Me too, my PC started to work slow and when i checked... more than 30 nw.exe in the admin tasks on WIn7, and i have 16gb of ram so imagine
SO = Win7
C2 Version: Construct 2 r220
NW.js Version: v0.13.0 beta 3 (Chromium 47) [Requires r217+] - 13th January 2016
Bought from the Scirra webpage no Steam.
To reproduce it is simple, I press F4 to execute the project using NW.js and then i press ESC to exit using an event for it using the BROWSER->Close. SO every time i made this action a nw.exe is created and after close the app the nw.exe i still running.