Calculation method for partition heat transfer of inner wall face of low-heat-dissipation diesel engine cylinder

Abstract

The invention provides a calculation method for partition heat transfer of an inner wall surface of a low-heat-dissipation diesel engine cylinder, which comprises the following steps of: S1, constructing a quasi-dimensional combustion heat release model in an engine cylinder, and calculating the heat release amount of mixed gas and gas temperature in in-cylinder sub-regions corresponding to a piston, a cylinder wall and a cylinder cover; s2, a multi-area heat network transfer model of a fuel gas-piston / cylinder wall / cylinder cover-cooling system in a low-heat-dissipation diesel engine cylinder is established, and the heat exchange heat flux density between the piston, the cylinder wall, the cylinder cover and the cooling system of the low-heat-dissipation diesel engine and the temperature of the wall faces of the piston, the cylinder wall and the cylinder cover are calculated; and S3, establishing a partition Woschni heat transfer model, and calculating the convective heat transfer coefficient and heat transfer capacity of the partition on the inner wall surface of the cylinder. According to the calculation method for partition heat transfer of the inner wall face of the low-heat-dissipation diesel engine cylinder, the space-time effect of flames, the cylinder cover, the piston and the cylinder sleeve is considered, calculation of the temperature difference and the high-temperature contact area is more accurate, and the calculation result of the heat transfer amount of each area is close to a CFD result.

Description

Technical Field

[0001] The present invention belongs to the field of application and research and development of vehicle internal combustion engines, and particularly relates to a calculation method for the partition heat transfer of the inner wall surface of a low-heat-dissipation diesel engine cylinder. Background Art

[0002] The calculation and analysis of the heat exchange amount of the diesel engine body are of great significance to engine design. By accurately calculating the heat exchange between the wall surface and the gas and coolant, the thermal loads of components such as the cylinder wall, piston, and cylinder head can be effectively evaluated, thereby optimizing thermal management, preventing overheating or thermal imbalance, and improving the engine efficiency and durability. In addition, analyzing the heat exchange amount can also help design an efficient cooling system, improve combustion efficiency, reduce emissions, and enhance the overall power performance. For the calculation of the heat exchange of the diesel engine body, a three-dimensional numerical simulation method of fluid-structure coupling or a calculation method based on the coupling of one-dimensional and three-dimensional is generally used. Due to the need to consider factors such as complex in-cylinder combustion and the complex structure of the cooling flow path, the three-dimensional numerical simulation method has a huge calculation cost and is difficult to be used for engine design and optimization. Since the current Woschni heat transfer model cannot distinguish the heat transfer characteristics of different spaces, the existing calculation methods based on the coupling of one-dimensional and three-dimensional cannot accurately calculate the heat transfer amount of the wall surface in different regions of the cylinder. Summary of the Invention

[0003] In view of this, the present invention aims to propose a calculation method for the partition heat transfer of the inner wall surface of a low-heat-dissipation diesel engine cylinder to solve the problem that the existing methods cannot accurately calculate the heat transfer amount of the wall surface in different regions of the cylinder.

[0004] To achieve the above object, the technical solution of the present invention is realized as follows: A calculation method for the partition heat transfer of the inner wall surface of a low-heat-dissipation diesel engine cylinder includes the following steps: S1. Construct a quasi-dimensional combustion heat release model in the engine cylinder, consider the spray penetration and wall impingement effects and the mixture spatial distribution state, and calculate the heat release amount of the mixture and the gas temperature in the corresponding sub-regions of the cylinder for the piston, cylinder wall, and cylinder head; S2. Define the engine piston, cylinder wall, and cylinder head as independent thermal nodes, consider the insulation layer material, thickness, and material thermal properties, and establish a multi-region thermal network transfer model of the in-cylinder gas-piston / cylinder wall / cylinder head-cooling system to calculate the heat transfer flux density between the piston, cylinder wall, cylinder head of the low-heat-dissipation diesel engine and the cooling system and the wall surface temperatures of the piston, cylinder wall, and cylinder head; S3. Consider the effect of the in-cylinder liquid spray flame on the cylinder head, piston, and cylinder wall surfaces, calculate the flow velocities and heat transfer areas of the flame zone and non-flame zone on each wall surface, establish a partitioned Woschni heat transfer model, and calculate the partitioned convective heat transfer coefficient and heat transfer amount of the inner wall surface of the cylinder.

[0005] Further, the specific method for constructing the quasi-dimensional combustion heat release model in the cylinder of the engine in S1 is as follows: S11. Set the key structures and operating parameters of the engine; S12. Set the quasi-dimensional combustion heat release model; S13. Calibrate the parameters of the quasi-dimensional combustion model; S14. Calculate the convective heat transfer coefficients and heat transfer amounts on the inner wall surfaces of the corresponding sub-regions in the cylinder for the piston, cylinder wall, and cylinder head.

[0006] Further, the method for defining the engine piston, cylinder wall, and cylinder head as independent thermal nodes and establishing the multi-region thermal network transfer model of the in-cylinder gas-piston / cylinder wall / cylinder head-cooling system in S2 is as follows: S21. Import the geometric model of the engine cylinder head; S22. Divide the key regions according to the functions and structures of the cylinder head; S23. Set the grid size; S24. Generate the grid, divide the cylinder head geometry into small grid units, and ensure that each region is reasonably divided; S25. After completing the grid, perform verification and ensure it is correct; S26. Import the grid files of the cylinder, piston, and cylinder head, and set the boundary conditions.

