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Assessment of a Wave Rotor Topped Demonstrator Gas Turbine Engine Concept
Abstract
A wave rotor topped gas turbine engine has been identified which incorporates five basic requirements of a successful demonstrator engine. Predicted performance maps of the wave rotor cycle have been used along with maps of existing gas turbine hardware in a design point study. The effects of wave rotor topping on the engine cycle and the subsequent need to rematch compressor and turbine sections in the topped engine are addressed. Comparison of performance of the resulting engine is made on the basis of wave rotor topped engine versus an appropriate baseline engine using common shaft compressor hardware. The topped engine design clearly demonstrates an improvement in shaft horsepower and SFC. Predicted off design part power engine performance for the wave rotor topped engine is presented including that at engine idle conditions. Operation of the engine at off design is closely examined with wave rotor operation at less than design burner outlet temperatures and rotor speeds. Challenges remaining in the development of a demonstrator engine are addressed.
Copyright © 1996 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
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... Therefore, a possibility of wave rotor topping cycle is described in this section. Wave rotors are expected to have a potential to improve drastically the performance of a gas turbine system [15][16][17]. A wave rotor consists of a rotor and ports as shown in Figure 12, and the rotor consists of many narrow channels called "cells", in which the working gas compression / expansion takes place. ...
... Figure 14 is the "wave diagram" which shows the pressure wave dynamics schematically with characteristic lines. This figure shows the diagram of 4-port through flow wave rotor [17], which is supposed to be appropriate for a gas turbine cycle. In this figure, an example of the total pressure and temperature in each port, which are normalized by the condition in Air-LP (Air Low Pressure Port), are indicated just for reference. ...
... With the present 1D numerical model, a wave rotor for kW size ultra-micro gas turbines was designed in reference to the NASA wave rotor [17]. This micro wave rotor is shown in Table 15 and Figure 25. ...
A synthesis of elementary heat balances along the gas path to establish a global energetic on the PowerMEMS type ultra-micro gas turbines (UMGT) is discussed, emphasizing, in particular, a new formulation of turbo-component performance in the presence of intense heat transfer. A potential benefit to integrate a wave rotor into UMGT system will be also presented to demonstrate the design feasibility according to numerical and experimental studies on a scaled-up model.
... Each port charges or discharges the fresh air or combustion gas respectively, and shock and expansion waves are generated and propagate in the cells by the pressure difference between the ports. Figure 1 shows the wave diagram of the four-port through-flow wave rotor [4] , which is one of the promising candidates for gas turbine cycle. In this figure, the cells are moving downward and the pressure wave propagation is drawn with characteristic lines. ...
... As for the LDA measurement, the flow velocity in the axial direction was measured. The system was set to measure the flow velocity in [12] NASA & Allison [4] Kentfield [12] G.E. [11] Comprex® (ABB) [13] Visualization 4). The detail of this LDA measurement was described in the reference [6] . ...
... These three are the dominant performance loss factors, according to the previous experimental works [11][12] , and the value of each parameter shows the amplitude of each performance loss. Table 1 shows the comparison of these parameters with other wave rotors [4,[11][12][13] . As shown here, the present apparatus covers wide range of τ parameter value by changing the rotor speed ( Table 2). ...
Wave rotor is expected to improve the performance of micro gas turbines drastically. In the wave rotor design, the rotor speed
is determined principally by the tube length. Therefore, a longer tube is preferable for miniaturized wave rotors to avoid
the difficulty in bearings and lubrication system, while it may yield thicker wall boundary layer, shock wave dissipation
and so on. In the present study, an experimental apparatus was built to visualize the wave rotor internal flow dynamics in
a narrow tube by schlieren method and Laser Doppler Anemometry. In addition, different lengths of the tube were adopted and
compared to investigate the effect of wall friction. Finally, 2D numerical simulation was performed and the results were compared
with those of experiments.
