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ABSTRACT
Since two-three decades, hydrogen has been identified as a versatile potential fuel competent to the conventional fuel such as gasoline. In order to implement it fully and to develop the combustion based power devices which may supply much higher energy density than others, the understanding of hydrogen air combustion mechanism is very essential. In this seminars, Computational Fluid Dynamics (CFD) based numerical simulations have been performed to study the combustion of non-premixed hydrogen-air mixture with different equivalence ratios and different mass flow rates and its effect on different species formation, peak temperature & NOx formation. The performance of the combustor is evaluated by using CFD package FLUENT under adiabatic wall condition. Here we used generalized finite rate chemistry model to analyze the hydrogen-air combustion system. The combustion is modeled using a global one step reaction mechanism, assuming complete conversion of fuel to H2O.Through such a systematic analysis, a proper controlled operation condition for the combustor is suggested which may be used as a guideline for combustor design. Results reported in this work illustrate that the CFD simulation can be one of the most powerful, beneficial & economical tool for combustor design, optimization & performance analysis.
CHAPTER 1
INTRODUCTION
Over the past three decades there has been considerable effort in the world to develop and introduce alternative transportation fuels to replace conventional fuels such as gasoline and diesel, environmental issues, most notably air pollution and limited availability of conventional fuels are among the principle driving forces behind this movement.
If one tries to find for the definition of perfect fuel, hydrogen probably satisfies most of the desirable characteristics of such a fuel. Plentiful and clean burning, hydrogen has very high energy content.
Due to difficulties in conducting spatially resolved measurements of combustion characteristics in devices, the numerical simulation can be cost effective approach to study the combustion mechanism.
In this work, Computational Fluid Dynamics (CFD) based numerical simulations have been performed to study the combustion of non-premixed hydrogen-air mixture in cylindrical chamber.
The performance of the combustor is evaluated by using CFD package FLUENT 6.0 under adiabatic wall condition at various equivalence ratios and mass flow rates of hydrogen & air.
1.1 Objectives and Scope
The objective of this work is to study the fundamentals of Computational Fluid Dynamics (CFD), Numerical modeling, combustion phenomenon and various aspects in order to use them for solving the realistic problems. The objectives of this research effort are:
¢ The understanding of the basics of Hydrogen-oxygen reaction mechanism, its combustion and the geometry of the cylindrical chamber used in this study is very important for simulating hydrogen-air combustion system.
¢ To develop a two dimensional numerical mesh and flow model which adequately and accurately represent the physical model of combustion chamber and is simple enough to limit the amount of computational time for obtaining a solution.
¢ To prepare a mathematical model for hydrogen-air combustion system.
¢ The objective of this study is to find and apply appropriate model that improve the simulation of combustion with the commercial CFD-package FLUENT.
¢ Generate numerical data/solutions which correlate as much as possible with the experimental data for various conditions including equivalence ratios, mass flow rates of hydrogen-air mixture.
The main objective of the research presented in this seminars, is the development of a numerical infrastructure for the multidimensional numerical simulation of combustion processes with the maximum level of accuracy.
CHAPTER 2
HYDROGEN AS A FUEL
Figure 1: Periodic Table
Hydrogen is a colorless, odorless, tasteless, and nonpoisonous gas under normal conditions on Earth. It typically exists as a diatomic molecule. Hydrogen is the most abundant element in the universe, accounting for 90 percent of the universe by weight. However, it is not commonly found in its pure form.
2.1 Properties of Hydrogen as a fuel
Hydrogen has several important chemical properties that affect its use as a fuel:
¢ It readily combines with oxygen to form water, which is absolutely necessary for life on this planet.
¢ It has a high-energy content per weight (nearly 3 times as much as gasoline), but the energy density per volume is quite low at standard temperature and pressure. Volumetric energy density can be increased by storing the hydrogen under increased pressure or storing it at extremely low temperatures as a liquid. Hydrogen can also be adsorbed into metal hydrides.
¢ Hydrogen is highly flammable; it only takes a small amount of energy to ignite it and make it burn. It also has a wide flammability range, meaning it can burn when it makes up 4 to 74 percent of the air by volume.
¢ Hydrogen burns with a pale-blue, almost-invisible flame, making hydrogen fires difficult to see.
