The different heat transfer regions in two-phase flow

5 The different heat transfer regions in two-phase flow 

We shall now consider subcooled liquid fed into the bottom of a vertical evaporator tube, that is uniformly heated along its entire length. The heat flux q is assumed to be low and the tube should be long enough such that the liquid can be completely evaporated. Fig. 4.53 shows, on the left, alongside the various heat exchange regions that have already been explained, the profiles of the liquid and wall temperatures. 

As long as the wall temperature stays below that required for the formation of vapour bubbles, heat will be transferred by single-phase, forced flow. If the wall is adequately superheated, vapour bubbles can form even though the core liquid is still subcooled. This is a region of subcooled boiling. In this area, the wall temperature is virtually constant and lies a few Kelvin above the saturation temperature. The transition to nucleate boiling, is, by definition, at the point where the liquid reaches the saturation temperature at its centre, and with that the thermodynamic quality is r h = 0. In reality, as Fig. 4.53 indicates, the liquid at the core is still subcooled due to the radial temperature profile, whilst at the same time vapour bubbles form at the wall, so that the mean enthalpy is the same as that of the saturated liquid. As explained in the previous section, the 

4 Convective heat and mass transfer. Flows with phase change 

As soon as the liquid film at the wall is completely evaporated, the temperature 

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