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Transferts de Chaleur et de Masse

Phase Change and Interfaces

Flow boiling in Microchannels

Contact: David Brutin

 

Recent publications

  • Barber J., Brutin D., Sefiane K., Tadrist L., Bubble confinement during flow boiling of FC-72 in a single rectangular microchannel of high aspect ratio, Experimental Thermal and Fluid Science, Vol. 34, (8), pp. 1375-1388, 2010.
  •  Luciani S., Brutin D., Le Niliot C., Tadrist L., Rahli O., Boiling Heat Transfer in a Vertical Microchannel: Local Estimation During Flow Boiling with a Non-Intrusive Method, Multiphase Science and Technology, Vol. 21, No. 3, 2009.

  • Barber J., Sefiane K., Brutin D., Tadrist L., Hydrodynamics, heat transfer and infrared measurements during flow boiling instabilities in a single microchannel, Applied Thermal Engineering, Vol. 29, (7), pp. 1299-1308, 2009.

  • Luciani S., Brutin D., Rahli O., Le Niliot C., Tadrist L., Flow Boiling In Minichannels Under Normal, Hyper and Microgravity: Local Heat Transfer Analysis Using Inverse Methods, Journal of Heat Transfer, Vol. 130, No. 10, 2008.

  •  Brutin D., Flow Boiling Instability, Encyclopedia of Micro- and Nano-Fluidics, Springer, ISBN: 978-0-387-49000-7, pp. 687-695, 2008.

Flow Boiling in Minichannels (S. Luciani Ph.D Thesis)

 

Flow boiling is an efficient heat transfer mechanism. The literature on flow boiling in mini and microchannel is presently growing due to industrial needs in miniaturization and increased use of micro heat exchangers for heat transfer. Few studies have been performed to analyze the influence of the gravity level on flow boiling in channels of several millimeters. For future space applications, the influence of gravity level on flow boiling pressure drop, heat transfer and flow stability are required.

The experiences are performed in the frame of the MAP Boiling project founded by ESA. The objective is to provide a better understanding of the influence of gravity on flow boiling. For this purpose, we designed an experimental setup to observe and take data measurements when flow boiling occurs in mini and microchannels during hyper and microgravity. Several techniques to achieve our goal are used such as: fast flow visualization, pressure and temperature measurements. The two-phase frictional pressure drop increase with the gravity is related to the flow patterns modifications using the flow visualization.

The visualization channel is used here to analyze the two-phase flow structures. We compare here two pictures took from the fast video recording at a given heat flux and mass flow rate. Differences mainly on the bubble size are encountered. The exit vapour quality in both cases is 0.2. The hypergravity frame at the bottom and the middle of the channel shows a lot of small bubbles, whereas for the microgravity frame big slugs are observed. The bubble sizes observed during the hypergravity period can be explained by the force balance on the bubble. The bubble departure from the nucleation sites is also influenced by gravity. The heating surface is vertical; the buoyancy force tends to detach the bubble from its nucleation site whereas the capillary force tends to maintain the bubble shape.
 


Two-phase flow patterns in the 0.84 mm diameter channel during hyper and microgravity (Qw = 32 kW/m2, Qm = 95 kg.s-1.m-2)

 

The two forces (buoyancy and capillary) are balanced in a non-dimensional number: the Bond number which compares the gravitational force to the surface tension force. For both cases, the forced convection due to the constant mass flow rate injected, will detach the bubble. However without gravity, a bubble can grow on the vertical surface and become larger than compared to the situation with gravity. Also, an increase in the number of bubbles induces greater liquid-vapour frictional pressure loss. These observations are proposed to explain the frictional pressure loss increase with the gravity level.

Flow boiling instabilities in microchannels (J. Barber Ph.D Thesis)

A powerful tool in achieving better comprehension of the mechanisms is detailed imaging and analysis of the two phase flow at a fundamental level. Boiling is induced in a single microchannel geometry (hydraulic diameter 727 µm), using a refrigerant FC-72, to investigate the effect of channel confinement on bubble growth. A transparent, metallic, conductive deposit has been developed on the exterior of the rectangular microchannel, allowing simultaneous uniform heating and visualisation to be achieved. In conjunction with obtaining high-speed images and videos, sensitive pressure sensors are used to record the pressure drop across the microchannel over time. Bubble nucleation, growth and oalescence, as well as periodic slug flow, are observed in the microchannel test section. Phenomena such as periodic pressure fluctuations caused by bubble dynamics and vapour blockage, and the variation of the aspect ratio and Reynolds number of a vapour bubble over time, are all observed and analysed. From analysis of our results, images and video sequences with the corresponding physical data obtained, it is possible to follow visually the nucleation and subsequent both ‘free’ and ‘confined’ growth of a vapour bubble during flow boiling of FC-72 in a microchannel.


FC-72 bubble growth in a microchannel

 



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