Two phase flow in vertical risers are common place in oil and gas industry and many other process industries. Depending on the flow rates of the phases, there could be several flow patterns could exist inside the riser. These could vary from bubbly flow to annular flow with slug and churn flow in between. When the liquid phase flow rate is higher the bubbly flow exists while the annular flow is dominated by higher gas flow rate that forms a distinct gas core in the middle of the vertical riser. Of these flow regimes, churn flow is of particular interest as it is not well understood. The paper will report findings of an experimental campaign investigating the development of churn flow. Experiments were carried out in a closed loop flow facility with a 127 mm ID, 11 m long vertical test section. The maximum flow rates achievable in the system were 17 and 1.2 m/s for gas and liquid phases respectively. Compressed air was used as the gas phase while water and water/glycerol mixtures were used as the liquid phase. The mixtures of water and glycerol were used to investigate the influence of the viscosity on the flow regime investigated. The flow was investigated using a Wire Mesh Sensor (WMS), an intrusive measurement device that can map the cross sectional distribution of phases. Void fraction measurements were made at several axial locations for a number of flow rate combinations from onset of churn flow until it turns into annular flow. A region in flow rates where large liquid ligaments (wisps) suspended in the gas core was found and the breakup mechanism has been observed. Furthermore, huge waves were observed in this region. Analysis of results shows that the frequency of both huge waves and the wisps entrained in the gas core increase along the axial distance. The changes to the flow behaviour with the increase of viscosity and other findings will be presented in detail in the paper.