Investigation On The Characteristics Of Subcooled Flow Boiling Heat Transfer In The Water Jacket Of Internal Combustion Engines

With the increasing requirements of energy saving and emission reduction,the need for a higher explosion pressure in the internal combustion engine cylinder is rising as well.The cylinder head,which directly faces the explosion pressure and thermal load impact caused by combustion,is one of the key components of the internal combustion engine with harsh working conditions.The development of the cylinder head is facing more and more severe challenges and requires more in-depth basic theoretical support.Theories and experiments show that subcooled flow boiling heat transfer phenomenon exists in the bridge area of the cylinder head water jacket above the fire plate.The boiling heat transfer phenomenon in the cylinder head water jacket is more prominent with high power density and high explosion pressure of the internal combustion engine.However,the mechanism of boiling heat transfer is complicated,and there are many influencing factors.At present,the understanding of boiling heat transfer cannot be completely unified.Therefore,the research on the mechanism of subcooled flow boiling heat transfer in the engine water jacket is of great significance to the development and design of high power density engines.According to the characteristics of boiling heat transfer in the water jacket of the engine,an experiment apparatus was built accordingly.The experiment used a mixture of ethylene glycol and water,which is commonly used in engine coolant with a volume ratio of 50/50,as the working fluid,and the heating surface is made of aluminum and cast iron,which are commonly used materials for engine cylinder heads.The effects of coolant temperature,flow rate,pressure and heating surface material on subcooled flow boiling heat transfer are experimentally studied.The results show that in the singlephase flow heat transfer and partially development of boiling region,reducing the coolant temperature and increasing the flow rate can increase the wall heat flux;however,in the fully developed boiling region,the effect of the influencing factors of the coolant temperature and flow rate disappears.Increasing the system pressure will have an inhibitory effect on boiling and delay the onset of boiling.For the studied aluminum and cast iron heating surfaces,the above-mentioned cooling fluid operating conditions have similar effects on boiling heat transfer.However,under the same experimental conditions,the boiling heat transfer of the cast iron heating surface is smaller than that of the aluminum heating surface,which is mainly caused by the difference in the thermal properties of the heating surface materials.In this study,the predicted values of the Chen model and the three power-type addition boiling heat transfer models were compared with the experimental values.It was found that in the high heat flux region,the predicted values of the above models have large deviations,with the maximum deviation higher than ±30%.Based on this,this study optimized the power-type addition boiling heat transfer model,adding the heating surface thermal property parameter influence factor to its nucleate boiling heat transfer term,and refitted the formula parameters according to the experimental data.The optimized formula can reasonably reflect the influence of the operating conditions of the coolant and the thermal properties of the heating surface on the boiling heat transfer.Compared with the experimental value,the maximum deviation is within±20%,and the average deviation is 8.19%,which can provide a prediction basis for the engine cylinder head water jacket boiling heat transfer.Based on the theory of the single-phase homogeneous boiling heat transfer model,and,a conserved control equation of vapor mass rate is established through theoretical analysis and mathematical derivation.According to the results of visual boiling heat transfer experiments,the characteristic of bubble condensation heat transfer during the boiling process was studied.The original bubble condensation heat transfer coefficient equation was optimized,and the optimized equation was used to derive the condensing source term of the homogenous boiling model.The model can be easily embedded in CFD calculation software,and it can be solved together with other single-phase flow control equations in the entire flow field range.According to the relationship between vapor mass rate and volume fraction,the distribution of bubbles in the flow field can be obtained indirectly.Based on the experimental data verification,the established homogenous boiling model can accurately predict the boiling heat transfer heat flux of the heating wall,and can predict the distribution of the void fraction in the channel as well.One-dimensional simulation software is used to provide the boundary conditions of the gas side in the cylinder;the above-mentioned homogenous boiling model is used to calculate the water side heat transfer.Through the program compiled by python language,the mutual information transfer of fluid and solid calculation results on the fluid-solid coupling surface is realized.Finally,the fluid-solid coupling simulation analysis model of the solid heat conduction of the cylinder liner and the boiling flow heat transfer of the cooling water jacket is established.The predicted value of the proposed model is compared with the experimental results of cylinder liner temperature measurement,it shows that the model has high accuracy and the maximum deviation is 1.65%.The fluid-solid coupling calculation method is then used to study the flow and heat transfer characteristics of an OPOC engine water jacket.Studies have shown that due to the action of the spiral fins of the water jacket of the OPOC engine,there is a secondary flow phenomenon in the area between the fins in the water jacket,which enhances the convective heat transfer performance.Increasing the pitch of the spiral fins of the water jacket can enhance heat transfer and reduce the maximum temperature of the cylinder liner,but at the same time it will increase the pressure difference between the inlet and outlet of the water jacket.Under high power density conditions,there is a boiling heat transfer phenomenon in the cooling water jacket.