The boundary layer equations for plane, incompressible, and steady flow are 

$$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Boundary Layer Theory

The effects of surface roughness on gas turbine performance are reviewed based on publications in the open literature over the past 60 years. Empirical roughness correlations routinely employed for drag and heat transfer estimates are summarized and found wanting. No single correlation appears to capture all of the relevant physics for both engineered and service-related (e.g., wear or environmentally induced) roughness. Roughness influences engine performance by causing earlier boundary layer transition, increased boundary layer momentum loss (i.e., thickness), and/or flow separation. Roughness effects in the compressor and turbine are dependent on Reynolds number, roughness size, and to a lesser extent Mach number. At low Re, roughness can eliminate laminar separation bubbles (thus reducing loss) while at high Re (when the boundary layer is already turbulent), roughness can thicken the boundary layer to the point of separation (thus increasing loss). In the turbine, roughness has the added effect of augmenting convective heat transfer. While this is desirable in an internal turbine coolant channel, it is clearly undesirable on the external turbine surface. Recent advances in roughness modeling for computational fluid dynamics are also reviewed. The conclusion remains that considerable research is yet necessary to fully understand the role of roughness in gas turbines.

A Review of Surface Roughness Effects in Gas Turbines

Health monitoring is an essential part of condition-based maintenance and prognostics and health management for gas turbines. Various health monitoring systems have been developed based on the measurement and observation of the fault symptoms including turbine performance parameters such as heat rate, and nonperformance symptoms such as structural vibration. This paper focuses on surveying state-of-the-art condition monitoring, diagnostic and prognostic techniques using performance parameters acquired from gas-path data that are mostly available from the operating systems of gas turbines. Performance parameters and the corresponding effective factors are presented in the beginning. Structure of performance monitoring and diagnostic systems are systematically laid out next, and the recent developments in each section are surveyed and discussed. Observing the importance of the prognostics in the recent trend of health monitoring research, an emphasis is given on the prognostic frameworks and their implementation for the remaining useful life prediction. A conclusion along with a brief discussion on the current state and potential future directions is provided at the end.

Performance-Based Gas Turbine Health Monitoring, Diagnostics, and Prognostics: A Survey

Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Fouling is caused by the adherence of particles to airfoils and annulus surfaces. Particles that cause fouling are typically smaller than 2 to 10 microns. Smoke, oil mists, carbon, and sea salts are common examples. Fouling can be controlled by appropriate air filtration systems, and can often be reversed to some degree by detergent washing of components. The adherence of particles is impacted by oil or water mists. The result is a build-up of material that causes increased surface roughness and to some degree changes the shape of the airfoil (if the material build up forms thicker layers of deposits). Fouling mechanisms are evaluated based on observed data, and a discussion on fouling susceptibility is provided. A particular emphasis will be on the capabilities of modern air filtration systems.Copyright © 2011 by Solar Turbines Inc.

Fouling Mechanisms in Axial Compressors

A simple physics-based model for particle rebound and deposition in turbomachinery

Gas turbine performance deterioration can be a major economic factor. An example is within offshore installations where a degradation of gas turbine performance can mean a reduction of oil and gas production. This paper describes the test results from a series of accelerated deterioration tests on a General Electric J85-13 jet engine. The axial compressor was deteriorated by spraying atomized droplets of saltwater into the engine intake. The paper presents the overall engine performance deterioration as well as deteriorated stage characteristics. The results of laboratory analysis of the salt deposits are presented, providing insight into the increased surface roughness and the deposit thickness and distribution. The test data show good agreement with published stage characteristics and give valuable information regarding stage-by-stage performance deterioration.

