Short-notes-for-Heat-transfer_docx-97_docx-93-wate.pdf-47.pdf

Empirical correlation for free convection heated

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Empirical Correlation for Free Convection Heated surface up or cooled surface down Laminar flow 2 × 10 5 < Gr.Pr < 2 × 10 7 Nu = 0.54 (Gr Pr) 0.25 Turbulent flow 2 × 10 7 < Gr.Pr < 3 × 10 10 Nu = 0.14 (Gr Pr) 0.33 Heated surface down or cooled surface up Laminar flow 3 × 10 5 < Gr.Pr < 7 × 10 8 Nu = 0.27 (Gr Pr) 0.25 Turbulent flow 7 × 10 8 < Gr.Pr < 11 × 10 10 Nu = 0.107 (Gr Pr) 0.33 Vertical plates and Large cylinder Laminar flow
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10 4 < GrPr < 10 9 Nu = 0.59 (GrPr) 0.25 Turbulent flow 10 9 < GrPr < 10 12 Nu = 0.13 (GrPr) Empirical Correlation for Forced Convection Laminar Flow over Flat Plate Hydrodynamic boundary layer thickness Laminar Flow over Inside Tube o Constant heat flux, Nu = 4.36 Fouling Factor (R f ) Fin Efficiency and Fin Effectiveness η fin = (actual heat transferred) / (heat which would be transferred if the entire fin area were at the root temperature)
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For a very long fin, effectiveness : And i.e., effectiveness increases by increasing the length of the fin but it will decrease the fin efficiency. Expressions for Fin Efficiency for Fins of Uniform Cross-section: o Very long fins: For fins having insulated tips Logarithmic Mean Temperature Difference (LMTD) LMTD Capacity Ratio
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Capacity ratio c = mc , where c = Specific heat Effectiveness of Heat Exchanger: o If m c c n < m h c h c min = m c c c o If m c c n < m h c h c min = m h c h Number of Transfer Units (NTU): U = Overall heat transfer coefficient A = Surface area C min = Minimum capacity rate If m h c h < m c c c c min = m c c c If m h c h < m c c c c min = m h c h Effectiveness for Parallel Flow Heat Exchanger
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Effectiveness for the Counter Flow Heat Capacity: Heat Exchanger Effectiveness Relation: Concentric tube: o Parallel flow: o Counter flow: Cross flow (single pass): o Both fluids unmixed: o C max mixed , C min unmixed: o C min mixed, C max unmixed: Total Emissive Power (E) It is defined as the total amount of radiation emitted by a body per unit time and area.
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E = σ T 4 W/m 2 σ = Stefan Boltzmann constant σ = 5.67 × 10 -8 W/m 2 K 4 Monochromatic (Spectral) Emissive Power (E λ ) It is defined as the rate of energy radiated per unit area of the surface per unit wavelength. Emission from Real Surface The emissive power from a real surface is given by E = εσAT 4 W ε = Emissivity of the surface, T = Surface temperature Emissivity (ε) It is defined as the ratio of the emissive power of any body to the emissive power of a black body of same temperature.
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