I found a relevant PhD supervisor who refused to supervise my PhD project by saying that he has no open position in his research group. Is it OK to mail him again and request to accept me as a student, because the work he is doing in his research group is extremely relevant for my research interests. I have received funding and just need space in a research group.
You can always ask.
However, be prepared that the answer may easily be still "no". Direct funding is by no means the only resource that an advisor has to pour into a student. There is also the time investment of actually advising a student, assorted costs for overheads and utilities, as well as presumably some money to fund conference trips. This isn't even to speak of simple inter-personal matters, such as that the professor may feel that you are not a good fit for the lab culture.
If you had already obtained funding prior to the first request, and the professor was aware of this, you have to seriously question why you were refused a position. Possible reasons are:
- The professor feels you won't fit in to the group or get along well with him as a supervisor.
- The professor doesn't have enough time to supervise another student. You don't want a supervisor that has no time for you.
- The professor doesn't want another student doing the stuff that you're doing. Maybe he wants to diversify into some different areas of your field.
Sure, you could ask him again (and perhaps you should, if you have only obtained funding after the first request). But you should also ask why you were rejected in the first place, because the answer to that question could tell you whether or not you are likely to enjoy your time in his research group.
Assuming the funding is new information, I would ask for an appointment to discuss your situation and any concerns/hesitations the professor might have. Face-to-face meeting has advantage of a dialog that email lacks. You can ask for professor's concerns, express your enthusiasm, and perhaps come up with a partial solution.
Some faculty is hesitant accepting students that they haven't worked with before (from a class), for example. If that is the case, you can either take a class, or ask to be admitted on a trial basis. Other times, faculty might not accept new students because they plan on leaving their current job or retiring.
The solution for you will depend on the specific circumstances that you can only find out through a face-to-face, honest discussion.
In this case I think it's best to first ask for permission to ask. Now, I don't mean to break your request into multiple communications; that wastes everyone's time.
But do start off with mentioning the additional funding and asking whether having funding makes any difference. That way you don't show a lack of respect for his earlier refusal. And if he can give you an answer just from that, he needn't waste his time reading further.
Farther down you can lay out any other arguments, pleading, etc.
Yes you can and you should. Although the group may be full (which can be a good sign) showing persistence and motivation is a good sign for the group leader: they are often very much interested in a motivated student - after all, having groups from which no one finishes the PhD (as may happen with non-motivated students) is not a good thing for their personal results evaluation.
I have a recent story for you: Päivi Rissanen - a former psychiatry closed ward inmate got finally, after a LOT of convincing a place under professor Antti Karisto (http://blogs.helsinki.fi/akaristo/in-english/) in the Department of Social Policy in University of Helsinki and will soon be a PhD, according to Helsingin Sanomat newspaper.
A strong will may meld the world around it to its liking. Do it.