Experimental Analysis of Wet Compression in Axial Compressor Stage

Authors:Roumeliotis I., Mathioudakis K.

Abstract

Water injection at compressor inlet has received significant attention in recent years, as it has found increasing industrial application. In order to obtain the full benefits, further understanding on the effect of water injection on compressor stage behavior is needed. In the present paper, experimental work concerning the effect of water injection on a compressor stage operation is presented. The measurements were obtained in an axial compressor stage, rotating al low speed so no evaporation may occur throughout the stage and allowing the assumption of incompressible flow. The effect on compressor stage performance is examined for injection rates up to 2%. The behavior of the air flow in the blade rows is examined through aerodynamic measurements. The results indicate that although the water injection appears to not have any significant effect on the flow pattern and to the stage pressure characteristic there is a measurable increase on the consumed power which results to degradation on overall stage efficiency. The consumed power increase due to water injection is analyzed in order to acquire further insight of the phenomenon and verify the hypothesis that the losses can be attributed to water centrifugation by the rotor.

Analysis of moisture condensation during air expansion in turbines

Authors:Roumeliotis I., Mathioudakis K.

Abstract

Water is always present in the atmospheric air in the form of vapour. As air of low temperature expands through the turbine condensation may occur. Condensation gives rise to several problems in materialization of the air cycle as it results in temperature rise thus to different conditions than the expected at the turbine exit. Also in most air cycle applications, liquid water must be removed from the air stream before it gets to the cooled space. In the case that turbine exit temperature is very low; there is the possibility of ice particles formation. A method allowing the evaluation of air properties when condensation occurs is required in order to examine its effect on the cycle and optimize the operating envelope. The present paper describes a thermodynamic equilibrium method for predicting the occurrence of condensation and calculating the mixture properties once condensation has occurred. The method has been validated against the experimental results from a turbocharger turbine. The experimental results show that condensation can cause significant alteration to the turbine exit conditions. It is demonstrated that condensation can be predicted and the mixture properties after condensation can also be accurately evaluated.

Turbocharger Unstable Operation Diagnosis Using Vibroacoustic Measurements

Authors:Aretakis N., Mathioudakis K., Kefalakis M., Papailiou K.D.

Abstract

The possibility to detect unstable operating condition (stall or surge) of an automotive turbocharger using vibration or acoustic measurements is studied. An experimental study is performed, in order to acquire and analyze test data, to find out whether vibration or acoustic measurements can be correlated to aerothermodynamic operating condition. An instrumentation set allowing the definition of the operating point on the map of the compressor of the turbocharger is used. Hot wires the compressor inlet serve as flow condition indicators and provide a clear indicator of the presence or not of instabilities, such as rotating stall or surge. Accelerometers are mounted on the casing and microphones are placed in the vicinity of the compressor casing, to measure vibration and sound emission. Data covering an extensive range of the compressor performance map have been collected and analyzed. Signal features from the different measuring instruments are discussed. Using such features, a bi-parametric criterion is established for determination of whether the compressor operates in the stable part of its performance characteristic or in the presence of unstable operation phenomena (rotating stall, surge). The possibility of generalizing the validity of observations is supported, by presenting results from testing a second turbocharger, which is shown to exhibit similar behavior.

Moisture Condensation Effect on Turbine Performance Tests

Authors:Roumeliotis I., Mathioudakis K.

Abstract

Water is always present in the atmospheric air in the form of vapour, affecting the operation of turbomachinery components in gas turbine engines. Due to water presence in the working medium, condensation may occur, which can influence the thermal performance of the component and alter the measurements taken for calculations. This can lead to erroneous evaluation of component performance parameters during development performance tests. Procedures to detect condensation and if possible to correct the measurements during engine or component test should be used to avoid such situations. A method allowing the prediction of condensation and the correction of the measurements for low speed expansion is presented. The method is implemented in turbine testing measurements where condensation occurs and the results show that condensation may be predicted and its effects corrected.

Design and Development of a Small Jet Engine Test Cell

Authors:Mathioudakis K., Argyropoulos P.

Abstract

The process of designing a test cell for a small jet engine is described. The test cell was designed to support educational activities in the field of gas turbines. The design was undertaken by a student, as a part of his requirements for obtaining a Mechanical Engineering Degree. The requirements for the design are first laid out, according to the characteristics of the engine to be tested and the restrictions posed by the installation location. The phases of designing and construction are presented. Technical aspects of design choices for setting up the test chamber are presented. The choice of instrumentation and the way it is arranged is discussed. Results from the first tests conducted on the finished facility are presented, first, to check how design targets have been met and second, to show what kind of information can be provided to support gas turbine courses. The fact that a multidisciplinary knowledge is applied in the project is Educational benefits for the student that has undertaken the project are discussed, while particular choices made for a better suitability for subsequent educational use are highlighted.

Turbomachinery Flow Measurements Using Long-Nose Probes

Authors:Doukelis A., Mathioudakis K.

