The overall goal of this experimental procedure is to present a thermography technique, that utilizes a well designed test section to accurately measure the local instantaneous convective heat transfer coefficients, in a single or two phase pipe flow. This method can help answer key outstanding questions in transient convective heat transfer, in single and multi phase flows. The main advantage of this technique, is that it's non invasive and can measure fast variations in wall temperature, resulting from time dependent flows.
Demonstrating this procedure will be Avram Balas, and Yakov Levi, technicians from our laboratory. Begin with a segment of pipe 70 centimeters long or more. It's diameter and wall thickness, should match the pipe of the experimental facility.
Mill four adjacent narrow windows along the pipe. Each window is six millimeters wide and 80 millimeters long. The gap between windows is 25 millimeters.
Next get 12 micrometer thin stainless steel foil, and cut a long thin strip. To help weld the heating wires the strip is coated with gold near each end of the strip. Continue by measuring the foil's electrical resistance with an ometer.
Now obtain a rigid tube that fits into the pipe. The tubes outer diameter should equal the inner diameter of the pipe. Cut out a long segment of the rigid tube to create an arch.
This long arch, the basis, fits into the pipe, and will support and protect the foil. To put the foil in the test section, coat the basis with grease to aid in detaching the foil later. Then place the foil on the basis, to transfer to the test section.
Flatten the foil and use a cloth and alcohol to clean excess grease. Next, apply adhesive on the foil periphery, so it will adhere to the pipe. In addition, identify regions corresponding to the bridges between windows on the pipe and apply adhesive to them.
When ready orient the basis, so the foil faces the same direction as the test section windows. Then carefully insert the basis into the test section. Ensure the foil is aligned with the windows.
Check that each gold stripe is at an edge of an outer window. Once the foil is in place, clamp the basis to each end of the pipe. Next obtain a bicycle inner tube, folded to fit into the pipe.
Carefully insert the tube into the pipe under the basis. When done, inflate the tire. As the tire inflates, observe the adhesive spreading across the foil and reaching the basis.
Use a cloth to clear any excess adhesive in the window openings. After 24 hours of drying deflate the tire and extract it. Open the clamps holding the basis and pipe together.
Test each end of the basis to decide which side is easier to disconnect. Then use a long roller and start on the easier end. Place the roller between the basis and the pipe, and move it slowly into the pipe, until the entire basis is separated.
The foil should remain undamaged on the pipe interior. As a test of the seal, close one end of the pipe. Then fill the pipe with water and check for leaks.
Continue after unblocking the pipe and cleaning its interior. Connect electrical heating wires to the golden strips on the foil. Also connect T-Type thermocouples to the bottom of each window.
Finally, spray paint the outer side of the foil with a black matt. Now put the test section in the experimental facility. Connect the pipes with flanges.
Before continuing, fill the system with water. Move on to set up a computer for control and data acquisition. Then connect the heating wires from the foil, to a DC power supply.
Next, place a computer controlled infra red camera at a window of the test section. For calibration, place it a few centimeters from the test section surface. Measure the ambient temperature, with a thermocouple held near the experiment.
Next, focus the camera on a window of the test section to measure its temperature. Adjust the emissivity of the IR camera, so the camera and the temperature measured by the thermocouple on the pipe window match. With the camera set up, prepare the optical probes.
Each probe consists of a laser and a photodiode, that is to be connected to the data acquisition computer. Turn the laser on and point the beam at the diode. Check that the computer records a positive signal.
Place an optical probe along the test pipe, that is now filled with water. The final set up is represented in this schematic. The measurement section is in addition to the regular setup.
The computer, in addition to data acquisition from the camera, thermocouples and optical probes, governs the synchronization of bubble injection and wall temperature measurement with the camera. Now establish the desired conditions for the experiment, and perform a single phase pipe flow reference measurement. After that inject a Taylor bubble in the setup, for a two phased flow measurement.
This is an example of the optical sensor output, for a single Taylor bubble rising in a vertical pipe filled with stagnant water. The initial drop represents the sensor circuit opening due to the Taylor bubble tip. The rise represents the bubble's passing.
Later shorter drops represent bubbles in the liquid wake behind the Taylor bubble. The delay of the features in the bottom curve, is due to the sensors being in different positions. This plot represents the ensemble average results of the local convective heat transfer coefficient, due to the passage of a single Taylor bubble rising in stagnant water in a vertical pipe.
The two phase flow convective coefficient results are normalized by the single phase flow coefficient. The distance from the bubble tail moving, in a frame of reference with the Taylor bubble is normalized by the pipe diameter. This video presented measurements of the local instantaneous heat transference and slug flow.
The method can provide information on heat transfer and study stage single phase flow, as well as more complex two phase flows. Once mastered, the manufacturing of the test pipe can be done in a few hours. While attempting this procedure on transient flow, its important to remember to automate thew entire process, to insure repeated results which can later be used for an assembling averaging process.
After watching this video you should have a good understanding of how to manufacture a heat transfer test pipe, which allows a quantitative investigation of convective heat transfer in pipe flows. Don't forget that this technique of convective heat transfer measurement, can be expanded to many flow configurations, using supplementary equipment as optical prompts and high speed cameras.
