Did dinosaurs exist at time of the flood?

No. The accepted scientific consensus is that the dinosaurs died out almost 65 million years before people appeared on earth. That means the dinosaurs were gone long before Noah would have loaded his ark. You can read many interesting facts about dinosaurs at the related link provided below.

The physical size of the ark and the lack of reference to dinosaurs specifically makes it unlikely that, should the ark be real, dinosaurs would have been on it.