[0007] Further, the specific method for calculating the flow velocities and heat transfer areas of the flame zone and non-flame zone on each wall surface, establishing the zoned Woschni heat transfer model, and calculating the convective heat transfer coefficients and heat transfer amounts of the zoned inner wall surface of the cylinder in S3 by considering the effect of the in-cylinder liquid spray flame on the cylinder head, piston, and cylinder wall surfaces is as follows: S31. Consider the effect of the liquid spray flame on the piston, cylinder wall, and cylinder head walls in the cylinder, and establish the mathematical model of the flow velocity on each wall surface; S32. Establish the mathematical model of the heat transfer area of the flame zone and non-flame zone of the piston, cylinder wall, and cylinder head walls.

[0008] Further, the specific process of establishing the mathematical model of the flow velocity on each wall surface by considering the effect of the liquid spray flame on the piston, cylinder wall, and cylinder head walls in S31 is as follows: Based on the convective heat transfer theoretical model: ; Since the Pr number of the gas varies little with temperature, for example, the Pr numbers at 30 °C and 1000 °C are 0.71 and 0.68 respectively, the influence of Pr on Nu is ignored; ; where and are model constants; this model does not consider the influence of radiation; Because: ; Therefore: ; wherein, is the fluid velocity, is the characteristic length, is the fluid thermal conductivity; Considering that the fluid density is a function of temperature and pressure, and the fluid viscosity and thermal conductivity are functions of temperature: ; ; ; According to the derivation of Woschni, the heat transfer coefficient can be expressed as: ; wherein the influence of piston movement on the in-cylinder gas flow movement needs to be considered , and at the same time, the influence of combustion on heat transfer also needs to be considered ; The mathematical model of fluid velocity is: ; ; In the formula is the heat transfer coefficient (W / (m 2 ·K)), is the gas pressure (Pa), is the gas temperature (K), is the instantaneous in-cylinder volume (m 3 ), represents the influence of the change in the mass of the working fluid in the cylinder, that is, the influence of fuel injection on the combustion temperature can be considered; is the instantaneous reverse-dragging in-cylinder instantaneous pressure, is the in-cylinder instantaneous pressure under combustion conditions; is the single-cylinder displacement; , and are the reference values at a certain moment when the pressure, volume and temperature are all known, and generally the intake valve closing moment or the combustion start moment is taken; is the engine stroke, is the rotational speed; other variables are model constants, generally determined according to tests; The model coefficient setting of the original Woschni model: ; ; = 6.18 (scavenging stage), 2.28 (compression, combustion and expansion stages); =0 (scavenging and compression stage), 3.24e-3 (combustion and expansion stage); Since the in-cylinder flow during the combustion heat release process of a diesel engine is mainly affected by the spray and its jet flame; the differences in the interactions between the liquid spray flame and the cylinder head, cylinder liner, and piston are significant, so the influence of the liquid spray flame on the flow in the near-wall regions of the cylinder head, cylinder liner, and piston can be considered to construct a zoned near-wall flow model to more accurately describe the effect of combustion on heat transfer .

