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We have developed a loop thermosyphon for cooling electronic devices. The cooling performance of a thermosyphon deteriorates with an increasing amount of non-condensable gas (NCG). Design of a thermosyphon must consider NCG to provide guaranteed performance for a long time. In this study, the heat transfer performance of a thermosyphon was measured while changing the amount of NCG. The resultant performances were expressed as approximations. These approximations enabled us to predict the total thermal resistance of the thermosyphon by the amount of NCG and input heating. Then, using the known leakage in the thermosyphon and the amount of dissolved NCG in the water, we can predict the amount of NCG and the total thermal resistance of the thermosyphon after ten years. Although there is a slight leakage in the thermosyphon, we are able to design a thermosyphon with a guaranteed level of cooling performance for a long time using the proposed design method.

Electronic devices need to be more compact these days, but it has become difficult to cool them sufficiently with existing air cooling systems that use only heat conductivity. We have developed a loop thermosyphon to provide high cooling performance for electronic devices [1,2]. The thermosyphon has some important features. One is that it has a porous structure that enhances evaporative heat transfer for the evaporation section to decrease thermal resistance. Another is that the position of the radiator can be freely adjusted by extending the joint pipes. This is highly beneficial when designing the server cooling system. Cooling systems for highly reliable electronic devices like servers must offer guaranteed performance for more than 10 years. However, the thermosyphon’s cooling performance deteriorates with an increasing amount of non-condensable gas (NCG) [^{−10} Pa-m^{3}/s. This value represents a typical small leakage. However, over a period of 10 years, a large amount of gas will leak and build up in the thermosyphon.

There has been a lot of research on cooling systems for electronic devices. Naphon et al. [

In this study, the effect of the amount of NCG on heat transfer performance of the loop thermosyphon was investigated. Then, the amount of NCG flowing into the thermosyphon and the change in long term heat transfer performance after 10 years were predicted. We propose a design method of a thermosyphon with a guaranteed level of cooling performance for a long time.

We have developed a loop thermosyphon to provide high cooling performance for electronic devices [1,2]. Figures 1 and 2 show the thermosyphon. As the choice of refrigerant is environmentally important, we used degassed water. The thermosyphon was made of copper to resist corrosion by water. For the evaporation section, we used a porous structure for the evaporation surface. The evaporation surface was attached to the object to be cooled by evaporation. The vapor generated by evaporation flowed along the vapor tube (the larger upper tube in

We measured the thermosyphon’s cooling performance while changing the amount of input heating, the flow rate of the cooling air, and the amount of NCG. As shown in

processing unit) in a server. Thus, the following performance experiments were performed based on the amount of input heating. Moreover, to examine the thermosyphon in the same conditions as actual use conditions, there was no thermal insulation on the thermosyphon’s surface. Three amounts of input heating were tested in this experiment: Q = 50, 100, and 200 W. A uniform cooling air flow into the condenser was obtained from a double chamber wind tunnel. Flow rates of U = 0.3, 0.5, and 0.9 m^{3}/min. were used. The temperature measuring points are shown in _{h} is the temperature of the heater, T_{e} is the temperature of the evaporation plate center, T_{v} is the temperature of the vapor (we assume the vapor tube surface temperature is the same as the vapor temperature), and T_{a-in} is the temperature of the cooling air in front of the condenser. We divided the measuring areas of the condensation tube equally into three and measured the temperatures in these condensation tube regions, T_{c-up} (upper area), T_{c-mid} (middle area), and T_{c-low} (lower area). Moreover, we measured the exhaust air flow temperatures, T_{a-up}, T_{a-mid}, and T_{a-low}, corresponding to these regions. To simplify the comparison, we used the mean temperature of the condensation tube T_{c} and the mean exhaust air flow temperature T_{a-out}.

We evaluated the thermosyphon’s cooling performance by dividing it into three regions: one for evaporation performance, the second for condensation performance in the condensation tube and air cooling performance at the condenser, and the third for the heat leakage from the outside surface of the thermosyphon. Finally, the total thermal resistance of the thermosyphon was discussed.

We evaluated the evaporation heat transfer coefficient of the evaporation surface. The evaporation heat transfer coefficient was defined by the following equation.

Here, S_{e} is the porous surface area of the evaporation surface (S_{e} = 1.51 × 10^{−3 m2}), Q is the input heating, T_{v} is the vapor temperature, and T_{e} is the evaporation surface temperature. The experimental results of the evaporation heat transfer coefficient are shown in _{0} was defined as P_{0} = P_{NCG} + P_{v}. Here, P_{NCG} is the partial pressure of NCG, and P_{v} is the pressure of saturated vapor, which is given by the temperature of vapor T_{V}. The dashed line in _{e}, the pressure P_{0}, and the heat flux of the evaporation surface q = Q/S_{e }, by using the least squares method.

