الفهرس 
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Abstract
Flow instabilities are most common operating problems in two-phase flow systems such as conventional boilers, steam generators and nuclear reactors. They are undesirable since they can degrade the system performance and cause serious control problems.
In this study, a theoretical procedure for predicting the onset of flow excursion instability in downward and upward flows, through a vertical heated channel of a specified MTR-Reactor at low-pressure and low-flow conditions was presented. A detailed analysis of the pressure DROP components for a downward and upward flow in the heated channel was carried out to study the possibility of unstable transition from single-phase flow to high-quality two-phase flow, i.e. flow excursion. Both the developed model and RELAP5/MOD3.2 code were used to study the effects of different parameters like reactor power, inlet subcooling and flow rate on the onset of flow instability. It was found that low flow rate and high subcooling are the two important conditions for the occurrence of this type of instability.
Also the present study introduces an experimental investigation, to predict the occurrence of flow instability and to determine the effects of operating parameters on its onset, was conducted. This investigation was applied on a system of parallel channels to guarantee a constant pressure head for all channels. The geometry investigated was rectangular channels of 800 mm heated length, and a 70 mm by 3 mm cross section. Subcooled water was used as the coolant, with inlet temperature ranging from 30 oC to 40 oC for the OFI experiments, and the heat flux ranged from 8.9 to 26.8 kW/m2.
It was found that, keeping all the operating parameters constant, OFI occurred at higher mass fluxes as the surface heat flux applied to the test section was increased. Comparisons were performed between experimental results and RELAP5/MOD3.2 theoretical results for the test section with the same operational conditions. The deviations at the OFI point ranged between 27.4 and 46.8 Pa for the channel pressure DROP. These errors are acceptable since the pressure DROP measurement errors are around 0.2 mbar, i.e; 20.6 Pa. For the mass flux, the maximum deviation is about 30%. So, it was concluded that acceptable agreement, between RELAP5 results and the experimental results, was achieved.