Boiling Enhancement/Micro-Porous Coatings

Boiling offers an attractive way to effectively cool high power electronic devices and maintain a uniform temperature distribution in applications requiring a high degree of isothermality such as lasers.   As such, pool (immersion) and two-phase flow boiling systems have been developed using refrigerants as the working fluid.  Some refrigerants, however, are not ideal for boiling due to their high wettability, low contact angle, and low specific heat.  To address this issue, attention has focused on enhanced boiling heat transfer by altering the microstructure of the surface.  This is known to promote nucleate boiling by increasing nucleation site density.  In addition, the critical heat flux (CHF) – the maximum heat flux beyond which dry-out occurs – can also be increased with the use of advanced coatings and surface modification.

Benefits:  At Advanced Cooling Technologies, Inc. (ACT), micro porous sintered metal powder coatings have been developed and tested.  Specifically, compared to otherwise identical uncoated surfaces, experiments at ACT demonstrate that the use of micro-porous coatings can:

  • Increase heat transfer coefficients up to 2.5 times
  • Increase CHF by 1.5 – 2 times
  • Substantially improve thermal stability (suppress boiling instabilities)

A representative coated mini-channel heat sink is shown in Figure 1.  In this example, copper powder is sintered to form a micro-porous coating having a thickness of 35-70 microns.  An SEM image of the surface of the coating is shown in Figure 2.   Note that the coating is much thinner than the hydraulic diameter of the channels (1.8mm) for this case.

Figure 9.  Microporous metallic coating on minichannel heat sink

Figure 1. Microporous metallic coating on minichannel heat sink

Figure 10.  SEM image of the microporous coating shown in Figure 9.

Figure 2. SEM image of the microporous coating shown in Figure 1.

Micro-porous coated heat sinks have been tested in pumped two-phase cooling loops and results for quantitatively compared to uncoated heat sinks under otherwise identical conditions.  Representative results are reported below.

Micro-porous Coatings Increase Heat Transfer Coefficients and Extend CHF: The heat transfer coefficient versus heat flux for a fixed coolant flowrate in a mini-channel heat sink is shown in Figure 3.  Results of a bare copper heat sink and a coated (sintered copper powder – mesh 200-230) are compared. As shown, the heat transfer coefficient depends on the input heat flux and can be increased by a factor of 2 with the application of the coating. Also note that the coating extends the critical heat flux yet has a negligible effect on the pressure drop across the heat sink since the coating thickness is very think compared to the hydraulic diameter.

Figure 11.  Microporous coating shown in increase heat transfer coefficient and extend CHF compared to uncoated mini-channel heat sinks

Figure 3. Microporous coating shown in increase heat transfer coefficient and extend CHF compared to uncoated mini-channel heat sinks

Microporous Coatings Improve Thermal and Flow Stability: In addition to improving thermal performance of the heat sink, the use of microporous coatings can also suppress instabilities in two-phase cooling systems by enhancing the boiling process.  To illustrate improved stability with coatings, two videos are shown below focused on two mini-channel heat sinks in a pumped two-phase test loop.  One heat sink is coated and one is not, yet otherwise identical.  Clearly, the flow and thermal instabilities are suppressed in the coated heat sink.

Figure 4 – Video 1. Flow oscillates wildly in an uncoated heat sink, with vapor flowing in the reverse direction.

Figure 5 – Video 2. The flow is stabilized with a porous coating, minimizing pressure and temperature spikes.

 

If you are interested in learning more, please contact ACT today.