Parametric study on fluidized bed thermal storage

In the present work, the numerical model developed earlier by the same authors [5] is refined and recast in non-dimensional form. The refined model is calibrated with recent experiments at different bed air-inlet temperatures. Excellent agreement between the numerical results and experiments is obtained. The refined model is then used to conduct an extensive parametric study. The objectives of the parametric study are: (i) to determine the effect of the non-dimensional parameters (α₂, α₃ and α₄) on the bed performance; (ii) to indicate the conditions required for favourable bed operation, and (iii) to compare the fluidized-bed performance with a small-particle packed bed performance. The numerical results are presented as time-histories of average bed temperature ( \(\bar \theta _b \) ) and bed efficiency(η). The performance histories are given for different value of each parameter (α₂, α₃, and α₄). The study shows that the fluidized bed behaves favourably for α₂ < 10, α₃> 30 and α₄ < 10. Moreover, it was concluded that the small particle packed bed, in general, offers better performance behaviour over a fluidized bed having the same bed size and heat input.     

Parametric study of pulsed thermal bumps in supersonic boundary layer

A three-dimensional numerical study is performed to explore the effect of pulsed spanwise-periodic surface thermal perturbation (also denoted as thermal bump) in a Mach 1.5 flat plate laminar boundary layer. A high-resolution upwind-biased Roe method is used with the compressive Van Leer harmonic limiter on a suitably refined mesh. The dependence of flow stability characteristics on the variation of thermal bump geometry (shape and dimension) and pulsing properties (disturbance amplitude and frequency) is assessed. It is shown that the finite-span thermal bumps generate streamwise vortices. When the thermal bump is pulsed, vortex shedding is observed, and the streamwise vorticity grows with the downstream distance. Analysis of the integrated disturbance energy indicates that the streamwise kinetic disturbance energy dominates over those associated with other two velocity and thermodynamic components. Immediately downstream of the bump, the dominant frequency corresponds to that of the imposed excitation while higher harmonic components are observed farther downstream. An analysis of parametric variation of bump shape and dimension indicates that finite bump span is important in injecting three dimensionality and that the rectangular shape results in faster disturbance growth than the circular one. The study also concludes that disturbance growth is non-linear with bump temperature and has a strong connection with pulsing frequency.     

Thermal constriction resistance between two solids for random distribution of contacts

An infinite interface of randomly distributed contacts is modeled as a finite square region with randomly placed contacts inside it. The contacts outside the region are treated as continuum of contacts. The continuum approximation allows for an interaction between the contacts within the square and those outside it. An analytical solution is obtained for the temperature field, and the contact resistance is analyzed for randomness effects. This is the first such analytical model developed to study random distribution of contacts. The result shows an excellent agreement when tested against the the available analytical solution for the case of periodic arrangement of contacts. For the random case, the resistance is observed to be a strong function of the area fraction of contact. Received on 16 March 1998     

Analytical solution for constriction resistance with interstitial fluid in the gap

An analysis of the thermal constriction resistance between two semi-infinite solids in contact has been carried out to study the effects of the interstitial medium. The thermal contact is modeled as a three-media problem with finite resistivities for each medium, together with complete interfacial coupling between the solids and the interstitial fluid. Analytical solutions are obtained for a periodic array of two-dimensional gaps of circular segment profiles flanked by flat contacting surfaces. A corrective-iterative technique is used to account for the interaction between the gaps to obtain a closed form expression for the contact resistance in terms of known parameters. For the special case of zero-thickness adiabatic gaps, the present solution compares very well with the available analytical solution for gap fraction values of the κ _g ≤0.7. The resistance is observed to be a strong function of gap conductivity and gap thickness, especially at lower values of these parameters. Received on 24 November 1997     

Parametric study of unsteady laminar flows

Results of a parametric study of unsteady laminar flows are analyzed. Three-dimensional unsteady equations of hydromechanics for a compressible medium are solved. The range of the characteristic Reynolds number Re = 400–900 is considered. It is demonstrated that the laminar flow in a plane channel ceases to be steady at Re = 415. As the Reynolds number increases, the unsteady processes become more intense, disturbances penetrate inward the channel, and separation zones lose their stability. In the vicinity of the channel exit, however, the flow tends to stabilize, though it remains unsteady. No transition to a turbulent flow occurs in the examined range of Reynolds numbers.     