... Full, steady state, wave rotor topped engine cycle deck investigations have been performed but these can only state whether the engine will operate at one point or another, they cannot answer questions regarding transient response, stability, or controllability. 4,5 For example, in conventional compressor-combustorturbine engines rapid fuel flow rate changes are restricted due to compressor surge limit considerations. Is there a similar limitation for wave rotor topped engines? ...
... 2 Simulation Description Both the engine code and wave rotor code have been described extensively in the literature. [4][5][6][7] As such, only brief descriptions will be given below. ...
... Details of the Case 1 wave rotor design has been described in the literature. 4,5 The wave rotor was designed to be selfdriven, that is, requiring no external drive motor or transmission. In the present investigation however, the rotor was assumed to run at constant speed throughout The compressor and turbines were simulated in the GETRAN code by use of the global compressor and turbine subroutines available in the code. ...
The dynamic behavior of a wave rotor topped turboshaft engine is examined using a numerical simulation. The simulation utilizes an explicit, one-dimensional, multi-passage, CFD based wave rotor code in combination with an implicit, one-dimensional, component level dynamic engine simulation code. Transient responses to rapid fuel flow rate changes and compressor inlet pressure changes are simulated and compared with those of a similarly sized, untopped, turboshaft engine. Results indicate that the wave rotor topped engine responds in a stable, and rapid manner. Furthermore, during certain transient operations, the wave rotor actually tends to enhance engine stability. In particular, there is no tendency toward surge in the compressor of the wave rotor topped engine during rapid acceleration. In fact, the compressor actually moves slightly away from the surge line during this transient. This behavior is precisely the opposite to that of an untopped engine. The simulation is described. Issues associated with integrating CFD and component level codes are discussed. Results from several transient simulations are presented and discussed.
... Welch also has established one-dimensional and a twodimensional analysis models to estimate the performance enhancements of wave rotors. 13,14,[21][22][23] In 1996, Snyder and Fish 17,24,25 have evaluated the Rolls-Royce Allison 250 turboshaft engine as a potential platform for a wave rotor demonstration, predicting a 18...20% increase in specific power and a 15...22% decrease in specific fuel consumption. They have used a detailed map of the wave rotor cycle performance accomplished by Wilson and Paxson. ...
... In turbojet engines, by considering the mechanical transmission efficiency, the compressor shaft work equals the turbine output work: (23) Therefore, the total temperature of the gas leaving the turbine can be calculated as: (24) To find the total pressure of the gas leaving the turbine (p t5 ), the value of the turbine isentropic efficiency (η T ) is needed. There are two ways to calculate η T for a turbine: It is preferred to use the turbine polytropic efficiency η PT to obtain p t5 . ...
A performance analysis is performed for a small turbojet engine topped with various wave rotor cycles. Five different advantageous implementation cases for a four-port wave rotor into the given baseline engine are studied. The compressor and turbine pressure ratios, and the turbine inlet temperatures vary in the thermodynamic calculations according to the anticipated design objectives of the five considered cases. Advantages and disadvantages are outlined. Comparison between the theoretic performance results of wave-rotor-topped engines and the baseline engine shows a significant performance enhancement for almost all the cases studied. The highest gain is obtained for the case in which the topped engine operates with the same turbine pressure ratio, inlet temperature and the same physical compressor like that of the baseline engine. A general design map is generated showing the design space and optima for the baseline and topped engines.
... However, the complex ducting to and from the combustor and additional weight may be seen as drawbacks to this approach [32,33]. The aerodynamic and mechanical design of such transition ducts have been investigated for accommodation of circumferential variation in flow properties across the ports of particular cycle designs [32][33][34][35]. ...
... In 1995, Rolls-Royce acquired Allison Engine Company in the United States, which had been collaborating with NASA on toppingcycle wave rotors [32][33][34]. Pulse detonation engine wave rotors [81,82] were investigated by Rolls-Royce in Indianapolis, Indiana for advanced supersonic turbofan engines [43,79] and subsonic/ supersonic missiles [83]. A feasibility study on using a detonative wave rotor integrated to a supersonic turbofan engine estimated [79] 15% reduction in specific fuel consumption at sea level and a 5% reduction at a cruise condition of Mach 2.4 at 60,000 ft altitude. ...
