Iterative Method to Derive the Equivalent Centrifugal Compressor Performance at Various Operating Conditions: Part I: Modelling of Suction Parameters Impact

This paper introduces a new iterative method to predict the equivalent centrifugal compressor performance at various operating conditions. The presented theoretical analysis and empirical correlations provide a novel approach to derive the entire compressor map corresponding to various suction conditions without a prior knowledge of the detailed geometry. The efficiency model was derived to reflect the impact of physical gas properties, Mach number, and flow and work coefficients. One of the main features of the developed technique is the fact that it considers the variation in the gas properties and stage efficiency which makes it appropriate with hydrocarbons. This method has been tested to predict the performance of two multistage centrifugal compressors and the estimated characteristics are compared with the measured data. The carried comparison revealed a good matching with the actual values, including the stable operation region limits. Furthermore, an optimization study was conducted to investigate the influences of suction conditions on the stage efficiency and surge margin. Moreover, a new sort of presentation has been generated to obtain the equivalent performance characteristics for a constant discharge pressure operation at variable suction pressure and temperature working conditions. A further validation is included in part two of this study in order to evaluate the prediction capability of the derived model at various gas compositions

An Iterative Method to Derive the Equivalent Centrifugal Compressor Performance at Various Operating Conditions: Part II: Modeling of Gas Properties Impact

This is the second part of a study conducted to model the aerothermodynamic impact of suction parameters and gas properties on a multi-stage centrifugal compressor’s performance. A new iterative method has been developed in the first part to derive the equivalent performance at various operating conditions. This approach has been validated to predict the compressor map at different suction pressures and temperatures using the design characteristics as reference values. A further case is included in this paper in order to emphasize the validity of the developed approach to obtain the performance characteristics at various gas compositions. The provided example shows that the performance parameters at different gas mixtures can be predicted to within ±1.34%. Furthermore, the conducted optimization in this paper reveals that the proposed method can be applied for the compressor design evaluation corresponding to the expected variation in suction conditions. Moreover, the examined case study demonstrates the effect of gas properties’ variation on the operating point and aerodynamic stability of the entire compression system. In order to achieve that, a simple approach has been established to assess the contribution of gas properties’ variation to the inefficient and unstable compressor performance based on the available operational data

Improving the Accuracy and Scope of Control-Oriented Vapor Compression Cycle System Models

The benefits of applying advanced control techniques to vapor compression cycle systems are well know. The main advantages are improved performance and efficiency, the achievement of which brings both economic and environmental gains. One of the most significant hurdles to the practical application of advanced control techniques is the development of a dynamic system level model that is both accurate and mathematically tractable. Previous efforts in control-oriented modeling have produced a class of heat exchanger models known as moving-boundary models. When combined with mass flow device models, these moving-boundary models provide an excellent framework for both dynamic analysis and control design. This thesis contains the results of research carried out to increase both the accuracy and scope of these system level models. The improvements to the existing vapor compression cycle models are carried out through the application of various modeling techniques, some static and some dynamic, some data-based and some physics-based. Semiempirical static modeling techniques are used to increase the accuracy of both heat exchangers and mass flow devices over a wide range of operating conditions. Dynamic modeling techniques are used both to derive new component models that are essential to the simulation of very common vapor compression cycle systems and to improve the accuracy of the existing compressor model. A new heat exchanger model that accounts for the effects of moisture in the air is presented. All of these model improvements and additions are unified to create a simple but accurate system level model with a wide range of application. Extensive model validation results are presented, providing both qualitative and quantitative evaluation of the new models and model improvements.Air Conditioning and Refrigeration Project 17

