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With rapid development of the semiconductor technology, more efficient cooling systems for electronic devices are needed. In this situation, in the present study, a loop thermosyphon type cooling system, which is composed mainly of a heating block, an evaporator and an air-cooled condenser, is investigated experimentally in order to evaluate the cooling performance. At first, it is examined that the optimum volume filling rate of this cooling system is approximately 40%. Next, four kinds of working fluids, R1234ze(E), R1234ze(Z), R134a and ethanol, are tested using a blasted heat transfer surface of the evaporator. In cases of R1234ze(E), R1234ze(Z), R134a and ethanol, the effective heat flux, at which the heating block surface temperature reaches 70°
C
, is 116 W/cm^{2}, 106 W/cm^{2}, 104 W/cm^{2} and 60 W/cm^{2}, respectively. This result indicates that R1234ze(E) is the most suitable for the present cooling system. The minimum boiling thermal resistance of R1234ze(E) is 0.05 (cm^{2}·K)/W around the effective heat flux of 100 W/cm^{2}. Finally, four kinds of heat transfer surfaces of the evaporator, smooth, blasted, copper-plated and finned surfaces, are tested using R1234ze(E) as working fluid. The boiling thermal resistance of the blasted surface is the smallest among tested heat transfer surfaces up to 116 W/cm^{2} in effective heat flux. However, it increases drastically due to the appearance of dry-patch if the effective heat flux exceeds 116 W/cm^{2}. On the other hand, in cases of copper-plated and finned surfaces, the dry-patch does not appear up to 150 W/cm^{2} in effective heat flux, and the boiling thermal resistances of those surfaces keep 0.1 (cm^{2}·^{}K)/W.

Recent development of semiconductor technology brings the remarkable performance improvement and miniaturization of electronic devices. This leads to the increase of the heat flux dissipated from electronic devices. Therefore, the development of highly efficient cooling system for electronic devices is required in order to operate them normally.

Many researchers have been studied on various cooling systems for electronic devices such as air cooling systems, liquid cooling systems, heat pipe systems, etc. Mudawar [^{2}∙K)/W) at heat input 30 W. Kawaguchi et al. [^{2}∙K)/W) at heat load 200 W, was as high as that of conventional air cooling fin unit. They also proposed a calculation method to predict the performance of the proposed cooling unit. Matsushima and Usui [^{2}∙K)/W) at heat input of 256 W. Chan et al. [^{2}∙K)/W) at heat load 203 W and an air flow rate 0.98 m^{3}/min.

In order to improve performances of cooling systems applying the boiling phenomenon, enhancing the boiling heat transfer in evaporator is one of the most important issues. Parker and El-Genk [^{2}). They reported that CHF increased linearly with liquid subcooling and CHF on porous graphite surface was higher than that of smooth copper surface; at liquid subcooling of 30 K, CHF on porous graphite was 57.1 W/cm^{2}, while that of smooth copper was 29.5 W/cm^{2}. They also compared their data of CHF with other researchers’ data of copper surface, silicon surface, micro-finned silicon surface, etc. Mori and Okuyama [^{2} as the height of honeycomb porous plate was decreased to 1.2 mm; this value of CHF was approximately 2.5 times higher than that of plane surface. They also proposed the CHF prediction model based on consideration of the mechanism of CHF enhancement by honeycomb-structured porous plate. Saiz Jabardo [^{2}) of 171 mm thick layer was 17 or more times higher than those reported on plane surfaces. Li and Peterson [_{2}O_{3}-water/ethanol) pool boiling experiments, and measured their wetting and wicking characteristics. Then, they tested the pool boiling heat transfer characteristics of their nanocoating surfaces in pure water and found that the CHF of nanocoating developed in ethanol nanofluid (193 W/cm^{2}) was larger than that of nanocoating developed in water-based nanofluid (190 W/cm^{2}) due to the quasi-static contact angles of the nanocoating and surface wettability. Hendricks et al. [^{2} and it was larger than that (23.2 W/cm^{2}) of bare Al surface (contact angle 104˚), and presumed that nanostructured surfaces created high nucleation site densities and bubble frequency and led to enhancement of CHF. Hosseini et al. [^{2}. Furberg and Palm [^{2}. Then, they reported that the dendritic and micro-porous surface produced smaller bubbles and higher bubble frequency and it led to improvement of the boiling heat transfer coefficient. Jun et al. [

