An organic Rankine cycle (ORC) machine is similar to a conventional steam cycle energy conversion system, but uses an organic fluid such as refrigerants and hydrocarbons instead of water. In recent ...years, research was intensified on this device as it is being progressively adopted as premier technology to convert low-temperature heat resources into power. Available heat resources are: solar energy, geothermal energy, biomass products, surface seawater, and waste heat from various thermal processes. This paper presents existing applications and analyzes their maturity. Binary geothermal and binary biomass CHP are already mature. Provided the interest to recover waste heat rejected by thermal devices and industrial processes continue to grow, and favorable legislative conditions are adopted, waste heat recovery organic Rankine cycle systems in the near future will experience a rapid growth. Solar modular power plants are being intensely investigated at smaller scale for cogeneration applications in buildings but larger plants are also expected in tropical or Sahel regions with constant and low solar radiation intensity. OTEC power plants operating mainly on offshore installations at very low temperature have been advertised as total resource systems and interest on this technology is growing in large isolated islands.
We investigate the role of the laminar/turbulent interface in the interscale energy transfer in a boundary layer undergoing bypass transition with the aid of the Kármán–Howarth–Monin–Hill (KHMH) ...equation. A local binary indicator function is used to detect the interface and employed subsequently to define two-point intermittencies. These are used to decompose the standard-averaged interscale and interspace energy fluxes into conditionally averaged components. We find that the inverse cascade in the streamwise direction reported in an earlier work arises due to events across the downstream or upstream interfaces of a turbulent spot. However, the three-dimensional energy flux maps reveal significant differences between these two regions: in the downstream interface, inverse cascade is stronger and dominant over a larger range of streamwise and spanwise separations. We explain this finding by considering a propagating spot of simplified shape as it crosses a fixed streamwise location. We derive also the conditionally averaged KHMH equation, thus generalising similar equations for single-point statistics to two-point statistics. We compare the three-dimensional maps of the conditionally averaged production and total energy flux within turbulent spots against the maps of standard-averaged quantities within the fully turbulent region. The results indicate remarkable dynamical similarities between turbulent spots and the fully turbulent region for two-point statistics. This has been known only for single-point quantities, and we demonstrate here that the similarity extends to two-point quantities as well.
We present a study of the turbulence cascade on the centreline of an inhomogeneous and anisotropic near-field turbulent wake generated by a square prism at a Reynolds number of
$Re=3900$
using the ...Kármán–Howarth–Monin–Hill equation. This is the fully generalised scale-by-scale energy balance which, unlike the Kármán–Howarth equation, does not require homogeneity or isotropy assumptions. Our data are obtained from a direct numerical simulation and therefore enable us to access all of the processes involved in this energy balance. A significant range of length scales exists where the orientation-averaged nonlinear interscale transfer rate is approximately constant and negative, indicating a forward turbulence cascade on average. This average cascade consists of coexisting forward and inverse cascade behaviours in different scale-space orientations. With increasing distance from the prism but within the near field of the wake, the orientation-averaged nonlinear interscale transfer rate tends to be approximately equal to minus the turbulence dissipation rate even though all of the inhomogeneity-related energy processes in the scale-by-scale energy balance are significant, if not equally important. We also find well-defined near
$-5/3$
energy spectra in the streamwise direction, in particular at a centreline position where the inverse cascade behaviour occurs for streamwise oriented length scales.
The Kármán–Howarth–Monin–Hill equation is employed to study the production and interscale energy transfer in a boundary layer undergoing bypass transition due to free-stream turbulence. The energy ...flux between different length scales is calculated at several streamwise locations covering the laminar, transitional and turbulent regimes. Maps of scale energy production and flux vectors are visualised on two-dimensional planes and three-dimensional hyper-planes that comprise both physical and separation spaces. In the transitional region, the maps show strong inverse cascade in the streamwise direction near the wall. The energy flux vectors emanate from a region of strong production and transfer energy to larger streamwise scales. To provide deeper insight into the origin of the inverse cascade process, we decompose the energy flux vector into components arising from nonlinear interactions between velocity fluctuations, mean flow inhomogeneity, pressure and viscous effects. The inverse cascade is mainly due to the nonlinear interaction component, and in the earliest stages of transition this component competes with that due to mean flow inhomogeneity. By superposing the instantaneous velocity fields and the energy flux vectors, we relate the inverse cascade process to the growth of turbulent spots. Once the transition process is complete, the maps become very similar to those observed in other fully developed turbulent flows, such as channel flow. Finally we characterise the nonlinear interaction term using probability density functions (PDFs) evaluated at different wall-normal heights. The PDFs are asymmetric and wide-skirted as in homogeneous isotropic turbulence, but are skewed towards positive values reflecting the inverse cascade.