For some decades, efforts have been made to exploit nonsteady combustion and gas dynamic phenomenon. The theoretical potential of nonsteady-flow machines has led to the investigation of various oscillatory flow devices such as pulse detonation engines, wave rotors, pulse jets, and nonsteady ejectors. This paper aims to provide a progress review of past and current research in developing a particular combustion concept: the wave rotor combustor. This pressure-gain combustor appears to have considerable potential to enhance the performance and operating characteristics of gas turbine and jet engines. After attempts in the mid-twentieth century were thwarted by mechanical problems and technical challenges identified herein, recent successes in Switzerland and efforts in the United States benefited from design expertise developed with pressure-exchange wave rotors. The history, potential benefits, past setbacks, and existing challenges and obstacles in developing these nonsteady combustors are reviewed. This review focuses on recent efforts that seek to improve the performance and costs of future propulsion and power- generation systems. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
... This design, shown schematically in Fig. 1, is attractive in that, aside from the difficulty of partial to full annular transition ducts, it can be readily integrated into existing gas turbine engines as another spool 5 . ...
... In fact, the rotor geometry (e.g., length, mean radius, number of passages, etc.) was taken from a previously published four-port, through-flow design. 4,5 As such, no optimization of the bypass cycle was performed in the manner of Ref. 3; only the port positions and rotor speed were varied to obtain correct wave timing. Both the four-port and bypass cycles were designed to be freewheeling, meaning that there is no independent drive motor. ...
A wave rotor cycle is described that avoids the inherent problem of combustor exhaust gas recirculation (EGR) found in four port, through-flow (uniflow) pressure-gain wave rotor cycles currently under consideration for top ping gas-turbine engines. The recirculated hot gas is eliminated by the judicious placement of a bypass duct that transfers gas from one end of the rotor to the other, The resulting cycle, when analyzed numerically, yields a mean absolute temperature for the rotor that is 18% below the already impressive value (approximately the turbine inlet temperature) predicted for the conventional four-port cycle. The absolute temperature of the gas leading to the combustor is also reduced from the conventional design by 17%. The overall design-point pressure ratio of this new bypass cycle is approximately the same as the conventional cycle. This paper will describe the EGR problem and the bypass-cycle solution, including relevant wave diagrams. Performance estimates of design and off-design operation of a specific wave rotor will be presented, The results were obtained using a one-dimensional numerical simulation and design code.
... Snyder and Fish established how a wave rotor-topped engine demonstrator could developed using the Allison model 250 turboshaft engine as a baseline, predicting a 13 -20% increase in SP and a 20 -32% reduction in SFC [16,17]. ...
... Berchtold, Tausing, and Pearson et al. [9] conducted a preliminary discussion on the combination feasibility of wave rotor and aviation gas turbine and analyzed the performance improvement of different gas turbines connected to wave rotor. Moritz [9], Snyder [10], and Welch et al. [11] successively combined the wave rotor and Allison Model 250 turboshaft engine and assessed the working condition and overall benefits from aspects of performance, compatibility and operability. Welch et al. [12] researched the performance gains of Allison Model 250-C30 turboshaft engine assembled with wave rotor, which are an increase of slight power and a decrease of substantial fuel consumption. ...
Breaking through the limit of conventional compression and combustion, wave rotor and trapped vortex combustors are able to improve the thermal efficiency of gas turbines. Detailed two-dimensional numerical simulations based on Ansys Fluent were performed to study the flow and combustion characteristics of the wave rotor–trapped vortex combustor system. The calculated pressure characteristics agree with the experimental results giving a relative error for average pressure of 0.189% at Port 2 and of 0.672% at Port 4. The flow stratification characteristics and the periodic fluctuations were found to benefit the zonal organized combustion in the trapped vortex combustor. For the six cases of different rotor speeds, as the rotor speed increased, the oxygen mass fraction at the combustor inlet rose and then fell. The proportion of exhaust gas recirculation fell at first and then rose, and the combustion mode became unstable with the dominant frequencies of the fluctuations increasing.
