Multiphase flows have been an attractive choice of fluid system to mitigate the temperature rise in electronic devices. We propose a particle-laden flow (i.e., a dense suspensions) comprising boron nitride (BN) particles of micrometer dimensions suspended in a 3M Fluorinert FC-43, which is a dielectric fluid commonly used for cooling electronics. A computational two-fluid model is developed in OpenFOAM to enable high-fidelity predictive simulation of such suspension flows. We perform 3D computational fluid dynamics simulations of coupled particle migration and heat transfer in three different microchannel geometries: uniform cross-section channels, channels with converging and diverging sections, and herringbone-patterned microchannels designed to enhance mixing. Our simulations address thermal management solutions for uniform heat input, as well as for a local hotspot in the system. The converging-diverging channel improves the thermal performance due to enhancement of the local suspension Reynolds number, while the herringbone channel induces secondary flows in the cross-section, which further enhance mixing and promote heat transfer. In particular, our simulations indicate that, for the same channel inlet dimensions and mass flow rate, out of the three designs, the herringbone geometry keeps the system coolest for a fixed heat input: the maximum temperature in the channel for a hotspot at the bottom wall of the channels is limited to 325 K for converging-diverging, 324 K for herringbone, and  332 K for uniform cross-section channels. While the converging-diverging channel exhibits a similar maximum temperature to the herringbone design, it requires the largest pumping power of 26.7 uW. The herringbone channel is the most efficient at 8.6 uW due, in part, to the increased channel cross-section in the herringbone design. As a point of reference, simulations with pure (clear) FC-43 fluid were performed in the same channels. For the herringbone channel, we found a maximum temperature of 430 K at a slightly lower pumping power of 5.8 uW. For the pure fluid in the converging-diverging channel, the maximum temperature was 360 K at $12.5 uW. Clearly, suspensions of high-thermal-conductivity BN particles improve the thermal management of electronics devices reducing the surface temperature by up to 100^oC for the same volumetric flow rate. Further investigation and optimization of the combined flow physics, particle migration, and thermal transport could lead to efficient microelectronics cooling systems.