Current Status, Design and Performance Trends for the Regenerative Flow Compressors and Pumps

Regenerative flow compressors and pumps, hereafter called RFC/RFP have found many applications in industry; still they are the most neglected turbomachines in the family of dynamic compressors. The number of publications existing in literature is very small compared to the large number of papers about the centrifugal and axial turbocompressors. This paper gives a detail discussion of fundamentals and working principle of regenerative turbomachines. Regenerative compressors are compared with centrifugal compressors and the importance of regenerative turbomachines in low specific speed range is emphasized. The major findings of available literature on regenerative turbomachine are summarized. The current status, limitations and some of the challenges faced by RFC/RFP are assessed in context of performance improvement. The paper concludes with an overview of ongoing research and future directions to be followed for performance improvement of this neglected class of turbomachines.

Cooling of a Finned Cylinder by a Jet Flow of Air

The paper describes a patented proposal to use jets of air in the cooling system of heavy trucks. Preliminary tests have been carried out, in the Heat Transfer Laboratory of the University of Rome “Tor Vergata”, to evaluate the heat transfer characteristics of a jet flow of air, impinging onto an externally finned cylinder. The cylinder is internally heated with an electric system. Thermocouples, located inside the cylinder, allow to measure the wall temperatures, in order to calculate the local and average convective heat transfer coefficients. A preliminary design of the practical apparatus, applied to heavy trucks, has been done in cooperation with Iveco. Nozzles are designed to be put after the fan of heavy trucks to converge air, in the form of jets, onto the tube where the charged air is flowing from the outlet of the turbo-compressor. The efficiency of the jet flow increases the cooling performances but, due to the high temperature at the outlet of the turbo-compressor, it may not be enough. The heat transfer cooling performances are enhanced if the tube to be cooled is externally finned. Some preliminary experiments have been carried out in a real scale bank test of an heavy truck engine at the Engineering Testing Laboratories Department of Iveco. Comparisons are done between the experiments and a simple theoretical model. Some conclusions are drawn about the cooling at different fluid dynamics conditions of the impinging jets.

A Parametric Study of Surge and Flow Instabilities in Gas Pipeline Compression Systems: The Effect of Pipe Parameters on Surge Margins

There is a need to understand the effect of coupling of the flow characteristics of a compressor with that of the pipeline and how this coupling effect the stability of the flow in a compression system. This study addresses such a need by carrying out a numerical simulation of the flow in the whole compression system including the compressor, the pipeline, and the other associated flow elements. A nonlinear, one-dimensional mathematical model is adopted for the present study. In this model, the gas flow inside the pipeline is assumed one-dimensional, viscous, and compressible. A parametric study is carried out using the proposed model, with air as the working fluid, to predict the surge margins for a subscale compression system and to study the effect of pipe length and diameter on these margins. Furthermore, the effect of these geometrical parameters on the amplitude and frequency of the flow oscillations are also established by numerical experimentation.

The Challenge of Design and Optimisation of Centrifugal Compressors for the UK’s Largest Producing Offshore Gas Field

The South Morecambe Field, owned and operated by Hydrocarbon Resources Ltd (a wholly owned subsidiary of Centrica PLC), has proved and probable reserves of 5.3 tcf and is located offshore in the East Irish Sea. With a plateau production rate of 1800mmscfd, the field delivered up to 20% of the UK peak gas demand.

Opportunities and Challenges in Micropump Technology

The rapidly growing applications in micro fluidic systems, biotechnology, micro chemical analysis systems, drug delivery systems, and chip-integrated cooling systems has introduced new opportunities and challenges in developing micropumps. Micropumps are one of the crucial components for moving liquids in these miniaturized systems. In this article, the focus is placed on both fundamental scientific and application specific issues. First, a brief review of the growing need for micropumps and the state of the art materials and fabrication technologies is presented. Next, the various pumping techniques, their working principle and their potential applicability to the future development of cost-effective miniature systems are discussed. Finally, selected results of an electrohydrodynamic micropump are presented.

VLSI Hotspot Cooling Using Two-Phase Microchannel Convection

Two-phase microchannel heat sinks are promising for the cooling of high power VLSI chips, in part because they can alleviate spatial temperature variations, or hotspots. Hotspots increase the maximum junction temperature for a given total chip power, thereby degrading electromigration reliability of interconnects and inducing strong variations in the signal delay on the chip. This work develops a modeling approach to determine the impact of conduction and convection on hotspot cooling for a VLSI chip attached to a microchannel heat sink. The calculation approach solves the steady-state two-dimensional heat conduction equations with boundary conditions of spatially varying heat transfer coefficient and water temperature profile. These boundary conditions are obtained from a one-dimensional homogeneous two-phase model developed in previous work, which has been experimentally verified through temperature distribution and total pressure drop measurements. The new simulation explores the effect of microchannels on hotspot alleviation for 20 mm × 20 mm silicon chips subjected to spatially varying heat generation totaling 150 W. The results indicate that a microchannel heat sink of thickness near 500 μm can yield far better temperature uniformity than a copper spreader of thickness 1.5 mm.