... Earlier investigations started by the Brown Boveri Company in the 1940s sought to apply wave rotors as a topping device in gas turbines aiming at increasing system efficiency [15]. The concept has been further developed by Rolls Royce [16][17][18], ONERA [19,20], and NASA [21][22][23]. In addition to gas turbines, wave rotors have successfully been applied as pressure wave superchargers to both light-and heavy-duty internal combustion engines Fig. 2 a Schematic of wave rotor turbine arrangement within a gas turbine as one possible application for wave rotor turbines. ...
Wave rotors are unsteady flow machines that exchange energy through pressure waves. This has the potential for enhancing efficiency over a wide spectrum of applications, ranging from gas turbine topping cycles to pressure-gain combustors. This paper introduces an aerodynamic shape optimisation of a power generating non-axial microwave rotor turbine and seeks to enhance the shaft power output while preserving the wave rotor's capacity to function as a pressure-exchanging device. The optimisation considers six parameters including rotor shape profile, wall thickness, and number of channels and is done using a hybrid genetic algorithm that couples an evolutionary algorithm with a surrogate model. The underlying numerical model is based on a transient, reduced-order, quasi-two dimensional computational fluid dynamics model at a fixed operating condition. The numerical results from the quasi-two-dimensional optimisation indicate that the best candidate design increases shaft power by a factor of 1.78 and imply a trade-off relationship between torque generation and pressure exchange capabilities. Further evaluation of the optimised design using three-dimensional computational fluid dynamics simulations confirms the increase in power output at the cost of increased entropy production. It is further disclosed that increased incidence losses during the initial opening of the channel to the high-pressure inlet duct compromise the shock strength of the primary shock wave and account for the decrease in pressure ratio. Finally, the numerical trends are validated using experimental data.
... The application range for wave rotors outlined by literature is diverse. The bulk of early studies focused on pressure exchangers with straight passage profiles for gas turbine topping cycles [1][2][3][4][5][6][7][8] and supercharging devices for internal combustion engines [9][10][11][12][13][14][15][16]. In recent years the application to refrigeration cycles [17][18][19] and pressure-gain combustors [20][21][22] has come into the focus of consideration. ...
A wave rotor is a shock-driven pressure exchange device that, whilst relatively rarely studied or indeed, employed, offers significant potential efficiency gains in a variety of applications including refrigeration and gas turbine topping cycles. This paper introduces a quasi one-dimensional wave action model implemented in MATLAB for the computation of the unsteady flow field and performance characteristics of wave rotors of straight or cambered channel profiles. The purpose here is to introduce and validate a rapid but reliable method of modelling the performance of a power-generating wave rotor where little such insight exists in open literature. The model numerically solves the laminar one-dimensional Navier-Stokes equations using a two-step Richtmyer TVD scheme with minmod flux limiter. Additional source terms account for viscous losses, wall heat transfer, flow leakage between rotor and stator endplates as well as torque generation through momentum change.
Model validation was conducted in two steps. First of all, unsteady and steady predictive capabilities were tested on three-port pressure divider rotors from open literature. The results show that both steady port flow conditions as well as the wave action within the rotor can be predicted with good agreement. Further validation was done on an in-house developed and experimentally tested four-port, three-cycle, throughflow micro wave rotor turbine featuring symmetrically cambered passage walls aimed at delivering approximately 500 W of shaft power. The numerical results depict trends for pressure ratio, shaft power and outlet temperature reasonably well. However, the results also highlight the need to accurately measure leakage gaps when the machine is running in thermal equilibrium.
... The application range for wave rotors outlined by the literature is diverse. The bulk of early studies focused on pressure exchangers with straight passage profiles for gas turbine topping cycles [1][2][3][4][5][6][7][8] and supercharging devices for internal combustion engines [9][10][11][12][13][14][15][16]. In recent years, the application to refrigeration cycles [17][18][19] and pressure-gain combustors [20][21][22] has come into the focus of consideration. ...
