The most direct way to evaluate the performance of heat transfer medium is to test its working temperature, critical heat flux, and thermal resistance in a heat pipe.

In this section, the working temperature refers to the heater temperature at different input power, where water coolant (15^{o}C, 500 ml/min) is applied at the condensation section of the heat pipe. The objectives of this test are to compare at what temperature (^{o}C) the heat transfer medium can conduct normal isothermal (±3^{o}C) heat transfer, that is, the Leidenfrost temperature point and is defined as: working condition temperature.

The experimental setup is as shown in Figure 1.

Fig. 1 Schematic experimental setup for heat pipe testing.

The description and function of the equipment ?-? are shown in the following table.

In this section, the same test pipe without any wick structure is used to evaluate the heat transfer performance of deionized water first and then the quantum medium. Deionized water (DI water) and quantum medium (QM) are filled into the heat pipe respectively for performance comparison. The deionized water heat pipe and quantum heat pipe are named as DIWHP, QHP, respectively. A dual-heating jacket with a total power of 3000W will be used in this section, the schematic experimental setup is as shown in Figure 1. Testing temperature is from 20 to 600 ?. The heat pipe temperature is monitored and collected by an infrared thermal imaging camera (US. FLIR, T1040). The temperatures of the heater, the evaporator, and condenser of the heat pipe are monitored simultaneously. Using an infrared camera to record the temperature could reduce the wire-connecting and testing errors caused by thermocouples. The temperature of the coolant at the inlet and outlet is monitored by a data acquisition unit (US, Agilent 34972A).

Chemical Composition | C | Si | Mn | P | S | Ni | Cr | N |

% | 0.035max | 1.00max | 2.00max | 0.045max | 0.030max | 8.0~11.0 | 18.0~20.0 | 0.10~0.16 |

DIWHP: 100 ml of DI water

Set the same input power for two heating jackets through power supply unit (manufactured by German). For deionized water heat pipe, the input power is set at 350 and 400W, the total heating power is 700 and 800W. For the quantum heat pipe, set the input power at 350, 400, 700, 1050 and 1400 W, the total heating power is 700, 800, 1400, 2100 and 2800W. Monitor the heater and heat pipe temperatures with FLIR 1040 thermal imaging camera; record the inlet and outlet temperature of the cooling water by Agilent 34972A data acquisition unit.

Set a constant input power to heat the heat pipe, and record the maximum input power (Q) when there is a temperature difference of 3 ℃ along the heat pipe or when the heat transfers significantly deteriorated from the analysis of the graph of heater temperature. The position of temperature measuring points of the heat pipes is shown in Figure 2.

Fig. 2 Temperature measuring points.

Heat transfer area of the pipe:

Heat transfer area of the medium:

Where T^{1}*Heater* is the working temperature of Quantum Heat Pipe (Heater) (^{o}C), T^{1}*Heater* is the working temperature of deionized water heat pipe (Heater) (^{o}C).

The heat flux is calculated according to the formula as follows:

Where q is the heat flux (W/m^{2}), is the input power (W), A is the heat exchange area (m^{2}), that is, the cross-sectional area of the heat pipe.

The total heat transfer resistance of the heat pipe is calculated as follows:

Where R is the thermal resistance (^{o}C/W), *T*we, *T*we are the temperatures (^{o}C) of evaporator and condenser sections, respectively. Q is the input power (W).

Set a total heating power of 700 W for both deionized water heat pipe (DIWHP) and quantum heat pipe (QHP), and record the heat pipe temperature and the coolant temperature by the thermal imaging camera and Agilent data acquisition unit, respectively. The results are displayed in Figure 3, the short red line in the figure shows the maximum temperature at the heater surface. *T*we and *T*we represent the evaporator and condenser temperatures of the heat pipe. The detailed position of the temperature measuring points is shown in Figure 2. The temperature difference between the evaporator and condenser of the heat pipe and the temperature difference between the inlet and outlet of the coolant are displayed in the right-side part of the figure.

Fig. 3 Comparisons of the DIWHP and QHP: 700W vs. 700W.

