This  article numerically investigates the flow boiling behavior in an  intermediate-scale vertical tube for an evaporator of a separated type  heat pipe. In this study, the volume of fluid (VOF) model and the  vapor-liquid phase change model (according to a user defined function)  are used to build a numerical model of a single vertical tube with a  50 mm inner diameter and 5000 mm length for simulating the flow regime.  Different two-phase flow characteristics are present in the vertical  tube working in low heat flux. We analyze the bubble movement and  classify the two-phase flow regimes in an intermediate-scale vertical  tube into four patterns: bubbly flow, cap flow, slug flow and churn  flow. Furthermore, the local characteristic of boiling heat transfer  along the tube and the effect of the bubble regimes on the heat transfer  are studied. The results indicate that saturated nucleate boiling is  the dominant boiling form under low heat flux.