A wave rotor is a shock-driven pressure exchange device that offers potential efficiency gains in a variety of applications including refrigeration and gas turbine topping cycles. This paper introduces a quasi one-dimensional model for the computation of the unsteady flow field and performance characteristics of wave rotors of straight or cambered channel profiles. The purpose here is to introduce and validate a rapid but reliable method of modelling the performance of a power-generating wave rotor where little such insight exists in open literature. The model numerically solves the laminar one-dimensional Navier-Stokes equations using a two-step Richtmyer TVD scheme with minmod flux limiter. Source terms account for viscous losses, flow leakage between rotor and stator endplates as well as torque generation through momentum change. Model validation was conducted in two steps. First of all, unsteady and steady predictive capabilities were tested on three port pressure divider rotors from open literature. The results show that both steady port flow conditions as well as the wave action within the rotor can be predicted with good agreement. Further validation was done on an in-house developed and experimentally tested four-port, three-cycle, throughflow micro wave rotor turbine featuring symmetrically cambered passage walls aimed at delivering approximately 500 W of shaft power. The numerical results depict trends for pressure ratio, shaft power and outlet temperature reasonably well. However, the results also highlight the need to accurately measure leakage gaps when the machine is running in thermal equilibrium.
... They obtained an increase of 1 to 2% for the engine thermal efficiency and 10 to 16% for the specific power. Snyder & Fish [77] discuss the performance benefits of the wave rotor cycle in a small turboshaft engine. The engine topped with the wave rotor showed a considerable reduction of 22% in specific fuel consumption (SFC) compared to the baseline engine. ...
... They obtained an increase of 1 to 2% for the engine thermal efficiency and 10 to 16% for the specific power. Snyder & Fish [61] discuss the performance benefits of the wave rotor cycle in a small turboshaft engine. The engine topped with wave rotor showed a considerable reduction of 22% in specific fuel consumption (SFC) compared to the baseline engine. ...
... 89 In 1995, Rolls Royce acquired Allison Engine Company, which had been collaborating with NASA GRC on topping-cycle wave rotors. [90][91][92] This was followed by an investigation of pulse detonation engine wave rotors 93,94 for advanced supersonic turbofan engines 39,88 and subsonic/supersonic missiles. 95 A feasibility study on using a detonative wave rotor integrated to a supersonic turbofan engine estimated 88 15% reduction in specific fuel consumption at sea level and a 5% reduction at a cruise condition of Mach 2.4 at 60,000 feet altitude. ...
In recent decades, considerable efforts have been made to exploit the non-steady combustion and gasdynamic phenomenon. The superior performance potential of non-steady flow machines has led to development of various intermittent devices such as pulse detonation engines, wave rotors, pulse jets, and non-steady ejectors. This paper aims to provide a succinct progress review of past and current research in developing a particular combustion concept: the wave rotor combustor. This pressure-gain combustor appears to have considerable potential to enhance the performance and operating characteristics of gas turbine engines. The history, potential benefits, past setbacks, and existing challenges and obstacles in developing these non-steady combustors are reviewed. This review particularly focuses on recent efforts, highlighting needs to improve the performance and costs of future propulsion and power generation systems.
... In 1996, Snyder and Fish evaluated the Rolls-Royce Allison 250 turboshaft engine as a potential platform for a wave rotor demonstration, predicting a 18...20% increase in specific power and a 15...22% decrease in specific fuel consumption [18,25,26]. They used a detailed map of the wave rotor cycle performance accomplished by Wilson and Paxson [17,19]. ...
