Methods of filling the heat pipes

. The paper deals with individuals methods of filling heat pipes methods (filling by evaporation of the working fluid, filling by vacuuming of working fluid, freezing of working fluid – vacuuming). The second part of the paper is devoted to the comparison of the power of heat pipes after filling by individual methods.


Filling of heat pipes
The heat pipe may be filled in several ways to compare the extent to which the way the heat pipe function is affected. In all filling methods, distilled water is used as the working fluid, due to its availability and good thermophysical properties. For each filling method, the tube will be filled three times [1].

Filling by evaporation of the working fluid
This method of filling is followed by the total weight of the heat pipe.
• Check the weight of the heat pipe in the empty state, calculating the total volume according to the relationship (1), and determining the required quantity of the working fluid from it. Usually it is 15 % to 25 % of the total volume [2].

= . 2 .
(1) • Fill the tubes with a syringe at a higher value than required.
• Verify weight after filling. The difference in weight before and after is the amount of working medium in the tube.  During filling TT1, the pressure dropped to 41 mb, TT2 to 32 mb and TT3 dropped to 21 mb. The evaporation and subsequent closing times vary depending on the amount of working medium in the tube, the heating intensity and the room conditions.

Filling by vacuuming of working fluid
This method is based on the principle of extracting air from the heat pipe by rotary vacuum pump via the container with liquid nitrogen [2]. Fill of heat pipe: • The heat pipe is connected via a three-way valve to the metering vessel with the working fluid and the pump. The exchanger is connected to a container with liquid nitrogen to which a pressure meter is attached. • The three-way valve is fully open. The pressure on the pressure gauge is monitored when it drops to a value close to 0 hPa. The three-way valve is switched so that the paths between the tube and the measuring vessel are permeable. Fill in the required amount that you calculate in the same way as in the first method. • After filling, the filling tube is pressed and begins to start.  The vacuuming time for each tube varied between TT1: time -32min, pressure -36mb, TT2: time -36min, pressure -42mb and TT3: time -41mi, pressure -28mb. Room temperature at 18 °C.
During the measurement, the vacuum pump was cleaned, exchange the starter capacitor, the overload coil and the winding of the motor were changed. Despite these measures, we did not achieve the required pressure in the tubes, the pump overheated and lost the pull during suction.

Freezing of working fluid -vacuuming
It works on a similar principle as the "filling by vacuuming of working fluid", with the difference that the tube is filled with the exact amount of the working fluid. The working fluid is freeze and the air is pump out of the tube [2].
Fill of heat pipe: • The tube is filled to the required amount, calculated as in the previous methods. • Using liquid nitrogen is a cooled thermo-isolating vessel, after cooling, returns the nitrogen to the dewar vessel. Put the tube into the thermal insulator, lock it off with a 20 minute insulating plug.
• The pipe connects to the pump as in the second method, pump out the air and closed.

Testing the power of different types of heat pipes
Power measurement was implemented at 0°, 15°, 30°, 60°, 90°, and it was repeated several times in order to achieve the most accurate results.
• Set JULABE to 80 °C. • Record input and output temperatures along with flow rates every 10 seconds for 20 minutes (heating and cooling). • When the measurement is complete, the device turns off. Close the hot water supply and open the cold water inlet. Cool the system to 30 °C. • After cooling, a new working position of 15° is set and the measurement is repeated from point 2. In the first method filling (Figure 8) heat pipe TT1 reached higher performance than heat pipe TT2 and TT3, which have about the same power range. The TT2 and TT3 were measured on a modified measuring device, while the TT1 tube on the original device. By more accurate results, the measurement device passed through the adjustment. This is also seen when comparing TT1 with TT2 and TT3. Several measurements were made on the original device to confirm that the device must be edited. Only one of these measurements was selected to indicate the problem [3].
The device has been modified to prevent flow changes due to hose wounds when adjusting work positions and to allow better temperature sensing. The treatment concerned the heating section, where hoses and valves were replaced by water-jet galvanized fittings.  The TT1, TT2 and TT3 heat pipes of the second fill method (Figure 9) had similar performance ranges. In the TT2 heat pipe, the power dropped sharply at 15° from 214 W to 136W. Fig. 9. Power of heat pipes filled by second method.
The TT3 heat pipe curve for the third filling method ( Figure 10) has a decreasing character all the time, the performance achieved is significantly lower than that of the TT1 and TT2 heat pipes even though the best parameters have been reached during the TT3 heat pipe.

Evaluation of results
First method of filling (Filling by evaporation of the working fluid): The average pipe outputs are: TT1 = 413.5 W, TT2 = 279 W and TT3 = 243 W. The heat pipe TT1 was measured on the original measuring device. It points out that the measuring device has to be adjusted to achieve the most accurate results. The heat pipe TT2 had a higher power output of 36W even though the initial pressure was higher by 10 mb than heat pipe TT3. In heat pipe TT1 and TT3, the smallest difference in power of 0.6 watts occurred with an initial pressure difference of 8 mb.
Third method of filling (Freezing of working fluid -vacuuming): In the third method of filling, the smallest performance was achieved. At heat pipe TT3, the lowest power was achieved at 163 W on average. These low power values can be caused by air bubbles created in the tube when they freeze the working fluid, which after thawing, has increased the internal pressure.

Conclusion
The filling technology also has a big influence on the heat pipe performance. Based on the different filling methods (filling by evaporation of the working fluid, filling by vacuuming of working fluid, freezing of working fluid -vacuuming) and the number of repetitions of each filling method, it was found that the best values of the transmitted power were achieved in the first filling method (filling by evaporation of the working fluid). The advantage of this method is that it is not as technologically demanding as other methods of filling. In the second filling method more accurate results would be obtained in the case of a better vacuum pump condition.
Article work was financially supported by the APVV-15-0778 project -The limits of control and convection cooling through phase changes of the working fluid in the thermosyphon LOOP heat pipe.