This paper presents a design and off-design study of a geothermal field consisting of a number of wells, a steam network which collects the geothermal fluid, and a power plant to generate electricity. The geothermal field considered is located in Italy and the fluid is a mixture of steam and non-condensable gases. The power plant is a conventional type for vapor dominated fields and has a compressor-extractor to remove non-condensable gases from the condenser. The condenser has wet cooling towers to remove heat. In this study, computer codes developed at the University of Perugia were used to simulate the behavior of geothermal field as a whole. The wells are modeled with second order functions in order to describe pressure-flow rate and temperature-flow rate correlations. The geothermal fluid network is calculated by setting pressures and thermal losses in all the branches. The power plant is simulated with all its components: steam turbine, condenser, gas extractor, cooling towers and auxiliaries. All the components of the geothermal field are simulated at both design and off-design conditions. The fluid network is solved with an algorithm developed by the authors, which allows the definition of boundary conditions by means of curves based on experimental data. The advantage in comparison with conventional techniques, requiring a fixed pressure or flow rate as boundary condition, is that the solution of the network and the power plant is always a real solution. The results show how changes in ambient conditions or in the characteristic curve of one or more plant components may influence power production and the exploitation of the geothermal source.

A steady-state model was developed to predict the performance of the high-pressure side of vapour-compression air-conditioning systems. The model consists of two segments; the compressor model and condenser model. The compressor model consists of a single empirical equation, for reciprocating compressors, operating with R-22, and having a cooling capacity from 2.6 to 3.5 kW. An important advantage of the approach is that the compressor performance indexes are based on operating conditions. The condenser model displays an exact method to determine physical dimensions of heat exchangers, and a simple, accurate manner to calculate the heat transfer variables. The correctness of the condensing temperature is obtained by an iterative procedure, using terms from both the compressor and condenser studies. The feasibility of the proposed model is demonstrated via a comparison with experimental data and a simulation study. Results indicate a good correlation between the mathematical model and its counterparts.

This study involves the experimental verification of a universal heat exchanger accumulator design procedure. A heat exchange accumulator was manufactured according to a mathematical model developed by the authors and installed into a small air conditioning system. Experimental results show that the addition of the heat exchanger accumulator results in a liquid overfeeding operation that replaces the previously utilized direct expansion operation. It provides an improved air conditioning system that has a 7.5% increase in coefficient of performance and a 4.4% increase in refrigerant mass flow rate. A pressure ratio reduction has a positive effect on the compressor performance and life span. Liquid overfeeding increases the cooling capacity of the system by 6.5%. When compared to direct expansion systems, this basic heat exchanger accumulator provides a reduction in cycling losses and power consumption, an increase in suction pressure and an improvement in isentropic compressor efficiency.

A vane latching mechanism for unloading a rotary vane compressor is described. Mechanical unloading is proposed as a potential alternative to electronic control of motor speed for variable compressor capacity. Performance of a demonstration compressor operating at part capacity in a residential air conditioning system is described.

It is proposed that consideration be given to an alternative, streamlined manner for mathematical modeling of the performance of energy conversion and transfer equipment. We make the case, here, by application to compressors. It is advocated that, instead of using an expression for efficiency as one of the governing equations, performance can be accounted for directly, with entropy production. It is shown that (1) the modeling is more straightforward, using fewer relations, and (2) that compressor performance (e.g. maps) can be represented equally well.

Split pistons allow more precise manufacturing and easier assembly of conjugate-piston multi-stage reciprocating compressors. Their disadvantage is that during start up, the high pressure stage piston may impact with both its cylinder head and the low-pressure stage piston. To determine the magnitude of these impacts and how they can be minimized, a simple mathematical model was constructed to analyze the pressure build-up within the compressor and the movement of the high-pressure stage piston. It is shown that above a minimum threshold value of pressure in the high-pressure stage, the pistons remain attached and move together without impact. The calculation of this minimum pressure along with the contact stresses, during rounds of impacts when the forces are greatest is demonstrated. A mathematical model is also provided to help minimize the impact stresses involved in such an arrangement. It is shown that by appropriate piston design and choice of piston material, selection of the optimum stroke and volume of the discharge system as well as the electric motor, split-conjugate pistons can be used in high-pressure multi-stage compressors.

Unsteady flow of oil and refrigerant gas through radial clearance in rolling piston compressors has been modeled as a heterogeneous mixture, where the properties are determined from the species conservation transport equation coupled with momentum and energy equations. Time variations of pressure, tangential velocity of the rolling piston and radial clearance due to pump setting have been included in the mixture flow model. Those variables have been obtained by modeling the compression process, rolling piston dynamics and by using geometric characteristics of the pump, respectively. An important conclusion concerning this work is the large variation of refrigerant concentration in the oil-filled radial clearance during the compression cycle. That is particularly true for large values of mass flow rates, and for those cases the flow mixture cannot be considered as having uniform concentration. In presence of low mass flow rates homogeneous flow prevail and the mixture tend to have a uniform concentration. In general, it was observed that for calculating the refrigerant mass flow rate using the difference in refrigerant concentration between compression and suction chambers, a time average value for the gas concentration should be used at the clearance inlet.

