Study On Mechanism Of Single Bubble Growth In Pool Boiling And Flow Boiling In Vertical Narrow Channel

Boiling is a violent vaporization process in which a gas phase is generated inside a liquid and converted from a liquid state to a gaseous state.According to the flow state of the liquid,the boiling can be divided into pool boiling and flow boiling.In any boiling,the generation and behavior of the bubble may increase the heat transfer effect between the wall and the fluid.The bubbles may also form a gas film on the heating wall,and lead to the wall burnout.So the study of bubbles is necessary..The following work on the bubble have been done in this paper:First,numerical simulations of single bubble growth process under subcooled boiling condition and saturated pool boiling condition in the literature were carried out by using the Volume of Fraction(VOF)method in ANSYS Fluent under two-dimensional condition.The User Defined Functions(UDF)were used to realize the conversion of gas phase and liquid phase.An additional steam source was added to the near-wall mesh according to the experimental results to simulate the evaporation of microlayer.Then a pool boiling test bench for researching single bubble growth was set up.The growth and condensation processes of single bubble in low subcooling water under normal pressure were recorded using a high-speed camera in the experiment.The temperature of the bottom wall at the nucleation point was recorded by thermocouples.Based on the bubble condensation process,the change of the bubble cap heat flow during the bubble growth process was calculated,and the obtained heat flow provided conditions for simulating the growth and condensation process of bubbles in two examples.Finally,a flow boiling test bench was set up and the flow boiling experiment was carried out in two vertical rectangular flow channels with gaps of 1.80mm and 2.80mm respectively.A high-speed camera was used to record the growth process of a single bubble around the Onset of Nucleate Boiling(ONB)point.The effects of various factors on bubble growth and wall superheat at ONB point were studied,and a typical bubble growth process was selected from each of the two flow channels for numerical simulation.In numerical simulation,the change of bubble morphology and size with time was obtained.At the same time,the temperature field,the velocity field and the heat flow of the ball cap were obtained.In pool boiling experiment,numerical simulation showed that the contribution of microlayer evaporation to the total evaporation was significant.The bubble cap heat flow prediction formula was also used to derive the trend of the normalized diameter of the bubble in the bubble condensation process with the Fo number.In flow boiling experiment,it was found that the heating power,mass flux and inlet fluid temperature have an effect on the growth of the bubble.The height of the channel has no significant effect on the size of a single bubble,but it will affect the detachment of the bubble.The ONB point position and the wall superheat were determined according to the measured temperature.And it was found that the wall superheat at ONB site was closely related to the heat flux,mass flux and inlet fluid temperature,while the relationship between that the wall superheat at ONB site and heat flux is not obviously affected by the channel height.