Handbook of Turbomachinery Second Edition Revised - Ventech!

addition, quantitative flow velocity measurements can be made simultaneously
if the visualization technique allows for the motion of discrete
particles to be viewed by high-speed optical devices. This study is performed
usually inside an up-scaled transparent model. One of the rather simple
visualization techniques can be based on application of neutrally buoyant
helium-filled soap bubbles of controlled size that are produced by a separate
bubble generator. The bubbles trace air-flow patterns at speeds as high as
200 fps. Since they follow the flow streamlines, the bubbles rarely collide
with objects in the airstream and are extremely durable. The motion of the
bubbles at lower speeds, up to 30 fps, can be photographed using
conventional film and lighting. For higher speeds, special film and lighting
are required. With an interrupted light source, the motion appears as broken
streaks on the photograph, which can be used to obtain a quantitative
measurement of local velocities. The primary objective of these experiments
is to quickly identify any stagnant zones, recirculation, and separation, or
any anomalies in the flow behavior that could compromise the projected
cooling performance. If any of these flow phenomena exist, it may be
necessary to incorporate design changes to overcome them.
A highly recommenced experimental technique, which is known as
thermal imaging, was introduced by Hippensteele et al. [126] and since then
has been used extensively for direct local convective heat-transfer measurements
within cooling passages. Improvements in video systems brought
tremendous enhancement of the imaging method. The Thermochromic
Liquid Crystal (TLC) coating may be described as an organic compound
existing in the state between its solid and liquid phases. When its molecules
are in a disturbed state (caused, for example, by increased temperature or
mechanical shear), their axes are rotated, leading to bright reflected colors of
light. The visible effect is shown through a change from colorless to red. As
the temperature is raised further, the colors pass through a range of the
visible spectrum to blue/violet and finally to colorless again at higher
temperatures. A primary advantage of the liquid-crystal coating is its
reversible state, which allows a number of experiments to be performed
without reapplying the coating and also provides redundant data during the
same test if transitions of more than one color are recorded. Data reduced
from thermal imaging can be presented in a pseudo-color plot of very finesize
pixels, representing the heat-transfer matrix of the entire airfoil internal
passage or one particular area. Each color corresponds to a particular range
of heat-transfer coefficients. These values define internal heat-transfer
boundary conditions required for thermal analysis.
The liquid crystal (for example, Hallcrest, R35C1W) is sprayed evenly
on the inner surface of the transparent model before testing. For an
optimum visual background for the liquid crystal display, the test surface is
Copyright © 2003 Marcel Dekker, Inc.