Transport of a passive scalar across a protective wall-jet in a pipe. Part II: Analysis and interpretation

First part of this paper (Tesař, 2011) described experimental investigations, combined with numerical flowfield computations, of a pipe flow in which a parallel wall-jet is blown along the inner surface of the pipe. The aim is to separate the main central flow and prevent its contact with the pipe wall. Although originally motivated by the problem of preventing transport of radioactivity to a radiation detector and its activation, the investigations used heat/mass transport analogy so that the actual subject of investigation was transport of heat from warm central air flow across a cool wall-jet flow into the (also cool) wall of the flow containing pipe. This heat transfer is itself also a problem which may be of interest in applications. In the previous part, apart from the description of the experiments, were also presented the basic characteristics of the time-mean velocity and temperature profiles. This second part analyses the obtained combined experimental and computational data to deduce information about general character and intensity of the radial transport across the turbulent protective wall-jets.

Research highlights
► Strategy of low-cost experiment measuring time-mean quantities combined with concurrent numerical computations of turbulent fluctuations and values very near to walls.
► Velocity profiles are similar but shape variations of fluctuation energy make inaplicable simplification based on similarity transformation.
► Temperature profiles and radial temperature gradients make possible representing the convective turbulent transport by equivalent conduction.
► Coefficent of equivalent effective conduction decreases with -0.5th power of axial distance.
► Dimensionless characterisation of thermal insulation (by means of Nu) varies with −0.25th power of axial distance.