Cooling of a Finned Cylinder by a Jet Flow of Air

Experimental tests have been carried out to evaluate the heat transfer characteristics on an externally finned cylinder impinged by a jet flow of air. The cylinder is internally heated with an electric system. Thermocouples located inside the cylinder allow to evaluate the wall temperature distribution, in order to calculate the local and average convective heat transfer coefficients.

Effects of Inlet Flow Field Conditions on the Stall Onset of Centrifugal Compressor Vaned Diffusers

This paper considers the performance and operating range of vaned diffusers for use in high performance centrifugal compressors. An experimental and numerical investigation is performed to determine the effects of inlet flow field conditions on pressure recovery and stall onset of different type vaned diffusers, such as discrete-passage and straight-channel diffusers. Diffuser inlet flow conditions examined include Mach number, flow angle, blockage, and axial flow non-uniformity. The investigation was carried out in a specially built test facility, designed to provide a controlled inlet flow field to the test diffusers. Unsteady pressure measurements showed the operating range of a compressor stage was limited by the onset of rotating stall, triggered by the loss of stability in the vaned diffuser, independent of the impeller operating point. For both diffusers investigated, loss of flow stability in the diffuser occurred at a critical value of the momentum-averaged flow angle into the diffuser. To provide additional information on diffuser flow development and to complement previous experimental work performed on straight-channel type diffuser, a computational investigation has been undertaken and important results are presented.

Heat and Mass Transfer Design of a Silicon Carbide Micro-Channel Heat Exchanger

Efficiency and emissions of advanced gas turbine power cycles can be improved by incorporating high-temperature ceramic heat exchangers. In cooperation with the DOE, a highly effective microchannel ceramic recuperator for a microturbine is under development. In this recuperator, the use of microchannel architecture will improve heat transfer and provide a more uniform temperature distribution. This will result in overall higher productivity per unit volume compared to conventional hardware. The use of ceramic for the recuperator will allow higher temperature operation than available in conventional microturbines. Based on a model for a typical microturbine, these changes may improve the overall system efficiency from about 27% to over 40%.

Numerical Investigation of Generalized Correlations for Turbulent Flow Pressure Drop and Heat Transfer Applied in Helically Coiled Tube Systems

A numerical analysis using a CFD package (Fluent v5.5) has been employed to investigate the turbulent pressure drop and heat transfer characteristics in the helically coiled tube system. The validity of using the techniques for creating coiled tube geometry and the corresponding volume mesh developed in this work is verified by explicitly comparing the numerically calculated results with those obtained from the experiments and correlations related to the straight and toroidal tubes. The authors’ previous work [5] includes the collection and summary of the general and application-specific published research and correlations. The information describing the pressure drop and heat transfer phenomena related to turbulent forced convection were combined and re-expressed into more generalized correlations using multiple linear regression techniques. In this paper, a numerical research effort using a commercial CFD package has been employed to reassess the actual phenomena with those predicted by the previously developed generalized correlations. The numerically predicted pressure drop and heat transfer coefficients at various Reynolds numbers are about 5–10% lower than those obtained by using existing generalized correlations [5]. For purposes of engineering calculations an error level of 15% or less is appropriate. The level of accuracy of the CFD modeling technique developed in this work is justified to investigate thermal-fluid phenomena in a coiled tube system.

Application and Validation of CFD in a Turbomachinery Design System

CFD (Computational Fluid Dynamics) has enjoyed widespread use in the turbomachinery industry for some time. When coupled with other solvers, such as meanline and streamline curvature, it can be an integral part of a comprehensive design and analysis system. The pbCFD (Pushbutton CFD®) product is the CFD component of Concepts NREC’s Agile Engineering Design System®. It is a structured grid CFD flow solver optimized for turbomachinery analysis. Concepts NREC has made an extensive validation effort over a wide range of diverse turbomachinery stages including, compressors, pumps, and turbines for both radial and axial machines. Detailed comparison to test data of 10 different stages is shown in this paper and clearly demonstrates the high performance of pbCFD in quantifying fluid dynamic losses and pressure changes over a wide range of geometries and flow conditions.

