Nickel Superalloy Turbine Blades Engineering Essay

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Nickel superalloy turbine blades are highly engineered components. Industrial gas turbine blades required the blades with longer time creep rupture properties and good hot-corrosion resistance. [1] To survive in harsh and extremely high temperature environment, some of the turbine blades undergo heat treatment after the casting process to achieve optimal balance of its mechanical properties. Gas quenching becoming the common heat treatment process used due to the advantages in comparison to the conventional heat treatment process such as oil and polymer quench. The advantages of gas quenching include the controllable cooling parameters of the furnace. The flexibility of controlling the direction of flow, furnace pressure and the mass flow rate of the gas make gas quenching much more favorable compared to the conventional methods. [4] Furthermore, the surface quality of the components undergo gas quenching will be retained after the treatment process. [6] Hence, the industry which required the good surface finish of the component will be beneficial from gas quenching, especially turbine blades where the surface roughness of turbine blades affects from the flow of fluid. Gas quenching is more environmentally friendly compared to conventional quench methods as there is no harmful gases are used or produced during the heat treatment process. [6]

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Heat treatment involves heating and rapid cooling or quenching of the components. The heating up process in required increasing the temperature of the furnace and then the components from room temperature to the set temperature through radiation. The time period for the components undergo heat treatment to reach the set temperature uniformly is called thermal hysteresis time. [9] After the thermal hysteresis time, heat is provided continually during the 'hanging time' to allow the microstructure of the component develops to austenite which is also called gamma phase. [9] In this case, the nickel-base alloy in the turbine blade undergo the phase transformation from body-centred cubic (BCC) to face-centred cubic (FCC) configuration of gamma phase. [1]

Gas quenching is controlled rapid cooling of heat-treated component. [6] The flow of gas during quenching is very complex due to the placement of the components along with the other devices that are used to control and perform the heat treatment process such as control thermocouples, heat source, and the basket for containing the turbine blades. The complex flow path of quenching gas leads to the complex heat transfer from the turbine blades to the gas. The furnace must be designed to deliver uniform distribution of flow to all the quenched components.

The couplings chart in shown on figure (1) can be used to predict the residual stress and distortions. According to that, temperature distributions and temperature gradients in the quenched components generated thermal stresses and phase transformation during quenching. Therefore, it is necessary to investigate the temperature distributions in the turbine blades. However the phase transformation is neglected in the investigation of thermal stresses due to temperature gradients. [5] The investigation is mainly focus on the root block of the turbine blades where the change of geometry thickness along the length is the highest. The distortions and cracks could be mainly occurred in the region. Experimental methods for determine the temperature gradients of the turbine blades is done on the fastest and the slowest cooling rate during quenching. The down side of the experimental methods is that the stress developments are not able to be accessed during the quenching process. Hence, a simple model to describe the quenching process is done on the root block of turbine blade. According to experienced engineers in Doncaster, they believed the cracking happened during the first five minutes of the quenching started. Therefore, the analysis is done for the first twenty five minutes of the quenching.

Figure 1: Thermal, metallurgy, and mechanical couplings in heat treatment. [5]

Quenching, phase transform

Methods

Experiment

There are a few experiments had been designed and performed in order to achieve the aim. To start with, the heat transfer rate in the root of the turbine blades at different positions in the furnace is investigated. During the heat treatment process, the turbine blades are arranged in array in the furnace. Hence, an experiment is conducted to determinate the cooling rate of every root block placed in the furnace. However, due to the limitation of the maximum of twelve thermocouples that is able to support by the furnace, the thermocouples are placed and labeled as shown in the figure (1). The experiment is begun by drilling a hole into the centre of root block of the turbine blade to accommodate the thermocouples. In this way, the temperatures in the root blocks are able to measure correctly. The thermocouples placed in the root are fixed to be 84 mm of depth and the depth of the insert is set to be constant for all turbine blades. The reason of this is to set the thermocouple inserts depth to be controlled. Hence, the factor is not causing any error to the readings. The turbine blades with the thermocouples is then loaded to the cage and placed in the middle of the furnace. The serial number of the thermocouples is recorded for and the sockets are connected. The door is closed and the furnace is run for heating and quenching process using the recipe recommended for the turbine blade. The data is sampled once every second using the Euroterm Datalogger for the furnace along the heat treatment process. Once the process is done, the data is downloaded from the company database for analysis and the experiment is then repeated to validate the results before proceed to the next experiment.