This paper discusses the thermodynamics of power cycles where steam or water are mixed with air to improve the performance of stationary gas turbine cycles. A general computer simulation program is presented; several components (compressor, combustor, turbine, recovery boiler, saturator, recuperator, inter-cooler, after-cooler) are modelled obtaining a transfer function between in-coming and out-coming fluids. Coupled with this simulation program, an optimisation is carried out using a decomposition strategy based on the second law of thermodynamic and a concept for costing the components of the cycles. A large number of decision variables can by optimised with enhanced convergence to an optimum. As results, a large number of simulations and optimisations are performed for three different plants (STIG, HAT and a cycle with water injection both in the compressor and in the combustor). A wide number of comparisons shows the difference between the cycles and the power level at which each plant gives the lowest exergoeconomic costs.

Multiphase transportation technology enables to transfer the mixture of crude oil, gas, water and sand in a single pipeline. Compared to the conventional method, it is more efficient since the multi-pipelines are not required. Multiphase compressor is the key machine in a multiphase transportation system, and the twin screw multiphase compressor/pump has proved to be a suitable machine for this application. This paper investigates the twin screw multiphase compressor, mainly focusing on the rotor profile. The calculating methods for mesh line length and blow hole area between the two rotors are presented. The geometrical characteristics of the rotor profile are also discussed in the paper.

A software package for the design of twin screw compressors is introduced in this paper. Functions of its main program include profile generation, geometrical parameter calculation, performance simulation, dynamic analysis, rotor deflection calculation, and cutter shape calculation. It is shown that the design work of twin screw compressor can be done quickly and precisely with the help of this software package. It is also illustrated how to obtain an optimum design by making use of the package.

Centrifugal compressors for multi-stage industrial applications must have both high efficiency and stable operation over a wide flow range. CFD analyses were used to evaluate the stall margins of candidate impeller designs compared to a baseline semi-inducer design. With this guidance, a new full-inducer impeller was designed with predicted wider operating range and slightly higher peak efficiency. Prototype tests confirmed these predictions. This paper presents predictions from two CFD codes and test results, for both the original and the redesigned impeller, and discusses the application of CFD for predicting impeller stall.

The preliminary design and analysis of a meso-scale refrigerator is presented here. The device is to be designed out of layers of silicon wafers bonded together and is to be fabricated through the techniques of microelectronics. The intended application of the device is an integrated heat removal system for electronics or photonic chips or modules. The paper presents a functional decomposition of the entire system, thermodynamic feasibility analysis, alternative configurations for two of the functions: actuation and compression, and parametric analysis for two alternative candidates for compressor actuation. A set of reasonable design requirements is first formulated. Overall function of the devices is decomposed into nine major sub-functions. Comparison of different alternatives for compression and actuation suggests that electrostatic actuation integrated with centrifugal compression is a viable option. Two different ways to implement electrostatic actuation are considered in details: variable capacitance motor and electrostatic induction motor. A set of design relations and criteria needed to obtain the optimal design of each motor is presented along with a discussion on relative effects of the main design parameters.

In this paper a general flowrate formula for lobe pumps is derived in terms of the pitch and deviation functions of the pump rotor. This formula applies to both circular and noncircular rotor pitch profiles. In order to relate the pump flowrate only to rotor geometry, a dimensionless flowrate expression, called “specific flowrate”, is adopted. This enables the flowrate analysis of lobe pumps without the ad-hoc effects of individual pump size and rotor speed. It is found that pump rotors can be grouped by the similarity of their deviation functions. The specific flowrates of a given group of pump rotors can be expressed solely in terms of their corresponding lobe noncircularities. This formulation enables a systematic and efficient design process of new lobe pumps. Classes of new lobe rotors based on some typical deviation functions are designed, and their corresponding specific flowrate functions are also provided.

This paper examines unsteady response of an experimental two-stage centrifugal compressor with vaneless diffusers. A thorough investigation of pressure fluctuations along the compressor channel is carried out in order to examine the onset of unstable flow conditions. Rotating stall structures are localized, with special attention paid to identifying the most sensitive area of the multi-stage compressor channel which may serve as a precursor to unstable compressor operation. It is shown that both stages may develop rotating stall structures simultaneously, however the 2 nd stage tends to destabilize first and reveals higher magnitudes of the unsteady response.

This paper presents the results of a series of numerical simulations exploring the stability of fluid flow in vaneless diffusers of centrifugal compressors. Three-dimensional, time-dependent, Reynolds-averaged Navier-Stokes simulations were performed for a fixed diffuser width ratio. Effective flow angle was varied from simulation to simulation in order to predict the threshold angle at which the instability — rotating stall — is manifested and maintained. The predicted threshold angle or stability limit is in excellent agreement with experimental data. The simulations reveal a two-stall cell configuration at the threshold condition.

