Bernoulli equation.
The percent error due to Bernoulli approximation.
Answer to Problem 116P
The pressure difference using Bernoulli equation is
The percent error due to Bernoulli approximation is
Explanation of Solution
Given information:
Ignore irreversible head loss, kinetic energy correction factor
Write the expression for Bernoulli equation.
Write the expression for error percentage.
Here, pressure difference using energy equation is
Calculation:
Substitute
Substitute
Bernoulli equation is not applicable here because of pressure head loss and kinetic energy correction factor as greater
Conclusion:
The pressure difference using Bernoulli equation is
The percent error due to Bernoulli approximation is
(c)
The nature of Energy Grade line.
The nature of Hydraulic Grade line.
Answer to Problem 116P
The Energy Grade Line (EGL) decrease from the inlet to the exit and Energy grade Line goes down.
The Hydraulic Grade Line (HGL) increase from the inlet to the exit and Hydraulic grade Line goes up.
Figure-(1)
The Figure (1) shows the nature of Energy Grade Line (EGL) and Hydraulic Grade Line (HGL).
Explanation of Solution
Given information:
Write the expression for net Energy Grade Line (EGL).
Substitute
Write the expression for net Hydraulic Grade Line (HGL).
Substitute
Calculation:
Substitute
Substitute
Conclusion:
The Energy Grade Line (EGL) decrease from the inlet to the exit and Energy grade Line goes down.
The Hydraulic Grade Line (HGL) increase from the inlet to the exit and Hydraulic grade Line goes up.
Figure (1)
The Figure (1) shows the nature of Energy Grade Line and Hydraulic Grade Line.
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Chapter 5 Solutions
Fluid Mechanics: Fundamentals and Applications
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