During the 360-720s of the test process, the maximum temperature difference between *T*we and *T*we for deionized water heat pipe is 1.5^{o}C; while the maximum temperature difference between *T*we and *T*we for quantum heat pipe is 0.8^{o}C. These temperature differences mean that both deionized water heat pipe and quantum heat pipe can transfer heat normally.

The heat flux of deionized water heat pipe and quantum heat pipe can be calculated as:

The thermal resistance of the deionized water heat pipe is

The thermal resistance of the quantum heat pipe is R =

The working temperature of deionized water heat pipe is *T*'Heater = 176^{o}C

The working temperature of quantum heat pipe is *T*Heater = 174^{o}C

Fig. 4 Comparison of the DIWHP and QHP: 800W vs. 800W.

When increasing the total input power to 800W, the testing results are shown in Figure 4. During the 360-720s of the test process, the maximum temperature difference between *T*we and *T*we for deionized water heat pipe is 4.3^{o}C, which means the heat transfer starts to deteriorate for the deionized water heat pipe; thus, it is determined that the heat flux for normal heat transfer from a deionized water heat pipe is 700W; while the maximum temperature difference between *T*we and *T*we for quantum heat pipe is 0.8^{o}C, which means the quantum heat pipe can transfer heat normally.

The heat flux of deionized water heat pipe and quantum heat pipe can be calculated as:

The thermal resistance of the deionized water heat pipe is

The thermal resistance of the quantum heat pipe is. R =

The working temperature of deionized water heat pipe is T^{1}'Heater = 198^{o}C

The working temperature of quantum heat pipe is T^{1}Heater = 194^{o}C

Fig. 5 Comparison of the DIWHP and QHP: 700W vs. 1400W

When increasing the total input power of quantum heat pipe to 1400W, the testing results are displayed in Figure 5. During the 360-720s of the test process, the maximum temperature difference between *T*we and *T*we for quantum heat pipe is only 1.4^{o}C, the quantum heat pipe can transfer heat normally.

The heat flux of near-field thermal radiation heat pipe can be calculated as:

The thermal resistance of the quantum heat pipe is

The working temperature of quantum heat pipe is T^{1}Heater = 311^{o}C

Fig. 6 Comparison of the DIWHP and QHP: 700W vs. 2100W.

When increasing the total input power to 2100W, the testing results are displayed in Figure 6. During the 360-720 s of the test process, the maximum temperature difference between *T*we and *T*we for quantum heat pipe is only 2.0^{o}C, the quantum heat pipe can transfer heat normally.

The heat flux of quantum heat pipe can be calculated as:

The thermal resistance of the quantum heat pipe is R =

The working temperature of quantum heat pipe is T^{1}Heater = 451^{o}C

Fig. 7 Comparison of the DIWHP and QHP: 700W vs. 2800W.

When increasing the total input power to 2800W, the testing results are displayed in Figure 7. During the 360-720s of the test process, the maximum temperature difference between *T*we and *T*we for quantum heat pipe is only 2.6^{o}C, the quantum heat pipe can transfer heat normally.

The heat flux of quantum heat pipe can be calculated as:

The thermal resistance of the quantum heat pipe is R =

The working temperature of quantum heat pipe is T^{1}Heater = 580^{o}C

At the input power of 700 W, the working temperature of the deionized water heat pipe is 176^{o}C. At the input power of 2800 W, the working temperature of the quantum heat pipe (working condition) is 580^{o}C. The working temperature of the quantum heat pipe increased by 230% compared with deionized water heat pipe under the same condition. The results are graphed in Figure 8.

Fig. 8 Working temperature comparison

The critical heat flux of the deionized water heat pipe is 1.26MW/m^{2} (700 W). The critical heat flux of the quantum heat pipe is 5.04MW/m^{2} (2800 W), which is more than 700% higher than the deionized water heat pipe (see Figure 9).

Fig. 9 Critical heat flux comparison

At the input power of 700W, the thermal resistance of the deionized water heat pipe is 0.0021^{o}C/W, while the thermal resistance of the quantum heat pipe is only 0.0010^{o}C/W. At the input power of 2800W, the thermal resistance of the quantum heat pipe is 0.0009^{o}C/W, the thermal resistance of the deionized water heat pipe is 2.3 times larger than that of the quantum heat pipe. The results are graphed in Figure 10.

Fig. 10 Thermal resistance comparison