The performance of a centrifugal compressor can be seriously affected by inlet flow distortions due to the unsatisfactory nature of the inlet configuration and the resulting inlet flow structure. In the previous work, experimental tests were carried out for the comparison of a centrifugal compressor stage performance with two different inlet configurations: one of which was a straight pipe with constant cross-sectional area as an ideal model and the other is a 90-degree curved pipe with nozzle shape as an actual model. The comparative test results indicated significant compressor stage performance difference between the two different inlet configurations. In addition, the numerical simulation part of the previous work clearly showed that the inlet flow distortion is caused by the pressure driven secondary flow developed in the curved section in the case of the bend inlet, resulting in locally concentrated incidence at the impeller inlet and thus the compressor stage performance degradation. An improved inlet model with the design method has been proposed based on the comparisons of the designated flow properties. In the present work, further numerical simulations on the compressor stage including the impeller and the diffuser with three different inlets are carried out to investigate the performance behavior of the compressor exposed to different inlet configurations. The three different inlet systems include the original bend inlet as well as the proposed inlet model based on the developed design method. Since the flow from the bend inlet is not axisymmetric due to the circumferential and radial distortion on the cross-section, the impeller and the diffuser are modeled with fully 360-degree passages, which accommodates the inlet flow distortion and the impeller-diffuser interaction influence on the entire flow passage of the compressor. The stage performance with the different inlet systems are evaluated and compared with the previous experimental result. The diffuser performance and the flow properties in the vaneless region are compared among those inlet models. The proposed inlet system indicated the benefit of performance improvement over the original inlet system.

A design study of various water vapor compressor configurations — including single and multistage centrifugal compressors and multistage axial compressors — is used to determine geometry and performance trends as a function of design speed, compressor configuration, and intercooling method. The intercooled multistage centrifugal machines led to the lowest required power levels, though the large outer diameter of these machines and the addition of an intercooler may reduce their appeal. The non-intercooled multistage axial configurations offered high design point performance and a much smaller outer diameter than the centrifugal machines, though at the expense of large overall axial length and limited flow range. Ultimately, the selection of the right machine for a given application is a tradeoff between the performance and geometry trends illustrated in this paper, as well as the cost of the machine.

The use of a virtual test rig to numerically test turbomachinery hardware can be extremely cost effective if the results obtained are physically correct and relatively accurate. The literature clearly shows that a lot of emphasis has been placed on single component validation optimized for a single operation point. There are few studies, however, that have clearly documented the numerical issues surrounding the modeling of a complete stage of a centrifugal compressor across its operating range. This effort uses a generic low flow stage design to demonstrate the accuracy to expect from the current state-of-the-art technology found in both commercial and research computational fluid dynamics (CFD) software. Even effects stemming from secondary flow paths are considered in this study. For design and off-design operation toward surge, 360-degree transient calculations are compared to those obtained from using the steady state fixed-rotor approximation. Finally, all work is ultimately compared to detailed test data obtained from single stage testing.

The objective of this work was to establish an advanced testing facility for studying the flow structure and loss mechanism in a single stage centrifugal compressor. A Trane’s CVHF 1280, two stage centrifugal compressor was modified to a single stage for laboratory environment. This modification included new fluid medium, driving motor, inlet and outlet designs. In this study, experiments were performed in order to evaluate the performance of the vaneless diffuser and volute. However, the modifications enable the flow structure investigation in all components of the compressor as well. In addition the testing facility would accommodate installation of the rest of the unit for investigation of the flow in the vaned diffuser and return channel of the two-stage compressor in the laboratory environment. Experiments were performed in three speeds and eight mass flow rates per speed. Static and total pressures were measured at the inlet and outlet of the stage. Static pressure distributions were mapped on the vaneless diffuser and volute casings. These data evaluates the characteristics of these components after the modifications.

Turbocompressor manufacturers as well as customers have a constant need for improved aerodynamic component analysis for production and development environments. These predictive tools have historically been grounded with experimental data. Both hardware and tests costs, however, continue to spiral upward, and the difficulties of obtaining reliable data remain unchanged. This has forced the turbocompressor manufactures to use Computational Fluid Dynamics (CFD) as an alternative approach for conducting stage parametric studies. In order for such an approach to be successful, a clear understanding of the relationship between the traditional performance parameters and the numerical solution has to be achieved. Ultimately, an industry standard similar to the test codes should be established. The focus of this work is to contribute into laying the foundation for such an effort. Based on energy conservation equations the relationships between CFD-based loss calculations and efficiencies are developed and discussed. Example of CFD-based performance prediction and flow field analysis of low flow compressor stage is then presented and directions of future work are outlined. This paper is the first part in a series of two. Second part containing more computational results and comparison with test data will follow.