Techno-economic assessment of radial turbomachinery in process gas applications

This research aims to assess the causes of inefficient and unstable operation of centrifugal compressors and turboexpanders in process gas applications in order to provide a solution for performance restoration and enhancement. It encompasses thermodynamic and flow evaluations to examine the efficiency and operating range improvement options of new units. Besides, this work is complemented by a technoeconomic analysis to provide a rounded outcome from these studies. In order to achieve the desired objectives, a novel integrated approach has been developed to assess the design and performance of multi-stage centrifugal compressors. The proposed systematic methodology involves five basic elements including evaluation of compressor selection, compressor sizing and casing structure, performance prediction at the design and off-design conditions, modelling of efficiency and head deterioration causes; and stage design evaluation. This will contribute towards evaluating the geometrical parameters of the new units’ designs at the early preliminary design phase, and thus, will be useful to identify the options for efficiency and operating range enhancements. For installed units, this approach can be implemented to assess the cause of inefficient and unstable operation by assessing the available operation data. A method was developed to predict the performance curve of multi-stage centrifugal compressor based on a stage stacking technique. This approach considers the advantages of Lüdtke and Casey-Robinson methods with an incorporation of a methodology for compressor selection and sizing to generate more accurate results. To emphasize the validity of the developed model, it has been evaluated for both low and high flow coefficient applications. The obtained results show a significant improvement in the estimated efficiency, pressure ratio, shaft power and operating range as compared with the existing methods. The centrifugal compressor is designed to run under various operating conditions and different gas compositions with the primary objective of high efficiency and reliability. Therefore, a new iterative method has been developed to predict the equivalent compressor performance at off-design conditions. This technique uses the performance parameters at design conditions as a reference point to derive the corresponding performance characteristics at numerous suction conditions with less dependency on the geometrical features. Through a case study on a gas transport centrifugal compressor, it was found that the developed approach can be applied for design evaluation on the expected variation of working conditions, and for the operation diagnosis of installed units as well. Furthermore, a parametric study has been conducted to investigate the effect of gas properties on the stage efficiency, surge margin, and compressor structure. The obtained results support the need for considering the gas properties variation when the off-design performance is derived. To evaluate the impact of internal blockage on the performance parameters, this study proposed an approach to model the effect of non-reactive deposits, which has been qualified using four operation cases and the obtained results are compared with the internal inspection findings from the stage overhauling process. This also covers the influential aspects of flow blockage on the technical and economic values. Since the main challenge here is to analyze the process gas composition in real time, the influences of the non-reactive deposits have been compared with the effect of the unanticipated gas composition change. Subsequently, it has turned out that the pressureratio parameter is not enough to assess the possibility of flow blockage and unexpected gas properties change. Moreover, it was observed that the stage discharge pressure was more sensitive to the fouled aftercooler comparing with suction and internal blockage. However, the effect of contaminated aftercooler on the surge point and discharge pressure and temperature of the upstream stage was found greater than its impact on the shaft power. Thus, a substantial surge margin reduction was detected when the first stage was operating with a fouled aftercooler comparing with the measured reduction as a result of unanticipated gas properties change. Furthermore, a larger pressure ratio drop was measured in the case of liquid carryover which revealed a more significant impact of the two phases densities difference comparing with the gas volume fraction (GVF) effect. The possibility of hydrate formation has been assessed using hydrate formation temperature (HFT) criteria. Additionally, this research highlights a number of challenges facing the selection of typical centrifugal stage design by assessing the contribution of design characteristics on the operating efficiency and stable flow range. Besides, an empirical-based-model was established to select the optimum impeller and diffuser configurations in order to make a compromise decision based on technical and economic perspective. It was concluded that there is no absolute answer to the question of optimum rotor and stator configuration. The preliminary aerothermodynamic evaluation exposed that the selection of the optimum impeller structure is governed by several variables: stage efficiency, pressure loss coefficient, manufacturing cost, required power cost, resonance frequency and stable operating range. Hence, an evaluation is required to compromise between these parameters to ensure better performance. Furthermore, it was argued throughout this study that the decision-making process of the typical stage geometrical features has to be based upon the long-term economic performance optimization. Thus, for higher long-term economic performance, it is not sufficient to select the characteristics of the impeller and diffuser geometry based on the low manufacturing cost or efficiency improvement criterion only. For turboexpanders, a simple and low cost tool has been developed to determine the optimum turboexpander characteristics by analysing the generated design alternatives. This approach was used in designing a turboexpander for hydrocarbon liquefaction process. Moreover, since the turboexpanders are expected to run continuously at severe gas conditions, the performance of the selected turboexpander was evaluated at different inlet flow rates and gas temperatures. It has turned out that designing a turboexpander with the maximum isentropic efficiency is not always possible due to the limitations of the aerodynamic parameters for each component. Therefore, it is necessary to assess the stage geometrical features prior the construction process to compromise between the high capital cost and the high energetic efficiency.SATM Prize winne

Impeller blade design method for centrifugal compressors

The design of a centrifugal impeller with blades that are aerodynamically efficient, easy to manufacture, and mechanically sound is discussed. The blade design method described here satisfies the first two criteria and with a judicious choice of certain variables will also satisfy stress considerations. The blade shape is generated by specifying surface velocity distributions and consists of straight-line elements that connect points at hub and shroud. The method may be used to design radially elemented and backward-swept blades. The background, a brief account of the theory, and a sample design are described