Results are presented predicting the significant performance enhancement of two small gas turbines (30 kW and 60 kW) by implementing various wave rotor topping cycles. Five different advantageous implementation cases for a four-port wave rotor into given baseline engines are considered. The compressor and turbine pressure ratios, and the turbine inlet temperatures vary in the thermodynamic calculations, according to the anticipated design objectives of the five cases. Advantages and disadvantages are outlined. Comparison between the theoretic performance (expressed by specific cycle work and overall thermal efficiency) of wave-rotor-topped and baseline engines shows a performance enhancement by up to 33%. The results obtained show that almost all the cases studied benefit from the wave-rotor-topping, but the highest gain is obtained for the case in which the topped engine operates with the same turbine inlet temperature and compressor pressure ratio as the baseline engine. General design maps are generated for the small gas turbines, showing the design space and optima for baseline and topped engines.
... Snyder and Fish have evaluated the Rolls-Royce Allison 250 turboshaft engine as a potential platform for a wave rotor demonstration, predicting a 18...20% increase in specific power and a 15...22% decrease in specific fuel consumption , Snyder and Fish, 1996, Welch, 2000. They used a detailed map of the wave rotor cycle performance accomplished by Paxson (1993, 1996). ...
This paper shows a preliminary design procedure for four-port reverse-flow wave rotors for implementation in gas turbine applications. First, a thermodynamic cycle analysis evaluates the performance improvement of a 30 kW microturbine by implementing various wave-rotor topping cycles. Five different advantageous implementation cases for a four-port wave rotor into the given baseline engine are considered. Advantages and disadvantages are outlined. The results obtained show that almost all the cases studied benefit from the wave-rotor-topping, but the highest gain is obtained for the case in which the topped engine operates with the same turbine inlet temperature and compressor pressure ratio as the baseline engine. Then, a one-dimensional analytical gas dynamic model of the high-pressure phase (charging zone) is employed to calculate flow characteristics inside the channels. Charts and explanations are presented for optimum design. Useful design parameters such as port widths and rotor size are determined by formulating traveling times of the waves inside the channels.
... In 1996, Snyder and Fish [10,191] of Allison Engine Company evaluated the Allison 250 turboshaft engine as a potential platform for a wave rotor demonstration, predicting an 18...20% increase in specific power and a 15...22% decrease in specific fuel consumption. They used a detailed map of the wave rotor cycle performance accomplished by Wilson and Paxson [8,158,179]. ...
The objective of this paper is to provide a succinct review of past and current research in developing wave rotor technology. This technology has shown unique capabilities to enhance the performance and operating characteristics of a variety of engines and machinery utilizing thermodynamic cycles. Although there have been numerous efforts in the past dealing with this novel concept, this technology is not yet widely used and barely known to engineers. Here, an attempt is made to summarize both the previously reported work in the literature and ongoing efforts around the world. The paper covers a wide range of wave rotor applications including the early attempts to use wave rotors, its successful commercialization as supercharges for car engines, research and development for gas turbine topping, and other developments. The review also pays close attention to more recent efforts: utilization of such devices in pressure-gain combustors, ultra-micro gas turbines, and water refrigeration systems, highlighting possible further efforts on this topic. Observations and lessons learnt from experimental studies, numerical simulations, analytical approaches, and other design and analysis tools are presented.
View Video Presentation: https://doi.org/10.2514/6.2022-1089.vid Gas turbines present a unique opportunity to generate power outputs and have been widely used in power plants, stationary power generation, and propulsion systems. Increasing concern regarding global warming has urged the gas turbine industry to reduce CO2 emissions from gas turbines. A leading solution is the use of hydrogen as a carbon-free fuel or low-carbon mix in gas turbines. New Wave Hydrogen (NWH2) has developed a revolutionary fuel reformer that thermally decomposes, or cracks, natural gas into hydrogen and carbon black. The innovative method is based on wave rotor technology utilizing shock waves to compress and heat natural gas to temperatures sufficient for methane pyrolysis. This paper introduces the new concept of producing hydrogen using a high-pressure gas source and the NWH2 wave reformer, integrated with a gas turbine, in a flexible range of designs for power generation. Preliminary numerical modeling using an in-house quasi-one-dimensional wave rotor code support the possibility of cracking natural gas in a wave reformer utilizing the energy contained in the combustor burned gas.
