Research On Flow And Heat Transfer In Diesel Engine Cylinder Head And Improvement Of Water Jacket Structure

As the diesel engines tends to high combustion pressure and high power density output,the heat load of key components of modern diesel engines has become increasingly serious.The cylinder head is one of the key components of the diesel engine's extremely harsh working environment.It suffers extreme heat loads during the actual operation of the diesel engine,and the heat problem is particularly serious.This can seriously affect the reliability of the diesel engine and its durability.Therefore,in the design and improvement of modern diesel engine cooling system,it is of great significance to accurately predict and evaluate the heat transfer status of diesel engine cylinder head and control its heat load reasonably and accurately.In this paper,taking a four-cylinder water-cooled diesel engine as the research object,the temperature field test of the cylinder head was carried out.The simulation of the flow and heat transfer of the coolant in the cooling water jacket of the cylinder head and the improved design of the cooling water jacket structure of the cylinder head were carried out.The main work is as follows:?1?In the area where the temperature on the fire circle is high and the temperature gradient changes greatly,20 measuring points are arranged and the installation of the hardness plug on the fire circle is completed.The cylinder head temperature measurement test is performed under the calibration conditions and the temperature field distribution of the cylinder head is obtained.The test results show that the temperature at the measuring point 14 of the"bridge zone"area of the two exhaust valves of the third cylinder is the highest,reaching 580 K.The temperature at the exhaust port side of the fire circle is above 550 K,and the intake port side is below 550K.Therefore,the measurement results show that the temperature test results meet the actual conditions of the highest temperature in the"bridge zone"area of fire circle and the exhaust port side temperature is generally higher than the intake port side temperature.?2?ProE software was used to establish a three-dimensional model of diesel engine block,cylinder head,intake and exhaust ports.The STAR-CCM+software was used to mesh the fluid and solid domains of the cooling system.Under the same boundary conditions,the traditional convection heat transfer and Euler two-phase flow boiling heat transfer models were used respectively to simulate the flow field and temperature field of the cylinder head,and the measured and calculated values of the cylinder head fire circle were compared.The results show that the fractional errors error between the calculated results of the traditional convection heat transfer model and the measured values are greater than 5 percent,which is a large deviation from the experimental value.The calculation errors of the Euler two-phase flow boiling heat transfer model are less than 5 percent,and the calculation results are closer to the test.The simulation results of the Euler two-phase boiling heat transfer model are closer to the experimental values.The calculation results of the Euler two-phase boiling heat transfer model are more in line with the actual conditions of the cooling fluid flow and heat exchange in the cylinder head.?3?On the basis of comparative analysis of the calculation accuracy of traditional convection heat transfer and Euler two-phase flow boiling heat transfer,the flow field,pressure field,temperature field,etc.of the two-phase flow boiling heat transfer in the cylinder head cooling water jacket were analyzed in detail.The results show that the distribution of coolant velocity and flow in the cooling water jacket of the cylinder head is not uneven.Because of the unreasonable layout of the upper water hole,the low flow velocity zone and the flow dead zone with a flow velocity of less than 0.5 m?s^-1 exist in the bridge zone area of the cooling water jacket.Due to the inertia of the coolant flow and the large volume of the cooling water jacket,the coolant flow at the bottom of the cylinder head cooling water jacket is small.The uneven distribution of coolant flow rate and flow causes the local temperature of the cooling water jacket of the cylinder head to be high and a large area of high temperature occurs.In the actual operation of the diesel engine,it is prone to thermal fatigue and other problems,which causes some damages to the body.?4?Aiming at the problems existing in the cooling water jacket of the cylinder head of the original engine,the cooling water jacket of the cylinder head was improved and two improvement schemes were proposed.The improvement scheme was evaluated through the flow field and temperature field.The results show that the two improved schemes have improved the flow velocity of the cooling water jacket bridge zone to a certain extent.The maximum flow velocity of scheme 1 and scheme 2 were increased by 0.16 m?s^-1 and 0.27 m?s^-1,respectively.The areas with low flow velocity and flow dead zone were further reduced,and the maximum temperature was reduced by 1.85 K and 4.05 K.The area of high temperature is further reduced.Through the heat load in the high temperature area.Through the comparison and analysis of the flow field and the temperature field,scheme 2 has good results in improving the low flow velocity area and reducing the heat load in the high temperature area.The improved scheme provides a reference example for the improved design of cooling water jacket of cylinder head.