Project Report2011 MVK160 Heat and Mass Transport

May 19, 2011, Lund, Sweden

Internal Blade Cooling Technology on Gas Turbines

Le Gloanec ErwanDept. of Energy Sciences, Faculty of Engineering,

Lund University, Box 118, 22100 Lund, Sweden

ABSTRACT

Cooling technologies are a major issue in gas turbine devices. Certain parts of them undergo high variations of temperature and, by a consequence, need to be efficiently cooled in order to avoid serious damages.

This project will focus on turbine blades. Several cooling technologies are available to guarantee a safe running environment. They are divided into two main parts: external and internal cooling. Much more material would be needed to present both of them. Thus, this work will only develop two common technologies which take part of the internal cooling section, the pin fins and the turbulence promoters.

INTRODUCTION

Cooling technologies on gas turbines are still a major concern. Thus, gas turbine manufactures don’t present their own technology, even if the basic principles are similar from one to another.

For a modern gas turbine, the blade cooling airflow is about 10 to 15% of the total compressor flow. A major challenge in achieving high turbine efficiency is to minimize turbine cooling airflow with the best utilization of its cooling potential.

PROBLEM STATEMENT

The following illustration presents a section of a turbine blade. The inlet cooling air comes from the bottom of the blade and then, follows different channel networks: this is an internal cooling.

Figure 1 - Turbine Blade section

The inlet airflow is divided into three parts. The first one, on the right of the blade which corresponds to its thinnest part, goes directly through small holes which are distributed all along the downstream part of the blade: these are the “Pin fins”. The second flow, in the middle part of the blade, is forced to pass through a complex network of pipes which contain obstacles whose role is to generate turbulences and consequently, to increase cooling effects: this is the reason why they are called “Turbulence promoters”. The third flow, on the left part of the blade, goes either through turbulence promoters or, thanks to

Copyright © 2011 by Erwan Le Gloanec

internal connections, to a region marked “Impingement cooling” on the previous picture. The role of the latter one is to create an air cooling film on the upstream part of the blade.

LITERATURE SURVEY

A previous study was realized on blade cooling technologies, entitled “Refroidissement des turbines à gaz – Techniques et efficacité” by Bruno Facchini and Luca Innocenti. It presents both internal and external cooling technologies used on gas turbine devices. Moreover, it introduces the way of conceiving a turbine blade by using CFD calculations.

Another survey has been used for this work, entitled « Simulation d’un couche limite perturbée par un obstacle » by Abdelkader Lahcene, Miloud Aminallah and Samir Laouedj. The purpose of this project was to study the effects on pressure and the thermal exchange coefficient of the presence of obstacles inside pipes, which are subject to a turbulent air stream flow.

PROJECT DESCRIPTION

Pin FinsAerodynamic losses on airfoils depend on the thickness of the downstream part of the blade. For this reason, the latter has to be as thin as possible. Consequently, multi-pass channels are not used on this part of the blade to avoid the thickening of the downstream part of the airfoil. Thus, small cylinders (the so-called “pin fins”) are introduced in a narrow channel from the root of the blade. They are placed in an orthogonal way compared with the airflow in order to considerably increase the turbulence effects: this phenomenon leads to increase the thermal exchange coefficient which improves the cooling effect.

Besides, depending on the ratio between the height and the length of the pin fin (h/d), the thermal exchange area may arise. Pin fins have a ratio 0.5<h/d<4. If h/d<2, the pin fins are qualified as short ones: in this case, the heat is mainly exchanged by the lateral area and the

overall exchange area is reduced by the presence of pin fins.

Most of the time, pin fins are not used in a single row. Those small cylinders are placed on several series, along a straight line or a shift line as illustrated in the following picture:

Figure 3 - Two different placements of pin fins

Using shift lines increases the turbulence effects on the airflow which, as we mentioned previously, also increases the thermal exchange coefficient. From the root of the blade, the latter is relatively limited on the first lines of pin fins. Then, it progressively increases at each stage of pin fin lines because of wakes and turbulences generated by the upstream pin fins. This phenomenon stabilizes from the fourth or the fifth stage.

Most of studies about this subject use pin fins which have the same height in order to simplify computations. But in reality, pin fins inside the

downstream part of the blade have a variable height within the pipe. The acceleration of the stream tends to decrease the thermal exchange coefficient by reducing the Nusselt number dependency on the Reynolds number.

Metzger developed a relation between the Nusselt number and the Reynolds number for 10 pin fins:

Nud=0.135∗ℜD0.69∗( X

d )−0.34

The previous relation is valid for short pin fins with 1.5<X/d<5, Y/d=2.5 and h/d=1. It can be applied to other number of pin fins by modifying the Nusselt number, as illustrated in the following graphic:

Copyright © 2011 by Erwan Le Gloanec

Figure 2 - Pin fins along a tur-

bine blade

Airflow

Airflow

Figure 4 - Evolution of the Nusselt number

The Nusselt number represents the ratio between the thermal transfer by convection and the thermal transfer by conduction. The convective part represents the heat transfer through the airflow along pin fins. The ratio Nu / Nu (which is the ratio between the local Nusselt number and the average one) tends to stabilize around one as the number of lines of pin fins increases. This means heat transfer occurs both thanks to the convective and conductive way. The maximum value of the ratio is obtained for four lines of pin fins: since its value is larger than one, the convective part is a little bit more significant than the conductive part.

