Biomedical subjects

Haruhiko Ohta

Publications and source records attributed to Haruhiko Ohta.

6 recordsLinked to original sources

PubMed2006

Structure of high-performance evaporators for space application.

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 x 10(6) 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.

Journal Article↗

PubMed2006

A microgravity experiment of the on-orbit fluid transfer technique using swirl flow.

The cryogenic fluid transfer technique will prove useful for flexible and low-cost space activities by prolonging the life cycle of satellites, orbital transfer vehicles, and orbital telescopes that employ cryogenic fluids, such as reactants, coolants, and propellants. Although NASA has conducted extensive research on this technique to date, a complicated mechanism is required to control the pressure in the receiver tank and avoid a large liquid loss by vaporization. We have proposed a novel fluid transfer method by using swirl flow combined with vapor condensation facilitated by spray cooling. This technique enables gas-liquid separation in microgravity and effectively facilitates vapor condensation without any special device like a mixer. In addition, since the incoming liquid flows along the tank wall, the tank wall would be cooled effectively, thereby minimizing the liquid loss due to vaporization. In this paper, the influence of the number of inlet points, fluid velocity at the inlet, fluid type, and boiling condition on swirl flow under microgravity conditions is investigated experimentally. The results indicated that the new fluid transfer technique using the swirl flow proposed by us is effective for cryogenic fluids that generally exhibit low surface tension and good wettability. In addition, it is possible to apply this technique to the real system because the swirl flow conditions are determined by the Froude number, which is dimensionless. Thus, the fundamental technique of fluid transfer by using the swirl flow under microgravity conditions was established.

Journal Article↗

PubMed2004

Development of a high-performance boiling heat exchanger by improved liquid supply to narrow channels.

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.

Cold Temperature↗

PubMed2004

A study of subcooled pool boiling of water: contact area of boiling bubbles with a heating surface during a heating process.

The contact area of bubbles with a transparent heating surface was optically measured during subcooled pool boiling of water on the ground. In the experiments, boiling bubbles were attached to the heating surface with a bubble holder and nearly reproduced the bubble behavior observed in low gravity. DC power was applied to the ITO heater and increased until the heater surface burned out. In quick heating, that is about 20 second until burnout and equal to the heating time during the low gravity period, the contact area was smaller than that for long time heating at the same heat flux. The experimental results suggest the reason why the critical heat flux in pool boiling is higher than the widely accepted predictions in microgravity. In a drop shaft experiment with constant heating, the contact area increased dramatically at the start of microgravity and became constant. Boiling bubbles coalesced and remained just over the heating surface.

Earth, Planet↗

PubMed2002

Review of existing research on microgravity boiling and two-phase flow: future experiments on the international space station.

This paper describes research objectives on boiling and two-phase flow under microgravity conditions from both scientific and technological points of view. Existing research for various systems in flow boiling are briefly reviewed and problems indicated for research conducted by available facilities with short microgravity duration. A wide range of experimental subjects that become possible by using different types of test sections is clarified and the validity of long term experiments is emphasized. A first step outline is described for a test loop with interchangeable test sections, such as transparent heated tubes, transparent flat heating surfaces, and narrow channels optimized for individual objectives and restricted by specifications of the facility.

Journal Article↗

PubMed2002

Heat transfer mechanisms in microgravity flow boiling.

The objective of this paper is to clarify the mechanisms of heat transfer and dryout phenomena in flow boiling under microgravity conditions. Liquid-vapor behavior in annular flow, encountered in the moderate quality region, has extreme significance for practical application in space. To clarify the gravity effect on the heat transfer observed for an upward flow in a tube, the research described here started from the measurement of pressure drop for binary gas-liquid mixture under various gravity conditions. The shear stress acting on the surface of the annular liquid film was correlated by an empirical method. Gravity effects on the heat transfer due to two-phase forced convection were investigated by the analysis of velocity and temperature profiles in the film. The results reproduce well the trends of heat transfer coefficients varying with the gravity level, quality, and mass velocity. Dryout phenomena in the moderate quality region were observed in detail by the introduction of a transparent heated tube. At heat fluxes just lower and higher than CHF value, a transition of the heat transfer coefficient was calculated from oscillating wall temperature, where a series of opposing heat transfer trends--the enhancement due to the quenching of dried areas or evaporation from thin liquid films and the deterioration due to the extension of dry patches--were observed between the passage of disturbance waves. The CHF condition that resulted from the insufficient decrease of wall temperature in the period of enhanced heat transfer was overcome by a temperature increase in the deterioration period. No clear effect of gravity on the mechanisms of dryout was observed within the range of experiments.

Journal Article↗