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Figure 1: The arrangement of turbine blades in furnace during heat treatment.

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Experiment 3

The continuation from the previous experiment, the cooling rate of the turbine blade root block varies with the positions in the furnace is able to be determined and calculated. The next experiment is to find the temperature distribution within the blade and the root block at the location with fastest and slowest cooling rate. Two freshly-scrapped turbine blades are used for instrumenting and holes and grooves are made and labeled in the specific places in the part of the blade as shown in figure (2). A thin nickel alloy plate is welded to location 4, 5, 6, 4a, 5a and 6a to form a holder for the thermocouple. The hole in location 1 and 1a is drilled through from the side of the turbine blade. During quenching process, the momentum of the gas on the thermocouple may causes the vibrations and changes in place of the thermocouple. This may cause in the experiment failure and errors in the data recorded. The purpose of the holder is to hold the thermocouples in place as well as to isolate the thermocouple from the direct contact of the thermocouple with the gas that is flowed into the furnace during quenching process. The blades are placed in the position where the root block has the fastest and slowest cooling rate respectively according to the first experiment. The thermocouples are made sure to be in close contact with the blade to avoid incorrect measurements. The fully loaded cage is then placed in the middle of the furnace. The serial number of the each thermocouple is recorded and the sockets are connected. After the door is closed, the furnace is run for heating and quenching using the same recipe in the first experiment. Similar to the first experiment, the data is sampled once every second using the Eurotherm Datalogger for the furnace and the data is downloaded from the company database after the process for analysis.

[304]

C:\Users\Soh\Desktop\tc placement 1.PNGC:\Users\Soh\Desktop\tc placement 1.PNG

Figure 2: Diagrammatic view of the turbine blade with the placements of thermocouples for second experiment

Computer Modeling

For the full heat treatment process and accurate simulation, it is necessary to model the full furnace with all the turbine blades and devices to describe both the heating process and the quenching process. [5] However, due to the time constrain and very powerful computer is needed to simulate the both heating and quenching process, a simple simulation is modeled. Since the heat transfer from the root block during quenching is mainly due to convective heat transfer, a computational fluid dynamic (CFD) analysis is able to use to simulate the heat transfer from the surface of the root block. A k-epsilon turbulent flow and heat transfer model of the turbine blade root block during the quenching process is modeled to predict the temperature distribution over the root block of turbine blade by using Fluent software. [3]

To begin with, the root block model is imported to Gambit software. Gambit is the software to model and mesh the model which is compatible with Fluent software. A simple furnace block is constructed around the root block. Because of the side symmetry of the furnace and the root block, half of the model is used. The root block is meshed with finer tetrahedral mesh due to its complex geometry and for more accurate results during simulation while the furnace is meshed with coarser tetrahedral mesh. Furthermore, the gas is defined to flow from the bottom of the furnace to the top which is similar to the setting for the first and second experiment. The meshed model is exported to Fluent software for numerical simulation for CFD analysis. The boundary conditions set up for the simulation could be referred to table (1).

Table 1: Boundary conditions used in CFD analysis.

Part

Type

Wall of the furnace

Heat flux - 0

Stationary

Non slip

Root block

Coupled

Initial Temperature - 1232 °C

Stationary

Inlet

Velocity inlet - evenly distributed 1 m/s

Outlet

Pressure Outlet - 3 x 105 Pa

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Symmetry

Symmetrical plane

Operating condition

3x105 Pa

The convective heat transfer between the root block and the flow gas can be calculated by using Newton's law of cooling,

(1)

where Q is the heat flux, h is the heat transfer coefficient, A is the area of the part, and is the temperature difference between the surface of the component and the flow gas. [4] From the equation, the heat flux is proportional to the area of part times the difference between the surface and the gas temperature.

