Testing the Effectiveness and Survivability of the Mini MineWolf

The following test results illustrate the Mini MineWolf’s viability in the field. First, Mini MineWolf’s ability to neutralize simulated anti-personnel mines was tested in three different soil types and at varying depths. The machine was then subjected to live anti-tank blasts by the German Army and the Canadian Centre for Mine Action Technologies

Thermal and fluid dynamic analysis of partially premixed turbulent combustion driven by thermo acoustic effects

Thermo-acoustic instability can be caused by the feedback mechanism between unsteady heat release, acoustic oscillations and flow perturbations. In a gas turbine combustor limit cycles of pressure oscillations at elevated temperatures generated by the unstable combustion process enhance the structural vibration levels of the combustor. In this paper, the behavior of turbulent partially premixed flames in a laboratory-scale lean partially premixed combustor (called as LIMOUSINE combustor) operating on natural gas- methane fuel mixtures is studied by using CFD methods. Depending on the operating conditions, the flame shows a stable or an unstable behavior. In order to predict the frequency and magnitude of the thermo-acoustic instability, and also to capture the reacting flow physics within the combustor, the influence of operating conditions on combustion characteristics is examined by using unsteady three-dimensional RANS solution of the conservation equations. To understand the effects of operating conditions on the observed stability characteristics, the time averaged velocity fields were measured with Particle Image Velocimetry (PIV) for the thermoacoustically stable and unstable operating conditions of the combustor. The comparison of the CFD calculations with the mean velocity fields shows good agreement. The results of the present study demonstrate the relationship between the flame structure, the mean velocity filed and pressure fluctuations under different operating conditions

The effect of fractal grid generated turbulence on the structure of premixed flames

In this thesis fractal grids as a new type of turbulence generators for premixed combustion applications are investigated. Fractal grids produce turbulence fields which differ from those formed by regular turbulence grids such as perforated plates or meshes. Fractal grids generate high turbulence intensities over an extended region downstream of the grid at relatively low blockage ratios (sigma ≈ 35%). Additionally, the integral scale of the flow remains almost constant downstream of the grid. This thesis examines the effect of fractal grid generated turbulence on the structure of premixed flames using a set of four different fractal square grids and one regular square grid which acts as a reference case. It is found that for flames stabilised at the same downstream position, flames in the turbulence field of fractal grids show more intense corrugation, a higher flame surface density and a larger turbulent burning velocity compared to flames in regular grid generated turbulence. This is the direct result of the increased turbulence level produced by the fractal grids and demonstrates the potential benefit of using fractal grids for applications in premixed combustion. As an example for possible applications, fractal grids are used as turbulence generators to investigate the geometric alignment between flames and the principal strain-rate axes of a turbulent flow. The statistical analysis reveals that turbulence-flame alignment strongly depends on the distance between the flame surface and the location where the strain-rate field is evaluated. This dependency also helps to interpret findings of previous alignment studies.Open Acces

Sensitivity of the numerical prediction of flow in the limousine combustor on the chosen mesh and turbulent combustion model

The objective of this study is to investigate the sensitivity and accuracy of the combustible flow field prediction for the LIMOUSINE combustor with regards to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed bluff body stabilized natural gas combustor designed to operate at 40-80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermo-acoustic instabilities is a very time consuming process. For that reason the meshing approach leading to accurate numerical prediction, known sensitivity, and reduced amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the numerical model. Typically, the structural mesh topology allows using much less grid elements compared to the unstructured grid, however an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter the studies are extended to combustible flows using standard available ANSYS-CFX combustion models. To validate the predicted variable fields of the combustor's transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under non-reacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow resolved with the use of the hexahedral mesh presents better agreement with experimental data and demands less computational effort. Finally in the paper the performance of the combustion model for reacting flow as a function of mesh configuration is presented, and the main issues of the applied combustion modeling are reviewed