Designing the chemical kinetics calorimeter as a tool for advanced thermochemical research.

Rugescu, Radu ; Silivestru, Valentin ; Aldea, Sorin 等

1. INTRODUCTION

The process of fast burning solid propellant samples within the limited closed space of a calorimeter is a typically static process. No chemical relaxation phenomena are expected and the equilibrium between the gaseous combustion products has no reason to be unfulfilled (Beckstead 2006; Schuricht 2001). Except for the fast self-burning of solids at the beginning of the process, when incomplete combustion sometimes occurs (Kubota 2007; Kuhl et al, 2006), the residing time of the chemical species considerably surpasses the time of chemical reactions (Westenberg & Favin 1962). It was therefore a surprise to find that the residual gas within the calorimeter, although slowly cooled down to the room temperature, presents a chemical composition of a high temperature gas (Rugescu 2005). The processes within the static calorimeters are further investigated, which involve a complex chemical behavior. Currently considered for high speed flows only (Bray 1959; Williams 1965; Hill et al. 1967), the chemical relaxation also occurred in these small and static devices, offering a simple and cheep means to investigate these processes in the laboratory. Considering the complex and very fast initial flow development within the combustor, we propose a multiple point measuring system with adequate pressure transducers and visualization means. This type of combustor is called Chemical Kinetics Calorimeter and such a precursor, simplified device was designed and manufactured by the COMOTI-UPB team for experimental investigation of the process.

The following criteria and principles were adopted for designing the CKC device:

1. Closed, high pressure vessel with V = 283 cmc;

2. Easy and fast dismounting of the hood;

3. Secured sealing for pressures up to [p.sub.max] = 200 bar;

4. Electrical connector for sample ignition;

5. Overpressure release membrane security valve;

6. Manual valve for vacuum pump and gas discharge;

7. High rate variable pressure transducer mounting;

8. High strength, stainless steel vessel construction;

9. Inner surface covering for ceramic catalyst studies;

10. Electric remote operation of the combustor.

These design conditions were embedded into the device manufactured by the Works for Powders in Fagaras, Romania.

[FIGURE 1 OMITTED]

The combustor assembly includes the independent valves and igniter mounting that actually are not parts of the set.

2. TECHNOLOGY OF THE ISOCHORIC COMBUSTOR

The general draft of the set is given in fig. 1. The set consists of two main parts, connected through a screw mounting, secured with two standard "o-ring" seals beneath the upper hood. This highly simplifies the combustor's block design. The sub-assemblies are numbered and described below.

The numbered items in Fig. 1 represent the following parts:

1. Electrical fuse igniter mounting (aerospace standard);

2. Dome nipple elastic seal;

3. Dome (hood) with mounting lateral 04 mm holes;

4. Hood high-pressure elastic seals;

5. Overpressure double o-ring seals;

6. Manual valve for vacuum pump and gas discharge;

7. Elastic seal for the overpressure release security valve;

8. Overpressure explosive membrane security valve;

9. Dome nipple for electrical igniter mounting;

10. Main body of the isochoric combustor;

11. Fast pressure variation transducer mounting.

In the improved version of the device the upper electric connector in the hood is replaced with a sealed, quartz translucent visualization port, where a video CCD camera is fitted, while the igniter itself is re-located to the fourth lateral position within the enlarged flange from the middle body of the combustor, now free.

The material used tor manufacture the body and hood is a high strength, stainless C-120 Chromium steel, equivalent to the 205 Cr 115 alloy. The entire set is fixed with screws on the working table, as seen in Fig. 2 from below. The device is intended for use on the fast variable inner pressure recordings but not intended in the measurement of heat released through isochoric combustion. These constant volume heat measurements are performed in separate, standard calorimetric devices, through known standard procedures.

The electric fuse igniter deserves a special attention. It comprises a standard detonating primer, based on 2 grams of colloidal double base powder (uppermost piece in Fig. 2). The effect of this quantity of powder is adding it to the combustion effect of the main propellant cartridge and is consequently taken into account while performing the pressure variation measurements and its comparison with the numerical simulation. The heating effect released by the electric fuse, previously determined, is also considered, as a heat added to the heat released by combustion. The melted fuse, consisting of a coper alloy, does not add gaseous species in the mixture however and the presence of the mass and volume of melted fuse metal in the vessel is consequently neglected.

The explosive membrane safety valve is a standard type also, involving a calibrated coper membrane that covers an orifice with a diameter of 6 mm, just large enough to assure a safe gas discharge when an overpressure arises during propellant sample combustion. Between combustion runs, the safety valve is covered with a protection cap which is removed before any run of the CKC. The check-in list includes the removal of this protection cap. This enters routine safety procedures.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. EXPERIMENTAL TECHNOLOGY

The management of experimental measurements with the CKC device is developed under a national CNCSIS grant proposal. The flow chart in Fig. 3 (PF is for the factorial research plan, SM for the measuring system, EI for the isochoric evolution and RT for the technical report) shows that the PF is targeted on proving with high confidence the temperature of chemical freezing, as a basis for its explanation.

4. CONCLUSIONS

Technology of the CKC for simple and cheap chemical kinetics research is presented. The device may easily be used in the laboratory, replacing high costs reaction engines experiments for propellant combustion studies. It may open new avenues for accessible chemical kinetics studies, with major consequences for an improved combustion control with reduced emissions.

5. REFERENCES

Beckstead, M. (2006), Recent progress in modeling solid propellant combustion, Combustion, Explosion, and Shock Waves (CESW), Vol. 42, Nr. 6, pp. 623-641 (19).

Bray, K. N. C. (1959), Atomic recombination in a hypersonic wind-tunnel nozzle, J. Fluid Mech., 6(1), p. 1-32.

Hill F. K.; Unger H. F. & Dickens W. (1967), AIAA J. 5(5) 54.

Kubota N. (2007), Propellants and Explosives-Thermochemical Aspects of Combustion, 2nd Ed., Wiley, N. York.

Kuhl, A., Neuwald, P., & Reichenbach, H. (2006), Effectiveness of combustion of shock-dispersed fuels in calorimeters of various volumes, CESW, 42(6), p. 731.

Rugescu, R. D. (2005), Thermal Freezing in the Technology of Aerospace Propellants, Eng. Meridian, TUM Chisinau, 4(4)

Schuricht, S. R. (2001), Numerical simulation of high speed chemically reacting flows, Purdue University, 162 pages; AAT 3075724.

Westenberg ,A. A. & Favin,S. (1962), Nozzle Flow with Complex Chemical Reaction, JHU, Appl. Phys. Lab, AD0275464.

Williams, J. C., III (1965), Correlation of the sudden freezing point in nonequilibrium nozzle flows, AIAA J. 3(6), p. 1169.