| The valve is an important component of the engine,which is used to control the inflow and outflow of gases,especially the exhaust valve is subject to thermal loading due to high temperature and pressure inside the cylinder that must tolerate the limit of material temperature for a sustainable and optional operation.The turbocharged gasoline direct injection(TGDI)engines have particularly high in-cylinder temperatures.Valves at the outlet of the combustion chamber must be durable at high temperatures of about 1000 °C.For this reason traditional solid steel valves are a limiting component since the alloys used are not durable under the highest temperatures.Since sodium inside the hollow valve melts at about 97.5 °C during engine operation it can be used as working fluid in order to improve the heat transfer in the valve.The periodic nature of the valve movement which maintains this cooling technique leads to a better distribution of the heat in the valve by avoiding extremely hot areas in the valve stem.The influence of several relevant influencing factors such as the valve speed,the valve angle,the valve pipe diameter and the liquid fill level were studied.The present study constitutes a contribution to predict the temperature maps of exhaust valves considering the real conditions of an engine operation.As an application,this methodology is used to highlight the temperature maps and to show the region of extreme temperature and heat flux in the aim of avoiding any damage.To our knowledge,no similar temperature maps predictions of sodium-cooled are published in the literature.This research was supported by the Graduate Scientific Research and Innovation Foundation of Chongqing,China,“Numerical Study on Heat and Mass Transfer in Liquid Sodium-cooled Valve of Direct Injection Turbocharged Gasoline Engine”(Grant No: CYS19029).The main research contents are as follows:Firstly,the temperature distribution of the valve axial and radial of the solid valve and sodium-cooled valve were examined by means of the hardness test.Experiments were performed using a 1.5 L TGDI engine bench test.The test results showed that the temperature of the sodium-cooled valve is lower than that of the solid valve at the same position in the axial and radial direction of the valve.The maximum axial temperature of the sodium-cooled valve is 667 °C,which is 153 °C lower than that of the solid valve,and the position of the maximum temperature is shifted.The maximum temperature of the sodium-cooled valve is located at the neck of the valve stem,but that of the solid valve is at the first hot spot(the intersection of the extension of the valve seat cone angle and the valve axis);The maximum radial temperature of the sodium-cooled valve is 67.8 ° C lower than that of the solid valve.The maximum temperature is at the center of the bottom surface of the valve head(second hot spot).Then,in order to predict the temperature maps of the solid valves,the present study uses third boundary conditions(the heat transfer coefficient and adiabatic wall temperature)considering the complex surrounding working environment of the cylinder head,since the first boundary conditions and second boundary conditions are not applicable.An adequate subdivision of the valve is used to better assess the effect of each part of the cylinder head.According to the heat transfer of the valve working in the real environment,considering the difference parameters affecting the heat transfer through the valve,seven zones are considered in theory,but in order to study conveniently,in this paper,we divided the valve into four zones: Stem-guide,Stem-port,Seat and Combustion face.Therefore,the instantaneous heat transfer coefficient and adiabatic wall temperature for each subdivision are evaluated during the four-stroke of an engine.The average values of the boundary conditions for each zone are determined during one cycle of the engine(T = 4?),and the average value as the boundary condition of the finite element analysis to calculate the valve temperature field.The corrected average value is used as the external boundary condition of the sodium-cooled valve.Further,for the boundary conditions calculation inside the sodium-cooled valve cavity,in this study,we assume that the valve remains stationary,liquid sodium and air reciprocate relative to the valve,and a dynamic mesh model is established.The interaction of liquid sodium and air is a gas-liquid two-phase flow problem,a volume of fluid(VOF)multiphase flow model is established for this purpose.Finally,the internal boundary conditions are calculated,combined with the external boundary conditions of the valve,and the temperature field of the sodium-cooled valve is finally calculated.The calculation results show that the maximum temperature error between the calculated temperature and the measured temperature is 45.5 °C,the relative error is7.52% in the axial of the valve,and the maximum error is 37.2 °C,the relative error is5.79% in the radial direction and their maximum temperature relative error is less than8%.Finally,the influence of several relevant influencing factors such as the valve speed,the valve angle,the valve pipe diameter and the liquid fill level were studied.It was found that the liquid fill level is one of the most influencing factors regarding the efficiency of the heat transfer whereas the influence of the valve speed and in particularly the valve angle and the valve cavity diameter were almost negligible.The ideal liquid sodium fill level is about 30%-50%. |
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