Transport phenomena in combustion
From Thermal-FluidsPedia
Combustion is an exothermic chemical reaction process between fuel and oxidant. If combustion involves a liquid fuel, the liquid fuel does not actually burn as a liquid; it is vaporized first and diffuses away from the liquid-vapor surface. Meanwhile, the gaseous oxidant diffuses toward the liquid-vapor interface. Under the right conditions, the mass fluxes of vapor fuel and gaseous oxidant meet and the chemical reaction occurs at a certain location known as the flame (Lock, 1994; Avedisian, 1997, 2000). The flame is usually a very thin region with a color dictated by the temperature of the combustion.
The temperature and mass concentration distributions during a combustion process can be represented as shown in Fig. 1. The initial temperature of a liquid fuel is T_{0}, and the temperature of the liquid–vapor interface, T_{I}, is at the dew point temperature of the fuel. The temperature reaches a maximum at the location of the flame, and decreases with increasing x until it reaches T_{∞}. The mass fraction of fuel, ω_{f}, is maximal at the liquid-vapor interface and decreases as location of the flame is approached. The mass fraction of the oxidant, ω_{o}, on the other hand, is maximal at infinity and decreases as location of the flame is approached. The mass fractions of both vapor fuel and oxidant reach their minimum values at the location of the flame. The mass fractions of the products of combustion, ω_{p}, are at their maximum at the location of the flame and decrease as x either increases or decreases.
The transport phenomena involved in combustion include heat and mass transfer in both the liquid fuel and the gaseous mixture. The governing equations for the gaseous phase will be discussed below. Firstly, the gaseous mixture must satisfy the continuity equation, i.e.,
and the gas flow is governed by the momentum equation,
where the shear stress can be obtained from eq. from Momentum equation and gravity is the only body force considered.
The energy equation for combustion is
which is a simplified version of eq. (20), with internal heat generation and viscous dissipation neglected. The effect of the substantial derivative of pressure on the energy is also neglected. The specific enthalpy for the mixture is related to the specific enthalpy of each component in the mixture by
It is a common practice in thermodynamic and heat transfer analyses to consider the change in enthalpy during a chemical reaction process rather than the absolute values of enthalpy. For a process that does not involve chemical reaction, we can choose any reference state for an individual substance and define the enthalpy at that reference state as zero. For example, the reference state for water is often chosen at the triple point, while the reference state for an ideal gas is often chosen as zero K. However, when chemical reaction is involved in a process, as is the case with combustion, the composition of the system changes during the chemical reaction; therefore, the reference states for all reactants and products must be the same.
One convenient option in such a situation is to segregate the enthalpy of any component into two parts: (1) the enthalpy due to its chemical composition at the standard reference state (at 25 ˚C and 1 atm), and (2) the sensible enthalpy due to any temperature deviation from the standard reference state. Therefore, the enthalpy for the i^{th} component in the mixture can be expressed as
where is the enthalpy of formation for the i^{th} component, i.e., the enthalpy due to its chemical composition at the standard reference state. The enthalpy of formation for selected substances is shown in Table 1.
The sensible enthalpy, , is related to temperature by
Substituting eqs. (5) and (6) into eq. (4), the enthalpy of the mixture becomes
where c_{p} is the average specific heat of the mixture, defined as
Substituting eq. (7) into eq. (3), the energy equation for combustion becomes
Table 1 Enthalpy of formation ho for selected substances at 25 ˚C and 1 atm
Substance | Formula (phase) | Enthalpy of formation (kJ/kmol) |
Acetylene | C_{2}H_{2} (g) | 226,730 |
Benzene | C_{6}H_{6} (g) | 82,930 |
Carbon | C (s) | 0 |
Carbon monoxide | CO (g) | -110,530 |
Carbon dioxide | CO_{2} (g) | -393,520 |
Ethyl alcohol | C_{2}H_{5}OH (g) | -235,310 |
Ethyl alcohol | C_{2}H_{5}OH () | -277,690 |
Ethylene | C_{2}H_{4} (g) | 52,280 |
Ethane | C_{2}H_{6} (g) | -84,680 |
Hydrogen | H_{2} (g) | 0 |
Methane | CH_{4} (g) | -74,850 |
Nitrogen | N_{2} (g) | 0 |
Nitrogen | N (g) | 472,650 |
n-Dodecane | C_{12}H_{26} () | -291,010 |
n-Octane | C_{8}H_{18} (g) | -208,450 |
n-Octane | C_{8}H_{18} () | -249,950 |
Oxygen | O_{2} (g) | 0 |
