The STAR-CCM + software is used to investigate the flow inside a cooling water jacket of an in-line six-cylinder diesel engine. The results show that the average flow velocity of the cooling water inside the jacket is 1.669 m/s while the flow velocity distribution is not uniform for each cylinder. Moreover, the fluid velocity in proximity to the cylinder head is too low, thereby affecting the cooling performances of the water jacket. Two corresponding structural optimization schemes are proposed to mitigate this issue and the post-optimization performances of the water jacket are discussed in detail.

At present, internal combustion engines (ICEs), and diesel in particular, have come under significant emissions pressure. With the increasingly stringent emission regulations and the development of various new technologies, diesel engines are developing in the direction of high power, light weight and low emissions [

Generally, for diesel engines, either air or liquid cooling systems are employed, both with their own advantages and disadvantages. The liquid cooling systems have the advantage of being able to keep the temperature of all of the cylinders quite even. In addition, the coolant is usually thermostatically controlled, so that the engine heats up quickly when starting, and always works all the time around the optimal operating temperature [

Since the cooling water jacket is completely enclosed inside the engine body, it is difficult to use test methods to study the flow characteristics inside [

In this paper, a certain in-line six-cylinder diesel engine is used as the research object. The flow condition of the coolant inside the cooling water jacket is simulated and calculated in the STAR-CCM + software, and the flow field distribution of the cooling water jacket is given. The unreasonable flow area is given structural improvement measures according to the calculation results, and the flow conditions of the optimized model and the original machine model are compared and analyzed.

In this paper, a certain in-line six-cylinder diesel engine is the research object, the main technical parameters are shown in

Diesel model | Direct injection, in-line, six-cylinder, turbo-charged, water-cooled, four-stroke |
---|---|

Bore × piston stroke (mm × mm) | 160 × 225 |

Delivery capacity | 27.14 |

Compression ratio | 14.5 |

Rated power/rated speed (kW/(r⋅min^{−1})) |
184/1000 |

Label each cylinder in turn according to the distance from the water inlet. The flow route of the coolant is as follows: The coolant flows in from the water distribution pipe inlet on the exhaust side of the cylinder block water jacket, goes into the cylinder block water jacket of Cylinders 1 to 6 through the water distribution pipe, goes through the up-per water holes on both sides that connect the cylinder block and the cylinder head water jacket to flows into the cylinder head water jacket, and finally flows out of the cooling water jacket from the water outlet of the water col-lection pipe.

The structure of the cooling water jacket model in this calculation is complicated, for its calculation area is com-posed of irregular curved surfaces. Considering this, the paper uses Hyper Mesh and STAR-CCM + software to mesh the geometric model of the cooling water jacket [

The generated fluid area volume mesh is shown in

According to the actual working conditions of the engine cooling water jacket of the research object, the following assumptions are made on the flow condition of the coolant inside the cooling water jacket without affecting the authenticity during the simulation calculation [

Regardless of the boiling effect of the coolant, the simulation calculation of the cooling water jacket is a steady-state process, that is, its flow and heat transfer do not change with time, and the final result tends to be convergent and stable.

Regardless of the boiling effect of the coolant, the flow of the coolant inside the cooling water jacket is a single-phase flow, assuming that all the internal coolant is in liquid state, and the influence of the vapor state is ignored.

The coolant is forced to circulate in the water jacket through the water pump. The coolant Flow velocity is set at the inlet of the fluid area during the simulation calculation, and the velocity direction is the normal direction of the water inlet.

In the CFD simulation calculation of the cooling water jacket, the calculation condition is selected as the rated operating condition of the diesel engine. Liquid water is selected as the coolant, and the physical parameters are set. For the physical parameters that change with temperature (density, dynamic viscosity, etc.), select the value at 353 K. The specific values are shown in

Physical parameters | Value |
---|---|

Density (kg⋅m^{−3}) |
971.8 |

Dynamic viscosity (Pa⋅s) | 3.551 × 10^{−4} |

Specific heat capacity at constant pressure (J⋅(kg⋅K)^{−1}) |
4195 |

The flow of the coolant is under a three-dimensional steady state with incompressible viscous turbulence. The boiling state of the liquid water and the wall roughness of the water jacket are not considered in the calculation process according to the basic assumptions.

Using the Segregated Flow Solver, which solves the integral conservation equations of mass and momentum in a sequential manner. The segregated solver employs a SIMPLE-type pressure-velocity coupling algorithm, where the mass conservation constraint on the velocity field is fulfilled by solving a pressure-correction equation [

Reynolds-Averaged Navier-Stokes (RANS) Turbulence Models provide closure relations for the Reynolds-Averaged Navier-Stokes equations, that govern the transport of the mean flow quantities [^{′}:

The steady Navier-Stokes equations are shown in

_{i} and _{j} are the velocity components in the _{i} and _{j} directions, respectively, and

The turbulence model of the simulation uses the _{t} is the turbulent viscosity,

Turbulent kinetic energy equation:

Turbulent dissipation rate equation:_{k} is the turbulent production, _{b} is the buoyancy production, _{M} is the compressibility modification. For incompressible flow, _{k} = 0 , _{M} = 0.

According to the coefficient suggested by Launder and Sharma, the value of constant _{ε1}, _{ε2}, _{ε}, _{k} are as follows:

For the area near the wall, two-layer full y+ wall processing is used, and the maximum number of calculation steps is set to 500 steps.