Recently, new substances with extremely low GWP such as R1234ze(E), R1234ze(Z), were synthesized as environmentally acceptable working fluids. Therefore, this study aims to confirm whether these newly synthesized substances are suitable to use as working fluids of electronic device cooling systems. In this study, a loop thermosyphon type cooling system, which consists mainly of an evaporator and an air-cooled condenser, is investigated experimentally. Four type heat transfer surfaces in the evaporator are tested in order to assess the performance of the loop thermosyphon type cooling system using four kinds of working fluids. The heat transfer surfaces tested are smooth, blasted, copper-plated and finned ones, and the working fluids tested are R1234ze(E), R1234ze(Z), R134a and ethanol.

bottom of the evaporator through thermal grease. Two cartridge heaters (200 W × 2) inserted in the heating block generate the heat load to the evaporator. The heat load is controlled by a DC power supply. The charging process of the working fluid into the experimental apparatus is conducted by two steps mainly. At the first step, the experimental apparatus and the connecting hose between the experimental apparatus and the working fluid container are evacuated by an oil-sealed rotary vacuum pump. At the second step, the working fluid is introduced into the apparatus, weighing its mass.

_{eff} is the effective heat flux, T_{hb}, T_{eo}, T_{base}, T_{r} and T_{a} denote the heating block surface temperature, the bottom surface temperature of the evaporator, the heat transfer surface temperature, the saturation temperature of the working fluid and the ambient air temperature, respectively, and R_{CONT}, R_{EBW}, R_{BOIL} and R_{COND} are the contact thermal resistance of thermal grease between the heating block and the evaporator, the thermal resistance of the evaporator bottom wall, the boiling thermal resistance between the heat transfer surface and the working fluid, and the

Surface | Surface Thickness (mm) | Inner Diameter of Heat Transfer Surface (mm) | Contact Area with Heating Block (mm^{2}) | Average Surface Roughness (mm) | Method^{*} |
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Smooth | 5.2 | 20 | 196 (14 × 14) | 7.75 | Laser |

Blasted | 5.2 | 20 | 196 (14 × 14) | 3.52 | Contact |

Copper-plated | 5.2 | 20 | 196 (14 × 14) | 40.8 | Laser |

Finned | 5.2 | 20 | 196 (14 × 14) | 0.68 | Contact |

^{*}method to measure average surface roughness

Step | Current Density (mA/cm^{2}) | Current Value (mA) | Duration (min) |
---|---|---|---|

1 | 10 | 60.0 | 1 |

2 | 200 | 1200.0 | 5 |

3 | 5 | 30.0 | 2 |

Heat Transfer Surface | Smooth, Blasted, Copper-plated, Finned |
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Working Fluid | R1234ze(E), R1234ze(Z), R134a, Ethanol |

Volume Filling Rate^{*} (%) | 20.2 - 72.4 |

Average Air Velocity (m/s) | 1.4, 2.5 |

Input Heat Flux^{**} (W/cm^{2}) | 15 - 165 |

^{*}Ratio of saturation liquid volume to internal volume of the system; ^{**}Calculated based on contact area between the evaporator and the heating block.

thermal resistance of condenser between the working fluid and the ambient air, respectively. In this study, q_{eff}, T_{hb}, T_{eo} and T_{base} are calculated from the temperature distribution in the heating block based on the assumption of the one dimensional heat conduction in the heating block and the evaporator bottom wall, and T_{r} is calculated from the measured pressure of the working fluid.

Thermal resistances to evaluate the heat transfer performance of the cooling system are defined as follows,

The system performance of the cooling system is evaluated by the total thermal resistance R_{SYS}, expressed as,

First, experiments were carried out in order to find the optimum volume filling rate of the working fluid. Then, effects of the average air velocity of condenser on thermal resistances were tested. The experimental results are compared in order to select the suitable working fluid and the highest performance heat transfer surface of the evaporator for the present cooling system.

