Thermal Properties Test And Thermal Conduction Mechanism Research Of ZrO₂/Toluene Nanofluids

The traditional pure liquid heat exchanger has low thermal conductivity,so it is difficult to meet the high load requirement of the heat exchanger.Increasing the thermal conductivity of the heat transfer properties is expected to develop a new generation of heat exchange technology.Since the thermal conductivity of solid particles is several orders of magnitude higher than that of liquids.The effective thermal conductivity can be improved by adding particles to the liquid.Nanoparticles have a small size effect that are close to liquid molecules.Dispersing nanoparticles into a liquid medium to form a new type of heat exchange working medium can not only improve effective thermal conductivity but also increase the practical applicability.In this thesis,the thermal conductivity of ZrO₂/toluene nanofluids was measured experimentally,and studied the thermal conductivity mechanism of nanofluids.The ZrO₂/toluene nanofluids studied in this paper has the characteristics of high concentration.The ZrO₂/toluene nanofluids were characterized by transmission electron microscopy and selected area electron diffraction.The nanoparticles with polycrystall ine structure were observed to be spherical,and the measured particle size was 3.2 nm.The nanoparticles were uniformly dispersed in the base liquid.The thermal conductivity of ZrO₂/toluene nanofluids was measured with a Hot Disk thermal constant analyzer in the temperature range of 23.6-62.9?and the concentration range of 2.20%-13.21%,found that:(1)At the same concentration,the thermal conductivity of ZrO₂/toluene nanofluids gradually increased with increasing temperature.(2)At the same temperature,the thermal conductivity of ZrO₂/toluene nanofluids increases with the increase of volume fraction,and the thermal conductivity of nanofluids increases the fastest in the range of 6.61_vol%-8.81_vol%.(3)At 62.3?,the thermal conductivity of ZrO₂/toluene nanofluids with a volume fraction of 13.21%was increased by a maximum of 59.9%compared with toluene solution.The thermal conductivity mechanism of ZrO₂/toluene nanofluids was studied by using the Hamilton-crosser model,Davis model,and Lu-Lin model to compare the predicted results with the experimental results.At low temperature,the theoretical model is in good agreement with the experimental results,indicating that temperature,thermal conductivity and volume fraction of particles are the main factors affecting the thermal conductivity of nanofluids.However,under high temperature conditions,the prediction results of the above models were not in good agreement with the experimental results.The Shukla model not only considers the shape and volume fraction of nanoparticles but also adds the effect of particle Brownian motion and micro-convection caused by Brownian motion on the thermal conductivity of the nanofluids.The model is in good agreement with the experimental results,indicating that the Brownian motion of particle s contributes more and more to the thermal conductivity of nanofluids with the increase of temperature.When a modifier with thermally sensitive properties is added to the nanofluids,the modifier has the property of stretching with increasing temperature,which in turn affects the phase change of the nanofluid s.Therefore,the thermal conductivity of the phase change with a 6.5_vol%was determined.From room temperature to the critical temperature,the thermal conductivity of the phase change nanofluids increases with increasing temperature,and above the critical temperature decreases with increasing temperature.At room temperature,the thermal conductivity of ZrO₂/toluene nanofluids with a 6.5_vol%was 23.6%higher than that of the basic liquid.The maximum thermal conductivit y around the critical temperature increased by 46.5%.