Ezekiel Villarreal

PhD Dissertation Defense

Mechanical Engineering Graduate Program

Title:

 Ultra-Fast Transient Pool Boiling under High Pressure and Subcoolings

ABSTRACT:

Growing global energy consumption and demand has led to growing interest in a variety of energy options for various applications. Of particular interest presently are energy sources that produce little to no carbon emissions. Nuclear energy is a vital component of the repertoire of zero-carbon energy sources. Increased safety and efficiency of present and future nuclear reactors will be critical in meeting energy needs. Under accident conditions nuclear reactors can undergo high-energy, fast power transients. These transients can lead to fuel cracking or melting and cladding ballooning or rupture, thereby reducing the effectiveness and safety of the reactor. The goal of this work is to investigate ultra-fast transient pool boiling regimes to enhance the fundamental understanding of this phenomenology and to establish a basis to evaluate accident scenarios. 

In this work, temperature, heat transfer, and high-speed imagery of transient boiling events are collected during ultra-fast (up to 250,000°C/s) heating pulses to define the behavior and mechanisms occurring. While pool boiling responses have been studied previously at longer time scales, this work provides fundamental understanding at a timescale faster than nuclear accident conditions. To accomplish this a high-pressure, variable temperature, transient boiling system is created to experimentally study a variety of factors related to ultrafast transient boiling. These factors include combinations of pressures and water subcoolings, the heater surface condition, the heating shape, and the heating level. The water is heated by, and measurement are taken from, a thin wire inside a pressure vessel. Parameter comparison studies are performed to provide a relative importance of the tested parameters on a variety of experimental outcomes. Additional investigations of the material failure mechanisms under extreme heating rates are presented.

Ultimately, this work presents the transient boiling heat transfer and associated phenomena of numerous experimental conditions under ultra-fast heating. This will provide a fundamental evaluation of an area of pool boiling that has not been previously explored. The results can provide a basis to drive growth and future study both in the nuclear industry and in the heat transfer industry, where there is an ever-increasing demand for higher heat transfer and controllable configurations.