Heat Transfer Of The Cooling System With Flowing And Nucleate Boiling In Engines

The quality of the engine cooling performance directly affects the performance of engine combustion, and the reliability and life of engine components, especially important for the performance of fuel consumption and emission behavior, it also is an important factor that restrict further increase of internal combustion engine specific power. Actually, traditional engine cooling system is mainly based on mechanical pump, mostly is single-phase forced circulation water-cooling system. With the development of electronic control equipment of internal combustion engine, the control capability of cooling system is significantly increased; at the same time with the increasing of engine power, the demand on the cooling system is significantly increased, which lead to the more efficient cooling method like flow boiling cooling method is paid attention. The reach results show that, if nucleate boiling engine cooling system is adopted, the cooling efficiency of engine will be strongly increased, the weight and the cost of engine will be reduced, and fuel consumption and pollutants will also be reduced and can get uniform body temperature. Therefore, the study of nucleate boiling engine cooling system has important theoretical and application value.Because the cooling channels in engine cooling system have complex structure, the cooling heat transfer analysis does not always use traditional experimental methods, but mainly rely on computational fluid dynamics (CFD) numerical methods. Through using CFD method to do simulation of nucleate boiling in engine cooling channels, it can get complicated flow field in engine cooling channels, and more important is it can analyze heat transfer and boiling, for example, it can get the number of bubbles, the magnitude of the heat transfer coefficient, etc. Therefore, in this paper, CFD method will be used to study heat transfer of engine cooling channels with flowing and nucleate boiling. In this paper, numerical simulation combined with the theoretical analysis has been applied to do analysis of the heat transfer of flowing and nucleate boiling in the engine cooling channels. The main work includes:Firstly, a mathematical model of nucleate boiling is established based on nucleate boiling heat transfer theory, and this model combined with flow heat transfer to achieve CFD calculation of flow boiling heat transfer. Through studying of the transformation between vapor and liquid in two phase flow, the transformation conditions and the transformation rate between the two phases have been determined. Through writing UDF program and load it into the FLUENT software, the heat transfer with flowing and nucleate boiling can be simulated. By making use of void fraction, the number of bubbles can be expressed. Based on the study of chen model, an innovative method of combination of void fraction and inhibitory factor can make the results of chen model more accurate when bubble is not fully developed.Secondly, according to foreign researchers Tao Bo's the experimental results, the same size rectangular channel model with the experiment is established, and the nucleate boiling heat transfer model in this paper was used to do analysis. The simulation results have good agreement with the experimental results, and error is in the allowable range. The verification shows that:the nucleate boiling heat transfer model in this paper can be used to analyze the heat transfer of flowing and nucleate boiling in the engine cooling channels.The solid-fluid conjugate heat transfer method has been used to study the heat transfer of flowing and nucleate boiling in the engine cooling channels in different velocity conditions and different heating wall temperature conditions. The results show that:Lower flow velocity and higher heating wall temperature are important trends of nucleate boiling occurring direction. The effect of flow boiling heat transfer is much larger than the effect of convection. Clearly, the boiling heat transfer cooling method will be benefit for engine cooling, that is, low pump power consumption, reduced cooling heat loss and improve thermal efficiency.Engine cooling system has complex structure, in order to understanding the basic flow boiling heat transfer law in engine cooling system, the flow channels in engine cooling system are summarized into several typical flow channels and cooling surface morphology, which are used to carry out basic features and characteristics analysis of heat transfer. For the typical cooling surface morphology, flow channel with square bosses, trapezoidal bosses, triangular bosses, circular bosses and flow channel with no boss, five different channels are studied in this paper. The results show that:in the channels with bosses, channel with circular bosses has best heat transfer effect in the same numerical calculation conditions.In this paper, the influence of the curvature of the flow channel was studied. Flow channel with low curvature, moderate curvature and high curvature and no curvature, four different channels are compared and analyzed. The results show that:in the same numerical calculation conditions, the higher curvature of the flow channel cross-section, the smaller boiling heat transfer coefficient, flow channel with no curvature should be used as far as possible in nucleate boiling engine cooling system.Based on the analysis of cooling surface, the actual cooling channel is used to do numerical simulation. In this paper, take rectangular, circular and ellipse channel as an example, the differences in the cooling process and the influence of flattening of the cooling channel was studied. The results show that:the more flat of the cooling channel, the more favorable heat transfer. In the same condition of flow area and flatting degree, the heat transfer effect in rectangular channel is better than the heat transfer effect in ellipse channel.