Flow Boiling Heat Transfer

• Boiling observed during flow of water is heated tube is called Flow Boiling.
• Typical Flow patterns observed when boiling occurs inside tubes subjected to constant heat flux are shown below.

• At the begining single-phase flow occurs.
• As bubble tries to nucleate, it condenses when released.
• This is called subcooled boiling.
• As the liquid temperature increases to the saturated value, bubbles move in the bulk liquid.
• This region is called saturated boiling region.
• The flow pattern changes from bubbly to slug flow.
• Slug flow implies that the bubbles are of the size of the radius of the tube.
• As the bubble become large, the liquid film becomes thin and nucleation is no longer seen.
• As the vapour core region becomes large and the film small, the flow pattern is called annular flow.
• As the vapour velocity increases, it creates waves on the liquid film and tears away drops. This is called entrainment.
• Gradually the film dries off and only entrained droplets are present.
• Finally, the drops also evaporate leading to single-phase vapour flow.
• At the dryout point, the heat transfer coefficient suddenly deteiorates and a large increase in wall temperature is seen.
• The methods used to compute the heat transfer coefficient are complex and hence not treated in this first level course.