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

ScienceDirect

ViewPDF

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.

(Video) Burning metals for carbon-free power

## 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.

## Principles of fuel MLR and flame HRR measurements

With gravity, a mass balance can be used to measure the mass-loss rate ($\stackrel{˙}{m}$) of the burning solid and liquid fuels as$\stackrel{˙}{m}=\frac{1}{g}\frac{dG}{dt}$where $G$ 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 (${\stackrel{˙}{V}}_{F,g}$) 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 ${\stackrel{˙}{m}}_{F}$ calculated with the three balance-free methods to the $\stackrel{˙}{m}$ 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

## References (33)

• M. Thomsen et al.Buoyancy effects on concurrent flame spread over thick PMMA

### Combust Flame

(2019)

• G. Jomaas et al.Fire safety in space-beyond flammability testing of small samples

### Acta Astronaut

(2015)

• P.B. Sunderland et al.Analysis and measurement of candle flame shapes

(Video) Air Pollution 101 | National Geographic

### Proc. Combust. Inst.

(2011)

• M.C. Johnston et al.Gravimetric measurement of solid and liquid fuel burning rate near and at the low oxygen extinction limit

### Fire Saf J

(2017)

• P. Dehghani et al.Theoretical analysis and predictions of burning in microgravity using a burning emulator

### Combust Flame

(2021)

• F.G. RoperThe prediction of laminar jet diffusion flame sizes: part I. Theoretical model

### Combust. Flame

(1977)

• S.L. Olson et al.High-speed video analysis of flame oscillations along a PMMA rod after stagnation region blowoff

### Proc. Combust. Inst.

(2019)

• X. Huang et al.Transition from opposed flame spread to fuel regression and blow off: effect of flow, atmosphere, and microgravity

### Proc. Combust. Inst.

(2019)

• S. Bhattacharjee et al.Radiative, thermal, and kinetic regimes of opposed-flow flame spread: a comparison between experiment and theory

### Proc. Combust. Inst.

(2017)

• S.L. Olson et al.Microgravity flammability boundary for PMMA rods in axial stagnation flow: experimental results and energy balance analyses

### Combust. Flame

(2017)

• J.C. Yang et al.Reduced gravity combustion of thermoplastic spheres

### Combust. Flame

(2000)

• D.L. Dietrich et al.Droplet combustion experiments in spacelab

### Symp. (Int.) Combust.

(1996)

• J.M. Citerne et al.Fire safety in space – Investigating flame spread interaction over wires

### Acta Astronaut.

(2016)

• O. FujitaSolid combustion research in microgravity as a basis of fire safety in space

### Proc. Combust. Inst.

(2015)

• P. Sun et al.Microgravity combustion of polyethylene droplet in drop tower

### Combust. Flame

(2020)

• D.L. Urban et al.Flame spread: effects of microgravity and scale

(Video) Insects and Wildfire in the Boreal Forest Sept. 2022

(2019)

• ## Cited by (1)

• Smart evaluation of building fire scenario and hazard by attenuation of alarm sound field

2022, Journal of Building Engineering

The audible fire alarm system of the building makes a sharp sound to alert all occupants when fires occur. According to the fire code, the fire alarm should be loud enough to be heard in any corner of the building. Thus, this work explores a smart technology of using alarm attenuation to reveal the fire scene information. Different alarms with frequencies from 500 to 2000Hz are tested. The propanol pool fires of different sizes and shapes are selected as the detecting targets. Results show that the sound pressure attenuation by the fire plume is positively correlated with the fire scene heat release rate. The sound-pressure attenuation is also greater if the flame thickness is larger along the sound path. Hence, a sound-field fire monitoring model is proposed and verified by experiments using larger wood-crib fire and liquid-pool fire. This work provides a practical sound-based fire monitoring model and helps establish a scientific framework for the smart technology of using the existing audible alarm system to continuously monitor the building fire development.

## Recommended articles (6)

• Research article

Numerical simulation on response characteristics of coal ignition under the disturbance of fluctuating heat

Combustion and Flame, Volume 237, 2022, Article 111870

Owing to coal commonly existing in a fluctuating thermal environment, a two–step parallel reaction model for coal ignition was established to explore the development of coal ignition under fluctuating thermal convection, fluctuating thermal radiation, and convection–radiation coupling (CRC). To be specific, this included temperature evolution in a coal mass and fluctuating periodic characteristics. The nonlinear gain of coal spontaneous combustion caused by oscillatory heat flux conditions was researched. The results show that the thermal fluctuation at 0.1Hz is conducive to coal ignition. The ignition delay time (IDT) exponentially decreases with the growth of both the thermal convection velocity and radiation heat flux. Under external thermal disturbance, the temperature, convection velocity, oxygen concentration, and depth of the sensitive area all vary periodically. Under different types of heat disturbance, the temperature and convection velocity fluctuate with an increasing trend, while the oxygen concentration fluctuates with a decreasing trend. There is a low phase difference between the temperature and the convection velocity while the phase difference between the temperature and oxygen concentration is about 90° Besides, the peak fluctuation of the oxidation rate of coal changes inversely with the depth of fluctuations of the sensitive area.

