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Multiphase flow simulations were run in an eccentric annulus. The dimensions of the annulus were 0.1 and 0.05 m for the outer and inner cylinders, respectively, and the mixture velocities were varied between 1.2 and 4.2 m/s. The simulations were compared with fully eccentric and completely concentric experiments conducted at the Institute for Energy Technology in Norway. The purpose of this paper is to explore the effect of the holdup fraction and interior pipe's position on the pressure gradient and flow regime. The comparisons indicate that moving the pipe from an entirely eccentric to partially eccentric configuration has a drastic impact on the pressure gradient. In all cases where the inner pipe was changed from a completely eccentric geometry to a less eccentric configuration, we notice an increase of 48-303 % of the mean pressure gradient. Comparatively, the cases where the pipe was moved from a concentric to a more eccentric configuration result in less drastic pressure gradient changes. 2 cases were within 22 % of the experimental results for mean, maximum, and minimum pressure gradient, while the last two cases exceeded the minimum and mean pressure gradients by 25-250 %, respectively. We rarely observed a change of flow regime as an effect of moving the inner pipe; 2 out of the 8 horizontal cases indicate either a transition from wavy flow to slug flow or significantly larger waves. The most prominent and frequent discrepancies identified were altered slug and wave frequencies. Through the simulations, we notice that there is an increased pressure gradient accompanying an increased holdup fraction when the phase-averaged velocities were the same. Corresponding to a fractional holdup increase of 0.177, 0.244, 0.063, and 0.073, the increase in simulated pressure gradient for each case of the same mixture flow rate and mesh density was 80, 300, 614 and 367 Pa/m respectively or 116, 244, 61.5 and 25 %.

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