EXPERIMENTAL AND THEORETICAL STUDY OF PARAMETRIC EFFECTS ON THE TRANSITION BOILING REGIME FOR FLOW BOILING IN A ROD BUNDLE UNDER LOSS OF COOLANT CONDITIONS

Open Access
Author:
Kumru, Hanife Tugba
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 08, 2017
Committee Members:
  • Fan-Bill Cheung , Thesis Advisor
Keywords:
  • minimum film boiling temperature
  • transition boiling regime
  • RBHT
  • spacer grid
  • heat transfer
Abstract:
Post-dryout heat transfer is an important phenomenon in the analysis of nuclear fuel rods during a hypothetical loss of coolant accident (LOCA). Heat transfer in transition boiling increases as intermittent wetting of the heated surface takes place. In film boiling, a vapor film covers the heated wall and prevents direct contact heat transfer from the liquid to the rod. The boundary between these two heat transfer regimes occurs at Tmin, the minimum film boiling temperature, sometimes called the Leidenfrost point. Understanding the sensitivity of Tmin to pressure, subcooling, peak power, surface conditions and flow rates is critical because Tmin determines when the vapor film breaks, and the heat transfer significantly improves. In this work, the minimum film boiling temperature has been determined for different ranges of pressure, subcooling, flow rate and peak power for various flooding rate tests. Pressure, liquid subcooling and flow rate were varied from 0.14 MPa to 0.42 MPa (20 to 60 psia), 5 K to 83 K (9 to 150°F), 0.076 m/s to 0.2032 m/s (3 to 8 in/s), and 1.31 kW/m to 2.3 kW/m (0.4 to 0.7 kW/ft) respectively with the peak power varying from 1.31 kW/m to 2.3 kW/m (0.4 to 0.7 kW/ft) and the initial peak rod temperature varying from 1033 K to 1144 K (1400 to 1600 °F). The experiments were performed using an electrically heated 7×7, 3.66 m (12 ft) Inconel rod bundle array with constant and variable flooding rate capabilities at the NRC-PSU Rod Bundle Heat Transfer (RBHT) facility. The latter was also equipped with seven mixing vane spacer grids with which to study the effects on Tmin. Data reduction was performed by using an inverse heat conduction code to calculate the surface temperature and heat flux. The results indicate that the system pressure, inlet subcooling, flooding rate and spacer grids have appreciable impact on the minimum film boiling temperature. The experimental results presented in this work can be used to determine the transition boiling regime in the limit of TCHF<T<Tmin.