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July 2005

High Heat Flux Removal by Microjet Impingement


this is the second in a series of articles reporting on a number of micro-fabricated devices for handling high heat fluxes.   this article series will cover:

  • microjet impingement cooling with liquids
  • cooling loops with liquid microjets
  • microjet impingement cooling with air
  • microfabrication techniques for creating micro-structured thermo-fluid devices
  • ultra-compact microchannel liquid-gas heat exchangers for high heat flux cooling loops

in the first article we gave a general overview of a number of micro-fabricated devices for high heat flux systems.  in this article we focus on impingement cooling with liquid microjets.

the devices and systems described in these articles are past the prototype and testing stage and are currently being deployed in commercial and military applications.  if you want white papers covering the above topics or would like to discuss ways these techniques can be adapted to specific applications, please contact us at [email protected].

background

in single phase heat transfer, impingement cooling is known to provide very high heat transfer coefficients.  these high heat transfer coefficients are achieved over an area under the jet, the so-called stagnation zone, where the impinging action of the jet onto the target surface results in a very thin boundary layer. the stagnation zone is, roughly, the same size as the impinging jet.  outside the stagnation zone, however, the heat transfer coefficient decays as the fluid flows over the target and the boundary layer thickness increases. 

there are a large number of correlations for jet-impingement heat transfer reported in the literature.  however, in general the following scaling relationship holds for stagnation zone heat transfer in laminar flows: nu ~ re0.5 pr 0..3, with the jet diameter serving as the length scale.  thus the heat transfer coefficient scales as:

 hj ~ ( vj /dj )1/2

where vj is the jet velocity and dj the jet diameter.  the inverse relationship between heat transfer coefficient and jet diameter is the motivation for going to small diameter jets.    

although the heat transfer coefficient increases with decreasing jet diameter, the stagnation zone size decreases.  thus, one needs to closely pack together a large number of microjets if the benefit of their small diameter is to be obtained over a large area.  this introduces a major problem which, until recently, had blocked the use of large area microjet arrays.  the problem is generically referred to as cross-flow.  that is, after one jet has impinged on the target the fluid will have to travel parallel to the target, and perpendicular to the other impinging jets, to be collected at the periphery of the target area.  in a large array consisting of hundreds of microjets per square cm, the cross flow of the central jets is so strong that the flow from the outer-lying jets gets disrupted before impacting the target.  in effect, the central jets wash out the impinging effect of the other jets.  another problem is the maldistribution of flow in-between the jets.  the central jets face a higher back pressure than the outer-lying ones, and therefore, if all jets are fed from the same header, the central jets get a lower flow rate than the outer-lying jets. 

the micro jet cooling array (mjca) solution

mjca is a micro-fabricated three-dimensional structure that not only allows one to go to jet diameters as low as 200-300 microns,  but also allows one to locally exhaust the fluid after impingement and eliminates the above mentioned jet-to-jet interactions.  mjcas are produced by the liga technique by international mezzo technologies.

mjca is a honeycomb structure which contains an array of small diameter channels (e.g. 300 microns).  the fluid is forced through these channels and leaves the mjca as an array of closely packed jets (e.g. 200 jets/sq. cm).  on the back side of the honeycomb, each jet conduit outlet is surrounded by a number of return (exhaust) ports.  these return ports are connected to the interior volume of the honeycomb.  thus, after the fluid from a jet has impinged onto the target, it flows back towards the mjca, and is conducted into the honeycomb.  in this manner, each jet is effectively isolated from the neighboring ones, and the cooling effect of a single microjet is replicated over a large area.  figure 1(a)  shows the side view of mjca where the jet conduits and the exhaust ports in the lower plate are clearly visible.  figure 1(b) shows the lower surface of the mjca with the microjet ports (larger diameter holes) surrounded by the return ports.  figure 1(c) shows a typical 1cm x 1cm mjca.


figure 1(a) side view of the fabricated mjca.  the conduits connecting the top and lower surfaces are the jet inflow conduits.  the holes in the lower surface are the exhaust ports.

figure 1(b) bottom view of the mjca with the jet impingement holes (large holes) surrounded by the exhaust ports (small holes).

figure 1(c)fabricated 1x1 sq. cm mjca

the performance of mjca

the performance of mjca has been exhaustively tested, using a high heat flux system capable of delivering heat fluxes in excess of 1.5 kw/cm2.   test results for a 300 micron diameter mjca are shown in figure 2 where we plot the heat transfer coefficient versus flow rate per unit of target area.  two features stand out.  the first is that very high heat transfer coefficients are obtained at moderate flow rates, and as predicted by the scaling laws  hj~vj0.5 .  the high heat transfer coefficients demonstrated by mjca are the values required for handling heat fluxes in the range of 1kw/cm2.

the second outstanding feature is that the jet-target separation gap plays an important role; indeed one can get a 50% increase in the value of hj through judicious selection of the jet-target distance.   our experimental results (for two microjet sizes) indicate that the optimum dimension of the jet-target gap is the jet diameter.


figure 2.  experimental results on performance of mjca

enhanced target surface

the experimental results given in figure 2 are for a smooth target surface.  we have also conducted experiments with a modified target surface to increase the area available for heat transfer, in principle similar to standard “finning” of the target.  when dealing with microjets with diameters in the range of a few hundreds of microns, enhancing the target surface is a non-trivial task.  in figure 3 we compare the experimental results of the first modified target surface we have tried with the smooth target results, and significant increases in the overall heat transfer coefficient are seen.  at a constant flow rate, the heat transfer coefficient is increased by as much as 90%.  or alternatively, the same heat transfer coefficient is obtained with 60% less coolant flow rate.  our analysis indicates that even higher performance can be achieved by optimizing the geometric features of the enhanced target surface, as is shown by the blue line in the figure.

figure 3.  the effect of using finned target surface on heat transfer coefficient.

mjca-based packaging solutions

mjca has a very small form factor.  the array itself can be less than 1 mm thick, and it can be placed as close as 300 microns from the target surface.  thus, it can be packaged to meet a wide range of geometric constraints.  further, mjca as well as its packaging can be designed to meet a wide range of flow rate and pressure drop constraints.  in the next article we will report on cooling loops with mjca cold plates.  we will show (a) how a single mjca can be packaged to serve as a stand-alone cold plate to handle high heat fluxes, and (b) how a number of arrays can be integrated into a single cold plate allowing for the removal of high heat fluxes from a number of sources.  we will also discuss the theoretical framework for designing cooling loops with mjca. 

to contact the author send an email to [email protected]

 



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