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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·
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 ·
velocity it y Nucleate pool
boiling regime io n oc
L ity Fre e
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
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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- Spring '10