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© 2018 Elsevier B.V. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Supplementary Raw Research Data, is open data under the CC BY license http://creativecommons.org/licenses/by/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (October 2018) in accordance with the publisher’s archiving policy, The start-up flow of a Bingham plastic in a channel is considered and Safronchik’s solution [1] for the initial evolution of the yield surface and the core velocity is revisited. Stricter time bounds for the validity of the above solution are derived and the solution is extended to include the velocity profile in the evolving yielded zone. Comparisons are made with another approximate solution derived under the assumption that the velocity in the yielded zone is parabolic adjusting with the evolving yield surface. This approximation performs well for small values of the yield stress, or, equivalently, for large values of the imposed pressure gradient.

The purpose of this study is to investigate the potential of a small-scale, combined-cooling-heating-and-power system, consisting of a 1 MWe gas turbine subsystem coupled to a 0.5 MWe photovoltaic (PV) subsystem for application in Cyprus. The proposed system is completely autonomous, without any interconnections to a central power grid. To allow maximum utilization of the electricity generated by the PV subsystem, an electrolyzer unit is coupled to the system to convert excess renewable electricity to hydrogen. The generated hydrogen is injected to the natural gas supply for the gas turbine. For the generation of useful cooling and heating, the system recovers heat from the flue gas exiting the gas turbine the recovered heat is supplied to a heat-activated absorption chiller/heater to generate cooling or heating. An electric chiller/heater is integrated to the system to supplement thermal energy when necessary. The thermal energy is supplied to nearby buildings through a district energy network. The annual average primary energy ratio of the proposed system is 0.806. For an assumed system lifetime of 20 years, the lifecycle cost of the proposed system is 11.12 million USD, resulting to a unit cost of electricity at 0.06 USD/kWh, which is a 62% reduction of the current cost in Cyprus. The results of the parametric study suggest that the economic performance of the proposed system is highly dependent on price fluctuations of the unit cost of natural gas, while the specific cost of the electrolyzer unit is also critical. The proposed system could become an important candidate for power and thermal energy generation in Cyprus as a measure to reduce the presently high cost of electricity. © 2018 Elsevier Ltd 188 443 455 443-455

We provide benchmark results for a transient variant of the lid-driven cavity problem, where the lid motion is suddenly stopped and the flow is left to decay under the action of viscosity. Results include Newtonian as well as Bingham flows, the latter having finite cessation times, for Reynolds numbers Re ∈ [1, 1000] and Bingham numbers Bn ∈ [0, 10]. The finite-volume method and Papanastasiou regularisation were employed. A combination of Re and Bn, the effective Reynolds number, is shown to convey more information about the flow than either Re or Bn alone. A time scale which characterises the flow independently of the geometry and flow parameters is proposed. © 2015, Springer-Verlag Berlin Heidelberg. 55 1 51 66 Cited By :6

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Decentralized, liquefied natural gas-fueled, trigeneration plants are considered as alternatives to centralized, electricity-only generating power plants to improve efficiency and minimize running costs. The proposed system is analyzed in terms of efficiency and cost. Electrical power is generated with a gas turbine, while waste heat is recovered and utilized effectively to cover heating and cooling needs for buildings located in the vicinity of the plant. The high quality of cooling energy carried in the LNG fluid is used to cool the air supply to the air compressor. Waste heat is recovered with heat exchangers to generate useful heating in the winter period, while in the summer period an integrated double-effect absorption chiller converts waste heat to useful cooling. For the base system (10 MWe), net electrical efficiency is up to 36.5%, while the primary energy ratio reaches 90%. The payback period for the base system is 4 years, for a lifecycle cost of 221.6 million euros and an investment cost of 13 million euros. The base system can satisfy the needs of more than 21,000 average households, while an equivalent conventional system can only satisfy the needs of 12,000 average households. © 2014 Elsevier B.V. All rights reserved. 86 257 267 257-267

The objective of this presentation is to show and explain why semisolid slurries can fill thin sections at seemingly unlimited flow velocity the suitability of SSM parts with very thin sections is a characteristic of the process that is often overlooked by the industry. This characteristic provides a unique opportunity for new advanced applications of the process, not possible by other existing metal-mold casting methods. © (2015) Trans Tech Publications, Switzerland. 217-218 159 165 159-165

