Capillary And Heat Transfer Performance Of Fe₃O₄ Water-based Nanofluids To Microgrooves Heat Sinks

The microgroove heat sink with the composite phase change heat transfer has an advantage of high heat transfer coefficient and stable operation,so it is considered to be one of the effective means to solve the heat transfer of high-performance semiconductor chips.The surface of the open capillary microgroove heat sink is processed with a micron-sized open microgrooves array.The capillary pressure gradient formed by the groove structure can drive the working fluid flow along the microgrooves.At present,most researchers believe that the heat transfer characteristics of microgroove heat sinks are closely related to capillary performance,so a lot of research has been carried out on the capillary performance of micro groove heat sinks.The research on the capillary performance of the microgroove heat sink can be roughly divided into three categories:the study of the capillary performance of the shape and size of the microgroove heat sink;the study of the micro-nano surface structure of the microgroove heat sink on the capillary performance;the study of the new working fluid of heat transfer.Nanofluids have a higher heat transfer coefficient than traditional fluids under laminar flow conditions.In the micro-devices that require increased heat load,such as rectangular microgroove heat sinks,the volume of the device is very important.The application of nanofluids can reduce the heat exchange volume of the device and thus reduce the volume of the device,so it is favorable to use nanofluids instead of conventional fluids.However,research reports on the effect of nanofluids on the capillary wetting and flow in the microgroove heat sink are relatively rare.In this paper,the Fe30O4 water-based nanofluid that can be prepared in large quantities is applied to the microgroove heat sink,and its enhanced wetting characteristics and heat transfer characteristics are studied theoretically and experimentally.The aim is to enhance the heat transfer of the microgrooves based on the nanofluid,which is of great significance for solving the heat dissipation problem of high-power density electronic devices.In this paper,a precision grinding process and a two-step method were used to prepare heat sinks of borosilicate glass with groove dimensions(groove width 0.3 mm,groove depth 0.6 mm,and groove spacing 0.4 mm)and six particle volume concentrations(0.00125 vol%,0.0025 vol%,0.00375 vol%,0.005 vol%,0.0075 vol%,0.01 vol%)Fe3O4 water-based nanofluid.Through X-ray diffraction,thermogravimetric analysis technology,dynamic light scattering analysis of the hydraulic characteristics of the nanofluid,the results show that the weight fraction of attached citric acid is about 4.2%,the hydrodynamic diameter of the particles is mainly distributed below 100 nm,and the average diameter is 47 nm.Compared with the size of individual particles,this indicates that the particles have experienced a certain degree of aggregation in the base fluid.Subsequently,the difference in the enhancement of the axial wetting length and capillary wetting characteristics of the microgroove heat sink with different nanofluid particle concentrations under the three fluid and heat sink temperature conditions at 25℃,40℃,and 60℃ was carried out in this paper.The experimental results show that Fe3O4 water-based nanofluid mainly relies on reducing the flow friction resistance and increasing the permeability to enhance the capillary performance of the microgroove heat sink;By establishing the theory of liquid capillary process in the microgroove structure under different temperature conditions The model quantitatively evaluates the improvement of the capillary performance of the microgrooves structure by the nanofluid.At the three temperatures,the increment percentage of the capillary pressure and permeability of the nanofluid is consistent with the change of the concentration,showing a trend of first increasing and then decreasing,and the maximum increment percentage occurs when the concentration is 0.00375%.After analysis,it is believed that this should be attributed to the increase of particle aggregation,which hinders the generation of structural dissjoining pressure and the migration of particles to the groove wall.Increasing the temperature is beneficial to the nanofluid to enhance the capillary pressure.When the temperature increased from room temperature to 60℃,compared with the base fluid,the average increment percentage of capillary pressure increased from 4.4%to 17.2%,and the increment percengtage increased to 23.3%at the optimal concentration.In the different concentration ranges,the effect of temperature on the enhanced permeability of nanofluids is different.At lower concentrations,increasing the temperature will result in a continuous decrease in the increment percentage of permeability.When the concentration is higher than 0.0025%,the permeability increment percentage appears to the lowest value at 40℃,and increasing the temperature will increase the increment percentage instead.Considering that in practical applications,the surface of the microgroove heat sink will be 10℃-20℃ higher than the temperature of the working fluid,so the different temperature difference in the microgroove is performed for the microgroove group heat sink.Through experimental comparison and theoretical analysis,the flow pattern of nanoparticles in the cross section of a single hotter microgroove is obtained,and the tangential flow,viscous resistance,and viscous resistance that play a major role in the migration of nanoparticles are obtained.The DLVO force is theoretically deduced.The study found that the hotter microgrooves can easily reduce the friction of the nanofluid and promote the reduction of Po number.The heat from the microgrooves will reduce the viscosity of the fluid near the surface of the microgrooves,which is beneficial to the particles.Under the action of DLVO force,it migrates to the surface of the microgrooves,thereby reducing friction.When the microgrooves are hotter than the fluid,the capillary pressure will be slightly affected by the nanofluid.This may be due to the counterbalance between the tangential flow and Marangoni flow on the cross section of the microgrooves,which prevents the particles from moving towards the evaporation film region.The heating from the microgrooves should also encourage particles to gather near the surface of the microgrooves,especially at high concentrations.Once the negative effect due to aggregation exceeds the positive effect due to the decrease in viscosity,it can be expected that the migration of particles to the surface of the microgrooves will be inhibited.At the end of this article,an experimental study on the heat transfer performance of nanofluid-enhanced microgroove heat sinks has been carried out.It is found that the composite phase-change heat condition is more conducive to Fe3O4 water-based nanofluids in enhancing the axial wetting in the microgroove heat sink.The wetting length and heat transfer performance,and the increment percentage with the particle concentration change law is consistent with the law presented in the capillary performance test.When boiling occurs,the force generated by the bursting of the vapor bubble will push the working fluid above the vaporization core to surge upward along the grooves.However,the excessively high particle concentration prevents the Fe3O4 water-based nanofluid from enhancing the heat transfer performance of the heat sink.