For the flow calculation in the fluid area of the cooling water jacket, the flow inlet uses the velocity inlet, and the inlet velocity is set to 7 m/s according to the parameters of the diesel engine water pump. The inlet temperature is set to 338.8 K. The flow outlet uses the pressure outlet, the outlet pressure value is set to 0, this boundary pressure can be regarded as the static pressure of the environment into which the fluid enters, and the outlet temperature is set to 344.8 K. The turbulence designation method of the inlet and outlet adopts the method of turbulence intensity + length scale, the turbulence intensity is 0.01, and the turbulence length scale is 10 mm. The wall of the cooling water jacket follows the non-slip wall boundary condition. For a given wall temperature, the specific values are shown in

Area | Temperature (K) |
---|---|

Cylinder head water jacket | 383 |

Cylinder block water jacket | 363 |

Water distribution pipe | 338.8 |

Water collection pipe | 344.8 |

Unstructured polyhedral meshes are used for this calculation. To determine the final mesh configuration, different density of mesh has been built for the Grid Refinement Study [

Mesh type | Mesh density | Total grid number | Average flow velocity (m/s) |
---|---|---|---|

1 | 0.5 | 3766505 | 1.672668 |

2 | 1.0 | 4591961 | 1.669431 |

3 | 1.5 | 5509422 | 1.677105 |

When the number of grids was increased from 3,766,505 to 5,509,422, the relative deviation of average flow velocity was less than 1%. Within the allowable range of the project, the number of grids used for the cooling water jacket was 4,591,961.

To validate the simulation model, according to Zhang et al.’s calculation model settings [

Parameters | Cylinder number | |||||
---|---|---|---|---|---|---|

1 | 2 | 3 | 4 | 5 | 6 | |

Simulation value (kg⋅s^{−1}) |
64.21 | 67.12 | 62.74 | 68.01 | 70.56 | 72.32 |

Test value (kg⋅s^{−1}) |
64.56 | 65.32 | 64.76 | 66.55 | 68.43 | 70.23 |

Error (%) | −0.542 | 2.756 | −3.536 | 2.194 | 3.244 | 2.976 |

The flow condition of the coolant inside the cooling water jacket determines the temperature distribution of the cooling water jacket. The higher the Flow velocity of the coolant in the water jacket is, the less dead space in-side the cooling water will be, and better cooling effect will the cooling system will get. According to calculation experience in relevant reference, it is usually required that the Flow velocity of coolant in the water jacket of the cylinder block is greater than 0.5 m/s, and the highest average Flow velocity is less than 2 m/s, while the Flow velocity of the cooling water in the area with greater thermal load in cylinder head water jacket is greater than 1 m/s [

The streamline distribution of the coolant inside the cooling water jacket is shown in

The cloud diagram of the flow velocity distribution of the coolant is shown in

The cloud diagram of the coolant velocity distribution on the upper part of the water jacket of the cylinder block is shown in

For the cooling system, if the pressure loss is too large, it means that the flow resistance of the internal coolant is also large, which will reduce the mechanical efficiency of the engine and increase the requirements for the performance of the water pump and the sealing surfaces in the cooling water jacket. If the pressure loss is too small, the cooling effect will be insufficient, and it will not meet the design requirements [

The pressure distribution cloud diagram of the cooling water jacket is shown in

Through the analysis of the simulation results of the original engine model and the problems found, according to the structural characteristics of the cooling water jacket combined with the previous theoretical data and experience [

In view of the uneven flow velocity of each cylinder of the original engine model, the main consideration is to keep the flow velocity of each cylinder as consistent as possible. During the flow of coolant, due to the resistance loss along the way, the flow velocity continues to decrease. So the farther away from the water inlet, the lower the flow velocity is in the water jacket, and the lower the flow into the water jacket at the same time, which affects the cooling effect.

The optimization plan is to adjust the diameter of the water inlet holes on both sides of each cylinder to increase the flow velocity and improve the cooling effect. The optimized positions of the water inlet holes of each cylinder are shown in

In view of the low-velocity area problem of the cylinder head water jacket of the original engine model, the optimization plan is to adjust the size of the processing hole at the lower section of the cylinder head water jacket from ø7 to ø10, which is used to strengthen the cooling of the key area of the lower section of the cylinder head water jacket. The location is shown in

Location | Cylinder 1 | Cylinder 2 | Cylinder 3 | Cylinder 4 | Cylinder 5 | Cylinder 6 |
---|---|---|---|---|---|---|

Original | 16 | 16 | 16 | 16 | 16 | 16 |

Optimized | 16 | 16 | 18 | 18 | 20 | 20 |

The original engine model’s and optimized water jacket flow velocity distribution cloud diagrams are shown in

The original engine model’s and optimized plans for the cross-sectional velocity distribution cloud diagram of the lower section of the cylinder head water jacket are shown in

The coolant flowing into the water jacket of the cylinder head from the water inlet holes of Cylinders 1 to 4 has been significantly enhanced, the low-velocity area of the cylinder head water jacket has been reduced, and the flow uniformity has also been improved. This proves that the optimized solution can achieve the improvement purpose of cooling water.

Based on the CFD simulation calculation of the flow condition of the cooling water jacket of the engine in the STAR-CCM + software, the flow velocity distribution and pressure distribution of the coolant in the cooling water jacket of the original engine model are obtained, which would provide a basis for the optimal design of the cooling water jacket.

The flow velocity of each cylinder of the original engine model is uneven, and the local flow velocity of the water jacket of the cylinder head is too low, which affects the cooling effect. Therefore, two structural optimization schemes are proposed for these two problems. By comparing the calculation results of the original engine model and the calculated results, an optimized scheme was obtained to improve the flow performance of the cooling water jacket.

Personally, I would like to thank support from our team members, and the reviews whose constructive and detailed critique contributed to the quality of this paper.