¢ The combustion of hydrogen does not produce carbon dioxide (CO2), particulate, or sulfur emissions. It can produce nitrous oxide (NOX) emissions under some conditions.
¢ Hydrogen can be produced from renewable resources, such as by reforming ethanol (this process emits some carbon dioxide) and by the electrolysis of water (electrolysis is very expensive).
¢ Energy Content for 1 kg (2.2 lb) of Hydrogen = 424 Standard Cubic Feet (Reacting with oxygen to form water).
Higher Heating Value Lower Heating Value
141,600 KJ 119,600 KJ
Table 1: Heating Values of H2
Properties of Hydrogen as a fuel
The properties of hydrogen are listed in table 2. along with conventional fuels i.e.Gasoline & Diesel and other alternative fuels such as CNG, LPG, and Biogas.
Properties
CNG
Hydrogen
Gasoline
LPG
Biogas
Lower Heating Value (KJ/Kg)
50000
12000
42000
46000
5000
Density (Kg/m3)
0.69
0.09
720-750
2.24
1.1
Flame Speed (cm/sec)
34.0
265-325
38.25
25.0
Stoichiometric A/F (Kg/Kg)
17.3
34.3
14.6
15.5
6
Flammability limit (% vol of air)
5.3-15
4-75
1.4-7.6
2.15-9.6
7.5-14
Octane No.
130
130+
86-94
103-105
120
Auto Ignition Temp.(0C)
730
585
222
428
700
Latent Heat of Vaporizations (KJ/m3)
509
375
428
493-549
Molecular Weight
18.88-17.05
100
55-60
Specific Gravity
0.424
0.07
0.72-0.78
Boiling Temp. (F)
-259
-423
80-437
Table 2: Properties of fuels
2.1.1 Limits of Flammability
The limits of flammability are one of the most important properties of a fuel. These parameters are a measure of the range of the fuel/air ratios over which an engine can operate. Hydrogen has wide range of flammability in comparison with other fuels. One of the significant advantages is that hydrogen engine can run on a lean mixture.When engine is run on slightly lean mixtures fuel economy is greater and the combustion reaction is more complete. Additionally, the final combustion temperature can be lowered by using ultra-lean mixtures, reducing the amount of NOx emissions.
2.1.2 Minimum Ignition Energy
The minimum energy required for ignition for hydrogen is about an order of magnitude less than that required for gasoline. This enables hydrogen engines to run well on lean mixtures and ensures prompt ignition. Unfortunately, since very little energy is necessary to ignite a hydrogen combustion reaction, and almost any hydrogen/air mixture can be ignited due to wide limits of flammability of hydrogen, hot gases and hot spots on the cylinder can serve as sources of ignition, creating problems of premature ignition and flashback.
2.1.3 Quenching Gap or Distance
In the combustion chamber, the combustion flame is typically extinguished at certain distance from the cylinder wall due to heat losses called as quenching distance. For hydrogen, the quenching distance is less than that of gasoline, so that flame comes closer to the wall before it is extinguished. Thus it is more difficult to quench a hydrogen flame than a gasoline flame.
2.1.4 Self Ignition Temperature
The self ignition temperature is the temperature that a combustible mixture must reach before it will be ignited without an external source of energy. For hydrogen, the self ignition temperature is relatively high. The high self-ignition temperature of hydrogen allows larger compression ratios to be used in hydrogen engine without increasing the final combustion temperature beyond the self ignition temperature and causing premature ignition. The hydrogen is difficult to ignite in a compression ignition or diesel configuration, because the temperatures needed for this type of ignition relatively high.
2.1.5 Flame Speed
The flame speed of hydrogen is nearly an order of magnitude higher than that of gasoline. For stoichiometric mixtures, hydrogen engines can more closely approach the thermodynamically ideal engine cycle. At leaner mixtures, the flame velocity decreases significantly.
2.1.6 Diffusivity
Hydrogen diffusivity, or its ability to disperse in air, is considerably greater than that of gasoline.The high diffusivity is advantageous for two main reasons. First, it facilitates the formation of uniform mixture of fuel and air. Secondly, if a hydrogen leak does develop, the hydrogen will disperse rapidly. Thus unsafe conditions can either be avoided or minimized.