Este trabajo presenta un analisis preliminar sobre los beneficios de ia implementacion de un rotor de onda en una micro-turbina convencional. Este estudio se enfoca en la consecución de un punto de disefio por medio de cálculos térmicos para así lograr una geometría adecuada, la cual podra ser evaluada por medio de Dinámica de Flujo Computacional CFD para determinar la viabilidad de la implementacion de este dispositive en un motor con micro-turbina a gas de 30 lb, de empuje.
The transition duct from the low-pressure exhaust port of a four-port wave rotor to the downstream high-pressure turbine constitutes a critical element for wave-rotor-topped engines. The transition duct must be compact, aerodynamically efficient, and have sufficient margin for off-design operation. At both on- and off-design operation, the transition duct processes a highly non-uniform, swirling flow. A transition duct, designed using a DOE-CFD approach in an earlier effort, was tested. Design-point performance was found to be lower than intended. The transition duct was found to be robust in terms of off-design performance. Measured transition duct loss levels and their impact on the total-pressure of the flow at turbine entry highlight the importance of the transition duct performance in realizing the thermodynamic benefits offered to gas turbine engines by wave-rotor topping.
A wave rotor may be used as a pressure-gain combustor, effecting wave compression and expansion, and Intermittent confined combustion, to enhance gas-turbine engine performance. It will be more compact than an equivalent pressure-exchange wave-rotor system, but wilt have similar thermodynamic and mechanical characteristics. Because the allowable turbine blade temperature limits overall fuel-air ratio to subflammable values, premixed stratification techniques are necessary to burn hydrocarbon fuels in small engines with compressor discharge temperatures well below autoignition conditions. One-dimensional, nonsteady numerical simulations of stratified-charge combustion are performed using an eddy-diffusivity turbulence model and a simple reaction model incorporating a flammability limit temperature. For good combustion efficiency, a stratification strategy is developed that concentrates fuel at the leading and trailing edges of the inlet port. Rotor and exhaust temperature profiles and performance predictions are presented at three representative operating conditions of the engine: full design load, 40% load, and idle. The results indicate that peak focal gas temperatures will cause excessive temperatures in the rotor housing unless additional cooling methods are used. The rotor temperature will be acceptable, but the pattern factor presented to the turbine may be of concern, depending on exhaust duct design and duct-rotor interaction.
The design of a wave rotor requires an understanding of the pressure
wave dynamics in the rotor passages. The present paper describes a
two-dimensional numerical simulation and an experimental visualization
of the wave rotor compression process. First, a unique experimental
apparatus with fixed cells and rotating ports was constructed for
visualization and direct measurements; this arrangement is opposite to
the conventional setup. Next, experimental and numerical results were
compared to verify the simulation modelling, particularly with regard to
the propagation velocity of pressure waves in the passages. Lastly, the
effects of gradually opening the passage to the ports and leakage
through the clearance, which are considered to be dominant factors in
wave rotor operation, on the pressure wave dynamics were carefully
investigated. The results showed that the gradual passage opening
greatly influences the primary shock wave, whereas the leakage mostly
influences the secondary (reflected) shock wave. Moreover, it was
revealed that the leakage generates an extra pressure wave during the
compression process due to the interaction between adjacent passages.
The precise estimation of pressure waves generated in the passages is a
crucial factor in wave rotor design. However, it is difficult to
estimate the pressure wave analytically, e.g. by the method of
characteristics, because the mechanism of pressure-wave generation and
propagation in the passages is extremely complicated as compared to that
in a shock tube. In this study, a simple numerical modelling scheme was
developed to facilitate the design procedure. This scheme considers the
three dominant factors in the loss mechanism —gradual passage
opening, wall friction and leakage— for simulating the pressure
waves precisely. The numerical scheme itself is based on the
one-dimensional Euler equations with appropriate source terms to reduce
the calculation time. The modelling of these factors was verified by
comparing the results with those of a two-dimensional numerical
simulation, which were previously validated by the experimental data in
our previous study. Regarding wave rotor miniaturization, the leakage
flow effect, which involves the interaction between adjacent cells, was
investigated extensively. A port configuration principle was also
examined and analyzed in detail to verify the applicability of the
present numerical modelling scheme to the wave rotor design.
