Thesis (D.Eng.)--Chulalongkorn University, 2002

The instability of the two-phase flow natural circulation due to the effect of heat flux, pressure, and subcooling in the proposed channel configuration was investigated. A 1 meter long non-heated riser above the heated section was proposed in this experiment. The heat flux was increased from 50 to 550 kW/m2. The pressure was varied from 0.1, 0.2, 0.4, 0.5, and 0.7 MPaA and the subcooling was varied from 5, 10, and 15 K. The numerical model was developed by using two-fluid model written in FORTRAN programming language to investigate the effect of channel diameter, system pressure and subcooled condition on stabitlities of two-phase natural circulation, which is caused by boiling. The semi-implicit scheme was utilized for finite difference equations. Newton Block Gauss Seidel (NBGS) method was employed to solve the system equations for unknown variables. The time step was fixed at all time at 1 millisecond and the mesh size was 100 mm. The results from the two-fluid model were compared with the experimental results. The effect of channel diameter was investigated by using the two-fluid model. The channel diameter was varied from 18, 20, 22, and 32 mm. It is confirmed that the increase in system pressure and subcooling stabilize the system. In addition the two-fluid model can give the good results and are in good agreement with experimental results. The prediction of the effect of channel diameter on the flow oscillation from the two-fluid model indicated that the change in the channel diameter affected the flow instability pattern at the heat flux higher than 300 kW/m2. As the frictional pressure drop was dominant at the high heat flux, the flow instability map changed from natural circulation to density wave oscillation. The new discovery received from the experiment indicated that the geysering can occur at the system pressure higher than 0.35 MPaA and at the velocity higher than 0.2 m/s due to the effect of channel geometry.