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CFD fluid dynamics analysis of engine cooling water jacket

Abstract: in the development process of modern automobile engine, in order to shorten its development cycle, calculation and analysis are applied more and more in the whole development process. CFD (Computational Fluid Dynamics) analysis of engine cooling water jacket has become an essential means of calculation and analysis in the process of engine development. This calculation and analysis can ensure that there is good coolant flow around the combustion chamber and exhaust duct with high engine heat load, and the pressure loss is relatively low. The engine analyzed is a medium-sized series diesel engine newly developed by FAW technology center, which can meet Euro II emission regulations. In the conceptual design stage, CFD is used to analyze the engine cooling water jacket. This paper mainly introduces the process of using CFD analysis to optimize the engine cooling water jacket

key words: CFD analysis of engine cooling water jacket

1 preface

in the development process of modern automobile engine, in order to shorten its development cycle, calculation and analysis are applied more and more in the whole development process. CFD (Computational Fluid Dynamics) analysis of engine cooling water jacket has become an essential means of calculation and analysis in the process of engine development. This calculation and analysis can ensure that there is good coolant flow around the combustion chamber and exhaust duct with high engine heat load, and the pressure loss is relatively low

the engine analyzed is a medium-sized series diesel engine newly developed by FAW technology center, which can meet Euro II emission regulations. In the conceptual design stage, CFD is used to analyze the engine cooling water jacket. This paper mainly introduces the process of using CFD analysis to optimize the engine cooling water jacket

2 calculation model and boundary conditions

the commercial software FLUENT is used for calculation, and the standard k is selected for turbulence model- ε The model has good convergence and accurate results for most flow problems. There are three models to choose from close to the wall: Standard wall functions; Non-Equilibrium Wall Functions； Two layer zonal models have different requirements for grids close to the wall. Standard wallfunctions is selected in this calculation. This model is very applicable to many flow problems in engineering. It requires the size of y+ to reflect the lattice close to the wall. Y+ is related to the wall shear stress and the distance between the lattice close to the wall and the wall

specific calculation formula: y+= △ YP/υ sqrt（ τ w/ρ）

in the formula, △ YP - the distance between the lattice close to the wall and the wall

υ—— Kinematic viscosity

τ W - wall shear stress

ρ—— Density

in order to get more accurate results, the value of y+ should be within a certain range when selecting the standard wall equation. It can be adjusted by the grid size close to the wall

because the structure of the engine water jacket is very complex, in order to speed up the pretreatment, the four sided body is usually used. In order to obtain more accurate results, the second-order difference scheme must be selected. In this analysis, the calculation mode is steady state, the flow type is adiabatic, incompressible and turbulent flow, and the coolant is 45% water and 55% hexanediol additive

CFD analysis of engine cooling water jacket can not only give the flow situation, velocity, flow distribution and pressure drop of the whole flow field, but also provide boundary conditions of heat transfer coefficient for finite element thermal analysis

the calculation formula of heat transfer coefficient (HTC) of wood flour as an organic filler of plastic is as follows:

htc= CP* ρ· sqrt( τ w/ρ)/Pr*(v/sqrt( τ w/ρ)+ 9.24*((Pm/Pr)^0.)* (1+0.28*e^ (-0.007*pm/pr))

where CP - specific heat capacity

ρ—— Density

pr - effective Prandtl number

pm -- molecular Prandtl number

Τ W - wall shear stress

v - speed

the heat transfer coefficient is only related to the flow of the fluid. In the finite element calculation, the boiling effect model needs to be used locally to correct it. The boiling effect is related to the temperature difference between the fluid and the metal

