Test set-up for evaluating the performance of the heat pipes

The test section of the wind tunnel was used as the testing rig for carrying out the experimentation. The set-up comprised of 19 cylindrical heat pipes arranged vertically at an angle of 90° with respect to ground. The separator plate was located at the centre of the pipes providing an evaporator and condenser sectional lengths of 400mm. Seven K-type thermocouple exposed wires were located upstream and downstream of the heat pipes for recording temperatures and the values were logged using the data acquisition system and connected to the computer.

The data logger comprised of a total of sixteen channels or ports which meant that up to sixteen thermocouple readings were possible during a single test run, out of which seven were used on measuring the air temperatures due to convection. Two thermocouple wires were also attached to the heat pipe surface (one at the evaporator end and one at the condenser end) in order to measure its thermal response when conducting the transient testing for a period of 24 hours. The wind tunnel axial fan with variable motor was able to provide air speeds ranging up to 12m/s while the heating elements provided temperatures up to 323K which was essential for carrying out the experimentation. All other openings unused during the test-run were sealed to prevent hot air from escaping and mixing with the ambient.

Two distinct sets of experimentation were carried out on the heat pipes with the first one being normalised steady-state and the second one involving transient evaluation. Initially, normalised air temperature tests were conducted wherein the source temperature streams at the evaporator section were varied according to a desired set- point temperature. The condenser section of the heat pipe was kept at a steady sink temperature ranging between 288K and 291K.

Using an inlet air speed of 2.3m/s, each test run was carried out for 200 seconds following the stabilisation of heating elements at the set-point level. Tests were carried out at three different heating temperatures including 305K (32°C), 308K (35°C) and 314K (41°C) in order to validate the CFD model at different source temperatures found in Doha, Qatar for the months of November, April and June (The Weather Channel,

2012). The heat pipes were arranged in three different spanwise thicknesses ranging from 48mm to 52mm. Water and R134a were used as the heat pipe internal fluids and were tested systematically one at a time during the run-time. In total, eighteen individual test-runs were conducted in the first experimental phase with nine separate runs for each working fluid.

The second set of experimental testing included a dynamic thermal model replicating the hourly temperatures for 21st June, 2012 found in Doha, Qatar (Weather History for Doha, Qatar, 2012). Inlet temperatures from the heating elements were varied every 1,800 seconds as per the available climatic data and the thermal performance of the heat pipes were monitored by connecting thermocouples upstream and downstream of the physical domain. The transient investigation was conducted in order to determine the responsive behaviour of the heat pipes in relation to external temperature variations between the evaporator and condenser sections over a period of 24 hours or 86,400 seconds.

Figure 4.28 and Figure 4.29 illustrate the experimental set-up for the performance evaluation of the heat pipes in relation to varying inlet temperature conditions.

Figure 4.28 Experimental set-up for heat pipe testing (front view) Thermocouple wires

Data logging devices

Test section

Laptop

Flow

Figure 4.29 Experimental set-up for heat pipe testing (isometric view)

Thermocouple locations were kept identical to the CFD measuring points in order to compare the readings. The origin was the base of the test section directly underneath the central heat pipe. Thermocouples were located 0.15m upstream and downstream of the heat pipes (X-direction), spaced 0.05m apart in the Z-direction. The Y-direction was kept constant at 0.25m. Table 4.6 displays the values of the measurement co-ordinates in the X, Y and Z direction.

Table 4.6 Co-ordinates of the measurement points

Profile X (m) Y (m) Z (m) I1 -0.15 0.25 0.05 I2 -0.15 0.25 -0.05 O1 0.15 0.25 0.10 O2 0.15 0.25 0.05 O3 0.15 0.25 0.00 O4 0.15 0.25 -0.05 O5 0.15 0.25 -0.10 Thermocouple wires Heat pipes Cold sink

The schematic view of the thermocouple wire locations in the X and Z direction along with their connection to the data logging device and the computer is further illustrated in Figure 4.30.

Figure 4.30 Schematic representation of thermocouple positions

4.11 Summary

This chapter described the method used for carrying out the experimentation phase of the study. A closed-loop wind tunnel was designed and commissioned and was characterised for its flow and thermal profile. The major components of the wind tunnel testing rig were labelled and the overall pressure loss was summarised. In addition, the work underlined the improved flow quality of air through the test section after the addition of the honeycomb and screen device. The study confirmed that the velocity non-uniformity coefficient in the test section was reduced from 6.6% to 0.9% after adding the flow straightener while mean turbulence intensity was 0.49% which was under the acceptable range associated with wind tunnels. The mean reduction in

Data logging device Computer

Thermocouple wires Flow direction Inlet plane Outlet plane (o) Measurement points I1 I2 O1 O2 O3 O4 O5 X Z Origin

velocity was 15.2% as the temperature was increased from 20°C to 50°C while the average variation in test section temperatures in response to heating elements was recorded at 5.4% (further information detailed in Appendix B and C).

The fluid flow and thermal evaluation was conducted primarily to comprehend the profiles of the test section prior to carrying out heat transfer experimentation. The chapter further illustrated the data acquisition devices and apparatus which were used for measuring and recording data along with the experimental set-up that was created for conducting the tests. Individual test-runs including stabilised steady-state and transient evaluation were described. Measurement point locations for velocity, pressure and temperature recordings were provided in order to achieve a direct comparison with the numerical results which will be discussed in the following chapters.

Chapter 5