Boiling and two-phase flow attract much attention for the improvement of cooling and heat dissipation processes on the ground and in space. However, there is very limited number of experimental data and no systematic or no coherent data needed for the design of space thermal management systems. In addition to establish the framework of database for boiling and two-phase flow in microgravity, the clarification of dominant force regime map is of top importance. Once the operating conditions free of gravity effect are clarified, reliable space systems can be developed by the iteration of ground tests. Furthermore, two-phase mixtures involving vapor phase can be transported independent of its orientation in the terrestrial cooling systems. Flow boiling experiments is planned onboard ISS by JAXA in 2016. In the present paper, individual subjects in the regimes of nucleate boiling and two-phase forced convection and the subjects in liquid-vapor interfacial behaviors are discussed by the citation of the results from the experiments of short microgravity duration, and the design concept of experimental setup under the restrictions inherent in the space experiment is described including the structures of test sections already integrated in PFM.
Thermal design and evaluation test for condenser system was conducted for the planned ISS experiments on two-phase boiling flow promoted by JAXA. Here perfluorohexane is planned to be used as a working fluid. In this condenser, eight rectangular copper tubes where circular channel of 6mm in diameter was installed on a flat type cold plate; same one which had be operated as thermal control system of onorbit experimental modules. Arithmetic thermal model of condenser was established considering tube wall conductivity, heat transfer coefficient on condensation and cooling water flow. Specification of the condenser was determined for 400 W of the maximum heat transport requirements through the analysis by this arithmetic model. Evaluation tests using BBM and EM were conducted to verify thermal performance of condenser. In the BBM test FC72 was used as a working fluid. System requirement for condenser; that is, liquid subcooling at outlet is 10 K and more in consideration of suction performance of circular pump was proved to be satisfied.
An observation section has been developed for the detail observation of gas-liquid interfacial structure in boiling and two-phase flow experiments onboard International Space Station. A stereoscopic photography method was applied for 3D measurement using one high framerate camera, and the accuracy in measurement of liquid film thickness was evaluated. It was confirmed in the preliminary experiment that liquid film thickness could be measured from images within ± 5 % accuracy. Moreover, to evaluate the development of the flow structure from the exit of the heating section, flow observation and cross-sectional average void fraction measurement was performed at four locations at the downstream of the heating section. As the result, under the scondition where slug flow was observed in the developed region, the fluctuation band of void fraction was small just at downstream of the exit of the heating section due to many small bubbles in liquid slugs. Under the conditions where annular flow was observed in the developed region, annular flow had been already formed at the exit, and then liquid film thickness became thicker.
Experiments were performed to verify the performance of experimental apparatus for the acquisition of reference data for flow boiling heat transfer under the terrestrial condition which is to be compared with that obtained under the microgravity condition onboard International Space Station (ISS) by using another apparatus with the same specification. Test section is a circular tube made of copper with an inner diameter of 4 mm and a heated length of 368 mm and oriented vertically on ground. To improve the accuracy of local heat fluxes, the compensation of heat flux distribution along the tube axis is discussed on the basis of the experimental results on the local heat transfer coefficients for a single-phase liquid flow. Correlations for local heat transfer coefficient of flow boiling are proposed here as functions of boiling number and Martinelli parameter in the regions of nucleate boiling and two-phase forced convection, respectively. Because the discrepancy of local heat transfer coefficient obtained from the apparatus for the terrestrial and the space experiments is caused by the difference of surface roughness in nucleate boiling region, a compensation factor is introduced in the correlation. The local heat transfer coefficients predicted by the proposed correlation are agreed well with those obtained by both apparatus.
Boiling is a very efficient mode of heat transfer because of the phase change involved and is a promising method for certain thermal management systems. However, the effects of gravity on two-phase flow phenomena in such systems have not been clarified in detail. To clarify these effects, boiling two-phase flow experiments onboard the Japanese Experiment Module “KIBO” have been proposed as an official Japanese Aerospace Exploration Agency project; this project concerns the effects of dissolved air on flow boiling heat transfer, which remains to be clarified in the case of a tube with an inner diameter of 4 mm. The abovementioned study involved conducting subcooled flow boiling experiments using various concentrations of dissolved air in perfluorohexane to clarify its effects on flow boiling heat transfer through a tube with an inner diameter of 4 mm. The excess temperature, boiling curve, and condensing heat transfer were determined and discussed. Consequently, dissolved air was demonstrated to affect boiling characteristics. In particular, dissolved air significantly affected the boiling curve in the low-heat-flux region: the boiling curve continuously decreased with increasing effective heat flux for mass fluxes of both G = 100 and G = 300 kg/m2 s. Moreover, dissolved air substantially affected the onset of boiling. These effects include decreasing the cluster radius required to form a nucleus. Furthermore, irrespective of the dissolved air concentration, no difference was observed among the boiling curves in the high-heat-flux region because of the fully developed flow regime, where a boiling curve determined according to the correlation proposed by Kandlikar effectively matched the experimental results in the case of a fluid-surface parameter Ffl of 1.7. This paper includes the contents of the proceedings of the 44th International Conference on Environmental Systems1) .