Propane | C_{3}H_{8} (g) | -103,850 |
Water | H_{2}O () | -241,820 |
Water vapor | H_{2}O (g) | -285,830 |
If the fuel is consumed at a rate of per unit volume, h_{c} – the heat of combustion – is defined by
Substituting eq. (5) into eq. (10) gives
The contribution of the second term on the right-hand side of eq. (11) is negligible, since . Dropping the second term from the right-hand side of eq. (11) and substituting the result into eq. (4), the energy equation becomes
The mass fraction of each component (fuel, oxidant, and product) is dominated by
where the subscript i can be f (fuel), o (oxidant), or p (product). The ratio of the rates of oxygen and fuel consumption is defined as the oxygen/fuel ratio:
The above analysis applies to combustion occurring on a planar surface. For many applications, combustion of the liquid fuel is usually preceded by breaking up a fuel jet into liquid droplets so that combustion occurs around a spherical liquid droplet. Combustion of a falling liquid droplet, as shown in Fig. 2.14, is analyzed here. The liquid fuel droplet vaporizes at the dew point, T_{d}, which is the saturation temperature corresponding to the partial pressure of the fuel vapor in the mixture near the liquid-vapor interface. To simplify the analysis, it is assumed that the temperature in the liquid fuel droplet is uniformly equal to the saturation temperature of the fuel at the total system pressure, i.e., the mass fraction of the fuel at the liquid-vapor interface equals one. It is further assumed that the shapes of both the liquid fuel droplet and the flame are spherical, which allows for application of a one-dimensional symmetric model (Lock, 1994), which is presented here.
If the combustion process is assumed to be in a quasisteady state (neglecting the transient term in the governing equation), the energy equation for combustion becomes
where u is the velocity of the gaseous mixture in the radial direction.
The mass fraction of the oxidant can be obtained by simplifying eq. (13), i.e.,
where D_{o} is the mass diffusivity of the oxidant in the gaseous mixture. Multiplying eq. (16) by h_{c} / γ and combining the resulting equation with eq. (15) yields
where eq. (14) was used to eliminate the rates of fuel and oxygen consumption.
Assuming that the Lewis number [Le = k / (ρc_{p}D_{o})] equals one and the specific heat is constant, eq. (17) can be simplified as follows:
where
is a modified temperature in the combustion process.
Equation (18) can be rearranged to the following form:
The continuity equation requires that
Since it has been assumed that the mass fraction of the fuel in the mixture at the liquid-vapor interface equals one, the mass flow rate reflects the mass flow rate of the fuel. The mass flux is often used in combustion analysis, and it is defined as
Equations (20) and (21) can be rewritten in terms of mass flux, i.e.,
Integrating eq. (23) from the liquid fuel droplet surface (r = r_{I}) to an arbitrary radius (r > r_{I}) yields
Substituting eq. (24) into eq. (25), one obtains
The mass flow rate at the surface of the liquid fuel droplet, , is the same as the fuel burning rate, , because the mass fraction at the surface of the droplet is one. Introducing the excess modified temperature,
eq. (26) becomes
Introducing a new dependent variable,
eq. (28) becomes
Integrating eq. (30) over the interval of , one obtains
The fuel burning rate, G_{f}, in eq. (31) is related to the heat transfer at the liquid droplet surface as follows
Substituting eq. (32) into eq. (29), the new dependent variable becomes
Substituting eq. (33) into eq. (31) and letting r = r_{I}, an equation for the fuel burning rate is obtained:
Equation (34) can also be used to determine the transient liquid fuel droplet size, because the fuel burning rate is related to the size of the droplet by
where ρ_{f} is the density of the liquid fuel.
Substituting eq. (35) into eq. (34), one obtains
Integrating eq. (35) and considering the initial condition r_{I} = r_{i} at t = 0, the liquid droplet radius becomes
Equation (37) can be used to estimate the time needed to completely burn the liquid fuel droplet:
References
Avedisian, C.T., 1997, “Soot Formation in Spherically Symmetric Droplet Combustion,” Physical and Chemical Aspects of Combustion, edited by Irvin Glassman, I., Dryer, F.L., and Sawyer, R. F., pp. 135-160, Gordon and Breach Publishers.
Avedisian, C.T., 2000, “Recent Advances in Soot Formation from Spherical Droplet Flames at Atmospheric Pressure,” Journal of Propulsation and Power, Vol. 16, pp. 628-656.
Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA
Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.
Lock, G.S.H., 1994, Latent Heat Transfer, Oxford Science Publications, Oxford University, Oxford, UK.