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In transition boiling, heat flux decreases due to which of the following? 1. Low value of film heat transfer coefficient at the surface during 100°C to 120°C surface temperature. 2. Major portion of heater surface is covered by vapour film which has smaller thermal conductivity as compared to liquid. 3. Nucleate boiling occurs very fast. Select the correct answer using the code given below.

In transition boiling, heat flux decreases due to which of the following? 1. Low value of film heat transfer coefficient at the surface during 100°C to 120°C surface temperature. 2. Major portion of heater surface is covered by vapour film which has smaller thermal conductivity as compared to liquid. 3. Nucleate boiling occurs very fast. Select the correct answer using the code given below. Correct Answer 2 only

Four different boiling regimes are 

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Natural Convection Boiling: Boiling starts as soon as the saturation temperature is reached at that pressure, but bubbles do not form until the liquid is heated few degrees above the saturation temperature, therefore the liquid is evaporated when it rises to the free surface, this fluid motion is due to the natural convection currents and heat transfer is taking place between the heated surface and the liquid is due to the natural convection.

Nucleate Boiling: In nucleate boiling the bubbles start forming at an increasing rate at an increasing number of nucleation sites. This regime can be separated into two regions, between A-B isolated bubbles are formed and dissipated into the liquid after they separate from the surface. The space vacated by the bubbles id filled by the liquid in the vicinity of the heater surface and the process is repeated. In B-C the temperature is further increased and bubbles form at such great rates at such a large number of nucleation sites that they form numerous continuous columns of vapour in the liquid.

Transition Boiling: Once the temperature is increased past point C the heat flux decreases because a large portion of the surface is covered by the vapor film, which acts as an insulation due to the low thermal conductivity of the vapor relative to the water. During the Transition boiling, a water vapour blanket is formed which acts as an insulation and decreases the heat transfer rate and reaches a minimum point known as the Leidenfrost point.

Film Boiling: In this region, the heater surface is completely covered by the continuous stable vapor film, point D, where the heat flux reaches a minimum. The heat transfer rate increases with the increasing temperature due to radiation heat transfer between the vapour film and the liquid.

Related Questions

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