Title page for ETD etd-05032005-171633


Type of Document Master's Thesis
Author Novak, Julia Ann
Author's Email Address jnovak@vt.edu
URN etd-05032005-171633
Title Cavitation and Bubble Formation in Water Distribution Systems
Degree Master of Science
Department Environmental Engineering
Advisory Committee
Advisor Name Title
Edwards, Marc A. Committee Chair
Diplas, Panayiotis Committee Member
Loganathan, G. V. Committee Member
Keywords
  • Gaseous Cavitation
  • Bubbles
  • Corrosion
  • Dissolved Gas
Date of Defense 2005-04-08
Availability restricted
Abstract
Gaseous cavitation is examined from a practical and theoretical standpoint. Classical cavitation experiments which disregard dissolved gas are not directly relevant to natural water systems and require a redefined cavitation inception number which considers dissolved gases. In a pressurized water distribution system, classical cavitation is only expected to occur at extreme negative pressure caused by water hammer or at certain valves. Classical theory does not describe some practical phenomena including noisy pipes, necessity of air release valves, faulty instrument readings due to bubbles, and reports of premature pipe failure; inclusion of gaseous cavitation phenomena can better explain these events. Gaseous cavitation can be expected to influence corrosion in water distribution pipes.

Bubbles can form within the water distribution system by a mechanism known as gaseous cavitation. A small scale apparatus was constructed to track gaseous cavitation as it could occur in buildings. Four independent measurements including visual observation of bubbles, an inline turbidimeter, an ultrasonic flow meter, and an inline total dissolved gas probe were used to track the phenomenon. All four measurements confirmed that gaseous cavitation was occurring within the experimental distribution system, even at pressures up to 40 psi. Gaseous cavitation was more likely at higher initial dissolved gas content, higher temperature, higher velocity and lower pressure. Certain changes in pH, conductivity, and surfactant concentration also tended to increase the likelihood of cavitation. For example, compared to the control at pH 5.0 and 30 psig, the turbidity increased 295% at pH 9.9. The formation of bubbles reduced the pump’s operating efficiency, and in the above example, the velocity was decreased by 17% at pH 9.9 versus pH 5.0.

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