Why doesn't "convergent–conical nozzles" have the definite article? Surely, Spotts et al. must have investigated some specific countable convergent–conical nozzles and the author of the paper is talking about those nozzles which is not generalized for us at least in this paper then Why?
You seem to have misunderstood when a definite article is used. Definite articles are not used when a specific item is mentioned, it's used when a specified item is mentioned. If I say "I bought a car", I use the indefinite article because, although I know what specific car I'm talking about, you don't. If, after I've said "I bought a car", I then say "The car was red", I use the definite article because I've told you what car I'm talking about: the car that I bought.
The definite article is used before a noun phrase when that noun phrase is sufficient to specify which object it's referring to, or when the noun phrase refers to a previously identified object. For instance, "I have a history book. The book lists all of the presidents of the United States of America." The first instance of "history book" can refer to any history book, so takes an indefinite article. The second instance of the word "book" refers back to the previously mentioned book, so takes a definite article. There is only one United States of America, so that gets a definite article. There is only one set of all the presidents, so that also gets a definite article.
It has to be the noun phrase itself that narrows it down. "The president who was in office during the Civil War" takes the definite article because "who was in office ..." is part of the noun phrase. If it's instead "A president was in office during the Civil War", then "in office" is now a subject complement, rather than a clause subordinate to "president". Even though "was in office during the Civil War" is true for only one president, the indefinite article is used because the qualification isn't part of the noun phrase. This is a subtle distinction, so I can see how it can be confusing.
Why has "effect" got the definite article?
You are told what effect is meant: it's the effect of the nozzle pressure ratio and nozzle angle on the nozzle performance.
The author has used the definite article before "nozzle pressure ratio" and "nozzle angle" because we are talking about some nozzle pressure ratios and nozzle angles which are used in that investigation.
It can be interpreted as saying "For several combinations of nozzle pressure ratios and nozzle angles, we looked at how that nozzle pressure ratio and that nozzle angle affected that nozzle performance". If we were given a particular combination, then we would know what nozzle pressure ratio, nozzle angle, and nozzle performance are being talked about: the nozzle pressure ratio, nozzle angle, and nozzle performance of that combination. We use the definite article when a statement is distributed over a set, and the value is determined by what element of that set is considered. For instance, "The square of the hypotenuse of any right triangle is equal to sum of the squares of the legs". Although we aren't talking about a particular hypotenuse overall, once we have "of any right triangle" we use the definite article because for any particular right triangle, "hypotenuse" refers to a clearly defined object.
Why doesn't "smaller nozzle angles" have the definite article? We are talking about the smaller size of those nozzles of the investigation.
Again, although there are specific nozzle angles, there aren't specified nozzle angles.
if not, then how "discharge coefficient" and "choked nozzle pressure ratio" have got the definite article?
In "the discharge coefficient for smaller nozzle angles", "the discharge coefficient" is specified. For a particular smaller nozzle angle, there is only one discharge coefficient.