2.1.7 Density
Hydrogen has extremely low density. This creates two problems: (1) a very large volume is necessary to store enough hydrogen to give a vehicle an adequate driving range, (2) the energy density of hydrogen air charge and hence the power output is reduced.
2.1.8 Flame characteristics
Hydrogen flames are very pale blue and are almost invisible in daylight due to the absence of soot. Visibility is enhanced by the presence of moisture or impurities (such as sulfur) in the air. Hydrogen flames are readily visible in the dark or subdued light.
Figure 2: Invisible Hydrogen Flame Igniting Broom
Hydrogen flames are almost invisible in daylight.Hydrogen fires can only exist in the region of a leak where pure hydrogen mixes with air at sufficient concentrations. For turbulent leaks, air reaches the centerline of the leakage jet within about five diameters of a leakage hole, and the hydrogen is diluted to nearly the composition of air within roughly 500 to 1000 diameters. This rapid dilution implies that if the turbulent leak were into open air, the flammability zone would exist relatively close to the leak. Therefore, when the jet is ignited, the flame length is less than 500 diameters from the hole (for example, for a 0.039 in/1 mm diameter leak, the flame length will be less than 19.7 in/0.5 m).
In many respects, hydrogen fires are safer than gasoline fires. Hydrogen gas rises quickly due to its high buoyancy and diffusivity. Consequently hydrogen fires are vertical and highly localized. When a car hydrogen cylinder ruptures and is ignited, the fire burns away from the car and the interior typically does not get very hot. Gasoline forms a pool, spreads laterally, and the vapors form a lingering cloud, so that gasoline fires are broad and en-compass a wide area.
Figure 3: Hydrogen Flame from Ruptured Fuel Cylinder
2.2 Benefits of Hydrogen Economy
Widespread use of hydrogen as an energy source in this country could help address concerns about energy security, global climate change, and air quality. Fuel cells are an important enabling technology for the Hydrogen Future and have the potential to revolutionize the way we power our nation, offering cleaner, more-efficient alternatives to the combustion of gasoline and other fossil fuels. These benefits are:-
¢ Strengthen National Energy Security
¢ Reduce Greenhouse Gas Emissions
¢ Reduce Air Pollution
¢ Improve Energy Efficiency
2.3 Hydrogen Storage and Delivery
In engine applications the storage and portability of adequate mass of hydrogen for practical applications remain one of the most difficult problems yet to be overcome. Hydrogen can be stored as a compressed gas in suitably designed high-pressure vessels. However, the very low density of hydrogen in comparison to other gaseous fuels, dictates that extremely high-pressure cylinders that are sufficiently light in weight and compact in volume need to be devised and used. The compression of the gas to such high pressures requires the expenditure of much expensive compression work and the provision of the necessary infra structure. Also, these hydrogen gas cylinders would add significantly to the total weight, cost and bulkiness of the fuel installation.
Hydrogen also can be carried on board vehicles and engine installations in the form of various metallic hydrides that would permit the controlled release of hydrogen through the supply of heat, often from the engine exhaust gas or its cooling water. These methods are of limited usefulness as they add much cost and weight while reducing the flexibility of the fuel system and contributing to an increase in undesirable emissions. The carrying of hydrogen as a cryogenic liquid has its serious limitations also. The work and infrastructure required to liquefy hydrogen are much too expensive and energy intensive to become widely usable. The energy consumed in the liquefaction process can be up to around 30% of the heating value of the hydrogen. Also, the cryogenic tanks needed to carry the liquid hydrogen, despite the very substantial progress made in recent years in their design, safety and manufacture, remain relatively expensive and bulky.
2.3.1 Hydrogen Delivery Methods
The hydrogen currently in the marketplace for industrial use is transported as a gas at low (100-300 psig) or high (3000-5000 psig) pressure or as a cryogenic liquid via gas pipelines, gas or cryogenic liquid trucks, tube trailers, barge, or rail cars. At high volumes, hydrogen delivery by pipeline is currently the lowest cost option. Liquefaction is often cost-effective in situations where lower volumes are needed.
¢ Compressed Gas and Cryogenic Liquid Storage
Hydrogen can be physically stored as either a gas or a liquid. Storage as a gas typically requires high-pressure tanks (5000-10,000 psi tank pressure). Storage of hydrogen as a liquid requires cryogenic temperatures, since the boiling point of hydrogen at one atmosphere pressure is -252.80C.
