Date of Award

12-2014

Document Type

Thesis - Open Access

Degree Name

Master of Science in Mechanical Engineering

Department

Mechanical Engineering

Committee Chair

Dirce Dikici, Ph.D.

First Committee Member

Sathya Gangadharan, Ph.D.

Second Committee Member

Marc Compere, Ph.D.

Abstract

The aim of this study is to investigate pool boiling performance of water under atmospheric pressure by two techniques. The first method is by adding small amounts of surfactants, and the second way is by using structured surfaces.

The first technique is investigated experimentally with environmentally friendly surfactants. The surfactants chosen for the study are an ionic sodium lauryl sulfate (SLS), nonionic ECOSURF™ EH-14, and nonionic ECOSURF™ SA-9. It is observed that adding a small amount of surfactant alters the water boiling phenomenon significantly. Boiling curves for different concentrations are shifted to the left. The wall temperature drops greatly with an increase in the concentration of aqueous surfactant solutions. Also, it is found that the optimum boiling heat transfer augmentation of SLS is higher than that of EH-14 and SA-9 compared to water. The maximum enhancement obtained is 66.27% for 300ppm aqueous SLS solution. However, the maximum enhancement in heat transfer coefficient is 24.31% for 1600ppm EH-14 and 22.09% for 200ppm SA-9. Boiling visualization shows that boiling with surfactant solutions compared with that in pure water is more vigorous. Bubbles are smaller in size, activate continuously, and collapse quickly. Also, the bubble departure frequency is higher than that of pure water. Compared with water, it is found that time required to reach boiling point for surfactant concentrations is reduced significantly. The maximum reduction for each surfactant was 14.6% for 100ppm SLS, 9% for 800ppm EH-14, and 12.49% for 300ppm SA-9.

For the second technique, an experimental study is conducted to investigate the performance of various structured surfaces in pool boiling. Surfaces with rectangular channels, holes, and mushroom fins are manufactured first and then studied. The results show that boiling heat transfer can be augmented by structured surfaces. The maximum enhancement is 51.66% achieved by Holed 3 surface compared with plain surface. As the spacing between channels or holes is decreased, the heat transfer coefficient is increased. The bubbles with holed surfaces and mushroomed surface have almost spherical shape, while in plain and grooved surfaces they have an irregular shape. Time to reach boiling point is measured. It is found that some enhanced surfaces show a higher reduction to others. For heat flux of 27.91 kW/m2, 8.58% enhancement in time to reach boiling point for Grooved 1 surface is attained, while at a heat flux of 35.08 kW/m2 the maximum reductions achieved are 8.74% for Mushroomed surface and 8.19% for Holed 1 surface. Also, the different regimes of pool boiling are observed by droplet dropping tests.

Also, the total evaporation time of a water droplet was measured and compared with the results of other studies. The results show that the evaporation times in the natural convection and nucleate boiling regimes are significantly shorter than those in film boiling regime. For natural convection and nucleate boiling regimes, the droplet evaporation time varies between 1s to 31s, while for film boiling regime it varies between 104s to 123s.

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