Single Stage Centrifugal Compressor Research Articles

Centrifugal Compressor Inlet Guide Vanes for Increased Surge Margin

This paper describes the results of compressor rig testing with a moderately high specific speed, high inducer Mack number, single-stage centrifugal compressor, with a vaned diffuser, and adjustable inlet guide vanes (IGVs). The results showed that the high-speed surge margin was considerably extended by the regulation of the IGVs, even though the vaned diffuser was apparently operating stalled. Simplified one-dimensional analysis of the impeller and diffuser performances indicated that at inducer tip Mach numbers approaching and exceeding unity, the high-speed surge line was triggered by inducer stall. Also, IGV regulation increased impeller stability. This permitted the diffuser to operate stalled, providing the net compression system stability remained on a negative slope.

Experiments With a Low-Specific-Speed Partial Emission Centrifugal Compressor

This paper describes experiments that were conducted several years ago on a single-stage open-face centrifugal partial emission compressor with a nondimensional specific speed of 0.15 (19.3 dimensional). The purpose of the experiments was experimentally to evaluate the partial emission compressor as a possible candidate for a high-speed low-flow natural gas fuel pump, the alternative to which was a larger heavier reciprocating compressor. A review and re-analysis of the test data were conducted. They used updated compressor data reduction performance procedures and flow models to redefine the experimental compressor as a reference case in the low-specific-speed range of single-stage centrifugal compressors. Normalized performance characteristics are presented for the experimental compressor in terms of efficiencies, diffuser recovery, and head coefficients versus diffuser relative flow parameter, or choke margin. Peak efficiency, obtained at a De Laval number of 1.16, and nondimensional specific speed of 0.15, was 34.5 percent. Stage performance is found to be dominated by diffuser geometry and choke margin, plus windage losses.

Development of a 30PS Class Small Gas Turbine and Its Power-Up Version

Due to the recent popularity of small and medium-sized industrial gas turbines in many fields, gas turbines below 100 SHP have been employed as prime movers, a power range traditionally reserved for diesel and gasoline engines. Generally speaking, however, small gas turbines have many design difficulties in thermal efficiency, high rotational speed, compact auxiliary equipment, etc., derived from limitations of their dimensions. Small gas turbines S5A-01 and S5B-01, which have 32 PS output power at standard conditions, have been developed and are being produced. Presently, a 30 percent growth rated power producer for S5A-02 and S5B-02 gas turbines is under development. These engines’ configurations are as follows: single-stage centrifugal compressor; single-stage radial turbine; single can combustor; hybrid fuel nozzle with pressure atomizer and airblast atomizer; fuel control valve with pulse width modulation system; electric motor drive fuel pump. In this paper, the authors describe the design features and development history of the base engine and the experimental results with the growth rated version.

Experimental investigation on return passage of multi-stage centrifugal compressor.

The key problems of improving the performance characteristics of a multistage centrifugal compressor are an efficient conversion of dynamic energy into static pressure within the return passage and an improved matching with the downstream stage. To achieve these goals, a single-stage centrifugal compressor with a return passage attached upstream was first examined. The pressure ratio and the impeller efficiency were determined with varying the geometry of the return passage and the inlet swirl. The results were compared to those without a return passage. Detailed measurements of flow fields within the return passage were then performed. The cause of the performance decline and the optimum flow conditions are discussed. The usefulness of the one-dimensional analysis for a practical prediction of losses in the return passage is confirmed.

Improvements in Performance Characteristics of Single-Stage and Multistage Centrifugal Compressors by Simultaneous Adjustments of Inlet Guide Vanes and Diffuser Vanes

Nowadays, multistage geared centrifugal compressors are most often equipped with three-dimensional impellers and adjustable inlet guide vane cascades, at least upstream of the first stage. Optimum stage efficiencies are made possible by optimum axial in-flow into each stage and freely selectable pinion shaft speeds. Combined with intercooling of the medium, the result is high machine efficiency with good operating ranges. Additional increases in efficiency can be achieved by means of vaned diffusers. Due to the attendant restriction to the working range, this solution is not common in production compressors. Nevertheless, the working range can be distinctly expanded by adjusting the diffuser vanes. In addition, the combination of simultaneous adjustment to inlet guide vanes and diffuser vanes enables an increase in machine efficiency over the entire operating range as compared with regulation using only inlet guide vanes or diffuser vanes. This paper reports on the development of centrifugal compressor stages equipped with vaned diffusers. The impellers have backward-curved blades. Experimental determination of suitable schedules for simultaneous adjustment of both inlet guide vanes and diffuser vanes, depending on the desired performance characteristic, will be dealt with in detail. Furthermore, some examples of the overall performance maps for multistage inter-cooled geared compressors will be shown as a result of combining the performance characteristic curves of the individual stages. The operating ranges and regions of maximum efficiency are optimally matched to the requirements in question by means of suitable adjustment schedules.