However, heat transfer coefficient between irregular surfaces of the root block with the quenching gas is almost impossible to determine directly. [5] However in Fluent software, transient convection problem is able to overcome the problem. Hence, the simulation of the heat transfer through convection is able to make with coupled and transient solver in Fluent software. Four points are placed on the similar places to the holes of the root block in the second experiment where the temperature gradient of the turbine blades is investigated. The temperature distribution and the temperature of the points in the root block for every time steps are recorded for the first 300 seconds of the quenching process. Further 1200 seconds is simulated to obtain the temperature of the points placed in the root block for comparison with the results from second experiment.

Results

Table 2: Table of cooling rate for Experiment 1 and Experiment 2

Position

Experiment 1

Experiment 2

Mean absolute percentage error (%)

A

Cooling rate for first 300s (°C/s)

0.4277

0.5860

7.8099

Cooling rate for 300s to 1500s (°C/s)

0.5803

0.5993

0.8053

B

Cooling rate for first 300s (°C/s)

0.5277

0.5337

0.2827

Cooling rate for 300s to 1500s (°C/s)

0.6080

0.6010

0.2895

C

Cooling rate for first 300s (°C/s)

0.6410

0.8167

6.0256

Cooling rate for 300s to 1500s (°C/s)

0.6514

0.6399

0.4453

D

Cooling rate for first 300s (°C/s)

0.5593

0.5547

0.2095

Cooling rate for 300s to 1500s (°C/s)

0.6177

0.5847

1.3723

E

Cooling rate for first 300s (°C/s)

0.9690

1.2643

6.6119

Cooling rate for 300s to 1500s (°C/s)

0.6492

0.6144

1.3751

F

Cooling rate for first 300s (°C/s)

0.9580

1.2980

7.5355

Cooling rate for 300s to 1500s (°C/s)

0.6564

0.6143

1.6558

G

Cooling rate for first 300s (°C/s)

0.7720

0.9553

5.3068

Cooling rate for 300s to 1500s (°C/s)

0.6543

0.6322

0.8615

H

Cooling rate for first 300s (°C/s)

0.7610

0.9303

5.0059

Cooling rate for 300s to 1500s (°C/s)

0.6606

0.6435

0.6550

I

Cooling rate for first 300s (°C/s)

0.6443

0.9867

10.4946

Cooling rate for 300s to 1500s (°C/s)

0.6450

0.6408

0.1653

J

Cooling rate for first 300s (°C/s)

0.8957

1.1587

6.4011

Cooling rate for 300s to 1500s (°C/s)

0.6544

0.6278

1.0399

K

Cooling rate for first 300s (°C/s)

0.9883

1.3210

7.2027

Cooling rate for 300s to 1500s (°C/s)

0.6536

0.6069

1.8511

L

Cooling rate for first 300s (°C/s)

0.8190

1.0973

7.2621

Cooling rate for 300s to 1500s (°C/s)

0.6537

0.6271

1.0378

Figure 2: Graph of Cooling Rate vs Time for Experiment 1.

C:\Users\Soh\Desktop\slow coolrate.tif

Figure 3: Graph of Cooling rate vs Time for Experiment 3 for Turbine Blade at Position A (Slowest Cooling Rate)

C:\Users\Soh\Desktop\fastest coolrate.tif

Figure 4: Graph of Cooling rate vs Time for Experiment 3 for Turbine Blade at Position K (Fastest Cooling Rate)

Figure 5: Graph for Experiment 3

Figure 6a: Graph from simulation.

J:\Root block meshed symm 1500s.tiff

Figure 6b: Graph from simulation.

