(97e) Optimization of Metallic Microchannel Array Evaporators

Flow boiling in microchannels and microchannel arrays is an effective
way of achieving the high heat transfer rates required in compact
evaporator designs. One objective in designing compact
evaporators is to generate dry vapor at a temperature not exceeding the
saturation temperature more than necessary, at the lowest possible
surface temperature. Water is used as a model substance in this study,
in which different evaporator designs are compared primarily with the
aid of high-speed videography in the visible wavelength range.

While the evaporation in single microchannels has been the subject of
intensive research over the last years, relatively little effort has
been expended on the situation in arrays of many (several dozen)
microchannels. For a practical application, however, pressure loss over
the device is often an important concern, and for a commercially
relevant throughput range, there is often no alternative to designs
involving such large numbers of channels. Microchannel arrays with
liquids undergoing flow boiling are well suited to the removal of heat
from devices such as semiconductor chips with high surface power
densities. For such heat removal applications, complete dryout of the
microchannels is undesirable and can be highly detrimental. For
evaporator applications, however, the situation is quite the opposite
since complete dryout of the microchannels towards their downstream ends
is what a good design should provide. This goal can always be achieved
by making the microchannels sufficiently long. The requirement of
minimized pressure loss mentioned above, however, also puts restraints
on the length for the individual microchannels in such arrays.

In this paper, several microchannel arrays with different channel
lengths and layouts are compared.
To allow for comparison between the different array designs, several
parameters were held constant. The arrays always consisted of
68 channels, each 200 μm wide and 100 μm deep
separated by 100 μm wide ridges, and were made by mechanical
micromachining of steel or brass, respectively. Array lengths varied
between 20 mm and 80 mm.

The experimental set-up consists of a steel frame with electrically
powered cartridge heaters, an exchangeable microstructure foil with the
microchannel arrays, and a window of glass or
Makrolon^™. Water was supplied by an HPLC pump at a
constant flow rate of 0.5 kg h^-1, corresponding to
a mass flux of 61 kg m^-2s^-1.The two
phase flow phenomena can be observed through the window and microscope
optics by a high-speed video camera operating at up to 1825 frames
per second at a resolution of 1024×256 pixels. Illumination
can be provided alternatively by halogen lamps with gooseneck light
guides, or by a stroboscope lamp with a flash duration of approximately
10 μs.

Microchannel array evaporators for industrial applications will have to
consist of metal, such as stainless steel. When such a device is
connected to the (inlet) tubing via metallic connectors, heat conduction
against the direction of water flow can cause boiling to start already
in the inlet plenum. Once this happens, relatively large bubbles are
formed in the inlet plenum, periodically breaking up at the microchannel
array entrance. The pulsation resulting from these events is primarily
responsible for the observed periodic breakdown of complete
evaporation. There are two possible ways of mitigating this effect:

• Minimizing the size of the inlet
  plenum, and hence the residence time there, should move the boiling front
  downstream and into the microchannel array proper.
• Alternative multistage evaporator designs should cope better with a
  feed already containing some vapor.

Both methods have been implemented in a number of foils whose
performances as evaporators are compared in this paper.