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MODULE 3 |
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Section 1: |
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Boundary Layer and Turbulence |
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Section 2: |
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Pressure Drop and Friction Factor |
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Section 3: |
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Heat Transfer Coefficient |
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Section 4: |
| Convection With Change of Phase | |
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The Boiling Curve
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Figure 8 shows the main features of the boiling curve or the relationship between heat flow and temperature difference (Tw -Tsat) One-phase Natural convection At very low temperatures (Tw - Tsat < 4 ºC) the heat transfer occurs without the appearance of bubbles. In this regime liquid in contact with the heater surface becomes superheated and rises in the form of natural convection currents. The relationship between heat flux and temperature difference is determined by employing the correlations of one-phase natural convection. Nucleate Boiling Following the abscissa towards the region of larger temperatures, the next regime is the nucleate boiling. This regime is characterized by the generation of bubbles in specific points of the heater surface. Probable nucleation sites are microscopic cracks on the surface of the heater element. At these sites a relatively large heater area per unit of liquid volume surrounds the trapped liquid. At the beginning of the nucleate boiling curve, the boiling process consists of isolated bubbles. As temperature increases, the bubble frequency increases and the nucleation sites multiply. Isolated bubbles start to interact with other bubbles merging into slugs and columns of vapor. The formation of more and more vapor close to the surface of the heater element has the effect of gradually insulating the surface. This effect gradually decreases the slope of the nucleate boiling part of the curve. The point where the heat flux is maximum (of the order of 106 W/m2) is denominated "Burnout point" and the heat flux "Critical Heat Flux". The temperature difference at this point is approximately 30 ºC.
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