On the Enhancement of Downward Facing Boiling Heat Transfer Using Micro-Porous Coating
Open Access
- Author:
- Sohag, Faruk Ahmed
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 19, 2016
- Committee Members:
- Fan-Bill Cheung, Dissertation Advisor/Co-Advisor
Fan-Bill B Cheung, Committee Chair/Co-Chair
Horacio Perez-Blanco, Committee Member
Gita Talmage, Committee Member
Albert Eliot Segall, Outside Member
Timothy John Eden, Special Member - Keywords:
- Downward-facing boiling
Micro-porous coating
Cold Spray
CHF limit
Nucleate boiling
Film boiling
Stability
Two-phase flow
IVR - Abstract:
- The concept of in-vessel retention (IVR) by passive external reactor vessel cooling (ERVC) in a flooded cavity during a severe accident has been considered as a viable approach to retain the radioactive core melt within the reactor vessel. The feasibility of IVR-ERVC depends primarily on the critical heat flux (CHF) distribution on the external bottom surface of the reactor vessel. It is now widely recognized that micro-porous coatings can enhance the local CHF limits for downward-facing boiling on the reactor vessel outer surface under IVR-ERVC conditions which in turn enhances reactor safety margins. The existing coating techniques, however, require heat treatment on the vessel wall which is not practical or feasible for commercial-size reactors. In this study, a new and versatile coating technique known as “Cold Spray” has been used to form a micro-porous coating on a hemispherical test vessel. Steady-state and quenching experiments were performed under simulated IVR-ERVC conditions using a bare vessel and a micro-porous coated vessel of identical dimensions. The downward-facing boiling and CHF phenomena observed in the steady-state and quenching experiments were analyzed qualitatively and quantitatively. Based on the results of visualizations in the quenching experiments, the coated vessel was found to quench much faster than the bare vessel under identical experimental conditions, indicating a higher downward-facing boiling heat transfer (i.e., cooling rate) for the coated vessel. It was also observed that the local rate of boiling and the local CHF limit tended to vary appreciably along the outer surface of the test vessel. Quantitative measurements showed that for all angular locations on the vessel outer surface, the local CHF values for the coated vessel were consistently higher than the corresponding CHF values for the bare vessel. More than 90% enhancement in the local CHF limit was obtained by using micro-porous coatings formed by the Cold Spray technique. Careful inspection of the test vessel confirmed that the micro-porous coating formed by the Cold Spray technique was very durable with its micro-structure unchanged after many cycles of heating and cooling. The Cold Spray process and related equipment are portable, flexible, and scalable, and hence the micro-porous coatings explored in this study can be utilized in new industrial designs, as well as existing reactors provided adequate cavity space is available below the RPV lower head. In addition to the experimental work, a stability analysis of the thin vapor film blanketing the outer surface of the vessel surrounded by ambient liquid was performed to obtain the most dangerous wavelength for breakup of the vapor film. Heat and mass transfer along with the normal and shear stresses at the interface were considered to accurately predict the stability behavior of the vapor film. A film boiling heat transfer model was developed based on the stability analysis. Furthermore, a scaling analysis was performed by utilizing the experimental data obtained in this study to develop a generalized correlation for downward-facing nucleate boiling. The correlation predicts the long-term behavior of reactor pressure vessel under IVR-ERVC conditions.