Flow boiling heat transfer of nanofluids in microchannels under electric fields

This study aims to explore the boiling heat transfer characteristics of nanofluids in the microchannels under the action of electric fields. The linear electrodes were arranged in the geometric center of microchannels to generate a 0-800V non-uniform electric field, and the SiO2-R141b nanofluids with the different mass fractions were prepared by a two-step method. A flow boiling experiment was performed on the microchannel with a cross-section of 2 mm×2 mm under the system pressure of 140 kPa, the mass flow rate of 317.71 kg/(m2•s), the heat flux density ranges from 11.53 to 29.14 kW/m2, and the inlet temperature of 32℃. A systematic investigation was made on the average saturated boiling heat transfer coefficient and the range of effective heat transfer heat flux density in the microchannel with the different mass fractions of nanofluids under the action of different electric voltages. The enhanced heat transfer mechanism of the nanofluid flow boiling was determined to simulate the detachment rate of the bubbles, the velocity of the confined bubbles, and the aspect ratio of the microchannels under the action of different voltages and electric fields. The comprehensive performance evaluation of the heat transfer (PEC) method was used to enhance the heat transfer of microchannels by different strengthening techniques. Both the electric field and the nanofluid were used to enhance the flow boiling heat transfer in the microchannel. Electric fields presented a better effect on the nucleate boiling heat transfer than single-phase heat transfer. The nanofluids were stable and enhanced heat transfer performance in the single- and multi-phase flow. An optimal mass fraction of nanoparticles was achieved in the nanofluids. The combination performance was better than that of the electric field or the nanofluid alone. The average saturated boiling heat transfer coefficient of nanofluids under the action of an electric field was higher than that without an electric field, which increased with the increase of voltage. The maximum coefficients of saturation boiling heat transfer increased by 44.9%, 20.9%, and 58.0%, respectively, for the electric field enhancement alone, nanoparticle enhancement alone, as well as the electric field and nanoparticle addition enhancement, compared with pure refrigerant. There was a significantly larger effective range of heat transfer enhancement that combined electric field and nanoparticles, compared with the electric field alone and the addition of nanoparticles to the pure refrigerant. The electric field can be expected to accelerate the bubble detachment frequency and the speed of the confined bubble, thereby reducing the aspect ratio of the confined bubble. At the same time, the nanoparticles can be used to improve the thermal conductivity of the working medium for the high heat exchange between the heat exchange wall and the working medium, indicating the enhanced heat transfer under the combined action. The nanofluid presented the highest enhancement performance of heat transfer under the electric field, where the average heat transfer comprehensive performance evaluation factor was 1.34. Consequently, the electric field combined with the addition of nanoparticles can effectively enhance the heat transfer of flow boiling in the microchannels. The finding can provide new ideas to improve the heat transfer performance of microchannels under the combined electric field and nanoparticle composites.the heat transfer of flow boiling in the microchannels. The finding can provide new ideas to improve the heat transfer performance of microchannels under the combined electric field and nanoparticle composites.