¢ Materials-based Hydrogen Storage
Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption). In adsorption, hydrogen is attached to the surface of a material either as hydrogen molecules or as hydrogen atoms. In absorption, hydrogen is dissociated into H-atoms and then the hydrogen atoms are incorporated into the solid lattice frame work. Hydrogen storage in solids may make it possible to store larger quantities of hydrogen in smaller volumes at low pressure and at temperatures close to room temperature. It is also possible to achieve volumetric storage densities greater than liquid hydrogen because the hydrogen molecule is dissociated into atomic hydrogen within the metal hydride lattice structure.
¢ Current Technology
Current on-board hydrogen storage approaches involve compressed hydrogen gas tanks, liquid hydrogen tanks, metal hydrides, carbon-based materials/high surface area sorbents, and chemical hydrogen storage. Storage as a gas or liquid or storage in metal hydrides or high surface area sorbents constitute reversible on-board hydrogen storage systems, since hydrogen regeneration or refill can take place on-board the vehicle. For chemical hydrogen storage approaches (such as a chemical reaction on board the vehicle to produce hydrogen), hydrogen regeneration is not possible on-board the vehicle and thus these spent materials must be removed from the vehicle and regenerated off board.
CHAPTER 3
COMBUSTION
3.1 Combustion Phenomena
Combustion is a key element of many of modern society's critical technologies. Combustion accounts for approximately 85 percent of the world's energy usage and is vital to our current way of life. Spacecraft and aircraft propulsion, electric power production, home heating, ground transportation, and materials processing all use combustion to convert chemical energy to thermal energy or propulsive force.
Examples of combustion applications:
¢ Gas turbines and jet engines
¢ Rocket propulsion
¢ Piston engines
¢ Guns and explosives
¢ Furnaces and boilers
¢ Flame synthesis of materials (fullerenes, nano-materials)
¢ Chemical processing (e.g. carbon black production)
¢ Forming of materials
¢ Fire hazards and safety
Combustion is a complex interaction of physical (fluid dynamics, heat and mass transfer), and chemical processes (thermodynamics, and chemical kinetics). Practical applications of the combustion phenomena also involve applied sciences such as aerodynamics, fuel technology, and mechanical engineering.The transport of energy, mass, and momentum are the physical processes involved in combustion. The conduction of thermal energy, the diffusion of chemical species, and the flow of gases all follow from the release of chemical energy in the exothermic reaction. The subject areas most relevant to combustion in the fields of thermodynamics, transport phenomena, and chemical kinetics can be summarized as follows:
Thermodynamics:
¢ Stoichiometry
¢ Properties of gases and gas mixtures
¢ Heat of formation
¢ Heat of reaction
¢ Equilibrium
¢ Adiabatic flame temperature
Heat and Mass Transfer:
¢ Heat transfer by conduction
¢ Heat transfer by convection
¢ Heat transfer by radiation
¢ Mass transfer
Fluid Dynamics:
¢ Laminar flows
¢ Turbulence
¢ Effects of inertia and viscosity
¢ Combustion aerodynamics
Chemical Kinetics:
Application of thermodynamics to a reacting system gives us the equilibrium composition of the combustion products and maximum temperature corresponding to this composition, i.e. the adiabatic flame temperature. However, thermodynamics alone is not capable of telling us whether a reactive system will reach equilibrium. If the time scales of chemical reactions involved in a combustion process are comparable to the time scales of physical processes (e.g. diffusion, fluid flow) taking place simultaneously; the system may never reach equilibrium. Then, we need the rate of chemical reactions involved in combustion. Combustion processes can be sub-divided based on mixing as premixed, non- premixed and partially premixed. Combustion in homogeneous-charge spark-ignition engines and lean burn turbines is under premixed conditions. Contrastingly, combustion in Diesel engines or industrial furnaces is under non-premixed conditions. In the nonpremixed cases, fuel is injected into the combustion chamber along with air, where it is ignited due to pre-existing hot gases or auto-ignites due to high temperatures second criterion for subdividing the turbulent combustion relates to the ratio of turbulence to chemical reaction time scales. Above a certain cross-over temperature, hydrocarbon oxidation occurs by chain-branching. Chain-branching ceases when the temperature falls below this limit, thus causing extinction of flame. This crossover temperature increases with pressure. While the fast chemical processes can be simulated using equilibrium approach, slow chemical reactions require being modeled using kinetic expressions. Presence of slow and very fast reactions in the same reaction mechanism can pose problems in numerical solutions due to stiffness of the equations. Most industrial combustion processes involve turbulent flows. Laminar flows are encountered in few industrial cases and a large number of academic cases. Flow simulations require the solutions of balance equations (of mass, energy and momentum). These equations are mostly of partial differential form. Laminar flow cases are much simpler and straightforward and can often be approximated with one dimensional treatment. Presence of turbulence in the flow requires special treatment to account for the complex nature of turbulence. Combustion requires that fuel and oxidizer be mixed at the molecular level and in turbulent flows; this mixing is done not only by molecular (thermal) processes, but also by the turbulent fluctuations. Molecular mixing takes place at the interface of the smallest eddies.
