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Measuring and Modeling of Radiation in Hydrogen Combustion

1. Introduction

Burning hydrogen emits thermal radiation in UV, NIR and IR spectral range. Especially in the case of a large vapor cloud explosion, the risk of heat radiation is commonly underestimated due to the non-visible flame of hydrogen-air combustion based on to effects. In the case of a real explosion accident organic substances or inert dust might be entrained from outer sources to produce soot or heated solids to substantially increase the heat release by continuum radiation.

Besides pressure waves, radiation is the main effect to influence the surroundings and contribute to fire and explosion transfer and propagation. The radiative emission of hazardous fires and gas explosions is strongly variable in time at short scales down to milliseconds. In addition, it includes fundamental information concerning the mechanisms and progress of burning. A fast scanning analysis of the spectral flame radiation must be fast enough to detect pre-reactions, transient phenomena, starting explosions, and enables the understanding of propagation mechanisms as well as quenching or extinguishing mechanisms.

In contrast to pressure effects, time-resolved radiation of fires and explosions is not yet investigated to a sufficient extend, especially concerning spectral resolution. There are only some publications on time-resolved emission spectroscopy of gas explosions ((EisenreichN:1986), (KolarikP:1991), (EcklW:1995), (WeiserV:1999)). Equipment for time and spectrally resolved investigation and methods of data evaluation to obtain temperatures and species as well as radiative power are described in the following sections.

2. Calculation of NIR/IR Spectra using BAM-Code

A quantitative data analysis of infrared spectra measured from flames and plumes of pyrotechniques has to be based on band modeling. An appropriate code, BAM, was developed by ICT which calculates NIR/IR spectra and allows least squares fits to experimental ones with temperature and concentrations being the fitting parameters (WeiserV:2005a). The calculations use the data from the Handbook of Infrared Radiation from Combustion Gases ((LudwigCB:1973), (FerrisoCC:1965)) which cover the temperature range from 600 to 3000 K. The computer program BAM calculates NIR/IR spectra (1-10 μm)of inhomogeneous gas mixtures of H2O (with bands around 1.3, 1.8, 2.7 and 6.2 μm), CO2 (with bands around 2.7 and 4.3 μm), CO (4.65 μm), NO (5.3 μm) and HCl (3.5 μm) and can take into account emission of soot particles. Self absorption and pressure line broadening can be taken into account. The calculation of an emission or transmission spectrum along an optical path starts from the determination of the absorption coefficients resolved with respect to wavelength. The spectral bands of three-atomic molecules consist of thousands of single lines, e.g., for important flame constituents, HITRAN ((KneizysFX:1988), (RothmanLS:2003)) lists nearly 50,000 lines for H2O and 60,000 for CO2, and simplified models to quickly obtain line positions and strength are currently not available. Therefore tabulated data are used. The calculation of a spectrum of diatomic molecules is based on the anharmonic oscillator and corrections for vibration-rotation interaction to end up with averaged absorption coefficients and line densities derived from the approach described above for spectra in the UV/Vis spectral range (HerzbergG:1950) for band modeling. The molecules which emit in the UV/Vis and consisting of C, N, O and H are quite well documented and analyzed ((HerzbergG:1950), (PearseRWB:1963), (MavrodineanuR:1965)). Calculated band profiles allow the calculation of temperature, in case of time resolution also time profiles ((DiekeGH:1961), (CoxonJA:1980), (ChidseyIL:1980), (SchneiderH:1988), (EcklW:1992), (EcklW:1992a)).

The band models enable an effective calculation of radiation transport in inhomogeneous media in order not to apply a transport equation for each single line ((WeiserV:2005a), (LudwigCB:1973), (FerrisoCC:1965)). Line shapes, half widths and positions of lines contribute to the models. At high temperatures, a Random Band Model has proven to be appropriate using a Doppler-Lorentzian line shape. For the separated bands of the various molecules, the individual lines were accumulated to a Single Line Group, and for the Curve of Growth, the Curti-Godson approximation was used. Benefits, shortcomings and possible errors of this model are discussed in the Handbook of Infrared Radiation from Combustion Gases (LudwigCB:1973).

