Flash Boiling Spray Simulation Based on Void Fraction and Superheat Controlling

A new flash boiling spray model whose atomization criterion based on the void fraction and superheat while evaporation model based on the dual-zone method is established to simulate the flashing sprays. The model function is implemented in KIVA program. Flash boiling spray model predicts spray penetration and spray cone angle and its development trend, in good agreement with the experimental results. The model has a good capability in simulating flash sprays at low superheat conditions, which breakup is controlled by void fraction, as well as high superheat transition process. It can also predict flare flashing sprays to some extent at higher superheat conditions.

The Dynamics of Bubble Growth at Medium-High Superheat: Boiling in an Infinite Medium and on a Wall

At high superheat, bubble growth is rapid and the heat transfer is dominated by radial convection. This has been found, in the case of a droplet boiling within another liquid and in the case of a bubble growing on a heated wall, leading to similar bubble growth curves. Based on an experimental parametric study for the droplet-boiling case, an empirical model was developed for the prediction of bubble growth, within the radial convection dominated regime (the RCD model) occurring only at high superheat. This model suggests a dependence of R∼t1/3—equivalent to a Nusselt number decreasing over time (Nu∼t−1/3), as opposed to R∼t1/2 —equivalent to a highly-unlikely constant Nusselt number, in most other models. The new model provides accurate prediction for both the droplet boiling and nucleate pool boiling cases, in the medium-high superheat range (0.26<Ste <0.41, 0.19<Ste<0.30, accordingly). By comparison, the new RCD model shows a more consistent prediction, than previous empirical models. However, in the nucleate boiling case, the RCD model requires the foreknowledge of the departure diameter, for which a reliable model still is lacking.

Experimental Study of Nucleate Boiling Heat Transfer Using Enhanced Space-Confined Structures

For narrow space boiling, it is difficult to release bubbles from the narrow space, especially on a large-area surface. To solve this problem, a new structure is designed in the present paper. An experimental study of pool boiling on the novel copper enhanced structure, with the separate ordinary confined spaces and the open channels between them, was conducted with water and ethanol. High-speed visualizations are performed to elucidate the bubble flow. The results show that the boiling performance of both water and ethanol can be enhanced effectively. The visualizations indicated that most active nucleation sites emerged in the confined channels and rarely appeared at the bare surfaces not covered by enhanced structures even at high superheat. The bubble diameter, the bubble departure frequency, and the numbers of nucleation sites are obtained using statistical methods. The results suggest that the magnitudes of bubble diameter of water are almost the same on the smooth and enhanced surfaces. The amount of nucleation sites on the enhanced surfaces is remarkably increased, indicating its key role in the boiling enhancement of water. The bubble departure frequency is increased on one of the enhanced surfaces while not increased on another, showing that it is also a significant factor for heat transfer enhancement under certain conditions. While for ethanol, all the three parameters are increased on the enhanced surfaces.

A New Empirical Model for Bubble Growth: Boiling in an Infinite Medium and on a Wall at High Superheat

At high superheat bubble growth is rapid and the heat transfer is dominated by radial convection. This has been found, in the case of a droplet boiling within another liquid and in the case of a bubble growing on a heated wall, leading to similar bubble growth curves. Based on experiments conducted for the first case, an empirical model is developed for the prediction of bubble growth within the radial convection dominated regime (the RCD model), occurring only at high superheat (0.26<Ste<0.41). This model shows a dependence of R∼t1/3 equivalent to Nusselt number decreasing over time (Nu∼t1/3) as opposed to R∼t1/2 appearing in most other models, leading to a highly unlikely constant Nusselt number. The new model is shown to give accurate predictions for the first case and for the second case at medium-high superheat (0.19<Ste<0.30, experimental data taken from literature). A comparison of the RCD model to other models, shows a more consistent and accurate prediction. However, in the second case (nucleate boiling) the RCD model requires the foreknowledge of the departure diameter, for which a reliable model still is lacking.

Rapid Boiling of a Two-Phase Droplet in an Immiscible Liquid at High Superheat

This work studies experimentally the rapid boiling of a droplet rising in a host liquid environment, within a range of superheats (0.2<Ja∗<0.5) not previously investigated. The direct-contact rapid-boiling process has many advantages in the fields of heat exchange and multiphase flow. By taking into account the superheat, heat transfer, and hydrodynamics of the multiphase-droplet the aim of this study is to create greater insight into the character of this transient-boiling process, for the first time. The sudden depressurization of a water column led to the rapid boiling of liquid propane droplets rising by buoyancy. During this millisecond boiling distinct stages were identified. Appropriate critical times for the transition between stages were defined by a simplified model, among these a novel criterion for the sudden pause in boiling caused by the engulfing liquid-film's collapse. Good agreement was found between these predicted time-points and measured changes in the boiling profile. This form of boiling, though being very rapid and sustaining high heat transfer rates, is still calm in nature, therefore, more predictable and widely applicable. Understanding this form of boiling suggests that the “design” of the boiling curve may be possible by setting the initial parameters.