Enhancement of heat transfer and anti-scaling capabilities for pool boiling

Zhang, Yuheng

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https://hdl.handle.net/2142/115592 Copy

Description

Title

Enhancement of heat transfer and anti-scaling capabilities for pool boiling

Author(s)

Zhang, Yuheng

Issue Date

2022-04-22

Director of Research (if dissertation) or Advisor (if thesis)

Wang, Sophie

Doctoral Committee Chair(s)

Wang, Sophie

Committee Member(s)

• Wang, Xinlei
• Miljkovic, Nenad
• Feng, Jie

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Pool Boiling
• Critical Heat Flux
• Scaling
• Fouling
• Heat Transfer
• Enhancement

Abstract

Nucleate boiling is one of the most efficient modes of heat transfer. Durable enhancement of the boiling performance has always been a great interest in many applications. In this work, we extend the research on a non-coating-based methodology that aims to enhance the nucleate boiling heat transfer and antiscaling capabilities by harnessing the hydrodynamics and capillaries induced in the boiling process. A new vapor management approach is proposed to enhance convection and stabilize the vapor flow emitted from a boiling surface with robust shroud structures – 1. Straight, tubular vapor shrouds are found to improve the stability of the liquid feeding and increase the critical heat flux (CHF). Near the CHF, the flow structure inside the vapor blanket is visualized, showing liquid entrainment passing into the vapor blanket and splashing onto the heated surface, which provides an answer to the problem of liquid feeding mechanism on a small and non-wicking surface. Momentum and energy analysis shows the CHF scales with the resulting maximum liquid flow rate by the shroud structure; 2. Compact shrouds with curved shapes are shown to enhance the heat transfer coefficient in nucleate boiling by increasing bubble expansion, promoting bubble coalescence, and enhancing thin-film evaporation. Characteristics of the liquid-vapor two-phase flow inside the shroud is analyzed. The resulted liquid feeding mechanism near CHF is discussed, and an equation based on scaling analysis is derived to predict the effect of shroud on heat transfer The characteristics of scale deposition in nucleate boiling with high salinity aqueous solution is also experimentally investigated. A surface design with micro-machined grooves on a polished stainless-steel substrate demonstrates a strong suppression effect on scale nucleation. Different groove depth and spacing designs are examined. The effect on heat transfer, bubble nucleation and scale morphology characteristics are analyzed. It is shown that grooves with an appropriate depth can enhance bubble nucleation while reducing scale deposition rate, whereas on the smooth substrate, bubble nucleation can only cause scale deposition rate to increase. The similarity between the anti-scaling effect of grooves and re-entrant fins is analyzed, which indicates that the suppressed scale deposition on grooved surfaces is mainly contributed by 1. the cooling effect due to enhanced bubble nucleation, and 2. the reduced surface dryout due to well preserved liquid meniscus. This work expands the understanding of three key mechanisms in nucleate boiling – 1. film evaporation, 2. liquid feeding, and 3. deposition of solids. Robust control methodologies for enhancing film evaporation, increasing liquid feeding stability, and reducing scale deposition are proposed and examined. The demonstrated method for direct visualization of the flow structure inside a vapor blanket also opens new opportunities for studying the liquid feeding mechanism in nucleate boiling near the critical heat flux.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Yuheng Zhang

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