Douglas Carl Hofer

Turbine Efficiency for Unsteady, Periodic Flows

The definition of turbine efficiency for a machine subjected to unsteady periodic inflows is studied. Since mass and energy are conserved quantities over a period, there is no ambiguity in calculating the actual work output of the turbine over a period. The main difficulty lies in calculating the isentropic work output of an ideal turbine operating under the “same” conditions. Two definitions of ideal work output are presented. In the first, the ideal turbine is assumed to operate under the same time traces of inlet and exit total pressures as the actual turbine. The expression for the efficiency that results involves no averages of total pressure. In the second approach, the ideal turbine is assumed to operate under the same average conditions as the actual turbine. Total pressure averages that preserve the isentropic work output are derived and used to calculate an efficiency of the turbine. The two expressions are calculated explicitly for the case of a turbine blade row downstream of a pulse detonation tube. It is found that the definition of efficiency using averages is approximately 10 points lower than the first definition.

Performance Metrics for Pulse Detonation Combustor Turbine Hybrid Systems

This paper develops performance metrics for a Pulse Detonation Turbine Engine (PDTE) which incorporates a pulse detonation combustor capable of increasing the average pressure of the flow during the combustion process. Previously reported performance of the PDTE cycles and commercial gas turbine engines are compared to an ideal Brayton cycle. This comparison is extended to form a new performance metric combining the effect of combustor pressure increase and turbine efficiency. The PDTE design space including compressor pressure ratio, combustor heat addition, combustor pressure loss, and turbine efficiency is explored to determine the effect of these variables on the proposed definition. The primary benefit of the proposed definition over those considered previously is that by combining the combustor and turbine efficiencies the difficulties associated with determining the unsteady turbine inlet state are avoided in favor of using the steady compressor discharge state.

Aerodynamic Design and Testing of Three Low Solidity Steam Turbine Nozzle Cascades

Three sets of low solidity steam turbine nozzle cascades were designed and tested. The objective was to reduce cost through a reduction in parts count while maintaining or improving performance. The primary application is for steam turbine high pressure sections where Mach numbers are subsonic and high levels of unguided turning can be tolerated. The base line design A has a ratio of pitch to axial chord of 1.2. This is the pitch diameter section of a 50% reaction stage that has been verified by multistage testing on steam to have a high level of efficiency. Designs B and C have ratios of pitch to axial chord of 1.5 and 1.8, respectively. All three designs satisfy the same inlet and exit vector diagrams. Analytical surface Mach number distributions and boundary layer transition predictions are presented. Extensive cascade test measurements were carried out for a broad incidence range from -60 to +35 deg. At each incidence, four outlet Mach numbers were tested, ranging from 0.2 to 0.8, with the corresponding Reynolds number variation from 1.8X 10 5 to 9.0X10 5 . Experimental results of loss coefficient and blade surface Mach number are presented and compared for the three cascades. The experimental results have demonstrated low losses over the tested Mach number range for a wide range of incidence from -45 to 15 deg. Designs B and C have lower profile losses than design A. The associated flow physics is interpreted using the results of wake profile, blade surface Mach number distribution, and blade surface oil flow visualisation, with the emphasis placed on the loss mechanisms for different flow conditions and the loss reduction mechanism with lower solidity. The effect of the higher profile loading of the lower solidity designs on increased end wall losses induced by increased secondary flow, especially on low aspect ratio designs, is the subject of ongoing studies.

Aerodynamic Design and Prototype Testing of a New Line of High Efficiency, High Pressure, 50% Reaction Steam Turbines

The aerodynamic design and prototype performance testing of a new line of high efficiency, high pressure (HP), 50% reaction steam turbines is described in some detail. Three designs were carried out that can be used in a repeating stage fashion to form high efficiency steam paths. The designs were performed employing a blade master concept. The masters can be aerodynamically scaled and cut to cover a wide range of applications while maintaining vector diagram integrity. Three equivalent prototype flow paths, one each for Gen 0, 1 and 2, masters were designed and tested in a Steam Turbine Test Vehicle (STTV). These prototype designs are representative of high pressure steam turbines for combined cycle power plants. Design of experiments is used to optimize the flow path, stage counts and diameters for production designs taking into account multidisciplinary design constraints. Four such Gen 1 steam path designs have been executed to date as part of a structured series of combined cycle power plants. [1-5] There are two A14 HEAT* (High Efficiency Advanced Technology) steam turbine HP flow paths for GE’s 107FA combined cycle power plants and two A15 HEAT HP flow paths for the 109FB. The larger of the A14 HEAT steam turbine HP’s has recently been performance tested at a customer site demonstrating world class efficiency levels of over 90% for this low volume flow combined cycle turbine [1]. HP volume flows are likely to drop even lower in the future with the need to go to higher steam inlet pressure for combined cycle efficiency improvements so steam path designs with high efficiency at low volume flow will be increasingly important.Copyright

Efficiency Entitlement for Bottoming Cycles

A significant portion of the new electrical generating capacity installed in the past decade has employed heavy-duty gas turbines operating in a combined cycle configuration with a steam turbine bottoming cycle. In these power plants approximately 1/3 of the power is generated by the bottoming cycle. To ensure that the highest possible combined cycle efficiency is realized it is important to optimize the bottoming cycle efficiency and doing so requires a solid understanding of the efficiency entitlement. This paper describes a new technique for calculating the theoretical efficiency entitlement for a bottoming cycle that corresponds to the maximum possible bottoming cycle work and maximized combined cycle work and efficiency. This new method accounts for the decrease in ideal efficiency as the gas turbine exhaust is cooled as it transfers heat energy to the working fluid in the bottoming cycle. The new definition is compared to conventional definitions, including that of Carnot and an Exergy based second law efficiency, and shown to provide a simple and accurate analytical expression for the entitlement efficiency in a bottoming cycle. For representative cycle conditions, the entitlement efficiency for the bottoming cycle is calculated to be ∼45% compared to the Carnot efficiency for the same conditions of ∼67%. Although the new method is applicable to any power cycle obtaining its heat input from the exhaust stream of a topping cycle, special attention is given to the steam bottoming cycle traditionally used in modern gas turbine combined cycle power plants. Comparisons are made between the ideal bottoming cycle and variants of a steam cycle including a single pressure non-reheat and a three pressure reheat cycle. These comparisons explore the unavoidable loss in efficiency associated with constant temperature heat addition that occurs in the steam cycle.Copyright