Study on Heat Transfer Performance of Flat Plate Pulsating Heat Pipe with Graphene Oxide at Low Heating Power

Graphene oxide (GO) has high thermal conductivity and strong hydrophilicity which can enhance the heat transfer performance of the fluid. Standby heat transfer is a long-time, low-heat power and small heat area heat transfer, caused in electronic equipment. In this paper, the heat transfer performance of flat plate pulsating heat pipe (FPPHP) under the different concentrations of GO as working fluid is studied to solve the standby heat transfer and analyze the enhancement and the retardation of the GO to FPPHP. The FPPHP is made of aluminum, 2 tubes, with the concentration of GO, is 0.1%, 0.3%, 0.5%, a heating power range of 1∼10 W, and a filling rate (FR) is 30%. The results of the experiment indicate that under the condition of the concentration of 30% of GO, the FPPHP startup at 4 W and run stably at 6 W. When the heating power is 10 W, the wall temperature of evaporation is 89.75 °C, the thermal resistance is 1.18 °C/W.


Introduction
Heat standby [1] , the process of standby heat loss caused by electronic equipment in the long-time standby state, causing higher equipment temperature, belongs to the environment of small space and high heat flow.Pulsating heat pipe (PHP), proposed by Akachi in the 1990s [2] , is a passive heat transfer element with high thermal conductivity, high heat flow density, high environmental adaptability, simple structure, and simple fabrication, which can operate in heat standby environment [3] .However, the heat standby environment has the characteristics of low heat power (<10 W) and small heating area (<10 cm 2 ), which require the PHP to have a small structure, low starting power, and good heat transfer performance.Graphene oxide (GO) has high thermal conductivity (3,000~5,000 W/(m•K)) and good hydrophilicity [4] .As PHP works fluidly, it can significantly improve the startup and heat transfer performance [5] .The main effects of GO on improving the performance of PHP are as follows: significantly increase the thermal conductivity of the base liquid; nanoparticles increase the gasification core of the base liquid and enhance the boiling heat transfer [6][7] .However, GO particles will increase the viscosity of the fluid, slowing the flow rate and circulation of the working fluid in the PHP [8] .
In this paper, GO solution (0.1% wt, 0.3% wt, 0.5% wt) was suggested as the working fluid, and Flat plate pulsating heat pipe (FPPHP) was used as experiment PHP, flow rate (FR)=30%, heat power 1~10 W, measuring the temperature change of evaporation section and condensation section under different power and analyze the influence of different concentrations of GO on the startup and heat transfer performance of the FPPHP at low power.A new way to solve the heat dissipation problem of hot standby could be provided.

Experiment set
The structure of the experiment FPPHP built by aluminum (100 mm×30 mm×10 mm, 2 tubes and 3 mm×3 mm rectangular section) is shown in Fig. 1.The Evaporation section is 30 mm×30 mm, condensation section is 30 mm×30 mm, and the thermal insulation section is 30 mm×10 mm.The heating system is shown in Fig. 2, which includes a heating module, cooling module, and data acquisition module.The heating module mainly includes DC power (HY3005B, 30 V, 10 A) and a Ceramic electric heater, supplying the heating power in the experiment.The cooling module mainly includes a Cooling fan and cooling air duct, cooling the condensation section.The data acquisition module mainly includes a PC, data acquisition instrument (DAQ970A), K-type thermocouple (measurement error ±0.1 ℃), and measuring the temperature of the FPPHP.The FPPHP was placed vertically, with the evaporation section at the bottom and the condensation section at the top.The evaporation section heated on one side was bonded closely with the ceramic electric heater, which was connected to the DC power.The condensation section was set at the end of the cooling duct, and the cooling fan was set at the other end.In the middle of the cooling duct, a heat-wire anemometer was set to monitor the cooling air speed.The evaporation section and thermal insulation section were covered by glass fiber cotton and further wrapped with aluminium foil reflection film.

