Critical heat flux for flow boiling of water at low pressure in vertical internally heated annuli

Critical heat flux for flow boiling of water at low pressures on technically smooth surface tubes was experimentally investigated. The experiments were performed in a vertical annular test section of two coaxial tubes. The inner Zircaloy-4 tube with an outer diameter of 9.5 mm was directly heated over a length of 326 mm. Outer glass tubes of 13 mm and 18 mm inner diameter formed two different annular assemblies with length to heated equivalent diameter ratios of 39.3 and 13.2. The experimental parameters of inlet subcooling enthalpy, outlet pressure and mass flux were varied in the ranges of 100-250 kJ/kg, 115-300 kPa, and 250-1000 kg/(m2 s). The resulting critical heat flux values were between 0.66 and 2.83 MW/m2 depending particularly on the mass flux conditions. The results were compared with literature measurement data as well as with prediction methods using look-up tables for critical heat flux. The length to heated equivalent diameter ratio was found to be one of the important parameters for the comparison between different measurement data. The experimental values were smaller than the calculated critical heat flux values of the predictions methods. The measurements showed better agreement with the more recent CHF look-up tables.