Numerical studies on the hydrodynamic and heat transfer characteristics of two-phase flows in small
tubes and channels are reviewed. These flows are non-boiling gas–liquid and liquid–liquid slug flows.
The review begins with some general notes and important details of numerical simulation setups. The
review is then categorized into two groups of studies: circular and non-circular channels. Different
aspects such as slug formation, slug shape, flow pattern, pressure drop and heat transfer are of interest.
The primary purpose of the present review is to report the existing numerical studies in order to highlight
the research gaps and suggestions for the future numerical simulations. Judging and/or comparing the
different works and correlations can be conducted separately for each of the sections reported here.
According to this review, there are some large gaps in the research literature, including pressure drop
and heat transfer in liquid–liquid slug flows. Gaps in research are also found in applications of non-circular
ducts, pressure drop and heat transfer in meandering microtubes and microchannels for both gas–
liquid and liquid–liquid two-phase flows.