cen58933_ch10

# Heating element figure 1016 schematic for example 102

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Unformatted text preview: ce wires inside, as shown in Figure 10–16. Determine the maximum heat flux that can be attained in the nucleate boiling regime and the surface temperature of the heater surface in that case. P = 1 atm Water, 100°C d Ts = ? Heating element FIGURE 10–16 Schematic for Example 10–2. Peak Heat Flux in Nucleate Boiling · qmax SOLUTION Water is boiled at 1 atm pressure on a nickel plated steel surface. The maximum (critical) heat flux and the surface temperature are 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 0.0589 N/m (Table 10–1) and, from Table A-9, l v Prl 957.9 kg/m3 0.6 kg/m3 1.75 hfg l Cpl 2257 103 J/kg 0.282 10 3 kg · m/s 4217 J/kg · °C Also, Csf 0.0060 and n 1.0 for the boiling of water on a nickel plated surface (Table 10–3). Note that we expressed the properties in units specified under Eqs. 10–2 and 10–3 in connection with their definitions in order to avoid unit manipulations. Analysis The heating element in this case can be considered to be a short cylinder whose characteristic dimension is its radius. That is, L r 0.005 m. The dimensionless parameter L* and the constant Ccr are determined from Table 10–4 to be L* L g( l ) 1/2 (0.005) (9.81)(957.8 0.6) 0.0589 1/2 2.00 1.2 which corresponds to Ccr 0.12. Then the maximum or critical heat flux is determined from Eq. 10–3 to be q·max Ccr hfg [ g 2 ( l )]1/4 0.12(2257 103)[0.0589 1.02 106 W/m2 9.8 (0.6)2(957.9 0.6)]1/4 cen58933_ch10.qxd 9/4/2002 12:38 PM Page 529 529 CHAPTER 10 The Rohsenow relation, which gives the nucleate boiling heat flux for a specified surface temperature, can also be used to determine the surface temperature when the heat flux is given. Substituting the maximum heat flux into Eq. 10–2 together with other properties gives · q nucleate 1,017,200 Ts l hfg (0.282 g( l ) 1/2 10 3)(2257 4217(Ts 0.0130(2257 119°C Cpl (Ts Tsat) Csf hfg Prn l 103) 3 9.81(957.9 0.6) 0.0589 1/2 100) 103) 1.75 Discussion Note that heat fluxes on the order of 1 MW/m2 can be obtained in nucleate boiling with a temperature diff...
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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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