Essay About Nusselt Number Of The Test Section And Turbine Blade

Heat Transfer Analysis Using Thermochromic Liquid Crystal
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Heat Transfer Analysis using
Thermochromic Liquid Crystal
1 Introduction
The goal of this experiment is to calculate the heat transfer coecient of forced
convective cooling, which our case should also be representative for the cooling
mechanism of a turbine blade.
Turbine blades are often exposed to very high temperatures due to the created
combustion-heat. These thermal conditions can lead to deformation or even
melting of the blades, which would drastically decrease the lifetime. One solu-
tion to this problem is to run the turbine at lower temperatures. However, this
would decrease the performance. A better solution is to cool down the blades or
to cover them with a protective coating during the combustion to optimize the
eciency. This is the reason why the understanding of heat transfer mechanisms
connected with the dierent
ow species is fundamental. With this knowledge
we can reach a high eciency in cooling so that the turbine can run at higher
temperatures which increases the performance.
1.1 Turbine Blade Cooling-Systems
There are two common ways to cool down a turbine blade, where also combi-
nations of the two types are used nowadays.
1. The blades are externally cooled as gas (normally air) is pumped through
small holes in the front edge of the blade. The air creates a thin layer on
the surface. Unfortunately, this not only “protects” the blade from the
high temperature environment, but it also has a negative eect on the
eciency of the turbine. The higher the mass
ow of the cooling layer,
the lower is the eciency.
2. The blades are cooled down internally with water
owing through a tube
which is casted into the blades. The tube is formed to be as long as possible
to achieve a high heat transfer via forced convection, which depends on
the heat transfer coecient.
1.2 Requirements
In this experiment, we want to calculate the convective heat transfer coecient
for the second type of cooling system mentioned above. As the exact
ow and
cooling properties inside the blade are hard to measure, an experimental model
of the blade with comparable ducting is used in our experiment.
2 Laboratory Description
As the experiment has to be representative for a turbine blade, the non-dimensional
Reynolds-, Prandtl- and Nusselt number of the test section have to be equal.
The test section is a channel shaped as shown in Figure 1. On the backside
there is a layer of thermochromic liquid crystals (TLC), which is placed on a
back plate made of aluminum. On the front side it has a Plexiglas-layer so that
there is a free view on the TLC layer when cold water is pumped through the
test section. To build up a turbulent
ow, V-shaped obstacles are also placed
on the TLC layer. The whole test section is connected to a water circuit with
a pump, a valve to open and close the water
ow, a
ow-measuring unit in
order to determine the mass
ow of the water and a three-way valve. Via the
three-way valve, it can be switched between hot and cold water as two water
reservoirs are connected to it. In front of the plexiglas a CCD camera and a
lamp are placed. The camera is connected to a PC and records the hue changes
of the TLC as the water cools down the TLC layer. The lamp will make sure
the light intensity stays the same over the duration of the experiment.
Figure 1: Channel Shape (Source: Laboratory Introduction by Prof. T. Roesgen
and B. Laveau)
2.1 Operating Procedure
Before starting the simulation, water from a reservoir with 45-50°C is pumped
through the test section until the TLC has reached a steady-state temperature.
The three-way valve settings are then changed and cold water is introduced into
the channel to simulate the cooling of a turbine blade. At the same time the
CCD camera is activated to take multiple pictures of the TLC layer, while the
layer changes its color as it cools down. When the temperature is lower than
the range the TLC can visualize, the valve is closed and the pictures are directly
sent to a Matlab program. The program matches a certain temperature to every
picture. This is possible, because the system was calibrated before we started
the experiment.
With the temperature distribution over time, the program calculates the convec-
tion heat transfer