57 results found
Burridge HC, Sehmbi G, Fiuza Dosil D, et al., 2017, Determining the venting efficiency of simple chimneys for buoyant plumes, 38th AIVC Conference
We present preliminary results from an examination of the capture and venting of a buoyant plume by a chimney. The aim is to enable improved management of indoor pollutant sources –for instance, the plume rising from a cooking pan in a kitchen or a cooking fire in a hut. Using the principle of dynamic similarity, we precisely and controllably model the behaviour of indoor plumes by using saline solutions ejected into an enclosure containing freshwater. These well-established laboratory analogue techniques enable the location and concentration of tracer in the plume to be easily tracked, reflecting the evolution of pollutants carried in the plume. Focusing on a plume within a room containing a quiescent ambient environment, we identify two physical mechanisms potentially responsible for driving the removal of pollutants. The first, we describe as the capture of the plume, a process driven by the direct interaction between the plume and the evacuation opening; the second, we describe as the draining flow driven by a buoyant layer of fluid which may accumulate at the ceiling and is then evacuated through the effects of buoyancy. We first demonstrate that the addition of a simple cylindrical chimney that hangs downwards from an opening in the (analogue) ceiling increases the venting efficiency of these potentially polluting plumes.We go on to examine how the capture efficiency of these simple chimneys varies as the relative size of the plume and the chimney are altered, and demonstrate that simple model can provide predictionsof the observed variation in capture efficiency.
Logie WR, Abbasi-Shavazi E, Hughes G, et al., 2017, Turbulent contribution to heat loss in cavity receivers, ISSN: 0094-243X
© 2017 Author(s). For the prediction of convective heat loss from solar concentrating receiver cavities a number of empirical correlations exist. Geometry and the inclination angle determine the degree to which natural convection can infiltrate the cavity and remove stably stratified hot air out through the aperture. This makes the task of defining characteristic lengths for such Nusselt correlations difficult, neither does their use offer insight as to how one might reduce heat loss through the use of baffles, air curtains or small aperture-to-cavity-area ratios. Computational Fluid Dynamics (CFD) can assist in the design of better cavity receivers as long as the rules upon which it rests are respected. This paper is an exploration of the need for turbulence modelling in cavity receivers using some common linear eddy viscosity closure schemes. Good agreement was obtained with the CFD software OpenFOAMO® 3.0.1 for a deep cavity aperture but it under-predicted a shallow cavity. The experiments used for validation were in the Grashof region Gr ≈ 106, well below the region for transition to turbulence between 108< Gr < 109.
Pye J, Coventry J, Ho C, et al., 2017, Optical and Thermal Performance of Bladed Receivers, 22nd International Conference on Concentrating Solar Power and Chemical Energy Systems (SOLARPACES), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
Pye J, Coventry J, Venn F, et al., 2017, Experimental testing of a high-flux cavity receiver, ISSN: 0094-243X
© 2017 Author(s). A new tubular cavity receiver for direct steam generation, 'SG4', has been built and tested on-sun based on integrated optical and thermal modelling. The new receiver achieved an average thermal efficiency of 97.1±2.1% across several hours of testing, and reduced the losses by more than half, compared to the modelled performance of the previous SG3 receiver and dish. Near-steady-state outlet steam temperatures up to 560°C were achieved during the tests.
