Experimental study and heat transfer analysis of downward flame spread over PMMA under the effect of wall spacing

To study the effects of spacing on the downward flame spread over polymethyl methacrylate (PMMA), an experiment was conducted by pure PMMA (with 200 mm height, 50 mm width, and 1 mm thickness) with spacings of 7 mm, 10 mm, 13 mm, 16 mm, 19 mm, 22 mm, and 25 mm to observe the flame height, pyrolysis spread rate of fuels, and heat feedback from the wall. The heat feedback received by PMMA was used to analyze the influencing mechanism of wall spacing on flame spread. The results are as follows: (1) The average flame height decreases with the increase in distances (δ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\delta$$\end{document}). This decrease in average flame height cycles through two stages: a fast drop stage and a slow drop stage. (2) The average pyrolysis spread rate first increases with the increase in distance, and a maximum pyrolysis spread rate occurred in the 13 mm spacing scenario. Then, the average pyrolysis spread rate decreases monotonously when the distance between wall and sample exceeds 13 mm. (3) The heat flux received by the sample consists of both heat flux from the flame and heat feedback from the wall. With the increase in distance, the heat feedback from the wall follows a downward trend, while the heat flux from the flame first increases and then remains constant. Because of the effects of heat flux from flame and heat feedback from the wall, the heat flux received by the sample first increases and then decreases with the increase in distance.