I have a question regarding the equation for the heat transfer coefficient of metal-working fluids for direct cooling by Victor Astakhov. Unfortunately, there is no accuracy stated in the book:

V.P. Astakhov

Tribology of Metal Cutting

Elsevier (2006)

https://www.sciencedirect.com/bookseries/tribology-and-interface-engineering-series/vol/52/suppl/C

At page 341, equation for hfc.

How accurate is this correlation?

I couldn't find any other paper which examined this topic.