The objective of this study is to visualize the transient flow patterns and heat transfer behaviors at low mass fluxes and high heat fluxes. The silicon chip consists of the intercrossed microchannel array with 10 triangular microchannels with the hydraulic diameter of 155.4 mu m, and five transverse trapezoid microchannels, separating the triangular microchannels into six independent zones. The chip is horizontally positioned. Liquid acetone is used as the working fluid. Tests were performed in the range of mass flux 40-80 kg/m(2) s and heat flux 107-216 kW/m(2). It is found that all the microchannels repeat the flow patterns in the timescale of milliseconds, with three substages: liquid refilling stage, transient stratified flow stage and partial/fully dry-out stage. Stratified flow is the dominant flow due to the low liquid Froude numbers. The axial liquid film thickness for each separated microchannel zone is increased along the flow direction, caused by the large momentum force due to evaporation on the interface of the vapor and the settled liquid film, and the high vapor shear stress applied on the vapor/liquid interface. The non-uniform axial liquid film distributions cause the earlier dry-out of the triangular microchannel upstream. However, dry-out always takes place earlier in the downstream zone than that in the upstream zone. The vapor slug near the triangular microchannel exit may be entrained in the long liquid plug during the liquid refilling stage. Besides, the steep jumped liquid film thickness may take place near the channel exit for the stratified flow. Both indicate the strong geometry effects. Due to the compact size and high thermal conductivity of the silicon chip, the chip surface temperature variations versus time are identified by the infrared radiator image system, especially in the ending area of the thin film heater. (c) 2006 Elsevier Ltd. All rights reserved.