Overall thermodynamic model of an ultra micro turbine

To reduce the size and weight of power generation machines for portable devices, several systems to replace the currently used heavy batteries are being investigated worldwide. As micro gas turbines are expected to offer the highest power density, several research groups launched programs to develop ultra micro gas turbines: IHI firm (Japan), PowerMEMS Consortium (Belgium). At Onera, a research program called DecaWatt is under development in order to realize a demonstrator of a micro gas turbine engine in the 50 to 100 Watts electrical power range. A single-stage gas turbine is currently being studied. First of all, a calculation of the overall efficiency of the micro gas turbine engine has been carried out according to the pressure ratio, the turbine inlet temperature and the compressor and turbine efficiencies. With realistic hypotheses, we could obtain an overall efficiency of about 5% to 10% which leads to around 200 W/kg when taking into account the mass of the micro gas turbine engine, its electronics, fuel and packaging. Moreover, the specific energy could be in the range 300 to 600 Wh/kg which exceeds largely the performance of secondary batteries. To develop such a micro gas turbine engine, experimental and computational work focused on: • a 10 mm in diameter centrifugal compressor, with the objective to obtain a pressure ratio of about 2.5 • a radial inflow turbine • journal and thrust gas bearings (lobe bearings and spiral grooves) and their manufacturing • a small combustor working with hydrogen or hydrocarbon gaseous fuel (propane) • a high rotation speed micro-generator • the choice of materials Components of this tiny engine were tested prior to the test with all the parts assembled together. Tests of the generator at 700,000 rpm showed a very good efficiency of this component. In the same way, compressor testing has been performed up to 500,000 rpm and has shown that the nominal compression rate at the 840,000 rpm nominal speed should be nearly reached.

To reduce the size and weight of power generation machines for portable devices, several systems to replace the currently used heavy batteries are being investigated worldwide. As micro gas turbines are expected to offer the highest power density, several research groups launched programs to develop ultra micro gas turbines: IHI firm (Japan), PowerMEMS Consortium (Belgium). At Onera, a research program called DecaWatt is under development in order to realize a demonstrator of a micro gas turbine engine in the 50 to 100 Watts electrical power range. A single-stage gas turbine is currently being studied. First of all, a calculation of the overall efficiency of the micro gas turbine engine has been carried out according to the pressure ratio, the turbine inlet temperature, and the compressor and turbine efficiencies. With realistic hypotheses, we could obtain an overall efficiency of about 5% to 10%, which leads to around 200 W/kg when taking into account the mass of the micro gas turbine engine, its electronics, fuel and packaging. Moreover, the specific energy could be in the range 300 to 600 Wh/kg, which largely exceeds the performance of secondary batteries. To develop such a micro gas turbine engine, experimental and computational work focused on: (1) a 10-mm diameter centrifugal compressor, with the objective to obtain a pressure ratio of about 2.5; (2) a radial inflow turbine; (3) journal and thrust gas bearings (lobe bearings and spiral grooves) and their manufacturing; (4) a small combustor working with hydrogen or hydrocarbon gaseous fuel (propane); (5) a high rotation speed microgenerator; and (6) the choice of materials. Components of this tiny engine were tested prior to the test with all the parts assembled together. Tests of the generator at 700,000 rpm showed a very good efficiency of this component. In the same way, compressor testing was performed up to 500,000 rpm and showed that the nominal compression rate at the 840,000 rpm nominal speed should nearly be reached.

The design of a multi-level fuzzy-logic control system for the control of turbine engines using type-1 fuzzy-logic is presented in this paper. A Matlab-Simulink model of an advanced turbine engine was used to test and validate the performance of the developed fuzzy control system. The model provided comprehensive and complex model of a turbine engine. As with most modeling of turbine engines, it was assumed that the fundamental dynamic characteristics of a turbine engine can be achieved by control of the engine fuel flow. In this model a single input single output (SISO) system is used to control the engine fuel flow, where the input to the SISO system is the low spool speed error signal and the output is the fuel flow actuator control signal. A fuzzy controller was developed for the SISO system and its performance is compared with that of the existing proportional integral (PI) controller. Both controllers were tested under normal operating conditions and with introduction of engine faults embedded in the model.

Gas Turbine is a complex system and highly non linear in its overall performance. In order to study its impact on electric power quality under various load conditions, it is essential to create a high quality performance model of gas turbine to simulate its behavior in real time efficiently. This paper focuses on dynamic modeling of generic gas turbine model using alternate simulation environment for better feasibility. The model is developed on a virtual test bed which is an advanced dynamic simulation environment and can run a dynamic co-simulation effectively. The approach is by developing mathematical models of individual components of gas turbines and utilizing component performance map matching method to run the simulation. The paper discusses briefly about the VTB simulation environment and its use for dynamic modeling of gas turbine. The simulation studies carried out include design condition, off design condition and transient conditions. Working model of twin shaft turbine engine using compressor and turbine maps are validated with established gas turbine simulation software and results are shown.

