Heat Transfer Analysis Using Thermochromic Liquid Crystal

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

2

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