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英国论文代写范文精选-About Subcooled boiling

2016-06-17 | 来源:51due教员组 | 类别:更多范文

51Due英国论文代写网精选assignment代写范文:“About Subcooled boiling”,这篇论文讨论了过冷沸腾。在核工程领域中,过冷沸腾研究已经成为核系统安全性的重要问题。过冷沸腾是很难理解的,是气泡运动学中比较复杂的一项研究。因此,过冷沸腾,这是一个具有挑战性的问题,要对冷凝气泡有一个非常透彻的认识。

Subcooled boiling flows are encountered in many industrial applications such as boilers, nuclear reactors, and the new generation of electronic and computer systems. There have been many efforts to understand and model subcooled boiling flow.

In the field of nuclear Engineering, subcooled boiling flows have been studied as an important issue in the optimum design and safety of nuclear systems because the presence of vapor bubbles has a large effect on the heat transfer characteristics of a nuclear system as well as pressure drops, flow nstability, etc.

However, subcooled boiling flow is very difficult to understand due to the complex behavior of bubbles with heat and mass transfer through the bubble interface. Therefore, in order to understand subcooled boiling flow, which is a challenging problem, it is essential to acquire thorough knowledge on condensing bubbles. There have been many experimental analyses on bubble behavior. In these experimental studies, bubble behavior regarding the bubble size, shape, velocity and moving trajectory was investigated in various conditions.

However, it has been impossible to obtain complete information about the bubble behavior due to the existence of the bubble interface between the vapor and liquid phases. The shape and the area of the varying interface, which governs the behavior of each phase, are very complex and thus difficult to measure.

Moreover, in subcooled boiling flow, bubble condensation significantly affects the change of interface so this complicates the analysis of the behavior of condensing bubbles even more. Therefore, it is necessary to carry out numerical simulations for bubble behavior as a complement to experiments. Such numerical simulations may contribute to a better physical understanding of complex phenomena regarding the behavior of condensing bubbles.

One of the numerical methods for bubble simulation is the volume of fluid model proposed by Hirt and Nichols. It can deal with immiscible fluids with clearly defined interface; the direct simulation of the varying interface is possible. Many numerical studies have analyzed bubble behavior in terms of the bubble size, bubble shape, rise trajectory and bubble velocity using the VOF model.

Wachem and Schouten , Lorstad and Fuchs , and Gopala and Wachem examined the applicability of the VOF model to the analysis of a single rising bubble in liquid. These examinations confirmed that the VOF model can yield good predictions of the bubble shape and terminal velocity. The VOF model also has been used to analyze the following: the effect of a channel wall on bubble shapes and terminal velocities; the velocity distribution and the distribution of the local wall shear stress in slug flow; the effect of the liquid velocity field on bubble motion in a linear shear flow; the trajectories of bubbles with various initial diameters that rise in water; the coalescence of two gas bubbles; the effect of the bubble size/shape and neighboring bubbles on the magnitude and direction of the lift force; etc.

The simulation results showed good agreements with the experimental data. However, most bubble simulations were limited to cases of adiabatic systems such as air bubble-water flow, where the heat and mass transfer between each phase were not considered. Those simulation results for bubble behavior are only valid for analyzing bubble behavior in adiabatic systems. However, in subcooled boiling flow, bubble condensation is the key parameter to describe heat and mass transfer phenomena.

It significantly affects the shape and the area of the varying interface thus the behavior of the condensing bubble becomes different from that of the adiabatic bubble. Therefore, in order to understand bubble behavior in subcooled boiling flow, a numerical study of condensing bubbles considering heat and mass transfer through the bubble interface is required.

This study focused on how to simulate bubble condensation with a CFD code. Moreover, using the VOF model, the behavior of condensing bubbles in subcooled boiling flow was investigated. In order to simulate the condensing bubble with the FLUENT code, the bubble condensation was modeled using the user-defined function.

For the validation of the UDF of bubble condensation, the results of CFD simulation were compared with the SNU experimental results. Through the VOF model coupled with the UDF of bubble condensation, the fundamental behavior of condensing bubble in terms of the bubble velocity, rise distance and moving trajectory was investigated under various conditions.

The effects of condensation on bubble behavior were analyzed by comparing the behavior of condensing bubbles with that of adiabatic bubbles. In this study, the volume of fluid model in a commercial CFD code FLUENT 6.2.16 is used to simulate condensing bubbles in subcooled boiling flow. The gas and liquid phases are considered as incompressible fluids and the flow is assumed to be laminar.

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