Is use of "at" and "in" in a sentences together correct grammatically?
there is low separation at higher radius of blade (near tip) in variable thickness blade.
It's fine to use both "at" and "in", in the same sentence. Here's an example:
We will meet at 10 o'clock in the library.
Your sentence is difficult to understand, but not because of your use of prepositions. Instead you use different terms than we would say in English. For example instead of:
at higher radius of blade
we would say something like:
toward the outer tip of the blade
Also, you're missing an article in "in variable thickness blade". "A" or "the" is required. Plus, with many compound adjectives it's usually a good idea to connect them with a dash.
a variable-thickness blade
It's debatable whether "in" is the right preposition to use. To me "with" sounds better, but I feel like this is due to convention and not because of any rule. However you can shorten the sentence by removing a redundant "blade" and combine with "of":
there is a low separation toward the outer tip of a variable-thickness blade.