Date of Award

2019

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Amitabh Narain

Committee Member 1

Jamal Yagoobi

Committee Member 2

Hassan Masoud

Committee Member 3

Sunil S. Mehendale

Committee Member 4

Radheshyam Tewari

Committee Member 5

Ezequiel F. Medici

Abstract

Analysis of results from steady and steady-in-the-mean high heat-flux (15 - 70 W/cm2, with water as working fluid) shear driven annular flow-boiling experiments presented here - and low heat-flux (0.1- 1 W/cm2, with FC-72 as working fluid) experiments presented elsewhere – together lead to a key conclusion. The conclusion is that heat carrying nucleation rates go often undetected by the typically used visualization approaches for flow boiling – as such flows often involve µm- to sub- µm scale bubble diameters in millimeter-scale ducts. These nucleation rates play a significant role in most of the so-called convective annular regimes (of liquid thicknesses greater than 100 microns) of flow boiling. The perspective associated with conventional classifications of nucleate and convective (plug-slug, annular, etc.) regimes are based on flow-regime classifications that depend on: (i) the larger and detected liquid-vapor interface configurations at tube diameter levels, and (ii) heat-flux dependency in heat-transfer coefficient (HTC) correlations. Various reliable syntheses of experiments with simulations and/or correlations establish, as reported here, that thin film annular regime flow boiling at high heat-flux values yield experimentally measured heat-flux or HTC values that are significantly greater (often by 80-90%) than those associated with the assumption of no nucleate boiling contributions. This leads to following the new stricter definition of convective boiling. The stricter convective flow boiling regime definition is absence or near absence of heat carrying nucleation rates. The results reported here are for experiments done in the horizontal rectangular channel of height 5 mm and width 10 mm, with the length being 14 cm in the direction of water flow. Non-zero inlet quality (between 0.4 0.7), arranged by introducing, at the inlet, separate flows of nearly saturated liquid and vapor phases – at mass flow rates consistent with imposed heat-fluxes – ensure annularity over the entire length of the horizontal boiling-surface at the bottom of the test section. Results from a separate work dealing with a newly patented breakthrough – which is referenced here – supports the basic result that significant increases in the typically undetected nucleation rates (with the help of novel approaches) can lead to significant increases in boiling HTC.

Download

Share

COinS