Thermophysical Parameters And Evaporation Characteristics Of Pendent Droplets For Nanofuel

Nanofuel refers to the uniform and stable two-phase solid-liquid suspension containing particles with sizes of 10-100nm.The basic physical and chemical properties of fluid,such as viscosity,thermal conductivity,heat and mass transfer coefficient,can be greatly changed with the addition of nanoparticles.Therefore,it has been widely used in efficient heat exchange system,photothermal conversion,cold storage with phase change,lubricated antifriction,temperature-controlled printing and other fields.In combustion field,nanoparticles with high catalytic activity and strong oxygen storage capacity can promote the combustion of engines and reduce the pollutants.In this paper,CNT,CeO₂ and Co₃O₄ nanoparticles with the size of 20nm and 50nm were selected as fuel additives,due to their high thermal conductivity,oxygen storage capacity and catalytic activity.Cetyltrimethyl ammonium bromide served as surfactant.Nanofuels with the concentrations of 50mg/L,100mg/L and 150mg/L were prepared by two-step method.The effects of type,size and concentration for nanoparticle on the physicochemical properties of nanofuel and the evaporation characteristics properties of pendent droplets were investigated.Meanwhile,the general law of the interaction between nanoparticles and fuel in the thermodynamics process were explored.This paper is aimed to provide basic data for clean and efficient combustion.The main work of this paper are as follows:?1?The thermodynamic characteristics of various nanofuels were investigated using isothermogravimetrical analysis.The activation energy for evaporation,vapor pressure and vaporization enthalpy were figured out by the Arrhenius equation,the Antoine and Langmuir equations,Clausius-Clapeyron equation,respectively.Owing to the lower specific heat capacity of nanoparticles,the heat transfer from the surface to the inside was accelerated during the fuel evaporation,which delayed the volatilization of fuel molecules on the gas-liquid surface.As a result,the activation energy for nanofuel vaporization rises with the increase in nanoparticle mass fraction.Furthermore,nanoparticles have large specific surface area and require more energy to separate fuel molecules adsorbed on the surface of particles.nanoparticles in fuel,postponing the diffusion of fuel molecules into gas.Hence,nanofuel has a lower vapor pressure than base fuel.The vapor pressure of 20nm CeO₂ nanofuel with 50mg/L was declined by 6.71%compared with C14.In addition,evaporation enthalpy of 20nmCeO₂ nanofuel with 150mg/L was increased by 13.87%than that of C14,due to the enhancement of Van der Waals force and the hydrogen bond between fuel molecules and nanoparticles.?2?The surface tension of nanofuels were investigated using pendent droplet method.The effects of nanoparticle size,mass fraction and temperature on surface tension at the temperature of 100¹40?were explored.It was revealed that the cohesion in base fuel molecules was formed due to the attraction between nanoparticles,which contributed to the growth in surface free energy and thus increased the surface tension of base fuel.Besides,the surface tension of base fuel increased with the increase of nanoparticle mass fraction or particle size.As temperature rose,the molecular movement speeded up for the increased kinetic energy,which can reduce intermolecular forces and the cohesion energy of liquid molecules.Therefore,the surface tension of base fuel and nanofuels declined with the increase of temperature,which presented a linear relationship.Furthermore,nanoparticles species have effects on the surface tension.Under the same mass fraction,the number of CNT particles is larger than that of CeO₂and Co₃O₄,because CNT possesses the smaller volume density.Consequently,the surface tension of CNT nanofuel was the highest among the studied nanofuels.?3?Pendent droplet evaporation device was adopted to investigate the evaporation characteristics of nanofuel droplets.The evaporation deformation behavior of nanofuel droplets was studied.Meanwhile,the effects of nanoparticle species,particle size,concentration and temperature on the evaporation characteristics of base fuel were pointed out.It was found that the evaporation of the droplets for base fuel and nanofuel followed D² law.The surface tension of nanofuel was high at the low temperature.It can be explained that the addition of nanoparticles delayed the evaporation of fuel droplets.With the increase of temperature,the surface tension of nanofuel reduced and the thermal conductivity of nanofuel enhanced gradually compared with base fuel.Thus,the evaporation time of droplets was shortened.The evaporation rates of 20nm CNT nanofuel with the concentration of 50mg/L was decreased by 1.12%at 100?and increased by 0.39%at 180?compared with base fuel.The evaporation rates of nanofuel with the same nanoparticle concentration decreased with the increase of particle size.Besides,various nanoparticles have different physicochemical properties like density,polarity and thermal conductivity,which brought about different evaporation rates.The sequence of evaporation rates for various nanofuels was determined as:CNT nanofuel>CeO₂ and Co₃O₄nanofuels.In this paper,vapor pressure,enthalpy and activation energy for evaporation,surface tension and other thermodynamic parameters were researched.The influence mechanism of nanoparticles on morphologies and evaporation rates for fuel droplets were analyzed.This study provided the important basic data for optimizing atomization combustion of nanofuel.