Subjunctive mood: were or was?

Here, I copied a sentence from a book published by Cambridge Press: ''Nusselt number is the ratio of the actual heat transfer to what it would be if there were conduction but no convection.''

This may be a simple question for you. But I confused about the ''were'' in the subordinate clause and supposed it should be was since conduction is in singular form. Could you please explain this issue?
Thanks for your comments.

Answer

This is an example of an "unreal conditional clause".

See:

https://www.grammarly.com/blog/conditional-sentences-was-instead-of-were/