2.1. TBCs of Gas Passages and Flame Surface
TBCs of the cylinder head are very important for the thermal load analysis. They mainly include the TBCs of the gases passage, the flame surface, the outer surface, and the sealing surface with the cylinder liner. Due to the complicated structures of the cylinder head and the harsh working environment, the former two are generally obtained through numerical simulation instead of the experimental measurement.
Heat transfer gases with the cylinder head mainly include intake gas, exhaust gas, and combustion gas in the cylinder, which are, respectively, in contact with the intake passage, the exhaust passage, and the flame surface of the cylindrical head to achieve heat transfer. To obtain their accurate TBCs, it is necessary to perform calculation analysis and test validation in the working process of the diesel engine. The basic parameters of the marine diesel engine with a single-stage turbocharger are shown in
Table 1.
The working process of the diesel engine is complicated. It involves multiple physical and chemical processes such as fuel injection and evaporation, air-fuel mixing, combustion, intake, and exhaust flow [
22,
23]. To simulate this complex working process, a one-dimensional model of unsteady flow with conservation equations of mass, momentum, energy, and composition was established for the intake and exhaust systems of the diesel engine, and numerical method with finite volume was used to solve it. To improve the calculation accuracy of the TBCs of the intake, the exhaust, and the in-cylinder gases, the multi-zone model with a Woschni heat transfer model was used for the combustion processes. The graphical discrete data was input into the compressor module, and the establishment of the turbine module was based on the empirical formula of the flow rate and the efficiency changing with the expansion ratio. Therefore, the gas composition and engine performance parameters in the cylinder, exhaust pipe, and intake pipe were obtained from the model [
24]. A professional engine working process simulation software GT-Power could solve the heat transfer laws of the multi-cylinder combustion process by defining appropriate combustion, and heat transfer modules, the transient temperatures, and heat transfer coefficients of the cylinder head’s flame surface, intake, and exhaust passages that were obtained. Through the diesel engine structural parameters and test data, the sub-models of the intake system, the valve system, the combustion chamber, the exhaust system, the crankcase, the injector, and the turbocharger were established, and the whole simulation model of the diesel engine’s working process is shown in
Figure 1 by GT-Power software.
The routine test bench and measurement system of the diesel engine are composed as shown in
Figure 2. The cylinder pressure (CP), top dead centre (TDC), and crank angle (CA) are measured through Kistler7013C sensor and 5018B charge amplifier and SZMB-18 magnetoelectric sensor with combustion acquisition and analysis system. The pressures before and after the intercooler are mainly obtained by the PT301 sensors, and the pressures of turbine inlet and outlet are mainly obtained by the PT421 sensors. The temperature after intercooling is measured by the PT100 sensor, and the temperatures before and after the turbine are measured by K-type thermocouples sensors. The fuel consumption rate of diesel engine is obtained through a fuel consumption meter and hydraulic dynamometer. All of the physical parameters above are collected, converted, and analyzed by the test and control system of the bench.
Since the heat source of the diesel engine is mainly produced in the combustion stage, the deviations from cylinder pressure curves of the calculation and routine test of 50 °C before and after TDC are extracted. The comparison of the whole curves of the calculation and the test at the rated speed and 100% load of the diesel engine is shown in
Figure 3. The characteristic parameters of the cylinder pressure including the peak fire pressure and its angle, the pressure at TDC, and indicated mean effective pressure are shown in
Table 2. By extracting the performance and thermal parameters of the diesel engine, the comparison results of the calculation and routine tests are shown in
Table 3.
From the comparison results of
Figure 3 and
Table 2, it can be seen that the deviation of the cylinder’s pressure at each point is less than 9.5% during the combustion stage and the average value of them is 3.68%. The errors of the characteristic values are within ±5.5%. It indicates the calculated cylinder pressure curve in a positive agreement with the experimental result. The errors between the calculated value on the performance and thermal parameters of the diesel engine with the test results are within ±3.5% in
Table 3, which validates the accuracy of the model.
The intake passage gas transient temperature (
Ti), exhaust passage gas transient temperature (
Te), and in-cylinder gas transient temperature (
Tg) curves are shown in
Figure 4. The intake passage gas transient heat transfer coefficient (
Hi), the exhaust passage gas transient heat transfer coefficient (
He), and the in-cylinder gas transient heat transfer coefficient (
Hg) curves are shown in
Figure 5.
The steady-state load of the cylinder head is studied in this paper, and the average temperature and the heat transfer coefficient are processed based on the above curves. The average heat transfer coefficient (
Hav) and the average temperature (
Tav) of in-cylinder gas in a working cycle of the diesel engine can be obtained by using Formula (1) and Formula (2) [
25]. In addition, the average heat transfer coefficient and the average temperature of the intake and exhaust gases are obtained according to the same method. Therefore, the average heat transfer coefficient and the average temperature of intake gas, exhaust gas, and combustion gas are calculated.