Dossmann Y, G Rosevear M, Griffiths RW, et al., 2016, Experiments with mixing in stratified flow over a topographic ridge, JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, Vol: 121, Pages: 6961-6977, ISSN: 2169-9275
Henley RW, Hughes GO, 2016, SO2 flux and the thermal power of volcanic eruptions, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, Vol: 324, Pages: 190-199, ISSN: 0377-0273
Hughes G, Pye J, Kaufer M, et al., 2016, Reduction of Convective Losses in Solar Cavity Receivers, 21st International Conference on Concentrating Solar Power and Chemical Energy Systems (SolarPACES), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
Hughes GO, 2016, Inside the head and tail of a turbulent gravity current, JOURNAL OF FLUID MECHANICS, Vol: 790, ISSN: 0022-1120
Hughes GO, Linden PF, 2016, Mixing efficiency in run-down gravity currents, JOURNAL OF FLUID MECHANICS, Vol: 809, Pages: 691-704, ISSN: 0022-1120
Pye J, Hughes G, Abbasi E, et al., 2016, Development of a Higher-Efficiency Tubular Cavity Receiver for Direct Steam Generation on a Dish Concentrator, 21st International Conference on Concentrating Solar Power and Chemical Energy Systems (SolarPACES), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
Vreugdenhil CA, Hogg AM, Griffiths RW, et al., 2016, Adjustment of the Meridional Overturning Circulation and Its Dependence on Depth of Mixing, JOURNAL OF PHYSICAL OCEANOGRAPHY, Vol: 46, Pages: 731-747, ISSN: 0022-3670
Zhang JJ, Pye JD, Hughes GO, 2016, ACTIVE AIR FLOW CONTROL TO REDUCE CAVITY RECEIVER HEAT LOSS, 9th ASME International Conference on Energy Sustainability, Publisher: AMER SOC MECHANICAL ENGINEERS
Abbasi-Shavazi E, Hughes GO, Pye JD, 2015, Investigation of heat loss from a solar cavity receiver, International Conference on Concentrating Solar Power and Chemical Energy Systems (SolarPACES), Publisher: ELSEVIER SCIENCE BV, Pages: 269-278, ISSN: 1876-6102
Gayen B, Hughes GO, Griffiths RW, 2015, Mechanical energy budget of turbulent rayleigh-benard convection
Turbulent Rayleigh-Bénard convection (RBC) is examined in terms of its mechanical energy budget. Three-dimensional large-eddy and direct numerical simulations are conducted at moderately large Rayleigh numbers. An expanded view of the mechanical energy pathways for RBC convection is developed for the the first time by recognising that mechanical energy includes gravitational potential energy and that the available component of this potential energy (APE) is the energy source for convection. The partitioning of energy pathways between large and small scales of motion is also analysed based on their corresponding temporal scales. The relative magnitudes of different pathways change significantly over the range of Rayleigh numbers Ra ∼ 107- 1013. At Ra < 107small-scale turbulent motions are energized directly from APE via turbulent buoyancy flux while kinetic energy is dissipated at comparable rates by both the large- and small-scale motions. In contrast, at Ra ≥ 1010most of the APE goes into kinetic energy of the large-scale flow, and the large scales undergo shear instabilities that sustain small-scale turbulence. At large Ra one half of the total APE supply goes to viscous dissipation of kinetic energy and the other half to mixing of the thermal field. Therefore, mixing efficiency approaches 50% at large Ra, as also predicted by a scaling analysis. At large Rayleigh number the viscous dissipation is largely in the interior, while the irreversible mixing is largely confined to the unstable boundary layers. The inclusion of the mechanical energy in the budget provides new information on the roles of different length scales and on the mechanics of the interior and boundary layer.
Saenz JA, Tailleux R, Butler ED, et al., 2015, Estimating Lorenz's Reference State in an Ocean with a Nonlinear Equation of State for Seawater, JOURNAL OF PHYSICAL OCEANOGRAPHY, Vol: 45, Pages: 1242-1257, ISSN: 0022-3670
Wykes MSD, Hughes GO, Dalziel SB, 2015, On the meaning of mixing efficiency for buoyancy-driven mixing in stratified turbulent flows, JOURNAL OF FLUID MECHANICS, Vol: 781, Pages: 261-275, ISSN: 0022-1120
Gayen B, Griffiths RW, Hughes GO, 2014, Stability transitions and turbulence in horizontal convection, JOURNAL OF FLUID MECHANICS, Vol: 751, Pages: 698-724, ISSN: 0022-1120
McIntosh A, Hughes G, Pye J, 2014, Use of an air curtain to reduce heat loss from an inclined open-ended cavity
The use of an air curtain directed across the aperture of an inclined open-ended cavity is examined as a method to reduce convective losses from a heated cavity. Computational fluid dynamics (CFD) simulations were conducted in two-dimensions for a range of air curtain velocities and axial cavity orientations. The greatest relative reduction in convective losses with an air curtain resulted when the cavity aperture plane was vertical (i.e. horizontal cavity axis). For cavities whose axis was inclined to the horizontal, convective losses could still be lowered with an air curtain, but reduced jet velocities were required for optimum performance.
