Experimental And Numerical Model Investigation On Subcooled Boiling Heat Transfer In The Cooling Passages Of Engine Cylinder Head

As the demand of people and society for the power and emission of internal combustion engine in-creasing, many technologies have been applied to strengthen the engine power, such as turbocharging, ultra-high peak pressure, high pressure common rail fuel injection, electronic control unit etc. And also, the level of engine cylinder head thermal load has been significantly improved.by these technologies. In recent years, using the reasonable high temperature boiling heat transfer can achieve the high efficient heat exchanging and ensure the normal operation, which has been a new generation of cooling system design development trend for engine. However, boiling heat transfer is a very complicated phase-change phenomenon, and the knowledge about it is much less than the single phase convective. Moreover, if the boiling don't control, the boiling form will become film boiling and the components will be destroyed. So, it is necessary to carry out some experiments and set up numerical models for subcooled flow boiling in water jacket, so as to make an accurate prediction and assessment for boiling condition in advance.In this study, a heavy-duty natural gas engine is taken as the research object, which is used to meas-ure the temperatures to analyze the heat transfer characteristics in the cylinder head under different cool-ing conditions. The results of research show that the engine cooling condition has a direct impact on the engine cylinder head temperature. If the cooling temperature is low, the temperatures of the cylinder head's measuring points have the same increases as the increasing coolant temperature. When the coolant temperature is high, the measuring temperatures have hardly any difference from the increases in cooling temperature. When the cooling system pressure is improved, the cylinder head temperature shows a trend of increasing especially under high load condition. In addition, the relationship between wall heat flux and wall temperature is no longer linear under the high load working conditions, and the wall heat trans-fer efficiency increases with wall temperature. A series of experimental results indirectly indicated that local nucleate boiling appears in the water jacket.An experimental setup is designed using cast iron as heating block and a high frequency induction hearing device is fixed under the bottom of block to simulate the flow boiling in engine cooling passage. The experimental process investigate the heat transfer characteristics of subcooled water flow boiling water flow under different pressures, flow velocities and liquid subcooling. The research results show that the incipient boiling point position has a direct relationship with local flow parameters. The higher the speed and the lower the inlet temperature will result in higher wall superheat at incipient boiling point. Increasing the flow velocity and reducing the inlet temperature of cooling channels can strengthen the convection heat transfer, but it has little effect on boiling heat transfer characteristics in the fully developed boiling region. With the system pressure increasing, the onset of nucleate boiling will be delayed, and then keep the same developing pattern in the nucleate boiling region. So, the system pressure is an im-portant parameter to control boiling. In addition, the visualization experiment of boiling is carried at the same time. The study found that the normal growth curve of single bubble contains different stages, and different nucleation sites may produce entirely different bubble curves under the same flow conditions and heating power. As the hearing power increasing, the bubble behavior becomes more complex and changeful, such as bubbles coalescence, bubbles break, which result in the probability of bubble size dis-tribution curve become more flat and the bubble size distribution become broader.Based on the current experimental data, three types of boiling empirical correlations with primitive parameters including Chen, Franz and Shah models are utilized to predict the wall heat flux but with an unsatisfactory result In order to accurately predict the heat flux, a modified boiling model basing on Franz's correlation is proposed to adapt to the present equipment, and 360 data points from all cases are correlated with a mean deviation of 6.47%, suggesting that the accuracy of modified asymptotic model is greatly improved with original model. Using the wall function to replace the convective items of correla-tion, a new numerical model for boiling heat transfer is established, which can be applied to all complex geometry. The verified calculations indicate that the new model can accurately predict the relationship of wall heat flux and wall temperature within a wide range of conditions. Finally, the new numerical model is compiled in a 3D coupling heat transfer system to predict the status of boiling in cooling passages un-der different cooling conditions.Basing on the Euler-Euler two-fluid equations, a modified model has been proposed and imple-mented in a CFD code by a user defined function to predict the two-phase flow characteristics in the subcooled boiling flow. Many sub-models such as the bubble departure diameter, bubble diameter, mo-mentum exchange coefficients etc. are associating with local flow parameters, which can be adapt to complex channel. Then, the adaptation and precision of new model is verified using two sets of experi-mental data including different mass flow rates, pressures, subcooling, heat fluxes, section shapes, and flow directions. The calculations indicate that the new two phase boiling model can more accurately pre-dict the distribution of void fraction. Finally, the verified model is used to simulate two-phase characteris-tics in a practical cooling passage for a four-stoke water-cooled natural gas engine. The results show some phenomena of bubble aggregation exist in the engine cooling passages, such as near flame face, exhaust side of water jacket Due to the low flow velocity, the boiling phenomenon result in a high void fraction in these zones, it may bring some potential risks for the heat transfer of engine cylinder head.