We experimentally investigate vertical upward oil-water two-phase flow in a 20 mm inner diameter pipe. We first using vertical multiple electrode array conductance sensor measure the water holdup, and using mini-conductance probes define five observed flow patterns, i.e., very fine dispersed oil-in-water (VFD O/W) flow, dispersed oil-in-water (D O/W) flow, oil-in-water slug (D OS/W) flow, water-in-oil (D W/O) and transition flow (IF). Then we present an experimental flow pattern map with oil and water superficial velocity ranging from 0.258 m/s to 3.684 m/s and 0.184 m/s to 1.474 m/s, respectively. In addition, we obtain the flow pattern transition boundaries in terms of water holdup. Finally, we propose an effective quadric time-frequency representation, i.e., the adaptive optimal kernel time-frequency representation (AOK TFR) to investigate the complex behavior underlying vertical upward oil-water flow. In particular, we extract total energy and time-frequency entropy to characterize the evolutions of flow patterns. The results suggest that AOK TFR based method could potentially be a powerful tool for characterizing the dynamical characteristics of different vertical upward water-dominant oil-water flow patterns. (C) 2012 Elsevier Ltd. All rights reserved.