Some Effects of Size on the Performances of Small Gas Turbines

By the new millennia gas turbine technology standards the size of the first gas turbines of Von Ohain and Whittle would be considered small. Since those first pioneer achievements the sizes of gas turbines have diverged to unbelievable extremes. Large aircraft turbofans delivering the equivalent of 150 megawatts, and research micro engines designed for 20 watts. Microturbine generator sets rated from 2 to 200kW are penetrating the market to satisfy a rapid expansion use of electronic equipment. Tiny turbojets the size of a coca cola can are being flown in model aircraft applications. Shirt button sized gas turbines are now being researched intended to develop output powers below 0.5kW at rotational speeds in excess of 200 Krpm, where it is discussed that parasitic frictional drag and component heat transfer effects can significantly impact cycle performance. The demarcation zone between small and large gas turbines arbitrarily chosen in this treatise is rotational speeds of the order 100 Krpm, and above. This resurgence of impetus in the small gas turbine, beyond that witnessed some forty years ago for potential automobile applications, fostered this timely review of the small gas turbine, and a re-address of the question, what are the effects of size and clearances gaps on the performances of small gas turbines?. The possible resolution of this question lies in autopsy of the many small gas turbine component design constraints, aided by lessons learned in small engine performance development, which are the major topics of this paper.

Modelling and Simulation of Transient Performance of the Semi-Closed O2/CO2 Gas Turbine Cycle for CO2-Capture

One of the concepts proposed for capture of CO2 in power production from gaseous fossil fuels is the semi-closed O2/CO2 gas turbine cycle. The semi-closed O2/CO2 gas turbine cycle has a near to stoichiometric combustion with oxygen, producing CO2 and water vapor as the combustion products. The water vapor is condensed and removed from the process, the remaining gas, primarily CO2, is mainly recycled to keep turbine inlet temperature at a permissible level. A model for predicting transient behavior of the semi-closed O2/CO2 gas turbine cycle is presented. The model is implemented in the simulation tool gPROMS (Process System Enterprise Ltd.), and simulations are performed to investigate two different issues. The first issue is to see how different cycle performance variables interact during transient behavior; the second is to investigate how cycle calculations are affected when including the gas constant and the specific heat ratio in compressor characteristics. The simulations show that the near to stoichiometric combustion and the working fluid recycle introduce a high interaction between the different cycle components and variables. This makes it very difficult to analytically predict the cycle performance during a transient event, i.e. simulations are necessary. It is also found that, except for the shaft speed calculation, the introduction of gas constant and specific heat ratio dependence on the compressor performance map will have only a minor influence on the process performance.

Conjugate Flow and Heat Transfer Investigation of a Turbo Charger: Part I — Numerical Results

In this paper a three-dimensional conjugate calculation has been performed for a passenger car turbo charger. The scope of this work is to investigate the heat fluxes in the radial compressor which can be strongly influenced by the hot turbine. As a result of this, the compressor efficiency may deteriorate. Consequently, the heat fluxes have to be taken into account for the determination of the efficiency. To overcome this problem a complex three-dimensional model has been developed. It contains the compressor, the oil cooled center housing, and the turbine. 12 operating points have been numerically simulated composed of three different turbine inlet temperatures and four different mass flows. The boundary conditions for the flow and for the outer casing were derived from experimental test data (part II of the paper). Resulting from these conjugate calculations various one-dimensional calculation specifications have been developed. They describe the heat transfer phenomena inside the compressor with the help of a Nusselt number which is a function of an artificial Reynolds number and the turbine inlet temperature.

New Free-Piston Topping Cycle for Gas-Turbine Power Plants

A proposed topping cycle inserts a free-piston internal-combustion engine between the compressor and the combustor of a combustion turbine. The topping cycle diverts air from the compressor to supercharge the free-piston engine. Because the free-piston engine uses gas bearings to support the piston and is built of high-temperature materials, the engine can increase the pressure and temperature of the gas, exhausting it to a small expander that produces power. The exhaust from the topping-cycle expander is at a pressure that can be re-introduced to the main turbine, allowing almost complete recovery of waste heat. A capacity increase exceeding 35% is possible, and overall cycle efficiency can approach 70% when incorporated into a state-of-the-art combined-cycle plant. The cost of per incremental kW of the topping cycle can be dramatically lower than that of the base turbine because of the high power density and simplicity of the engine. Building on decades of progress in combustion turbines systems, the new cycle promises high performance without the engineering risks of manufacturing a completely new cycle.

The Feasibility of Using an Air Turbine to Drive an Afterburner Fuel Pump

Current fighter engine designs extract power to drive the afterburner fuel pump through the use of a gearbox. The presence of the gearbox only allows the fuel pump to operate at a fixed proportion of engine speed. In addition the fuel pump is continually rotating, although not pumping fuel, even when the afterburner is not engaged. This article investigates the feasibility of using an air turbine to drive the afterburner fuel pump in preparation for supporting an all-electric engine. Utilising performance data for a typical modern military engine, 1-dimensional design techniques were used to design several radial turbines to power the afterburner fuel pump. A choice of an axial or a radial air turbine is possible. Both were reviewed and it was determined that a radial turbine is optimum based on manufacturability and (theoretical) efficiency. Several design iterations were completed to determine the estimated weight and size based on various air off-take locations, mass flows, and rotational speeds. These iterations showed that increasing mass flow allows for lower rotational speeds and/or smaller diameter rotors, but with a corresponding increases in thrust penalties.

