Abstract: The Dynamics of Flame Spread in Microgravity
PI: Subrata (Sooby) Bhattacharjee
Mechanical Engineering Department
Co-PI: Prof. Kazunori Wakai, Prof. Shuhei Takahashi
Gifu University, Japan
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Despite intensive research on microgravity flame spread in the last decade, inspired primarily from fire safety concerns in manned space travel, a number of aspects of the flame spread problem in general still remain unexplored or unresolved. Some of these outstanding issues include the following.  (i) Recent space-based experiments have established that steady flame spread cannot be sustained over thick fuels. However, the mechanism of this observed self-extinguishment, hypothesized to be the separation between the species field and the thermal field, remains to be experimentally determined. (ii) Earlier experiments have established that flame spreads steadily over thin fuels. This obviously raises the possibility of a critical thickness below which steady spreads are unsustainable. No experiments have been conducted to determine this critical thickness of a fuel bed, which has clear implications to fire-safety in a gravity free environment.   (iii) The low flow velocity achievable under a given microgravity environment may be comparable to the induced velocity from the residual gravity levels. However, at present there are no established criteria in terms of the relevant non-dimensional numbers, which define a "Microgravity" environment.  (iv) Despite the progress made in opposed-flow flame spread theory, the spread rate data still suffers from the influence of buoyancy-induced flow as the opposed-flow is decreased. There is a need for clean spread rate data, free from the interference of buoancy, with fuel thickness and flow velocity as parameters to fill a void that exists today between the thin and the thick limits, and to verify a proposed spread rate formula that unifies different regimes. (v) With a few exception, wind-aided flame spread has been traditionally studied as a separate subject from wind-opposed flame spread, although the latter is relatively well understood. Microgravity offers a unique opportunity to link the two paradigms by studying transition between wind-opposed to wind-aided spread as the mild flow velocity, achievable only in the absence of buoyancy-induced flow, is gradually reversed. Based on scale analysis, a simple formula is proposed that relates the spread rates in these two configurations through the aspect ratio or slenderness of the flame. 

Funded by the Govt. of Japan, the PI's have been already collaborating on ground-based effort as preparatory work for this proposal. Here, we propose four space-based experiments on flame spread over thin sheets of PMMA placed in the center of a wind tunnel in which a fully developed flow of oxygen/nitrogen mixture with prescribed average velocities can be created. The experiments are made compact by moving the fuel, spooled on two rollers, opposite to the direction of the spread to make the flame stationary. As a result, experimental parameters, especially flow velocity and fuel thickness, can be varied in a single experiment without intervention from an operator. By simply changing the location of the igniter the experiment can be re-configured for concurrent-flow spread. The proposed algorithm of moving the fuel, while keeping the flame stationary, has been validated for downward spread experiments at San Diego State University.

The two pairs of experiments, one each for opposed-flow and concurrent-flow configuration, conducted at two different oxygen levels, 50% and 70% by volume, will yield 80 spread rate data points with fuel thickness, fuel width and flow velocity as parameters. Because of the active control (instantaneous spread rate information is available to the control algorithm), the experiment can be automatically restarted in case of an unexpected extinction ensuring successful data acquisition.  

The proposed diagnostics is completely non-intrusive.  While infrared imaging with a rotating filter wheel in front of the camera lens, developed recently at NASA, will be employed to determine the extent of the thermal field, absorption imaging, under development at Gifu University, Japan, will be employed to size the species field. Imaging the species and the thermal field will help verify the theoretical prediction that radiative losses at low flow velocities causes the thermal field to shrink relative to the vitiating species field resulting in flame extinguishment for fuel below certain critical thickness. Although only three different thickness of PMMA will be used in the experiments, changes in the environmental parameters, especially flow velocity, will result in adequate variation in the thermal thickness of the fuel.

The proposal is directly related to basic understanding of flame spread, wind-opposed and wind-aided, important for materials selection leading to minimization of risk of ignition and fire hazard in space stations and space voyages.  Moreover, the rich data set produced by the proposed experiments will help verify existing spread rate formulas and fuel development of new formulas that bring together the two regimes of concurrent-flow and opposed-flow flame spread. Currently such data is available only for flow velocities greater than the velocity induced by buoyancy driven flow. As a result flame spread formulas are unavailable or untested at the low velocity range most relevant to microgravity environment. The proposed imaging diagnostics, builds upon earlier development of emission pyrometry by the first PI, and is expected to produce improved non-obtrusive measurement of line-of-sight-averaged temperature and concentration field. 

San Diego State is an urban university with a Joint Doctoral Program with UCSD. Several undergraduate students, exchange students from Gifu University, Japan, as well as Masters and Ph.D. students will be involved in this project.