Manuel V. Heitor

Measurements of turbulent and periodic flows around a square cross-section cylinder

Laser-Doppler measurements of the velocity characteristics are presented for the turbulent flow around a square cross-section cylinder mounted in a water channel for Re=14000. The study involved spectral analysis and digital filtering of the LDV data obtained behind the cylinder. The purpose of the measurements is to separate and quantify the turbulent and the periodic, non-turbulent, motions of the wake flow, in order to improve knowledge of the nature of the fluctuations in the near-wake region of two-dimensional bodies. The results show, for example, that in the zone of highest velocity oscillations the energy associated with the turbulent fluctuations is about 40% of the total energy.

Thermocouples and sample probes for combustion studies

Abstract The importance and limitations of probe methods for making measurements in combusting systems are reviewed and discussed in terms of the contributions that they can make to improve the knowledge and the modelling of practical systems. Emphasis is on measurements of temperature, major species and ion concentrations, which are discussed in terms of the need to determine and minimize probe-induced disturbances in practical combusting flows. In addition, errors arising from the difficulty in interpreting the results in turbulent flames are discussed and the review also provides some insight into measurements of time-resolved scalar quantities.

Velocity, temperature, and species characteristics of the flow in a gas-turbine combustor☆

Abstract The isothermal and combusting flow characteristics of a model can-type gas-turbine combustor are reported. In isothermal flow a laser-Doppler velocimeter was used to measure the longitudinal and tangential velocity characteristics, and the mean concentration of a tracer of helium gas, injected through the fuelling device, was used to obtain the mixture fraction distribution. In combusting flow, with propane as fuel, the velocimeter measured density weighted velocity characteristics, thin digitally compensated bare-wire thermocouples measured unweighted temperature characteristics, and a sampling probe transported gas samples to various instruments which measured near density weighted concentrations of O 2 , UHC, H 2 , CO 2 , CO, and NO x . The mean velocity results show that, relative to isothermal flow, combustion increases the strength and decreases the width of the primary vortex and, further downstream, attenuates the magnitude and strength of the swirl by the longitudinal acceleration of the flow. Turbulence measurements indicate that the production of turbulent kinetic energy in the dilution zone of the combustor is due to “conventional” mechanisms by the interaction of shear stress with shear strain. In the recirculation zone, however, gradient transport is negligible and the effect of the mean pressure gradient is likely to be important in the balance of turbulent kinetic energy. The scalar field is dominated by the primary jets. An increase of 34% in the air-fuel ratio causes a similar increase in the pattern factor at the exit plane and a 14% decrease in the combustion efficiency. The pattern factor is improved by 15% and optimal combustion efficiency is obtained by increasing the inlet air temperature by 432K. In the primary zone, combustion is controlled more by physical than chemical kinetic processes and the formation of pollutants is well described by a partial equilibrium model. Downstream of the primary holes the fuel breakdown and the CO oxidation are reaction-rate limited, and the scalar field may be described by a constrained equilibrium model.

Wake flows behind two-dimensional model hills

Abstract Laser-Doppler measurements of velocity characteristics are reported for the recirculating flow established in the wake of two-dimensional, polynomial-shaped obstacles that are axisymmetrical about a vertical axis and mounted in a water channel downstream of a fully developed channel flow for Re = 6 × 10 4 . The study involves measurements of the mean and fluctuating flow properties in the streamwise and spanwise directions and includes comparison of the flow around a single obstacle with that resulting from the interaction of consecutive obstacles. The results include analysis of the turbulent stresses in local flow coordinates and reveal flow structure qualitatively similar to that found in other turbulent flows with a reattachment zone and, for the case of multiple hills, resembling the flow over wavy surfaces of large amplitude. The implications of the results for the calculation of turbulent flows over curved boundaries using turbulence models are discussed.

Swirl flow structure and flame characteristics in a model lean premixed combustor

The swirling flow in a lean premixed prevaporized (LPP) combustor model (LPP chamber) is studied, making use of high-speed photography, Laser Doppler Velocimetry (LDV), sound probes, fine-wire thermocouples, and suction probes for chemical gas analysis. The experimental study, which involved parametric changes of Re , S , and { , extends from the combustor primary zone to the premixing chamber and was conducted with and without reaction, burning either gaseous propane or liquid fuel assisted with preheated air at 300 °C. Measurements show that the swirling flow mixture that enters the primary zone of the combustor chamber shows evidence of a PVC structure, for S > 0.5, that embraces the central recirculation zone (CRZ). Furthermore, reaction test results show that increasing the swirl number decreases CO and NO x concentrations at the combustor exit and reduces flame stability limits to levels close to the lean limit because of flashback. In addition, a detailed study of the reacting condition ( S =1.05, { =0.5) was performed; the results indicate that the PVC is still present, embracing the base of the CRZ and staying in the inner shear layer, where the flame should also be stabilized, and contributing particularly to an abnormal radial distribution of W rms . Large gradients of mean temperature andUnburnedHydrocarbons(UHC) concentration were found to be limited to the base of the CRZ.

