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Chengyao Li

Bio: Chengyao Li is an academic researcher from Case Western Reserve University. The author has contributed to research in topics: Flame spread & Extinction (optical mineralogy). The author has an hindex of 4, co-authored 5 publications receiving 57 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth.

59 citations

Journal ArticleDOI
01 Jan 2019
TL;DR: In this paper, a 3D transient model was used to study the current-flow flame spread over a thin charring material in an unmanned spacecraft, and two sample sizes were used: 5 cm by 29 cm and 41 cm by 94 cm, the largest ever samples burned in controlled experiments in microgravity.
Abstract: Concurrent-flow flame spread over a thin charring material was studied in an unmanned spacecraft. Two sample sizes were used: 5 cm by 29 cm and 41 cm by 94 cm, the largest ever samples burned in controlled experiments in microgravity. A low-speed ambient airflow of 20 cm/s was used. The samples were ignited from their upstream ends and were allowed to burn for several minutes. Video recorded during the burning process and readings from thermocouples on the sample surfaces were examined. The results showed that flames reached a quasi-steady state with nearly constant flame length for both sample sizes. However, the pyrolysis length exhibited an initial overshoot before reaching steady state for the large sample. This phenomenon has not been reported or observed until now. While steady state pyrolysis lengths were similar, the 5 cm wide sample had a slightly larger spread rate (by ∼13%) and a shorter burnout time (pyrolysis length over spread rate) compared to the 41 cm wide sample. A previously developed three-dimensional transient model was used to conduct the numerical study. Detailed profiles of the gas and solid phases, including flow patterns, species concentrations, temperature, solid burning rate, and heat flux distributions are examined. The modeling results reveal that flow reduction in a growing boundary layer along the sample surface accounts for the overshoot of the flame length observed for the large sample. Compared to a wide sample, a narrow sample has more effective oxygen transport across the width of the sample. This results in a stronger flame, a shorter flame standoff distance from the sample surface, and larger heat feedback to the sample. This accounts for the shorter burnout time for the narrow sample compared to the large sample.

14 citations

Journal ArticleDOI
01 Jan 2021
TL;DR: In this paper, two-dimensional numerical simulations were performed to study concurrent-flow flame spread over a thin solid in microgravity, where the main variable is the ambient flow velocity.
Abstract: Two-dimensional numerical simulations were performed to study concurrent-flow flame spread over a thin solid in microgravity. The main variable is the ambient flow velocity. Results were validated against recent microgravity experiments (Saffire) where samples 41 cm wide were burned in two different flow velocities. The numerical results showed that, when flow velocity increases, the average radiative heat flux out of the solid surface in the pyrolysis region remains constant. The average convective heat flux decreases and the average radiative heat flux into the solid surface increases. The average net heat flux remains approximately constant. Additional simulations using different ambient pressures and oxygen percentages further showed that, while away from extinction conditions, the average net heat flux in the sample pyrolysis region and hence the average solid mass burning rate are insensitive to the ambient conditions (i.e., oxygen percentage, flow speed, pressure). Furthermore, the steady-state flame length is mainly controlled by the rate of oxygen entrainment into the gas-phase reaction zone and the average solid mass burning rate. Based on these observations, analytical model was developed to correlate the flame spread rate and flame length to ambient conditions. It predicts that, for concurrent-flow flame spread at steady state, the flame length and flame spread rate have linear dependency on the ambient pressure and the forced flow velocity, and a second order dependency on the ambient oxygen percentage. The proposed correlations were tested against numerical simulations as well as a collection of data from previous microgravity experiments in a wide range of ambient conditions.

9 citations

Journal ArticleDOI
TL;DR: In this article, an in-house three-dimensional transient numerical model with a two-stage solid pyrolysis was used to simulate upward flame spread over thin solids.
Abstract: Upward flame spread over thin solids is numerically studied using an in-house three-dimensional transient numerical model with a two-stage solid pyrolysis. The simulation results reveal fundamental...

6 citations

Journal ArticleDOI
TL;DR: In this article , an electrically-heated wire coil is placed beneath the sample in a slow forced flow to serve as both a heater for the solid sample and a gas-phase hot spot to ignite the oxidizer-fuel vapor mixture.

2 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth.

59 citations

Journal ArticleDOI
TL;DR: In this article, the authors revisited the problem of opposed fire spread under limited and excessive oxygen supply and reviewed various near-limit fire phenomena, as recently observed in flaming, smoldering, and glowing spread under various environment and fuel configurations.

40 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of pressure and microgravity on upward/concurrent flame spread over 10 mm thick polymethyl methacrylate (PMMA) slabs was investigated and correlated in terms of a non-dimensional mixed convection analysis that describes the convective heat transferred from the flame to the solid.

30 citations

Journal ArticleDOI
TL;DR: In this article, an array of 10 1.5 cm-long 5 cm-wide filter papers is uniformly distributed on a vertical sample holder subjected to double-sided burn, and the distance between the samples was varied from 0 to 4 cm.

18 citations

Journal ArticleDOI
TL;DR: In this article, a series of microgravity experiments of concurrent-flow flame spread over samples of ultra-low area densities are conducted using NASA Glenn Research Center's Zero Gravity Research Facility (the 5.18 ǫ s drop tower).
Abstract: There are no existing experimental studies of flame spread rate trends for ultra-thin solid samples. Previous theory has predicted that for concurrent flame in kinetic regime, the flame spread rate decreases as the sample thickness decreases and there is a critical thickness below which burning is not possible. To test this hypothesis, a series of microgravity experiments of concurrent-flow flame spread over samples of ultra-low area densities are conducted using NASA Glenn Research Center’s Zero Gravity Research Facility (the 5.18 s drop tower). The tested samples are cellulose-based materials of various area densities, ranging from 0.2 mg/cm2 to 13 mg/cm2, as low as one order of magnitude less than those ever tested before. Each sample is 30 cm long by 5 cm wide and is burned in a low-speed concurrent air flow (5 to 30 cm/s). The results show that the concurrent flame spread rate is proportional to the flow velocity relative to the flame and is inversely proportional to the sample area density. A theoretical formulation, provided in this work, suggests that the flame length has a linear relationship with the relative flow speed and has no direct dependency on the sample area density. The experimental data supports this conclusion. From the images recorded in the experiments, a unique flame base tubular structure directed upstream away from the burnout zone is observed for thin samples. This structure is suspected to be due to flame stretching and localized blowoff caused by the oxidative pyrolysis Stefan flows at the sample burnout. This can be an indication that the chemical time becomes comparable to the flow time of the Stefan flow and the tested samples are approaching the kinetically-limited thickness. For the thinnest tested sample (0.2 mg/cm2), flames with concurrent and opposed dual natures are observed when the air flow rate is low (< 20 cm/s). At the lowest tested flow rate (5 cm/s), the flame spread rate exceeds the air flow rate and the flame transits to an opposed flame in the concurrent flow. The dual nature and flame transition are presented and discussed. This study provides detailed examination through high-resolution images of the transition between the concurrent to opposed flame spread modes.

17 citations