Experimental Study On Thermophysical Properties And Convective Heat Transfer Characteristics Of Mixed Nanofluids

The low utilization rate of industrial waste heat is one of the main reasons for high energy consumption in China.Organic Rankine Cycle technology(ORC)using nanofluids as working fluids is one of the effective ways to improve heat transfer performance and recover and utilize industrial waste heat efficiently.Hybrid nanofluid is a new generation of heat transfer fluids with higher thermal conductivity and better comprehensive performance in nanofluid.In particular,it has the excellent characteristics of adjusting thermal properties(thermal conductivity,viscosity,etc.)through mixing means to better suit different conditions,making it more suitable for industrial applications.However,the mixing of various types of nanoparticles leads to complex internal interactions,unclear mechanism of thermophysical property change,and large density and surface charge difference that makes it difficult to uniformly suspend for a long time,which restrict its long-term development.Therefore,the influence of surfactant type and concentration on the stability of hybrid nanofluids was studied.Based on digital image statistical technology,a quantitative evaluation method for the stability of nanofluids was proposed.The effects of mixture ratio of base fluids and nanoparticles,volume fraction,temperature and nanoparticle size on viscosity and thermal conductivity were explored,and the synergistic mechanism of heat transfer enhancement of hybrid nanofluids was clarified.Finally,the heat transfer characteristics of laminar and turbulent convective heat transfer in single-phase tubes with hybrid nanofluids were studied.The main research contents are as follows:1.The effects of various surfactants on the stability of hybrid nanofluids at different mass concentrations and temperatures were investigated.Through multiple stability characterization methods,it was found that the hybrid nanofluid with PVP surfactant added had the best stability,and the stability time was up to 25 days.A new stability evaluation parameter,UCND(Uniformity Coefficient of Nanoparticle Distribution)was introduced by using uniformity theory and image analysis technology to quantitatively evaluate the distribution uniformity of nanoparticles in the fluid.By comparing UCND with the characterization methods of transmission electron microscopy and nanoparticle size measurement,it is found that UCND based on mathematical theory can be used to quantitatively analyze the nanoparticle distribution in the whole region,which is more time-saving and more objective.2.The influence of the mixture ratio of nanoparticles(NP)and base fluids(BF)on the thermal conductivity of hybrid nanofluids was studied.The results show that the proportion of NP mixture is an important factor that determines the arrangement of nanoparticles and affects the enhancement of thermal conductivity.From the perspective of economy,Al₂O₃-Cu O/EG-W hybrid nanofluid in laminar flow is the most efficient fluid,and Al₂O₃-EG/W single nanofluid is the best choice for turbulent flow.The essence of enhancing heat transfer of hybrid nanofluids,namely,the cooperative heat transfer mechanism,is analyzed.It is pointed out that the heat transfer is greatly enhanced by the reasonable arrangement of nanoparticles in the hybrid nanofluids,which forms a compact solid-liquid interface and a reasonable heat transfer network.3.By studying the rheological properties change,found that the hybrid nanofluids at low concentrations and under low shear rate showed a newtonian mechanics behavior.Analysis of variance(ANOVA)showed that the volume fraction has a significant effect on viscosity and temperature in the study,more than 45%volume fraction on the contribution of viscosity,and the effect of nanometer particle size of around 25%.Finally,it is found that the synergistic effect in the hybrid nanofluids affects the arrangement of nanoparticles.Therefore,in the low volume fraction range,the viscosity of the hybrid nanofluids is lower when adding small size nanoparticles,while in the high volume fraction range,adding large size nanoparticles can obtain low viscosity nanofluids.4.An experimental platform for forced convection heat transfer in a hybrid nanofluid tube was built,and the influence of the flow velocity and volume fraction of the nanofluid on the convection heat transfer coefficient and Nusselt number of the flow in the tube was explored by changing the flow velocity and volume fraction of the nanofluid in the tube.The results show that adding nanoparticles into water can enhance the heat transfer effect of working medium flow in the tube.Compared with the base liquid,the average value of the heat transfer coefficient of the hybrid nanofluid is about 15.75%.Moreover,the enhancement effect of Nu number in turbulent region is more obvious than that in laminar region.The enhancement of heat transfer coefficient in laminar flow region is 20.08%,and that in turbulent region is 33.71%.