Energy dissipation by heat loss is a typical design concern when increasing power output and efficiency of existing power systems on the micro electro-mechanical systems (MEMS) and nano electro-mechanical systems (NEMS) scale. Current methods to reduce heat loss in power electronics include implementation of working fluids and nanofluids in active cooling systems. Devices such as microchannel heat sinks (MCHSs) were developed to mitigate this common occurrence. These fluids serve as an additional thermal barrier via convection heat transfer. In addition to heat loss, the pressure drop along a microchannel has been noted as a significant limiting factor of heat sink performance. The pressure loss across microchannels is significantly high for this particular length scale. Conventional solutions to heat dissipation exist but are limited to a fixed cooling fin area for passive systems or supplying additional pumping power for active systems.

This study presents the use of hydrophobic nano structures as a solution to the pressure loss of a MCHS. The presence of nanostructures in microchannel geometry can enable a more favorable fluid flow for the case of MCHSs. These structures have been shown to have a direct impact on surface wettability in both static and dynamic applications. The impact of hydrophobic wall nanostructures is more than inducing slip; they can remove the no slip boundary condition typically associated convective heat transfer and internal flow. These nanostructures help experimenters characterize the drag reduction effect (*f*Re) and characteristic slip length(λ). These parameters characterize the effect of drag along the surface.

Numerical and experimental studies have shown a direct correlation between channel geometry and flow parameters. Optimization of several design variables and the introduction of hydrophobic nanostructures along the inner channel walls have been shown to significantly reduce the friction factor. These techniques reduce the heat sink’s primary dependence on pumping power and allow the designer’s choice of material and working fluid to have a more significant impact on the heat sink’s performance. The goal of this study is to visualize slip flow and quantify the impact of hydrophobic nanostructures on heat sink performance.

ANSYS FLUENT was used for the simulation of the heat transfer. The Reynolds number of the working fluid was set at a laminar range varying from 200 – 800. A heat flux of 90 W/cm^2^ was applied to the top wall to simulate a heat source. Adiabatic conditions were given to the side and bottom walls to simulate a 1 – D conjugate heat transfer problem. Channel geometry was generated with the intent of simulating parallel flow. The set parameters were measured or calculated based on initial conditions. After data was obtained for microchannels without nanostructures, geometry for a nanostructure grid was generated and analyzed with the same initial and boundary conditions.

The simulation data collected established results of both pressure and velocity fields over the nanostructure regions. In an attempt to determine the design with the smallest pressure loss, a design of experiments was established for an optimization of nanostructure geometry parameters. Minimization of the pressure loss is set as the objective function. Corresponding candidate design points are displayed in response surfaces.

 

 

The intended outcome of implementing nanostructures is to quantify the impact on pressure drop and required pumping power. A decreasing trend of heat energy removed as a function of Reynolds number is expected as well. Visualization of an inviscid boundary layer was produced as a result of transient flow simulation. Preliminary results from simulation display pressure drop as a function of Reynolds number. A correlation between pressure drop and nanostructures is established in this research. Also, plots of the heat transfer coefficient vs nondimensionalized constants are produced.


Comparative Study of Effective Cooling in Microchannel Heat Sinks (Mchs) With Nanofluid and Hydrophobic Nanostructures Using Computational Fluid Dynamics

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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