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Investigation of Hot Surface Ignition of a Flammable Mixture

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Investigation of Hot Surface Ignition of a Flammable Mixture Paper # 12S-39 Topic: Laminar Flames Western States Section of the Combustion Institute (WSSCI) Spring 2012 Meeting March 19-20, 2012, Arizona University, Tempe, AZ Investigation of hot surface ignition of a flammable mixture S.K. Menon1, P.A. Boettcher2, B.Ventura3, G. Blanquart1 and J.E. Shepherd2 1Department of Mechanical Engineering California Institute of Technology, Pasadena, CA, USA 2Graduate Aerospace Laboratories California Institute of Technology, Pasadena, CA, USA 3Daniel Guggenheim School of Aerospace Engineering Georgia Institute of Technology, Atlanta GA, USA Experimental and numerical studies are conducted to analyze hot-surface ignition of a flammable mix- ture. The experimental setup, equipped with temperature diagnostics and schlieren imaging, utilizes a glow plug to initiate ignition in a flammable mixture. The numerical simulation utilizes a tabulated chemistry approach to include detailed reaction kinetics for the fuel including low-temperature reaction pathways. The numerical approach is validated with a test case for a homogenous reactor and simula- tions of the experimental setup are carried out. Experimental data and simulation results are analyzed to determine minimum surface temperature for ignition and ignition delay time, and their dependence on fuel-air equivalence ratio and the thermal plume generated prior to ignition. A timescale analysis is conducted to understand the relative importance of convective, diffusive processes induced by the ther- mal plume, and chemical kinetics with and without the inclusion of low-temperature reaction pathways to yield the rate limiting processes leading to ignition. 1 Introduction The ignition of flammable mixtures under low-pressure, low-temperature conditions is relevant to safety considerations in various situations. In devices such as aircraft and automobile fuel tanks, gas pipelines, chemical reactors, and structures such as underground mines, it is critical to understand the underlying physics leading to hot surface ignition of a flammable mixture [1, 2]. The design considerations in these applications hinges on the potential for a disastrous fire or explosion caused by hot surface ignition [3]. The standard technique used to characterize the ignition hazard of a flammable mixture, in the absence of a spark or a flame, is using the autoignition temperature (AIT). AIT is defined as the maximum acceptable surface temperature in a particular area to prevent fire and explosion hazards [4]. Standard testing methods for AIT, however, utilize a sufficiently large vessel where the size of the hot surface and hence the rate of heat input is unimportant [5]. This makes understanding the hot surface ignition process and characterizing ignition criteria a separate study in itself. Considerable amount of experimental work has been done to investigate hot surface ignition of hydrogen-air mixtures. Studies have been conducted using a cylindrical wire [6] or rod [7], placed in a combustion bomb and heated till the mixture ignites. Other experiments have used hot metal spheres to initiate ignition by dropping them into a flammable mixture [8]. Hot surface ignition of hydrocarbon fuels have been investigated, in the presence of natural convection, by Rae [9] and in 1 WSSCI 2012 Meeting – Paper # 12S-39 Topic: Laminar Flames the presence of forced convection, by Kuchta [5] and Mullen [10]. A common observation is that ignition temperatures decrease with increasing surface area of the heat source. In situations where ignition occurs after the formation of a buoyant thermal plume, additional complications arise due to temperature stratification. Simple models of hot surface ignition have been proposed to correlate experimental data. One of these suggested by Laurendeau [11] assumes that ignition occurs when the rate of heat loss from the surface equals the rate of heat gain due to chemical reactions. This criterion, known as the Van’t Hoff criterion, has been used in previous work by Buckel [8] and Law [12] . Inspite of several simplifying assumptions and a steady state approximation, good agreement was found with regard to hot surface temperature required for ignition. While results appear encouraging, the transient nature of the ignition process motivates the use of a multi-dimensional fluid dynamics code with detailed finite rate chemistry to capture the complete physics. Recent development of flamelet based models utilizing tabulated chemistry allow multi-dimensional simulations for complex hydrocarbon fuels with detailed chemistry to be carried out at consider- ably lower cost and computation time while retaining all the necessary physics of the problem. These advantages have motivated the use of flamelet based models to study a variety of combus- tion problems [13, 14]. The applicability of tabulated approaches to model the thermal ignition process is explored in this work. The objective of this work is to gain insight into the hot surface ignition process by