An essential element for the integration of a semisolid process in the production of complex commercial components is the availability of accurate mathematical and computational tools that could describe both the rheological behavior and the material characteristics of the suspension, which are strongly affected from its internal structure and its evolution during deformation. In this study we considered the squeeze flow experiment, which is a standard method used to determine material properties of semisolid slurries, where a fixed amount of material is compressed from its topside either under constant load or constant velocity, while the bottom side remains fixed. Through high fidelity computational modeling we simulated the classical compression experiment by including the effects of thixotropy in order to demonstrate its role in determining material constants. More specifically a structural viscoplastic model based on the Bingham plastic constitutive equation is proposed. The yield stress is assumed to vary linearly with the structural parameter which follows a first-order rate equation accounting for the material structure break-down and build-up. The development of the yielded/unyielded regions in relation to material structural changes is analyzed. Furthermore, we performed also simulations, where the compression is interrupted for a short time, in order to study the material internal structure after a short relaxation time. © (2015) Trans Tech Publications, Switzerland. 217-218 121 129 Sponsors: Conference code: 107639

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A standard method used to determine material properties of semi-solid slurries is the squeeze flow experiment a fixed amount of material is squeezed under constant force or velocity and the relation between the force and the displacement of the sample provides information about the rheology of the slurry. The objective of this work is to contribute to the further development of the squeeze flow methodology in order to accurately determine material properties. This is achieved by a model that accounts for the finite yield stress and the thixotropy of the slurry. More specifically a structural viscoplastic model based on the Bingham plastic constitutive equation is proposed. The yield stress is assumed to vary linearly with the structural parameter which follows a first-order rate equation accounting for the material structure break-down and build-up. Numerical experiments of squeeze flow under either constant load or constant velocity are presented and discussed. Comparisons with their non-thixotropic counterparts are made in the case of compression under constant load. The development of the yielded/unyielded regions in relation to material structural changes is analyzed. The numerical results show that initially thixotropy does not affect the flow. However, once the structure is destroyed, the unyielded regions grow slower than the non-thixotropic case allowing for longer compression of the sample. Under constant force the structure may be destroyed at the early stages of the compression but at a later time it re-builds steadily till the cessation of the flow experiment. Under constant velocity, however, the structure is destroyed steadily. Depending then on the case, the final internal structure of the squeezed material can vary significantly. This is an important issue that needs to be taken into consideration in the evaluation of the material parameters. © 2012 Elsevier B.V. 193 103 115 Cited By :13

In this study a small-scale, completely autonomous combined cooling, heating, and power (CCHP) system is coupled to a photovoltaic (PV) subsystem, to investigate the possibility of reducing fuel consumption. The CCHP system generates electrical energy with the use of a simple gas turbine cycle, with a rated nominal power output of 1 MWe. The nominal power output of the PV subsystem is examined in a parametric study, ranging from 0 to 600 kWe, to investigate which configuration results in a minimum lifecycle cost (LCC) for a system lifetime of 20 years of service. The load profile considered is applied for a complex of households in Nicosia, Cyprus. The solar data for the PV subsystem are taken on an hourly basis for a whole year. The results suggest that apart from economic benefits, the proposed system also results in high efficiency and reduced CO2 emissions. The parametric study shows that the optimum PV capacity is 300 kWe. The minimum lifecycle cost for the PV-assisted CCHP system is found to be 3.509 million €, as compared to 3.577 million € for a system without a PV subsystem. The total cost for the PV subsystem is 547,445 €, while the total cost for operating the system (fuel) is 731,814 € (compared to 952,201 € for a CCHP system without PVs). Overall, the proposed system generates a total electrical energy output of 52,433 MWh (during its whole lifetime), which translates to a unit cost of electricity of 0.067 €/kWh.

In this work we revisit the issue of obtaining true material constants for semisolid slurries. Therefore, we consider the circular Couette flow of Herschel-Bulkley fluids. We first show how true constants can be obtained using an iterative procedure from experimental data to theory and vice versa. The validity of the assumption that the rate-of-strain distributions across the gap share a common point is also investigated. It is demonstrated that this is true only for fully-yielded Bingham plastics. In other cases, e.g., for partially-yielded Bingham plastics or fully-yielded Herschel-Bulkley materials, the common point for the fully-yielded Bingham case provides a good approximation for determining the material constants only if the gap is sufficiently small. It can thus be used to simplify the iterative procedure in determining the material constants. © 2016 Trans Tech Publications, Switzerland. 256 153 172 Sponsors: Conference code: 184809

