Dynamic behavior and flame acceleration of outwardly expanding flame in hydrogen-air mixture

• Published in: Kikai Gakkai ronbunshū = Transactions of the Japan Society of Mechanical Engineers Vol. 82; no. 835; p. 15-00612
• Main Authors: KATSUMI, Toshiyuki, KOBAYASHI, Hironori, AIDA, Takuro, KADOWAKI, Satoshi
• Format: Journal Article
• Language: English; Japanese
• Published: The Japan Society of Mechanical Engineers 2016
• Subjects: Cellular flame; Dynamic behavior; Flame acceleration; Flame propagation velocity; Flame radius; Flame stretch; Hydrogen; Intrinsic instability; Markstein length; Modeling; Risk assessment
• ISSN: 2187-9761; 2187-9761
• DOI: 10.1299/transjsme.15-00612

From the perspective of safety, it is necessary to assess the risk of hydrogen-air deflagration accurately. Especially, flame propagation velocity is one of the most important factors. Propagation velocity of outwardly expanding flame has been estimated from burning velocity of a flat flame considering influence of thermal expansion at a flame front; however, this conventional method is not enough to estimate an actual propagation velocity because flame propagation is accelerated owing to cellular flame front caused by intrinsic instability in hydrogen-air deflagration. Therefore, the dynamic propagation characteristics of hydrogen-air deflagration need to be understood. We performed explosion tests in a closed chamber which has 300mm diameter windows at atmospheric pressure and room temperature and in the range of equivalence ratio from 0.2 to 1.0. In the explosion tests, dynamic behaviors of flame propagation were observed by using high speed Schlieren photography. Analyzing the obtained Schlieren images, flame radius and flame propagation velocity were measured. As the result, cellular flame fronts formed and flame propagations of hydrogen-air mixture were accelerated in the range of equivalence ratio from 0.3 to 1.0. At the equivalence ratio of 0.2, a flame floated up and could not propagate downward because the influence of buoyancy exceeded a laminar burning velocity. In the range of smooth flame at small radius, considering the correlation of flame propagation velocity with flame stretch, flame propagation velocity of flat flame and Markstein length were obtained and critical flame radius was found. In the range of cellular flame at large radius, flame propagation velocity was fitted well by using a regression equation which including a logarithm of flame radius and characteristics of flame acceleration was discussed based upon the regression equation. Based upon these considerations, we propose this regression equation as simple model of flame propagation velocity.