Haruhiko Ohta

The use of immiscible liquids for cooling of surfaces with high heat generation density is proposed based on the experimental verification of its superior cooling characteristics in fundamental systems of pool boiling and flow boiling in a tube. For the purpose of practical applications, however, heat transfer characteristics due to flow boiling in narrow rectangular channels with different small gap sizes need to be investigated. The immiscible liquids employed here are FC72 and water, and the gap size is varied as 2, 1, and 0.5 mm between parallel rectangular plates of 30 mm × 175 mm, where one plate is heated. To evaluate the effect of gap size, the heat transfer characteristics are compared at the same inlet velocity. The generation of large flattened bubbles in a narrow gap results in two opposite trends of the heat transfer enhancement due to thin liquid film evaporation and of the deterioration due to the extension of dry patch in the liquid film. The situation is the same as that observed for pure liquids. The latter negative effect is emphasized for extremely small gap sizes if the flow rate ratio of more-volatile liquid to the total is not reduced. The addition of small flow rate of less-volatile liquid can increase the critical heat flux (CHF) of pure more-volatile liquid, while the surface temperature increases at the same time and assume the values between those for more-volatile and less-volatile liquids. By the selection of small flow rate ratio of more-volatile liquid, the surface temperature of pure less-volatile liquid can be decreased without reducing high CHF inherent in the less-volatile liquid employed. The trend of heat transfer characteristics for flow boiling of immiscible mixtures in narrow channels is more sensitive to the composition compared to the flow boiling in a round tube.

The employment of immiscible mixtures in nucleate boiling improves the cooling performance drastically. Increase of CHF is possible by high subcooling of less-volatile liquid compressed by high-vapor pressure of more-volatile component. In addition, the reduction of surface temperature from that of pure less-volatile liquid is resulted from the co-existed vapor of more-volatile component. And the increase of pressure above the atmospheric keeping low liquid temperature is possible to prevent the mixing of incondensable gases. Furthermore, boiling can be initiated at lower surface temperature, which is required for the cooling of e.g. automobile inverters accompanied by large variation of thermal load. The performance of immiscible mixtures in nucleate boiling are summarized with reference to new data for Novec7100/water and FC72/ethanol and existing data obtained by the present authors.

Boiling of Immiscible Mixtures for Cooling of Electronics | InTechOpen, Published on: 2016-06-15. Authors: Haruhiko Ohta, Yasuhisa Shinmoto, Daisuke Yamamoto, et

To improve the heat transfer performance during nucleate boiling applied to the cooling of semiconductor chips and of power electronics with high heat generation density, the present authors proposed a promising method of using immiscible mixtures. The nucleate boiling of immiscible mixtures with optimized fluid combination, composition and distribution of phases can make simultaneously possible i) drastic increase of CHF, e.g. 3×10^6W/m^2 for aqueous mixture without using enhanced surfaces, ii) reduction of surface temperature, iii) reduction of surface temperature overshoot at the incipience of boiling, and iv) elevation of system pressure preventing the mixing of air. Because detailed heat transfer characteristics remain unknown, the paper summarizes the recent results from pool boiling and flow boiling experiments performed in our laboratory.

Energy consumption in data centers has seen a drastic increase in recent years. In data centers, server racks are cooled down in an indirect way by air-conditioning systems installed to cool the entire server room. This air cooling method is inefficient as information technology (IT) equipment is insufficiently cooled down, whereas the room is overcooled. The development of countermeasures for heat generated by IT equipment is one of the urgent tasks to be accomplished. We, therefore, proposed new liquid cooling systems in which IT equipment is cooled down directly and exhaust heat is not radiated into the server room. Three cooling methods have been developed simultaneously. Two of them involve direct cooling; a cooling jacket is directly attached to the heat source (or CPU in this case) and a single-phase heat exchanger or a two-phase heat exchanger is used as the cooling jacket. The other method involves indirect cooling; heat generated by CPU is transported to the outside of the chassis through flat heat pipes and the condensation sections of the heat pipes are cooled down by coolant with liquid manifold. Verification tests have been conducted by using commercial server racks to which these cooling methods are applied while investigating five R&D components that constitute our liquid cooling systems: the single-phase heat exchanger, the two-phase heat exchanger, high performance flat heat pipes, nanofluid technology, and the plug-in connector. As a result, a 44–53% reduction in energy consumption of cooling facilities with the single-phase cooling system and a 42–50% reduction with the flat heat pipe cooling system were realized compared with conventional air cooling system.