A solar-heating-and-cooling (SHC) system, consisting of a flat-plate solar collector array, a hot water storage tank, and an absorption chiller unit is designed and modeled to satisfy thermal loads (space heating, domestic hot water, and space cooling). The system is applied for Nicosia, Cyprus, a location with prolonged summer-like conditions, where heating demand is moderate, while space cooling demand is comparatively very high. The study investigates the potential of a solar system installed and operated onsite in a detached single-family household to satisfy all necessary thermal loads. The hot water storage tank is also connected to an auxiliary heater (diesel-fired boiler) to supplement solar heating, when needed. The main purpose of the study is to model the overall system and contact a parametric study that will determine the optimum economic system performance in terms of design parameters. The system is compared, through a cost analysis, to an electric heat pump (EHP) system. It is found that the optimum system combination of solar collector area and volumetric capacity of the hot water storage tank is 70m2 and 2000L, respectively. The total annualized cost (in USD) for the optimum SHC system is $3,719. The sensitivity analysis showed that the SHC system would be unfavorable to compete with EHP technology, if the solar collector cost is above $360/m2. © 2015 Elsevier Ltd. 117 59 73 59-73

Three-dimensional direct numerical simulations of a Bingham fluid flowing past a confined circular cylinder have been used in order to investigate viscoplastic effects in the wake-transition regime. The case of a cylinder confined in a plane channel with a fixed blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2 has been studied, for values of the Bingham number and Reynolds numbers (based on the cylinder diameter and bulk flow velocity) in the range 0 Bn ≤ 5 and 150 ⩽ Re ⩽ 600 , respectively. The critical Reynolds number for the onset of three-dimensional flow regime has been shown to increase linearly with Bingham number, at least in the range of parameters considered. Two distinct modes of three-dimensional instability (modes A and B) have been identified in the flow, and the influence of viscoplastic effects on the evolution of these instabilities have been described. Significant deviations in the structure of the far wake from the Newtonian case have been observed and associated with a disruption of underlying processes that are known to operate in the Newtonian wake. Despite these structural differences, it has been shown that the evolution of the Strouhal number with Reynolds number in the viscoplastic fluid exhibits two discontinuous changes that are associated with the onset of the different instabilities in the flow, a behavior that mirrors the behavior in Newtonian fluids.

Systematic experimental work and modeling efforts have been conducted to characterize the yield behavior of commercial aluminum alloys in the semisolid state. In this study, extensive compression experiments were performed to measure the yield stress of semisolid aluminum slurries at high solid fractions (0.5 to 1.0), and a cone penetration method was employed to measure yield stress at low solid fractions (

A small-scale combined-cooling-heating-and-power (CCHP) plant is proposed as a possible alternative to large-scale, centralized, electricity-only power plants. The proposed system is based on a non-ideal gas turbine (Brayton) cycle, integrated with a cooling plant and a district energy network. The study analyzes whether the proposed system could be an ideal candidate for distributed generation applications, especially in locations which are distant from centralized power plants. Therefore apart from reducing transmission and distribution losses, waste heat could be recovered effectively to generate heating, or cooling (via a cooling plant based on absorption refrigeration technology). The system considers fueling with liquefied natural gas (LNG), which is a safe and transportable fuel option. The cooling energy in the LNG is recovered in a useful manner since LNG regasification is coupled to the ambient air feed to the air compressor. The study includes a basic thermodynamic analysis, followed by an exergy analysis and a cost analysis. The simulation results signify a potential for further investigation of the proposed system, since its performance results in significant thermodynamic and environmental improvements, when compared to an equivalent conventional system. The system operates in two modes: (a) winter operation, where recovered heat is distributed to the district energy network, (b) summer operation, where recovered heat is used to drive an absorption chiller cooling plant to generate cooling, which is also distributed to the district energy network. The average primary energy ratio of the proposed system is 0.91, while the net electrical efficiency is 0.356 and 0.365, for summer and winter operation, respectively. The average exergetic efficiency is 0.419, which is a 25% improvement when compared to the conventional system. The cost analysis shows that the payback period is within a reasonable time frame (approximately 4 years), for a total initial cost of 17.1 million €. © 2014 Elsevier B.V. 21 386 396 386-396

