Investigation of two-phase pressure drop across a spacer grid in the dispersed flow film boiling regime

The pressure drop associated with two-phase flow through a spacer grid under the dispersed flow film boiling (DFFB) regime is studied, using experimental results obtained under constant reflood conditions in a 7 × 7 rod bundle. The DFFB regime is characterized by dispersed liquid droplets entrained by superheated vapor, which is the continuous phase. In DFFB, the two phases are known to be in thermal-hydraulic non-equilibrium state. Therefore, the corresponding pressure drop behavior across spacer grid is expected to be different from other flow and heat transfer regimes and needs to be investigated separately. A series of reflood tests were performed in the Rod Bundle Heat Transfer (RBHT) Facility at various system pressures, degrees of inlet subcooling, flooding rates, and rod bundle power inputs. Effects of these parameters on pressure drop across spacer grid (SG) are studied in detail. At the SG of interest, the mass flow quality varies within 0.2 < x < 0.9 while the Reynolds number varies within 1400 < Re < 10000. The most pronounced effect on SG pressure drop during reflood has been adequately identified as the inlet flooding rate. The system pressure is also found to affect the SG pressure drop behavior. It is observed that the inlet subcooling and rod bundle peak power have a relatively insignificant effect on the grid loss coefficient. In addition, based on different slip ratios upon determining the two-phase flow void fraction from quality, effects of the two-phase thermal-hydraulic non-equilibrium on the SG pressure drop is discussed. The current experimental results have also been compared with the single-phase grid loss data as well as with available experimental results of post-CHF two-phase flow SG pressure drop. Relatively good agreement is observed from the comparison.