[0009] Furthermore, the process of establishing the mathematical model of the heat transfer area of the flame zones and non-flame zones of the piston, cylinder wall, and cylinder head wall in S32 is as follows: Based on the interaction relationships between the liquid spray flame and the cylinder head, cylinder liner, and piston, the following assumptions are made: 1) The interaction between the liquid spray flame and the cylinder head wall surface is approximated as a convective heat transfer process of a plane parallel incoming flow; 2) The interaction between the liquid spray flame and the bottom surface of the piston is approximated as a convective heat transfer process of a plane parallel incoming flow; 3) The interaction between the liquid spray flame and the firing surface of the piston; 4) The incoming flow in the cylinder is parallel to the cylinder liner, approximated as a convective heat transfer process of a plane parallel incoming flow; Therefore, the zoned heat transfer model needs to determine the average flow velocity , average gas temperature ; The average flow velocity result is used to calculate the convective heat transfer coefficient , and the average gas temperature is used to calculate the temperature difference required for the heat transfer amount ; With the help of the idea of a quasi-dimensional combustion model, calculate the average flow velocity , average gas temperature ; For the spray penetration distance in a high-temperature environment, due to the influence of spray evaporation, the penetration distance model can adopt the model established by Dent: ; Among them, is a model constant, defaulting to 3.01; is the pressure difference inside and outside the nozzle hole, is the nozzle hole diameter; Spray penetration velocity: ; The spray cone angle is calculated according to the Bracco model: ; Among them, is the nozzle hole length, and are the gas and fuel liquid densities respectively; The liquid spray flame is at 2 A cone with a cone angle propagates outward, and the flame propagation speed is calculated based on the heat release rate: ; Among them, is the average density of the in-cylinder mixture, which can be calculated based on the in-cylinder instantaneous volume and the total mass of the working fluid; is the combustion efficiency of the fuel, generally taking values from 0.9 to 1; is the heat of combustion per unit mass of the fuel (J / kg), and for diesel, it generally takes values between 42.5 - 44.0 MJ / kg; The ignition delay period of the fuel spray injected into the cylinder is calculated using a simplified Arrhenius expression: ; ; There is a lift-off length in the fuel spray flame, and the high-temperature flame zone exists from the lift-off position to the spray front edge; The lift-off length of the diesel jet: ; Among them, is generally from 0.1 to 0.25, and under high-pressure fuel injection conditions generally takes a value of 0.2; is the velocity of the spray leaving the nozzle (calculated based on the instantaneous injection pressure and the Bernoulli equation), is the gas temperature; 1. For the convective heat transfer between the liquid spray flame jet and the cylinder head wall: ; Among them, is the half-angle of the fuel bundle, is the model constant; Heat transfer area: The cylinder head is divided into a flame zone and a non-flame zone. It is assumed that the flame zone during the combustion process is a ring: the inner diameter is the lift-off length, and the outer diameter is the penetration length of the spray flame; After the fuel sprays out of the nozzle time later, the flame propagates downstream along the spray from the flame lift-off position at a velocity gradually, so the areas of the flame zone and the non-flame zone: ; ; Among them, is the relative time from the ignition moment, is the cylinder bore; The gas temperature in the flame zone: ; Among them, is the theoretical combustion temperature of diesel under the condition of an excess air coefficient of 1, taking values from 2300K to 2500K; is an empirical coefficient with a value range of 0.1 to 0.3; is the excess air coefficient; The gas temperature in the non-flame zone is determined according to the motoring condition; 2. For the convective heat transfer between the liquid spray flame jet and the piston bottom wall surface: ; where is the semi-apex angle of the piston bottom; When , it is assumed that the liquid spray flame does not contact the piston bottom. At this time, only the strengthening effect of the spray on the air flow needs to be considered for the heat transfer between the spray and the piston wall surface; the heat transfer area is the piston bottom area , and the gas temperature is; when , the liquid spray flame will impact the concave area of the piston bottom, and then the flame will roll up; the wall impact point A of the liquid spray flame is the demarcation point between the flame zone and the non-flame zone; the conical area within the wall impact point A is the non-flame zone, and the gas temperature is ; 3. For the convective heat transfer between the liquid spray flame jet and the piston fire surface: When , it is assumed that the liquid spray flame vertically impacts the concave wall surface of the combustion chamber, and the near-wall flow velocity: ; where is the velocity of the liquid spray flame jet parallel to the wall surface after hitting the wall; The boundary layer thickness at the jet wall impact point: ; where is the boundary layer thickness, generally takes 0.37, is the distance from the spray hole to the wall impact point, is the jet Reynolds number; According to the momentum conservation, the liquid spray flame (with a diameter of ) generates a flow parallel to the wall surface after hitting the wall, and its velocity is: ; The liquid spray flame moves along the piston wall surface after hitting the wall, which determines the contact area of the flame zone and is approximately circularly expanded: ; where is the relative time after the liquid spray flame hits the wall; Thus, the contact area of the non-flame zone: ; where is the total piston area; When When it is the case, it is assumed that the liquid spray flame expands upward along the impingement point of the cavity, approximated as a wall-parallel flow: ; The contact area of the flame zone is approximated as a runway-shaped expansion: ; The contact area of the non-flame zone: ; 4. For the convective heat transfer between the liquid spray flame jet and the cylinder liner The flow velocity near the wall surface: ; After the liquid spray flame impinges on the wall, it approximately expands circularly along the cylinder liner wall: ; Among them, is the relative time after the liquid spray flame impinges on the wall; The contact area of the non-flame zone: .

[0010] Compared with the prior art, the calculation method for the zonal heat transfer on the inner wall surface of the low-heat-rejection diesel engine cylinder according to the present invention has the following advantages: (1) The calculation method for the zonal heat transfer on the inner wall surface of the low-heat-rejection diesel engine cylinder according to the present invention can accurately calculate the heat transfer amount on the wall surfaces of different regions in the cylinder.

[0011] (2) For the calculation method for the zonal heat transfer on the inner wall surface of the low-heat-rejection diesel engine cylinder according to the present invention, the zonal Woschni heat transfer model takes into account the temporal and spatial interactions of the flame with the cylinder head, piston, and cylinder liner, and the calculation of the temperature difference and the high-temperature contact area is more accurate. The calculation results of the heat transfer amounts in each zone are also close to the CFD results. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings: Figure 1 is the flowchart of the calculation method for the zonal heat transfer on the inner wall surface of the low-heat-rejection diesel engine cylinder according to the embodiment of the present invention; Figure 2 is the flowchart of constructing a quasi-dimensional combustion heat release model in the engine cylinder according to the embodiment of the present invention; Figure 3 is the flowchart of establishing a multi-region thermal network transfer model of the gas-piston / cylinder wall / cylinder head-cooling system in the low-heat-rejection diesel engine cylinder according to the embodiment of the present invention; Figure 4 is the flowchart of establishing a zonal Woschni heat transfer model according to the embodiment of the present invention; Figure 5In (a), it is the calculation result of the existing Woschni heat transfer model, and in (b), it is the corresponding CFD calculation result; Figure 6 They are the heat transfer amounts of each wall surface in the cylinder calculated by the existing Woschni heat transfer model and CFD; Figure 7 In (a), it is the calculation result of the zoned Woschni heat transfer model in this embodiment; in (b), it is the corresponding CFD calculation result; Figure 8 They are the heat transfer amounts of each wall surface in the cylinder calculated by the zoned Woschni heat transfer model and CFD according to the embodiment of the present invention. Specific embodiments

[0013] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0014] The present invention will be described in detail below with reference to the drawings and in combination with embodiments.

