Stirred vessels are widely used throughout the process industry; mixing in stirred chemical reactors affects around 25% of all process industry operations. Insufficient or excessive mixing may result in wastage of processing time, raw material and/or formation of by-product.  In this context, our work has focused on improving the mixing performance of stirred vessels with the aid of accurate numerical simulations (that go beyond the standard RANS approach). Initial work was based on Large Eddy Simulation (LES), and more recent work on DNS. Our latest simulations show that impellers with irregular blades have both smaller energy consumption and also lead to shorter mixing times in comparison to impellers with regular blades. The simulations provide a detailed picture of the differences in the flow and scalar fields that can explain the improved performance characteristics. Part of this work was carried out in collaboration with Prof. J.C. Vassilicos.