10 solution water is boiled at 1 atm pressure on a

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Unformatted text preview: inn 104 Pore Tunnel Plai Th ed t ube erm qc (kcal/(m 2 h) ′′ oe xc elE Vapor Liquid 1 2 5 (Ts – Tsat ) (°C) FIGURE 10–14 The enhancement of boiling heat transfer in Freon-12 by a mechanically roughened surface, thermoexcel-E. 10 SOLUTION Water is boiled at 1 atm pressure on a stainless steel surface. The rate of heat transfer to the water and the rate of evaporation of water are to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat losses from the heater and the pan 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 2257.0 103 J/kg 0.282 10 3 kg · m/s 4217 J/kg · °C Also, Csf 0.0130 and n 1.0 for the boiling of water on a mechanically polished stainless steel surface (Table 10–3). Note that we expressed the properties in units specified under Eq. 10–2 in connection with their definitions in order to avoid unit manipulations. Analysis (a) The excess temperature in this case is T Ts Tsat 108 100 8°C which is relatively low (less than 30°C). Therefore, nucleate boiling will occur. The heat flux in this case can be determined from the Rohsenow relation to be · q nucleate Cpl (Ts Tsat) 3 Csf hfg Prn l 9.81 (957.9 (0.282 10 3)(2257 103) 0.0589 3 4217(108 100) 0.0130(2257 103)1.75 l hfg 7.20 g( l 1/2 ) 104 W/m2 The surface area of the bottom of the pan is A D2/4 (0.3 m)2/4 0.07069 m2 0.6) 1/2 cen58933_ch10.qxd 9/4/2002 12:38 PM Page 528 528 HEAT TRANSFER Then the rate of heat transfer during nucleate boiling becomes · Q boiling Aq·nucleate (0.07069 m2)(7.20 104 W/m2) 5093 W (b) The rate of evaporation of water is determined from · m evaporation · Q boiling hfg 5093 J/s 2257 103 J/kg 2.26 10 3 kg/s That is, water in the pan will boil at a rate of more than 2 grams per second. EXAMPLE 10–2 Water in a tank is to be boiled at sea level by a 1-cm-diameter nickel plated steel heating element equipped with electrical resistan...
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