Development, Validation, and Application of a Refrigerator Simulation Model

This report describes the further development and validation of the Refrigerator/Freezer Simulation (RFSIM) model. The reports also describes the first major application of the model as an analysis tool for new refrigerator designs; several aspects of multi-speed compressor operation were examined with the model. Several improvements were made to the model that facilitated the validation process and the examination of multi-speed compressors: the model was made more general so that it could operate in numerous configurations in addition to the original design and simulation modes; many improvements were made in the modeling logic and robustness of the capillary tube-suction line heat exchanger model; and the equation-of-statebased property routines that calculated the thermodynamic properties were replaced with interpolation routines that were much faster. The RFSIM model, in design and simulation mode, was validated with data from two refrigerators. In both modes, the average model errors were less than ??5% for several important variables such as evaporator capacity and coefficient of performance. The errors of the simulation mode were reduced from the previous model validation primarily by using a different void fraction correlation in the refrigerant charge equations. The results from the validated RFSIM model indicate that a two-speed compressor could yield energy savings of 4% to 14% due to the increased steady-state efficiency at the low speed and an additional 0.5 to 4% savings due to the decreased cycling frequency. The results also showed that the capillary tube-suction line heat exchanger, when designed for the low speed, did not adversely affect the pull-down capacity when the compressor operated at the high speed. Lastly, it was found that a refrigerator operating at low ambient temperatures could actually benefit from a decrease in the condenser fan speed. This change in fan speed increased the evaporator capacity by reallocating charge to the evaporator and subsequently reducing the superheat at the evaporator exit.Air Conditioning and Refrigeration Project 6

Runtime Assurance Protection for Advanced Turbofan Engine Control

This paper describes technical progress made in the application of run time assurance (RTA) methods to turbofan engines with advanced propulsion control algorithms that are employed to improve engine performance. It is assumed that the advanced algorithms cannot be fully certified using current verification and validation approaches and therefore need to be continually monitored by an RTA system that ensures safe operation. However, current turbofan engine control systems utilize engine protection logic for safe combustion dynamics and stable airflow through the engine. It was determined that the engine protection logic should continue to be used to provide system safety and should be considered as a part of the overall RTA system. The additional function that an RTA system provides is to perform diagnostics on anomalous conditions to determine if these conditions are being caused by errors in the advanced controller. If this is the case, the RTA system switches operation to a trusted reversionary controller. Initial studies were performed to demonstrate this benefit. The other focus was to improve the performance of the engine protection logic, which was deemed too conservative and reduced engine performance during transient operations. It was determined that the conservative response was due to poor tuning of one of the controller channels within the protection logic. An automatic tuning algorithm was implemented to optimize the protection logic control gains based on minimizing tracking error. Improved tracking responses were observed with no change to the existing protection logic control architecture

Centrifugal Compressor Performance Making Enlightened Analysis Decisions

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The effects of atmospheric turbulence on fuel consumption in extended formation flight

Includes bibliographical references.Extended formation flight (streamwise separations of between 10 to 40 spans), has been recently proposed as a method for reducing the induced drag of commercial aircraft. However, induced drag savings are not necessarily directly indicative of fuel savings. In a realistic environment, atmospheric turbulence will continuously perturb the formation’s aircraft and their wakes. As a result, each aircraft in the formation will experience fluctuations in aerodynamic loads. For an aircraft to maintain accurately its position within a formation, it must continually adjust its throttle setting. This dynamic throttling may result in inefficient engine operation, thereby detracting from the reductions in induced drag. In this work, a high-fidelity transient engine model, representative of a typical commercial high-bypass turbofan engine, has been incorporated within a simple twin-aircraft formation flight simulator. The aerodynamic interactions between aircraft were modelled using a horseshoe vortex method, specially adapted for extended formations. The aircraft were constrained to longitudinal motion, with altitude fixed. This created a two degree of freedom formation model that is analogous to wind tunnel experimentation. A simple proportional gain controller was used to manipulate the throttle settings, in an attempt to maintain the trail aircraft’s position relative to the leader, in a turbulent atmosphere. It was found that a fuel saving of approximately 25 may be achieved at a practical lateral separation of 1 span, corresponding to a stream-wise separation of 20 ± 0.3 spans, in moderate turbulence levels