• Research article

Computational analysis of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures

Combustion and Flame, Volume 238, 2022, Article 111933

The present study aims to gain insight of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures. A specific goal is to use the validated three-dimensional (3-D) numerical model to reveal fine details of the gas and liquid phase flows as well as the resulting flame characteristics, which are challenging to obtain experimentally. To facilitate the study, 3-D formulations have been developed to explicitly solve the transport equations in both phases. A compressible solver was formulated for flame propagation in the gas phase using a one-step chemical reaction expression and mixture-averaged diffusion coefficients for the gaseous species. An incompressible solver with temperature dependent thermo-physical properties was employed to describe the convective motions and heat transfer in the liquid fuel region. Validation has been conducted for both uniform and pulsating spreads over a narrow 1-propanol tray with varying fuel depths through comparing the predicted flame front evolution with published measurements. Further qualitative comparison has also been conducted for some predicted fine features of the gas and liquid phase flows and flame spreading characteristics with published experimental observations. For both the uniform and pulsating spread, the detailed flame structure including the main diffusion flame and a small stratified premixed flame at the front have been captured. Wherever relevant, the detailed predictions were also used to shed light on some discrepancies in previously reported features in different laboratory studies and numerical simulations. Finally, the detailed 3-D predictions were used to illustrate fine features of the subsurface convective flow and its relative position to the flame front, the relative magnitudes of the subsurface flow velocity and that of the spread rate as well as the role of the thermocapillary-driven subsurface flow in the flame spread mechanism.

• Research article

Merging dynamics of dual parallel linear diffusion flames

Fire Safety Journal, Volume 127, 2022, Article 103490

(Video) How effective were fuel treatments in the 2011 Wallow Fire?

The merging of flames is a widely observed fire phenomenon in building and wildland fires. This work explores the merging characteristics of two identical linear diffusion flames. The duel parallel linear burners are used (dimensions of 100mm×4mm and 200mm×2mm), and the heat release rate (HRR) of each buoyancy-driven flame varies from 0.7 to 10.9kW. The flame merging probability and merging flame height are quantified under different spacing of burners and HRRs. A new dimensionless HRR involving the aspect ratio is proposed to determine the merging probability. Moreover, decreasing the entrained air from the ground promotes the flame merging, which can be reflected by an increase of dimensionless HRR. The merging flame height is determined by the competition of buoyancy and pressure difference around the flame, which can be correlated by the effective entrainment perimeter and the dimensionless HRR. Such a correlation can also be used to explain the flame merging phenomena with different HRRs and aspect ratios in the literature.

• Research article

Scale model experiments of fire whirls over the non-fuel zone around an L-shaped fire source

Combustion and Flame, Volume 238, 2022, Article 111930

The Hifukusho-ato fire whirl (HAFW) is a catastrophic fire accident in Tokyo in 1923 and a representative fire whirl phenomenon that occurs over the non-fuel zone around an L-shaped fire source. Previous studies have mainly focused on fire whirls above fire sources, while very limited work reported the HAFW-type fire whirls, which were found to be scale-dependent (more easily to occur in a large-scale wind tunnel test facility than small-scale ones due to the strong viscous effects in the latter) and thus not precisely discussed. In this study, a scale-independent method is proposed by fitting an additional fire source in the inside bend of an L-shaped fire source to mimic the flame-overhang region observed in the large-scale tests; the HAFW-type fire whirl is then successfully reconstructed in a small-scale test model (1/10,000th of the HAFW prototype). Meanwhile, the results show that the average fire whirl occurrence position in the flow direction in a large-scale test model (1/2,000th of the HAFW prototype) does not collapse with those in a small-scale test model (1/10,000th of the HAFW prototype without additional fire sources installed), but with those in the aforesaid scale-independent test model; such collapse can be summarized by a linear fitting correlation between a normalized fire whirl occurrence position and a modified Froude number under different fire source scales and lateral wind velocities. Based on the above experimental facts, updated scaling law to reproduce HAFW-type fire whirls and similar fire whirl occurrence positions is provided. This work advances a fundamental understanding of fire whirls generated by an L-shaped fire source; thus, helping guide fire-rescue operations, and benefiting future fire whirl model development.

