Should I acknowledge institutions where I did internships on unrelated works? I'm a PhD student at a university. I have a main research topic that I started a while ago. I did a summer internship at another institution in my major field but unrelated to my main research topic. If I write a paper on my main research topic, should I mention the other institution? If so, as an affiliation or in the acknowledgements?
Be honest in your acknowledgments. Just don't add someone or some institute name just because you want to show your link with them. If they did help in any shape or form, well thank the institute and the exact people involve in your discussions. Something like the following: 


  I would like to thank Mr/Mrs X, from the institute Y, for their insightful/encouraging discussion on J,K, and Z.