Research on ash from burning of rice husks (RHA) has already demonstrated that it is one of the most promising supplementary cementing materials (SCM), given its high specific surface and great ...amount of silica soluble in alkaline conditions. Indeed, RHA could be a product of added value if it wasn't for its limited availability and periodically high residual carbon content, factors inhibiting its wider use in building materials. Most of the published work has exploited the effectiveness of RHA of very high specific surface and reactive silica, without really investigating the effect of these factors with respect to mechanical and durability characteristics of the derived cement and concrete. This is of crucial importance since someone could falsely rate RHA of moderate fineness and chemical reactivity as potentially reject, constituting thus a significant portion of this by-product unutilised. Reactive silica and fineness effect was assessed in this study by examining two different RHAs, both in blended cement and concrete level. It was found out that RHA is a material extremely “sensitive” to fineness changes; the higher being the fineness the more positive is the effect of RHA inclusion in the mix. Not surprisingly, active silica holds a key role especially for later-age strength gain, indicating that pozzolanic effect takes over from the “physical” effect of the pozzolan as hydration evolves. Pozzolanic potential, strength development of mortars and concrete, efficiency factors (k-values) estimation and resistance against chloride penetration are part of the testing performed and reveal the importance of the binary action of RHA in producing competitive blended cement and concrete.
We use direct numerical simulation data to study interscale and interspace energy exchanges in the near field of a turbulent wake of a square prism in terms of a Kármán–Howarth–Monin–Hill (KHMH) ...equation written for a triple decomposition of the velocity field which takes into account the presence of quasi-periodic vortex shedding coherent structures. Concentrating attention on the plane of the mean flow and on the geometric centreline, we calculate orientation averages of every term in the KHMH equation. The near field considered here ranges between two and eight times the width
$d$
of the square prism and is very inhomogeneous and out of equilibrium so that non-stationarity and inhomogeneity contributions to the KHMH balance are dominant. The mean flow produces kinetic energy which feeds the vortex shedding coherent structures. In turn, these coherent structures transfer their energy to the stochastic turbulent fluctuations over all length scales
$r$
from the Taylor length
$\unicodeSTIX{x1D706}$
to
$d$
and dominate spatial turbulent transport of small-scale two-point stochastic turbulent fluctuations. The orientation-averaged nonlinear interscale transfer rate
$\unicodeSTIX{x1D6F1}^{a}$
which was found to be approximately independent of
$r$
by Alves Portela
et al.
(
J. Fluid Mech.
, vol. 825, 2017, pp. 315–352) in the range
$\unicodeSTIX{x1D706}\leqslant r\leqslant 0.3d$
at a distance
$x_{1}=2d$
from the square prism requires an interscale transfer contribution of coherent structures for this approximate constancy. However, the near constancy of
$\unicodeSTIX{x1D6F1}^{a}$
in the range
$\unicodeSTIX{x1D706}\leqslant r\leqslant d$
at
$x_{1}=8d$
which was also found by Alves Portela
et al.
(2017) is mostly attributable to stochastic fluctuations. Even so, the proximity of
$-\unicodeSTIX{x1D6F1}^{a}$
to the turbulence dissipation rate
$\unicodeSTIX{x1D700}$
in the range
$\unicodeSTIX{x1D706}\leqslant r\leqslant d$
at
$x_{1}=8d$
does require interscale transfer contributions of the coherent structures. Spatial inhomogeneity also makes a direct and distinct contribution to
$\unicodeSTIX{x1D6F1}^{a}$
, and the constancy of
$-\unicodeSTIX{x1D6F1}^{a}/\unicodeSTIX{x1D700}$
close to
$1$
would not have been possible without it either in this near-field flow. Finally, the pressure-velocity term is also an important contributor to the KHMH balance in this near field, particularly at scales
$r$
larger than approximately
$0.4d$
, and appears to correlate with the purely stochastic nonlinear interscale transfer rate when the orientation average is lifted.
