HEAT RADIATION OF BURNING HYDROGEN/AIR MIXTURES IMPURIFIED BY ORGANIC VAPOUR AND PARTICLES Weiser, V., Kessler, A., Roth, E., Eckl, W., Langer. G

Indroduction Burning hydrogen forms hot water vapour (steam) Water emits heat radiation in (N)IR spectral range depending on volume of flame ball, temperature and water concentration If addition components where are heated (dust) or co-combust initiated from by the H2 combustion additional heat radiation occurs

Scope To provide data for estimate heat radiation Qrad of hydrogen flames or explosions impurified with other inert heated materials or co-combustion hydrocarbons as ? View factor as function of (reference) shape, distance y (see text books on heat radiation) ? A Emitting area or projected area Stefan-Boltzmann constant ? T effective emission temperature (combustion temperature) Total emissivity as function of emitting path length x, species concentrations ci ???

Experimental Set-Up defined H2-release H2-ignition Measurement techniques: Phantom high speed-camera (BOS) FLIR IR-camera, NIR- (HGS), Filter wheel-spectrometer, Release of organic substances Spray Powder

Measurement techniques DV-camera: visual flame shape IR-camera (FLIR SC500): IR-flame shape with 50 fps radiation in the MIR-spectral range of 7.5 to 13 µm Phantom V5 b/w digital high speed video camera => visualisation with BOS-method with 1000 fp

BOS-Method (Background-Oriented-Schlieren) Armin Kessler, Walter Ehrhardt, Gesa Langer; Hydrogen Detection: Visualisation of Hydrogen Using Non Invasive Optical Schlieren Technique BOS, International Conference on Hydrogen Safety (ICHS), 08.-10.09.2005, Pisa Density gradients induced by hydrogen release generate local virtual displacements of the random background pattern, which is computed using PIV algorithms in comparison to the undistorted case.

Measurement techniques DV-camera: visual flame shape Phantom V5 b/w digital high speed video camera => visualisation with BOS-method with 1000 fps IR-camera (FLIR SC500): IR-flame shape with 50 fps radiation in the MIR-spectral range of 7.5 to 13 µm NIR-spectrometer: fast scanning hot gas sensor (HGS) based on a Zeiss MCS 511 NIR spectral range 0.9 to 1.7 µm; scan rate of 300 spectra per second IR-spectrometer: filter wheel spectrometer rotating wheel with 3 interference filter segments; cover 2.45 to 14 µm; InSb/HgCdTe-sandwich-detector; 130 turns (=spectra) per second. Optic: “infinity to 1” projection realized by ZnSe-lenses. The spectrometer was adjusted to the position of ignition. intensity calibration of all spectrometer systems uses a technical black body radiator. => results in [intensity per wavelength] but not [intensity / wavelength area steradian]

Data analysis of NIR/IR spectra (ICT-BaM) Computer code for generation and fitting of NIR/IR spectra (1-10 µm): band modelling based on single line group model, Curtis-Godson- approximation and tabulated data of H2O and CO2 based on data of “Handbook of Infrared Radiation from Combustion Gases”, NASA inhomogeneous gas mixtures of - H2O (bands at 1.3, 1.8, 2.7 and 6.2 µm) - CO2 (bands at 2.7 and 4.3 µm) - CO (band at 4.7 µm) - NO (band at 5.4 µm) - HCl (band at 3.5 µm) - particles (e.g. soot) temperature range 300 - >3000 K emission or transmission calculations single or multi-layer model of radiation transfer Fitting parameter: Temperature, (concentration * path length)

Experimentals Test plan: Release of 1.5 l hydrogen in 300 ms; ignition at different positions (about 60 experiments) + 4 ml Dowanol CH3O[CH2-CH(CH3)O]2H + 5 and 10 g milk powder C1H1.86N0.08O0.53 +K, Ca, Cl, Na … + 0.5 and 1.5 g Aerosil (inert powder) silicic acid (SiO2 x n H2O) Variation of: Time of ignition (pure hydrogen) Ignition at different times of organic fuel and hydrogen release

Visual picture DV camera prints of hydrogen/air explosions (pictures at maximum emission) a) pure, b) plus Aerosil, c) plus DPM, d) plus milk powder

BOS evaluations pure hydrogen H2 plus DPM

Thermographs Pure hydrogen released in air

Thermographs

NIR-spectra for temperature meassurements using ICT-BaM pure hydrogen/air explosion hydrogen/air impurified with milk powder

Characteristic Time History NIR-Spectra ICT-BaM evaluation of spectra sequences received from the combustion of a) pure hydrogen/air explosion, b) H2/air plus milk powder

Mean temperatures (left) and maximum temperatures (right) of all experiments

Series of IR-spectra a) pure hydrogen/air explosion, b) hydrogen/air impurified with milk powder

ICT-BaM-analysis of IR-spectra series Infra red spectra received from hydrogen/air explosions with additional particles in the combustion zone compared with best least-squares fits resulting from ICT-BaM analysis Ratio of concentration length of water to CO2 resulting from spectra sequences of hydrogen/air explosions partially mixed with DPM and milk powder achieved by ICT-BaM analysis

In scope to discuss the radiative heat emission of free hydrogen/air explosions in small scale (1.5 dm3 H2) the experimental results can be summarized as follows: The reaction of the inhomogeneous mixed hydrogen in air takes place very quickly in a period of about 10 ms forming a hot gas volume that may be described as a sphere of 0.4 to 0.5 m diameter. The gas temperature amounts up to 2000 K. In 0.2 to 0.3 s the gas volume cools down to 1500 K without dramatic changes of shape and size. Even in the case of co-combustion of relatively small amounts organic species the main heat emission is caused by water band systems in spectral range 2.5 to 9 µm. In the case of pure hydrogen/air explosion the contribution of natural CO2 is negligible. Even in the case of co-combustion of a considerable amount of hydrocarbons the molar fraction of emitting CO2 does not exceed 10% of water. In the same case the contribution of continuum emission of soot or other particles can be described with a low emissivity of less than 0.02. => Input for e(l,T) of the water bands can be calculated e.g. with ICT-BaM code or RADCAL

Total emissivities of hot water gas calculated with ICT-BaM code as function of temperature, concentration and optic path length. values of the reported experiments

Conclusions Small scale explosions of 1.5 dm3 hydrogen in free air were investigated resulting in spherical flame balls of A = 0.4 m emitting with T = 1500 to 2000 K with a total emissivity in the order of magnitude e = 0.15. The injection of small amounts of co-combusting of inert or organic materials increases the emissivity to e = 0.20 with similar temperature. This data do not result in a dangerous heat radiation concerning these small scale explosions. But the reported method of investigation is also applicable to study much larger gas explosion which are presumed to emit more dangerous heat radiation.