2.2. TBCs of Cooling Water Jacket
The cooling water jacket in the cylindrical head has a greater influence on the TBCs. The calculation of the temperature and heat transfer coefficient between the water jacket and the wall of the cylindrical head directly affected the results of the thermal and mechanical load [
26]. Establishing the fluid-structure interaction model of the cooling water jacket and the cylindrical head was conducive to accurately obtaining the TBCs of the cylindrical head [
27,
28].
Although the flow and heat transfer of the cylinder head considering the fluid-structure interaction system are complicated, it follows three physical laws, which include the law of conservation of mass, the law of conservation of momentum, and the law of conservation of energy. The equations of the laws are, respectively, shown in Equations (3)–(5). Realising the heat transfer between the fluid and structure of the interface wall is the key to the fluid-structure interaction heat transfer calculation. From the conservation of energy, the heat transferred from the fluid should be equal to the heat absorbed by the structure at the boundary. The fluid-structure interaction heat transfer equation is obtained by the Fourier heat conduction equation and fluid convection heat transfer, shown in Equation (6) [
29].
where
is the velocity vector,
is the fluid pressure,
is the fluid dynamic viscosity,
is the fluid specific heat capacity,
is the fluid density,
is the heat transfer coefficient of the fluid,
is the mass force acting on the fluid in the gravity field,
= g,
is the fluid absorbed heat,
is the fluid temperature,
is the energy dissipation function,
is the thermal conductivity of the structure,
is the local transfer heat,
is the local heat transfer coefficient, and
is the wall temperature.
The unidirectional fluid-structure interaction method was adopted in this paper. The cooling water jacket was regarded as the fluid region, the cylinder head was regarded as the structural region, the simulation model of the fluid region was established, and the results of the fluid region were mapped on the structural region.
The cooling water jacket of the cylinder head is complex and the three-dimensional geometric model cannot be obtained directly from the model of the cylindrical head. It needs to be wrapped from the model of the cylindrical head in HyperMesh software. Other unrelated structures of the cooling water jacket were removed by delete command, and the inlet and outlet passages were closed by using the command of the surface-spline/filler, which was convenient for mesh and boundary completeness of the model. According to the mesh division and element attribute definition for the water jacket, the FE model of the fluid region with 43,696 nodes and 186,600 elements in total are shown in
Figure 6. The requirements of CFD calculation on mesh quality mainly include skewness, growth rate, aspect ratio, and alignment with the flow of the gird. Specifically, the grid skewness cannot be higher than 0.95, the growth rate cannot exceed 1.4, the aspect ratio is less than 5:1, and the alignment with the flow grid line is consistent with the flow direction to reduce false diffusion. At the same time, it ensures that the flow monitoring curve does not change drastically with the increase of the iteration numbers after the calculation residuals converge. The mesh quality of the cooling water jacket meets the requirements of the calculation and algorithm convergence.
The model was imported into the ANSYS-Fluent module. The flow field of cooling water could be regarded as a steady-state incompressible turbulent state. According to References [
3,
29,
30], the temperature and the heat transfer coefficient in the different areas were dissimilar. The cross-pipe area was close to the flame surface and the fuel injector, and its temperature was relatively high. The cooling water jacket area occupied the vast majority of the heat transfer area, but it was far away from the flame surface, and its temperature was relatively low. Therefore, different boundary conditions were set according to the different location of the areas to improve the calculation accuracy. The cooling water jacket was divided into two areas: the cross-pipe area (CPA) and the main cooling water passage area (MCPA).
The wall temperature of the CPA under the flame surface was calculated by the wall heat exchange formula (7) [
16]. The boundary condition of the cooling water jacket’s wall surface was determined by the energy conservation and ensures the experimental value of the outlet temperature of the cooling water.
where
is the wall temperature of cooling water (°C),
is the saturation temperature of the cooling medium (°C), and
is the total heat absorbed by cooling water (W).
The parameters of the cooling water jacket set in general, models, materials, cell zone conditions, and boundary conditions module of ANSYS-Fluent are shown in
Table 4. The parameter values are assigned to the inlet and outlet cooling water area and the CPA, mainly including the definition of the cooling water attribute, the inlet and outlet cooling water temperature, flow rate, other parameters, the CFD calculation method, the calculated control factor, and the number of iteration steps.
The temperature of the cooling water in the cylindrical head was high. It was easy to form turbulence or even boiling state [
31] as well as certain speed fluctuation, cavity flow, vortex, and rotation in the cooling water jacket of the cylindrical head existed at the same time. The CFD calculation of the cooling water jacket was suitable for double-precision, a steady-state coupled algorithm, and a realisable k-epsilon turbulence model. The cooling water jacket wall surface was set as a no-slip condition. The flow difference between the inlet and outlet was monitored to ensure that the flow monitoring curve did not change dramatically with the increase of the number of iterations after the residual convergences. The calculation contours of the steady-state temperature and heat transfer coefficient of the cooling water jacket are shown in
Figure 7 and
Figure 8.