► Effect of corrosion on bare B500c bars and embedded in concrete, under salt spray. ► Moderate loss of strength but significant reduction in ductility, on bare samples. ► Higher losses ...(strength/ductility) on embedded bars for same mass loss with bare. ► Development of pit depth measurement procedure, using advance imaging analysis. ► Pits significance analysis indicated more severe pitting corrosion on embedded bars.
In the current study the effects of chloride-induced corrosion, in terms of mechanical properties and pit depths, are evaluated on B500c steel bars embedded in concrete (embedded samples) and directly exposed (bare samples), immersed in a salt spray chamber. The results indicate that for the same level of mass loss, degradation of the “embedded” samples was found to be much more severe than that of the “bare samples”, in terms of losses in yield strength and uniform elongation. Analysis of the statistical significance of the pit depth and area values measured, based on a methodology developed using advanced imaging analysis, indicate that degradation of the steel bars embedded in concrete produced a more severe pitting corrosion in terms of depth of pitting, compared to the steel samples directly exposed to the same corrosive medium, for the same (on average) mass loss.
Various Rankine cycle architectures for single fluids and other improved versions operating with ammonia/water mixture are presented in this paper. Untapped heat resources and their potential for ...driving organic Rankine cycles are outlined. The nature – state and temperature of the heat source significantly influences the choice of the type of organic Rankine cycle machine. The temperature appears as a critical parameter during the selection process. Modules differ from one another from technology, size and cost viewpoints. The investment cost of an ORC project includes machine, engineering, system integration, capital costs, etc. and is closely linked to the application.
In this work the durability of Portland cement systems incorporating supplementary cementing materials (SCM; silica fume, low- and high-calcium fly ash) is investigated. Experimental tests simulating ...the main deterioration mechanisms in reinforced concrete (carbonation and chloride penetration) were carried out. It was found that for all SCM tested, the carbonation depth decreases as aggregate replacement by SCM increases, and increases as cement replacement by SCM increases. The specimens incorporating an SCM, whether it substitutes aggregate or cement, when exposed to chlorides exhibit significantly lower total chloride content for all depths from the surface, apart from a thin layer near the external surface. New parameter values were estimated and existing mathematical models were modified to describe the carbonation propagation and the chloride penetration in concrete incorporating SCM.
This paper investigates the dynamics of velocity gradients for a spatially developing flow generated by a single square element of a fractal square grid at low inlet Reynolds number through direct ...numerical simulation. This square grid-element is also the fundamental block of a classical grid. The flow along the grid-element centreline is initially irrotational and becomes turbulent further downstream due to the lateral excursions of vortical turbulent wakes from the grid-element bars. We study the generation and evolution of the symmetric and anti-symmetric parts of the velocity gradient tensor for this spatially developing flow using the transport equations of mean strain product and mean enstrophy respectively. The choice of low inlet Reynolds number allows for fine spatial resolution and long simulations, both of which are conducive in balancing the budget equations of the above quantities. The budget analysis is carried out along the grid-element centreline and the bar centreline. The former is observed to consist of two subregions: one in the immediate lee of the grid-element which is dominated by irrotational strain, and one further downstream where both strain and vorticity coexist. In the demarcation area between these two subregions, where the turbulence is inhomogeneous and developing, the energy spectrum exhibits the best
$-5/3$
power-law slope. This is the same location where the experiments at much higher inlet Reynolds number show a well-defined
$-5/3$
spectrum over more than a decade of frequencies. Yet, the
$Q{-}R$
diagram, where
$Q$
and
$R$
are the second and third invariants of the velocity gradient tensor, remains undeveloped in the near-grid-element region, and both the intermediate and extensive strain-rate eigenvectors align with the vorticity vector. Along the grid-element centreline, the strain is the first velocity gradient quantity generated by the action of pressure Hessian. This strain is then transported downstream by fluctuations and strain self-amplification is activated a little later. Further downstream, vorticity from the bar wakes is brought towards the grid-element centreline, and, through the interaction with strain, leads to the production of enstrophy. The strain-rate tensor has a statistically axial stretching form in the production region, but a statistically biaxial stretching form in the decay region. The usual signatures of velocity gradients such as the shape of
$Q{-}R$
diagrams and the alignment of vorticity vector with the intermediate eigenvector are detected only in the decay region even though the local Reynolds number (based on the Taylor length scale) is only between 30 and 40.
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