/pdf/axial-compressor-deterioration-caused-by-saltwater-ingestion-t0oics0xqh.pdf

Axial Compressor Deterioration Caused by Saltwater Ingestion

Efficient inlet air filtration is a key element for limiting fouling, erosion, and corrosion in the compressor section of offshore gas turbine installations. Current filtration systems are normally successful in preventing serious erosion and corrosion problems in the compressor section, but significant performance deterioration caused by compressor fouling still remains a challenge. This performance deterioration increases fuel consumption and emissions and has a particularly severe economic impact when it reduces oil and gas production. Operating experience from different offshore installations has shown that the deterioration rate in gas turbine performance increases when the turbines are operating in wet or humid weather and that the differential pressure loss over the intake system is affected by ambient humidity. An experimental test rig has been built in the laboratory at the Norwegian University of Science and Technology (NTNU) in order to increase understanding of the fundamentals related to gas turbine inlet air filtration. This paper presents the results from an experimental investigation of the performance of gas turbine inlet air filter elements that have been in operation offshore. Performance under both dry and wet conditions is assessed. Different types of filter elements show significantly different changes in differential pressure signature when exposed to moisture, and all of the tested filter elements demonstrate a loss of accumulated contamination after operating in wet conditions. Hence, contaminants originally accumulated by the filter elements are re-entrained into the airstream on the downstream side of the filters when they are exposed to moisture. The change in differential pressure signature as a result of operating in wet conditions demonstrates another weakness of solely applying differential pressure for condition monitoring of the filter system.Copyright © 2010 by ASME

Performance Deterioration of Intake Air Filters for Gas Turbines in Offshore Installations

Gas turbine performance deterioration caused by fouling in the compressor section is a well known phenomenon in offshore installations This performance deterioration not only increases fuel consumption and emissions but also has a severe economic impact when it reduces oil and gas production Because fouling in the compressor section is commonly caused by intake air contamination, gas turbines offshore have air inlet filtration systems in order to limit the amount of ingested contaminants Many different filtration systems from various suppliers are in operation offshore Manufacturers supply documentation for their filtration system based on several international standards, and it can be challenging for the operator to make a direct comparison of different filtration systems The comparison is further complicated by the fact that the characteristic offshore challenges related to salt and moisture in the intake air are not adequately covered in international standards, and these challenges are handled and documented differently among the manufacturers This paper analyzes the challenges related to choosing the best filter solution for an offshore gas turbine installation based on data from offshore sites in the North Sea The relevance of test requirements in applicable international standards and available supplier documentation is evaluated based on actual operating conditions offshore Deviations among international test standard requirements, available manufacturer documentation, and actual operating conditions offshore are identified, and improved test requirements are suggested In addition, this paper addresses the long-term effects of filter contamination and methods for intake filter monitoring based on data from offshore sites in the North SeaCopyright © 2009 by ASME

Filtration of Gas Turbine Intake Air in Offshore Installations: The Gap Between Test Standards and Actual Operating Conditions

Contamination in the intake air causes fouling in the compressor section of gas turbines. The amount and type of fouling present in the compressor section is site-specific, and knowledge of its composition is important in order to achieve efficient intake air filtration. This knowledge is also of great importance when optimizing both online and offline compressor wash regimes. This paper presents the results of an investigation of compressor fouling in two different offshore gas turbine installations. Fouling samples collected from various locations in the gas turbine air intakes, inlet guide vanes, and first compressor rotor stages were analyzed in a laboratory using an electron probe micron analyzer. The structure and composition of the analyzed compressor fouling is determined, and the probable sources of the different elements are identified.© 2009 ASME

Compressor Fouling in Gas Turbines Offshore: Composition and Sources From Site Data

There is currently no international standard for evaluating and documenting the performance of the complete gas turbine intake air system in offshore applications. Several suppliers document the performance of their filters in accordance with applicable Heating, Ventilation, and Air Conditioning (HVAC) air-filtration standards for general ventilation. These standards fail to address the offshore-specific challenges related to salt removal and moist and wet operation and cannot be used to accurately predict operational performance or life. It is therefore desirable to develop suitable test methods and standards that can be used to better predict operational performance and life before filters and complete inlet air systems are put into operation offshore. An experimental test rig has been built in the laboratory at the Norwegian University of Science and Technology (NTNU) in order to increase understanding of the fundamentals related to gas turbine inlet air filtration. This paper presents the results from an experimental test program where the test rig was used to evaluate the effect of accelerated deterioration of high-efficiency filter elements for gas turbine inlet air filtration. High-efficiency filter elements from different suppliers were deteriorated by ingesting a saltwater solution. The performance of the filters exposed to accelerated deterioration was evaluated for different levels of contamination and compared to the performance of filter elements that have accumulated comparable amounts of contaminants in offshore operation.Copyright © 2010 by ASME

Olaf Brekke

Papers

Axial Compressor Deterioration Caused by Saltwater Ingestion

Performance Deterioration of Intake Air Filters for Gas Turbines in Offshore Installations

Filtration of Gas Turbine Intake Air in Offshore Installations: The Gap Between Test Standards and Actual Operating Conditions

Compressor Fouling in Gas Turbines Offshore: Composition and Sources From Site Data

Accelerated Deterioration by Saltwater Ingestion in Gas Turbine Intake Air Filters