Simultaneous velocity and temperature measurements in a premixed flame

Procedures which allow the correlation of velocity signals from a laser anemometer and temperature signals from a compensated, small-diameter thermocouple are described together with the error sources associated with the use of the technique in premixed flames. The digital compensation procedure includes the effect of velocity and temperature on the time constant of the thermocouple and the influence of its exposure to the solid particles required by the laser anemometer are quantified and shown to be able to cause large differences in the measured probability-density-distribution of the reaction progress variable. The technique has been used to measure the probability-density-distribution of temperatures, conditioned by the arrival of velocity signals and velocity conditioned by the temperature signal and sample results are presented to help quantify the accuracy of the measurements.

On the Role of the University in the Knowledge Economy

This paper draws on recent conceptual approaches to economic growth, in which the accumulation of knowledge is the fundamental driving force behind growth, to examine the contemporary role of the university. It suggests that the functions that society commonly attributes to the university are beginning to be shared with a wide range of institutions in the context of the knowledge-based economies, so that the university is faced with demands that require a strengthening of its ability to create and disseminate knowledge. While the role of universities is in need of some rethinking, their institutional integrity must be preserved. To cope with the variety of demands and a continuously changing environment, it is argued that the higher-education system needs to be diversified. Specific policy proposals are made in terms of institutional arrangements that assure diversity, while maintaining institutional integrity. Copyright , Beech Tree Publishing.

Impingement of single and twin turbulent jets through a crossflow

Laser-Doppler measurements of velocity characteristics of the flowfield resulting from the impingement of single and twin jets against a wall through a low-velocity crossflow are presented and discussed together with visualization of the flow. The experiments have been carried out for a velocity ratio between the jet and the crossflow of 30, for a Reynolds number based on the jet exit between 60,000 and 105,000, and for the jet exit 5 jet diameters above the ground plate. In addition, calculations based on a two-equation turbulence model are presented for the three-dimensional flow characterized by the measurements, and comparison between experimental and numerical results show that the mean flowfield is well predicted. The calculation of the turbulent field requires, however, consideration of the individual stresses. 21 refs.

Influence of confinement on combustion instabilities of premixed flames stabilized on axisymmetric baffles

Abstract The frequency and strength of combustion-induced oscillations have been measured for premixed flames stabilized on baffles located on the axis of a pipe. With the baffle located between 5 and 20 diameters upstream of the pipe exit, the frequency corresponds to longitudinal (standing quarter-wave) acoustic waves in the cold gas column upstream of the baffle and the strength is only weakly dependent on axial location, area blockage ratio (0.25 and 0.5), forebody shape (disk or cone), and Reynolds number ( Combustion-induced oscillations, with a predominant frequency and large amplitude, were not observed when the baffle was located between approximately 1.5 and 5 pipe diameters from the pipe exit. As the baffle was moved from the 1.5 diameter location toward the pipe exit, increasingly strong oscillations were observed with equivalence ratios between 1.1 and 1.5. The frequency of these oscillations increased with decreasing equivalence ratio and increasing Reynolds number. Instability could be obtained with the baffle between 1.5 and 5 diameters of the pipe exit by constricting the diameter of the exit plane. The difference in behavior between confined and unconfined flames is ascribed to the existence of a self-induced favorable pressure gradient in the confining duct. It is proposed that the instantaneous value of this gradient influences the rate of heat released by the flame through changes in the rate of turbulent mixing due to the preferential acceleration of products of combustion over reactants.

Infrastructures, incentives, and institutions: Fostering distributed knowledge bases for the learning society

Abstract While much attention has been devoted to information and communication technologies, a more fundamental change at the start of the new millennium is the increasing importance of innovation for economic prosperity and the emergence of a learning society. The analysis in this paper shows that innovation should be understood as a broad social and economic activity: it should transcend any specific technology, even if revolutionary, and should be tied to attitudes and behaviors oriented towards the exploitation of change by adding value. We build on the idea of inclusive learning, which entails a process of shared prosperity across the globe following local specific conditions, and argue that it is crucial to understand the features of knowledge-induced growth in rich countries, as well as the challenges and opportunities for late-industrialized and less-developed countries. To achieve these objectives, we emphasize the relative importance of infrastructures and incentives , but considering the increasingly important role of institutions towards the development of social capital . This is because learning societies will increasingly rely on “distributed knowledge bases” as a systematically coherent set of knowledge maintained across an economically and/or socially integrated set of agents and institutions. This broad concept has motivated the work behind the present paper, which builds on material presented at the 5th International Conference on Technology Policy and Innovation (ICTPI), which was held in Delft, The Netherlands, in June of 2001. Under the broad designation of “critical infrastructures,” the Conference brought together a range of experts to discuss technology, policy and management in a context much influenced by the dynamics of the process of knowledge accumulation, which drives learning societies. Thus, this special issue includes a set of extended contributions to the Delft conference, and the aim of this introductory paper is to set the stage for these contributions, with an original contribution on possible views on the role critical infrastructures play to foster innovation in the learning society.