We extend our recent work on the creeping flow of a Bingham fluid in a lid-driven cavity, to the study of inertial effects, using a finite volume method and the Papanastasiou regularisation of the Bingham constitutive model (Papanastasiou, 1987) [7]. The finite volume method used belongs to a very popular class of methods for solving Newtonian flow problems, which use the SIMPLE algorithm to solve the discretised set of equations, and have matured over the years. By regularising the Bingham constitutive equation it is easy to extend such a solver to Bingham flows since all that this requires is to modify the viscosity function. This is a tempting approach, since it requires minimum programming effort and makes available all the existing features of the mature finite volume solver. On the other hand, regularisation introduces a parameter which controls the error in addition to the grid spacing, and makes it difficult to locate the yield surfaces. Furthermore, the equations become stiffer and more difficult to solve, while the discontinuity at the yield surfaces causes large truncation errors. The present work attempts to investigate the strengths and weaknesses of such a method by applying it to the lid-driven cavity problem for a range of Bingham and Reynolds numbers (up to 100 and 5000 respectively). By employing techniques such as multigrid, local grid refinement, and an extrapolation procedure to reduce the effect of the regularisation parameter on the calculation of the yield surfaces (Liu et al., 2002) [55], satisfactory results are obtained, although the weaknesses of the method become more noticeable as the Bingham number is increased. © 2014 Elsevier B.V. 208-209 88 107 Cited By :15

Herschel-Bulkley fluids are materials that behave as rigid solids when the local stress τ is lower than a finite yield stress τ0, and flow as nonlinearly viscous fluids for τ > τ0. The flow domain then is characterized by two distinct areas, τ τ0. The surface τ = τ0 is known as the yield surface. In this paper, by using analytic solutions for antiplane shear flow in a wedge between two rigid walls, we discuss the ability of regularized Herschel-Bulkley models such as the Papanastasiou, the bi-viscosity and the Bercovier and Engelman models in determining the topography of the yield surface. Results are shown for different flow parameters and compared to the exact solutions. It is concluded that regularized models with a proper choice of the regularizing parameters can be used to both predict the bulk flow and describe the unyielded zones. The Papanastasiou model predicts well the yield surface, while both the Papanastasiou and the bi-viscosity models predict well the stress field away from τ = τ0. The Bercovier and Engelman model is equivalent to the Papanastasiou model provided a proper choice of the regularization parameter δ is made. It is also demonstrated that in some cases the yield surface can be effectively recovered using an extrapolation procedure based upon an analytical representation of the solution. © 1999 The Society of Rheology. 43 463 483 463-483

We investigate the performance of the finite volume method in solving viscoplastic flows. The square lid-driven cavity flow of a Bingham plastic is chosen as the test case and the constitutive equation is regularised as proposed by Papanastasiou [J. Rheol. 31 (1987) 385- 404]. The numerical results obtained for Bingham numbers up to 10 and Reynolds numbers up to 5000 compare favourably with reported results obtained through other methods. The effects of the Reynolds and Bingham numbers are also investigated. It is shown that the convergence rate of the standard SIMPLE pressure-correction algorithm, which is used to solve the algebraic equation system that is produced by the finite volume discretisation, severely deteriorates as the Bingham number increases, with a corresponding increase in the non-linearity of the equations. Using the SIMPLE algorithm in a multigrid context [Syrakos & Goulas, Int. J. Numer. Methods Fluids 52 (2006) 1215-1245] dramatically improves convergence, although the multigrid convergence rates are not as high as those for Newtonian flows. 70 81 Sponsors: Conference code: 104618

We investigate the performance of the finite volume method in solving viscoplastic flows. The creeping square lid-driven cavity flow of a Bingham plastic is chosen as the test case and the constitutive equation is regularised as proposed by Papanastasiou [J. Rheol. 31 (1987) 385-404]. It is shown that the convergence rate of the standard SIMPLE pressure-correction algorithm, which is used to solve the algebraic equation system that is produced by the finite volume discretisation, severely deteriorates as the Bingham number increases, with a corresponding increase in the non-linearity of the equations. It is shown that using the SIMPLE algorithm in a multigrid context dramatically improves convergence, although the multigrid convergence rates are much worse than for Newtonian flows. The numerical results obtained for Bingham numbers as high as 1000 compare favourably with reported results of other methods. © 2013 Elsevier B.V.. 195 19 31 Cited By :18

The quasi-conservative stochastic averaging (QCSA) method is applied to a Bouc-Wen-Baber hysteretic system (BWB) under Gaussian white noise excitations. The stationary probability density of the system's response amplitude and energy is obtained for different excitation levels and different damping ratios. These results are compared with the studies presented by Cai and Lin in A New Solution Technique for Randomly Excited Hysteretic Structures, Technical Report on Grant No. NCEER-88-0012, State University of New York, Buffalo, NY, 1988. The first-passage time problem for the hysteretic system is also studied using the method of QCSA. The results for the expected time to failure as a function of the initial total energy are shown, for different excitation levels and for different threshold values of energy. The relationship between the expected time to failure when initial total energy is zero and excitation level is shown for a wide range of hysteresis shape parameters. © 1995. 10 161 170 161-170