Figure 1 shows a BAM simulation of emission spectra of homogeneous gas layers of 30 mol% water in air with an optical depth of 1 m at different temperatures. Hydrogen reacts with the oxygen in air to water, which is a well known strong emitter of bands in near and medium infrared spectral range. Co-combusting hydrocarbons additionally produce emission active products like CO2, CH, CO, and soot. Thermodynamic equilibrium calculations of hydrogen/air and hydrogen/hydrocarbon/air mixtures in stoichiometric compositions of air numbers between 0.5 and 4 executed with the ICT code (VolkF:1982) result in a typical temperature in the range of 2000 to 2800 K.

In a real explosion which might entrain dust and organics, these bands are to be expected to combine with bands of, for example, CH, CO2, CO depending on the special constraints of the experiment or accident. The analysis of the spectra enables to derive the progress of reaction, reaction front, hot reaction products and their expansion and temperature profiles. Well defined experiments should be performed to verify the spectral characteristics predicted by the calculations.

3. Experimental Techniques

Hydrogen as a homo-nuclear, diatomic molecule does not show any bands in the NIR/IR spectral range. But in case of combustion in air, the formed water has many well known bands in this range. Additionally, this reaction provides quite an amount of heat radiation, so emission spectra could be taken. In the UV range, diatomic radicals could be observed.

        Emission Spectroscopy in the UV/Vis-Range (OMA)

In the UV/Vis-spectral range, diatomic molecules emit specific systems whose structure is partially resolved. Determination of rotational and vibrational temperatures of various diatomic molecules and transitions from UV/Vis emission spectra is based on the calculation of line intensities and profiles.

For acquisition of emission spectra in the UV/Vis range, commercial diode array spectrometers are available (EcklW:1995). At the ICT, an ANDOR spectrograph (OMA) is used equipped with a CCO detector with 1024 elements. To get rotationally resolved spectra, a grating with 2400 lines/mm can be chosen (wavelength resolution 20 cm-1). Minimum time resolution of the system is 10 ms. A grating with 300 lines/mm can be used, if no rotational resolution was needed. Wavelength calibration is performed by standard lamps of Hg and Ne, and the intensity calibration by black body radiator or tungsten strip lamp. A typical spectrum of an OH band is shown in Fig. 2.

        Emission Spectroscopy in the NIR-Range (AOTF)

An AOTF spectrometer system enabling the acquisition of spectra in the NIR range 1000-2500 nm at rates of up to 1000 scans/s at a wavelength resolution of 2-3 nm is also available. The intensity calibration was carried out by recording reference spectra of a black body radiator. A spectra series of a hydrogen/air gas explosion is given in Fig. 3.

        Emission Spectroscopy in the NIR-Range (HGS)

The fast scanning hot gas sensor (HGS) based on a Zeiss MCS 511 NIR spectrometer (WeiserV:2005a) is a grating spectrometer with an InGaAs diode array as detector (spectral range 0.9 to 1.7 μm). The spectral resolution is about 15 nm at a scan rate of 300 spectra per second. The imaging optics consists of glas fibre optic with a narrow slit, which achieves a space resolution of about 1 mm. The quantitative calibration occurred by a commercial black body radiator. Temperature, concentration of water, and emissivity could be calculated by using the BAM code as it is described above (Fig. 4).

        Emission Spectroscopy in the IR-Range (Filter Wheel Spectrometer)

Spectra in the IR range are recorded applying a rotating filter wheel spectrometer. The spectrometer consists of a fast rotating wheel with three interference filter segments. They continuously vary the transparency in the wavelength regions from 2.4 to 5.5 μm (InSb detector) and 5.5 to 14 μm (InSb/HgCdTe sandwich detector). The system was developed at ICT to investigate fast combustion processes. The intensity calibration was carried out by recording reference spectra of a black body radiator.

IR spectra show the bands of reacting components and products more detailed. In case of hydrogen combustion, intense water band systems will emit. In case of co-combustion of organic materials, additional emission bands of CO and CO2 as well as a continuum radiation of soot and other particles occur and particularly increase the total thermal output drastically.

        Thermo Video Camera

An IR camera of AGEMA (Thermovision 900) recorded images of the radiation in the NIR/IR spectral range. The detector is sensitive between 3 and 5 ~μm wavelength. A filter could limit the detection to the CO2 band at 4.25 μm only. The frame rate is 30 s-1.

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