Abstract

The present work provides a detailed account of a pneumatic measuring technique appropriate for flow field measurements in turbomachinery configurations, making use of long-nose 5-hole probes. The way of obtaining flow quantities in a frame of reference on the sensing head of the probe is first addressed. Transformation of velocity co-ordinates from the probe frame to a stationary frame, customary for turbomachinery flows, is then discussed. Sources of error are also discussed, with particular attention on those that can be introduced by the nose geometry and the co-ordinate transformations. The potential of the measuring technique is demonstrated by presenting the application of the technique for measurements in an annular cascade facility. The results are compared to results obtained by a 3-D Laser-Doppler Anemometer.

Identifying Faults in the Variable Geometry System of a Gas Turbine Compressor

Authors:Tsalavoutas A., Mathioudakis K., Stamatis A., Smith M.K.

Abstract

The influence of faults in the variable geometry (variable stator vanes) system of a multistage axial compressor, on the performance of an industrial gas turbine is investigated. An experimental investigation has been conducted, by implanting such faults into an operating gas turbine. The faults examined are individual stator vane mistuning of different magnitude, and located at different stages. Fault identification is based on the aerothermodynamic measurement data and is achieved by employing two different techniques, namely adaptive performance modelling and monitoring the circumferential distribution of the turbine exit temperature. It is observed that the deviations of the modification factors, introduced to an adaptive performance model, form patterns that can be used to identify the faults. The patterns characterize both the kind and the magnitude of the fault. On the other hand, the turbine exit temperature profile is also influenced and its change can be used as additional information, to increase the confidence level of the diagnosis (contrary to customary practice, which expects temperatures profiles to reflect only burner or turbine malfunctions).

Classification of Radial Compressor Faults Using Pattern Recognition Techniques

Authors:Aretakis N., Mathioudakis K.

Abstract

An application of pattern recognition techniques for classification of faults in a radial compressor is presented. A number of mechanical alterations, simulating faults, are introduced in a test compressor. They include the insertion of an inlet obstruction, an obstruction in a diffuser passage, variation of impeller tip clearance and impeller fouling. Two kinds of measurements namely sound emission and casing vibration are examined. Three kinds of pattern recognition techniques with increasing complexity are used in order to correctly classify the examined faults according to engine condition. The possibility of using each one of these techniques for diagnosing faults in a radial compressor is also examined. It is demonstrated that minor faults, which do not affect performance, can be identified using the proposed techniques

Derivation of Signatures for Faults in Gas Turbine Compressor Blading

Authors:Aretakis N., Mathioudakis K., Dedoussis V.

Abstract

A procedure for the derivation of signatures for faults in the blades of a gas turbine is presented here. A variety of blade faults,corresponding to changes in the angle or spacing of one or more blades, are examined. A ?uid dynamic simulation model is used to derive the unsteady pressure signals sensed by a stationary transducer for the cases of faulty blades. The blade fault signatures are derived by processing these signals using Fourier techniques. The features of blade fault signatures are studied. Finally, the possibilities they o§er for the discrimination and identi?cation of di§erent possible blade faults are also examined.

Radial Compressor Fault Identification using Dynamic Measurement Data

Authors:Aretakis N., Mathioudakis K.

Abstract

The exploitation of different unsteady quantity measurements for identifying mechanical alterations on a radial compressor with a vaned diffuser is examined in this paper. Measurements of sound emission, casing vibration and unsteady inner wall pressure are performed. The mechanical alterations considered have been chosen in order to reproduce or simulate faults in the compressor. They include the insertion of an inlet obstruction, an obstruction in a diffuser passage, variation of impeller tip clearance, and impeller fouling. Processing these measurement data leads to the derivation of fault signatures which can be utilized for identifying them. The suitability of measuring each of the above physical quantities is discussed with respect to their sensitivity to particular faults. The dependence of the fault signatures on operating point is also examined. It's demonstrated that minor faults which do not affect compressor operation and are not detectable by performance monitoring, can possibly be detected by the proposed methodology.

Jet Engine Fault Detection with Discrete Operating Points Gas Path Analysis

Authors:Stamatis A., Mathioudakis K., Berios K., Papailiou K.D.

Abstract

A common feature of all Differential Gas Path Analysis methods is the necessity of measuring a number of performance variables greater or at least equal to the number of diagnostic parameters which have to be estimated. Discrete Operating Conditions Gas Path Analysis (DOCGPA) is an extended version of the conventional GPA algorithms, providing-among other things-the capability to overcome this problem. In the present paper, we describe how this method can be coupled with an engine computer model, in order to perform component directed fault diagnosis. Application to a commercial turbofan engine demonstrates the effectiveness of the proposed method.

Reliable experimental procedure for the evaluation of alternative fuel emissions in small-scale turbojet engines: A bio-kerosene blend case study

Authors: Villette S, Alexiou A, Aretakis N, Mathioudakis K.

