Abstract
This paper provides investigation of the dynamic effects of rapid changes in fuel flow rate and steam demand on the thermal and flow characteristics of the riser tubes in natural circulation water tube boilers. A nonlinear state space dynamic model which captures the important physical interactions of the main variables of steam generation in drum boilers was developed. The system under consideration includes the drum, the riser and downcomer as its major components. Also, a thermal model for the prediction of possible tube overheating was devised. A numerical scheme for the solution of the governing differential equations was established. The dynamic response of the system's state variables due to rapid changes in fuel and steam flow rates was investigated. The results show that the sudden rise in the heat flux is followed by an increase in the steam quality which is accompanied by a decrease in the circulation rate as a result of increase in the pressure. The riser temperature increases due to the increase in the steam temperature and due to the dynamic influence resulting from increase in the heat flux. The present calculations of the water level in the drum provide good comparison with those in the literature.