Characteristics of the high pressure ratio centrifugal compressor.

Compressors in small gas turbines are usually composed of single stage centrifugal compressors, and their high efficiency and wide operation range are very important for the gas turbines. Although much research on fundamental technique have been reported by many researchers, there are questions to solve by practice in designing high pressure ratio machines. In this paper the authors relate the aerodynamic design and the experimental result of the advanced high pressure ratio centrifugal compressor for gas turbines.

Experimental and Theoretical Study of Surge in a Small Centrifugal Compressor

Experimental results for deep surge in a small single-stage centrifugal compressor are compared to predictions based on the lumped parameter Greitzer model developed for axial compressors. Both negative and positive flow branches of the steady characteristic, being essential for the model, were measured. Predictions are in fair agreement with data when using a relaxation time smaller than the one proposed for axial compressors. The stability limit of the model equations have been studied for finite amplitude disturbances.

Experiments With a Model Free Rotating Vaneless Diffuser

Experimental testing of a model free-rotating vaneless diffuser, for application to high pressure ratio single-stage centrifugal compressors, was conducted to determine diffuser performance under braked and free rotating conditions at entry Mach numbers up to unity. The experimental test rig comprised a swirl generating nozzle upstream of the model vaneless diffuser rotor with an outer-to-inner diameter ratio of 1.3. Additional downstream diffusion was completed with stationary vaneless and vaned diffuser inserts. A significant improvement in diffuser performance was achieved under free-rotating conditions even though large wakes generated by upstream stationary swirl nozzles were present. Overall static pressure recovery for the complete diffusion system increased approximately 20 percent at free-rotating conditions corresponding to a tangential velocity ratio (diffuser rotor/incident stream) of 0.43.

Development of a Small Single- and Two-Shaft Gas Turbine for Military Applications

In the last two decades there has been a progressive increase in the scope of small gas turbine development and application, both in commercial and military fields. Technological advances in the military field have created demands for prime movers able to meet more stringent operational requirements. An environment now exists in the U.S. Armed Forces conducive to the acceptance and use of small gas turbines that can be adapted to the many specialized applications demanding distinctive physical characteristics. In 1957, the U.S. Bureau of Naval Weapons entered into a contract with Solar, a Division of International Harvester Company, for the design and development of a small, lightweight, single-shaft gas turbine engine to power a one-man helicopter. Rated at 55 s.h.p. for 100°F ambient conditions, the engine design requirements were based on simplicity of operation and maintenance, low production cost, and a short development time. These factors were considered more important than optimum specific power, fuel consumption and weight. A two-shaft version of this engine was also required to power a one-man flying platform. To facilitate the required low engine airflow, weight and rugged characteristics, a single-stage centrifugal compressor and radial inflow turbine, mounted back to back, were selected. A unique feature of the two-shaft engine was the simple method of achieving two-shaft capability by using the exducer portion of the radial turbine as a power turbine. This design resulted in both engines having the same nominal performance, weight and envelope dimensions, with maximum component interchangeability. A comprehensive aerodynamic development programme was undertaken to improve component efficiencies and various aspects of this programme are discussed. Based on favourable results from this programme, the building and development of an uprated engine was undertaken. By incorporating a modified compressor, a 38 per cent increase in s.h.p. and a 6 per cent decrease in specific fuel consumption (s.f.c.) was obtained. Probably the most important factor in the choice of a small gas turbine for military applications is its ability to develop more power, per unit weight and volume, than other engine types. In general, the specific weight of reciprocating engines, in the same power class as the subject gas turbine, average about 10–15 lb/hp with specific volumes ranging from 0.220 to 0.30 fr3/hp, compared to values of 0.53 lb/hp and 0.027 ft3/hp for the turbine. Over 20 different variations of this engine have been produced for such diverse applications as small aircraft and boat prime movers, aircraft and helicopter auxiliary power plants, mobile generator sets, and ‘winterization’ kits for U.S. Army field service. Intensive efforts are being made by companies on both sides of the Atlantic to develop the small gas turbine for a variety of applications. It seems reasonable to predict that these efforts will ultimately lead to the development of units with improved performance and lower initial cost.