[0015] A calculation method for the zoned heat transfer of the inner wall surface of a low-heat-rejection diesel engine cylinder, as Figures 1 to 4 shown, includes the following steps: S1. Construct a quasi-dimensional combustion heat release model for the engine cylinder, considering the spray penetration and wall impingement effects and the mixture spatial distribution state, and calculate the heat release amount and gas temperature of the mixture in the corresponding sub-regions of the piston, cylinder wall, and cylinder head; The specific method is as follows: S11. Set the key engine structure and operating parameters, and set the key operating and structural parameters of the engine in GT-POWER.

[0016] S12. Select and set the quasi-dimensional combustion heat release model. For the quasi-dimensional combustion model, the heat release amount is usually expressed by the local combustion rate, and the combustion rate is related to factors such as the pressure, temperature, fuel injection amount, and air-fuel ratio in the cylinder; S13. Calibrate the parameters of the quasi-dimensional combustion model according to the engine test pressure and heat release rate results; S14. Calculate the convective heat transfer coefficient and heat transfer amount of the inner wall surface of the corresponding sub-regions of the piston, cylinder wall, and cylinder head.

[0017] S2. Define the engine piston, cylinder wall, and cylinder head as independent thermal nodes, and considering the insulation layer material, thickness, and material thermal properties, establish a multi-region thermal network transfer model of the gas-piston / cylinder wall / cylinder head-cooling system in the low-heat-rejection diesel engine cylinder to calculate the heat transfer flux density between the piston, cylinder wall, cylinder head and the cooling system and the wall surface temperatures of the piston, cylinder wall, and cylinder head; The specific method is as follows: S21. Import the geometric model of the engine cylinder head, usually generated and imported into cool-3d using CAD software (such as STEP or IGES format); S22. Divide the key areas according to the functions and structures of the cylinder head, such as the cooling water channels, gas channels, and surface heat transfer areas; S23. Set the mesh size to ensure sufficient resolution in the key areas (such as cooling water channels and air flow paths) to improve the calculation accuracy; S24. Use the Cool-3D tool to generate the mesh, divide the cylinder head geometry into small mesh cells, and ensure that each area is reasonably divided; S25. After completing the mesh, export the mesh to GT-SUITE for verification and ensure it is correct for subsequent heat flow and fluid analysis; S26. Import the mesh files of the cylinder, piston, and cylinder head, and set the boundary conditions.

[0018] S3. Consider the effects of the in-cylinder liquid spray flame on the cylinder head, piston, and cylinder wall surfaces, calculate the flow velocities and heat transfer areas of the flame and non-flame zones on each wall surface, establish a partitioned Woschni heat transfer model, and calculate the convective heat transfer coefficients and heat transfer amounts of the partitioned inner cylinder wall surfaces.

[0019] Specifically, it includes two improvements to the original Woschni heat transfer model: S31. Consider the effects of the liquid spray flame on the in-cylinder piston, cylinder wall, and cylinder head wall, and establish a mathematical model for the flow velocity of each wall surface. S32. Establish a mathematical model for the heat transfer area of the flame and non-flame zones of the piston, cylinder wall, and cylinder head wall.

[0020] The specific process of establishing the mathematical model for the flow velocity of each wall surface by considering the effects of the liquid spray flame on the in-cylinder piston, cylinder wall, and cylinder head wall in S31 is as follows: Based on the convective heat transfer theoretical model: ; Since the Pr number of the gas changes very little with temperature, for example, the Pr numbers at 30°C and 1000°C are 0.71 and 0.68 respectively, the influence of Pr on Nu is ignored.

[0021] ; where and are model constants. This model does not consider the influence of radiation.

[0022] Because: ; Therefore: ; where is the fluid velocity, is the characteristic length, is the thermal conductivity of the fluid.

[0023] Considering that the fluid density is a function of temperature and pressure, the fluid viscosity and thermal conductivity are functions of temperature: ; ; ; According to the derivation of Woschni, the heat transfer coefficient can be expressed as: ; where the influence of the piston movement on the in-cylinder gas flow movement needs to be considered , and at the same time, the influence of combustion on heat transfer also needs to be considered .

[0024] The mathematical model of the fluid velocity is: ; ; In the formula is the heat transfer coefficient (W / (m 2 ·K)), is the gas pressure (Pa), is the gas temperature (K), is the instantaneous volume in the cylinder (m 3 ). represents the influence of the change in the mass of the working fluid in the cylinder, that is, the influence of fuel injection on the combustion temperature can be considered. is the instantaneous reverse-dragging in-cylinder instantaneous pressure, is the in-cylinder instantaneous pressure under combustion conditions. is the single-cylinder displacement. , and are the reference values at a certain moment when the pressure, volume and temperature are all known, and generally the intake valve closing moment or the combustion start moment is taken. is the engine stroke, is the rotational speed. Other variables are model constants and are generally determined according to experiments.

