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Unformatted text preview: in kg/h. Water 165°C Answer: 19.4 kg/h 10–21 To understand the burnout phenomenon, boiling experiments are conducted in water at atmospheric pressure using
an electrically heated 30-cm-long, 3-mm-diameter nickelplated 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 0.5.
10–22 Reconsider Problem 10–21. Using EES (or
other) software, investigate the effects of the
local atmospheric pressure and the emissivity of the wire on
the critical heat flux and the temperature rise of wire. Let the
atmospheric pressure vary from 70 kPa and 101.3 kPa and
the emissivity from 0.1 to 1.0. Plot the critical heat flux and the
temperature rise as functions of the atmospheric pressure and
the emissivity, and discuss the results.
10–23 Water is boiled at 1 atm pressure in a 20-cm-internaldiameter teflon-pitted stainless steel pan on an electric range. If
it is observed that the water level in the pan drops by 10 cm in
30 min, determine the inner surface temperature of the pan.
Answer: 111.5°C 10–24 Repeat Problem 10–23 for a polished copper pan. 10–25 In a gas-fired boiler, water is boiled at 150°C by hot
gases flowing through 50-m-long, 5-cm-outer-diameter mechanically polished stainless steel pipes submerged in water. If
the outer surface temperature of the pipes is 165°C, determine
(a) the rate of heat transfer from the hot gases to water, (b) the
rate of evaporation, (c) the ratio of the critical heat flux to the
present heat flux, and (d) the surface temperature of the pipe at
which critical heat flux occurs.
Answers: (a) 10,865 kW, (b) 5.139 kg/s, (c) 1.34, (d) 166.5°C 10–26 Repeat Problem 10–25 for a boiling temperature of
10–27E Water is boiled at 250°F by a 2-ft-long and 0.5-in.diameter nickel-plated electric heating element maintained
at 280°F. Determine (a) the boiling heat transfer coefficient, Hot gases FIGURE P10–25
(b) the electric power consumed by the heating element, and
(c) the rate of evapor...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
- Spring '10