What is this flange on the engine?
This is called a "strake" or "chine" (Aerodynamic fan cowl strake/chine, Nacelle strake/chine).
They allow the aircraft to generate more lift at lower speeds, which entails such positive consequences as lower stall speeds, lower landing speeds, lower take-off speeds and shorter runways.

Strakes should be used if the nacelles are mounted closely under the wing of the aircraft, see also Why don't all aircraft use/need nacelle chines?. Nacelles are usually mounted like that in order to


maximize the distance between the lower nacelle side and the runway, so that the engine/fan cowl/nacelle doesn't hit the ground during violent landings when the wings bend down, and in order to
minimize the risk for runway debris being sucked into the engine.
Source: Flugzeugtriebwerke, Bräunling Willy J.G., VDI-Buch, Springer Vieweg, 2015


From US Patent 20100176249: Engine Nacelle Of An Aircraft Comprising A Vortex Generator Arrangement:


  With an optimal arrangement and at high angles of attack, such vortex generators, which are known as nacelle strakes or chines, generate a powerful vortex that flows over the wing, where on a slat in front of said wing it delays airflow separation until the aircraft flies at greater angles of attack.




The idea dates back to 1971 (US Patent 3744745: Liftvanes):


  in operational conditions wherein high angles of attack are encountered, such as in landing or takeoff, the vanes oppose the strong upwash around the nacelle, reducing the flow separation on its upper areas, and providing a strong downwash marked by marginal trailing vortices




From R.S. Shevell. Aerodynamic Bugs: Can CFD spray them away? 1985


  DC-10 wind tunnel tests showed a significant loss in maximum lift
  coefficient in the flap deflected configurations, with landing slat
  extension, compared to predictions. This resulted in a stall speed
  increase of about 5 knots in the approach configuration. The initial
  wing stall occured behind the nacelles and forward of the inboard
  ailerons. The problem was traced by flow visualization techniques to the
  effects of the nacelle wake at high angles of attack and the absence of
  the slat in the vicinity of the nacelle pylons. The solution was
  developed in the NASA Ames Research Center 12 ft. pressurized tunnel and
  turned out to be a pair of strakes mounted forward on each side of the
  nacelles in planes about 45 degrees above the horizontal. The final
  strake shape was optimized in flight tests. The strakes are simply
  large vortex generators. The vortices mix the nacelle boundary layer air
  with the free stream and reduce the momentum loss in the wake. The
  vortices then pass just over the upper surface of the wing, continuing
  this mixing process. The counterrotating vortices also create a downwash
  over the wing region unprotected by the slat, further reducing the
  premature stall. The effect of the strakes is to reduce the required
  takeoff and landing field lengths by about 6%, a very large effect.