Flow boiling heat transfer characteristics of water are experimentally studied in a circular minichannel with an inner diameter of 1500 pin. The fluid flows upwards and the test section, made of the nickel alloy Inconel 600, is directly electrically heated. Thus, the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 50 and 100 kg/(m(2) s) and heat fluxes from 10 to 115 kW/m(2) at an inlet pressure of 3 bar are examined. Infrared thermography is applied to measure the outer wall temperatures of the minichannel. This experimental method permits the identification of different boiling regions, boiling mechanisms and the determination of local heat transfer coefficients. Measurements are carried out in single-phase flow, subcooled and saturated boiling regions. The experimental heat transfer coefficients in the region of saturated boiling are compared with correlations available in literature and with a physically founded model developed for convective boiling. (c) 2006 Elsevier Ltd. All rights reserved.