Research On Heat Transfer Of Subcooled Flow Boiling In The Cooling Passages Of Engine Cylinder Head Based On Bubble Behavior Dynamics

Since the development of the modern internal combustion engine is changing to high cylinder pressure and high power density output,more and more attention has been paid to the applying foreground of this heat transfer form in engine cooling systems.However,the possible shift to ‘transition boiling' should be seriously considered,which may occur under high power conditions and lead to thermal failure of the cylinder head.In the design of cylinder head,how to calculate the heat transfer in cylinder head accurately is an urgent problem in the design of cylinder head.Because of the local boiling phenomenon in the water jacket of cylinder head,it is difficult to calculate the heat transfer of cylinder head accurately.Based on the application background,this paper studies the boiling heat transfer based on the bubble behavior dynamics.An experiment is carried out to closely observe the boiling behaviors on horizontal aluminum heating surface using high-speed photography.A bubble tracking algorithm is developed,which efficiently determines the locations of active nucleation sites.Results show that values of active nucleation site density(NSD)and bubble departure frequency BDF are greater in conditions of elevated wall superheat degrees and system pressures.Comparatively,liquid subcooling and flow velocity have little effects on the active NSD and BDF.Regarding the inconsistencies between experimental data of the current study and predicted data of existing models,revised models are proposed.The newly proposed empirical formula of active NSD conforms well to the experimental data with a mean relative error of 25.27%.The modified model for BDF based on the dimensionless BDF and dimensionless heat flux agrees well with experimental data with a mean relative error of 21.25%.The experiment confirms that the linear relationship between bubble contact diameter and bubble departure diameter is still valid under high mass flux.The bubble contact diameter is 0.4027 times of the bubble departure diameter.A force balance model based on the newly modified model of bubble growth rate is used to calculate bubble departure diameter and bubble lift-off diameter,and its predicted values are in good agreement with the present measured values.The force balance model can be expressed by a series of dimensionless parameters: density ratio,Jacob number corresponding to wall superheat,Jacob number corresponding to subcooling,Prandt number,Reynolds number,and contact angle.Two new empirical formulas of bubble departure diameter and bubble lift-off diameter in a horizontal channel are developed based on the dimensionless parameters using the Buckingham theorem.The empirical formula of bubble departure diameter predicts the departure diameter obtained in a horizontal channel with an average relative error of 12.08%.The empirical formula of bubble lift-off diameter predicts the lift-off diameter obtained in a horizontal channel with an average relative error of 6.80%.In the study of single bubble growth process,it is found that the single bubble growth model established in this paper can accurately simulate the single bubble growth process in pool boiling and subcooled flow boiling.The growth of bubbles mainly comes from the evaporation of micro liquid layer and subcooled layer.The rapid growth stage of the bubble is mainly due to the rapid evaporation of the micro liquid layer.It is also found that the temperature at the bottom of the bubble and the thickness of the micro liquid layer in the subcooled flow boiling are different from that in the pool boiling.In subcooled flow boiling,the upstream wall temperature of the bottom of the bubble is different from that of the downstream.The same phenomenon exists in the thickness of the micro liquid layer.The evaporation rate in the upstream region of the micro liquid layer is faster than that in the downstream region,which is also the reason for the difference between the upstream temperature and the downstream temperature at the bottom of the bubble.A new wall heat flux partitioning model is developed using an experimentally based mechanistic approach.Wall heat transfer is divided into convective heat transfer,evaporation heat transfer,transient cooling heat transfer and bubble sliding heat transfer.Based on the two-fluid model and the new wall heat flux partitioning model,a numerical simulation of subcooled boiling under low-pressure conditions is performed on CFX with a user-defined program.The Multiple Size Group model considering the effects of vapor bubbles coalescence and breakup is used to describe the different sizes of bubbles in the liquid phase.The results show that the model can well predict the important parameters such as wall heat flux,void fraction,bubble diameter and so on.This lays a good foundation for Euler multiphase model used in the engine cooling system and provides a good theoretical basis for the optimization and design of a high-temperature cooling system.The multi-field sequential coupling strategy of engine cylinder head is established.The heat transfer and flow boundary conditions of the gas side and the cooling water side are separately obtained by calculating the working process in cylinder and the flow of cooling water.Based on those boundaries,the new wall heat flux partitioning model is applied to the cooling passage for a water-cooled diesel engine.Compared with the single-phase flow calculated in engine cooling passage,the new model reflects the real heat transfer and gives an accurate temperature field in engine cylinder head.The deviation between the temperature predicted by the new wall heat flux partitioning model and the measured temperature is less than 5%.It is found that the boiling area is mainly concentrated in the water channel near the fire surface and exhaust port.The distribution of vapor phase in cooling passage is obtained.The vapor fraction is 0.11 near the fire surface.The experiment provides data support for the correlation between bubble behavior and wall heat transfer.The newly established bubble growth model and wall heat flux partitioning model provide theoretical support for the study of enhanced heat transfer of subcooled boiling.The research methods and results of this paper play an important guiding role and practical application value in the design and optimization of engine advanced cooling system.