In the Newton-Richmann equation, the heat flux is equal to the product of the heat transfer coefficient by the temperature difference of hot and cold coolant the solid surface and the ambient fluid outer and inner wall surfaces inner and outer wall surfaces

The correct answer is: the solid surface and the ambient fluid.<br /><br />In the Newton-Richmann equation, the heat flux is equal to the product of the heat transfer coefficient and the temperature difference between the solid surface and the ambient fluid. This equation is commonly used to describe the heat transfer between a solid surface and a fluid (such as air or water) in various engineering applications, such as heat exchangers, cooling systems, and chemical processes.