integral relationship for cvmbahrami/ensc 283/lecture...integral relations for cv • control volume...
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Integral relations for CV
• Control volume approach is accurate for any flow distribution but is often based on the “one‐dimensional” property values at the boundaries. p p y
• It gives useful engineering estimates.
M. Bahrami ENSC 283 Spring 2009 1Integral Relations for CV
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System and control volume• A system is defined as a fixed quantity of matter or a region in space
chosen for study. The mass or region outside the system is called the surroundings
BOUNDARY SURROUNDINGS
surroundings.
SYSTEM
• System boundary: the real or imaginary surface that separates the systemf it di Th b d i f t b fi d blfrom its surroundings. The boundaries of a system can be fixed or movable.
• Open system or control volume is a properly selected region in space. Itusually encloses a device that involves mass flow such as a compressor
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usually encloses a device that involves mass flow such as a compressor.
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Control volume• control volume is an abstract concept and does not hinder the flow in any
way.
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Volume and mass flow rate
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Reynolds transport theorem
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One‐dimensional approximation
• In many situations, the flow crosses the boundaries of the control surface at simplified inlets and exits that are approximately one‐dimensional (the s p ed e s a d e s a a e app o a e y o e d e s o a ( evelocity can be considered uniform across each control surface).
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Example• A fixed control volume has three one‐dimensional boundary sections, as
shown in the figure below. The flow within the control volume is steady. The flow properties at each section are tabulated below Find the rate of changeflow properties at each section are tabulated below. Find the rate of change of energy that occupies the control volume at this instant.
Control surface type , kg/m3 V, m/s A, m2 e, J/kg1 inlet 800 5.0 2.0 3002 inlet 800 8.0 3.0 1003 outlet 800 17.0 2.0 150
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Conservation of mass, B=m• For conservation of mass, B=m and β=dm ⁄ dm=1. The Reynolds transport
equation becomes:
• If the control volume only has a number of one‐dimensional inlets and• If the control volume only has a number of one‐dimensional inlets and outlets, we can write:
• for steady‐state flow, ∂ρ⁄∂t=0, and the conservation of mass becomes:y ρ
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Average velocity• In cases that fluid velocity varies across a control surface, it is often
convenient to define an average velocity.
• If the density varies across the cross‐section, we similarly can define an average density:
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Example 2• In a grinding and polishing operation, water at 300 K is supplied at a flow rate
of 4.264×10‐3 kg/s through a long, straight tube having an inside diameter of D=2R=6 35 mm Assuming the flow within the tube is laminar and exhibits aD=2R=6.35 mm. Assuming the flow within the tube is laminar and exhibits a parabolic velocity profile:
u(r) uavg Rrr
R
L
r
• where umax is the maximum fluid velocity at the center of the tube. Using thewhere umax is the maximum fluid velocity at the center of the tube. Using the definition of the mass flow rate and the concept of average velocity, show that
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Linear momentum equation• For Newton’s second law, the property being differentiated is the linear
momentum, mV. Thus B =mV and β=dB⁄dm=V. The Reynolds transport theorem becomes:
• Momentum flux term
• If cross section is one dimensional V and ρ are uniform and over the area• If cross‐section is one‐dimensional, V and ρ are uniform and over the area, momentum flux simplifies
• For one‐dimensional inlets and outlets, we have
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Net pressure on CV
• If the pressure has a uniform value p all around the surface the net• If the pressure has a uniform value pa all around the surface, the net pressure force is zero.
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Momentum flux correction• To consider the effects of non‐uniform velocity, we introduce a correction
factor β.
• Values of β can be calculated using the duct velocity profile and the above definition for
• Laminar flow:
• Turbulent flow:Turbulent flow:
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