SCR System Mixing Devices Ford Practical Example

4 SCR System Mixing Devices Ford Practical Example 

Mixing space limitations in exhaust systems of Ford diesel trucks imposed a difficult set of design constraints. At the same time, fixed injector design parameters (injection pressure, spray angle, droplet size, etc.) and some combinations of exhaust flow and temperature resulted in very short residence times (below 20 ms) for droplet evaporation and urea decomposition. 

To evaluate urea mixing quality, a new test rig was designed and built. The rig allowed collection of 37 ammonia concentration values in about 1 h. The values were collected at the end of the mixing section followed by a short flow-straightening element (Fig. 15.23). A rotating sample probe was designed with a minimal resistance to the gas flow. The data was organized to produce a circular pattern that showed sensitivity to an interaction between the atomized liquid and the gas flow. 

Initial analysis of uniformity was based on the industry-wide uniformity index known as the gamma value. The index is generally expected to take into account local concentration values anywhere within the studied gas flow area. However, when compared to changes in uniformity pattern produced by various mixers, the index was not sufficiently sensitive to certain types of nonuniform distribution resulting in overly optimistic values. 

This led to the need to describe uniformity patterns produced by various mixers, and to characterize these patterns in some way. The rotating probe produced data that could be grouped according to the three diameters resulting from rotation of 

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