Experiment process
The structure and layout of the FPPHP remain unchanged in the experiment.Before the experiment begins, check the wires and insulation cotton set.The experiment process operates as follows: Turn on the PC and data acquisition instrument and collect the current temperature data till the data is stale.When the data is stable, it can be considered to be environment temperature.
Turn on the cooling fan.When the cooling fan runs stably, turn on the DC power, adjust the power to 1 W, and output.Observe the temperature change of the evaporation section and condensation section of the heat pipe after power output through the temperature-time curve on the PC.
When the power output time of 1 W reaches 1,800 s, adjust the power to 2 W to continue output and retime till 10 W is finished.
Change the FPPHP and repeat steps 1~3 to complete all experiments.

Data process and error analysis 2.3.1 Data process
There are 8 temperature measuring points in the experiment, point 1~4 is in the evaporation section and point 5~8 is in the condensation point.The following formula can be used to calculate the average temperature of the evaporation section and condensation section and heat transfer resistance during the stable operation of PHP.
The average temperature of the evaporation section and condensation section:

Error analysis
Uncertainty analysis of heating power.The voltage range of the heating DC power supply used in the experiment is 0~30 V, with an accuracy of 0.01.The current range of the heating DC power supply used in the experiment is 0~10 A, with an accuracy of 0.001 [9] .The lowest heating power is 1 W with 5.5 V, 0.18 A. The relative uncertainty of heating power is 0.01 0.001 0.58% 5.5 0.18 Thermal resistance uncertainty analysis.The thermocouple accuracy is 0.1 ℃, and the data acquisition instrument accuracy is 0.0256 ℃ after calibration.The absolute uncertainty of temperature measurement is Therefore, the maximum uncertainty of the experimental results is 4.84%.

Results and Analysis
The results of the change of evaporation and condensation temperature, temperature difference, and heat transfer resistance of FPPHP with different concentrations of GO (0.1% wt, 0.3% wt, 0.5% wt) under 1~10 W, FR=30% are shown in Fig. 3~4.The wall temperature of evaporation (T e ) and the wall temperature of condensation (T c ) of FPPHP increase rapidly (unsteady heat transfer) and then tend to be stable (steady heat transfer) with heating time and with the increase of heating power T e and T c increase till 10 W.
Within 1~3 W, T e of the 3 FPPHP have the same trend, and the temperature difference (ΔT) rises as a linear function which indicates FPPHP unstart and heat conduction is the main heat transfer.Within 4 W, the thermal resistance (R) decreases and ΔT rises as a convex function which indicates FPPHP gets started and heat transfer includes heat conduction and phase change heat transfer of working fluid.However, when the heating power is above 5 W, 3 kinds of FPPHP have different heat transfer characteristics.
Within 4~6 W, ΔT of 0.1%wt (ΔT 1 ) rises as a convex function and R of 0.1% wt (R 1 ) decreases slowly.While ΔT1 and R1 are both higher than those of 0.3% and 0.5%, indicating that the enhancement of GO of 0.1%wt to heat transfer of FPPHP is less than the enhancement of 0.3% or 0.5%.Within 7~9 W, Te of 0.1%wt (T e1 ) rises rapidly while Tc of 0.1%wt (T c1 ) rises slowly.The ΔT1 rises rapidly and higher than others and R1 begins to increase reversely, indicating that the retardation to heat transfer of FPPHP is higher than enhancement with GO of 0.1%wt.When the heating power is 10 W, Tc1 begins to decrease, and the rising rate of ΔT 1 and R1 furtherly increases, which indicates that the heat transfer performance and the heat transfer limit of FPPHP deteriorate.This is because the optimized effect of the GO to heat transfer by increasing the conductivity of the working fluid and gasification core is lower NESP-2023 Journal of Physics: Conference Series 2592 (2023) 012002 than the negative effect of the GO particles to heat transfer by hindering the circulation of the working fluid in the FPPHP.Within 4~10 W, the rising rate of T e and T c of FPPHP of 0.3% wt (T e2 and T c2 ) is slower than others.The temperature difference of 0.3% wt (ΔT 2 ) rises slowly and then be stable after the heating power is 9 W. ΔT 2 is the lowest temperature difference.The thermal resistance of FPPHP of 0.3%wt (R 2 ) decreases rapidly and is lower than other thermal resistance.Within 9 W, wall temperature can be observed obviously in the evaporation section, which indicates that FPPHP works at the stable heat transfer stage.Within 10 W, T e2 =89.75 ℃, T c2 =78 ℃, R 2 =1.18 ℃/W, which is the best performance in the experiment.This is because the number of GO particles increases, the enhancement to heat transfer of FPPHP strengthens, and the working fluid phase change at a lower temperature.The FPPHP could get started and run stably at a lower temperature.The lower the temperature, the lower the viscosity of GO.Therefore, the negative effect of the GO particles on heat transfer is lower than the optimized effect.
The rising rate of T e and T c of FPPHP of 0.5% wt (T e3 and T c3 ) are higher than those of 0.3% and lower than those of 0.1% within 4~7W.However, when the heating power is higher than 8 W, T e3 and T c3 rise rapidly at the same time till 10 W. The maximum of T e3 is equal to the maximum of T e1 with 10 W. The decrease rate of R of FPPHP of 0.5%wt (R 3 ) is between R 1 and R 2 and the rising rate of ΔT of FPPHP of 0.5% wt (ΔT 3 ) is between ΔT 1 and ΔT 2 with 4~10 W. This is because the enhancement and retardation to heat transfer of FPPHP are both strengthened because of the increase of concentration of GO.The difference between the enhancement and retardation is less than a difference of 0.3% wt, which indicates that the rising rate of ΔT 3 is lower than that of ΔT 2 and the decrease rate of R 3 is lower than that of R 3 .When the heating power is above 8 W, the gasification amount of working fluid is large enough and the liquid amount is relatively small, the relative viscosity increases, which indicates that the resistance of circulation increases.Therefore, T e3 and T c3 increase rapidly to increase pressure in FPPHP to overcome resistance to complete the circulation.