Figures 4(a)-(b) show the effects of the volume filling rate on the boiling thermal resistance, R_{BOIL}, and the thermal resistance of condenser, R_{COND}, respectively, where the volume filling rate is defined as the ratio of the liquid volume to the total inner volume of the present cooling system at ambient air temperature 20˚C. The experimental condition of these results is as follows: the working fluids is R1234ze(E), the heat transfer surface in the evaporator is smooth, and the average air velocity of condenser is 2.5 m/s. As shown in _{BOIL} is affected little by the volume filling rate. On the other hand, as shown in _{COND} is affected strongly by the volume filling rate. As the volume filling rate increases, the value of R_{COND} decreases once and reaches the minimum, and then it increases. The optimum volume filling rate in which R_{COND} is the minimum is about 40%. The optimum volume filling rate might be determined by the effective heat transfer area of the condenser and the circulating mass flow rate. On the other hand, the circulating mass flow rate, which is determined by the liquid head difference between the condenser and the evaporator, surges with the increase of the volume filling rate.

Figures 5(a)-(b) show the effects of the volume filling rate on R_{BOIL} and R_{COND}, respectively, in the case of the blasted surface. The experimental condition is the same as in

Figures 6(a)-(b) show the results of the breakdown of the system thermal resistance, R_{SYS}, in cases of the average air velocity of 1.4 m/s and 2.5 m/s, respectively. The other experimental conditions are as follows: the working fluid is R1234ze(E), the volume filling rate is about 40%, and the heat transfer surface is smooth. The boiling thermal resistance, R_{BOIL}, the thermal resistance of condenser, R_{COND}, the thermal resistance of the evaporator bottom wall, R_{EBW}, and the contact thermal resistance, R_{CONT}, are 15%, 40.7%, 24.7% and 19.6% at average air velocity of 1.4 m/s, respectively, while R_{BOIL}, R_{COND}, R_{EBW} and R_{CONT} are 18.6%, 33.3%, 26.8% and 21.3% at average air velocity of 2.5 m/s. From the comparison between Figures 6(a)-(b), it is found that only R_{COND} becomes smaller by increasing the average air velocity. This is due to the increase of air side heat transfer performance.

The experimental data of R1234ze(E), R1234ze(Z), R134a and ethanol are compared in order to select the suitable working fluid for the present cooling system. The data used in the comparison were obtained on the following experimental condition: the heat transfer surface is blasted one, the average air velocity of condenser is 2.5 m/s, and the volume filling rate is approximately 40%. The comparison results are shown in

_{BOIL} and q_{eff}. In cases of R1234ze(E) and R1234ze(Z), R_{BOIL}_{ }decreases once with the increase of q_{eff}, and then it increases due to the appearance of dry-patch on the heat trans-

fer surface. On the other hand, in cases of R134a and ethanol, R_{BOIL} decreases monotonously with the increase of q_{eff}. This is because the dry-patch does not appear in the present experimental ranges of R134a and ethanol. The minimum boiling thermal resistances of R1234ze(E), R134a, R1234ze(Z) and ethanol in the present experimental ranges are 0.05 (cm^{2}∙K)/W, 0.06 (cm^{2}∙K)/W, 0.1 (cm^{2}∙K)/W and 0.2 (cm^{2}∙K)/W, respectively. R_{BOIL} is the reciprocal of the boiling heat transfer coefficient. In general, the boiling heat transfer coefficient is strongly related with the reduced pressure. It is observed that the higher the reduced pressure, the larger the heat transfer coefficient. As shown in

_{COND} and q_{eff}. In the cases of R1234ze(E), R1234ze(Z) and R134a, R_{COND}_{ }increases a little with the increase of q_{eff}. On the other hand, in the case of ethanol, R_{COND} decreases with the increase of q_{eff}, and it is 1.5 or more times higher than in the cases of other working fluids.

