Experiments on two-phase heat transfer regimes in vertical downward flow: From nucleate to film boiling

The addition of latent heating to sensible heating in subcooled nucleate boiling results in high heat transfer efficiency, making it ideal for cooling applications such as nuclear power plants, spacecraft thermal management, HVAC systems, and electronics cooling. However, the end of nucleate boiling regime restricts the highest achievable heat transfer at the critical heat flux (CHF), where overcrowding of vapor on the heated wall causes a drastic drop in heat transfer and a rapid rise in system temperature. Understanding nucleate boiling and CHF is crucial for the safe operation of these cooling systems and to avoid any potential system damage. Post-critical heat flux (CHF), the system enters the film boiling regime where a stable vapor film covers the heated surface, preventing liquid contact and resulting in low heat transfer coefficients. This regime is significant in reactor safety analysis during a possible loss of coolant accident in pressurized water reactors. Knowledge of film boiling heat transfer and rewetting is crucial for accurately predicting reflooding during emergency cooldowns of water-cooled reactors. This thesis presents experimental data from a closed-loop facility for the flow boiling of water flowing vertically downward in a single-side heated square channel. Heat transfer characteristics in nucleate boiling regime, film boiling regime, CHF, and rewet are studied closely for a range of system conditions like mass flux from 195 kg/m²s to 587 kg/m²s, inlet subcooling from 5°C to 15°C, and pressure from 100 kPa to 500 kPa. Power supplied to the heated wall was increased in steps, allowing the system to stabilize after each increment, until CHF was reached. After CHF, heating power was decremented throughout the film boiling regime until rewetting occurred. Several quasi-stable data points are recorded by waiting for the system to reach a steady state after each power reduction. The study revealed that mass flux significantly affects heat transfer in the low wall superheat region of the boiling curve, while inlet subcooling predominantly influences the high wall superheat region. CHF values increase steadily with higher mass flux and inlet subcooling. In the film boiling regime, increased inlet subcooling enhances heat transfer but mass flux and pressure showed no significant influence. Additionally, higher inlet subcooling conditions led to rewetting at higher wall temperatures compared to lower subcooling conditions. In general, this study reveals the parametric dependence of the boiling curve of water in downward flow and contributes to the understanding of two-phase heat transfer.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Piuly, Sanbera Islam

Contributors dc:contributor

• Brooks, Caleb S.
• Miljkovic, Nenad

• Two-phase Heat Transfer
• Subcooled Flow Boiling
• Vertically Downward Flow
• Nucleate Boiling
• Chf
• Film Boiling

Statement dc:rights

• Copyright 2024 Sanbera Piuly

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/127251

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F127251