284 results found
Zadrazil I, Bismarck A, Hewitt GF, et al., 2012, Shear Layers in the Turbulent Pipe Flow of Drag Reducing Polymer Solutions, Chemical Engineering Science, Vol: 72, Pages: 142-154
A range of high molecular weight polymers (polyethylene oxide) was dissolved at very low concentrations – in the order of few wppm – in a solvent (water). The Newtonian character of the polymer solutions was confirmed by rheological measurements. The polymer solutions were then pumped through a long horizontal pipe section in fully developed turbulent conditions. The flow experienced a reduction in frictional drag when compared to the drag experienced by the equivalent flow of the pure solvent. Specifically, drag reduction was measured at Reynolds numbers ranging from 3.5×10^4 to 2.1×10^5 in a pressure driven flow facility with a circular tube section of internal diameter 25.3 mm. The turbulent flow was visualized by Particle Image Velocimetry and the resulting data were used to investigate the effect of the drag reducing additives on the turbulent pipe flow. Close attention was paid to the mean and instantaneous velocity fields, as well as the two-dimensional vorticity and streamwise shear strain rate. The results indicate that drag reduction is accompanied by the appearance of “shear layers” (i.e. thin filament-like regions of high spatial velocity gradients) that act as interfaces separating low-momentum flow regions near the pipe wall and high-momentum flow regions closer to the centerline. The shear layers are not stationary. They are continuously formed close to the wall at a random frequency and move towards the pipe centerline until they eventually disappear, thus occupying or existing within a “shear layer region”. It is found that the mean thickness of the shear layer region is correlated with the measured level of drag reduction. The shear layer region thickness is increased by the presence of polymer additives when compared to the pure solvent, in a similar way to the thickening of the buffer layer. The results provide valuable insights into the characteristics of the turbulent pipe flow of a solvent contai
Markides CN, Fokaides P, Neophytou MK-A, 2012, An experimental investigation of the flow and transfer processes in homogeneous urban street-canyon geometries using Particle Image Velocimetry, 7th International Symposium on Turbulence Heat and Mass Transfer (THMT), Publisher: BEGELL HOUSE, INC, Pages: 530-539
Solanki R, Galindo A, Markides CN, 2012, Dynamic modelling of a two-phase thermofluidic oscillator for efficient low grade heat utilization: Effect of fluid inertia, Publisher: ELSEVIER SCI LTD, Pages: 156-163, ISSN: 0306-2619
Markides CN, Smith TCB, 2011, A Dynamic Model for the Efficiency Optimization of an Oscillatory Low Grade Heat Engine, Energy, Vol: 12, Pages: 6967-6980
A simple approach is presented for the modeling of complex oscillatory thermal-fluid systems capable of converting low grade heat into useful work. This approach is applied to the NIFTE, a novel low temperature difference heat utilization technology currently under development. Starting from a first-order linear dynamic model of the NIFTE that consists of a network of interconnected spatially lumped components, the effects of various device parameters (geometric and other) on the thermodynamic efficiencies of the device are investigated parametrically. Critical components are highlighted that require careful design for the optimization of the device, namely the feedback valve, the power cylinder, the adiabatic volume and the thermal resistance in the heat exchangers. An efficient NIFTE design would feature a lower feedback valve resistance, with a shorter connection length and larger connection diameter; a smaller diameter but taller power cylinder; a larger (time-mean) combined vapor volume at the top part of the device; as well as improved heat transfer behavior (i.e. reduced thermal resistance) in the hot and cold heat exchanger blocks. These modifications have the potential of increasing the relevant form of the second law efficiency of the device by 50% points, corresponding to a 3.8% point increase in thermal efficiency.
