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

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. NOMENCLATURE Ac: Cross-Section area G: Mass flux of water through test section ( ) c A m m : Mass flow rate of water through test section (kg/s) Q”: Heat flux through channel walls to water (kW/m) TA: bulk temperature at the test section inlet (°C) TB: bulk temperature at the test section outlet (°C) TS: Average surface temperature of the test section (°C) x: vapor mass fraction at outlet ( ) fluid vapor m m INTRODUCTION The need to increase heat transfer coefficients on the evaporating liquid side in an evaporator is becoming increasingly important in many applications including automotive air conditioning, heat pipes, direct refrigeration cooling of electronic devices, and fuel cells. Although evaporation in small diameter channels received considerable attention in the 1960’s (for example, Bergles, 1964), its use in a compact evaporator configuration with multiple channels is currently receiving wide attention. The complexities associated with evaporation in multiple channel passages are not clearly understood. The present study focuses on providing an insight into the two-phase flow characteristics during evaporation of water in 1-mm hydraulic diameter, multiple channel, electrically heated evaporator section. LITERATURE REVIEW There are very few publications available in literature addressing the flow patterns in multi-channel evaporators with small diameter channels. Conventional compact evaporators are plate-fin type, with the evaporating liquid flowing between two parallel plates that have uniformly spaced bumps for brazing the plates together. These obstructions provide a heat transfer enhancement on the evaporation side. However, the evaporating liquid is free to flow across the plate width. The use of small diameter channels, each of 1-mm or less hydraulic diameter presents quite a different scenario. Here the pressure drop between the two manifolds is quite high and the evaporating liquid cannot flow across into another flow channel as in the case of a plate-fin type evaporator.