The major problems for the development of an ultra micro gas turbine system were discussed briefly from the stand point of the internal flow and the performance characteristics. Following to these, the development of ultra micro centrifugal compression systems for the ultra micro gas turbine is explained with the design and the manufacturing processes. The measured results of ultra micro centrifugal compressors are shown.

A comparative study of the influence of different means of turbine cooling on the thermodynamic efficiency and specific work of gas turbines is presented. A common general model of a simple open cycle gas turbine is used to compare the performance of turbines using different types of cooling; internal convection and impingement by air, film cooling by air, internal convection and impingement by steam, film cooling by steam and closed loop cooling by water. The results are also compared to the previously published results of the analysis of open loop water cooled gas turbines. The model evaluates the efficiency and specific work of simple cycle gas turbines as it is influenced by mixing losses of coolant with combustion gases, pumping work of coolant and heat transfer from the expanding gas. The study is performed in terms of dimensionless variables in order to achieve generality and to provide useful design guidelines and insights. Blades internally cooled by convection and impingement are treated as heat exchangers operating at constant metal temperature and the coolant exit temperature is simply expressed as a function of a heat exchanger effectiveness, an independent parameter which is normally a function of the intricacy of the layout of the cooling passages. The coolant requirements and heat transfer with film cooling are determined using a dimensionless correlation derived experimentally at M.I.T. Sample calculations give the optimum turbine inlet temperature of thermodynamic efficiency and specific work for different pressure ratios and typical dimensionless numbers. The data on specific work are significant because they can be readily used in evaluations of a given type of gas turbine in a combined cycle. The sensitivity of the efficiency and specific work to each key input parameter is reported. The use of superheated steam as a coolant can provide some performance advantages since the steam raised in a waste heat boiler expands with the combustion gases, increases the turbine mass flow and also provides a certain amount of heat regeneration. Performance results are also reported for this steam cooled gas turbine operating with mixed working fluid.

Generic Analysis Methods for Gas Turbine Engine Performance: The development of the gas turbine simulation program GSP

The gas turbine (Brayton) cycle is a steady flow cycle, wherein the fuel is burnt in the working fluid and the peak temperature directly depends upon the material capabilities of the parts in contact with the hot fluid. In the gas turbine, the combustion and turbine parts are continuously in contact with hot fluid. The higher the firing temperature, higher is the turbine efficiency and output. Therefore, increasing turbine inlet temperature (firing temperature) has been most significant thrust for gas turbines over the past few decades and is continuing in pursuing higher power rating without much increase in the weight or size of the turbine. Firing temperature capability has increased from 800 °C in the first generation gas turbines to 1,600 °C in the latest models of gas turbines. Higher firing temperatures can only be achieved by employing the improved materials for components such as combustor, nozzles, buckets (rotating blades), turbine wheels and spacers. These critical components encounter different operating conditions with reference to temperature, transient loads and environment. The temperature of the hot gas path components (combustor, nozzles and buckets,) of a gas turbine is beyond the capabilities of the materials used in steam turbines thus requiring the use of much superior materials like superalloys, which can withstand severe corrosive/oxidizing environments, high temperatures and stresses. However, for thick section components such as turbine wheels, which require good fracture toughness, low crack growth rate and low coefficient of thermal expansion, alloy steels are extensively used. But the wheels of latest models of gas turbines, operating at very high firing temperatures (around 1,300–1,600 °C), are made of superalloy, which offers a significant improvement in stress rupture, tensile and yield strength and fracture toughness required for the application.

The measurement of gas turbine power output is essential for original equipment manufacturers (OEMs) and plant operators to evaluate the performance of their gas turbines. Accurate knowledge of the power output allows for better gas turbine control based on customer requirements. By measuring the GT power, OEMs can predict critical internal performance parameters such as turbine inlet temperature, compressor inlet flow, compressor efficiency, and turbine efficiency. Furthermore, accurate prediction of steam turbine efficiency can also be achieved using a mass and energy balance approach. In a multi-shaft combined cycle power plant, gas and steam turbine power are measured separately, enabling analysis of the heat and mass balance approach within individual turbine boundaries. However, in a single-shaft combined cycle power plant, only the total cycle power is measured, making it difficult to predict gas turbine and steam turbine power output separately. Accurate gas turbine power output prediction is critical to prevent improper gas turbine operation, such as over-firing or under-firing. Over-firing the gas turbine can damage the hot gas path life of the turbine while under-firing can result in a loss of gas turbine power. Additionally, accurate power output prediction enables OEMs to provide precise performance guarantees, reducing the risk of paying liquidity damages. To validate the single-shaft approach for predicting gas turbine power output, data from a multi-shaft combined cycle power plant was used, and accurate gas turbine power output was predicted and compared with the power observed using multiple-shaft gas turbine power. The first approach of the paper is solely founded on the principles of the first law of thermodynamics. The objective is to validate this first law approach using only a limited data set from a multishift combined cycle power plant, such as compressor inlet temperature, fuel flow, fuel lower heating value, combined cycle power output, and gas turbine power output. The other methods are Hybrid modeling approach and Data driven approach. In the hybrid method a model has been obtained using mass and energy balance equation and the real gas turbine data is used to obtain the values of unknown parameters in the hybrid model. In the Data driven approach, real gas turbine’s data is used to obtain a black box model. Hybrid models are generally regarded as superior to data-driven models in terms of accuracy because they combine the use of physics-based principles and data. Data-driven approaches, on the other hand, offer a simpler way to model plants or processes as they do not rely on prior knowledge of the system and can be developed solely using process data.

