55 results found
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 ≈ 10 6 , well below the region for transition to turbulence between 10 8 < Gr < 10 9 .
Pye J, Coventry J, Ho C, et al., 2017, Optical and thermal performance of bladed receivers, ISSN: 0094-243X
© 2017 Author(s). Bladed receivers use conventional receiver tube-banks rearranged into bladed/finned structures, and offer better light trapping, reduced radiative and convective losses, and reduced tube mass, based on the presented optical and thermal analysis. Optimising for optical performance, deep blades emerge. Considering thermal losses leads to shallower blades. Horizontal blades perform better, in both windy and no-wind conditions, than vertical blades, at the scales considered so far. Air curtains offer options to further reduce convective losses; high flux on blade-tips is still a concern.
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
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
Coman MA, Griffiths RW, Hughes GO, 2010, The sensitivity of convection from a horizontal boundary to the distribution of heating, JOURNAL OF FLUID MECHANICS, Vol: 647, Pages: 71-90, ISSN: 0022-1120
Hughes GO, Prastowo T, Griffiths RW, et al., 2010, The effects of topography on mixing rates in controlled exchange flows, Pages: 69-74
We examine irreversible mixing in buoyancy-driven baroclinic exchange flows past bottom topography. In all cases the shear between the counter-flowing layers is unstable in the vicinity of a hydraulic control situated above the topography. The overall amount of irreversible turbulent mixing produced by overturning billows during a given time is determined from measurements of vertical density profiles after the exchange. The potential energy changes are used to evaluate the global mixing efficiency, defined as the fraction of the available potential energy released into kinetic energy that is converted to potential energy through irreversible mixing. In all cases examined, the mixing efficiency is no more than 11%±1%, which corresponds to the maximum mixing efficiency found in previous work for exchange flows through a lateral contraction. The mixing efficiency is found to reduce as the fractional depth of the topography increases and as the topographic slope decreases. © 2010 Taylor & Francis Group, London.
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