The Burning of a Thermoplastic Material Under a Forced-Flow Boundary Layer Flame

Abstract

REPORT DOCUMENTATION PAGEForm Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY)July 31,2002 REPORT TYPE Memorandum Report TITLE AND SUBTITLEThe Burning of a Thermoplastic Material Under a Forced-Flow Boundary Layer Flame 6.AUTHOR(S)Chuka C. Ndubizu,* Ramagopal Ananth, and Patricia A. Tatem PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)Naval SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)Office of Naval Research 800 North Quincy Street Arlington, VA 22217-5000 SPONSOR / MONITOR'S ACRONYM(S) SPONSOR / MONITOR'S REPORT NUMBER(S) DISTRIBUTION /AVAILABILITY STATEMENTApproved for public release; distribution is unlimited. SUPPLEMENTARY NOTES *GEO-CENTERS, Inc., Lanham, MD ABSTRACTThis report presents experimental results of the characterization of forced flow boundary layer burning of poly(methyl methacrylate) (PMMA) without water mist. Small samples (7.5 x 9.5 x 2.5 cm) of black PMMA were burnt at the exit of a wind tunnel at free stream velocities between 60 and 168 cm/s, in tests lasting between 2.5 min and 20 minutes. The results of the gas phase temperature mapping indicate that a stable flame is formed over the sample within a short time after ignition.The local surface regression rate is transient, decreasing with time near the leading edge and increasing with time downstream. For 60 < U M < 168 cm/s, the time averaged regression rate in the leading section decreased by about a factor of two between 5 and 20-minute tests. The transient burning rate in the leading section is due to the presence of a valley formed in this section as a result of non-uniform surface regression. On the other hand, the transient regression rate in the trailing section seems to be due mainly to in-depth solid heat conduction. SUBJECT TERMS

Pyrolysis and Boundary Layer Combustion of a Non-Charring Solid Plate Under Forced Flow

“…The combustion experiments were conducted for different values of the bum time to record the transient effects. Experimental details are given elsewhere [16,17,[33][34][35] and only a brief description is given here for completeness. The geometry is similar to that shown in Figure 1.…”

“…Measurements, however, were made well within the smooth region and along the center line of the sample. Some of the experimental results are summarized below [16,17,[33][34][35]:…”

“…We will focus here mainly on the theoretical aspects of the problem and provide only a brief description of the experiments. Further details of the experiments can be found in a separate NRL memo report, NRL/MR/6180-02-8630 [17].…”

“…Therefore, the surface regresses non-uniformly and an initially flat surface of a real material, such as PMMA, develops curvature as the time progresses. Indeed, experiments [16,17] performed in our laboratory have recently shown that the surface curvature affects local heat, mass and momentum transport, which in turn can have significant effect on the local burning rates for PMMA. Furthermore, Hegab et al [18] have recently obtained self-consistent solutions to describe buming of a sandwich propellant exposed to quiescent air and showed significant moving boundary effects on the buming rates.…”

“…Furthermore, Hegab et al [18] have recently obtained self-consistent solutions to describe buming of a sandwich propellant exposed to quiescent air and showed significant moving boundary effects on the buming rates. Therefore, our second objective is to make quantitative comparisons of the steady-state NS solutions, and Emmons solutions with our experimental measurements of regression rate distributions [16,17] at different bum times (5 to 20 min) to study the fransient effects and the validity of the approximations (flat surface and steady surface-pyrolysis). The experiments were performed on non-spreading, forced-flow, boundary layer flames, which were formed quickly by exposing the entire surface of a PMMA plate (10 cm) to uniform radiant flux rather than allow co-current flame spread.…”