In order to clarify flow behavior and heat transfer characteristics of flow boiling in microgravity, the boiling and two-phase flow experimental facility which will be conducted in International Space Station (ISS) is developed. The glass heated section as an evaporating test section is installed in this facility, and the heated section is consisted of three transparent heated tubes which have different heated length, 50 mm, 50 mm and 5 mm, respectively. The transparent heated tube is made by Pyrex glass and the inner wall of the tube is covered by extremely thin gold film as using not only a heater but also resistance thermometer; that is, the tube can be measured the averaged inner wall temperature of the entire heated area. In addition, gas-liquid behavior of flow boiling can be observed through the gold film. The transparent heated short tube, which has 5 mm heated length and 4 mm inner diameter is a newly developed for third segment of the heated section of ISS experiment. In this study, derivation of the inner wall temperature and image analysis of flow behavior by using the short tube is described. Then the relation between heat transfer characteristics and flow behavior by using this tube is discussed. As a result of single-phase heat transfer experiment, the electrode resistance of the heated tube should be considerd for estimation of the inner wall temperature of the short tube. Nusselt number considered the resistance of the electrode part has a good agreement with existing correlations compeared to Nusselt number derived from total resistance of the tube. By using the image analysis, useful imformation such as liquid film thickness of annular flow and passing of disturbance wave can be obtained with heat transfer characteristics data simultaneously. The results indicates that the heat transfer in the liquid film region of slug flow is insignificantly small compeared to turbulent liquid flow at the wake of slug under the conditions of this study.
This research involves microgravity experiments being conducted on flame spread over fuel droplet arrays at different pressures including low-pressure condition in order to improve understanding of flame-spread characteristics under high-altitude relight condition of jet engines. n-Decane was used as a fuel. The ambient pressure ranged from 25 to 200 kPa. The results show that the flame-spread rate and flame-spreadlimit distance decreased with an increase in ambient pressure. As the ambient pressure is increased, the thermal conduction time from the flame of a droplet to the next unburned droplet increases due to the decreased thermal diffusivity, and the droplet heating time also increases due to the increased fuel boiling point, resulting in a decrease in the flame-spread rate and -limit distance. The pressure effect on the flame-spread limit around two interactive burning droplets is also discussed.
In order to compare the limiting oxygen concentration, below which a flame cannot continue to spread stably over solid materials, between concurrent-flow flame and opposed-flow flame spreads in microgravity, we have conducted parabolic flight experiments on flame spreading over a thin paper sheet (thickness: 0.12 mm and width: 20 mm) by varying the ambient air flow velocity. The two suction wind tunnels were mounted on the same aircraft and the opposed-flow and concurrent flow flame spread experiments were performed simultaneously. The air flows were driven by the pressure difference between inside and outside of the aircraft cabin, and the cross-sectional mean flow rate was obtained up to 7 cm/s. The oxygen concentration in the wind tunnel was controlled by adding nitrogen gas to the air flow sucked from the aircraft cabin. As a result, within the velocity range of the ambient flow from 1 cm/s to 7 cm/s, both limiting oxygen concentrations decreases as the ambient airflow rate increases. Moreover, it is found that although the propagation rate of the concurrent-flow flame spread is higher than that of the opposed flow flame spread at near limiting oxygen condition, both limiting oxygen concentrations of the concurrent-flow and opposed-flow flame spreads show almost the same values at the same ambient flow
Fluid motions after an impact of a droplet on a thin liquid layer were investigated by experimental observation under low gravity condition using the parabolic flight of an airplane. Whether the droplet bounces or coalesces on the liquid layer was found to be dependent on the Weber number and also on the Bond number, which represents the relative importance of gravity. In the plane of the Weber and the Bond numbers, droplet bouncing occurred for larger Weber numbers when the Bond number was small. The appearance of secondary droplets, which are caused by a symmetry-breaking of the growing cylindrical liquid sheet, was found to be independent to gravity. Our experimental results do not find gravity-dependent instability to be a cause of the symmetry-breaking, because it was observed even under microgravity conditions.