Heat transfer in nucleate boiling and convective evaporation

6 Heat transfer in nucleate boiling and convective evaporation 

In the calculation of the heat transfer in saturated boiling it is presumed that the heat transfer is predominantly determined by the bubble formation. In addition, convection also has some influence on the heat transfer, although this may only be small. Models and empirical equations start from the heat transfer in boiling in free flow. The influence of forced flow is taken into account with an extra term. This is based on the concept that the growth of the bubbles is only slightly influenced by the flow, as long as the bubbles are smaller than the superheated boundary layer. The heat transfer coefficient is a combination of two parts 

4 Convective heat and mass transfer. Flows with phase change 

When vapour and liquid flow downwards in a vertical tube, the slip between the two phases is reduced because of the buoyancy. This leads to a deterioration in the heat transfer, and according to measurements made by Pujol [4.90] using the refrigerant R113, the heat transfer coefficient adown of the downward flow is smaller by a factor 0.75 than that for upward flow aup 

The region of saturated boiling is followed by that of convective evaporation. With the increasing vapour content the heat transfer from the wall to the fluid improves. The thermal resistance of the boundary layer decreases in comparison to the thermal resistances in nucleate boiling. Likewise, the wall temperature drops, cf. Fig. 4.53, so that only a few or no bubbles are formed at the wall. The heat transfer is predominantly or exclusively determined by evaporation at the phase boundary between the liquid at the wall and the vapour in the core flow. 

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