Executive Summary : | Helical coil tubes are widely employed in nuclear reactor steam generators, heat exchangers and chemical industry applications due to its high heat transfer characteristics and compactness. The study of heat transfer characteristics in a helical coil tube is important as the curvature of the tube significantly influences the coolant phase change and the vapour volume fraction distribution. For higher curvature tubes, the vapour volume fraction in the vicinity of the heated surface reach critical value depicting the departure from nucleate boiling (DNB). Due to the poor heat transfer characteristics of accumulated vapour, DNB causes sharp rise in temperature. The DNB location is not stationary and hence the helical tube wall experiences alternate rise and drop of temperature. These temperature transients due to the occurrence of DNB cause thermal stress fluctuations which may lead to deterioration of the tube surface due to thermal fatigue. Therefore, the present study focuses on predicting the inlet conditions that causes DNB and life expectancy of the helical coil tube for given DNB temperature loads. The Eulerian two fluid model coupled with wall heat flux partition model will be assessed for identifying the suitable closure models for the prediction of DNB in helical coil tubes through validation studies against the benchmark experimental data. The simulations will be further extended to identify the operating conditions that cause DNB and the sharp rise of surface temperature in the helical tube. The DNB temperature fluctuations are supplied as an oscillating load condition to the structural solver for fatigue analysis. The fatigue life expectancy for a given temperature oscillations is predicted using the von-Mises stress criteria from the design curve. The proposed project will result in a robust Eulerian model framework for the prediction of DNB and the life expectancy of helical coil tube subjected to DNB temperature fluctuations. A design map will be developed to identify the operating conditions that cause DNB in helical coil tubes and the corresponding life expectancy. These design maps are highly valuable to the engineers or analysts for the effective design of thermal components in nuclear reactors which results in increased thermal efficiency, safety and hence meets the power needs of the country. |