Yasuhisa Shinmoto

STUDY ON AERODYNAMIC MECHANISM OF HOVERING INSECTS

In order to clarify the fundamental mechanism of the hovering flight of a dragonfly, the numerical simulation of unsteady viscous flow around a tandem airfoil configuration oscillating in still air has been conducted by using a twodimensional Navier-Stokes code. It is shown that the mutual flow interactions between the fore- and hind-airfoils are playing the dominant role in generating the time mean aerodynamic force acting in the direction of the stroke plane, which is indispensable for the dragonfly to hover with the body axis horizontal. The total amounts of the lifting force and the necessary power are also estimated and shown to be very close to those estimated by other researchers. Some additional numerical simulations and discussions are also presented to explain why a true hover-fly that has, in contrast with a dragonfly, only a pair of wings can also hover with the body axis horizontal.

Thermal management systems for data centers with liquid cooling technique of CPU

Energy consumption in data centers has seen a drastic increase in recent years and approximately 40% of it is spent on cooling facilities. In data centers, server racks are cooled down indirectly by chilled air with an air conditioner. This technique is inefficient because IT equipment is not cool enough whereas the server room is overcooled. Under these circumstances, the authors have proposed liquid cooling systems in which CPUs are cooled down directly and exhaust heat is not radiated into the server room. Three cooling technique for CPUs are developed simultaneously. Two of them involve a single-phase heat exchanger and a two-phase heat exchanger as a cooling jacket. The other is flat heat pipes in order that the heat generated by CPU is cooled after being transported to the outside of the chassis. Condensation section of the heat pipe is cooled down by liquid with liquid manifold, so that there is no air cooling part in our new liquid cooling systems. Performance evaluation tests of the cooling devices have been conducted, which are single-phase heat exchanger, two-phase heat exchanger and two-types of flat heat pipes. Meanwhile, nanofluid technology for heat transfer enhancement and development of plug-in connectors for standardization of the systems are pursued. In addition, Verification tests have been carried out with using commercial servers. As a result, a 44% to 53% reduction in energy consumption of cooling facilities with single-phase cooling system was realized compared with conventional air cooling system.

EFFECTS OF DYNAMIC STALL PHENOMENA ON PROPULSIVE EFFICIENCY AND THRUST OF A FLAPPING AIRFOIL

Numerical simulation of dynamic stall phenomena around an airfoil oscillating in a coupled mode, where the pitching and heaving oscillations have some phase differences, has been performed using the Navier-Stokes code. The propulsive efficiency and the thrust have been calculated for various combinations of the phase difference and the reduced frequency for two different amplitude ratios, and the effects of the dynamic stall phenomena on the behaviors of the propulsive efficiency and thrust are discussed in detail by examining each flow pattern obtained. The highest efficiency has been observed for the case where the pitching oscillation advances 90 deg. ahead of the heaving oscillation, for which the flow separation is confined in the small region on the airfoil surface near the trailing edge in spite of the large amplitude oscillations. For phase angles other than 90 deg. the efficiency is degraded by the occurrence of the large scale leading edge separation.

Structure of High-Performance Evaporators for Space Application

Abstract:  A new structure of cold plates, where an unheated auxiliary channel is installed to supply liquid directly to the bottom of coalesced flattened bubbles in a narrow heated channel, is tested to investigate the increase in critical heat flux. Assuming the application to the laser solar power system, a large heating surface with a length of 150 mm in the flow direction is employed, and a narrow channel structure is adopted to reduce the size of cold plates, where the gap sizes are selected as 5 mm and 2 mm. Experiments are performed for water as a test liquid at inlet subcooling of 15 K under near atmospheric pressure. Inlet liquid velocity is varied from 0.065 m/s to 0.6m/s for the upward flow on ground. A value of critical heat flux of 2.2 × 106 W/m2 is obtained for 5‐mm gap size at the inlet velocity of 0.2 m/s. At low liquid flow rate, the structure realizes the CHF values larger by 2.5 times than those for the normal heated channel without additional liquid supply. A new method to evaluate the performance of cold plates is proposed to take account of the variation in the size of heating surface, inlet liquid velocity, and subcooling that influence the CHF values. The validity of the proposed structure of the cold plate for the increase in critical heat flux is confirmed.

