High-Speed photographic observation of flow boiling of water in parallel mini-channels

Abstract

The use of smaller passage dimensions is becoming more prevalent in flow boiling applications. Passages with hydraulic diameters on the order of 1mm provide higher heat transfer coefficients resulting in more compact heat exchanger arrangements. Passages with 1mm hydraulic diameters do not pose severe clogging or blocking constraints as opposed to micro-channels and have a less pronounced pressure drop penalty. The present paper explores the flow regimes during flow boiling of water in mini-channels. High-speed photography is used to obtain visual pictures of the flow phenomena from subcooled flow boiling all the way to critical heat flux conditions. The tests include one single-channel and a set of six parallel channels with electric heating from three sides of the channel. The top cover is made of Lexan to permit visual observations. The results are used to identify specific features of flow boiling in smaller diameter mini-channels.