Problem 54
A cylindrical rod is used for boiling water at \(1 \mathrm{~atm}\). The rod has a diameter of \(1 \mathrm{~cm}\), and its surface has an emissivity of \(0.3\). Determine the film boiling convection heat transfer coefficient at the burnout point. Evaluate the properties of vapor at \(1150^{\circ} \mathrm{C}\). Discuss whether \(1150^{\circ} \mathrm{C}\) is a reasonable film temperature for the vapor properties.
Problem 55
A 3 -mm-diameter cylindrical heater is used for boiling water at $100^{\circ} \mathrm{C}$. The heater surface is made of mechanically polished stainless steel with an emissivity of \(0.3\). Determine the boiling convection heat transfer coefficients at the maximum heat flux for \((a)\) nucleate boiling and \((b)\) film boiling. For film boiling, evaluate the properties of vapor at \(1150^{\circ} \mathrm{C}\).
Problem 56
To understand the burnout phenomenon, boiling experiments are conducted in water at atmospheric pressure using an electrically heated 30 -cm-long, 4-mm- diameter nickel-plated horizontal wire. Determine \((a)\) the critical heat flux and \((b)\) the increase in the temperature of the wire as the operating point jumps from the nucleate boiling to the film boiling regime at the critical heat flux. Take the emissivity of the wire to be 0.3.
Problem 58
What is condensation? How does it occur?
Problem 59
What is the difference between film and dropwise condensation? Which is a more effective mechanism of heat transfer?
Problem 60
How does the presence of a noncondensable gas in a vapor influence the condensation heat transfer?
Problem 61
Consider film condensation on a vertical plate. Will the heat flux be higher at the top or at the bottom of the plate? Why?
Problem 61
What is the modified latent heat of vaporization? For what is it used? How does it differ from the ordinary latent heat of vaporization?
Problem 63
Consider film condensation on the outer surfaces of four long tubes. For which orientation of the tubes will the condensation heat transfer coefficient be the highest: \((a)\) vertical, \((b)\) horizontal side by side, \((c)\) horizontal but in a vertical tier (directly on top of each other), or \((d)\) a horizontal stack of two tubes high and two tubes wide?
Problem 64
In condensate flow, how is the wetted perimeter defined? How does wetted perimeter differ from ordinary perimeter?
