The effects of transient conditions on the onset of intermittent dryout during blowdown

- This papers presents the results of an experimental investigation of transient critical heat flux in high quality and intermediate pressure water.
- In existing literature conclusions vary from those showing no effect of transient conditions to results which show 30-40% improvement in CHF.
- Along with new CHF data points in the liquid film dominated flow regime, the authors provide a methodology for producing bias free estimates of CHF based on existing correlations.
- With these bias free CHF estimates, comparisons are made between transient and steady-state CHF at comparable local conditions.
- The work concludes that based on consistently collected and analyzed data that quasi-steady CHF experiments adequately predict transient CHF using the same local thermalhydraulic conditions.

For a given set of conditions in a boiling system the point of liquid film dryout or departure from nucleate boiling corresponds to the change from convective or nucleate boiling to transition or film boiling. This change is associated with a rapid deterioration of the heat transfer coefficient and the heat flux at this transition is denoted the critical heat flux (CHF). Computer models used to predict station transients and CHF rely heavily on empirical correlations to predict the CHF. Liquid film CHF data are usually obtained using a quasi-steady method wherein the heat flux is incremented in small steps with each step being allowed to reach a new equilibrium until an abnormal temperature increase is detected on the experimental surfaces. In applying a correlation derived from steady-state experiments to transient analyses these codes implicitly assume that dryout will occur for the same local conditions during transients as during steady state conditions. There is some disagreement in literature as to the validity of this hypothesis. This paper provides new steady-state and transient experimental data for CHF in water at intermediate pressures and demonstrates that for high-quality CHF that indeed CHF occurs at comparable local conditions during transients and steady-sate.