Abstract

Most emission studies conducted on micro gas-turbine (MGT) testbeds report inconsistent or contradictory results. The primary cause is the entrainment of ambient air into the exhaust stream during sampling, since the analyzer probe must operate directly in the free jet discharged by the engine. This dilution alters the measured gas composition, increases uncertainty and can mask or even reverse the subtle emission effects associated with bio-kerosene and other Sustainable Aviation Fuel (SAF) blends. Consequently, it becomes difficult to reliably assess, extrapolate or interpret the influence of fuel composition on emission behaviour—despite the importance of accurate SAF evaluation for aviation decarbonization efforts. To address this limitation, an alternative sampling arrangement is proposed and experimentally validated. In the new configuration, the exhaust stream is first guided through a duct & chamber configuration, where the gas decelerates and becomes shielded from ambient dilution before being sampled. This modification significantly improves the stability and repeatability of the measurements. Using Jet-A fuel, the improved setup reduces emission uncertainty by up to 83% for CO2 and O2, and by 25 & 39% for NOx and CO, enabling consistent detection of fuel-dependent emission trends. A series of bio-kerosene blend tests were performed using both the conventional exposed-probe arrangement and the improved shielded-sampling configuration. With both sampling arrangements, the measured thrust–speed profile remains essentially identical across all fuel blends, indicating that neither setup alters engine performance evaluation. However, the corresponding emission profiles differ markedly. In the conventional exposed-probe configuration, the effect of increasing bio-kerosene concentration on emissions appears inconsistent and is often obscured by measurement variability caused by ambient-air dilution. In contrast, the improved shielded-sampling configuration yields a clearer and statistically consistent trend: NOx levels decrease with increasing blending ratio, reaching reductions of up to 16% for the B60 blend. Overall, the proposed sampling method enhances the reliability of emission measurements in small-scale turbojet engines and enables their use as a consistent and meaningful platform for SAF evaluation. Keywords: Micro-gas turbine; Small-scale turbojet engine; Emission measurement method; Emission analyzer probe placement; Bio-kerosene

Combustion Uniformity and Emissions Reduction in a Microgas Turbine Powered by Biojet Fuel Blends Derived From Used Cooking Oil.

Authors: Villette S, Alexiou A, Aretakis N, Mathioudakis K.

Abstract

This study experimentally investigates the impact of biokerosene blends derived from used cooking oil (UCO) on the performance and emissions of a microgas turbine, assessing their suitability as drop-in sustainable aviation fuels (SAFs). Engine tests were performed using jet-A and UCO-based biojet blends at volumetric ratios of 10%, 25%, 50%, and 60% in a fully instrumented and carefully calibrated small-scale turbojet test rig, capturing undiluted emissions as well as detailed performance parameters. Across the full operating range, thrust, fuel consumption, air mass flow, compressor pressure ratio, combustor pressure losses, exhaust gas temperature (EGT), and gaseous emissions (CO2, O2, NOx, CO) were systematically analyzed. Results show that overall engine performance remained effectively unchanged, with observed variations remaining within typical measurement uncertainty. Importantly, increased biojet content correlated with reductions of up to 16% in NOx and 12% in CO emissions, effects linked to enhanced combustion uniformity quantified via circumferential EGT deviation. This work therefore presents the first experimental demonstration of this correlation in a microgas turbine, alongside the first combustor pressure loss data for multiple biojet/jet-A blends. The findings clearly indicate that UCO-derived biojet blends up to 60% offer tangible environmental benefits without compromising engine performance, while also providing novel insights into fuel–air mixing effects directly relevant for future sustainable aviation fuel (SAF) deployment.

From Small-Scale Turbojet Biojet Experiments To Civil Aeroengine Applications: An Adaptive Physics-Based Approach..

Authors: Villette S, Alexiou A, Aretakis N, Mathioudakis K.

Abstract

This paper proposes a comprehensive and straightforward method to extrapolate biojet effects on civil aeroengines. The approach integrates experimental results from biojet blend evaluation on a micro gas turbine (MGT) test rig with low-order numerical aeroengine and combustor models, all developed and executed at the Laboratory of Thermal Turbomachines (LTT) of National Technical University of Athens (NTUA), forming an experiment-driven, physics-based methodology. The proposed biojet modeling strategy is validated at small scale by comparing MGT-produced emission measurements with corresponding predictions from a 1D burner model (properly adapted to simulate the MGT burner), which are obtained through a simple modification of the assumed fuel composition. Subsequently, a geared-turbofan (GTF) engine model, incorporating the 1D burner model, is calibrated for Jet-A operation against International Civil Aviation Organization (ICAO) performance and emission data. The calibrated model is then subjected to landing & take-off, off-design and mission analyses for both Jet-A and a 50% biojet blend, producing quantitative estimates of the biojet blend’s impact on engine and aircraft performance and emissions. The results reveal that, when powering a GTF engine with this specific biojet blend, overall mission CO2, NO𝑥 and CO emissions can be reduced by up to 1.4%, 3% and 3.5%, respectively, while fuel consumption is also slightly reduced for the same thrust requirement.