the calculated boundary conditions are: inlet: mass flow; Outlet: outlet

3 optimization design of water jacket of cylinder block

because the upper half of the engine cylinder block is close to the combustion chamber, its heat load is high, while the heat load of the lower half is low. According to the characteristics of its heat load, the structure of the cylinder water jacket should achieve high upper flow rate and low lower flow rate, but at the same time there should be no dead zone of no flow. The water jacket of the single cylinder block of the engine first proposed is called the basic structure, and its velocity distribution is shown in Figures 1 and 2. Figure 2 shows the expanded velocity distribution close to the inner wall surface. It can be seen that the water jacket structure of the cylinder block basically meets the requirements of high velocity in the upper part and low velocity in the lower part, but the velocity distribution on the left and right sides is not very uniform. The average heat transfer coefficients on the left and right sides of the inner wall surface are 3028 w/m2 · K and 4578w/m2 · K respectively, and the difference is also relatively large

Figure 1 velocity distribution of water jacket structure of basic cylinder block figure 2 velocity distribution of water jacket structure of basic cylinder block close to the inner wall

in order to make the velocity distribution on the left and right sides more uniform, remove the outlet hole on the exhaust side, this structure is called improvement scheme 1. The velocity distribution is shown in figures 3 and 4. It can be seen that the velocity distribution tends to be uniform. The average velocity in the upper half is about 1m/s, and the average velocity in the lower half is about 0.4m/s. The average heat transfer coefficients on the left and right sides of the inner wall are 3911 w/m2 · K and 4483 w/m2 · K respectively, and the difference is relatively small

Figure 3 cylinder water jacket improvement scheme 1: flow velocity distribution figure 4 cylinder water jacket improvement scheme 1: flow velocity distribution near the inner wall surface

in order to further improve the flow velocity in the upper half, improvement scheme 2 is proposed. Figure 5 and Figure 6 show the velocity distribution close to the inner wall of improved scheme 1 and improved scheme 2 respectively. It can be seen that the average velocity of the upper half of improved scheme 2 has increased. The average heat transfer coefficients of the left and right upper half of the inner wall surface in improved scheme 1 are 5258 and 5489 w/m2 · K respectively, and the average heat transfer coefficients of the left and right upper half of the inner wall surface in improved scheme 2 are

5608 w/m2 · K and 5956 w/m2 · K respectively, which are improved in improved scheme 2

Fig. 5 improvement scheme of cylinder water jacket 2: velocity distribution at the inner wall Fig. 6 improvement scheme of cylinder water jacket 2: velocity distribution at the inner wall

the shape of water outlet hole is also optimized in the analysis. Fig. 7 shows the shape of water outlet hole before optimization and its cross-sectional velocity vector diagram. It can be seen that the shape before optimization leads to strong vortex at the water outlet, so the pressure loss is large. Figure 8 shows the optimized outlet hole shape and its cross-sectional velocity vector diagram. It can be seen that the vortex is basically eliminated by the new structure. Figure 9 shows the comparison of pressure loss between the two. It can be seen that the optimized outlet structure reduces the pressure loss by nearly one third

Figure 7, figure 8 the shape of the water outlet and its cross-section before and after optimization. It communicates through the serial port (COM number) on the back of the computer. Velocity vector diagram

Figure 9 the comparison of pressure loss before and after optimization of the shape of the water outlet

4 optimization design of the water jacket of the cylinder head

the area with high heat load of the engine cylinder head is the bottom of the cylinder head corresponding to the cylinder barrel, especially the "nose bridge" area between the inlet and exhaust valves. Therefore, the design of the water jacket of the engine cylinder head should ensure good flow in the "nose bridge" area. The gas temperature on the exhaust side is higher, so it also needs better flow. At the same time, the front and rear cylinders should be as uniform as possible

the first designed water jacket structure of the engine cylinder head is called the original structure. The velocity distribution of the calculated bottom plane and the velocity distribution of the "nose bridge" area are shown in Figure 10 and Figure 11. Figure 12 shows the proportion of water flow from the water inlet hole of each cylinder to different channels

