I am always confused about which temperature to evaluate fluid properties at. Let's say I have a helical pipe and I know the inlet temperature, outlet temperature, and surface temperature and the inlet Reynold's number. I must determine the length of the pipe needed to satisfy the outlet temperature which means I must know the mass flow rate. I can do this by determining the inlet density and viscosity.
When I use the inlet temperature for these properties, the length is 1.046 m
When I use the average between the inlet and the surface, the length is 0.3994 m.
When I use the average between the inlet and the outlet, the length is 0.5768 m.
As you can see, the temperature I use drastically changes the pipe length.
Also, I am always confused as to what temperature to evaluate the properties at for the Nusselt number as well.
Best Answer
Usually the properties are taken at mean temperature (and pressure) between in- and outlet, often by iteration. If your problem is that sensitive to changes in thermal properties I would calculate the problem sectionwise to account for the non-linearity.
When calculating Nusselt Numbers, often the wall temperature needs to be taken into account as well. Wall temperatures are found by iteration as the heat transfer coefficients are inversely proportional to the T-gradients.