Parametric study of double cellular detonation structure

A parametric numerical study is performed of a detonation cellular structure in a model gaseous explosive mixture whose decomposition occurs in two successive exothermic reaction steps with markedly different characteristic times. Kinetic and energetic parameters of both reactions are varied in a wide range in the case of one-dimensional steady and two-dimensional (2D) quasi-steady self-supported detonations. The range of governing parameters of both exothermic steps is defined where a “marked” double cellular structure exists. It is shown that the two-level cellular structure is completely governed by the kinetic parameters and the local overdrive ratio of the detonation front propagating inside large cells. Furthermore, since it is quite cumbersome to use detailed chemical kinetics in unsteady 2D case, the proposed work should help to identify the mixtures and the domain of their equivalence ratio where double detonation structure could be observed.     

Parametric study on mass loss of penetrators

Earth penetration weapon （EPW） is applicable for attacking underground targets protected by reinforced concrete and rocks. With increasing impact velocity, the mass loss/abrasion of penetrator increases, which significandy decreases the penetration efficiency due to the change of nose shape. The abrasion may induce instability of the penetrator, and lead to failure of its structure. A common disadvantage, i.e. dependence on corresponding experimen- tal results, exists in all the available formulae, which limits their ranges of application in estimating the mass loss of penetrator. In this paper, we conduct a parametric study on the mass loss of penetrator, and indicate that the mass loss of penetrator can be determined by seven variables, i.e., the initial impact velocity, initial nose shape, melting heat, shank diameter of projectile and density and strength of target as well as the aggregate hardness of target. Further discussion on factors dominant in the mass abrasion of penetrator are given, which may be helpful for optimizing the target or the projectile for defensive or offensive objectives, respectively.     

Parametric studies of failure mechanisms in elastic EB-PVD thermal barrier coatings using FEM

Catastrophic failure of thermal barrier coatings (TBCs), usually occurs due to large scale buckling and spallation, primarily originating at the bond coat and TGO interface. Spallation in TBCs is preceded by a competition between buckling and interface delamination that is stimulated by the waviness of the interface. In the presence of thermal loading, the waviness is responsible for growth of interfacial delamination. In this paper, a finite element model of the two and three layer TBC’s is developed in the commercial code ANSYS to investigate the buckle and interface delamination mechanisms and develop a simplified parametric understanding of these mechanisms. The models for simulation are validated with analytical and experimental results. Parametric relations, in terms of geometric and material parameters representing constituents of the TBC, are developed in this paper for critical stresses and energies causing buckling and debonding initiated instabilities. Through these relations, critical parameters that control failure mechanics are identified for a fail-safe design space.     

Parametric study of the Hartmann–Sprenger tube

Experiments were conducted with a Hartmann–Sprenger tube (H–S) to study the effect of different parameters on the frequency and amplitude of acoustic fluctuations excited when the H–S underexpanded jet impinges on an in-line cavity. Time averaged shadowgraphs were acquired to study the flow field between the underexpanded jet and the cavity for varying parameters of the H–S tube. It was observed that the H–S tube primarily excited two different modes. The first mode corresponds to the jet regurgitant mode (JRG) where the frequency of oscillations scales as a function of the cavity depth. The other mode is screech where an oscillating shock is formed in front of the cavity. The screech mode excites a higher acoustic frequency than the JRG and it is observed to be a strong function of the pressure ratio R, and distance between the jet and the cavity X. At a fixed cavity length, varying standoff distance X could excite either the JRG or screech. At very low standoff distances (X/D_j<0.8), the current study indicates that there is a mode switch from screech to JRG. A cavity to jet diameter, D_c/D_j>1 was found to sustain JRG over a wide range of X. Diameter ratios D_c/D_j<1 sustained high frequency screech modes in a wide range of H–S tube parameters.     

Photoacoustic signal dependence on sample-backing thermal resistance

This paper presents theoretical analysis of the dependency of photoacoustic cell response excited with a modulated laser beam on the quality of thermal contact between the sample and the backing. Periodic changes of the solid sample temperature, resulting from absorption of a modulated laser beam warm up gas surrounding the sample. Changes of gas temperature are transferred into changes of its pressure, as the result of thermodynamic processes in the gas filling photoacoustic cell and are detected with a microphone in a typical photoacoustic experiment. Presence of thermal resistance substantially increases temperature inside the cell in the low frequency range of excitation beam, compared to the case when boundary conditions take into account only continuity of temperature and heat flux at the sample-backing interface. Received on 21 December 1999     

Parametric thermal analysis of a single molten metal droplet as applied to layered manufacturing

In the present study a parametric thermal analysis of a single molten metal droplet deposited on a large substrate has been performed with application to various solid freeform fabrication (SFF) processes employing droplet-based deposition. Simulation is conducted to investigate the effect of droplet shape, substrate thermal properties, substrate size and thermal boundary conditions of substrate base on the cooling rate of the droplet and the substrate. It is found that droplet shape and substrate thermal properties have significant effect only on the solidification time, whereas the steady-state conditions vary significantly with all the process parameters studied.     