Preliminary Results of a Cold Flow Test in a Fuel Cell Gas Turbine Hybrid Simulation Facility

The dynamic interdependencies created during the integration of fuel cell and a gas turbine in a hybrid power generation system are not well understood. Because these systems are new, there are risks that unexpected complications might arise during both steady state operation and transient events. A 250kW experimental fuel cell gas turbine simulation facility has been constructed at the National Energy Technology Laboratory (NETL), U.S. Department of Energy to examine the effects of transient events on the dynamics of these systems. A natural gas burner controlled by a real-time fuel cell model is used in the facility to simulate the thermal output of a solid oxide fuel cell during transient events. Pressure vessels are used for simulating the cathode and post combustion volumes, and are integrated into the system with a modified turbine and the fuel cell simulator. Preliminary results of system characterization are presented and discussed in context of the test scenarios proposed for experimental evaluation of thermal and mechanical transient impact on fuel cell and the gas turbine systems.

Condition Based Maintenance, Life Cycle Management, and Autolog

Condition Based Maintenance (CBM) was instituted by the United States Navy in a policy outlined in OPNAV INSTRUCTION 4790.16 dated 6 May 1998. The goal is to move from time-directed preventive maintenance to condition-directed maintenance. It is hoped this will optimize readiness while reducing maintenance and manning requirements. The concept is that use of sensors, algorithms, and automated reasoning and decision making models to monitor equipment operations will provide critical analyses to operators that will help prevent impending failure. Red flags to operators allows maximization of maintenance effort that will focus limited resources to areas most needed to ensure safety and mission readiness while simultaneously minimizing operating costs (O & S), labor, and risk of mission degrading failures (Hedderich [3]). For the U.S. Navy, there is a large chasm to bridge between vision and reality. CBM technology is being slowly tested and integrated. But testing, modifying and back fitting all the Navy’s critical systems with CBM technology will be long term and costly and will be constantly faced with the dilemma of having just integrated one technology as it is being replaced by newer ones. In the mean time, more focus could be paid to possible interim phases that could be more quickly and cheaply integrated and still move the Navy forward in utilizing CBM technology. Autolog is such an effort and is offered here as an application and process to gather more real time data needed for CBM from one source that can be quickly and easily provided to distant engineering support activities for Gas Turbines systems while also easing record keeping and data transmittal requirements for the fleet.

A New Concept of Impingement Cooling for Gas Turbine Hot Parts and Its Influence on Plant Performance

Significant improvements in gas turbine cooling technology are becoming harder as progress goes over and over. Several impingement cooling solutions have been extensively studied in past literature. An accurate and extensive numerical 1D simulation on a new concept of sequential impingement was performed, showing good results. Instead of having a single impingement plate, we used several perforated plates, connecting the inlet of each one with the outlet of the previous one. Main advantages are: absence of the negative interaction between transverse flow and last rows impinging jets (reduced deflection); better distribution of pressure losses and heat transfer coefficients among the different plates, especially when pressure drops are significant and available coolant mass flow rate is low (lean premixed combustion chamber and LP turbine stages). Practical applications can have a positive influence on both cooled nozzles and combustion chambers, in terms of increased cooling efficiency and coolant mass flow rate reduction. Calculated effects are used to analyze main influences of such a cooling system on global performances of power plants.

Centrifugal Compressor of a 100 kW Microturbine: Part 1 — Experimental and Numerical Investigations on Overall Performance

This paper describes on/off design performance of a centrifugal compressor of a 100 kW turbogenerator gas turbine engine used for small scale power generation. The compressor stage is made up of a radial impeller and a two-stage diffuser (radial and deswirl). Part 1 deals with the experimental and numerical tests on overall compressor and diffuser performance: An extensive test series with steady probe measurements at impeller exit and diffuser exit is performed at different operating points and rotational speeds. This makes it possible to characterize both overall compressor and diffuser. The numerical model is based on a mixing plane at impeller-diffuser interface and therefore neglects the effect of unsteadiness due to rotor-stator interaction. Then, in part 2 the true time-dependent interaction is investigated by means of a numerical model where a sliding mesh technique is adopted. The unsteady results are then processed and compared with the steady ones regarding the flow in the diffuser. Finally, in part 3 the geometry of the compressor diffuser is optimized using an evolutionary algorithm coupled with a CFD code in order to improve compressor performance.

Micro Humid Air Cycle: Part B — Thermoeconomic Analysis

Part A of this paper demonstrated that the HAT cycle, when applied to small-size gas turbines, can significantly enhance the efficiency and specific work of simple and recuperated cycles without the drastic changes to plant layout necessary in medium- and large-size plants. In this part B a complete thermoeconomic analysis is performed for microturbines operating in a Humid Air cycle. The capital cost and internal rate of return for both new machines and existing microturbines working in an mHAT-optimised cycle are presented and analysed. Three different scenarios are considered. The first scenario reflects a distributed electrical power generation application where cogeneration is not taken into account. Instead, the other two scenarios deal with CHP civil applications for different heat demands. The thermoeconomic results of the integrated mHAT cycle, based on a preliminary design of the saturator, demonstrate that microturbine performance can be greatly enhanced, while specific capital costs, in some cases, can be reduced up to 14%, without significant increase in layout complexity. Moreover, thanks to its operational flexibility (able to operate in dry and wet cycles), the mHAT is financially attractive for distributed power and heat generation (micro-cogeneration), particularly when heat demand is commutated in short period.