Streamlining of the Radial Inlet Design Process for Centrifugal Compressors

A radial compressor inlet represents an asymmetric and highly complex flow path, with high potential for flow disturbance. Due to the large computational resources and long lead times required for CFD analysis of such components, this resource has historically been reserved for conceptual or prototype designs. In the production environment, where compressor internals are often customized to a particular application, designers generally rely on geometric analysis of the flowpath. Low priority historically given to centrifugal inlet design is adequately illustrated in “mud etching” of the flow field in a retrofitted radial compressor inlet. An estimate of the potential for efficiency gain through inlet optimization, based on CFD predicted loss coefficient, is presented. It is noted that poor exit flow profiles can negatively impact performance, as well. Ill effects may include efficiency loss in downstream components, mechanical vibration, and compressor control issues. With continual improvement in CFD processing speed, the prospect of applying CFD based optimization techniques to production radial inlet designs becomes more feasible. In this investigation, CFD analysis is performed on an existing radial inlet design and validated with data from a flow visualization test rig. The subject inlet design is subsequently optimized through CFD analysis, with detailed attention being given to the impact of adjusting various geometric characteristics. A number of independent geometric parameters, which are determined to have significant impact on loss coefficient, are condensed into an optimization parameter. This optimization parameter serves as a preliminary indicator of design quality. Alternative brute force design methods are time prohibitive and may not provide the designer with feedback required to effectively alter geometry. Details of the CFD modeling and subsequent validation testing of the baseline inlet design are given. CFD results for a variety of modified inlet designs are presented. An overview of the optimization parameter and its application to a new radial inlet design are also presented. The potential for such an optimization parameter to limit design iteration is illustrated. Although additional refinement is suggested, the subject optimization parameter shows potential to direct the designer away from low efficiency designs.

Numerical and Experimental Study of Impeller Diffuser Interaction

The unsteadiness of the flow at the leading edge of a vaned diffuser represents a source of low efficiency and instability in a centrifugal turbomachine. Furthermore, the internal flow of the impeller can be affected by asymmetric downstream conditions, which results in extra flow unsteadiness and instabilities. Numerical and experimental data are obtained. The simulation of impeller diffuser interaction is performed using CFX-Tascflow. A frozen rotor simulation is used for the steady calculation and a rotor-stator simulation is used for the unsteady calculation using the steady results as an initial guess. The unsteady simulation is done not only for one impeller and diffuser blades, but also for the whole impeller and diffuser blades using Unix workstation. For the experimental work, a transparent fan is design and tested at The Turbomachinery Laboratory of SJTU. The test rig consists of a centrifugal, shrouded impeller, diffuser and volute casing all made of plexiglass. A particle image velocimeter (PIV) is used to measure the 2-D instantaneous velocity in the interaction region between impeller, vaned. A series of performance measurements were carried out at different speeds. The first trial of measuring the instantaneous flow field in a part of the impeller and vaned diffuser together at different relative locations between them is presented in this work at different flow rates. Obtaining detailed measurements in the interaction region between the impeller and diffuser can help in understanding the complex flow phenomena and improving centrifugal fan and compressor performance. Finally, the comparison between the unsteady measurements and unsteady calculations showed that the Rotor/Stator Model can predict the basic characteristics of unsteady flow in centrifugal fan but still need improvement to satisfy the true transient simulation for unsteady impeller diffuser interaction.

A New Approach to Predicting Centrifugal Compressor Sideload Pressure

This paper presents a new method of predicting pressure in a centrifugal compressor sideload. The method is compared with results from a CFD analysis and with test data points from five different sideloads. In all cases for which the sideload exit total pressure was known, the new method predicted the total pressure within 1 percent, usually within 0.5 percent. The new method is simple enough to use in unsophisticated prediction programs or spreadsheets or even hand calculations. Furthermore, the curvature factor developed for this method illustrates how sideload geometry affects sideload pressure.

Log Mean Temperature Correction Factor: An Alternative Representation

The Log Mean Temperature Difference (LMTD) correction factor, F, is traditionally expressed in terms of two non-dimensional parameters P=t2−t1T1−t1, and R=T1−T2t2−t1 in form of charts as the underlying equations are complicated. F shows strong functional dependence on both P and R, reducing the accuracy of reading the charts particularly in the steep regions. In this study it is shown that the LMTD correction factor F, can be expressed in terms of two new variables, φ=(T1−T2)2+(t2−t1)22[(T1+T2)−(t1+t2)] and ρ=T1−t2T2−t1. Expressed in terms of these variables, F correlations and charts are much better behaved. Furthermore, it is shown that for the shell and tube and cross flow of heat exchangers, over a wide range of operating conditions of practical interest (0.5 ≤ ρ ≤ 1.0), F can be approximated as a function of a single variable function φ, to within 4% accuracy.