[0025] Setting of the model coefficients of the original Woschni model: ; ; = 6.18 (scavenging stage), 2.28 (compression, combustion and expansion stages) = 0 (scavenging and compression stages), 3.24e-3 (combustion and expansion stages) Since the in-cylinder flow during the combustion heat release process of a diesel engine is mainly affected by the spray and its jet flame, and the interaction between the liquid spray flame and the cylinder head, cylinder liner, and piston is very different. Therefore, the influence of the liquid spray flame on the flow in the near-wall region of the cylinder head, cylinder liner, and piston can be considered to construct a partitioned near-wall flow model to more accurately describe the effect of combustion on heat transfer. .

[0026] The process of establishing the mathematical model of the heat transfer area of the piston, cylinder wall, and cylinder head wall flame zone and non-flame zone in S32 is as follows: Based on the interaction relationship between the liquid spray flame and the cylinder head, cylinder liner, and piston, the following assumptions are made: 1) The interaction between the liquid spray flame and the cylinder head wall surface is approximately a convective heat transfer process of a plane parallel incoming flow; 2) The interaction between the liquid spray flame and the bottom surface of the piston is approximately a convective heat transfer process of a plane parallel incoming flow; 3) The liquid spray flame acts on the firing surface of the piston; 4) The incoming flow in the cylinder is parallel to the cylinder liner, approximately a convective heat transfer process of a plane parallel incoming flow; Therefore, the partitioned heat transfer model needs to determine the average flow velocity in the above four regions , the average gas temperature . The average flow velocity result is used to calculate the convective heat transfer coefficient , and the average gas temperature is used to calculate the temperature difference required for the heat transfer amount . The average flow velocity and average gas temperature in the four regions are calculated by means of the idea of the quasi-dimensional combustion model . , the average gas temperature .

[0027] For the spray penetration distance in a high-temperature environment, due to the influence of spray evaporation, the penetration distance model can adopt the model established by Dent: ; where is the model constant, defaulting to 3.01. is the pressure difference inside and outside the nozzle hole, is the nozzle hole diameter.

[0028] Spray penetration velocity: ; The spray cone angle is calculated according to the Bracco model: ; where is the nozzle hole length, and are the gas and fuel liquid densities respectively.

[0029] The liquid spray flame is at 2 A cone with a cone angle propagates outward, and the flame propagation speed is calculated based on the heat release rate: ; Among them, is the average density of the in-cylinder mixture, which can be calculated based on the in-cylinder instantaneous volume and the total mass of the working medium; is the combustion efficiency of the fuel, generally taking values from 0.9 to 1; is the heat of combustion per unit mass of the fuel (J / kg), and for diesel, it generally takes values between 42.5 - 44.0 MJ / kg.

[0030] The ignition delay period of the fuel spray injected into the cylinder is calculated using a simplified Arrhenius expression: ; ; The fuel spray flame has a lift-off length, and the area from the lift-off position to the spray front is a high-temperature flame zone. The lift-off length of the diesel jet: ; Among them, is generally between 0.1 and 0.25, and under high-pressure fuel injection conditions generally takes a value of 0.2. is the velocity of the spray leaving the nozzle (calculated based on the instantaneous injection pressure and the Bernoulli equation), is the gas temperature.

[0031] 1) For the convective heat transfer between the liquid spray flame jet and the cylinder head wall: ; Among them, is the half angle of the fuel spray, is the model constant.

[0032] The heat transfer area: The cylinder head is divided into 2 regions (flame region and non-flame region). It is assumed that the flame region during the combustion process is a ring: the inner diameter is the lift-off length, and the outer diameter is the penetration length of the spray flame.

[0033] After the fuel sprays out of the nozzle time later, the flame gradually propagates downstream along the spray from the flame lift-off position at a velocity , so the areas of the flame region and the non-flame region: ; ; Among them, is the relative time from the ignition moment, is the cylinder bore.

[0034] The gas temperature in the flame region: ; Among them, is the theoretical combustion temperature of diesel fuel under the condition of an excess air coefficient of 1, with a value range of 2300K - 2500K; is an empirical coefficient, with a value range of 0.1 - 0.3; is the excess air coefficient.

[0035] The gas temperature in the non - flame zone is determined according to the motoring condition.

[0036] 2) For the convective heat transfer between the liquid - spray flame jet and the piston bottom wall surface: ; where, is the half - included angle of the piston bottom surface.

[0037] When , it is assumed that the liquid - spray flame does not contact the piston bottom surface. At this time, only the strengthening effect of the spray on the air flow needs to be considered for the heat transfer between the spray and the piston wall surface. The heat - transfer area is the area of the piston bottom surface , and the gas temperature is . When , the liquid - spray flame will impact the concave - pit area of the piston bottom surface, and then the flame will roll up. The wall - hitting point A of the liquid - spray flame is the demarcation point between the flame zone and the non - flame zone.

[0038] The conical area within the wall - hitting point A is the non - flame zone, and the gas temperature is .

