Study On Thermal Conductivity And Viscosity Of Graphite Suspension

With the rapid development of science, traditional heat transfer approaches cannot satisfy the increasing energy dissipation. Although increasing heat transfer area, various inserts can improve heat flux, larger heat exchanger weight and volume, even more pump power consumption appear. For electronic elements, temperature need to be controlled seriously under a fixed value to ensure its normal work, and therefore, heat transfer is much a challenge. From the object of heat exchanger with small volume, high heat transfer property and low weight, working fluid with high thermal conductivity is considered. Traditional working fluid, like water, ethylene glycol, and pump oil cannot be utilized in some crucial cases due to their relative lower thermal transport property. Choi coined the term of nanofluids which is mixture of nano-size particles and base fluid like water, ethylene glycol and can be kept stable in1995. Due to the fact that solid always have much higher thermal conductivity than liquid, nanofluids show the relative higher thermal conductivity than pure working fluid and can be utilized for many areas.Experiment results cannot be repeated and even occur to appear opposite conclusions due to the the complicated suspension structures and measuring approaches. Nanofluids made by different approaches demonstrate different structures and experiment results. Some data from one group usually cannot coincide with another group with the same particle, base fluid and volume fraction, even with manufacture approach. There is no uniform mechanism for nanofuilds heat transfer enhancement. Current researchers are always focus on the thermal conductivities of different nanofluids, while more pump power consumption turn out due to the addition of particles. Concerning the viscosities of different nanofluids, researchers always showed the description of the phenomena of the viscosity variation and no theories and mechanisms occurred for the viscosity research.According to current issue, thermal conductivity and dynamic viscosity of graphite suspension are investigated in this thesis. The contents of research are as follows.1) Manufacture of graphite suspension. By following procedures of sulfuric acid intercalation, microwave expansion and ultrasonication, graphite suspension is fabricated. 2) Thermal conductivity measurement of graphite suspension. By transient hot-wire method, thermal conductivity of graphite suspension is measured. Different volume fraction by diluting the concentrated suspension and different sonication time are used to investigate the thermal conductivity variation. Thermal conductivities of suspension measured at different temperatures are utilized to interpret the competition of Brownian motion and clusters effect.3) Thermal percolation phenomenon is analyzed by AC impedance spectroscopy. Sharpen kink point in the thermal conductivity enhancement plot is elucidated from energy view.4) Dynamic viscosity measurement of suspension. By a cone-plate meathod and AR-G2apparatus, dynamic viscosity is measured and compared with the thermal conductivity near percolation point. Shear thinning phonomenum occurs under the interpretation of particles" alignment.5) Temperature effect for viscosity is obtained by controlling the plate temperature and is analyzed through solvent intrinsic, Brownian motion, particles’alignment and energy variaion. The experiment results are further analyzed by bring in Stokesian Dynamic approach.In this thesis, thermal conductivity and viscosity are analyzed to investigate into the fluid properties under stationary and flowing state. These work supply the theory foundation to design stable suspensions with high thermal concutivity and low viscosity and to further achieve the heat exchangers for industry with performance of small volume, high hear transfer property and light weight.