Cavitating Flow of Liquid Nitrogen in Horizontal Rectangular Nozzle

The fundamental characteristics of the two-dimensional cavitating flow of liquid nitrogen through a horizontal rectangular nozzle are numerically and experimentally investigated to realize the further development and high performance of new multiphase cryogenic fluid applications. First, the governing equations of the cavitating flow of liquid nitrogen based on the unsteady thermal nonequilibrium two-fluid model are presented, and several flow characteristics are numerically calculated, taken into account the thermodynamic effect of cryogenic conditions. Next, the flow visualization measurement on unsteady cavitating flow of liquid nitrogen through a rectangular nozzle installed in a horizontal duct is carried out to clarify the basic cryogenic two-phase flow structure and fundamental characteristics of the transient growth process of cryogenic cloud cavitation.

Investigation of a Centrifugal Compressor Stage With Two Volutes and the Same Impeller

Volute scroll, conic diffuser and sudden expansion discharge loss account for 4–6 points of efficiency decrement in a typical centrifugal compressor stage. The flow in a volute is highly complex. It is strongly believed that understanding of the detailed flow structure in a volute will provide insights on minimizing the losses by isolating the mechanisms that contributes to entropy generation. The result will be a more efficient centrifugal compressor product for customers and users and a product at higher profitability levels for manufacturers. This paper presents the experimental and numerical investigation on the matching of two different overhung volutes to the same centrifugal compressor impeller. The experimental data were measured from flange to flange firstly, then three Kiel probes were installed on pinch position circumferentially. At the same time, a detailed numerical simulation of the performance of the two volutes has been carried out. A computational model, using the k-ε turbulence model and the wall function, has been used to predict the internal flow of the both volutes. A good agreement between experimental data and numerical simulation results is found. The overall performance of the two volutes was also discussed in detail.

Thermo-Mechanical Design of a High-Temperature Silicon Carbide Micro-Channel Heat Exchanger

Efficiency and emissions of advanced gas turbine power cycles can be improved by incorporating high-temperature ceramic heat exchangers (see Figure 1). In cooperation with the DOE, preliminary development and testing of SiC based structures has been completed. This program has focused on four initial areas: thermo-mechanical degradation as a function of the chemical operating environments, design of a layered microchannel heat exchanger, thermo-mechanical testing and analysis of these structures, and fabrication development through rapid prototyping techniques.

Application of a CFD-Based Program for the Optimization of a Centrifugal Impeller

A typical centrifugal impeller characterized by a low flow coefficient and cylindrical blades is redesigned by means of an intelligent automatic search program. The procedure consists of a Feasible Sequential Quadratic Programming (FSQP) algorithm [6] coupled to a Lazy Learning (LL) interpolator [1] to speed-up the process. The program is able to handle geometrical constraints to reduce the computational effort devoted to the analysis of non-physical configurations. The objective function evaluator is an in-house developed structured CFD code. The LL approximator is called each time the stored database can provide a sufficiently accurate performance estimate for a given geometry, thus reducing the effective CFD computations. The impeller is represented by 25 geometrical parameters describing the vane in the meridional and s-θ planes, the blade thickness and the leading edge shape. The optimisation is carried out on the impeller design point maximizing the polytropic efficiency with more or less constant flow coefficient and polytropic head. The optimization is accomplished keeping unaltered those geometrical parameters which have to be kept fixed in order to make the impeller fit the original stage. The optimisation, carried out on a cluster of sixteen PCs, is self-learning and leads to a geometry presenting an increased design point efficiency. The program is completely general and can be applied to any component which can be described by a finite number of geometrical parameters and computed by any numerical instrument to provide performance indices. The work presented in this paper has been developed inside the METHOD EC funded project for the implementation of new technologies for optimisation of centrifugal compressors.

Numerical Simulation of Hydraulically Actuated Natural Gas Compressors

A numerical simulation has been developed to assist in design of low-speed hydraulically actuated compressors delivering natural gas at overall pressure ratios of 90: 1. The model has been used to estimate the effect of heat transfer during compression, the effects of reduced inter-stage volume, and the effects of departures from ideal gas behavior during the last stage of compression. It estimates overall compression efficiency, volumetric efficiency and cylinder pressures as a function of time. The model is capable of estimating dynamic behavior in intercooler passages when inter-stage volumes are small, and has been adapted to two or three stages.