Gas Turbines for Missile Ground Support

HANDLING, testing, and launching of guided missiles require a multitude of services which are external to the missile proper. Specific requirements for such ground support equipment are dictated by the operational concept of the missile system and the detail design of the missile. In general, hydraulic, electric, mechanical, and pneumatic power are needed at the site of operation for erecting and aligning, fueling, calibration, checkout, and launching. The proper selection of the prime movers for such services is of great importance, considering the safety, reliability, quality, and control requirements. In addition, depending on the operational concepts, emphasis may be placed on such items as versatility, transportability, self-sufficiency, and logistics. Considering these, the advantage of using gas turbine prime movers is definitely indicated. The lightweight, compact engines are capable of using a wide variety of fuels and are easy to integrate with the driven components, such as pumps, compressors, and generators. Solar Aircraft Company has designed and is manufacturing a variety of gas turbine driven power units for diverse applications. Solar combines them in complete packages for ground, shipboard, and airborne electric generator sets, space heaters, smoke generators, air bleed power supplies, and pumps. The present production units use as the prime movers the 50-hp Mars and the 500-hp Jupiter engines. The Mars engine weighs approximately 100 lb, which includes the gearbox and the accessories, and its dimensions are 23 in. in height, 17 in. in width, and 23 in. in length. I t develops 50 bhp at a speed of 40,000 rpm, or 42 ppm bleed air with a bleed air pressure of 18.3 psig. Air mass flow, pressure ratio, and fuel consumption are 2.7 pps, 2.5:1, and 2.2 lb/bhp X hr, respectively. The Jupiter engine weighs approximately 990 lb, including gearbox and accessories; its dimensions are 42 in. by 32 in. by 91 in. I t develops up to 520 bhp at 20,000 rpm, or 150 ppm bleed air with a pressure of 45 psig. Air mass flow, pressure ratio, and fuel consumption are 9 pps, 4.6:1, and 0.92 lb/bhp X hr, respectively. Both engines are designed for operation over a temperature range from — 65 F to + 130 F. They operate on all commonly used fuels, such as gasoline, jet engine fuel, diesel fuel, and kerosene. Complete protection against overspeed, overtemperature, and loss of oil pressure is provided. Construction of both engines is simple and compact. The Mars engine employs a single-stage centrifugal compressor, single elbow combustor, and a singlestage radial inflow turbine. Turbine and compressor are mounted back-to-back on the same shaft. The Jupiter engine employs a ten-stage axial compressor, single combustor, and a three-stage axial turbine. For constant speed application, the three stages are coupled together, whereas for variable speed application the third stage of the turbine is not connected to the gas producer section but mounted on a separate power take-off shaft. Both engines have completely automatic electrical start systems, and power is available in less than one minute even at extreme low temperatures. Preheating is not required. The Mars engine is also available with a hand-crank starting system; the Jupiter engine is adaptable for pneumatic or fuel-air starting systems. Standard speed control systems for both engines incorporate a droop-type governor with 5 per cent droop from no load to full load and steady-state speed variation of ± 0 . 5 per cent. The system is adaptable for isochronous governing and parallel operation w^hen used for generator set applications. Other types and sizes of gas turbine prime movers are presently in development to satisfy power requirements different from those of the described units. I t is important that proper and timely consideration be given to the ground support requirements for guided missiles systerns so that optimization of the system be achieved, and an effective research tool or weapon be developed. The gas turbine prime mover should play a more important role in this concept in the future, due to its inherent characteristics and flexibility for integration into such a complex system.

On Performance Characteristics of a Centrifugal Compressor with Supersonic Impeller Tip Speed

It is very important to examine whether the performance characteristics of a centrifugal compressor should fall rapidly as soon as the absolute velocity of the air at the impeller exit exceeds the local sonic velocity. The experiments were performed by the present author with a centrifugal supercharger coupled with an electric dynamometer of 300 HP. The conclusions of these experimental study were that (1) the performance characteristics showed no rapid breakdown when the absolute velocity of the air at the impeller exit exceeds the local sonic velocity, this velocity being 400 m/s or so on account of the temperature rise of the air in the impeller channel, and (2) the maximum obtainable value of the pressure ratio of single stage centrifugal compressor was found to be 2.79 at the tip speed of the impeller of 435.3 m/s in this case.