The purpose of this study was to investigate inlet tube spacing and protrusion effects on multiple circular tubes in the laminar; transitional and turbulent flow regimes. An experimental set-up was built for this investigation and three configurations of test sections were investigated. The first was a single-tube test section for validation purposes, of which the results were compared with literature. The second was two multi-tube test sections with three tubes spaced at different pitches. The third configuration was similar to configuration two, except that the centre tube had a small protrusion. All the tubes had an inner diameter of 3.97 mm, and long tube lengths of 6 m were used to ensure fully developed flow. The tubes were electrically heated that ensured a constant heat flux heating condition. Water was used as the test fluid, and the Prandtl number varied between 3 and 7. The experiments were conducted at heat fluxes of 2, 3 and 4 kW/m(2) for Reynolds numbers between 1000 and 7000, to ensure that the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes, were covered. The tubes were spaced apart from each other at 1.25, 1.4 and 1.5 times the outer tube diameter, and the protrusion of the centre tube was 10% of the tube inner diameter. It was found that an increased pitch ratio dampened the inlet disturbances in the centre tube and reduced the flow asymmetry in the side tubes, therefore the differences in the critical Reynolds numbers and transition gradients of the three tubes decreased. As the inlet disturbances were damped in the centre tube, transitional was delayed compared to a single tube with a square-edged inlet. For the side tubes, the increased flow asymmetry led to increased critical Reynolds numbers, as well as increased transition gradients. The presence of a protrusion inlet in the centre tube significantly increased the asymmetry of the flow in the side tubes, which led to an additional increase in the critical Reynolds numbers and the transition gradients increased. Free convection effects also led to increased critical Reynolds numbers and transition gradients, as well as decreased differences between the results of the tubes in the multi-tube set-up when a square-edged inlet was used. However, free convection effects were not able to dampen the inlet disturbances caused by a protrusion inlet in the centre tube. (C) 2017 Elsevier Ltd. All rights reserved.