Experimental Investigations Into the Effect of Surface Roughness And Contact Force on Leakage Between Two Rigid Metallic Surfaces

Current research and development efforts in the field of heavy-duty gas turbines focus on increasing engine efficiencies, lowering combustor emissions and extending inspection intervals. To achieve further improvements in these development fields, it is crucial for gas turbine manufacturers to continuously build up knowledge on leakage air inside different components of gas turbines. Leakage air can by nature be hardly predicted and is usually only estimated indirectly in real engines. Therefore, investigations on representative test rigs remain a fundamental method for quantifying air leakages more accurately. This article deals with a specific air leakage, which can occur between two firmly pressed rigid surfaces. One challenge for the engineers is to predict the leakage for new surfaces but also used surfaces with fretting corrosion and wear marks as a function of the contact force. In this perspective, air mass flow measurements were performed in a leakage test rig for different contact geometries, pressure ratios and compression levels between the surfaces. The purpose of the analysis is to determine the potential effect of the roughness, length, curvature and contact forces of the pressed surfaces on the leakage amount. The presented measurement procedure and results contribute to the extension of the leakage characteristic database for generic gas turbine components.

Slipping–rolling transitions of a body with two contact points

In this paper, the general kinematics and dynamics of a rigid body is analysed, which is in contact with two rigid surfaces in the presence of dry friction. Due to the rolling or slipping state at each contact point, four kinematic scenarios occur. In the two-point rolling case, the contact forces are undetermined; consequently, the condition of the static friction forces cannot be checked from the Coulomb model to decide whether two-point rolling is possible. However, this issue can be resolved within the scope of rigid body dynamics by analysing the nonsmooth vector field of the system at the possible transitions between slipping and rolling. Based on the concept of limit directions of codimension-2 discontinuities, a method is presented to determine the conditions when the two-point rolling is realizable without slipping.

Leidenfrost droplet trampolining

A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.

Textural and Geochemical Evidence for Multiple, Sheet-Like Magma Pulses in the Limeira Intrusion - Paraná Magmatic Province, Brazil

Quantitative petrographic, structural, and textural parameters are integrated with geological, geochemical, and Sr-isotope data to examine the emplacement, growth processes, and the magmatic evolution of the high-Ti tholeiitic Limeira Intrusion, in the Paraná Magmatic Province - Southeastern Brazil. Our data strongly support a multiple-stage evolution, due to the nested emplacement of distinct crystal-bearing magma pulses that probably evolved independently, except at their boundaries. A stage of cooling and crystallization between magma injections originates a stepwise T-t path, leading to variations in the plagioclase residence times and effective growth rates inwards, also occasioning sudden changes in crystal shape and size at the boundaries of each magma pulse. The time delay between pulses allows preserving internal “chilled margins” and the development of near-rigid surfaces at their contacts, increasing the alignment and clustering of crystals during magma replenishment. Isotopic and textural data demonstrate a complex assembly history, in which the appearance of mixed plagioclase populations in between magma pulses coincides with the onset of initial Sr isotope ratio increase, which can be attributed to a locally enhanced cooling-rate, and the extraction of residual melts from the previous crystallizing batches and mixing with the younger pulses. Typical C- and S-shaped MgO (wt.%) compositional profiles within individual pulses indicate that the first pulse probably evolved by in situ fractional crystallization followed by melt migration inward, while the younger ones have contributions from both compaction of the lowermost crystallization front and compositional convection. Mafic globular structures are found at the boundaries of magma pulses and constituting the mafic-rich layers in layered rocks. They are interpreted as evidence for chemical disequilibrium, arguably associated with the trigger of silicate liquid immiscibility. The upwards compositional convection of the silica-rich residual liquid and the accumulation of the Fe-Ti-P-rich crystal-bearing end member in the bottom of the latest magma pulses might represent the most significant mechanism of differentiation in the Limeira Intrusion.

The numerical calculation of statically admissible slip-line field for plane strain compression of a three-layer symmetric strip between rigid plates

This paper present a method to build up statically admissible slip-line field (the field of characteristics) and, as a result, the field of statically admissible stresses of the compression of a three-layer symmetric strip consisting of two different rigid perfectly plastic materials between rough, parallel, rigid plates (for the case: the shear yield stress of the inner layer is greater than that of the outer layer). Under the conditions of sticking regime at bi-material interfaces and sliding occurs at rigid surfaces with maximum friction, the appropriate singularities on the boundary between the two materials have been assumed, then a standard numerical slip-line technique is supplemented with iterative procedure to calculate characteristic and stress fields that satisfy simultaneously the stress boundary conditions as well as the regime of sticking on the bi-material interfaces