Experimental validation of theoretical models in two-phase high-viscosity ratio liquid-liquid flows in horizontal and slightly inclined pipes”

Liquid–liquid flow literature proposes models developed to predict quantities and phenomena of interest,
once given fluid properties and the features of the flow domain. The validity of any model should be verified
through experimental observations, being this practice an effective way to evaluate the model conditions
of applicability and possible limitations. Despite the fact that several works have already been
proposed on the validation of theoretical models, most of them concern liquids characterised by low viscosity
ratio ~l, while in industrial realities (such as petroleum or food ones) the liquids involved are often
characterised by high viscosity ratios. The extension of low-~l results to high-~l flows is not straightforward,
so that it is necessary to validate the models for the latter case specifically. This work presents
experimental pressure drops and flow-pattern maps associated to the flow of oil and water in horizontal
and slightly inclined pipe, where the chosen liquids are characterised by an oil-to-water viscosity ratio of
about 800:1 at 20°C. Various theoretical models have been considered, with particular attention to coreannular
flow two-fluid model and oil-in-water dispersion homogeneous no-slip model for the prediction
of associated pressure drops, and flow-pattern map transition criteria involving the regimes encountered
in the experimental tests. The theoretical predictions have been then compared to the experimental
results. A satisfactory agreement has been found especially as concerns pressure drop comparisons. As
regards the predicted transition boundaries superimposed on the corresponding flow-pattern maps,
the ‘free’ parameters have been fitted on the basis of experimental results and observations, and the final
agreement is good in the prediction of both the core-annular flow region of existence and the transition
to oil-in-water dispersion. No conclusion can be expressed on transition criteria involving stratified flow,
which only seldom has been observed in the performed experiments.