A wave rotor may be used as a pressure-gain combustor effecting non-steady flow, and intermittent, confined combustion to enhance gas turbine engine performance. It will be more compact and probably lighter than an equivalent pressure-exchange wave rotor, yet will have similar thermodynamic and mechanical characteristics. Because the allowable turbine blade temperature limits overall fuel/air ratio to sub-flammable values, premixed stratification techniques are necessary to burn hydrocarbon fuels in small engines with compressor discharge temperature well below autoignition conditions. One-dimensional, unsteady numerical simulations of stratified-charge combustion are performed using an eddy-diffusivity turbulence model and a simple reaction model incorporating a flammability limit temperature. For good combustion efficiency, a stratification strategy is developed which concentrates fuel at the leading and trailing edges of the inlet port. Rotor and exhaust temperature profiles and performance predictions are presented at three representative operating conditions of the engine: full design load, 40% load, and idle. The results indicate that peak local gas temperatures will result in excessive temperatures within the rotor housing unless additional cooling methods are used. The rotor itself will have acceptable temperatures, but the pattern factor presented to the turbine may be of concern, depending on exhaust duct design and duct-rotor interaction.
The potential for improved performance of wave rotor cycles through the use of passage height variation is examined. A Quasi-one-dimensional CFD code with experimentally validated loss models is used to determine the flowfield in the wave rotor passages. Results indicate that a carefully chosen passage height profile can produce substantial performance gains. Numerical performance data are presented for a specific profile, in a four-port, through-flow cycle design which yielded a computed 4.6% increase in design point pressure ratio over a comparably sized rotor with constant passage height. In a small gas turbine topping cycle application, this increased pressure ratio would reduce specific fuel consumption to 22% below the un-topped engine; a significant improvement over the already impressive 18% reductions predicted for the constant passage height rotor. The simulation code is briefly described. The method used to obtain rotor passage height profiles with enhanced performance is presented. Design and off-design results are shown using two different computational techniques. The paper concludes with some recommendations for further work.
The performance benefits derived by topping a gas turbine engine with a wave engine are assessed. The wave engine is a wave rotor that produces shaft power by exploiting gas dynamic energy exchange and flow turning. The wave engine is added to the baseline turboshaft engine while keeping high-pressure-turbine inlet conditions, compressor pressure ratio, engine mass flow rate, and cooling flow fractions fixed. Related work has focused on topping with pressure-exchangers (i.e., wave rotors that provide pressure gain with zero net shaft power output); however, more energy can be added to a wave-engine-topped cycle leading to greater engine specific-power-enhancement The energy addition occurs at a lower pressure in the wave-engine-topped cycle; thus the specific-fuel-consumption-enhancement effected by ideal wave engine topping is slightly lower than that effected by ideal pressure-exchanger topping. At a component level, however, flow turning affords the wave engine a degree-of-freedom relative to the pressure-exchanger that enables a more efficient match with the baseline engine. In some cases, therefore, the SFC-enhancement by wave engine topping is greater than that by pressure-exchanger topping. An ideal wave-rotor-characteristic is used to identify key wave engine design parameters and to contrast the wave engine and pressure-exchanger topping approaches. An aerodynamic design procedure is described in which wave engine design-point performance levels are computed using a one-dimensional wave rotor model. Wave engines using various wave cycles are considered including two-port cycles with on-rotor combustion (valved-combustors) and reverse-flow and through-flow four-port cycles with heat addition in conventional burners. A through-flow wave cycle design with symmetric blading is used to assess engine performance benefits. The wave-engine-topped turboshaft engine produces 16% more power than does a pressure-exchanger-topped engine under the specified topping constraints. Positive and negative aspects of wave engine topping in gas turbine engines are identified.
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