• Research article

Effect of oxygen on the burning rate of wood

Combustion and Flame, Volume 234, 2021, Article 111591

The large-scale adoption of wood as a construction material for tall buildings could pave the way for sustainable construction. Its adoption, however, is hindered by a limited understanding of wood's behaviour in a fire. In particular, the effect of oxygen and heat flux on the burning (including pyrolysis) and ignition behaviour of wood is poorly understood. We addressed this gap by studying the effect of oxygen concentration and heat flux on the burning and ignition behaviour of particleboard experimentally and computationally. Particleboard was chosen as a proxy for all woody construction materials. We conducted over 60 experiments in an FPA on samples of particleboard spanning different oxygen concentrations (0–21%), heat fluxes (10–70kW/m2), sample densities (600–800kg/m2), and sample thicknesses (6–25mm). Only the heat flux and oxygen concentration significantly affected the charring rate, time-to-flaming ignition, and burning mode (pyrolysis, smouldering, flaming). To explore this effect further, we used a multi-physics model of particleboard charring developed in Gpyro. Combining the computational and experimental results, we showed that particleboard undergoes only pyrolysis in oxygen concentrations below 4%, smouldering between 4 and 15%, and flaming above 15% at a heat flux of 30 kW/m2. These oxygen concentration thresholds were found to decrease as the heat flux increases. We also showed that smouldering and flaming increases the charring rate by 25 and 37%, respectively. This means that the rate of loss of a section of structural wood, quantified by the charring rate, in a fire due to smouldering is similar to that of flaming combustion. In addition, we noted the existence of a triple point for the ignition of wood at which a slight change in environmental conditions can lead to either smouldering, flaming, or only pyrolysis. In summary, this paper quantified for the first time the contributions of the three modes of burning to the charring rate of wood and highlights the importance of smouldering for timber construction.

• Research article

Experimental study of downward flame spread and extinction over inclined electrical wire under horizontal wind

Combustion and Flame, Volume 237, 2022, Article 111820

Flame spread and extinction behavior are significantly influenced by both fuel inclination and wind velocity, however, the coupling effect of two has not been well quantified yet. In this paper, downward flame spread and extinction over inclined electrical wire under horizontal wind is investigated. Polyethylene(PE)-insulated wires (0.5mm/0.15mm in core diameter/insulation thickness) of two representative core materials (Copper/Nickel-Chrome: high/low conductivity) are used as samples. Flame geometrical characteristics (flame length ${L}_{f}$, pyrolysis length ${L}_{p}$, gas-phase thermal length ${L}_{g}$), as well as the flame spread rate (FSR) are quantified. Results show that as the horizontal wind velocity increases, FSR demonstrates a non-monotonic trend before extinction, where four regimes are identified based on different heat transfer controlling mechanisms. On the other hand, FSR of Cu-core wire and NiCr-core wire show different dependence on inclination angle. FSR reaches the maximum when the flame is pushed by the horizontal wind to be parallel to wire, which can be explained by a balance of horizontal wind and buoyancy-induced flow (i.e. inclination). A quantified model based on the three characteristic lengths (${L}_{f}$,${L}_{p}$,${L}_{g}$) and a proposed mixed-convective coefficient ${h}_{mix}$ combining the effect of inclination angle and wind velocity is established. The proposed model well represents the experimental FSR and interprets the controlling heat transfer mechanism. Moreover, the extinction limit is represented by a heat loss factor ${{R}_{l}}_{oss}$ as a function of strain rate a, showing an enhanced quenching effect and a weakened blow-off effect with increased inclination angle. The flame with Cu-core is more difficult to extinguish at small inclination but becomes easier at large inclination than that with NiCr-core, indicating a transition of inner core role from “heat source” to “heat sink”. This work provides essential knowledge on flame spread and extinction mechanism over inclined fuel under wind.

(Video) Effects of Fuel Treatments and Previous Fires on Subsequent Fire Management Costs

View full text

## Videos

1. Wildfires in California: Fire Ecology and the Economics of Suppression
(UCANR)
2. How to Start a Rocket
(ASTRA Bremen)
3. Lecture 13 Estimating Canopy Fuels
(Scott Ritter)
4. Heating With Coal. How to Start a Coal Fire
5. Montana Wildfire Risk Assessment
(NRFireScience)
6. Fuel for the Fire: Improved Understanding of Fire Behavior in Africa Based on Partitioned...
(AGU)

## Latest Posts

Article information

Author: Mr. See Jast

Last Updated: 10/07/2022

Views: 6394

Rating: 4.4 / 5 (55 voted)

Author information

Name: Mr. See Jast

Birthday: 1999-07-30

Address: 8409 Megan Mountain, New Mathew, MT 44997-8193

Phone: +5023589614038

Job: Chief Executive

Hobby: Leather crafting, Flag Football, Candle making, Flying, Poi, Gunsmithing, Swimming

Introduction: My name is Mr. See Jast, I am a open, jolly, gorgeous, courageous, inexpensive, friendly, homely person who loves writing and wants to share my knowledge and understanding with you.