Experiment on nucleate pool boiling in microgravity by using transparent heating surface ? Analysis of surface heat transfer coefficients

Investigation of mechanisms in nucleate boiling under microgravity conditions is essential for the development of the cooling systems handling a large amount of waste heat. A transparent heating surface with multiple arrays of 88 thin film temperature sensors and mini-heaters was developed for the clarification of boiling heat transfer mechanisms in microgravity. To investigate gravity effects on the microlayer behaviors and corresponding local heat transfer coefficients, images of liquid-vapor behaviors underneath attached bubbles and local heat transfer data were simultaneously obtained in microgravity pool boiling. The present paper reports the analysis of the data measured during the ESA parabolic flight campaign. It was found that the liquid-vapor behaviors were strongly affected by the direction and the level of residual gravity. Various patterns of liquid-vapor behaviours and corresponding enhancement or deterioration of the heat transfer are observed.

Heat Transfer Enhancement Observed in Nucleate Boiling of Alcohol Aqueous Solutions at Very Low Concentration

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.Copyright

Nucleate boiling of low-concentration alcohol aqueous solutions for the development of thermal management systems in space

To develop high-performance space thermal management systems using boiling phenomena, the heat transfer characteristics of low concentration alcohol aqueous solutions were investigated on ground. For mixtures of 1-Propanol/Water, 2-Propanol/Water and Water/1-Butanol, i.e. substantially positive mixtures at very low concentration range of alcohol, heat transfer enhancement was observed, while only heat transfer deterioration was reported in most of existing studies for nucleate boiling of mixtures. A concept of coexisting heat transfer enhancement due to Marangoni effect additionally to the heat transfer deterioration due to mass transfer resistance was emphasized. The concept seems to be true for mixture nucleate boiling independent of gravity level.

Fundamental experiments for the development of high-performance cold plates in a feasability study on space solar power system

To realize a concept of solar power satellite with high power generation, it is a promising method to transfer the power by laser beam using active mirror type amplifiers where cooling of the laser material accepting highly condensed sunray is inevitable. To remove high heat flux density from a large area, a structure of cold plate consisted of two parallel plates is devised and the effective liquid supply directly to the bottom of flattened bubbles due to nucleate boiling is realized by the auxiliary liquid feeder. The critical heat flux is increased by more than 1.5 times from that without the additional liquid supply. The technology can be applied not only to space but widely to the development of high-performance cold plates employed on ground.

Liquid cooling network systems for energy conservation in data centers

Energy consumption in data center has been drastically increasing in recent years. In data center, server racks are cooled down by air conditioning for the whole room in a roundabout way. This air cooling method is inefficient in cooling and it causes hotspot problem that IT equipments are not cooled down enough, but the room is overcooled. On the other hand, countermeasure against the heat of the IT equipments is also one of the big issues. We therefore proposed new liquid cooling systems which IT equipments themselves are cooled down and exhaust heat is not radiated into the server room. For our liquid cooling systems, three kinds of cooling methods have been developed simultaneously. Two of them are direct cooling methods that the cooling jacket is directly attached to heat source, or CPU in this case. Single-phase heat exchanger or two-phase heat exchanger is used as cooling jackets. The other is indirect cooling methods that the heat generated from CPU is transported to the outside of the chassis through flat heat pipes, and condensation sections of the heat pipes are cooled down by liquid. Verification tests have been conducted by use of real server racks equipped with these cooling techniques while pushing ahead with five R&D subjects which constitute our liquid cooling system, which single-phase heat exchanger, two-phase heat exchanger, high performance flat heat pipes, nanofluids technology, and plug-in connector. As a result, the energy saving effect of 50∼60% comparing with conventional air cooling system was provided in direct cooling technique with single-phase heat exchanger.Copyright

Development of a High-Performance Boiling Heat Exchanger by Improved Liquid Supply to Narrow Channels

Abstract: 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.