Water jacket optimization using CFD & FEM
Authors: Stefano Fontanesi, Vincenzo Gagliardi, Matteo Giacopini, Simone Malaguti and Reggio Emilia – University of Modena, Italy

A detailed understanding of the flow and thermal behavior in the water jacket of a turbocharged diesel engine can result in significant design improvements. If the velocities of the coolant flow drop too low then heat is not convected away from the metal engine block. Over time, the resulting high temperatures produce cracks in the structure, ultimately causing its catastrophic failure. The analysis reported here enabled the coolant flow path to be optimized, reducing the peak temperatures in key locations and resulting in a 20 % reduction in the peak thermal stresses.

Fig:01 - Simulation methodology

An optimization study involving both fluid-dynamic and thermo-structural aspects was carried out. Using a crossdisciplinary approach, the structural and thermodynamic problems were decoupled using an ad hoc methodology to trade-off computational effort with accuracy. This procedure allows a sensitivity study to be carried out, varying geometric parameters of the engine to obtain an optimized component.

Methodology
The adopted methodology (shown in Figure 1) decoupled the structural and thermodynamic simulations. In order to evaluate the temperature distribution of the metal cast, a CFD analysis of both the water circuit and the surrounding metal was performed. Boundary conditions from a 1-D simulation of the whole engine were imposed, while coolant/metal heat transfer was calculated using STAR-CD.

The temperature field was then passed to the FEM code, and structural analyses were carried out in order to assess the fatigue strength of the component. Finally, this methodology was applied to a comparison of the current circuit configuration and an improved design (where the water jacket flow has been optimized) in order to estimate the effectiveness of the design optimization on the fatigue strength of the component.

Fig 02: Isosurface of flow volume where velocities are below Vcrit for BASE and EVO designs

Fluid-dynamic preliminary analysis
CFD analyses were carried out to focus the flow distribution in the critical regions, i.e. the valve bridges and the pre-chamber areas. Initially an isothermal analysis was performed on the whole engine water jacket. In order to evaluate the effect of simple geometric modifications to the circuit layout on the cooling effectiveness, the original configuration (BASE) was compared to a modified one (EVO). The flow in the EVO configuration is forced to cross the whole engine block before entering the head jacket and only reaches the jacket exit after crossing the whole engine head (cross-flow).

A critical velocity Vcrit was defined, below which the local heat transfer is considered to be ineffective. The percentage of the coolant volume in which the velocity fell below Vcrit was compared for the two solutions.

Thermo-mechanical analyses
On completion of the CFD analysis of the water circuit and the surrounding metal cast, the temperature distribution in the iron cast was evaluated. Since the choice of boundary condition is responsible for the accuracy of the metal cast temperature calculation, the heat flux distribution was derived both from experimental measurements and numerical predictions. Experimental measurements were used to set the coolant temperature at both the gasket and the circuit outlet. For the heat flux, data from a 1-D GT-Power simulation of the whole engine at a given operating condition was imposed, while the coolant/metal heat transfer was directly calculated
in STAR-CD. Although the 1D model is unable to accurately account for three-dimensional effects and non-uniform cylinder-to-cylinder distributions, the decision to derive the boundary data from the 1D model was considered to be a good trade-off between accuracy and computational effort.

Please select a thumbnail to view the images on the left.

Results
CFD
Two main issues appear from the coolant/metal analysis (Figure 3):

• the highest temperatures in the solid domain are located at the junction between the pre-chamber and the combustion dome, towards the side opposite to the injector location; the high-temperature layer within the iron cast is quite thin,
  
• and obviously located towards the walls facing the
  combustion chamber, confirming the validity of the
  application of a uniform temperature at the solid domain
  outer walls.

CFD - FEM
Figure 4 shows a direct comparison between computational and experimental results in the pre-chamber 3 crack region; it is possible to observe a very convincing match in terms of maximum equivalent von Mises stress location and crack initiation, thus confirming the validity of the simulation methodology.

Optimization
As a final validation, a comparison of the stress distribution between the BASE and EVO circuit designs was made. Figure 5 clearly indicates in the EVO design peak tension values that cause the cracking has been significantly reduced: its value reduces from 240 MPa to less than 200 MPa (a 20% reduction). This clearly indicates the benefits resulting from the improved heat transfer coefficient in the critical area as a result of the optimized flow design.

Conclusion
An optimization study involving both fluid-dynamic and thermostructural aspects of a turbocharged diesel engine head was carried out. A cost-effective methodology was evaluated to correctly represent the fatigue-failure critical regions without excessive computational costs. Since the aim of the work was to trade-off solution accuracy and computational, the following conclusions were drawn:

• a proper choice of both fluid-dynamic and mechanical boundary conditions is required in order to deliver the required accuracy;
  
• comparisons with experimental data confirmed that the methodology adopted was able to accurately predict locations prone to cracking;
• the modeling procedure allowed a sensitivity study to be carried out of the engine head to variations of the gasket plate design;
• the modifications of the gasket passages, although very simple, allowed the cooling performance of the circuit to dramatically improve (Figure 6), almost liminating critical stress concentrations at the cylinder 3 pre-chamber, which was experimentally detected to crack when operating a full engine load.