Flashback Mechanisms in Lean Premixed Gas Turbine Combustion (eBook)

Ali Cemal Benim, Khawar Jamil Syed (Autoren)

eBook Download: PDF | EPUB

2014 | 1. Auflage
134 Seiten
Elsevier Science (Verlag)
978-0-12-800826-3 (ISBN)

Infos

Blending fuels with hydrogen offers the potential to reduce NOx and CO2 emissions in gas turbines, but doing so introduces potential new problems such as flashback.  Flashback can lead to thermal overload and destruction of hardware in the turbine engine, with potentially expensive consequences. The little research on flashback that is available is fragmented. Flashback Mechanisms in Lean Premixed Gas Turbine Combustion by Ali Cemal Benim will address not only the overall issue of the flashback phenomenon, but also the issue of fragmented and incomplete research.

• Presents a coherent review of flame flashback (a classic problem in premixed combustion) and its connection with the growing trend of popularity of more-efficient hydrogen-blend fuels
• Begins with a brief review of industrial gas turbine combustion technology
• Covers current environmental and economic motivations for replacing natural gas with hydrogen-blend fuels

Textauszug

Chapter 2

Concepts Related to Combustion and Flow in Premix Burners

Abstract

This chapter provides an overview of some fundamental concepts relating to the combustion and aerodynamics in premix gas turbine burners. First, the laminar flame speed is defined and the one-dimensional flame structure following the thermal theory is outlined. The role of species diffusion is discussed and the notion of “preferential diffusion” is introduced. The effect of flame stretch and flame curvature on laminar flame speed is outlined. Then, the structure of turbulent premixed flames is briefly discussed, identifying combustion regimes with different characteristics. Finally, the aerodynamics of swirl premix burners, where vortex breakdown is utilized for flame stabilization, is described.

Keywords

Flame stretch and curvature

laminar flame speed

preferential diffusion

swirl burner

turbulent flame speed

turbulent premixed combustion

vortex breakdown

2.1. Laminar premixed flames

2.1.1. The Laminar Flame Speed

The laminar flame speed is the speed at which a flame will propagate through a quiescent, homogeneous mixture of unburned reactants, under adiabatic conditions (Turns, 2012). The laminar flame speed for a planar, unstretched flame (L0) shall be dealt with first. Consider a one-dimensional, planar flame front within the laminar flow of a homogenous mixture, where the unburned reactants approach the flame front with the constant velocity u. Steady-state conditions, i.e., a spatially stationary flame front is obtained, when =SL0. In the following, as is common practice, a number of assumptions are made. It is assumed that the Mach number is low, and mechanical energies, the viscous dissipation, as well as the pressure difference across the flame front are negligible. Furthermore, it is assumed that the specific heat capacity, the thermal conductivity, and the diffusivity take constant values across the flame front (values corresponding to the unburned mixture are taken), the flame is thin (high activation energy), the Lewis number is unity (Le = 1), and the chemical kinetics is governed by a single-step irreversible reaction.

The initial theoretical analyses for the determination of the laminar flame speed date back to Mallant and Le Chatelier, who postulated that the combustion is sustained, i.e., the unburned mixture is continuously heated up to the “ignition temperature,” by the upstream propagation of heat through the layers of unburned gas (Glassmann and Yetter, 2008). Here, the flame is assumed to consist of two zones, i.e., a “preheat zone” and a “reaction zone,” as illustrated in Figure 2.1 for a one-dimensional flame.

Fig. 2.1 Schematic of a one-dimensional planar and unstretched flame front.

Expressions derived based on simplifying assumptions indicate that L0 strongly depends on TU and TB. Among the improvements of this theory, that postulated by Zeldovich and Frank-Kamanetskii has been most significant (Glassmann and Yetter, 2008), who considered the diffusion of molecules. In addition to heat diffusing from the reaction zone into the preheat zone, the reactants diffuse, in the opposite direction, from the preheat zone into the reaction zone, which is equally important for sustaining the combustion. The relative importance of both mechanisms is indicated by the Lewis number:

=αD

(2.1)