Discussions

The assumptions made before the experiment stated that the temperature of the blades at the symmetrical position would have the same heating and cooling rate. The reason of the assumption made is because the furnace supported the placement of twelve thermocouples only. However from table (2), cooling rate and heating rate for is differ for A and D, E and F, I and L. This may due to the shape of the turbine blades which has the concave and convex shape at one of their side. The view factor is used to account for the effect of orientation on radiation heat transfer between surfaces. Technically speaking, Fii = 0 for plane and convex surfaces while Fii has a value for concave surface. [2] Therefore, there is no radiation which able to leave the convex surface and hit back later. However, it is different for concave surface. The radiation left the surface able to hit back the concave surface. Therefore, the turbine blades which received the direct radiation from the heating element with concave surface will have higher heating rate and slower cooling rate. Besides, the variation of the temperature could be affected by the placement of the basket. The basket could be place off centred in the furnace. Furthermore, the overused of the basket may caused the distance between turbine blades is not constant to each other. This could be one of the factors which cause the variation of temperature in the symmetrical location of turbine blades.

The first experiment is repeated with five second data sampling to validate the result from the first experiment. Based on table (2), there are variations in the cooling rate for the positions in both of the first and the repeating experiment. The position of the slowest cooling rate in the root block is varies for both of the experiment. The root of the turbine blade in position A has the slowest cooling rate in the first experiment with 0.4277 °C/s. While in position D, the root block has the cooling rate of 0.5547 °C/s which made the root of the turbine blade with the slowest in the second experiment. However, the position with fastest cooling rate is still remained the same for both experiment which is at position K with 0.9883 °C/s and 1.3210 °C/s for the first and second experiment respectively. Pattern of the cooling rate based on the table (2) is the turbine blades at front row and middle position has the fastest cooling rate, and the cooling rate is gradually slower to the turbine blades at the side and the turbine blades at the back rows which is the furthest from the furnace door has the slowest cooling rate. According to graph in figure (2), the position with ranking of the fastest cooling rate to slowest cooling rate are shown in figure (7) where K is the position with fastest cooling rate, followed by E, F, J, L, H, G, I, C, D, B, and lastly A. The front row and the middle row of the turbine blades have the higher cooling rate compared to the back row due to the design of the inlet. The inlet is designed inclined towards the furnace door with several small baffles reflected the gas to the back of the furnace. Therefore, the inlet gas is more focused on the turbine blades at the front row. From graph (1), the cooling rate of the turbine blade root is at the maximum at 300 s. It is found that the cooling rate reaches maximum when the pressure of the furnace achieved 3 x 105 Pa from vacuum and the cooling rate fluctuated for the first 100 s when the gas is being pump into the furnace.

In the third experiment, the cooling rate is for the turbine blade is very different for the turbine blades in the position with the fastest cooling rate and the slowest cooling rate. According to figure (3) which is the graph for the turbine blade in position A, point 3 has the highest cooling rate at the first 200 s followed up by point 2, point 6, point 4, point 5 and lastly point 1. Point 6 has the highest cooling rate after 200 s the quenching process started which is due to the smallest cross section of the blade at the position.

However, for the turbine blade at the position K with the fastest cooling rate in experiment 1, it seems like the except for the point 6a and point 1a which has the fastest and slowest cooling rate respectively. Point 2a, 3a, 4a and 5a have the cooling rate that is very close to each other by referring to figure (4). Similar to the turbine blade at position A, all the points near to the surfaces has the maximum heat transfer rate. Refer to equation (1) above, the heat flux increases when the temperature difference between the surface temperatures with the flow gas increases. From both of the graphs in figure (3) and figure (4), the turbine blades at both positions started off with maximum cooling rate at the first 100s when the gas started flowing into the furnace for quenching. However, the turbine blade at position A seems to have gradual or flatter cooling rate and which finally achieve the uniform temperature gradient with point 1 which the point is placed in within the root block of the turbine blade. By looking at the figure (4), the slope of the graph seems to be much steeper compared to the graph in figure (3). In other words, the surface of the turbine blade at position K cooled much faster than the inner body of the root block of the turbine blade. Hence, a larger thermal stress would be generated due to the larger thermal gradient.

Compare with simulation

Position

A

B

C

D

Ranking

12

11

9

10

Position

H

G

F

E

Ranking

6

7

2

3

Position

I

J

K

L

Ranking

8

4

1

5

Door

Figure 7: Ranking of the position with fastest cooling to slowest cooling rate

Conclusions