3.2 Hydrogen Combustion
One the reasons for which we are interested in hydrogen is because its chemistry is considered a starting point for the more complex hydrocarbon chemistry. It is important to stress that in the auto ignition stages of any flame, the fuel air mixture may follow a low temperature reaction mechanism and in the latter stages, an explosive reaction due to the increase in temperature and/or pressure causing the operating point to shift between the regions of the graph of figure 4. This point is especially significant in hydrocarbon chemistry, because it is in the low-temperature regime that particular pollutant compounds are formed. Figure 4 depicts the explosion limits of a stoichiometric mixture, but equivalent plots can be obtained for many different mixture compositions.
Figure 4: Explosion Limits of Stoichiometric Hydrogen-Oxygen Mixture
The general characteristics are:
¢ The first and second limits are ones that correspond to conditions of very low pressures (up to an absolute pressure of about 0.3 bar) and will not be considered. Our lowest working pressure is atmospheric pressure, 1 bar.
¢ The third limit follows the trend that one would expect from simple density considerations. As the pressure increases, the initial densities of the reactants increase and a lower temperature is necessary for the reactions to become fast enough for explosion. Furthermore, noting the logarithmic axis, we can see that the effect of temperature is much stronger than that of pressure, a trend one would expect and correctly captured by the considerations of simplified one-step Arrhenius chemistry.
CHAPTER 4
GRID GENERATION AND MATHEMATICAL MODELING
4.1 Model geometry and mesh
The geometry of the cylindrical chamber used in this study is shown in figure 5.The non premixed hydrogen and air are injected into the cylindrical chamber from inlets located at one axial end as shown in figure 5. A small nozzle in the center of the combustor introduces hydrogen at 90m/s and air enters the combustor coaxially. Because of the axial symmetry of the combustion chamber, the geometry is modeled as a two-dimensional axi-symmetric model.
Figure 5: Schematic Diagram of the Combustion Chamber for Hydrogen Combustion Modeling
4.1.1 Grid Generation
Mesh or grid generation consists of creating a set of grid points along the boundaries and throughout the domain of interest. The numerical simulation of Navier-Stokes equation requires the gen-eration of grids in the flow domain. As the hydrogen injected centrally and air enters co-axially in the combustion chamber more variation in the properties will be seen along the central axis as well as at the inlet. So clustering is done from combustion wall surface towards central axis and from inlet towards exit.
For the scenarios analyzed in this study, the number of nodes and number of quadrilateral cells are 1705 and 1615 respectively used to mesh the model for the CFD simulations. This fine mesh size will be able to provide good spatial resolution for the distribution of most variables within the combustion chamber.
Figure 6: Combustion Chamber Grid
4.2 Mathematical Modeling
Modeling is the representation of a physical system by a set of mathematical relationships that allow the response of the system to various alternative inputs to be predicted. In Computational Fluid Dynamics, we model the physical system involving fluid flow within the definite boundaries by the set of mathematical equations usually in differential form and obtain the numerical solution of these governing equations describing the fluid flow by the use of computational methods. The governing equations may include: the set of the Navier-Stokes equations, continuity equation, and any additional conservation equations, such as energy or species concentrations. The fluid flow is modeled by the governing equations, which show the effect of the governing phenomena on the fluid flow. These governing phenomena may include: conduction, convection, diffusion, turbulence, radiation and combustion. The following is brief description of the governing equations.
