Engineering
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Uncoupling Coriolis Force and Rotating Buoyancy Effects on Full-Field Heat Transfer Properties of a Rotating Channel
Chapters
Summary October 5th, 2018
Here, we present an experimental method for decoupling the interdependent Coriolis-force and rotating-buoyancy effects on full-field heat transfer distributions of a rotating channel.
Transcript
This method can help answer key questions in the heat transfer field including many rated to internal cooling of cast turbine rotor blades. The main advantage of this technique is the full field heat transfer data collected and the proposed data reduction method. Those are able to reveal individual and interdependent effects of the Coriolis folds and rotating buoyancy on local heat transfer properties.
Demonstrating the procedure will be Kuo-Ching Yu, Wei-Ling Cai and Hong-Da Shen. Three graduate students from my laboratory. The protocol requires the use of a rotating rig that consists of a shaft driven by a motor.
The shaft drives a rotating platform that supports a test module. It also has a counterweight for rotational balancing. An infrared camera is in position to scan the test module.
Features of the test module used for data collection are depicted in this exploded view schematic. When constructed, the Teflon frame, sidewalls, dividers and top and back plates help define a square two past channel with S-shaped inlet and outlet legs. The module's base attaches to the rotating platform.
During experiments, stainless steel foil end walls carry current to generate heating flux. Copper plates help hold the foil in place. An air Plenum chamber supplies pressurized air flow through the base slightly off the centerline of the inlet leg.
Finally, exhaust from the outlet leg also goes through the base. Once assembled and in position, the exposed foil end wall is the leading end wall in the rotation. At the mounted test module, prepare to measure the thermal emissivity.
There hang heating foil between infrared camera and the test module. Make the electrical connections for heating. Access the side of the foil closest to the test module.
On that side at the center of the foil install a calibrated thermocouple. Next turn attention to the camera facing side of the foil. Prepare this side opposite the thermocouple by spraying a thin layer of black paint onto it.
Now employing enclosure to isolate the camera in heating foil during data collection. Feed electrical power to the heating foil to create a symmetrical flow field. Once the system is at steady-state, measure the temperature by thermocouple and infrared thermography.
Repeat the measurement with different heater powers. After completing the measurements, remove the thermocouple from the foil. Have the rig ready for making heat transfer tests.
This includes equipment for pressurizing the test channel. Be prepared to adjust the counterbalancing weight to establish balance of the rig. First verify or establish static balancing of the rotating rig.
Once achieved, the rotor will remain in any angular position that it is set to. For dynamic balancing, start the rig rotating at the desired constant rate. Begin infrared imaging and view the captured images.
When the rig is not in dynamic balance, the thermographic image from the measurements is not stable. To achieve dynamic balance, gradually adjust the counterweight. When dynamic balance is achieved, there will be a stable thermal image under the running conditions.
Remove the test module from the rotating rig and take it to a bench. Next access the modules coolant channel. Have thermal insulation material available, in this case insulating fiber.
Fill the coolant channel to the test module with the thermal insulation. Here, the channel is filled sufficiently for the next steps in the protocol. Reassemble the test module and prepare it for remounting on the rig.
Reconnect all power in instrument cables. Return the test module to the rotating rig, apply heating power and set the conditions for the measurement. Monitor the wall temperature over time, usually more than three hours.
The temperatures at the two time points of interest are plotted below the thermal image. When the temperature variation is less than 0.3 Kelvin, record the wall in ambient temperatures and the heater power. Make multiple measurements while systematically varying the heating, rotation speed and direction.
When done take the module back to a bench to remove the insulating material before mounting it on the rig again. Next, perform the heat transfer test with the test module. Start the spreadsheet developed for the experiment.
In the appropriate cells, define the geometric parameters associated with the test module. At the set up, start coolant flow into the test channel. In the spreadsheet, enter the measured values for ambient and fluid temperatures, coolant mass flow rate, atmospheric pressure and the measured coolant static pressure.
The software calculates the Reynolds number and displays it. If it is not the desired Reynolds number, change the coolant mass flow rate. Then reenter the measured parameters to find the new Reynolds number.
With the Reynolds number established, activate the thermography system. Next, supply and regulate the heating power to set the wall temperature. Check that the temperature has reached a steady-state at the predefined value by verifying the temporal wall temperature profile is flat.
In the spreadsheet, enter the averaged wall temperature over the scanned area. Also input the heating voltage and the heating current. For baseline tests, once the conditions are set save the data for post processing.
For the rotating heat transfer test continue by activating the motor to begin rotation. Enter the rotation speed of the shaft into the spreadsheet. The software will determine the rotation number for the current conditions.
Adjust the rotation speed in order to obtain the targeted rotation number. In order to achieve the desired Reynolds and notation numbers in a steady state condition, it may be necessary to fine-tune the coolant flow rate. Rotation speed and heat up power several times.
Save all the rotating heat transfer data for post processing. Continue to systematically collect data for different values of the experimental parameters. In these images of the S channel test module with coolant streams at different Reynolds numbers there is spatial variation of Nusselt numbers due to centrifugal forces induced by vortices.
These plots reflect the area averaged heat transfer properties over the leading and trailing end walls of the S channel module. The rotation to static Nusselt number ratio as a function of buoyancy number goes from below to above one for the leading-edge wall. For the trailing edge wall, the ratio is never below one.
Note, for fixed rotation number and different Reynolds numbers, the normalized Nusselt numbers vary over a small range. Different channel types have different behavior. For a given rotation number, extrapolation to zero buoyancy gives the heat transfer level due to Coriolis forces with vanishing buoyancy for the leading wall.
Similar analysis works for the trailing wall. Here is the variation of the rotation to static Nusselt number ratio at vanishing buoyancy as a function of rotation number for different channel geometries. The data revealed the uncoupled Coriolis force effects on the area averaged heat transfer properties of the leading and trailing edge end walls.
These data demonstrate the impact of buoyancy number on the heat transfer properties of a rotating channel is rotation number dependent. So this method can provide insight into cooling performance of rotating channel inside a cast turbine rotor blade. It can also be applied to other systems such as amature cooling of rich core motor.
Generally, individuals new to this method or cycle because the heat transfer measurement from a rotating surface is difficult. Once mastered, this technique can be done in 100 hours if it is performed properly. While attempting this procedure, it is important to remember to constantly check for coolant flow leakage.
After its development, this technique pave the way for researchers in the field of cast turbine engine to explore the full field Nusselt number distributions in rotor blades. After watching this video, you should have a good understanding of how to uncoupled effects of Coriolis folds and rotating buoyancy on full field agents for properties of the rotating channels and applications to cast turbine rotor blades.
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