No abstract

The basic objective of the paper is to identify the desired circulation ratio for the natural circulation of water tube boilers in different operating conditions. This requires the basic study of heat flux and the mode of the boiling heat transfer, and the phenomenon like departure from nucleate boiling and tube overheating. The parameters, which need to be studied are heat flux, pressure, dryness fraction, void fraction, liquid velocity and their impact on the required circulation ratio. For a natural circulation boiler, the circulation ratio is one of the most important design parameters as the other design parameter like critical heat flux and skin temperature are mainly derived from the circulation ratio. The required circulation ratio can vary with the boiler pressure, liquid velocity and maximum heat flux. This study is intended to provide input for the safe and optimum design of a natural circulation boiler.

In this present works, a generalized approach for the two-phase flow analysis of a natural circulation in hybrid boiler. The model uses the combination of node and loop equations and the Newton Raphson technique for the solution of the set of equations. Loop equations have been developed for each evaporator tube with the unique driving force and pressure drop of the concerned loop. Node equations are mainly developed for common risers and downcomers. A unique connectivity matrix has been used to correlate each branch flow with loop and node equations. The model is validated by a unique indirect method by comparing the actual water level and calculated water level. Experiments have been performed with uniform diameter of tubes in 53 channels circulating loop to find the volume of steam. The model shows good agreement with experimental investigations with a maximum of 3.57% absolute error. The model can be used for the design, analysis, and optimization of the natural circulation network.