4.3 Governing Equations
4.3.1 Continuity Equation
Considering the law of conservation of mass the continuity equation,
+ u.div () + .div (u) = 0
In the given equation the first term is the rate of change of density. In the second and the third terms the divergence div is the flux density or flux/volume. The first two terms show the two ways the density of the fluid element changes. If we assume the incompressibility condition i.e. density of the fluid is constant, the above equation reduces to, div (u) =0.
4.3.2 Momentum Equations
Also known as Navier and Stokes equations, these are derived for a viscous flow and give the relationships between the normal/shear stresses and the rate of deformation (velocity field variation).We can obtain these equations by making a simple assumption that the stresses are linearly related to the rate of deformation (Newtonian fluid), the constant of proportionality for the relation being the dynamic viscosity of the fluid. Following is stated the Navier and Stokes equation for i-th coordinate direction,
+ = - + + Fi
Where is the viscous force tensor and F represents a body force in the -th coordinate direction. In practical situations of combustion, all fluids are assumed to be Newtonian and the viscous stress tensor is:
= µ { + } - µ { }
Where µ is the molecular viscosity which depends on the fluid. The Kronecker delta is =1,if i = j, 0 otherwise.
4.3.3 Species
+ = - + (a=1, 2, 3¦ n)
Where n is the number of species, is the molecular diffusivity flux of the species a in the j-th coordinate direction, is the mass reaction rate of this species per unit volume, and is the mass fraction of species a.
The diffusive flux, ,can be approximated by:
= - = -
where is the Schmidt number of the species a, defined as:
=
Where D is the molecular diffusivity of the species a relative to the other species.
4.3.4 Standard k-e turbulence model
In this simple model, two additional transport equation are solved for the two turbulence quantities viz. the turbulent kinetic energy k and the energy dissipation rate e. These two quantities are related to the primary variables and can give a length scale and time scale to form a quantity with dimension of , thus making the model complete (no more flow-dependent specifications are required). This is a widely used model in CFD simulations.
=
The balance equation for k is:
+ ) = -(2/3) + s : + { } - +
4.4 Boundary conditions
The inlet temperature of hydrogen and air is considered to be uniform at 300 K. A fixed, uniform velocity 90 m/s is specified at the hydrogen inlet.Axis-symmetric boundary conditions are applied along the central axis of the combustion chamber. At the exit, a pressure outlet boundary condition is specified with a fixed pressure of 1.01325 * 105 Pa. At the chamber wall, no-slip boundary condition and no species flux normal to the wall surface are applied. The thermal boundary condition on the chamber wall is taken as adiabatic wall condition.
CHAPTER 5
CFD SIMULATION
A number of numerical simulations have been performed to study the combustion phenomena under adiabatic wall conditions when hydrogen air mixture changes from lean to rich and also at different mass flow rate of mixture. Figure. 7 shows the contours of temperature (K) on the cross section along central axis of combustion chamber at stoichiometric air fuel ratio i.e. at =1. And Figure 8 shows the gas temperature distribution along the central axis. It can be seen from Figures 7 and 8 that the highest temperature is obtained at the exit of combustion chamber. The flame temperature can be as high as 2365 K which is almost the same as the adiabatic flame temperature of the Combustion of non premixed stoichiometric hydrogen-air mixture. Figures 9 to show the contours of molecular species on the cross-section along the central axis at F=1 under adiabatic wall condition.
Figure 7: Temperature Contours at =1
Figure 8: Contours of Mole fraction of
Figure 9: Contours of Mole fraction of N2
Figure 10: Contours of Mole fraction of O2
Figure 11: Contours of Mole fraction of H2
Figure 12: Contours of Mole fraction of OH
Figure 13: Contours of Mole fraction of O
CHAPTER 6
CONCLUSION
In this work, the CFD based combustion simulations have been applied to analyze the combustion characteristics of non-premixed hydrogen-air in a 2D combustor. The CFD simulations, taking in to account the coupling of fluid dynamics, heat transfer and detailed chemical kinetics, are used to investigate the effects of various operating conditions. The combustor performance is evaluated by predicting the temperatures of exit gas of the combustor and outer wall of the combustor. To make the combustor operable, the heat output should meet the design criteria, the wall temperature should be lower than the material allowable temperature and the exit gas temperature should be high enough.
CHAPTER 7
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