Nicola Casari

Centrifugal pumps performance estimation with non-Newtonian fluids: review and critical analysis

Centrifugal pumps are used in many applications in which non-Newtonian fluids are involved, such as food industry and oil&gas applications, producing the pump performance derating. In order to give an overview of pros, cons of the different analytical approaches for pump performance derating a literature review on the most significant advances in this topic will be carried out. Moreover to deepen the knowledge about the internal flow and rheological behavior inside the centrifugal pumps working with non-Newtonian fluids, a detailed CFD analysis of two different pumps will be carried out. The analysis will be focus on the apparent viscosity correction involved in the performance derating with analytical methods and the effects of different types of fluid. Moreover the comparison of the results with two pumps with very different typology, field of application, and dimensions will help to generalize the meaning of the analysis.

AN ENERGY BASED FOULING MODEL FOR GAS TURBINES: EBFOG

Fouling is a major problem in gas turbines for aeropropulsion because the formation of aggregates on the wet surfaces of the machine affects aerodynamic and heat loads. The representation of fouling in CFD is based on the evaluation of the sticking probability, i.e. the probability a particle touching a solid surface has to stick to that surface. Two main models are currently available in literature for the evaluation of the sticking coefficient: one is based on a critical threshold for the viscosity, the other is based on the normal velocity to the surface. However, both models are application specific and lack generality. This work presents an innovative model for the estimation of the sticking probability. This quantitiy is evaluated by comparing the kinetic energy of the particle with an activation energy which describes the state of the particle. The sticking criterion takes the form of an Arrhenius-type equation. A general formulation for the sticking coefficient is obtained. The method, named EBFOG (Energy Based FOulinG), is the first ”energy” based model presented in the open literature able to account any common deposition effect in gas turbines. The EBFOG model is implemented into a Lagrangian tracking procedure, coupled to a fully three-dimensional CFD solver. Particles are tracked inside the domain and equations for the momentum and temperature of each particle are solved. The local geometry of the blade is modified accordingly to the deposition ∗Address all correspondence to this author, email: nicola.casari@unife.it, nicola.casari15@imperial.ac.uk rate. The mesh is modified and the CFD solver updates the flow field. The application of this model to particle deposition in high pressure turbine vanes is investigated, showing the flexibility of the proposed methodology. The model is particularly important in aircraft engines where the effect of fouling for the turbine, in particular the reduction of the HP nozzle throat area, influences heavily the performance by reducing the core capacity. The energy based approach is used to quantify the throat area reduction rate and estimate the variation in the compressor operating condition. The compressor operating point as a function of the time spent operating in a harsh environment can be in this way predicted to estimate, for example, the time that an engine can fly in a cloud of volcanic ashes. The impact of fouling on the throat area of the nozzle is quantified for different conditions.

Gas turbine blade geometry variation due to fouling

Solid particles ingestion is a severe problem for gas turbines. In the aero-propulsion field the main problems related to this phenomenon occur on the hot sections of the machinery. Impinging particles can stick or erode the blade material. The deposition on the turbine blades is the main issue among the two and the clogging of cooling holes can even speed up this process rising the blade surface temperature. An higher temperature affects negatively the deposition problems, increasing particle stickiness. In this paper an innovative approach to account for fouling and erosion effects on turbine vanes is presented. An energetic model to predict the sticking probability is used (EBFOG, from Energy Based FOulinG) and the erosion is evaluated through the model proposed by Tabakoff. Geometry variation of blades subject to fouling are investigated by means of a moving mesh technique which accounts for the boundary displacement of the blade surface.