Why do these conditions yield smaller heat transfer

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Why do these conditions yield smaller heat transfer rates than those for a hot upper surface or cold lower surface? (9.32)
Problem: Solar Receiver Problem 9.33: Convection and radiation losses from the surface of a central solar receiver. KNOWN:Dimensions and emissivity of cylindrical solar receiver. Incident solar flux. Temperature of ambient air. FIND:(a) Heat loss and collection efficiency for a prescribed receiver temperature, (b) Effect of receiver temperature on heat losses and collector efficiency. = h L / k
Problem: Solar Receiver (cont.) PROPERTIES:Table A-4, air (Tf= 550 K): k= 0.0439 W/mK, ν= 45.6 ×10-6m2/s, α= 66.7 ×10-6m2/s, Pr= 0.683, β= 1.82 ×10-3K-1. ANALYSIS:(a) The total heat loss is ()4radconvssssqqqAThATTεσ=+=+With RaL= gβ(Ts- T)L3/να= 9.8 m/s2(1.82 ×10-3K-1) 500K (12m)3/(45.6 ×66.7 ×10-12m4/s2) = 5.07 ×1012, the Churchill and Chu correlation, Eq. 9.26, yields (){}21/6228/279/160.3870.0439W/m K0.8250.82542.46.83W/mK12m10.492 /LRakhLPr=+=+=+"#$$$$%&$$'()*$$+,-.Hence, with As= πDL= 264 m2()()4282422264m0.25.6710W/mK800K264m6.83W/mK 500Kq=×××+×656radconv1.23 10 W9.01 10 W2.13 10 Wqqq=+=×+×=×
Cylinders The Long Horizontal Cylinder Boundary Layer Development and Variation of the Local Nusselt Number for a Hot Cylinder: ()21/6128/279/160.3870.601010.559 /DDDRaNuRaPr!"##=+<$%&'##+()*+(9.34) Spheres The Average Nusselt Number: ()1/44/99/160.589210.469 /DDRaNuPr=+!"+#$(9.35)
Boiling Chapter 10 Sections 10.1 through 10.5
Distillation: Convert sea water to fresh water?Why does water vapor enter small condenser tube?
Steam has 2,613kW Thermodynamic analysis
Heat transfer rate [W] = 357 kW In condenser (max) 2,613kW
Vapor Compression Distillation System Compressor work = mdot (h2-h1) 50 kW 2614 kW Lose 357 kW Lose 2,256 kW 2564 kW Assume
General Considerations General Considerations Agitation of the liquid by vapor bubbles is one mechanism that provides for large convection coefficients and hence large heat fluxes at low-to-moderate surface-to-fluid temperature differences. Special form of Newton’s law of cooling: ()satΔsseqh TThT!!==Air at 1 atm Heater Boiling Water @ 100oC Is water at the saturation temperature? Excess temperature Saturation temperature
Boiling Curve The Boiling Curve Saturated pool boiling case: "Little vapor formation. "Liquid motion is due principally to single-phase natural convection. Free Convection Boiling ()Δ5 CeT<oOnset of Nucleate Boiling - ()ONBΔ5 CeToWater at 1 atm Excess Temperature
General Considerations (cont.) Special Cases "Pool Boiling: Liquid motion is due to natural convection and bubble-induced mixing."Saturated Boiling: Liquid temperature is slightly higher than saturation temperature. "Forced Convection Boiling: Fluid motion is induced by external means, as well as by bubble-induced mixing. "Subcooled Boiling: Liquid temperature is less than saturation temperature.

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