Conclusion
This paper studied the heat transfer performance of FPPHP under the different concentrations of GO (0.1%, 0.3%, 0.5%) as working fluid within 1~10 W, FR=30%.It can be concluded that: Within 1~3 W, there is no FPPHP get a start and the heat conduct by the pipe is the main heat transfer.When the concentration of GO is 0.1%wt, the enhancement of the GO particles to heat transfer is less than the retardation to heat transfer of the FPPHP.When heating power is 7 W, T e1 increases rapidly, causing ΔT 1 and R 1 to increase reversely.When 10 W, T c1 begins to decrease.
When the concentration of GO is 0.3% wt, the enhancement of the GO particles to heat transfer is strengthened significantly, causing the startup of FPPHP at 4 W and heat transfer steadily at 6 W, with the lowest wall temperature.when 10 W, T e2 =89.75 ℃, T c2 =78 ℃, R 2 =1.18 ℃/W.
When the concentration of GO is 0.5% wt, the enhancement and retardation to heat transfer of FPPHP are both strengthened, and the difference between them is less than that of 0.3% wt, which indicates that the rising rate of ΔT 3 is lower than that of ΔT 2 and the decrease rate of R 3 is lower than that R 3 .With the power increases, the resistance of circulation increases, which indicates that T e3 and T c3 increase rapidly to increase pressure in FPPHP to overcome resistance to complete the circulation.

Fig. 1
Fig.1The structure of FPPHP and the distribution of measuring points


The average temperature difference and the heat transfer resistance of the FPPHP: difference between the evaporation section and condensation section is 3.03 ℃ when the heating power is 1 W. The maximum uncertainty of thermal resistance is

Fig. 3 Fig. 4
Fig.3The change of wall temperature of FPPHP with time