As one of the key devices in the high temperature gas turbine system, cross-corrugated recuperators provide high heat transfer capabilities with compact size, light weight, strong mechanical strength and are mandatory to achieve 30 % electrical efficiency or higher for micro turbine engines. Flow in such geometries is usually laminar with lower Reynolds numbers. In order to understand mechanisms of flowing and heat transfer, periodic fully developed fluid flow and heat transfer in two types of cross-corrugated structures with inclination angle at 90° are investigated numerically and experimentally. Periodicity was used to reduce the complexity of the channel geometry and enables the smallest possible segment of the flow channel to be modeled. The velocity and temperature distributions were obtained in the three-dimensional complex domain. Besides a detailed flow analysis, comparison of the local heat and mass transfer and the pressure losses for these geometries are presented. It is shown that the flow phenomena caused by the different geometries were of significant influence on the homogeneity and on the quantity of the local heat and mass transfer as well as on the pressure drop. As a recuperator for micro turbine engines, cross-corrugated sinusoidal channels are more preferable to triangular channels.

Waste energy utilization in heat-exchange gas turbine cycles

One of the ways that can be used to increase the efficiency of a shaft gas turbine engine is by installing a regenerative heat exchanger in one of the following two configurations: 1. after the low pressure turbine (usual case). 2. after the high pressure turbine (suggested). Analysis of ideal cycles as well as real cycle for both configurations is done by using a computer program, where the following parameters were studied: heat exchanger effectiveness, turbine efficiency, compressor efficiency, ratio of turbine inlet pressure to compressor delivery pressure (P3/P2), and maximum temperature ratio (T3/T1). From the sensitivity analysis for both configurations the usual configuration is inferior to the suggested in terms of the relative effects of compressor and turbine efficiencies on the overall efficiency and turbine inlet pressure on overall thermal efficiency and power output. However, the usual method is superior with respect to the relative effects of the compressor and turbine efficiencies on power output and flow path design and manufacture.

Second-law analysis (SLA) is an important concept in thermodynamics, which basically assesses energy by its value in terms of its convertibility from one form to another.[...]

Recent development of micro devices is remarkable as in the examples of Micro-TAS, Lab-on-a-chip or ultra micro gas turbine. In order to make the micro devices smaller and more effective, an appropriate use of a micro scale jet as an actuator can be a key technology. Aiming at the development of a measurement system of the micro flow control devices in the future micro aerodynamics, we have established a system to measure a continuous jet, a pulsed jet and a synthetic jet for the flow control in the low Reynolds number air flow with a micro length scale. The two-dimensional flow field around the micro jet using micro particle image velocimetry (PIV) was measured. The jet was injected through the device using an acoustic speaker. It was observed that a saddle point existed at the certain phase where the velocity is 0 at the boundary of the jet blowing and suction phase for the synthetic jet into a still air. It was found that the pulsed jet and the synthetic jet are more effective in the fluid mixing in the low Reynolds number flow than the continuous jet. The dead water region was observed downstream of the jet in case of the jet injection into cross flow. It was recognized that the synthetic jet at the certain oscillation frequency generated a vortex pair near the jet hole.

The mirror gas turbine proposed by Tsujikawa and Fujii extends the applications of turbo machinery. The characteristic component of a mirror gas turbine is a thermal generator, which is a kind of “inverted Brayton cycle”. The operating sequence of the thermal generator is reverse that of an ordinary gas turbine, namely, the hot working fluid is first expanded, and then cooled, compressed, and finally exhausted. In this work, we investigated the theoretical feasibility of inserting a thermal generator to a small reheat gas turbine of 30–100kW classes. Using process simulator software, we calculated and compared the thermal efficiency of this reheat gas turbine to that of a micro gas turbine under several conditions, turbine inlet temperature. This comparison showed that the performances of the both gas turbines are significantly influenced by the performance of the heat exchanger used for the recuperator. The efficiency of the micro gas turbine is also improved by using water injection into the compressor to cool the inlet gas. The resulting thermal efficiency of this reheat gas turbine is about 7% higher than that of a micro gas turbine with the same power unit.