Then the total heat flux becomes eq 107 qtotal qfilm

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Unformatted text preview: 10 157 W/m2 8 W/m2 · K4)[(250 273 K)4 (100 273 K)4] Note that heat transfer by radiation is negligible in this case because of the low emissivity of the surface and the relatively low surface temperature of the heating element. Then the total heat flux becomes (Eq. 10–7) q·total q·film 3· q 4 rad 5.93 104 3 4 157 5.94 104 W/m2 Finally, the rate of heat transfer from the heating element to the water is determined by multiplying the heat flux by the heat transfer surface area, · Q total · q · qmax High velocity Low velocity it y Nucleate pool boiling regime io n oc vel ow L ity Fre e onv ec ct ∆Texcess FIGURE 10–18 The effect of forced convection on external flow boiling for different flow velocities. 104 W/m2) Discussion Note that the 5-mm-diameter copper heating element will consume about 1 kW of electric power per unit length in steady operation in the film boiling regime. This energy is transferred to the water through the vapor film that forms around the wire. 10–3 lo c h ve Hig · Aq·total ( DL)q total ( 0.005 m 1 m)(5.94 933 W I FLOW BOILING The pool boiling we considered so far involves a pool of seemingly motionless liquid, with vapor bubbles rising to the top as a result of buoyancy effects. In flow boiling, the fluid is forced to move by an external source such as a pump as it undergoes a phase-change process. The boiling in this case exhibits the combined effects of convection and pool boiling. The flow boiling is also classified as either external and internal flow boiling depending on whether the fluid is forced to flow over a heated surface or inside a heated tube. External flow boiling over a plate or cylinder is similar to pool boiling, but the added motion increases both the nucleate boiling heat flux and the critical heat flux considerably, as shown in Figure 10–18. Note that the higher the velocity, the higher the nucleate boiling heat flux and the critical heat flux. In experiments with water, critical heat flux values as high as 35 MW/m2 have been obtained (compare this to the pool boiling value of 1.3 MW/m2 at 1 atm pressure) by increasing the fluid velocity. cen58933_ch10.qxd 9/...
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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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