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A cooling jacket for electronics devices, composed of a main heated channel with grooves and auxiliary unheated channels for additional liquid supply, was developed. The structure prevents dryout phenomena, i.e. the heat transfer limitation in boiling, by the reduction of substantial heated length. The heating surface, with a length of 30mm and a width of 30mm, was installed horizontally facing upwards in the cooling jacket. Test liquid was selected as FC72 to keep electric insulation from semiconductors integrated in the server. The measured critical heat flux (CHF) was larger than 33.3×10^4W/m^2 (300W) for inlet volumetric flow rate V_ = 0.8l/min and 1.0l/min (mass velocity G_ = 372kg/m^2 and 466kg/m^2s, respectively) at inlet liquid temperature T_ = 35℃ and test pressure P=0.15MPa. Larger CHF values compared to those for the channel without auxiliary unheated channels were confirmed.

Energy utilization in data centers, especially cooling systems for server racks, needs extensive improvement. The present authors proposed advanced cooling network systems for data centers, and R & D activities have been conducted under the so-called Green IT Project sponsored by NEDO (New Energy and Industrial Technology Development Organization). In the present concept, CPUs in servers are cooled down by either direct liquid cooling system or heat pipes with liquid cooling systems in the condensation region. The liquid cooling systems are integrated in each server rack and among server racks. A series of studies on both single phase and two phase narrow channel heat exchangers, high performance heat pipes with self-rewetting fluids and nanofluids for heat transfer enhancement are ongoing. In addition, a prototype server rack with the cooling network systems is also under development toward commercial products. This paper reports the updated status of the present R & D.

Experiments on saturated pool boiling were conducted by using alcohol aqueous solutions of 1-Propanol/Water, 2-Propanol/Water and Water/Ethylene glycol mixtures at atmospheric pressure. The heating surface was a horizontal upward-facing circular flat plate of 40mm in diameter. Despite that only the deterioration of heat transfer coefficient due to the existence of diffusion resistance was reported so far in nucleate boiling of mixtures, enhancement of heat transfer coefficients was observed for 1-Propanol/Water and 2-Propaonl/Water mixtures at extremely low alcohol concentrations. In these concentration ranges, Marangoni force acting on the vapor-liquid interface towards the three-phase interlines was expected to enhance the heat transfer, and this heat transfer enhancement can overcome the heat transfer deterioration due to the mass transfer resistance inherent in mixture boiling. In the intermediate concentration range, however, no heat transfer enhancement but only deterioration was observed, where Marangoni effect had no important role. On the other hand, the measured CHF values were gradually decreased with the increase of alcohol concentration in the moderate concentration range. In the low alcohol concentration range, however, CHF values were clearly decreased for 1-Propanol/Water and 2-Propanol/Water. To confirm the observed heat transfer trends of three different alcohol aqueous solutions, Marangoni effect on the nucleate boiling heat transfer mechanisms was discussed.

Heat generation density from semiconductor devices has been increasing with the rapid development of electronic technology. The cooling system using boiling two‐phase phenomena has attracted much attention because of its high heat removal potential. To develop compact and high‐performance cooling systems, we conducted experiments on the increase of critical heat flux (CHF) for flow boiling in narrow channels by improved liquid supply. A large surface of 150 mm in heated length and 30 mm in width with grooves of an apex angle of 90°, 0.5‐mm depth, and 1 mm in pitch was employed. A structure of narrow heated channel between parallel plates with an unheated auxiliary channel was devised and tested by using water for different combinations of gap sizes and volumetric flow rates, where inlet of the main heated channel and the outlet of auxiliary unheated channel were closed to prevent the flow instability observed frequently at low flow rate for parallel two channels. For the total volumetric flow rate more than 4.5 × 10⁻⁵ m³/s, higher values of CHF large than 2 × 10⁶ W/m² were obtained for gap size of 2 mm. For gap sizes of 2 mm and 5 mm at high volumetric flow rate larger than 6.0 × 10⁻⁵ m³/s, or mass velocity based on the cross section are of main heated channel 958.1 kg/m²s and 383.2 kg/m² s, respectively, the extension of dry patches was observed at the upstream location of the main heated channel resulting in burnout not at the downstream but at the upstream. By the increase in total volumetric flow rate, the pressure drop increased because of increasing in the flow rate passing through the sintered metal porous plates connecting both channels. The values of pressure drop for gap size of 2 mm were higher than that for gap size of 5 mm. When the performance of the cooling system was evaluated on the basis of pump power, ignoring its variation in the efficiency with volumetric flow rate, that is, the power defined as the product of the pressure drop and the total volumetric flow rate, higher values of CHF were obtained for gap size of 5 mm as far as the same pump power was concerned.