[0039] For the convective heat transfer between the liquid - spray flame jet and the piston fire surface: When , it is assumed that the liquid - spray flame vertically impacts the concave - pit wall surface of the combustion chamber, and the near - wall flow velocity: ; where, is the velocity of the liquid - spray flame jet parallel to the wall surface after hitting the wall.

[0040] The boundary - layer thickness at the jet wall - hitting point: ; where, is the boundary - layer thickness, generally takes 0.37, is the distance from the spray hole to the wall - hitting point, is the jet Reynolds number.

[0041] According to the law of conservation of momentum, the liquid - spray flame (with a diameter of ) generates a flow parallel to the wall surface after hitting the wall, and its velocity is: ; The movement of the liquid - spray flame along the piston wall surface after hitting the wall determines the contact area of the flame zone (approximately circular expansion): ; where, is the relative time after the liquid spray flame hits the wall.

[0042] Thus, the contact area of the non-flame zone: ; where, is the total piston area.

[0043] When , it is assumed that the liquid spray flame expands upward along the wall-hitting point of the pit, approximately as a parallel flow to the wall surface: ; The contact area of the flame zone expands approximately in a runway shape: ; The contact area of the non-flame zone: ; 4) For the convective heat transfer between the liquid spray flame jet and the cylinder liner: The flow velocity near the wall surface: ; After the liquid spray flame hits the wall, it expands approximately circularly along the cylinder liner wall: ; where, is the relative time after the liquid spray flame hits the wall.

[0044] The contact area of the non-flame zone: ; In summary, the partitioned heat transfer model established this time models the cylinder head, piston, and cylinder liner separately. The improved partitioned Woschni heat transfer model is compiled through the custom function of Gt-power and embedded into the Gt-power whole-machine model to realize the partitioned calculation of the heat transfer amounts of the in-cylinder piston, cylinder wall, and cylinder head.

[0045] Figure 5 The calculation results of the in-cylinder heat transfer coefficient of the low-emission heat diesel engine by the original Woschni heat transfer model and the CFD method are given. Since the original Woschni heat transfer model cannot distinguish the flow velocities in the cylinder head, piston, and cylinder liner regions, only an in-cylinder average heat transfer coefficient curve can be obtained (that is, the heat transfer coefficients in the cylinder head, piston, and cylinder liner regions are the same value). The results show that the original Woschni heat transfer model significantly underestimates the heat transfer coefficient and heat transfer amount in the cylinder head region and significantly overestimates the heat transfer coefficient and heat transfer amount in the piston and cylinder wall regions ( Figure 6 ). The prediction deviation of the original Woschni heat transfer model is higher than 10%. Since the improved partitioned Woschni heat transfer model considers the partitioned flow velocities (that is, separately models and calculates the near-wall flow velocities of the cylinder head, piston, and cylinder liner), the heat transfer coefficients of three different regions in the cylinder (cylinder head, piston, and cylinder liner) can be obtained.Figure 7 ), and the calculated results of the heat transfer coefficient are very close to the CFD results. The zoned Woschni heat transfer model takes into account the spatio-temporal interaction between the flame and the cylinder head, piston, and cylinder liner, and the calculation of the temperature difference and high-temperature contact area is more accurate. The calculated results of the heat transfer in each zone are also close to the CFD results ( Figure 8 ). Compared with the CFD calculation results, the prediction deviation of the zoned Woschni heat transfer model is less than 10%.

[0046] Therefore, the improvements of the zoned Woschni heat transfer model compared with the original Woschni heat transfer model are as follows: 1) The zoned Woschni heat transfer model takes into account the zoned flow velocity (i.e., separately modeling and calculating the flow velocities near the cylinder head, piston, and cylinder liner walls), so the heat transfer coefficients in three different regions in the cylinder (cylinder head, piston, and cylinder liner) can be obtained. Compared with the Woschni heat transfer model, the calculated results of the heat transfer coefficient of the zoned Woschni heat transfer model are close to the CFD results. 2) The zoned Woschni heat transfer model takes into account the spatio-temporal interaction between the flame and the cylinder head, piston, and cylinder liner, and the calculation of the temperature difference and high-temperature contact area is more accurate. The calculated results of the heat transfer in each zone are also close to the CFD results.

[0047] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A calculation method for the partitioned heat transfer of the inner wall surface of a low heat dissipation diesel engine cylinder, characterized in that: It includes the following steps: S1. Construct a quasi-dimensional combustion heat release model for the engine cylinder, consider the spray penetration and wall impingement effects and the mixture spatial distribution state, and calculate the heat release of the mixture and the gas temperature in the corresponding cylinder sub-regions of the piston, cylinder wall, and cylinder head; S2. Define the engine piston, cylinder wall, and cylinder head as independent thermal nodes, consider the insulation layer material, thickness, and material thermal properties, and establish a multi-region thermal network transfer model for the in-cylinder gas-piston / cylinder wall / cylinder head-cooling system of the low-heat-rejection diesel engine to calculate the heat transfer flux density between the piston, cylinder wall, cylinder head and the cooling system and the wall temperatures of the piston, cylinder wall, and cylinder head; S3. Consider the effect of the in-cylinder liquid spray flame on the cylinder head, piston, and cylinder wall surfaces, calculate the flow velocities and heat transfer areas in the flame and non-flame zones of each wall surface, establish a partitioned Woschni heat transfer model, and calculate the convective heat transfer coefficient and heat transfer amount for the partitioned in-cylinder wall surfaces.