Markides CN, Mastorakos E, 2010, Experimental Investigation of the Effects of Turbulence and Mixing on Autoignition Chemistry, Flow Turbulence and Combustion, Vol: 86, Pages: 585-608, ISSN: 1573-1987
The autoignition of acetylene, released from a finite-sized circular nozzleinto a turbulent coflow of hot air confined in a pipe, has been the subject of a recentexperimental study to supplement previous work for hydrogen and n-heptane. Aswith hydrogen and n-heptane, autoignition appears in the form of well-defined localizedspots. Quantitative information is presented concerning the effects of turbulenceintensity, turbulent lengthscale and injector diameter on the location of autoignition.The effects of these parameters on inhomogeneous autoignition have not beeninvestigated experimentally before. The present study establishes that increasingthe bulk velocity increases the autoignition length, as was reported for hydrogenand n-heptane. For the same turbulence intensity, the autoignition length increasesas the injector diameter increases and as the turbulent lengthscale decreases. Asimultaneous decrease in turbulence intensity and increase in lengthscale causesa reduction in autoignition length. Further, the frequency of appearance of theautoignition spots has also been measured. It is found to increase when autoignitionoccurs closer to the injector, and also at higher velocities. The observed trends areconsistent with expectations arising from the dependence of the mixture fraction andthe scalar dissipation rate on the geometrical and flow parameters. The data can beused for the validation of turbulent combustion models.
Fokaides PA, Markides CN, Neophytou M, 2009, Ventilation characteristics of the built environment and their effects on the urban microclimate, SUSTAINABLE DEVELOPMENT AND PLANNING IV, VOLS 1 AND 2, Vol: 120, Pages: 271-281, ISSN: 1743-3541
Markides CN, Mastorakos E, 2009, Experimental investigation of the effects of turbulence and mixing on autoignition chemistry, 6th International Symposium on Turbulence, Heat and Mass Transfer, Publisher: BEGELL HOUSE, INC, Pages: 605-608
Markides CN, Mastorakos E, 2008, Measurements of the statistical distribution of the scalar dissipation rate in turbulent axisymmetric plumes, FLOW TURBULENCE AND COMBUSTION, Vol: 81, Pages: 221-234, ISSN: 1386-6184
Markides C, 2008, Autoignition in Turbulent Flows: Experimental Observations and Investigation of the Effects of Turbulence and Mixture Inhomogeneities, Publisher: VDM Verlag, ISBN: 9783836494342
In this work gaseous fuels were released continuously and concentrically into confined annular co-flows of turbulent hot air. Following injection the fuel and air mixed and at some length downstream of the nozzle the reactive mixture autoignited. Original phenomena are reported of autoignition spots, unsteady flame propagation and extinction or flashback. The frequency of the spots was measured, as were their acoustic and chemiluminescence characteristics. Optical measurements of the autoignition locations were made and used to estimate mean delay times from injection. As would be expected by considerations of simple chemical kinetics and the mean concentration field, higher air temperatures and lower fuel velocities resulted in autoignition closer to the injector. However, as the air velocity and hence also turbulent fluctuations were increased, autoignition shifted downstream and was delayed, while its frequency and sound intensity decreased. Such and other situations are presented that cannot be explained purely in terms of chemical arguments, i.e. homogeneous delay times, highlighting the significance of the mixing field through the mixture fraction and scalar dissipation rate.
Markides CN, Mastorakos E, 2008, Flame propagation following the autoignition of axisymmetric hydrogen, acetylene, and normal-heptane plumes in turbulent coflows of hot air, JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME, Vol: 130, ISSN: 0742-4795
Markides CN, De Paola G, Mastorakos E, 2007, Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air, EXPERIMENTAL THERMAL AND FLUID SCIENCE, Vol: 31, Pages: 393-401, ISSN: 0894-1777
Markides CN, Mastorakos E, 2006, Measurements of scalar dissipation in a turbulent plume with planar laser-induced fluorescence of acetone, CHEMICAL ENGINEERING SCIENCE, Vol: 61, Pages: 2835-2842, ISSN: 0009-2509
Markides CN, Mastorakos E, 2006, Flame propagation following the autoignition of axisymmetric hydrogen, acetylene and normal-heptane plumes in turbulent co-flows of hot air, 51st ASME Turbo Expo, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 843-852
Markides CN, Mastorakos E, 2005, An experimental study of hydrogen autoignition in a turbulent co-flow of heated air, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 30, Pages: 883-891, ISSN: 1540-7489
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