Burning Hands is a tactical spell that only allows you to deal damage to a unit that is adjacent to the caster, and that is the way it works when you manually play a battle - it has no range.
However, I auto-resolved a battle and was surprised I had won, so I went into the "detail" of the battle and saw this, which seems to indicate that this spell was cast and caused damage while the enemy was still some distance away (and still moving towards the caster).
