Adiabatic-combustion chamber

This reaction takes place in an adiabatic-combustion chamber, shown in Fig. 21.1. This chamber has no tubes to absorb radiant heat. 

During the strike, the sulfur plant was shut down for minor repairs. I had to supervise its start-up. Mainly, I had to reheat the adiabatic-combustion chamber to 1800°F, before restoring the flow of H2S. This was done by burning a controlled amount of methane or natural gas, with a carefully regulated flow of air. The idea was to slowly heat up the combustion chamber with hot flue gas by 100 to 200°F per hour. This slow reheat was needed to avoid cracking the refractory bricks, because of uneven heating. To carry away a portion of the heat of combustion of the natural gas, we used pipeline nitrogen. 

Many choose to list the adiabatic combustion chamber temperature, the average molecular weight of the gases leaving the combustion chamber ft, and y, the ratio of the specific heats of these gases, in describing a given propellant combination. 

The adiabatic flame temperature of a fuel is the temperature that would be attained if the fuel were burned in an adiabatic combustion chamber and all of the energy released went into raising the temperature of the reaction products (as opposed to being absorbed by or transferred through the reactor wall). 

This reaction takes place in an adiabatic-combustion chamber, shown in Fig. 30.1. This chamber has no tubes to absorb radiant heat. Plenty of radiant heat is liberated, but only to the refractory brick walls. The bricks then reradiate the heat back into the gaseous products of reaction. This is called adiabatic combustion because no heat is lost from the combustion reaction to radiation. The adiabatic-combustion temperature for the preceding reaction [Eq. (30.1)] is about 2300°F. The refractory used to contain this high temperature is manufactured from 90 percent alumina. Such refractory may be exposed to temperatures of up to 2900°F without damage. 

An investigation showed that the thermowell—a steel or ceramic tube containing the thermocouple wires—was not fully inserted into the adiabatic-combustion chamber. The end of the thermowell was only halfway into the 12-in-thick refractory wall, as shown in Fig. 25.2. 

Because this reaction is highly exothermic, the equiUbrium flame temperature for the adiabatic reaction with stoichiometric proportions of hydrogen and chlorine can reach temperatures up to 2490°C where the equiUbrium mixture contains 4.2% free chlorine by volume. This free hydrogen and chlorine is completely converted by rapidly cooling the reaction mixture to 200°C. Thus, by properly controlling the feed gas mixture, a burner gas containing over 99% HCl can be produced. The gas formed in the combustion chamber then flows through an absorber/cooler to produce 30—32% acid. The HCl produced by this process is known as burner acid. 

An equation representing an energy balance on a combustion chamber of two surface zones, a heat sink Ai at temperature T, and a refractory surface A assumed radiatively adiabatic at Tr, inmost simply solved if the total enthalpy input H is expressed as rhCJYTv rh is the mass rate of fuel plus air and Tp is a pseudoadiabatic flame temperature based on a mean specific heat from base temperature up to the gas exit temperature Te rather than up to Tp/The heat transfer rate out of the gas is then H— — T ) or rhCp(T f — Te). The... 

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

It follows that the efficiency of the Carnot engine is entirely determined by the temperatures of the two isothermal processes. The Otto cycle, being a real process, does not have ideal isothermal or adiabatic expansion and contraction of the gas phase due to the finite thermal losses of the combustion chamber and resistance to the movement of the piston, and because the product gases are not at tlrermodynamic equilibrium. Furthermore the heat of combustion is mainly evolved during a short time, after the gas has been compressed by the piston. This gives rise to an additional increase in temperature which is not accompanied by a large change in volume due to the constraint applied by tire piston. The efficiency, QE, expressed as a function of the compression ratio (r) can only be assumed therefore to be an approximation to the ideal gas Carnot cycle. 

The liquid propellant rocket combination nitrogen tetroxide (N204) and IJDMII (unsymmetrical dimethyl hydrazine) has optimum performance at an oxidizer-to-fuel weight ratio of 2 at a chamber pressure of 67 atm. Assume that the products of combustion of this mixture are N2, C02, H20, CO, H2, O, H, OH, and NO. Set down the equations necessary to calculate the adiabatic combustion temperature and the actual product composition under these conditions. These equations should contain all the numerical... 

