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MODULE 2 |
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Section 1: |
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Boundary Layer and Turbulence |
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Section 2: |
| Pressure Drop and Friction Factor | |
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Section 3: |
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Heat Transfer Coefficient |
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Section 4: |
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Convection With Change of Phase |
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Pressure Drop and Friction Factor
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Several formulations are available to estimate the friction factor for turbulent flows. Correlations that reasonably approximate the smooth surface condition are: f = 0.316Red-0.25 Red < 2x104 (3.14) f = 0.184Red-0.2 Red < 2x104 (3.15)
Another option to estimate the friction factor is to use the Moody diagram, figure 6. This graphical approach is a compilation of several empiric formulations. In addition to depending on the Reynolds number the friction factor is a function of the tube surface condition. It is a minimum for smooth surfaces and increases with increasing surface roughness.
Fig. 6. Moody Diagram The friction factor is also temperature dependent and Seider and Tate suggest the correction: (3-16)
fiso is the friction factor for the isothermal case, mw the fluid viscosity close to the wall and mf the average fluid viscosity.The power required to overcome the pressure drop is estimated by: In a circuit the total pressure drop is estimated by the algebraic sum of the distributed losses along the tubes (estimated by 3.12) and the frictional losses (due to singularities such as expansions, contractions, valves, bands, etc.).
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