We evaluated the condensation performance in the condenser tube. The amount of input heating Q and the condensation heat transfer rate Q_{cond} are different because of the heat leakage from the thermosyphon. Therefore we measured the condensation heat transfer rate Q_{cond} by using the temperature increase of the cooling air (T_{a-out}- T_{a-in}). The total condensation heat transfer coefficient h_{c-total} was defined by the following equation.

Here, S_{c} is the total area of the inner condensation tube surface (S_{c} = 4.95 × 10^{−3 m2}), T_{v} is the vapor temperature, and T_{c} is the mean temperature of the condensation tube. The relation between the condensation heat transfer coefficient and the partial pressure of NCG is shown in _{c-total}, the partial pressure of NCG (P_{NCG}), and the condensation heat transfer rate, by using the least squares method.

The approximation is shown with solid lines in

Next, we evaluated the air cooling performance on the outside of the condenser tube. The condenser has offset fins attached to the outside of the condensation tube. The forced convection heat transfer coefficient h_{f} on the offset fins was obtained from the following equation.

Here, S_{fin} is the total area of the offset fins, T_{c} is the mean temperature of the condensation tube, and T_{a-in} is the temperature of the cooling air in front of the condenser. Manglik et al. [_{f} on offset fins for different air flow rates U are shown in

The condensation heat transfer Q_{cond} is less than amount of the input heating Q because of heat leakage from the thermosyphon. The fraction of this difference was 0% - 8% when the NCG pressure was 0 - 1 kPa and 10% - 19% when the NCG pressure was 1 - 5.5 kPa. We obtained an expression of the approximation of the heat leakage (Q-Q_{cond}) as a function of the temperature difference of the vapor temperature, the cooling air temperature (T_{v}-T_{a-in}), and the air flow rate U from our experimental data by the following equation.

The total thermal resistance θ of the thermosyphon is defined by the following equation.

Here, Q is the amount of input heating, T_{e} is the evaporation surface temperature, and T_{a-in} is temperature of the cooling air in front of the condenser. Using the abovementioned approximations, we can predict the total thermal resistance θ of the thermosyphon for given values of the partial pressure of NCG (P_{NCG}), the amount of input heating Q, the temperature of the cooling air in front of the condenser T_{a-in}, and the air flow rate U. The predicted and experimental results of the total thermal resistance of the thermosyphon for the air flow rate U = 0.008 m^{3}/s are shown in

Here, by using the above-mentioned results, the thermal design approach for thermosyphons with slight NCG leakage is shown as follows. First, we predict the amount of partial pressure of NCG (P_{NCG}) in the thermosyphon after 10 years with a constant leakage rate r_{leak} and amount of dissolved NCG in injected water (DO). The leakage rate r_{leak} gives the amount of molecules that flow into the thermosyphon when the inside is a vacuum and the outside pressure is atmospheric pressure P_{a}. Assuming that the leakage rate is proportional to the difference between the inner pressure P_{0} and the external pressure P_{a}, the following expression is obtained.

Therefore, the inner pressure of the thermosyphon after t seconds is given by the following equation.

Here, P_{d} is the initial partial pressure of the dissolved NCG in injected water, and P_{v} is the saturation vapor pressure at 25˚C. The calculation result of the amount of partial pressure of NCG (P_{NCG}) is shown in

Using Figures 6 and 7, we can predict the change in cooling performance of a thermosyphon with NCG leakage. _{leak} = 3 × 10^{−9} Pa-m^{3}/s, dissolved oxygen DO = 2 mg/L, and air flow rate U = 0.008

m^{3}/s), the total thermal resistance θ was over 0.25 K/W after 10 years. The total thermal resistance after 10 years increased by 25% of the initial value. When the dissolved oxygen DO = 0 mg/L as in situation B, the total thermal resistance θ was less than 0.25 K/W after 10 years. When the air flow rate U = 0.015 m^{3}/s in situation C, the total thermal resistance θ was also less than 0.25 K/W after 10 years. We proposed a new design method for a thermosyphon with a guaranteed level of cooling performance for a long time.

We measured the heat transfer performance of a loop thermosyphon with non-condensable gas (NCG), and the resultant performances were expressed as approximations. Then, we proposed a design method for a thermosyphon with a guaranteed level of cooling performance for a long time. We obtained the following results:

1) The predicted total thermal resistance of the thermosyphon using the obtained approximations agreed with the experimental results within an error of 8% - 17%.

2) The total thermal resistance of the thermosyphon increased by 25% after 10 years when leakage rate r_{leak} = 3×10^{−9} Pa-m^{3}/s, dissolved oxygen DO = 2 mg/L, and air flow rate U = 0.008 m^{3}/s.

3) Although there was a slight leakage in the thermo syphon, we were able to design a thermosyphon with a guaranteed level of cooling performance for a long time using the proposed design method.