^{1}

^{*}

^{1}

^{1}

The height of the pool fire depends on the amount of heat feedback from the flame to the fuel. In order to predict flame height in a partial gravity environment, we investigated the heat feedback amount of a small pool flame experimentally under normal to partial gravity conditions; using the drop tower at Hirosaki University in Japan to obtain arbitrary partial gravity condition, which varied from 1 G to 0.55 G. We performed the measurement of the flame shape with a digital camera. Based on the experiment result, we expected the amount of fuel vapor from the amount of heat feedback of the pool flame calculated and to establish the prediction formula of the flame height in the partial gravity environment.

In recent years, as space research and technology progress, activity range of human spreads to various gravity environments. Although a fire in a spacecraft and station is a greatly feared hazard, flame characteristics under various gravity environments are not clarified entirely. Because there is no buoyant force under microgravity environment; flame behaviour is completely different than under normal gravity environment. To predict phenomena of fires, it is important to understand the effect of gravity on flame behavior which differs depending on the buoyancy. To clarify the relationship between flame behavior and gravity, some combustion research under varied gravity environments using drop towers, parabolic-trajectory airplanes, the Space Shuttle, and centrifuges has done [

From previous research, it is clear that the flame height of small-scale pool fires decreases as gravity levels decline [

Arbitrary partial gravity experiments were performed using the drop tower facility at Hi-rosaki University [^{2}. As a result, the partial gravity field is established in the test package. When the package falls 8 m, we can keep the low gravity condition for about one second. By controlling the weight of the test package, we can change the gravity level from 0.5 G to 0.7 G.

after ignition.

The distance from the fuel surface to the top of the luminous flame is defined as the flame height H. The flame surface area obtained from the image is defined as S. In 1 G and 0.55 G, the flame heights are defined as H_{1} and H_{0.55}, The flame surface area are defined as S_{1} and S_{0.55}. _{0.55}/H_{1} and S_{0.55}/S_{1} and pool diameter.

From previous research, it is clear that the flame height of pool fires decreases under partial gravity environments [

We calculate the amount of heat feedback at each pool diameter. Calculation method was based on Yoshida’s research [

flame by gravity level decrease, but it near the center of the flame base is less influenced by the decreasing of the gravity level. Therefore, it is expected that the amount of convective heat feedback near the center of the flame base is less influenced by the decreasing of the gravity level. In this study, it is assumed that flame distribution of center of flame does not change. From previous research, it anticipated temperature distribution as

The amount of radiative heat feedback was calculated using the representative flame temperature (ethanol G = 1:1190 K, G = 0.55:1128 K, acetone G = 1:1222 K, G = 0.55:1188 K) which obtained by the previous study [

The heat flux multiplied by the area of each heat receiving surface is taken as the amount of heat feedback from the flame to the liquid surface and is shown in

In this research, we assumed the liquid phase of the fuel as steady, and considered the response of an infinitely thin fuel surface. Experiments of this study were performed after 1 min because the flame stabilized after 1 min. At the same time, we can also consider that this period is when the liquid phase of the fuel becomes steady. Based on this assumption, we can ignore sensible heat and heat

loss. It is assumed that a pool flame is a jet diffusion flame that blows evaporated fuel. The amount of heat feedback which determined in Section 3.1 is used as the amount of heat flowing into the liquid surface. By dividing it by evaporation latent heat (ethanol: 854.8 kJ/kg, acetone: 551.9 kJ/kg), the mass flow rate of the fuel vapor from the fuel liquid surface is obtained. Calculate the volumetric flow rate of fuel vapor by dividing the mass flow rate by the density of fuel (ethanol: 2.074 kg/m^{3}, acetone: 2.614 kg/m^{3}). The flow velocity of the fuel vapor can be obtained by dividing the volumetric flow rate by the area of the fuel surface. _{f} [m/s] and the pool diameter d [mm].

Since the flow velocity of the fuel vapor depends on the heat flux, it tends to be close to

Regarding the flame height with the gravity level G as the experimental parameter, Altenkirch derived the following equation [

We would like to organize the pool flames assumed as jet diffusion flames in Section 3.3, in the same way as Altenkirch. However, in this study, we should consider flame shape change due to buoyancy rather than inertia in order to deal with flames with a particularly small jet velocity. Therefore, Altenkirch’s formula is modified as follows.

Grashof number

^{2}/s]. Flame is a state in which fuel and air are mixed, but it is difficult to estimate the ratio. Therefore, the values of

Using the above parameters, the influence of Richardson number on flame height and the pool diameter in laminar combustion region without Puffing was summarized.

From the inclination of the straight line in

tained as follows.

words it is inversely proportional to Grherhof number Gr to the power of 0.1, and Reynolds number Re to the power of 0.2. Since it is proportional to Grashof number, the flame height H also decreases as the gravity level G decreases. This is consistent with the experimental result.

As the gravity level decreases, at transport of fuel and oxidizer, the diffusion become that it is more dominant than buoyancy. Therefore, in the microgravity environment, the prediction formula obtained in Section 3.4.2 may not be valid.

The experimental formula is valid within the range of

In this study, we estimated the amount of heat feedback of small-scale pool fire, and obtained the experimental formula that in order to expect the flame height. The following conclusions are summarized here.

1) The flame height of the pool flame decreases in the partial gravity environment, but the surface area does not shrink unconditionally.

2) For a laminar flow diffusion flame without Puffing, we could obtain an experimental formula that expresses the relation of the flame height. It is proportional to the number of Richardson’s number 0.1.

3) In the microgravity environment, diffusion of fuels and oxidizing dominates rather than buoyancy. Therefore, the range of the number of Richardson

that the prediction formula derived in this study is effective is

Takahashi, R., Torikai, H. and Ito, A. (2017) Effect of Gravity on Flame Structure of Small-Scale Pool Fires. Open Journal of Safety Science and Technology, 7, 96-105. https://doi.org/10.4236/ojsst.2017.73009