Date of Award

2017

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

Sajjad Bigham

Committee Member 2

Scott Miers

Committee Member 3

Radheshyam Tewari

Committee Member 4

Scott Wagner

Abstract

Steady and steady-in-the-mean shear driven annular flow-boiling is experimentally investigated in this thesis. By annular flow-regime one means separated liquid and vapor flows with the flowing liquid film staying on the boiling-surface.

The goals for the steady annular flow-boiling operations are to better understand relative importance of heat-transfer mechanisms (nucleation versus convective phase-change, with convective meaning absence of nucleation) and to develop better quantitative characterizations/correlations for heat-transfer rates. The goals for the steady-in-the-mean annular investigations was to explore improved means of significantly increasing heat transfer rates - while retaining the flow-regime's annularity.

The experimental investigations (with FC-72 as a working fluid) were carried out within a horizontal test-section of 50 cm length and rectangular cross-section (depth = 15 mm and height = 2 mm). Temperature controlled heating (by using reversed and controllable thermo-electric coolers) at low heat-fluxes (in the range of 0.1 – 1 W/cm2) were used.

The results establish enhancements between pulsatile (steady-in-the-mean) and corresponding steady cases to be up to 110% for local heat-flux measurements (at 40 cm location) and up to 18.3% for the overall heat-flux. The new physic based decomposition of heat transfer mechanisms – between micro-scale nucleate boiling and convective boiling (assuming no nucleation) – was established by synthesizing experimental data with corresponding data obtained from a very accurate 2D CFD convective modeling (assuming no nucleation) and simulation technique from Dr. Narain’s computational team. It is found that invisible micro nucleation plays a very big role in thin liquid film (400 – 40 µm thick) annular flow-boiling – and is responsible, typically, for removing about 70 to 90% of the total heat-flux at each location. Experimental data is also used to propose superior heat-transfer coefficient correlations.

For steady-in-the-mean cases, it can be concluded that to enhance the heat-flux in an efficient way, inlet liquid flow rate pulsations are best - particularly if the liquid pulsator frequency at suitably small amplitude is set close to the predominant noise frequency (found from the Fast Fourier Transform of the dynamic differential pressure measurement across the test-section) already present in the steady realization.

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