05 immersed in water as shown in figure 1017 if the

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Unformatted text preview: erence of less than 20°C. P = 1 atm EXAMPLE 10–3 Film Boiling of Water on a Heating Element Water is boiled at atmospheric pressure by a horizontal polished copper heating element of diameter D 5 mm and emissivity 0.05 immersed in water, as shown in Figure 10–17. If the surface temperature of the heating wire is 350°C, determine the rate of heat transfer from the wire to the water per unit length of the wire. SOLUTION Water is boiled at 1 atm by a horizontal polished copper heating element. The rate of heat transfer to the water per unit length of the heater is to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat losses from the boiler are negligible. Properties The properties of water at the saturation temperature of 100°C are hfg 2257 103 J/kg and l 957.9 kg/m3 (Table A-9). The properties of vapor at the film temperature of Tf (Tsat Ts)/2 (100 350)/2 225°C 498 K (which is sufficiently close to 500 K) are, from Table A-16, 0.441 kg/m3 1.73 10 5 kg/m · s Cp k 1977 J/kg · °C 0.0357 W/m · °C Note that we expressed the properties in units that will cancel each other in boiling heat transfer relations. Also note that we used vapor properties at 1 atm pressure from Table A-16 instead of the properties of saturated vapor from Table A-9 at 250°C since the latter are at the saturation pressure of 4.0 MPa. Analysis The excess temperature in this case is T Ts Tsat 350 100 250°C, which is much larger than 30°C for water. Therefore, film boiling will occur. The film boiling heat flux in this case can be determined from Eq. 10–5 to be 100°C Heating element Vapor film FIGURE 10–17 Schematic for Example 10–3. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 530 530 HEAT TRANSFER q·film 0.62 gk3 ( )[hfg 0.4Cp (Ts D(Ts Tsat) l Tsat)] 9.81(0.0357)3 (0.441)(957.9 0.441) [(2257 103 0.4 1977(250)] 0.62 (1.73 10 5)(5 10 3)(250) 5.93 104 W/m2 1/4 (Ts Tsat) 1/4 250 The radiation heat flux is determined from Eq. 10–6 to be q·rad 4 (Ts4 Tsat) (0.05)(5.67...
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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.

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