Question

explain the regims of pool boiling with help of neat sketch

Answer 1

Answer:

The theory of boiling is complex and not yet fully understood. The boiling process depends on factors such as mass flow, vapor content and the temperature difference between the refrigerant and the heating surface. The available research is based on empirical experiments on smooth tubes. Understanding the evaporation process inside plate heat exchangers is further complicated by the swirling flow that the plate pattern induces.

However, the processes of evaporation could be explained by a combination of the available research on pool boiling and flow boiling. SWEP has developed algorithms based on these theories and empirical laboratory tests that resemble the heat transfer.

Pool boiling

The heat transfer coefficient for pool boiling has a characteristic curve that displays heat flux versus the temperature difference between the evaporating and the secondary media (see Figure 6.4). As the temperature difference increases, the energy flow per heat transfer area (kW/m2) will increase until an unstable maximum value for heat transfer is reached, the critical heat flux. At this point, vapor will form to such an extent that it hinders contact between the liquid refrigerant and the heat transfer area. As the temperature difference increases, a continuous gas film will form and the heat flux will reach a minimum. A further increase in the temperature difference will increase the heat flux only slowly, because the heat transfer coefficient will depend on radiation. High heat transfer coefficients after this point are achieved only with a temperature difference of >800K, where BPHEs are not a suitable heat transfer solution. Preferably, a BPHE evaporator should be designed to operate below the critical heat flux to avoid the unstable performance of partial film boiling and the low heat transfer coefficient of complete film boiling.

Explanation:

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