If i daven nusach sefard in a ashkenaz minyan which tachanun should I pray?
You should pray in the Ashkenaz nusach. However, some write that you are permitted to add the 13 middos.
Sources: As with any conspicuous change from the custom of a shul, a person davening in a nusach Ashkenaz minyan should not daven the Sefard tachanun (due to the prohibition of lo tisgodedu). This halachah is mentioned in Iggros Moshe (Orach Chaim Vol. 3) concerning the reverse question–somebody who usually davens Ashkenaz and is present in a Sefard minyan. However, the Steipler is quoted (Orchos Rabbeinu Vol. 1, p. 41) as stating that it is permitted to add the 13 middos, because this is considered an addition, and not a change.