Powertrain & Energy - January 25, 2012 - 17

at lawrence livermore national laboratory, reducing run times. The company has patents on its automatic mesh generation and algorithms for treating moving boundaries with automatic mesh generation. “Helping our clients develop alternative fuels means our software must model multiple injections, with droplets from those injections composed of different liquids,” said Lee. CONVERGE recently added models that capture liquids, each with its own evaporation rate. This models, say, a dual-fuel model of gasoline and diesel where one evaporates more slowly than the other does. Another feature the company added recently is film evaporation—modeling what happens when a droplet hits a cylinder wall and evaporates. He also sees boosting as a significant trend. In response, CONVERGE now interfaces with GT Power from gamma technologies, treating boosted air into the cylinder as a boundary condition computed by GT Power. “We also feed our solutions back to GT Power,” he said. He sees future developments continuing in spray modeling, especially modeling CFD inside injectors. “As you get into different fuels and different properties, you could have very different things happening inside the nozzle that affects the spray as it leaves.” Sibendu Som, Mechanical Engineer, Argonne national laboratory, is working to develop models on that very phenomenon. “We are developing spray and combustion models for biodiesel, including a near-nozzle flow model,” he said. “This models two-phase cavitation and turbulence from inside the nozzle

and couples it with aerodynamic breakup, and eventually with spray and combustion models.” Why is this important? According to Som, in the past, modeling only predicted aerodynamic breakup, which is not accurate enough. The near-nozzle spray has much influence over performance and emissions. “When you have dual fuels, such as diesel and biodiesel, the physical and chemical properties are very different. This means the flow inside the injector is very different for each,” he said, with liquid penetration and liquid length higher for biodiesel. He noted that he coded the model inside the CONVERGE code from Convergent Science, taking advantage of the automatic meshing capabilities of CONVERGE.

Future—more, more, more

As good as today’s modeling is, better is needed. “Our customers are asking us to provide more chemical and spatial fidelity, along with faster solutions,” said Reaction Design’s Meeks. This is all driven by a storm of requirements changes, particularly in automotive. Besides the obvious chemical differences in alternatives such as CNG or ethanol, conventional fuels are changing as well. Gasoline derived from tar sands or shale oil may be different from that derived from petroleum, according to Meeks, making it—in essence—an alternative fuel. All this means designers need to truly understand how chemical kinetics drives what happens inside the combustion chamber, according to Meeks. This is driving the continuing need for better models that run faster, as Meeks attests to continued interest in the MFC consortium as well as interest in their software offerings. Lee from Convergent Science agrees. “Fifteen or 20 years ago, engineers were not doing as much alternative fuel research and so you could [compose models with] fewer cells,” he said. This set an expectation. Engineers got used to compute times of less than two days for a complete intake-and-combustion cycle. “Now, with these complex alternative fuels and advanced injection strategies, compute time is again getting significant. Fuel injection now may have three or four pulses in a cycle vs. one,” said Lee.

SAE Powertrain & Energy

January 25, 2012

Powertrain & Energy - January 25, 2012

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