Simulation Experimental Study On Boiling Heat Transfer Of Nano-fluid In The Bridge Zone Of Internal Combustion Engine

The cylinder cover of internal combustion engine is withstanding high alternating thermal load in the working process.If heat doesn't transfer in time,the service life,reliability,performance and fuel economy of internal combustion engine will be seriously challenged.Based on boiling cooling system,for water,the efficiency of boiling heat transfer still can be further improved.Some existing criterion correlations have their own limitations so that simulation experiment needs to be continued for the future internal combustion engine.In addition,the traditional working medium can be replaced by nanofluids,whose effects on improving the efficiency of cooling system still have a contradiction.To figure out this contradiction is the key to lift the cooling efficiency to a higher level.In allusion of these problems,under the background of strengthening heat transfer in the cooling channel of the bridge zone of internal combustion engine,an experimental system to research the subcooled boiling heat transfer is set up according to the parameters in cooling channel of the physical machine such as surface temperature,cooling water temperature and flow velocity.The simulation experiment which includes measurements of the convective heat transfer coefficient and friction factor and high-speed photography visualization ensures that the Reynolds number,the Prandlt number and the Nusselt number are consistent with the parameters in the real internal combustion engine.In the water experiment,it is shown that increasing the Reynolds number and reducing the subcooled temperature of the medium can improve the heat transfer.The rising Reynolds number will augment the density of bubbles and the frequency of bubble generating.It contributes to the disintegration of the gas film as well.Bubbles leave the heating surface quickly with much heat,then more boiling gasification cores are set up to absorb heat again.The combination of bubbles enlarges the volume of the individual bubbles and increases their buoyancy to push the bubbles away from the wall.At the meantime,larger volume produces larger heat exchange area,which is conducive to heat exchange.On the one hand,medium with higher temperature can bring the boiling point forward so that the mode of heat transfer turn to boiling heat transfer from single phase heat transfer and that can make the heat coefficient improve greatly.On the other hand,it also gets into film boiling state early which is harmful to heat transfer.There are several kinds of models of empirical formula for boiling heat transfer and the experiment data don't fit them well.After comparison,available experiment data are refitted and new empirical experience parameters are obtained by selecting a progressive model with relatively smaller deviation.The new formula can predict more accurately.The influences of nanofluids on heat transfer have both positive and negative aspects,which have a considerable relationship with the concentration of nanoparticles.Nanofluids of 0.50% volume fraction shows a positive influence on heat transfer.But nanofluids of other volume fractions both weaken the heat transfer.Nanoparticles will increase the turbulent kinetic energy and the disturbance of the liquid.Addition of nanoparticles makes the thermal conductivity and other physical properties of liquid changed,leads to faster heat transfer and higher frequency of bubbles generating.At the same time,large disturbance accelerates the bubbles separating from the heating surface that can promote new bubbles generation.The deposition of nanoparticles is a crucial factor in preventing convective heat transfer.The thicker deposition is,the weaker heat transfer is.Excessive deposition of nanoparticles will cut down the number of the activation nucleus and extremely increase the heat transfer resistance on the surface.These sedimentary nanoparticles become a huge obstacle for bubbles to absorb heat and it will restrict the growth of bubbles.Inadequate bubbles will hardly leave the wall and cause a further impediment of heat transfer,which results in new bubbles generating gets restrained.To prevent the heating surface from sedimentary nanoparticles,the surface is covered by a kind of hydrophobic coating.The effect of prevention is not perfect as hypothesis,but still,the heat transfer efficiency of nanofluids is a little higher than water.If the surface could be modified better to create a super-hydrophobic structure,the efficiency of subcooled boiling heat transfer will be profoundly promoted.