Specific pipes to increase turbu-lence effects

Certain blades are fitted with specific pipes: instead of being slick, they contained bumps whose role is to enhance the turbulence phenomenon (those bumps are also called ribs). Indeed, using ribs considerably increases the level of turbulence and, at the same time, the value of the thermal exchange coefficient. Thus, by using limited airflow, the cooling efficiency is better than the one observed with slick pipes. However, because it is technically more complex to settle, using ribs inside pipes is more expensive.

Two types of ribs can be used: they are either stuck on the inner surface of the pipe or removed from it.

Figure 5 - Stuck ribs

Figure 6 - Removed ribs

If the ribs are stuck on the inner surface of the pipe, a recirculated airflow is going to be created just after each rib, which will decrease the thermal exchange coefficient in this specific area. On the contrary, if the ribs are removed from the inner surface of the pipe, the airflow will be separated into two parts, on both side of each rib. Therefore, because it doesn’t generate a secondary and stationary airflow, the cooling effect will be more efficient in this case due to a higher thermal exchange coefficient. This situation is illustrated through the following graphic: it represents the thermal exchange coefficient as a function of x which is the airflow direction indicated on the two previous sketches.

Figure 7 - Evolution of the thermal exchange coefficient

Due to the recirculated and stationary airflow, the thermal exchange coefficient presents strong variations after each rib. This is not the case with removed ribs and that is probably the reason why it is more efficient in terms of cooling. Blades undergo both convective and conductive phenomenon which mainly determine their running temperature. To ensure a high efficiency, the area close to the combustion chamber imposes really high temperature to the closest compressor and turbine blades. Those temperatures are not so far from the melting point of the blade material: thus, it is extremely important not to have undesired variations of temperature due to strong evolutions of the thermal exchange coefficient. As a consequence, removed ribs seem to be more used than stuck ribs.

Copyright © 2011 by Erwan Le Gloanec

As shown on the illustration on the first page, turbulence promoters are often used in multi-pass devices. The shape of the heat exchanger is likely to be a streamer which contains several pipes whose axis is radial. Alternately crossed by centrifugal and centripetal airflow, those pipes are connected thanks to bends, most of the time slick ones. Streamers are located in the central part of the blade, where the thickness is not a crucial factor and thermal solicitations are less important than the ones on the front part of the blade.

In the case of multi-pass streamers, the effects of the rotation modify the thermal exchange mechanism. Because of the presence of both centrifugal and centripetal pipes, the Coriolis force alternately increases thermal exchanges on the side of the pipe which is turned up (the leading surface) and on the one which is turned down (the trailing surface) as illustrated in the following picture:

Figure 8 - Multi-pass streamers

Those types of devices have a great cooling efficiency and the coolant fluid undergoes high variations of temperature. Here, there are not any analytical methods to calculate the thermal exchange coefficient for a turbulent flow. Indeed, the presence of ribs generates flow movement really complex like airstream separations or recirculation of fluid. Thus, most of surveys use thermal exchange coefficient determined from semi-empiric correlation which results from known geometries.

In 1988, J.C. Han and J.S. Park developed such correlations to predict the behavior of a rectangular pipe which contained ribs on both

side of the inner surface and placed orthogonally to the inlet airstream, in a fully developed turbulence flow. The thermal exchange coefficient is computed after having determined the Nusselt number:

Nu=a∗ℜ−b∗Pr−c

The previous relation is valid for a slick pipe. In the presence of ribs, the “b” exponent on the Reynolds number has to be lower of approximately 4% and the “a” coefficient has to be multiplied by 2.7.

CONCLUSIONS

The use of pin fins is one of the only technologies available to cool the downstream part of the blade since it needs to be really thin.

Concerning the turbulence promoters, using removed ribs seems to be a better solution than stuck ribs in order to limit the variations of the thermal exchange coefficient.

REFERENCES

[1] Klaus Brun and Rainer Kurz, 1999, Introduction to Gas Turbine Theory – An overview of fundamental concepts. Book

[2] Bruno Facchini and Luca Innocenti, Refroidissement des turbines à gaz – Techniques et efficacité, Techniques de l'Ingénieur editions, TI-bm4566

[3] Abdelkader Lahcene, Miloud Aminallah and Samir Laouedj, 2005, Simulation d’une couche limite perturbée par un obstacle. Paper in Conference ProceedingsAvailable on the website http://web.univ-ubs.fr/limatb/EG2M/Disc_Seminaire/CFM2005/articles/157.pdf  

[4] www.wikipedia.com

[5] Course Material

Copyright © 2011 by Erwan Le Gloanec