2. The calculation method for the partitioned heat transfer of the inner wall surface of a low heat dissipation diesel engine cylinder according to claim 1, wherein: The specific method for constructing the quasi-dimensional combustion heat release model for the engine cylinder in S1 is as follows: S11. Set the key engine structure and operating parameters; S12. Set the quasi-dimensional combustion heat release model; S13. Calibrate the parameters of the quasi-dimensional combustion model; S14. Calculate the convective heat transfer coefficient and heat transfer amount for the wall surfaces in the corresponding cylinder sub-regions of the piston, cylinder wall, and cylinder head.

3. A calculation method for zonal heat transfer on the inner wall surface of a low heat dissipation diesel engine cylinder according to claim 1, characterized in that: The method for defining the engine piston, cylinder wall, and cylinder head as independent thermal nodes and establishing a multi-region thermal network transfer model for the in-cylinder gas-piston / cylinder wall / cylinder head-cooling system of the low-heat-rejection diesel engine in S2 is as follows: S21. Import the geometric model of the engine cylinder head; S22. Divide the key regions according to the function and structure of the cylinder head; S23. Set the mesh size; S24. Generate the mesh, divide the cylinder head geometry into small mesh elements to ensure that each region is reasonably divided; S25. After completing the mesh, perform verification and ensure it is correct; S26. Import the mesh files of the cylinder, piston, and cylinder head, and set the boundary conditions.

4. A calculation method for the zonal heat transfer of the inner wall surface of a low heat dissipation diesel engine according to claim 1, characterized in that: The specific method for considering the effect of the in-cylinder liquid spray flame on the cylinder head, piston, and cylinder wall surfaces in S3, calculating the flow velocities and heat transfer areas in the flame and non-flame zones of each wall surface, establishing a partitioned Woschni heat transfer model, and calculating the convective heat transfer coefficient and heat transfer amount for the partitioned in-cylinder wall surfaces is as follows: S31. Consider the effect of the liquid spray flame on the in-cylinder piston, cylinder wall, and cylinder head walls, and establish a mathematical model for the flow velocity of each wall surface; S32. Establish a mathematical model for the heat transfer area of the flame and non-flame zones of the piston, cylinder wall, and cylinder head walls.

5. According to the calculation method for the partitioned heat transfer of the in-cylinder wall of the low-heat-rejection diesel engine described in claim 4, it is characterized in that: The specific process for considering the effect of the liquid spray flame on the in-cylinder piston, cylinder wall, and cylinder head walls in S31 and establishing a mathematical model for the flow velocity of each wall surface is as follows: Based on the convective heat transfer theoretical model: ； Since the Pr number of the gas changes very little with temperature, for example, the Pr numbers at 30°C and 1000°C are 0.71 and 0.68 respectively, the influence of Pr on Nu is ignored; ； Among them, and are model constants; the influence of radiation is not considered in this model; Because: ; Therefore: ; Among them, is the fluid velocity, is the characteristic length, is the fluid thermal conductivity; Considering the fluid density is a function of temperature and pressure, and the fluid viscosity and thermal conductivity are functions of temperature: ； ； ； According to the derivation of Woschni, the heat transfer coefficient can be expressed as: ； Among them the influence of the piston movement on the in-cylinder gas flow needs to be considered meanwhile, the influence of combustion on heat transfer also needs to be considered ; The mathematical model for the fluid velocity is: ； ； where is the heat transfer coefficient (W / (m 2 ·K)), is the gas pressure (Pa), is the gas temperature (K), is the instantaneous volume in the cylinder (m 3 ), represents the influence of the change in the mass of the working fluid in the cylinder, that is, the influence of fuel injection on the combustion temperature can be considered; is the instantaneous pressure in the cylinder during motoring, is the instantaneous pressure in the cylinder under combustion conditions; is the displacement of a single cylinder; , and are the reference values at a certain moment when the pressure, volume and temperature are all known, and generally the intake valve closing moment or the combustion start moment is taken; is the engine stroke, is the rotational speed; other variables are model constants, generally determined according to tests; Setting of the model coefficients of the original Woschni model: ； ； =6.18 (scavenging phase), 2.28 (compression, combustion and expansion phases); =0 (scavenging and compression phase), 3.24e-3 (combustion and expansion phase); Since the in-cylinder flow during the combustion heat release process of a diesel engine is mainly affected by the spray and its jet flame; the differences in the interactions between the liquid spray flame and the cylinder head, cylinder liner, and piston are significant, it is thus possible to consider the influence of the liquid spray flame on the flow in the near-wall regions of the cylinder head, cylinder liner, and piston to construct a partitioned near-wall flow model to more accurately describe the effect of combustion on heat transfer 。 6. A calculation method for the partition heat transfer of the inner wall surface of a low heat dissipation diesel engine cylinder according to claim 4, characterized in that: The process of establishing the mathematical model of the heat transfer area of the piston, cylinder wall, cylinder head wall flame zone and non-flame zone in S32 is as follows: Based on the action relationship between the liquid spray flame and the cylinder head, cylinder liner, and piston, the following assumptions are made: 1) The action of the liquid spray flame on the cylinder head wall surface is approximately a convective heat transfer process of a plane parallel incoming flow; 2) The action of the liquid spray flame on the piston bottom surface is approximately a convective heat transfer process of a plane parallel incoming flow; 3) The action of the liquid spray flame on the piston fire surface; 4) The incoming flow in the cylinder is parallel to the cylinder liner, approximately a convective heat transfer process of a plane parallel incoming flow; Therefore, the zonal heat transfer model needs to determine the average flow velocity in the above four regions , the average gas temperature ; the average flow velocity result is used to calculate the convective heat transfer coefficient , and the average gas temperature is used to calculate the temperature difference required for the heat transfer amount ; Calculate the average flow velocity in four regions by means of the idea of the quasi-dimensional combustion model , the average gas temperature ; For the spray penetration distance in a high-temperature environment, due to the influence of spray evaporation, the penetration distance model can adopt the model established by Dent: ； Among them, is a model constant, and its default value is 3.01; is the pressure difference inside and outside the nozzle hole, is the diameter of the nozzle hole; Spray penetration speed: ； The spray cone angle is calculated according to the Bracco model: ； Among them, is the spray hole length, and are the gas and fuel liquid densities respectively; The liquid spray flame propagates outward in a cone with a 2 cone angle, and the flame propagation speed is calculated based on the heat release rate: ； Among them, is the average density of the in-cylinder mixture, which can be calculated based on the instantaneous in-cylinder volume and the total mass of the working fluid; is the combustion efficiency of the fuel, generally taking a value of 0.9 to 1; is the heat of combustion per unit mass of the fuel (J / kg), and the value of diesel generally ranges between 42.5 - 44.0 MJ / kg; The ignition delay period of the fuel spray injected into the cylinder is calculated using a simplified Arrhenius expression: ； ； There is a lift-off length in the fuel spray flame. The area from the lift-off position to the spray front edge is the high-temperature flame zone; the lift-off length of the diesel jet: ； Among them, is generally from 0.1 to 0.

