The purpose of erecting or building a chimney is to make way for smoke or other gases to flow out into the higher atmosphere and prevent the atmospheric layer close to the ground from getting polluted heavily.
Thus, for the fact that a chimney helps the gases to 'flow' out, the term 'flue' can be used as a synonym for the word 'chimney'.
The word 'flu' refers to a common disease comprising of cold and cough and has nothing to do with the functioning of a chimney.
The word 'fluid' means a matter that does not have a definite shape of itself. Most gases and liquids fall under the category of fluids but the term, in general, does not represent the functioning of a chimney.
