Heat flux partition based on onset of significant void

Abstract

The thermal log-law ${\Theta }_{+}\left(y_{+}\right)=\beta +2.12log\left(y_{+}\right)$ is valid in flow boiling with a value of $\beta$ that evolves as the flow develops. Using a multiphase flow cross-literature database, this constant is shown to be ${\beta }_{OSV}=-7$ at the point of onset of significant void (OSV). This means that at the OSV the liquid is at saturation temperature up to $y_{+}\simeq 30$. The OSV predictions using this model have a similar mean average error as the Saha and Zuber 1974 correlation for one less fitted constant for channel, pipe and annular flows for pressure from 1 to 147 bar and Peclet numbers from $3.5\cdot 10^3$ to $4\cdot 10^5$. This model is used to build a heat flux partitioning (HFP) inspired from system-scale codes (Lahey 1978). It predicts the distribution of the heat flux between the liquid phase and the evaporation term when the total heat flux is known. It does not give information on the total heat flux as a function of wall temperature and cannot be used to draw a boiling curve. In imposed flux conditions, this partition provides more coherent flux distribution between the evaporation and liquid terms than Kurul and Podowski (1990) based approaches and improves void fraction predictions in high-subcooling regions on the DEBORA database (Garnier et al. 2001) and on experiments by Bartolomei and Chanturiya (1967) and Bartolomei et al. (1982). When the wall temperature is imposed, it must be coupled with an empirical boiling total heat flux correlation to replace a traditional HFP. The prediction of the total heat flux is then as good as that of the correlation.