Evaluation of burning rate in microgravity based on the fuel regression, flame area, and spread rate (2022)

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Combustion and Flame

Volume 237,

March 2022

, 111846

Abstract

The fuel burning rate and heat-release rate (HRR) play key roles in determining the fire intensity and hazard. On Earth, the burning rate of a condensed fuel is normally measured by the mass loss, but in microgravity, the impossibility of measuring the weight loss with a balance makes the measurement of burning rate challenging. This work proposes three methods to quantify the burning rate of condensed fuels in microgravity by measuring (i) the regression rate of the fuel surface, (ii) the spread rate of the flame leading edge, and (iii) the flame-sheet area, which all rely on video imaging of the flame or fuel surface geometry. The accuracies of these methods are quantified first in the ground-based tests with representative fuels, 1) solid candle and PMMA rods with diameters from 8 to 15mm, 2) liquid fuels including propanol, hexane, and kerosene, and 3) the methane and propane gases. Results show that the burning rate obtained optically by tracking the flame leading edge and the fuel regression were less accurate due to strong sensitivity to camera resolution and background light. Comparatively, measuring the flame-sheet area is easier and gives more accurate results, and microgravity PMMA-rod flame (BASS-II project in the International Space Station) show that the fuel mass flux across the flame sheet is almost constant (0.15mg/cm2-s) for a given fuel configuration and environment. This work offers a useful way to measure fuel burning rate and HRR in spacecraft and provides a path for the performance-based spacecraft fire safety design.

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Introduction

On Earth, gravity-induced buoyancy can complicate the flame dynamics [1,2]. Thus, conducting combustion experiments in a microgravity environment makes the flame simpler and helps reveal the fuel-burning and flame characteristics [3], [4], [5], [6], [7]. Furthermore, the fire hazard in microgravity is an important concern for space travel, and it will increase as the travel time spent in space is increased with the proposed space missions [8], [9], [10], [11], [12]. On the other hand, combustion experiments in a microgravity environment are difficult and expensive to conduct. For this reason, the burnings of small solid fuel samples have been investigated in the limited amount of microgravity combustion experiments, where the flame spread rate and extinction limits are primarily explored [8,[13], [14], [15], [16], [17], [18], [19], [20], [21]].

Although the growing spacecraft experiments continue to yield insights into the microgravity flame and burning characteristics of gaseous fuels, very limited data are available about the burning rate of solid and liquid fuels and the corresponding heat-release rate (HRR). Based on the classical Burke-Schumann's theory [22,23], it is possible to correlate the fuel flow rate and flame shape for a pure diffusion flame. Several studies [24], [25], [26] have also combined the theoretical analysis and the flame geometrical information to estimate the fuel burning rate, while these analytical models require very accurate input parameters. Considering the experimental approach, the HRR of a burning condensed fuel can be measured by the mass-loss rate [27] or the oxygen depletion of fume gases under the principle of oxygen calorimetry [28]. Nevertheless, a microgravity environment disables the use of regular mass balance to obtain the mass-loss rate (MLR). Using oxygen calorimetry requires two oxygen sensors mounted in the wind tunnel before and after the flame with a fast response speed [5,29,30], which is also challenging. Moreover, if not all the fuel gases are consumed in the flame, the accuracies of both fuel-based and oxygen-based HRR measurements are reduced. Thus, new indirect methods are needed to quantify the MLR of burning fuels and estimate the HRR in microgravity. The accurate measurement of flame HRR will be valuable for estimating the fire hazards in spacecraft and the performance-based fire safety design of spacecraft facilities.

In most flame-spread and fuel-burning experiments on Earth or in microgravity, video cameras are always used, which record the variation of flame, the spread of flame front, and the regression of fuel sample (e.g., [4,6,7,31]). If the relationships between these parameters measured from videos and the fuel-burning rate can be established, there would be other methods to calculate fuel MLR and fire HRR in microgravity. However, the accuracy and reliability of these methods are still unknown, considering the quality of video footage varies from test to test and can be interfered by experimental and environmental conditions, such as the sizes of flame and fuel, image resolution, and background light.

For this reason, this work aims to explore the feasibility of using the above three parameters, i.e., flame geometry, flame spread, and fuel regression, to measure MLR for different burning fuels. The accuracies of these methods are first examined on the ground with a precision balance for liquid and solid fuels, and a controlled flowmeter for gaseous fuels. The mechanism behind the pros and cons of each method are discussed. Finally, PMMA-rod experiments previously conducted in microgravity (BASS-II project in the International Space Station [14,19]) were used to verify the feasibilities of these three methods in space.

Section snippets

Principles of fuel MLR and flame HRR measurements

With gravity, a mass balance can be used to measure the mass-loss rate (m˙) of the burning solid and liquid fuels asm˙=1gdGdtwhere G is the weight of fuel sample and g is the gravitational acceleration. If all gasified fuels are completely consumed in the flame, the MLR is also the burning rate [25,26]. For gaseous-fuel flame, the mass flow rate or volume flow rate (V˙F,g) is normally controlled and pre-set, Thus, its burning rate under complete combustion is known or proportional to the gas

MLR estimation of a burning candle (base case)

The flame produced by a 3-mm candle was first analysed to directly compare the m˙F calculated with the three balance-free methods to the m˙ measured by the load cell. A thin candle as the base case was selected because it can melt and produce a stable laminar flame without dripping. Besides, the candle flame has a wick (see Fig.4), which keeps a distance between the flame and fuel top surface, so both the flame and the fuel-regression surface can be clearly captured by the camera.

The burning

Demonstration of PMMA flame in microgravity

In our previous works [14,19], the microgravity tests on PMMA flame have been conducted in the International Space Station (ISS) as a part of the Burning and Suppression of Solids-II (BASS-II) project [16], [17], [18], which is intended to understand the mechanism that governs the flame spread over the surface of solid combustible materials. The tests were carried out with rods of black PMMA with diameters of 6.4mm, 9.5mm, and 12.7mm and a length of 57mm. Due to the absence of gravity,

Conclusions

This paper is motivated by estimating the burning MLR of condensed combustible materials in microgravity, where the fuel weight loss cannot be used in assessing the flame HRR. Three potential alternate methods are proposed, i.e., (1) by measuring the spread rate of the flame leading edge, (2) by measuring the regression rate of condensed fuel, and (3) by measuring the area of the flame sheet.

The feasibility of each method has been first examined by calibration experiments in normal gravity with

CRediT author statement

Caiyi Xiong: Investigation, Writing - Original Draft, Formal analysis. Haoran Fan: Investigation, Resources. Xinyan Huang: Methodology, Conceptualization, Formal analysis, Supervision; Writing-Review & Editing. Carlos Fernandez-Pello: Methodology, Writing-Review & Editing.

Declaration of Competing Interest

The authors declare that they do not have any conflicts of interest.

Acknowledgments

C.X. is funded by the National Natural Science Foundation of China (NSFC) Grant No. 52006185; X.H. is funded by HK PolyU Emerging Frontier Area (EFA) Scheme of RISUD (P0013879), and CFP is supported by NASA Grants NNX10AE01G and NNX13AL10A

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