In the theories outlined earlier, a problematical quantity is the ignition temperature, which cannot easily be determined. There are less sophisticated approaches that still lead to some useful estimations. The assumption of a linear temperature profile across the whole flame front, along with the assumption of equal sizes for the preheat and reaction zones, implies an ignition temperature that is equal to the arithmetic average of the unburned and burned mixture temperatures. Equating the heat conducted from the reaction zone (which is now straightforward based on the assumed linear temperature profile and Fourier’s law) to the energy required to raise the temperature of the unburned gases (that flow toward the flame front) to the ignition temperature, and considering the equality of flow and flame speeds, for a stationary flame, the following relationship between the laminar flame speed and flame thickness is obtained (Turns, 2012):

L=αδPH=αδR=2αδL

(2.2)

The equality δPL = δR is, of course, a strong simplification. Nevertheless, the proportionality δ ∼ α/SL is an important correlation, which is quite often used as equality δ = α/SL for estimating orders of magnitudes.

An analysis of the energy balance assuming single-step kinetics enables the establishment of a proportionality between the unburned reactant temperature TU, burned product (adiabatic flame) temperature TB (which, in turns, depends on the fuel composition, the unburned mixture temperature, and the equivalence ratio), the pressure p, and the laminar flame speed SL (Turns, 2012), given as follows:

L∼TU+TB20.375⋅TU⋅TB−(n/2)⋅exp−EA2RTB⋅p(n−2)/2

(2.3)

As can be seen from the above equation, the temperature influence is governed by the exponential term. The pressure influence depends on the overall reaction order n. The overall reaction order can be determined experimentally or by numerical analysis based on detailed reaction mechanisms. Strictly speaking, the overall reaction order is not constant and depends on further variables such as equivalence ratio and pressure (Law, 2006). For single-step kinetics of methane, Westbrook and Dryer (1981) give n = 1, which results in SL ∼ p−0.5. This value, which may be considered to loosely correspond to the ranges provided in Law (2006), is widely assumed and generally used for methane (Turns, 2012). For higher hydrocarbons, n is generally assumed to take values smaller than 2 (Peters and Rogg, 1993), implying a general decrease of the laminar flame speed with pressure. For hydrogen–air flames, according to the analysis of Law (2006), n shows even stronger variations with pressure, where under lean conditions, for pressures lower than 50 atm, values lower than 1 are indicated.

Based on the flame thickness and speed, a chemical timescale τC can be defined shown in the following. This corresponds to a residence time in the flame zone, and can also be written in terms of ν, assuming Pr = 1, as follows:

C=δLSL=αSL2≍νSL2≍δL2ν

(2.4)

The previous expressions assume, among other things, a Lewis number of unity. Beyond the discussion of the mixture Lewis number being unity or not, the calculation of a meaningful effective Lewis number can further be complicated, if the individual unburned species exhibit considerably different mass diffusivities. This phenomenon is generally referred to as “preferential diffusion.” Hydrogen fuel blends typically exhibit this behavior. Since hydrogen possesses a comparably much larger mass diffusivity, it can diffuse much more rapidly to the flame zone, leading to a local hydrogen enrichment and shifting of the local mixture composition.

Beyond the analytical theories, a one-dimensional laminar premixed flame can be analyzed in detail by solving the complete set of transport equations numerically, using a chemical simulation tool such as CHEMKIN (www.reactiondesign.com) or Cantera (www.cantera.org), utilizing a detailed reaction mechanism such as GRI-Mech (www.me.berkeley.edu/gri-mech). This leads to an accurate prediction of the laminar flame speed, considering the species diffusion accurately, including Lewis number effects and the preferential diffusion. However, if (Eq. 2.4) is then used to estimate the chemical timescale, it should be recalled that the expression still assumes a Lewis number of unity.

2.1.2. Effect of Flame Curvature and Stretch on the Laminar Flame Speed

A planar and unstretched flame front is an idealization. In most real cases, the flame front is curved/wrinkled and stretched. This is especially true for turbulent flames, due to the action of turbulent eddies. Curvature and stretch can affect the flame speed. A stretching of the flame occurs if the points on the flame surface “glide” along it due to a tangential velocity component that exhibits a spatial variation. Thus, a Lagrangian flame surface area A spanned by a number of points is continuously deformed. The flame stretch rate is defined as...

Produkt-Details

• Erscheint lt. Verlag: 1.12.2014
• Sprache: englisch
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 0-12-800826-1 / 0128008261
• ISBN-13: 978-0-12-800826-3 / 9780128008263