Heat transfer downstream of a fluid withdrawal branch in a tube

Experiments have been performed to study how fluid withdrawal at a branch point in a tube affects the turbulent heat transfer characteristics of the main line flow downstream of the branch. Air was the working fluid. The experiments were carried out for several fixed test section Reynolds numbers and at each Reynolds number the ratio of the withdrawn flow to the test section flow (hereafter designated as the flow split number) was varied systematically. Local heat transfer coefficients were determined both around circumference and along the length of the tube, and circumferential average coefficients were also evaluated. The circumferential average Nusselt numbers in the thermal entrance region are much higher than those for a conventional turbulent pipe flow having the same Reynolds number, and the differences are accentuated at higher values of the flow split number. When normalized by the corresponding fully developed value, the axial distribution of the circumferential average Nusselt number is relatively insensitive to the Reynolds number for a fixed flow split. The thermal entrance lengths, based on a five percent approach to fully developed conditions, are in the 20 to 30 diameter range, which is substantially greater than that for conventional turbulent airflows. Circumferential variations on the order of ±20 percent are induced by the fluid withdrawal process. For the most part, these variations are dissipated upstream of x/D = 10.