combining experimental data with numerical simulations. More specifically, the focus of this paper is placed on identifying the rate controlling processes that determine the onset of ignition. Additionally, the simulations will be utilized to study the effect of global parameters such as hot surface temperature and fuel-air equivalence ratio on the onset of ignition. The paper is organized as follows. First, the experimental setup is discussed, and the results are compared with predictions from an analytical model. Next, the reaction chemistry and its tabula- tion are discussed. Two test cases, one for a zero-dimensional homogeneous reactor and another for a one-dimensional hot-surface ignition problem are presented to validate the computational ap- proach. The setup of the two-dimensional simulations of the experiment is discussed. Simulation results are presented and the effects of hot surface temperature and mixture equivalence ratio are analyzed. Finally, conclusions are presented and discussed. 2 Experiment 2.1 Setup Figure 1(a) shows the layout and Fig. 1(b) shows a photograph of the closed 2 liter combustion vessel utilized in the experiment. The hot surface is simulated by a high temperature glow plug made by Bosch (part number 978801-0485). The glow plug is mounted on an aluminum stagnation plate giving a reference plane to facilitate comparison with the computational results. The vessel incorporates four access ports for schlieren visualization, and is instrumented with a fast response pressure transducer at the top, and four K-type thermocouples in contact with the glow plug for measurement of surface temperature at ignition. Figure 2(a) shows the geometry of the glow plug and Fig. 2(b) shows temperature traces as a function of time at the four locations on the glow plug 2 WSSCI 2012 Meeting – Paper # 12S-39 Topic: Laminar Flames (a) Schematic of the setup (b) Photograph of the setup Figure 1: Details of the experimental apparatus. surface. The temperature of the glow plug is controlled by a power source which steadily increases the current passing through the glow plug to give a constant heating rate of about 220 K/s. From these traces, it may be observed that the temperature around the glow plug is fairly constant (within a few percent) and only decreases at the base close to the stagnation plate. The vessel is first evacuated and then filled to atmospheric pressure with a mixture of hexane, oxygen, and nitrogen, depending on the desired equivalence ratio, using the method of partial pressures. A circulation pump runs for about 2 minutes to produce a homogeneous mixture. This mixture is allowed to rest for another 2 minutes before heating up the hot surface. Hexane is utilized as the fuel in the experiments due to its strong similarity to aviation and indus- trial hydrocarbon fuel. Additionally it is simple to handle experimentally. The hexane sample con- sists of 89% n-hexane and 10.91% other hexane isomers according to manufacturer specifications. Mixtures of hexane and air at equivalence ratios between 0.56 and 3.0 having fuel concentrations between 1.2% and 6.48% [15] respectively were investigated in the experiments. Since the hexane vapor pressure of 15.6 kPa is higher than the partial pressure of hexane used in the experiments (2.5 kPa) at room temperature, none of the fuel is expected to condense [16]. 2.2 Results Figure 3 shows a schlieren image obtained using a vertical knife edge for a fuel-rich (φ = 1:74) hexane-air mixture just after ignition. A plume of hot gas is generated by natural convection and an ignition kernel is seen in contact with the tip of the glow plug. Results from multiple tests show that ignition always occurs after the formation of a thermal plume. Furthermore, ignition is typically seen to occur at the top of the glow plug. Data recorded by the pressure transducer show negligible change in pressure during the ignition process as only small pockets of gas ignite at a time. 3 WSSCI 2012 Meeting – Paper # 12S-39 Topic: Laminar Flames (a) Glow plug dimensions (b) Transient temperature profiles Figure 2: Glow plug details. Figure 3: Schlieren photograph of the vessel just after ignition for φ = 1:74. 4 WSSCI 2012 Meeting – Paper # 12S-39 Topic: Laminar Flames Figure 4: Hot surface temperature at ignition from experiments and correlation using the Lauren- deau [11] ignition criterion. Figure 4 shows thermocouple measurements of surface temperature at ignition for hexane-air mix- tures with equivalence ratios between 0.75 and 3.0. The lower flammability limit is observed for φ = 0.6 with the mixture at φ = 0.5 not igniting after heating for 30 seconds and the glow plug surface reaching a temperature of 1520 K. This observation is consistent with previous work by Zabetakis [15]. Figure 4 shows the surface temperature for ignition to be about 920 +/- 20K across the range of equivalence ratios. The response time of the thermocouple is 0.5 seconds, which is found by measuring its response to a propagating flame. Since the heating rate is about 220 K/s, the time response of the thermocouple introduces an ignition temperature uncertainty of about + 110K.
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