Combustion and Flame 146 (2006) 302–311 www.elsevier.com/locate/combustﬂame Measurements of laminar burning velocities for natural gas–hydrogen–air mixtures Zuohua Huang a,b,∗ , Yong Zhang a , Ke Zeng a , Bing Liu a , Qian Wang a , Deming Jiang a a State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, People’s Republic of China b School of Energy and Power Engineering, Xi’an Jiaotong University, 710049 Xi’an, People’s Republic of China Received 9 August 2005; received in revised form 18 January 2006; accepted 15 March 2006 Available online 17 April 2006 Abstract Laminar ﬂame characteristics of natural gas–hydrogen–air ﬂames were studied in a constant-volume bomb at normal temperature and pressure. Laminar burning velocities and Markstein lengths were obtained at various ratios of hydrogen to natural gas (volume fraction from 0 to 100%) and equivalence ratios (φ from 0.6 to 1.4). The inﬂuence of stretch rate on ﬂame was also analyzed. The results show that, for lean mixture combustion, the ﬂame radius increases with time but the increasing rate decreases with ﬂame expansion for natural gas and for mixtures with low hydrogen fractions, while at high hydrogen fractions, there exists a linear correlation between ﬂame radius and time. For rich mixture combustion, the ﬂame radius shows a slowly increasing rate at early stages of ﬂame propagation and a quickly increasing rate at late stages of ﬂame propagation for natural gas and for mixtures with low hydrogen fractions, and there also exists a linear correlation between ﬂame radius and time for mixtures with high hydrogen fractions. Combustion at stoichiometric mixture demonstrates the linear relationship between ﬂame radius and time for natural gas–air, hydrogen–air, and natural gas–hydrogen–air ﬂames. Laminar burning velocities increase exponentially with the increase of hydrogen fraction in mixtures, while the Markstein length decreases and ﬂame instability increases with the increase of hydrogen fractions in mixture. For a ﬁxed hydrogen fraction, the Markstein number shows an increase and ﬂame stability increases with the increase of equivalence ratios. Based on the experimental data, a formula for calculating the laminar burning velocities of natural gas–hydrogen–air ﬂames is proposed.  2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Laminar burning velocity; Markstein length; Stretch; Constant-volume bomb 1. Introduction With increasing concern about the energy shortage and environmental protection, research on improving * Corresponding author. E-mail address: zhhuang@mail.xjtu.edu.cn (Z. Huang). engine fuel economy and reduction of exhaust emis- sions has become the major research aspect in the combustion community and engine development. Due to limited crude oil reserves, the development of al- ternative fuel engines has attracted more and more concern. Alternative fuels usually are the clean fu- els compared to diesel fuel and gasoline in the engine combustion process; thus the introduction of these al- 0010-2180/$ – see front matter  2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustﬂame.2006.03.003