Posted by: Mehul Patel | Posted on: December 10th, 2014

As global emission levels continue to rise at an alarming rate, automotive manufacturers are strained to leverage their contribution towards developing greener vehicles. The top contributing pollutants from a vehicle includes Carbon Monoxide (CO), Unburned Hydrocarbons (UHC) and Oxides of Nitrogen (NOx). The emission of these harmful gases is a result of incomplete combustion inside the engine, which is practically very much difficult to control. In order to reduce CO and HC concentration in exhaust gases, catalytic converter is employed in most vehicles available today.

However, NOx emissions are usually controlled using EGR systems. An EGR or an Exhaust Gas Re-circulation system has been in use since many decades and is employed to circulate a portion of exhaust gas back to the intake manifold. This process allows diluting Nitrogen in the exhaust gas as well as providing inert gases to the combustion, which also helps in reducing peak engine temperatures.

The EGR system utilizes a cooler to reduce the temperature of the exhaust gases, before it is fed to the engine intake. This cooler is nothing but a compact shell and tube heat exchanger, utilizing engine coolant to cool the incoming exhaust gases. While this technique offers excellent control of NOx formation, there are several critical design parameters that affect its performance. These include:

Limited pressure drop in the cooler to ensure heat transfer uniformity

Compactness of the design with high thermal efficiency

Limitations in coolant flow rate and preventing the occurrence of boiling

Resistance of the cooler against high thermal stress for better fatigue cycle

Resistance to fouling due to deposition of residue on heat transfer walls

In order to ensure that the EGR system is designed for maximum performance and life cycle, physical tests are indeed necessary. However, the uses of CFD permits to identify critical factors that otherwise go unnoticed or are not visible. The prime requirement of an EGR cooler is to achieve maximum heat transfer through the arrangement of tubes inside a shell that circulates engine coolant.

Simulation techniques allow visualizing the flow inside the shell and tube bundles comprehensively. Using a coupling of liquid-to-solid-to-liquid heat transfer model, the effectiveness of the heat exchanger can be simulated.

Critical factors such as uniformity index, i.e. how well the exhaust gas receives uniform cooling through indirect cooling can be assessed. Hot spots or regions that restrict the flow or drop the pressure beyond limited values can be ascertained, in order to incorporate required design changes in the EGR system. Since there will be a detailed information about the heat transfer process occurring inside the cooler, the required number and size of the tubes can be optimized to reduce the overall weight and develop compact systems.

The simulations should also be extended to study the flow behavior inside the inlet and outlet manifolds for the EGR to better understand the effect of turbulence and pressure on the system performance. Another feature of CFD is to determine particle deposition using particle tracking simulation that provides assistance in determining servicing schedules. The heat transfer data obtained through CFD simulations can be utilized as an input for fatigue analysis to determine the impact of stresses due to high temperature for useful life cycle.

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