Running the test rig: experimental procedures

This section describes the procedures involved in running the test rig. Additionally, in order to prepare the rig before running an experiment a degassing procedure needs to be followed.

3.7.1 Degassing procedure

Assuming that the microchannel test section is already installed in the test rig, a degassing procedure should be carefully followed to ensure that the flow loop system is free from gases. This procedure is given below as a sequence of instructions.

1. Switch on the computer and run the LabView program so that the main power supply in the test rig control box is turned on.

2. Close all valves on the rig and isolate the main reservoir from the flow loop by closing the valves connecting the reservoir and the flow loop.

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3. Evacuate the reservoir by using the vacuum pump. When the gauge pressure inside the reservoir is reduced to approximately -30 inHg (-100 kPa), close the vacuum pump valve and switch off the vacuum pump.

4. Fill the main reservoir with deionized water through the degassing line until the water level inside the reservoir reaches the water level mark on the sight glass. 5. Boil the liquid in the reservoir over a period of time and periodically open the

degassing line vent valve on the top of the reflux condenser to release any trapped air or gas to the atmosphere. This step should be repeated until the pressure inside the reservoir does not change at the corresponding saturation temperature, e.g. 102°C.

6. Switch on the chiller unit to supply chilled water-glycol coolant to the subcooler.

7. Open valves between reservoir and flow loop and switch on the gear pump. Note: that the temperatures of water at the inlet pump should be less than 50oC. 8. Open the valves on the local pressure transducer purging lines. Water will then

flow through the pressure transducers so that gas or air trapped inside the transducers and the tubes connecting the transducers to the test section is flushed away.

9. Continue circulating water around the flow loop. The vent valve on the top of the reflux condenser should be opened periodically so that air or gas in the loop or reservoir can escape to the atmosphere. This step should be repeated until the pressure inside the reservoir does not change at the corresponding saturation temperature.

10. The rig is now ready to be used for conducting experiments.

3.7.2 Sequence of steps for running an experiment

In this section, the experimental procedures are grouped separately for the single-phase and two-phase flow boiling tests.

3.7.2.1 Single-phase experiments

The objectives of the single-phase experiments are to obtain adiabatic and non-adiabatic measurement data to establish the pressure drop, friction factor and heat transfer

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characteristics. The following procedures are adopted, again given in the form of instructions.

For adiabatic experiments:

1. Switch on the computer and run the LabView program.

2. Open the degassing vent valve on the main reservoir so that the pressure inside the reservoir is approximately equal to the ambient pressure. The water inside the main reservoir does not need to be boiled.

3. Run the pump to circulate the water through the flow loop.

4. Flush any air, trapped inside the local pressure transducers, or inside the tubes connecting the pressure transducers to the test section, by opening the purging valves one-by-one.

5. Stop the pump and open the valve connecting the water manometer to the flow loop. Turn on the lamp inside the pressure transducer enclose to keep the temperature inside at approximately 50°C. Check the calibration of all local pressure transducer against the water manometer.

6. Close the water manometer valve. 7. Re-start the pump.

8. Adjust the pump speed to give the required mass flow rate. Switch on the pre- heaters one or more to raise the water temperature at the channel inlet to the required level.

9. After approximately 15 minutes, the flow and the heating process in the preheater will be steady; the steady condition is achieved when the mass flow rate, inlet temperature and pressures show constant values.

10. Record all data, including temperatures, pressures and mass flow rate using the LabView program.

11. Repeat procedures 8 and 9 for different inlet temperatures and mass flow rates, e.g. 20 g/min, 30 g/min, 40 g/min, and further as required.

12. After finishing the experiment, switch off the pre-heaters, decrease the mass flow rate and turn off the pump. Shut off the valves connecting the main reservoir and the flow loop and the degassing vent valve.

93 For non-adiabatic experiments:

In this group of experiments, the aim is to obtain single-phase heat transfer coefficients. Hence, power should be applied to the microchannel test section to heat the water flowing through it.

1 – 8. Follow the same steps as for the adiabatic experiments.

9. Switch on the electrical supply to the test section and use the Variac to adjust the power to the required value, e.g. 11 W. The power setting chosen should not cause boiling to occur inside the test section, even for the minimum mass flow rate selected. Run a mass flow rate, e.g. 20 g/min, and wait until conditions are steady. This condition is reached when there are no changes with time in the mass flow rate, pressure and temperature readings. After steadiness is reached, record all data using the LabView program. Increase the mass flow rate, e.g. 30, 40, 50 g/min and further as desired, and record all data for every mass flow rate used.

10. Repeat step 9 for different input power setting, e.g. 12, 15, 20, 24, 30 W.

3.7.2.2 Flow boiling experiments

The objectives of these experiments are to investigate the mechanisms of flow boiling in microchannel and to obtain heat transfer coefficient, two-phase pressure drop and instability data for a range of mass fluxes and heat fluxes. In addition, flow boiling patterns are visualized to inform the construction of flow pattern maps. To fulfil the above objectives, the experiment should be conducted as follows:

1. If the main reservoir is empty, fill with deionized up to the water level mark on the sight glass.

2. Open the degassing vent valve so that the pressure inside the reservoir is approximately equal to the ambient pressure. Switch on the lamp inside the pressure transducer enclosure.

3. Run the pump and operate the purging valves to flush out air/gas trapped inside the local pressure transducers and the tubes connecting the transducers to the test section.

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5. When the temperature inside the transducer enclosure reaches approximately 50°C, check the calibration of all the local pressure transducers against the water manometer.

6. Closed all valves on the flow loop and the degassing vent valve, but do not turn off the lamp inside the transducer enclosure.

7. Perform degassing procedures.

8. Adjust the speed of the pump so that the required water flow rate is delivered to give a mass flux of 200 kg/m²s.

9. Switch on the pre-heaters (one or more) and adjust the pre-heaters to raise the water temperature at the channel inlet rise to 98°C.

10. Switch on the Variac power supply and adjust the test section heating to the desired value. After steadiness is reached indicated by no changes in the mass flow rate and all temperatures, record all the required measurement data using the LabView program. Increase the power carefully by adjusting the Variac setting. Wait until the flow rate and temperature are steady and then again record all the measurement data. Increase the power further in steps until the fluid starts to boil. Stop increasing the power when either the test section wall temperatures reach values of approximately 120°C, or the pressure in the main reservoir cannot be decreased further. Record all data and the ambient pressure at each step as the test section heating power is increased. Under flow boiling conditions, increasing the heat flux causes the mass flux to decrease and the inlet pressure to increase. Consequently, both the mass flux and inlet pressure need to be adjusted to maintain constant values. The test section inlet pressure can be decreased by reducing the system pressure or the temperature in the main reservoir.

11. For flow boiling conditions, at each step of increasing power input, images of bubble movements should be captured and recorded at three different locations along the test section (i.e. near inlet, middle and near outlet) using the Phantom V6.0 high-speed camera.

12. Decrease the power input to the test section step-by-step until the Variac auto transformer is at the zero setting position. Record all data and the ambient pressure at each step. Switch off the Variac power supply and turn off the pre- heaters.

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13. Repeat steps 8 to 12 for different mass fluxes, i.e. 300, 400, 500 kg/m²s, and further as required.

14. After finishing the experiments, turn off power to all heaters and after the test section wall temperatures fall below 90°C, switch off the pump and close all valves on the rig. Shut down the LabView program and the main power supply to the test rig. Finally, turn off the Coriolis flowmeter, the camera lamps and the camera.