Volume, constant, combustion

M. Ducros, H. Tachoire. Calorimetrie de Combustion a Volume Constant dans TOxygene (lere Partie). L Actualite Chim. 1978, 9. 

Figures 12-14 show the results after 2 hours of leakage. The figures show surfaces of constant gas concentration that represents the lean limit of combustion. It can be seen that the volume of combustible gas created by the hydrogen-fueled vehicle is much smaller than the volume created by the LPG-fueled vehicle. This was true regardless of which of the two propane flow rates was assumed.

The volume of combustible gases produced by the hydrogen-fueled vehicle had reached steady state after 1 hour as seen in Figure 15. Figure 15 shows the surface of constant 4.1 % hydrogen concentration, which is the lean limit of combustion for hydrogen. 

Sometimes, the terms heat of explosion and heat of detonation get mixed-up and are often (wrongly ) used as if they were equivalent. The heat of explosion and the heat of detonation are both heats of reaction but are obtained under different conditions (by applying Hess law). In the detonation run, the heat of reaction corresponds to the heat of reaction at the C-J point , and is called heat of detonation. The heat of reaction obtained under constant volume condition combustion is usually called the heat of combustion or heat of explosion . There is one additional term which is worth mentioning here and that is the heat of complete combustion . This term is usually used to denote constant volume combustion in an oxygen atmosphere. 

G. D Errico, D. Ettorre, and T. Lucchini. Simplified and Detailed Chemistry Modeling of Constant-Volume Diesel Combustion Experiments. SAE Paper, 2008-01-0954, 2008. 

A fireball is assumed to bum with a constant temperature in the isothermal fireball model of Lihou and Maund (1982). Combustion is controlled by the supply of air and ceases after a time which is correlated empirically with the mass of flammable gas in the initial vapor sphere. It is assumed that a fraction (1 — /c) of the fuel is used to form soot, and the remaining fractionbums stoichiometrically, producing an increase of /ij moles per mole of flammable gas. The stoichiometric molar ratio of air to flammable gas is p, and dVIdt is the volumetric rate of air entrainment. The rate of increase of volume can now be written as ... 

Qt = stoichiometric composition of combustible vapor in air expressed as a volume percent Co = sonic flow discharge orifice constant, varying VvTth Reynolds number... 

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. 

FIGURE 6.12 A bomb calorimeter is used to measure heat transfers at constant volume. The sample in the central rigid container called the bomb is ignited electrically with a fuse wire. Once combustion has begun, energy released as heat spreads through the walls of the bomb into the water. The heat released is proportional to the temperature change of the entire assembly. 

We have seen that a constant-pressure calorimeter and a constant-volume bomb calorimeter measure changes in different state functions at constant volume, the heat transfer is interpreted as A U at constant pressure, it is interpreted as AH. However, it is sometimes necessary to convert the measured value of AU into AH. For example, it is easy to measure the heat released by the combustion of glucose in a bomb calorimeter, but to use that information in assessing energy changes in metabolism, which take place at constant pressure, we need the enthalpy of reaction. 

An overly simplified model of fluidized-bed combustion treats the solid fuel as spherical particles freely suspended in upward-flowing gas. Suppose the particles react with zero-order kinetics and that there is no ash or oxide formation. It is desired that the particles be completely consumed by position z = L. This can be done in a column of constant diameter or in a column where the diameter increases or decreases with increasing height. Which approach is better with respect to minimizing the reactor volume Develop a model that predicts the position of the particle as a function of time spent in the reactor. Ignore particle-to-particle interactions. 

Dunn-Rankin, D. and Sawyer, R.E, Interaction of a laminar flame with its self-generated flow during constant volume combustion, AlAA Volume from Tenth ICDERS, 115-130,1985. 

Figure 6-17 illustrates a constant-volume calorimeter of a type that is often used to measure q for combustion reactions. A sample of the substance to be burned is placed inside the sealed calorimeter in the presence of excess oxygen gas. When the sample bums, energy flows from the chemicals to the calorimeter. As in a constant-pressure calorimeter, the calorimeter is well insulated from its surroundings, so all the heat released by the chemicals is absorbed by the calorimeter. The temperature change of the calorimeter, with the calorimeter s heat capacity, gives the amount of heat released in the reaction. 

C06-0057. Acetylene (C2 H2) Is used In welding torches because it has a high heat of combustion. When 1.00 g of acetylene bums completely in excess O2 gas at constant volume, it releases 48.2 kJ of energy, (a) What Is the balanced chemical equation for this reaction (b) What is the molar energy of combustion of acetylene (c) How much energy is released per mole of O2 consumed ... 

C06-0068. Constant-volume calorimeters are sometimes calibrated by ranning a combustion reaction of known A E and measuring the change in temperature. For example, the combustion energy of... 

C06-0071. An electric heater adds 19.75 kJ of heat to a constant-volume calorimeter. The temperature of the calorimeter increases by 4.22 °C. When 1.75 g of methanol is burned in the same calorimeter, the temperature increases by 8.47 °C. Calculate the molar energy of combustion of methanol. 

---