Numerical Investigation of Boundary Layers in Wet Steam Nozzles

Condensing nozzle flows have been used extensively to validate wet steam models. Many test cases are available in the literature, and in the past, a range of numerical studies have dealt with this challenging task. It is usually assumed that the nozzles provide a one- or two-dimensional flow with a fully turbulent boundary layer (BL). The present paper reviews these assumptions and investigates numerically the influence of boundary layers on dry and wet steam nozzle expansions. For the narrow nozzle of Moses and Stein, it is shown that the pressure distribution is significantly affected by the additional blockage due to the side wall boundary layer. Comparison of laminar and turbulent flow predictions for this nozzles suggests that laminar–turbulent transition only occurs after the throat. Other examples are the Binnie and Green nozzle and the Moore et al. nozzles for which it is known that sudden changes in wall curvature produce expansion and compression waves that interact with the boundary layers. The differences between two- and three-dimensional calculations for these cases and the influence of laminar and turbulent boundary layers are discussed. The present results reveal that boundary layer effects can have a considerable impact on the mean nozzle flow and thus on the validation process of condensation models. In order to verify the accuracy of turbulence modeling, a test case that is not widely known internationally is included within the present study. This experimental work is remarkable because it includes boundary layer data as well as the usual pressure measurements along the nozzle centerline. Predicted and measured boundary layer profiles are compared, and the effect of different turbulence models is discussed. Most of the numerical results are obtained with the in-house wet steam Reynolds-averaged Navier–Stokes (RANS) solver, Steamblock, but for the purpose of comparison, the commercial program ansys cfx is also used, providing a wider range of standard RANS-based turbulence models.Engineering and Physical Sciences Research CouncilThis is the author accepted manuscript. It is currently under an indefinite embargo pending publication by the American Society of Mechanical Engineers (ASME)

Microscopic pathways for stress relaxation in repulsive colloidal glasses

Residual stresses are well-known companions of all glassy materials. They affect and, in many cases, even strongly modify important material properties like the mechanical response and the optical transparency. The mechanisms through which stresses affect such properties are, in many cases, still under study, and their full understanding can pave the way to a full exploitation of stress as a primary control parameter. It is, for example, known that stresses promote particle mobility at small length scales, e.g., in colloidal glasses, gels, and metallic glasses, but this connection still remains essentially qualitative. Exploiting a preparation protocol that leads to colloidal glasses with an exceptionally directional built-in stress field, we characterize the stress-induced dynamics and show that it can be visualized as a collection of “flickering,”mobile regions with linear sizes of the order of ≈20 particle diameters (≈2 μm here) that move cooperatively, displaying an overall stationary but locally ballistic dynamics