Criteria for Onset of Flow Instability in heated vertical narrow rectangular channels at low pressure: An assessment study

The ONB-based relationships are largely conservative as expected since the ONB always precedes the OFI. The NVG criteria can provide relatively good results, but a crucial issue is related to the value of Peclet number at which the transition between the thermally and the hydro-dynamically driven bubble detachment takes place in narrow rectangular channels. In view of this, the standard Saha-Zuber correlation cannot predict OFI for Peclet numbers lower than 70,000; while the Saha-Zuber KIT correlation, whose transition Peclet number is smaller, identifies the OFI for all the experiments. The approach that makes use of a FDB correlation can capture the OFI in most of the cases, although its performance also depends on the type of correlation that is applied for the single-phase heat transfer. The Flow Instability Ratios (FIRs) like the ones developed by Whittle-Forgan or Stelling et al., are of particular interest because they only require global system parameters, and because they are shown to be a valid option for determining the flow excursion in the experiments included in this study. For instance, the Stelling FIR with the Saha-Zuber KIT correlation estimates OFI in all the tests. Finally, best-fitting procedures of the available data were also introduced in order to optimize such FIRs.