Mechanistic modelling of water–oxygen bubbly flow in horizontal pipes: Deviation analysis from experimental correlations and performance comparison with CFD

Two correlations have been considered in the literature (Martin et al., 1971 and Kim et al., 2001) for the computation of heat transfer in liquid–gas bubbly flow in horizontal pipes. Motivated by the benefits of these two correlations, we investigate the horizontal two-phase bubbly flow in a double-pipe heat exchanger with inner diameter of d = 21 . 6 mm and length L = 16 ⋅ d using the mechanistic one-dimensional cell model. The results of the model have been compared with the correlations to assess the two approaches considered. Firstly, the first approach was assumed, whereby the properties of water–oxygen mixture can be considered as those of a single pseudo-fluid. This was achieved by appropriately averaging the thermodynamic properties of the two involved phases. Secondly, the second approach was considered, whereby the properties of the liquid phase were used directly. This was done after it was realized that it had not been commonly addressed in the literature. The results demonstrate that this approach yields satisfactory heat transfer coefficients in the case of small gas fractions (up to 6%). Secondly, a similar study utilizing the computational fluid dynamics (CFD) method based on the discrete phase method (DPM) is conducted, which has not been addressed in previous works for the heat transfer investigation. The findings indicate that the CFD method successfully recovers a highly accurate heat transfer estimation, with a slight improvement in heat transfer (up to 5%) with increasing gas fraction. The mechanistic model, being numerically inexpensive compared to CFD, can predict heat transfer and pressure drop with good agreement, proving its competitiveness. The results presented here are useful for understanding and optimizing heat loss in the cooling circuit of PEM electrolysis.