Volume, constant, combustion chamber

As referred to in the previous chapter, in bomb combustion calorimetry the reaction proceeds inside a pressure vessel—the bomb—at constant volume, and in this case the derived quantity is Ac U°. In flame calorimetry the reaction occurs in a combustion chamber, which is in communication with the atmosphere, and the measurements lead to ACH°. The methods of combustion calorimetry will be described in the following paragraphs. 

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. 

Figure 2. Time-resolved Schlieren photographs of a constant volume combustion chamber at k atm. pressure. The vertical dimension of each frame is 0.3 cm.

First, we shall use a quasi-stationary approach already mentioned earlier, based on the assumption that characteristic times of heat and mass transfer in the gaseous phase are much shorter than in the liquid phase, since the coefficients of diffusion and thermal conductivity are much greater in the gas than in the liquid. Therefore the distribution of parameters in the gas may be considered as stationary, while they are non-stationary in the liquid. On the other hand, small volume of the drop allows us to assume that the temperature and concentration distributions are constant within the drop, while in the gas they depend on coordinates. Another assumption is that the drop s center does not move relative to the gas. Actually, this assumption is too strong, because in real processes, for example, when a liquid is sprayed in a combustion chamber, drops move relative to the gas due to inertia and the gravity force. However, if the size of drops is small (less than 1 pm) and the processes of heat and mass exchange are fast enough, then this assumption is permissible. As usual, we assume the existence of local thermodynamic equilibrium at the drop s surface, as well as equal pressures in both phases. The last condition was formulated at the end of Section 6.7. 

In more efficiently designed and operated sulfur burners such as those used in the paper industry and in the manufacture of sulfuric acid (Shreve, 1944), the sulfur is melted, atomized in compressed air, and burnt in a separate combustion chamber. In these burners gas of a constant composition, 19-20 volume per cent SOa, without any sublimation and with only 0.14% of the sulfur transformed into SOs, can be obtained. Sulfur dioxide is a colorless gas having a characteristic odor, a normal molecular volume of 21.89 1., and a molecular weight of 64.06 g. It is soluble to the extent of 36.4 volumes in one volume of water at 20° C. Its solubility in water decreases from 8.6% by weight at 20° C. to 0.1% at 100° C. At atmospheric pressure SO2 liquefies at —10° C. at 20° C. liquid SO2 exerts a pressure of 3.25 atm. or 40.6 p.s.i. The solubility of sulfur dioxide in water has been determined accurately by Beuschlein and... 

The determination of the combustion pressure of propellants (or pyrotechnic compositions) at constant volume conditions is performed in a specially designed closed vessel. The vessel has a combustion chamber with a volume up to 2.5 dm and it can withstand the dynamic pressure of as much as 5000 bar. Such vessels are so-called closed bombs, ballistic bombs, or manometric bombs. 

It should be emphasised that the pressure inside the combustion chamber may be kept constant by means of the regulating valve. If a chamber without a regulating valve is used, then, strictly speaking, the pressure inside the combustion chamber is not constant. Namely, during combustion, the pressure inside the chamber is somev at increased. However, the increase in the pressure can minimised by the choice of an adequate sample size and the volume of the combustion chamber. 

The study on the spray combustion characteristics of 10% CPO blended with diesel fuel was conducted in a constant-volume combustion chamber. With the fixed experimental conditions such as spray ambient pressure and injection events, the effects of 10% CPO diesel at the injection line pressure of 100 MPa on spray combustion and flame stmcture were investigated using a photo diode and ICCD camera. Two-color method was also employed to predict combustion flame temperatures and KL factors. 

In the combustion reaction as carried out in the calorimeter of Figure 7-2, the volume of the system is kept constant and pressure may change because the reaction chamber is sealed. In the laboratory experiments you have conducted, you kept the pressure constant by leaving the system open to the surroundings. In such an experiment, the volume may change. There is a small difference between these two types of measurements. The difference arises from the energy used when a system expands against the pressure of the atmosphere. In a constant volume calorimeter, there is no such expansion hence, this contribution to the reaction heat is not present. Experiments show that this difference is usually small. However, the symbol AH represents the heat effect that accompanies a chemical reaction carried out at constant pressure—the condition we usually have when the reaction occurs in an open beaker. 

Anon. 2007b. D 6890 Standard Test Method for Determination of Ignition Delay and Derived Cetane Number (DCN) of Diesel Fuels Oils by Combustion in a Constant Volume Chamber. In Annual Book of ASTM Standards (05.05). West Conshohocken PA ASTM International. 

Figure 3. A pressure (p) time (t) history of combustion in a constant volume chamber for a methane-air mixture with X = 1.3. (Reproduced with permission from Ref. 20. Copyright 1983, Combustion Science and Technology.)...

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