Stewart KD, Saenz JA, Hogg AM, et al., 2014, Effect of topographic barriers on the rates of available potential energy conversion of the oceans, OCEAN MODELLING, Vol: 76, Pages: 31-42, ISSN: 1463-5003
Gayen B, Hughes GO, Griffiths RW, 2013, Completing the Mechanical Energy Pathways in Turbulent Rayleigh-Benard Convection, PHYSICAL REVIEW LETTERS, Vol: 111, ISSN: 0031-9007
Griffiths RW, Hughes GO, Gayen B, 2013, Horizontal convection dynamics: insights from transient adjustment, JOURNAL OF FLUID MECHANICS, Vol: 726, Pages: 559-595, ISSN: 0022-1120
Hughes GO, Gayen B, Griffiths RW, 2013, Available potential energy in Rayleigh-Benard convection, JOURNAL OF FLUID MECHANICS, Vol: 729, ISSN: 0022-1120
Gayen B, Griffiths RW, Hughes GO, et al., 2012, Direct numerical simulation of horizontal convection driven by differential heating
A numerical study based on three-dimensional direct numerical simulations are performed to investigate horizontal thermal convection in a long channel at a large Rayleigh number, Ra. Differential thermal forcing is applied at the bottom boundary over two equal regions. The steady-state circulation is achieved after the net heat flux from the boundary becomes zero. A stable thermocline forms above the cooled base and is advected over the heated part of the base, confining small-scale three-dimensional convection to the heated base and end wall region. At the endwall a narrow turbulent plume rises through the full depth of the channel. The less energetic return flow is downward in the interior, upon which eddy motions are imposed. This work, for the first time, focuses on the three dimensional instabilities and structures of the flow. The conversions of mechanical energy are examined in different regions of the flow (boundary layer, plume and interior) and help to understand overall circulation dynamics.
Saenz JA, Hogg AM, Hughes GO, et al., 2012, Mechanical power input from buoyancy and wind to the circulation in an ocean model, GEOPHYSICAL RESEARCH LETTERS, Vol: 39, ISSN: 0094-8276
Stewart KD, Hughes GO, Griffiths RW, 2012, The Role of Turbulent Mixing in an Overturning Circulation Maintained by Surface Buoyancy Forcing, JOURNAL OF PHYSICAL OCEANOGRAPHY, Vol: 42, Pages: 1907-1922, ISSN: 0022-3670
Griffiths RW, Maher N, Hughes GO, 2011, Ocean stratification under oscillatory surface buoyancy forcing, Journal of Marine Research, Vol: 69, Pages: 523-543, ISSN: 0022-2402
Laboratory experiments with overturning circulation driven by oscillatory heat fluxes at one boundary are used to explore implications, for the ocean stratification, of a cyclic fluctuation in sea-surface buoyancy forcing. Fluctuations having a range of periods spanning the timescale for global recycling of the ocean volume through the thermocline are considered, with emphasis on inter-hemispheric 'see-saw' oscillations. Episodic sinking of dense water in the oceans is represented by convection in a channel with a base that is cooled over a central region and subjected to oscillatory heating near both ends, while providing a constant total heat input. For this simplified system the time-average interior temperature is found to be insensitive to the forcing period, but does vary with oscillation amplitude, whereas the interior fluctuations increase with forcing period. The circulation and density field are significantly different from those given by a steady forcing equal to the time-average of the actual oscillatory forcing, even for high-frequency oscillations. The results indicate that the overall stratification lies between that expected from the strongest phase of deep sinking and that given by symmetric sinking in both hemispheres. Glacial cycles are predicted to involve significant temperature fluctuations in the abyssal ocean. However, they are too short for the ocean to remain in quasi-equilibrium with the changing boundary conditions.
Griffiths RW, Maher N, Hughes GO, 2011, Ocean stratification under oscillatory surface buoyancy forcing, JOURNAL OF MARINE RESEARCH, Vol: 69, Pages: 523-543, ISSN: 0022-2402
Paitoonsurikarn S, Lovegrove K, Hughes G, et al., 2011, Numerical Investigation of Natural Convection Loss From Cavity Receivers in Solar Dish Applications, JOURNAL OF SOLAR ENERGY ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 133, ISSN: 0199-6231
Stewart KD, Hughes GO, Griffiths RW, 2011, When do marginal seas and topographic sills modify the ocean density structure?, JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, Vol: 116, ISSN: 0148-0227
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