An Experimental Study On The Thermal Properties And Heat Transfer Characteristics Of Water-based Graphene Nanofluids

The theory that heat transfer can be enhanced by adding the nano-particles to the heat transfer medium has been widely accepted by the researchers,and is also widely used in the fields of chemical industry,energy,building and the like.Graphene as a kind of nano-material with special lamellar structure and high thermal conductivity has been paid high attention in recent years,and it used as nano-fluid additive has also been extensively studied.The results show that by adding the graphene nano-particles to the traditional heat transfer medium,the thermal physical properties and the heat transfer characteristic of the fluid can be obviously improved,and the heat transfer efficiency and the economic efficiency of the heat exchange system can be effectively improved.This paper prepared the graphene nano-fluid with different concentration by two-step method,and its basic physical properties are tested and studied.For studying the heat transfer characteristic of graphene nano-fluid,the experimental platform for the convection heat exchange in the horizontal circular tube and the rectangular small-channel channel is designed and set up,Through the analysis of the experimental data,the strengthening and heat transfer mechanism of the graphene nano-fluid is studied,and the relation of the convection heat exchange between the nano-fluid in the horizontal circular tube and the rectangular small channel is fitted,and a certain theoretical foundation is provided for the later study on the strengthening and heat exchange of the graphene nano-fluid.The main contents of the work are as follows:Firstly,the properties of the graphene was basically describes,the results were analyzed briefly,and then through the way of two steps of preparation of water based graphene nanofluid with different concentration,we also has carried on the analysis and research on thermal conductivity and dispersion stability through the Zeta potential and thermal conductivity test.The results show that the thermal conductivity of water based graphene nanofluid increases with the increase of the concentration of graphene,it increases by 3% in concentration of 0.05% when compared to the based fluid,but the thermal conductivity of fluid in concentration of 0.1% decreased due to the poor dispersion stability of solution,thermal conductivity increases only 1.5% compared to the base fluid.Compared with Maxwell calculation model the results found that two have the same rules,but for the unique sheets structure making results slightly higher than the calculated results.Then the heat transfer experiment platform in horizontal circular tube was designed and set up,and the heat transfer experiment was carried out with the graphene nano-fluid as the heat transfer medium.The wall temperature of the circular tube was tested under different experimental conditions,and the change of the heat transfer efficient was analyzed by the theoretical calculation.The results show that the convective heat transfer increases with the increase of the mass concentration,and there is a different degree of improvement in the inlet section and the full development section as compared with water.When Re is between 500-2000,the heat transfer efficient of fluid with 0.01% concentration in the inlet section increases by 6%,increases by 1.9.5% in the full development section,the convection heat transfer of the whole pipeline increases by 1.7%,and heat transfer efficient of fluid with 0.03% concentration increases by 2.8% in the inlet section,in the fully developed section,for the effect of viscosity is higher than that of the thermal conductivity,the thickness of the thermal boundary layer is increased,and the convective heat transfer is inhibited,so the convective heat transfer strength of the whole pipeline can be increased by only 19.5%.And a heat transfer relation was fitted by combining experimental data and theoretical models which is suitable for the graphene nano-fluid in the horizontal circular tube.Thirdly,two small-sized rectangular channels were designed and processed,and the convective heat transfer experiment was carried out with the graphene nano-fluid as the heat transfer medium.The change of the convective heat transfer coefficient in the rectangular small channel was analyzed by the experimental data.The results show that,in the range of Re = 500-2000,the graphene nano-fluid exhibits a better heat transfer capacity than water.When the mass concentration is 0.03%,the channel-fin ratio is 1: 1,and the temperature reduction rate is up to 16%.When the heating power is 210 W,the local heat transfer coefficient with the graphene nano-fluid as the heat transfer medium is up to 36.6%.The change of the local heat exchange coefficient under the condition of comparing the channel-fin ratio of two different channels can be seen that the 1: 2 channel is smaller in size than the rectangular small channel of the 1: 1 channel-fin ratio,so that the flow rate of the fluid in channel is larger,the mixing is stronger,and the local heat exchange coefficient can be higher.Because of the Brownian motion and high thermal conductivity of the graphene nano-particles,the heat transfer can be obviously enhanced.Under the same conditions,the Nu of the graphene nano-fluid with the mass concentration of 0.03% in the rectangular small channel with the channel-fin ratio of 1: 1 has increased by 69.6%.A heat transfer relation was fitted by combining experimental data and theoretical models.