Flow Instability and Critical Heat Flux for Flow Boiling of Water in a Vertical Annulus at Low Pressure

We investigated the critical heat flux (CHF) for flow boiling of water in a vertical annulus. The coaxial annulus has a diameter ratio of 1.37 and the inner zircaloy tube is heated directly over a length of 325 mm. CHF can occur prematurely due to flow instabilities. Therefore, we analyzed the flow stability at different heat input conditions using two types of pumps, a rotary and a gear type pump. The unstable CHF occurred at 61% and 90% of the stable value for the rotary and the gear type pump, respectively. Consequently, the following CHF experiments were conducted at stable flow conditions. The outlet pressure was constant at 120 kPa, the mass flux varied from 250 to 1000 kg/(m²s) and the inlet subcooling was at 102, 167, and 250 kJ/kg. The CHF results increase with mass flux from 0.67 to 2.62 MW/m² and show similar trends compared to literature data. However, the experimental data for flow boiling in annuli at low pressure are limited. Additionally, we measured the dynamic contact angle between the zircaloy tube surface and water using the Wilhelmy method.