The applicability of the Dittus-Boelter and other duct equations in estimating convection heat-transfer coefficients for beam windows used in radioisotope production targetry was investigated. Calculated predictions were compared with experimental measurements for fully-developed flow of helium in a round-duct geometry. It was found that calculations by means of the duct equations are unrealistic unless thermal entrance effects are taken into account.
Steyn, G. F., & Stevens, C. J. (1996). The importance of thermal entrance effects in gas-cooled beam windows for radioisotope production targets. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 373(1), 10–17. https://doi.org/10.1016/0168-9002(95)01395-4