In order to predict the performance of cooling method using an evaporative latent heat, which is applied in various acceleration environments, fundamental of nucleate boiling must be understood. This study aims at the analysis of boiling by a numerical method of the two-phase flow considering phase change. CIP-LSM (CIP-based Level Set & MARS) which is a numerical method for free surface flows using interface tracking method is made to resolve an interface sharply in order to estimate the mass transfer and energy transfer more faithfully. It is stable and robust even for large density and viscosity rations on the order of 1000 to 1. Additionally, CIP-LSM implemented a phase change model is used to handle boundary conditions imposed at liquid-gas interface, including velocity jump conditions due to volume change and heat flux jump conditions due to latent heat and can simulate a Stefan program of evaporation successfully. This paper presents an analyses related to single bubble behavior during nucleate boiling at and near a heated surface in order to validate the reliability of this model. About the effect of the phase change model, good agreement was obtained between numerical
result and experimental results.
In order to obtain the fundamental knowledge of free-surface flows under low-gravity conditions, the unsteady
deformation of liquid surface in cylindrical vessels were experimentally observed with a drop tower. Main concern was placed on the dynamic behavior of the liquid driven by surface force and wetting. The deformation of the ethanol surface driven by surface tension was confirmed to be governed by the similarity rule described by Weber number. Even when the operating fluid was changed to ethyl benzene or FC-77, the scaled displacement has good correlation with the scaled time.
Among the operating fluids with the similar values of advancing contact angle, the similarity law was well established in the present cases.
The drift-flux model and the flow regime transition model are of practical importance for two-phase flow analyses under microgravity conditions. This report outlines our previous studies on development of prediction models for gas-liquid twophase flow under reduced gravity and microgravity conditions. First, the drift flux model which takes the effects of gravity and frictional pressure loss into account are summarized. Next, the flow regime transition criteria which take the frictional pressure loss effect are briefly introduced. To evaluate the applicability of these models to microgravity condition, comparisons of the models with various experimental datasets taken under microgravity conditions are performed.
Sample computations of the models are also performed under various gravity conditions such as microgravity and lunar, Martian and Earth surface gravities.
In order to classify the flame spread behavior over a thermally thin material against the opposed flow, scale analysis was conducted and the results were compared with those of drop experiments. Two dimensionless number, Rrad and Da, which represented the radiation loss factor and the Damkohler number, respectively, were introduced to estimate the transition flow velocity from the thermal-regime to the microgravity-regime or the kinetic-regime in a given oxygen level. The result of scale analysis showed that the thermal-regime, in which the flame became most robust, located in low ambient flow condition. This predicted that the robust flame could exist in microgravity with a mild flow. The result of the drop experiment agreed with the prediction; the flame spread in CO2 balance condition was observed even at 21% oxygen level in microgravity whereas the downward spread was impossible at the same oxygen level.
Spontaneous ignition of fuel droplets was numerically studied as a fundamental study for fuel spray. First, spontaneous ignition of an isolated droplet in a closed cell filled with hot fuel/air premixed gas was calculated. Namely, the inter-droplet interaction in a spray was expressed through the interference of the outer boundary of the cell. Fuel was n-heptane.
Detailed chemical kinetics including the low-temperature oxidation reactions were employed, and therefore cool-flame ignition delay τcf and hot-flame ignition delay τig were evaluated. Initial liquid-phase temperature, gas-phase equivalence ratio and initial pressure were 300 K, 0.4 and 3 MPa, respectively. Since the premixed gas was fuel-lean, the existence of the droplet might either promote the ignition through a role as fuel source or hinder the ignition through a role as heat sink.
When initial droplet diameter do was relatively large, both τcf and τig were not different from those of only premixed gas. It means that such large droplet required relatively long time for vaporization compared with chemical characteristic time, and the ignition occurred at the outer boundary of the cell that the fuel vapor from the droplet did not reach. However, as d0 decreased, τcf and τig increased and decreased, respectively, first. Thus interaction between spray and premixed gas was recognized. With increasing liquid-phase equivalence ratio, τig either increased monotonically or had minimal value depending on initial gas-phase temperature. Next, spontaneous ignition of a droplet pair was studied both experimentally and numerically. The experiments were performed in microgravity in order to get rid of the effects of natural convection using n-decane as fuel.
Both τcf and τig increased with decreasing inter-droplet distance, while the duration between coolflame appearance and hot-flame appearance decreased, which is supposedly caused by higher cool-flame temperature. The experimental results can be explained thorough fuel vaporization behavior simulated by numerical calculations. Thus, droplet number density of spray can have the opposite effects on spontaneous ignition depending on conditions.
Three kinds of general approaches for identifying the flammability limits of combustible mixture, i.e., flames in a
propagation-tube, those in a constant-volume bomb and counterflow flames were introduced. Based on various studies, flammable boundaries of different types of the flames were found to be significantly dependent on the flame configurations due to the combined effects of Lewis number and radiative heat loss. Further understandings accomplished through microgravity experiments, such as SEF (self-extinguishing flame), flame ball and C- & G-curve behavior in counterflow flames were introduced and current limitation of our understandings on the flammability limits were discussed. Finally, future Japanese space experiment for the constructions of the comprehensive flammability-limit theory was introduced.
IKAROS is a small solar power sail demonstration spacecraft developed by JAXA. The spacecraft launched on May 2010 succeeded in deploying a thin solar power sail membrane with 14 meters in width and 7.5 micrometers in thickness. The membrane was folded and wrapped around the spacecraft and unfurled by centrifugal force in two stages. Since the deployment test of the large thin membrane is impossible on the ground, a huge amount of numerical simulations were performed before launch by employing the multi-particle system model (MPM) and the nonlinear elasto-dynamic finite element analysis (NEDA). MPM approximates a thin membrane with a spring-mass-damper system and enables fast numerical simulations. NEDA is based on the energy momentum method which preserves the total energy, the linear momentum and the angular momentum and enables reliable simulations of flexible multi-body systems. In this paper, their numerical results are compared with flight data and the deployment behaviors of the membrane are discussed.