Assume a process for each of the four devices (1) compressor as adiabatic with efficiency of 85%, (2) combustion chamber as isobaric, (3) turbine as adiabatic with efficiency of 89%, and (4) heat exchanger as isobaric on both hot and cold sides. Input the given information (1) working fluid is air, (2) inlet pressure and temperature of the compression device are 14.7 psia and 60°F, (3) inlet pressure and temperature of the turbine are 120 psia and 2000°F, (4) mass flow rate of air is 1 Ibm/sec, (5) exit pressure of the turbine is 14.7 psia, (6) display the exit temperature of the compressor (it is 562.5°F), and (7) input the exit temperature of the exhaust turbine gas... 

Assume compressor as adiabatic and 80% efficient, combustion chamber and mixing chamber as isobaric, and turbines as adiabatic and 80% efficient. 

In the simple two-component system of PVC binder and oxidizer, the important propellant properties of specific impulse, density, adiabatic flame temperature, and burning rate increase with an increase in solids loading. This is shown in Figure 8, where theoretical calculated values of specific impulse, adiabatic flame temperature, and density are given for a range of oxidizer content for PVC plastisol propellants comprised of only binder and oxidizer. [Calculated values of specific impulse reported throughout this paper are for adiabatic combustion at a rocket chamber pressure of 1000 p.s.i.a. followed by isentropic expansion to 1 atm. pressure with the assumptions that during the expansion process chemical compo-... 

In a rocket combustion chamber the chemical reaction is essentially adiabatic. This has been expressed by Siegel and Schieler (37) as follows ... 

When an explosive is initiated either to burning or detonation, its energy is released in the form of heat. The liberation of heat under adiabatic conditions is called the heat of explosion, denoted by the letter Q. The heat of explosion provides information about the work capacity of the explosive, where the effective propellants and secondary explosives generally have high values of Q. For propellants burning in the chamber of a gun, and secondary explosives in detonating devices, the heat of explosion is conventionally expressed in terms of constant volume conditions Qv. For rocket propellants burning in the combustion chamber of a rocket motor under conditions of free expansion to the atmosphere, it is conventional to employ constant pressure conditions. In this case, the heat of explosion is expressed as Qp. 

Then, the calculated temperature of the combustion chamber is 3800°F. This is called the adiabatic flame temperature. Such a temperature is quite sufficient to turn even bricks into a high-viscosity, lava-type, semisolid fluid. 

In deflagration, the limiting temperature is the adiabatic flame temperature. Figure 11 (41) presents some temperature measurements made with an optical pyrometer on the exit wall of the same ceramic combustion chamber for which pressure loss data were presented in Figure 5. Temperatures w ithin 500° F. of the adiabatic flame temperature were obtained. 

Autothermal reforming combines partial oxidation and adiabatic steam reforming for conversion of the hydrocarbon feedstock into synthesis gas free of soot and higher hydrocarbons. The ATR reactor design consists of burner, combustion chamber, and catalyst bed placed in a refractory lined vessel, as illustrated in Fig. 10. The hydrocarbon feedstock with steam is reacted with oxygen in a substoichiometric flame, often... 

Figure 9-25 shows the adiabatic flame temperature T)-of GAP propellants containing boron particles as a function of e, where e is the air to fuel ratio in the secondary combustion chamber. As e increases, Tf increases rapidly in the region e< 5. However, Tf decreases with increasing e in the region e > 5. The maximum temperature is 2550 K at e = 5 for the propellants without boron and is 2650 K for (0.1) and 2730 K for (0.1) and (0.2), respectively, where (B) is the mass fraction of boron. 

Two basic types of combustion chambers may be seen when power burners are tested. The first group consists of completely insulated adiabatic chambers. The second group is combustion chambers allowing for part of the heat released by combustion to be absorbed in the cooling medium (see Figure 20.7). Water is typically used as the cooling medium, which circulates through the area between the shells of the combustion chamber whereas the internal shell on the side of the flame is not... 

---