cen58933_ch10

# 05 immersed in water as shown in figure 1017 if the

This preview shows page 1. Sign up to view the full content.

This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

## 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.

Ask a homework question - tutors are online