The GraVent explosion channel operated at the Institute of Thermodynamics, TUM, is similar to other typical explosion test facilities with high aspect ratio (length-to-diameter/width ratio). The basic idea of reaching fast combustion regimes at laboratory scale is to provide a high degree of confinement and congestion. These are realized as an entirely closed channel equipped with evenly spaced obstacles. This type of experiment was extended by mechanisms for the generation of transverse concentration gradients and for water mist injection.

The channel is composed of up to six segments. Solid plates, referred to as ignition and end plate, close the channel at both ends. The following coordinate system is used: axial direction x, vertical direction y and lateral direction z as marked in Fig. E.1. Standard segments with a length of 0.9 m and one optical segment (OS) with a length of 0.6 m are available. The optical segment can be placed at arbitrary positions along the channel, denoted OS1 (optical segment at position 1, x = 0 m to x = 0.6 m), OS2 etc.

The explosion channel cross-section is 0.3 m wide and 0.06 m high. Located below this channel is an additional channel for investigating transverse venting. The explosion and venting channels are separated by solid plates. Only experiments without venting are provided in the DDT database, thus the separation plates are permanently installed.

The mixture is ignited using a low energy spark plug centered at the ignition plate. The flame travels through the channel towards the end plate. Measurement systems are triggered off the ignition signal.

Figure E.5. illustrates the generation of concentration gradients. The last obstacle of BR60S300 (x = 2.05 m) and the first injection manifold of the unobstructed channel section (x = 2.15 m) are depicted. In a first step, the channel is filled with ambient air. Using a vacuum pump, the explosion volume is partially evacuated. Initial pressure prior to H₂ injection depends on the requested H₂ concentration and is calculated by the method of partial pressures. Then, H₂ at a pressure of 8 bar from a gas cylinder is injected through a regular pattern of injection ports in the facility's top plate (1). This pattern is shown in Fig. E.6. for a standard segment. Each row of ports comprises three ports across the channel width. Axial positions of rows coincide with notch positions in the top plate. Distributors connect 9 injection ports each to the H₂ supply system. In each distributor a 0.9 mm choked aperture controls the flow rate. The H₂ flow is deflected inside the channel, forming a compact horizontal H₂ layer along the channel top (2). At obstacle positions in the obstructed channel section, H₂ deflection is achieved by slots in the upper obstacles. Positions between obstacles as well as top plate notches in the unobstructed section are equipped with manifolds protruding into the channel at the upper wall (cp. Fig. E.3. (c)). These manifolds do not significantly influence the DDT process. Vertical concentration gradients form due to diffusion (3). The orientation of resulting gradients (4) is thus normal to the main direction of explosion front propagation.

Gradients of defined slope can be generated by controlling the diffusion time t_d between H₂ injection and ignition. A diffusion time of 60 s yields a homogeneous mixture, whereas a diffusion time of 3 s results in a steep concentration gradient. Further diffusion times of 10 s, 7.5 s and 5 s have been investigated.

The method for concentration gradient generation has been characterized experimentally and numerically in previous work by Vollmer et al. [1] and Ettner et al. [2], respectively. Figure E.7. gives a first impression of concentration gradients.

Profiles computed in the latter work were transformed into polynomials that describe the concentration profiles:

The general notation is as follows:

X_H2(y) = p₁×y⁴ + p₂×y³ + p₃×y² + p₄×y +p₅

where y is the vertical position in the channel in [m] according to the coordinate system introduced in Fig. E.1. and p_i are polynomial coefficients given in the following table:

Between the given average H₂ concentrations, values can be interpolated for each diffusion time with a square fit.

[1] K. G. Vollmer, F. Ettner, and T. Sattelmayer. Deflagration-to-Detonation Transition in Hydrogen/Air Mixtures with a Concentration Gradient. Combustion Science and Technology, 184:1903–1915, 2012.

[2] F. A. Ettner. Effiziente numerische Simulation des Deflagrations–Detonations–Übergangs. Ph.D. thesis, Lehrstuhl für Thermodynamik, Technische Universität München, 2013.

The GraVent facility was equipped with a water mist injection system for one specific measurement campaign. The goal was to investigate the influence of water mist on flame acceleration, DDT and detonation propagation. This is of particular interest for nuclear reactor safety since water sprinklers are part of the containment safety concept for severe accidents in several types of reactors. Results from this study are also incorporated in the DDT database. Only pressure traces could be obtained since the photodiodes did not acquire sufficient flame radiation due to the water mist.

For the injection of water mist, the venting channel of the setup is equipped with 14 ultrasonic atomizers. An air flow circulating in a closed loop, driven by a diaphragm pump, transports the water mist generated in the venting channel into the explosion channel. Over a time of 60 s, water mist accumulates in the explosion volume so that a water loading ratio of 0.11 kg/m³ can be established. The water droplet size spectrum was analyzed using a Malvern Particle Sizer, see Fig. E.8. The Sauter mean diameter (SMD) is 13μm. After water injection, the H₂-air mixture is prepared as described above. During each experiment the water loading ratio shortly before ignition is determined by measuring extinction. For more details on the injection method, please refer to [3].

Note that the value of the diffusion time after H₂ injection of 10 s does not result in a concentration gradient in these experiments due to a different H₂ injection strategy.

If you are interested in simulating one of the very lean experiments from the H₂O measurement campaign, please contact us in advance. It will be necessary to consider more details of the explosion channel geometry when equipped with the water mist injection system.

[3] L.R. Boeck, A. Kink, D. Oezdin, J. Hasslberger, T. Sattelmayer, Influence of Water Mist on Flame Acceleration, DDT and Detonation in H₂-Air Mixtures, International Journal of Hydrogen Energy, 40:21, pp. 6995-7004, 2015