25. Under high-pressure fuel injection conditions the general value is 0.2; is the velocity of the spray leaving the nozzle (calculated according to the instantaneous fuel injection pressure and Bernoulli equation), is the gas temperature; 1. For the convective heat transfer between the liquid spray flame jet and the cylinder head wall surface: ； Among them, is the half angle of the fuel spray, is the model constant; Heat transfer area: The cylinder head is divided into a flame zone and a non-flame zone. It is assumed that the flame zone during the combustion process is a ring: the inner diameter is the lift-off length, and the outer diameter is the spray flame penetration length; After the fuel is sprayed out of the nozzle After a time, the flame propagates downstream along the spray from the flame lift-off position at a velocity gradually, and thus the areas of the flame zone and the non-flame zone are as follows: ； ； Among them, is the relative time from the ignition moment, is the cylinder bore; The gas temperature in the flame zone: ； Among them, is the theoretical combustion temperature of diesel under the condition that the excess air coefficient is 1, and the value ranges from 2300K to 2500K; is an empirical coefficient, and the value ranges from 0.1 to 0.3; is the excess air coefficient; Gas temperature in the non-flame zone Determined according to motoring condition 2. For the convective heat transfer between the liquid spray flame jet and the piston bottom wall surface: ； Among them, is the half included angle of the piston bottom surface; When Assume that the liquid spray flame does not contact the piston bottom surface. At this time, only the strengthening effect of the spray on the air flow needs to be considered for the heat transfer of the spray to the piston wall surface; the heat transfer area is the area of the piston bottom surface , and the gas temperature is; When the liquid spray flame impacts the concave area of the piston bottom surface, and then the flame curls upwards; the wall-impact point A of the liquid spray flame is the demarcation point between the flame zone and the non-flame zone; The conical region within the wall collision point A is a non-flame zone, and the gas temperature is ; 3. For the convective heat transfer between the liquid spray flame jet and the piston fire surface: When it is assumed that the liquid spray flame vertically impacts the wall surface of the combustion chamber recess, and the flow velocity near the wall surface is: ； Among them, is the velocity parallel to the wall after the liquid spray flame jet hits the wall; The boundary layer thickness at the jet wall impingement point: ； Among them, is the boundary layer thickness, generally taken as 0.37, is the distance from the nozzle to the wall impact point, is the jet Reynolds number; According to the law of conservation of momentum, after the liquid spray flame (with a diameter of ) hits the wall, a flow parallel to the wall surface is generated, and its velocity is: ； After the liquid spray flame impinges on the wall, it moves along the piston wall, determining the contact area of the flame zone, which is approximately circular expansion: ； wherein, is the relative time after the liquid spray flame hits the wall; Thus, the contact area of the non-flame zone: ； Among them, is the total piston area; When occurs, it is assumed that the liquid spray flame expands upward along the impingement point of the cavity, approximately as a wall parallel flow: ； The contact area of the flame zone is approximately runway-shaped expansion: ； The contact area of the non-flame zone: ； 4. For the convective heat transfer between the liquid spray flame jet and the cylinder liner The flow velocity near the wall surface: ； After the liquid spray flame impinges on the wall, it approximately expands circularly along the cylinder liner wall: ； Among them, is the relative time after the liquid spray flame hits the wall; The contact area of the non-flame zone: 。

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