Changing Channels

By Massimiliano Di Paola, Senior CAE Analyst, and Nazario Bellato, Simulation Manager, Magneti Marelli Powertrain S.p.A., Bologna, Italy 
Bhartendu Tavri, Simulation Engineer, Magneti Marelli India Pvt. Ltd., Gurgaon, India

Modeling the full geometry of the complex wave channels in a new, integrated intake-manifold–intercooler design for fuel-efficient cars previously required lengthy solution times. Magneti Marelli engineers used the directional loss model in ANSYS CFD software to simulate the channels as a porous medium in one-third of the time.

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Intake-manifold–intercooler velocity plot

Based on government regulations and customer demand, automobile manufacturers need to improve fuel economy and reduce emissions for vehicles. For this reason, many are adding turbochargers to their cars. However, to ensure reliable operation and performance of these turbocharged vehicles, other changes to the engines are required.

"The current design delivers a substantial increase in heat exchange through the intercooler, reducing outlet temperature by 8 percent to improve engine performance."

TURBOCHARGERS AND INTERCOOLERS

In most piston engines, intake gases are pulled into the cylinder by the pressure reduction caused by the downward piston stroke. Turbochargers and superchargers increase the performance and efficiency of internal combustion engines by compressing the air prior to entry into the intake manifold, so that more air is forced into the cylinder and each engine cycle generates more power. However, as turbochargers compress air, the temperature of the air increases, leading to reduced density and limiting the air mass that can be forced into the cylinder. This in turn affects combustion ability. High intake air temperatures also increase the risk of pre-ignition or knocking, which can cause serious damage to engines. For many years, the most advanced turbocharged and supercharged engines, such as those used in racing, have used an intercooler to remove heat after the air has been compressed to increase air density. The most common type of intercoolers, called air-to-air intercoolers, use atmospheric air to cool the intake air. This type is relatively simple and inexpensive, but its efficiency is limited by the amount of contact with and the temperature of ambient air.

Intercooler configurations
 
Integrating the air-to-water intercooler with the air intake manifold provides a substantial reduction in cost and weight.

Air-to-water intercoolers provide considerably higher performance by using water to extract heat from the air. But air-to-water intercoolers have rarely been used in production vehicles in the past because they require a pump, radiator, fluid and plumbing that add considerable cost and weight to the vehicle. A significant recent trend in automotive design is the use of air-to-water intercoolers that are lighter, more compact and less expensive because they are integrated into the intake manifold.

But efficiently integrating the intercooler and intake manifold is challenging. The intake air and cooling water must be channeled through the integrated intake-manifold– intercooler in such a way that heat transfer is maintained at high levels to keep intake air temperature low while minimizing pressure losses that reduce engine efficiency. Magneti Marelli engineers use ANSYS computational fluid dynamics (CFD) software to optimize the performance of a new, integrated intake-manifold– intercooler in a fraction of the time that would have been required using previous simulation methods.

MODELING A COMPLEX SYSTEM

Designing this complicated system using conventional build-and-test iterations would be very expensive and time-consuming because of the large number of design parameters and the limited amount of information that could be collected during physical testing. On the other hand, the product presents a difficult simulation challenge because of its complex internal geometry, which includes tiny wave channels that move air through the intercooler on a tortuous path to transfer as much heat as possible to the surrounding liquid. Accurately capturing flow behavior in regions such as these, where abrupt changes are expected in key variables such as velocity, pressure and temperature, normally requires that the mesh be refined by generating an inflation (boundary) layer. In the past, Magneti Marelli engineers used a hybrid mesh in these applications, with hexahedral elements in the boundary layer and less computationally intensive tetrahedral elements in the rest of the flow volume.

But in this case the geometric complexity of the wave channels was so great that a good quality hybrid mesh would yield long solution times, even with large computing resources.

A hex-dominant mesh was not practical because of the presence of flow obstructions that were designed to improve heat transfer by generating turbulence. Even by ignoring the flow obstructions, and meshing the microchannels as regular fluid passages with an assumption of trapezoidal cross-sections, there would have been a very high element count and nonuniform mesh density, resulting in long solution times.

The directional loss model produced the velocity plot shown here in one-third of the time required in the past.
 
The directional loss model produced the velocity plot shown here in one-third of the time required in the past.

MULTIPHYSICS SIMULATION

Magneti Marelli engineers overcame this problem by using the directional loss model in ANSYS CFX, which is based on Darcy’s momentum loss equation for fluids flowing in a porous medium. Using this model, a generalized form of the Navier-Stokes equation together with Darcy’s law were solved over a given domain in a form that accounts for the volume porosity of the media and expresses pressure drop in terms of Darcy’s law. Engineers ran the simulation and compared the results to physical testing. Then they adjusted the linear and quadratic loss coefficients that control the intercooler’s porosity levels in the simulation to match the air pressure loss across the intercooler, as measured by physical experiments. After calibrating the model, it correlated well with physical measurements, while taking a fraction of the solution time that would have been required to model the complete geometry of the intake-manifold–intercooler. The simulation provided far more accurate performance estimates of design alternatives than were obtained in the past with hand calculations. The CFD model based on trapezoidal elements took three days to solve, while the model based on Darcy’s law had 12 million fewer elements and took only one day to solve.

Intercooler wave channels
 
Tiny wave channels in the intercooler provide a difficult analysis challenge.
Trapezoidal meshing
 
A more traditional approach involving meshing the wave channels with a trapezoidal cross-section yielded excessive solution times.

With the model validated, Magneti Marelli engineers used it to evaluate the impact of many design parameters on heat transfer and pressure drop through the intercooler. They also exported the pressure fields to ANSYS Mechanical and performed structural simulations to evaluate the stability of the structure. At various stages of the design process, engineers used ANSYS DesignXplorer to rapidly iterate through the design space and identify design parameter values that best met the specified design objectives. The team is now using simulation to study several other complex phenomena, such as the condensation effect inside the air intake manifold and the water hammer effect in the intake system while the engine is being cranked by the starter motor.

Using simulation early in the design process saved time and money by making it possible to optimize the design at an earlier stage than was possible in the past. The company expects to achieve higher performance while creating fewer prototype iterations than would have been necessary using previous design methods. The current design delivers a substantial increase in heat exchange through the intercooler, reducing outlet temperature by 8 percent compared to the previous intercooler design, which, in turn, improves engine performance. The new design also reduces overall pressure loss to improve fuel economy by 5 percent. The project is continuing as engineers currently focus on optimizing the structural and thermal stability of the system. Magneti Marelli has received a very favorable response from major automotive manufacturers on integrated intake-manifold–intercooler technology, and is working to implement the new design in upcoming vehicle models.

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