Numerical Study of Erosion due to Online Water Washing in Axial Flow Compressors

Gabriele³

et al. 2021

E3S Web Conf.

Self Cite

Gas turbines performance losses are mainly due to the deposition of dirt on the compressor blades that needs to be periodically removed. This is the reason motivating the presence of water washing systems (WWS) in most of the compressor gas turbines. Water washing is generally achieved by installing a number of nozzles on the compressor casing and spraying water that clean the dirty surfaces of the compressor. The side effect of such a technique is the rising risk of erosion due to the impact of water droplets on the compressor blades which is even more pronounced when dealing with online water washing systems that is done while the unit is at normal load. The design of these systems must balance benefits and disadvantages associated to the process itself. The benefits can be measured in terms of water washing efficiency that is a quantity not uniquely defined. In previous works, the authors introduced some indices useful to evaluate the spatial cleaning coverage (the wet to the total surface) and the quantity of water mass actually impacting the dirty surfaces (the impacted to injected mass). On the other hand, water washing erosion is a complex phenomenon depending on several parameters, such as the mechanical properties of the blade material, the impact velocity and angle and the droplet diameter. For this reason, the WWS are strongly influenced by the adopted nozzles and by the injection conditions. The present paper aims at assessing water washing for six different injection conditions in the first stage of a real axial compressor. Two-phase CFD simulations are carried out with Ansys Fluent where a User Defined Function implemented by the authors is used to properly model water droplet erosion mechanism and to obtain all the quantities needed to evaluate the washing quality. Results confirm the strong influence of the injection conditions on the main features of the washing system. The study is part of an ongoing partnership between Baker Hughes and Sapienza University of Rome aiming at maximizing the washing of the compressor blades while maintaining the erosion under specific thresholds.

Section: Resultsmentioning

confidence: 99%

Section: Washing Efficiencymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Evaluation of water washing efficiency and erosion risk in an axial compressor for different water injection conditions

Gabriele³

et al. 2021

E3S Web Conf.

Self Cite

“…[28]) Fig. 3 Different droplet impact regimes identified by the adopted model [19] the compressor. Droplets are quickly dragged through the domain, and, because of the different injection positions, each cloud of droplets follows different patterns resulting in low values of the volume fraction in most of the numerical domains.…”

Section: Models and Methodologymentioning

confidence: 99%

“…[15], we developed an erosion model based on experiments available in the open literature [16,17]. The WDE model was applied to several compressor blade geometries in the past [15,[18][19][20][21]. In all these simulations, the computation of the erosion mechanism was carried out by assuming that the profile did not change due to the erosion itself thus neglecting the variation in the fluid flow due to the profile modification.…”

Section: Introductionmentioning

confidence: 99%

Numerical Prediction of Long-Term Droplet Erosion and Washing Efficiency of Axial Compressors Through the Use of a Discrete Mesh Morphing Approach

Castorrini

Journal of Turbomachinery

et al. 2022

Self Cite

Online water washing represents an operation strategy commonly used to reduce compressor performance deterioration due to blade fouling. Since this kind of washing is applied when the machine operates close to full load conditions, injected droplets are strongly accelerated and consequently impact the rotor blades at high velocity, thus inducing undesirable phenomena like erosion. Here, we present a novel technique to study long-term water droplets erosion by also considering the geometry modification caused by droplets impacts. Two-phase unsteady numerical simulations were carried out, considering the injection of water droplets and their transport across the fluid flow in the first part of a real compressor, which is modeled in the region extending from the inlet to the rotor blades of the first stage. Simulations are performed on the whole machine to account for the asymmetric distribution of the spray injectors, the machine struts, inlet guide vanes (IGVs), and rotor blades. The k−ɛ realizable turbulence model with standard wall functions was coupled with the discrete phase model to track injected droplets motion. Droplets-wall interaction is modeled following the Stanton–Rutland approach aiming at detecting the effect of droplet impact (deposit, rebound, and splashing) depending on the impact conditions. Moreover, a semi-empirical erosion model developed by the authors was used to evaluate the erosion induced by the droplets injection. Material removal due to erosion is converted into nodal mesh displacement that is used by a secondary routine to implement the mesh morphing scheme. The mesh modification is applied at discrete steps to reduce the computational load. This technique is adopted to account for the blades geometry modification due to water droplet erosion leading to performance losses. Moreover, an estimation of the compressor operating life before maintenance operations is given and the water washing efficiency during the whole life of the machine is evaluated by means of proper indices. At the end of the simulation workflow, erosion phenomena are observed in all the compressor regions, especially in the rotor where erosion peaks are reached at the hub of the blades leading edge. The rotor blades wet surface was found to remain almost constant at around 50% during compressor water washing. Erosive phenomena were proved to evolve nonlinearly with time indicating the need to account for the geometry modification for obtaining an accurate prediction of the long-time process.

Water washing of axial flow compressors: numerical study on the fate of injected droplets

Gabriele³

et al. 2020

E3S Web Conf.

Self Cite

In turbomachinery applications blade fouling represents a main cause of performance degradation. Among the different techniques currently available, online water washing is one of the most effective in removing deposit from the blades. Since this kind of washing is applied when the machine is close to design conditions, injected droplets are strongly accelerated when they reach the rotor blades and the understanding of their interaction with the blades is not straightforward. Moreover, undesirable phenomena like blades erosion or liquid film formation can occur. The present study aims at assessing droplets dragging from the injection system placed at the compressor inlet till the first stage rotor blades, with a focus on droplets impact locations, on the washing process and the associated risk of erosion. 3D numerical simulations of the whole compressor geometry (up to the first rotor stage) are performed by using Ansys Fluent to account for the asymmetric distribution of the sprays around of the machine struts, IGV and rotor blades. The simulations are carried out by adopting the k-ε realizable turbulence model with standard wall functions, coupled with the discretephase model to track injected droplets motion. Droplets-wall interaction is also accounted for by adopting the Stanton-Rutland model which define a droplet impact outcome depending on the impact conditions. The induced erosion is evaluated by adopting an erosion model previously developed by some of the authors and implemented in Fluent through the use of a User Defined Function (UDF). Two sets of simulations are performed, by considering the rotor still and rotating, representative of off-line and on-line water washing conditions, respectively. In the rotating simulation, the Multiple Reference Frame Model is used. The obtained results demonstrate that the washing process differs substantially between the fixed and the rotating case. Moreover, to quantify the water washing effectiveness and the erosion risk, new indices were introduced and computed for the main components of the machine. These indices can be considered as useful prescriptions in the optimization process of water washing systems.