Evaporation Characteristics and Morphological Evolutions of Fuel Droplets After Hitting Different Wettability Surfaces

To solve the wall-wetting problem in internal combustion engines,the physical and chemical etching methods are used to prepare different wettability surfaces with various microstructures.The evaporation characteristics and morphological evolution processes of diesel and n-butanol droplets after hitting the various surfaces are investigated.The results show that the surface microstructures increase the surface roughness(Ra),enhancing the oleophilic property of the oleophilic surfaces.Compared with n-butanol droplets,the same surface shows stronger oleophobicity to diesel droplets.When a droplet hits an oleophilic property surface with a lower temperature,the stronger the oleophilicity,the shorter the evaporation time.For oleophilic surfaces,larger Ra leads to a higher Leidenfrost temperature(TLeid).The low TLeid caused by enhanced oleophobicity,dense microstructures and increased convex dome height facilitates droplet rebound and promotes the evaporation of the wall-impinging droplets into the cylinder.The evaporation rate of the droplets is not only related to the characteristics of the solid surfaces and the fuel droplets but also affected by the heat transfer rate to the droplets in different boiling regimes.The spreading diameter of a droplet on an oleophobic surface varies significantly less with time than that on an oleophilic surface under the same surface temperature.

Morphological Development of Fuel Droplets after Impacting Biomimetic Structured Surfaces with Different Temperatures

To improve the controllability for the evaporation process of fuel spray impinging on the cylinder wall,an experimental study on the development of morphological process of different fuel droplets on aluminium alloy surfaces is carried out.The metal surfaces with different wettability are prepared by laser etching and chemical etching for the experiments.In total,three different fuels are tested and compared under different surface temperatures,including diesel,n-butanol and dimethyl carbonate(DMC).The results show that under a lower wall temperature,the surface wettability,viscosity and surface tension of the fuels have significant effects on spreading and rebounding behaviour of the droplets.As the wall temperature rises over the boiling points of the fuel but below its Leidenfrost temperature,the contact angles between the fuels and surfaces are varying according to the surface wettability,boiling point and Leidenfrost temperature of the fuels.When the temperature of the surface exceeds the Leidenfrost temperature of all the fuels,after impacting the surfaces,different fuel droplets tend to have the same development pattern,regardless of the surface wettability.The rebound level is mainly affected by the amount of fuel vapour generated during the wall-hitting process.Viscosity,surface tension and other properties of the fuel have little effect on post-impacting behaviour of the droplet when the wall temperature is higher than the Leidenfrost temperature of the fuel.

Evaporation of liquid fuel droplet at supercritical conditions

A comprehensive analysis of liquid fuel droplet evaporation at supercritical conditions is performed.The numerical model is based on complete time-dependent conservation equations,with a full account of variable thermophysical properties and vapor-liquid interfacial thermodynamics.And the model employs the Peng-Robinson(PR)equation of state(EOS).As a specific example,problems involving n-heptane droplet in nitrogen gas are investigated.The results indicate that the increase of ambient pressure and temperature results in the increase of surface temperature rise rate and surface regression rate.The transition from subcritical state to supercritical state can occur at the droplet surface when the droplet evaporates in a strongly supercritical environment.

Numerical Simulation of Vortex Engine Flow Field:One Phase and Two Phases

Aiming at improving efficiency in combustion systems, the study on droplet behavior and its trajectory is of crucialimportance. Vortex engine is a kind of internal combustion engine which uses swirl flow to achieve highercombustion efficiency. One of the important advantages of designing vortex engine is to reduce the temperatureof walls by confining the combustion products in the inner vortex. The scopes of this investigation are to studyvortex engine flow field as well as effective parameters on fuel droplet behavior such as droplet diameter, dropletinitial velocity and inlet velocity of the flow field. The flow field is simulated using Reynolds Stress TransportModel (RSM). The Eulerian-Lagrangian method and the one-way coupling approach are employed to simulatetwo phase flow and dispersed phase in the chamber, respectively. A new method, based on computing pressureforce exerted on the droplet surface, is introduced to determine the distinction between using one-way andtwo-way coupling approaches. The results showed that the droplets with smaller diameter are more likely to followthe flow stream lines than bigger droplets, thus evaporate completely in the chamber. Moreover, droplets withgreater initial velocity have higher evaporation rate, yielding the existence of evaporation and combustion in theinner vortex. Additionally, the higher inlet velocity of continuous phase results in higher centrifugal force, leadsdroplets in question to deviate towards the wall faster.