Study On Influence Of Wall Condition On Fuel Wall-wetting Process

As fuel consumption regulations and emission regulations are getting increasingly stringent currently,new combustion modes,such as homogeneous charge compression ignition(HCCI),premixed charge compression ignition(PCCI),reactivity controlled compression ignition(RCCI)and low temperature combustion(LTC),have attracted attention widely.These new combustion modes can improve thermal efficiency of internal combustion(IC)engines and avoid the problem of the trade off relationship between particulate matter and NO_x emissions.However,because of some of the characteristics of their combustion organization,the fuel wall-wetting phenomenon is promoted,which causes a heterogeneous distribution of air fuel ratio,a decreasing combustion stability and a higher HC and CO emissions.Due to the unavoidable fuel wall-wetting phenomenon,in order to optimize the performance of new combustion modes,it is essential to find out how to take fuel wall-wetting process under control.The project of National Natural Science Foundation of China(5167060406)on which this paper is based proposed taking advantage of the surface wettability to control the solid-liquid coupling process after fuel impacting the wall.On the basis of previous tests and simulations,this paper used molecular dynamics simulation method to study the process of fuel droplets wetting the wall under different wall conditions.First of all,the simulations under environmental temperature condition were carried out.In the simulations of fuel droplets of different diameters wetting the smooth wall,the results show that a higher solid-liquid interaction coefficient leads to stronger oleophilicity of the surface.And droplet diameter has little effect on the intrinsic contact angle within the range of droplet size this paper studied.When it comes to the rough surfaces,the equilibrium state is obtained first.The results show that as the solid-liquid interaction coefficient decreases,the height of pillar rises,or the distance between the pillars decreases,the wetting state of the droplet on the surface tends to be in Cassie state.When in Wenzel state,under the same the solid-liquid interaction coefficient condition,a higher pillar or a larger distance between pillars leads to a higher apparent contact angle.In contrast,the structure has little effect on contact angle when in Cassie state.And then,a lateral force is added to each atom in the droplet and the contact angle hysteresis phenomenon is studied.The results show that the shape of the droplets can be classified into three types under different wall conditions.When the solid-liquid interaction coefficient decreases,the surface has a weaker pinning effect on the droplet,which causes a lower contact angle hysteresis.Especially when the solid-liquid interaction coefficient is low enough and the droplet on the rough surface is in Cassie state,both of the values of advancing contact angle and receding contact angle are extremely close to the apparent contact angle,which means the contact angle hysteresis is really low.If the droplet keeps in Wenzel state when moving on the surface,a higher pillar rises the energy barrier the droplet has to cross,which leads to a higher advancing contact angle.When in Cassie state during moving process,the structure of the surface has little effect on contact angle hysteresis.Here are the simulations of high wall temperature and this paper analyzes the morphological development process of the droplet and the total energy of droplet molecules curve along time.The results show that a higher solid-liquid interaction coefficient promotes the spreading process and enlarges the solid-liquid contact area.Additionally,a higher solid-liquid interaction coefficient can also weaken thermal resistance of the solid-liquid interface.Both of the factors can enhance heat transfer.Meanwhile,a higher solid-liquid interaction coefficient can strengthen the ability of the surface to trap droplet molecules,which weakens evaporation.In contrast,a lower solid-liquid interaction coefficient helps boiling.High wall temperature can speed up heat transfer in a certain temperature range,however,when the wall temperature is too high so that the bottom of the droplet evaporates rapidly,the vapor phase weakens heat transfer.The effect of the surface structure reflects mainly in its ability of trapping the droplet molecules.The higher the height of pillars is and the shorter the distance between pillars is,the stronger trap effect of the surface.If there are too many molecules trapped in the micro-structure,the total energy will be low when in convergence state.