Figure 10 flow velocity distribution in the bottom plane of the original structure of the engine cylinder head Figure 11 tangential velocity distribution and average velocity in the "nose bridge" area of the original cylinder head

it can be seen from Figure 10 that the flow velocity between the front and rear cylinders is uneven, because the cylinder head is longitudinal water, and all water must go out from the front of the exhaust side of the first cylinder, so the unevenness between the front and rear cylinders is inevitable. However, the gap can be reduced by properly adjusting the water inflow of the six cylinders

it is found from Figure 11 that the average speed in the "bridge of the nose" area is between 0.45 ~ 0.76 M/s. according to experience, this speed is slightly lower. From the flow distribution ratio between various channels in Figure 12, more water flows away from the underside of the inlet

in order to improve the flow velocity in the "nose bridge" area and reduce the flow from the channel below the inlet, an improvement scheme 1 is proposed

Figure 12 flow distribution ratio between channels of each cylinder in the original cylinder head Figure 13 tangential velocity distribution and average velocity in the "nose bridge" area of cylinder head improvement scheme 1 Figure 13 and Figure 14 show the velocity distribution in the "nose bridge" area of improvement scheme 1 and the flow distribution ratio between channels. It can be seen that the flow through the channel below the inlet is greatly reduced, and the flow through the main "bridge of the nose" area is increased, with an average flow rate of about 0.1m/s

Figure 14: flow distribution ratio between channels of each cylinder in cylinder head improvement scheme 1 Figure 15: cylinder head improvement scheme 2: tangential velocity distribution and average velocity in the "nose bridge" area

from the velocity distribution diagram of the "nose bridge" area in figures 11 and 13, it can be found that the flow velocity in the lower part of the "nose bridge" area of each cylinder is lower than that in the upper part, but the lower part needs higher flow velocity. In order to achieve this purpose, it is proposed to change the operation and facilitate the entry scheme 2. Because the geometric drawing was not modified, it was only repaired from the lattice of improvement scheme 1, so the shape was not particularly smooth. Figure 15 shows the flow velocity distribution in the "bridge of the nose" area of improvement scheme 2. It can be seen that the improvement measures have increased the flow velocity and its average flow velocity in the lower part of the "bridge of the nose" area

5 analysis of integral water jacket

the previous analysis is a single cylinder block and cylinder head, and some boundary conditions are assumed. In order to test the real flow situation, the analysis of integral water jacket is required

the integral water jacket includes cylinder block, cylinder head, cylinder gasket and oil cooler, which are divided into grids in fluent's preprocessor gambit and independent preprocessing software ICEM. The requirements for the test of cylinder block and insulator material peeling and tearing are as follows: the shape of cylinder gasket is relatively standard, and the six sided body is used; The shape of the oil cooler box and cylinder head is complex, and the four sided frame is used. The total number of grids is about 2.5 million

because the lattice of each component is divided separately, interface is used at the connection of components, which makes it easier to solve complex models

the flow conditions of oil cooler box, cylinder block and cylinder head in the calculation of integral water jacket are introduced below

5.1 flow analysis of oil cooler box

the main purpose of oil cooler box is to ensure that the oil can be properly cooled. Of course, it can only reflect the cooling condition on the water side. The final cooling condition of oil is also related to the flow of oil in the oil plate. Only the water side and oil side have good flow, the cooling of oil can meet the requirements. In order to ensure the cooling of engine oil, it is usually required that there must be a flow rate of about 1m/s between engine oil cooler plates, but the flow rate should not be too high to ensure the strength of engine oil plates

Figure 16 shows the shape of the oil cooler box as viewed from the side of the engine. After the water from the water pump enters the oil cooler, part of it passes through the oil plate, and part of it enters the cylinder block water jacket of six cylinders respectively

Figure 16 structure of oil cooler box Figure 17 flow velocity distribution along the length of oil cooler

Figure 17 and Figure 18 show the flow velocity distribution along the section and cross section along its length, with two round holes

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