Surface orientation effects on bubble behaviors and critical heat flux mechanism in saturated water pool

•CHF (Critical Heat Flux) decreases with the heated surface orientation.•Vapor behaviors prior to CHF can be characterized into three regions.•Bubbles generated from the vertical surface were drifted upward freely.•Bubbles generated from the heater facing downward were forced upward.•Bubbles generated from the horizontal facing downward were remained on the heated surface. A visualization study on boiling phenomena at various surface orientations was conducted to investigate the bubble behaviors and the critical heat flux (CHF) mechanism. The vapor behavior observed in this study indicates that the pool boiling can be categorized into three regions: upward facing (0° ≤ θ