Yasuhisa Shinmoto

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.

Recent development in electronic devices with increased heat dissipation requires severe cooling conditions and an efficient method for heat removal is needed for the cooling under high heat flux conditions. Most researches are concentrated on small semiconductors with high heat flux density, while almost no existing researches concerning the cooling of a large semiconductor, i.e. power electronics, with high heat generation density from a large cooling area. A narrow channel between parallel plates is one of ideal structures for the application of boiling phenomena which uses the cooling for such large semiconductors. To develop high-performance cooling systems for power electronics, experiments on increase in critical heat flux (CHF) for flow boiling in narrow channels by improved liquid supply was conducted. To realize the cooling of large areas at extremely high heat flux under the conditions for a minimum gap size and a minimum flow rate of liquid supplied, the structure with auxiliary liquid supply was devised to prevent the extension of dry-patches underneath flattened bubbles generated in a narrow channel. The heating surface was experimented in two channels with different dimensions. The heating surfaces have the width of 30mm and the lengths of 50mm and 150mm in the flow direction. A large width of actual power electronics is realizable by the parallel installation of the same channel structure in the transverse direction. The cooling liquid is additionally supplied via sintered metal plates from the auxiliary unheated channels located at sides or behind the main heated channel. To supply the liquid to the entire heating surface, fine grooves are machined on the heating surface for enhance the spontaneous liquid supply by the aid of capillary force. The gap size of narrow channels are varied as 0.7mm, 2mm and 5mm. Distribution of liquid flow rate to the main heated channel and the auxiliary unheated channels were varied to investigate its effect on the critical heat flux. Test liquids employed are R113, FC72 and water. The systematic experiments by using water as a test liquid were conducted. Critical heat flux values larger than 2×106W/m2 were obtained at both gap sizes of 2mm and 5mm for a heated length of 150mm. A very high heat transfer coefficient as much as 1×105W/m2K was obtained at very high heat flux near CHF for the gap size of 2mm. This paper is a summary of experimental results obtained in the past by the present authors.

For a drastic energy conservation in data centers (more than 30%), a new advanced concept of thermal management systems of integrated cooling network in server racks is proposed, and a series of developmental studies toward the realization of the concept have been conducted. The concept consists of the following technological items: - Plug-in thermal network cables, - Narrow channel heat exchangers (single-phase and two-phase), - Thin flat-type heat pipes, - Development of nano-fluids for heat transfer enhancement. In this concept, CPUs in server racks are directly cooled with the aid of advanced 1.5 mm thick flat-type heat pipes or 3 mm thick narrow channel heat exchangers, though CPUs in server racks, at present, are cooled down with massive heat sinks by forced air flow in the racks. In the case of heat pipes, condensation regions are cooled by either single-phase or two-phase narrow channel heat sink instead of air-cooled finned heat sink. These cooling units are accommodated in server racks. In addition, plug-in thermal network cables, in which a working fluid for narrow channel heat exchangers is running, are integrated among server racks, and thermal energy dissipated in server racks are inclusively merged and managed by the thermal network cables. The working fluid heated up by the dissipated heat is recuperated in an outdoor cooling system. An attempt at the applications of silver nano-fluids as a working fluid in heat pipes and thermal network is also conducted in the present study. Silver nano-fluids are thermochemically synthesized by a microwave heating technique, which allows for superior stable suspension characteristics.

A two-phase flow loop is a promising method for application to thermal management systems for large-scale space platforms handling large amounts of energy. Boiling heat transfer reduces the size and weight of cold plates. The transportation of latent heat reduces the mass flow rate of working fluid and pump power. To develop compact heat exchangers for the removal of waste heat from electronic devices with high heat generation density, experiments on a method to increase the critical heat flux for a narrow heated channel between parallel heated and unheated plates were conducted. Fine grooves are machined on the heating surface in a transverse direction to the flow and liquid is supplied underneath flattened bubbles by the capillary pressure difference from auxiliary liquid channels separated by porous metal plates from the main heated channel. The critical heat flux values for the present heated channel structure are more than twice those for a flat surface at gap sizes 2 mm and 0.7 mm. The validity of the present structure with auxiliary liquid channels is confirmed by experiments in which the liquid supply to the grooves is interrupted. The increment in the critical heat flux compared to those for a flat surface takes a maximum value at a certain flow rate of liquid supply to the heated channel. The increment is expected to become larger when the length of the heated channel is increased and/or the gravity level is reduced.

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