_{hb}, and q_{eff}. In all cases of working fluids, T_{hb} increases with the increase of q_{eff}. The effective heat flux, at which T_{hb} reaches70˚C, is as follows: 60 W/cm^{2}, 104 W/cm^{2}, 106 W/cm^{2} and 116 W/cm^{2}^{ }in cases of ethanol, R134a, R1234ze(Z) and R1234ze(E), respectively. This result indicates that R1234ze(E) is the most suitable for the present cooling system among test working fluids.

The experimental data of smooth, blasted, copper-plated and finned surfaces are compared in order to find the suitable heat transfer surface of evaporator in the present cooling system. The data used in the comparison were obtained on the following experimental condition: the working fluid is R1234ze(E), the average air velocity of condenser is 2.5 m/s, and the volume filling rate is approximately 40%. The comparison result is shown in

_{BOIL} and q_{eff}. In the cases of smooth, copper-plated and finned surfaces, R_{BOIL} gradually decreases with the increase of q_{eff}, and then it is kept approximately at 0.1 (cm^{2}∙K)/W.

This characteristic corresponds to the transition from sub-cooled boiling to the saturated boiling. In the case of the blasted surface, the same characteristic is observed up to 116 W/cm^{2} of the effective heat flux, and then R_{BOIL} increases drastically due to the appearance of dry-patch. In this case the minimum boiling thermal resistance is 0.05 (cm^{2}∙K)/W. The blasted surface shows the best performance up to 116 W/cm^{2}, but the dry-patch appears at the effective heat flux above 116 W/cm^{2}.

_{hb} and q_{eff}. T_{hb} of all heat transfer surfaces increases with the increase of q_{eff}. The effective heat flux, at which T_{hb} reaches 70˚C, is as follows: 102 W/cm^{2}, 115 W/cm^{2}, 110 W/cm^{2} and 113 W/cm^{2} in cases of smooth, blasted, copper-plated and finned surfaces, respectively. The effective heat flux, at which T_{hb} reaches 80˚C, is as follows: 123 W/cm^{2}, 120 W/cm^{2}, 140 W/cm^{2} and 135 W/cm^{2} in cases of smooth, blasted, copper-plated and finned surfaces, respectively. As a result, it is found from Figures 8(a)-(b) that copper-plated and finned surfaces are suitable for the heat transfer surface of the evaporator.

The loop thermosyphon type cooling system for high heat flux was investigated experimentally in order to evaluate the cooling performance. The main findings are as follows.

・ With the change in the volume filling rate, the thermal resistance of condenser is more greatly influenced than the boiling thermal resistance, and these indicate that the optimum volume filling rate is approximately 40%.

・ The boiling thermal resistance, R_{BOIL}, the thermal resistance of condenser, R_{COND}, the thermal resistance of the evaporator bottom wall, R_{EBW}, and the contact thermal resistance, R_{CONT}, are 15%, 40.7%, 24.7% and 19.6% at average air velocity of 1.4 m/s, respectively, while R_{BOIL}, R_{COND}, R_{EBW} and R_{CONT} are 18.6%, 33.3%, 26.8% and 21.3% at average air velocity of 2.5 m/s. R_{COND} occupies the largest part in the system thermal resistance, and it becomes smaller as the average air velocity is larger.

・ With the combination of R1234ze(E) and the blasted heat transfer surface of the evaporator, the boiling thermal resistance is the smallest up to 116 W/cm^{2} of the effective heat flux, and the minimum boiling thermal resistance is 0.05 (cm^{2}∙K)/W around 100 W/cm^{2} of the effective heat flux.

・ With R1234ze(E), the boiling thermal resistance of the blasted surface drastically increases after the appearance of the dry-patch at 116 W/cm^{2}, while it of plated and finned surfaces is maintained as 0.1 (cm^{2}∙K)/W up to approximately 150 W/cm^{2}.

This study was supported by Fuji Electric Co., Ltd. and Central Glass Co., Ltd., Japan. We would like to express our sincere thanks for their supports.