Phonon cross-plane transport and thermal boundary resistance: effect of heat source size and thermal boundary resistance on phonon characteristics

Phonon cross-plane transport across silicon and diamond thin films pair is considered, and thermal boundary resistance across the films pair interface is examined incorporating the cut-off mismatch and diffusive mismatch models. In the cut-off mismatch model, phonon frequency mismatch for each acoustic branch is incorporated across the interface of the silicon and diamond films pair in line with the dispersion relations of both films. The frequency-dependent and transient solution of the Boltzmann transport equation is presented, and the equilibrium phonon intensity ratios at the silicon and diamond film edges are predicted across the interface for each phonon acoustic branch. Temperature disturbance across the edges of the films pair is incorporated to assess the phonon transport characteristics due to cut-off and diffusive mismatch models across the interface. The effect of heat source size, which is allocated at high-temperature (301 K) edge of the silicon film, on the phonon transport characteristics at the films pair interface is also investigated. It is found that cut-off mismatch model predicts higher values of the thermal boundary resistance across the films pair interface as compared to that of the diffusive mismatch model. The ratio of equilibrium phonon intensity due to the cut-off mismatch over the diffusive mismatch models remains >1 at the silicon edge, while it becomes <1 at the diamond edge for all acoustic branches.     

Parametric study of the interaction of bridges and moving vehicles

In this paper we develop a mathematical model for the analysis of the dynamic response of a bridge structure as it interacts with moving vehicles. The vehicles are modeled both for fixed axle distances (e.g. Cooper Loadings) and for variable axle distances (e.g. multi-vehicles). We then generate an algorithm to solve the resulting equations of motion. Results obtained using our theoretical model are shown to compare very well with measured field data.Associate Professor of Mech. Eng.Professor of Mech. Eng.Associate Professor of Civil Eng.     

Destabilization of film boiling by means of a thermal resistance

Described is the type of vaporisation which takes place when a thermal resistance, consisting in a film of a substance of low heat conductivity, is placed between the surface of a quenched sample and the cooling liquid. This type of vaporisation, larvate boiling, is characterised by an alternate wetting/non-wetting of the solid surface.Two conditions are necessary for larvate boiling: thermal resistance and surface effusivity.Substituting larvate boiling for film boiling allows the heat flux between a solid surface at high temperature and the cooling liquid to be greatly increased.     

A numerical study of pulsatile laminar flows in a pipe with a ring-type constriction

Numerical simulations have been carried out to study pulsatile laminar flows in a pipe with an axisymmetric ringtype constriction. Three types of pulsatile flows were investigated, namely a physiological flow, a pure sinusoidal flow and a non-zero mean velocity sinusoidal flow. The laminar flow governing equations were solved by the SIMPLE algorithm on a non-staggered grid and a modified Crank-Nicolson approximation was used to discretrize the momentum equations with respect to time. The maximum flow Reynolds numer (Re) is 100. The Womersley number (N_w) ranges from 0 to 50, with the corresponding Strouhal number (St) ranging from 0 to 3·98. The constriction opening ratio (d/D) and thickness ratio (h/D) are fixed at 0·5 and 0·1 respectively. Within the time period investigated, all these pulsatile flows include both forward and backward flows. The unsteady recirculation region and the recirculation points change in size and location with time. For N_w ≤ 1 and St≤ 1·56 x 10^?3 the three pulsatile flows have the same simple relation between the instantaneous flow rate and pressure loss (Δp) across the constriction and the pressure gradient in the axial direction (dp/dz) in the fully developed flow region. The phase angles between the flow rate and pressure loss and the pressure gradient are equal to zero. With increasing N_w and St, the phase angle between the flow rate and the dp/dz becomes larger and has its maximum value of 90° at N_w = 50 and St = 3·98. The three pulsatile flows also show different relations between the flow rate and the pressure gradient. The pure sinusoidal flow has the largest maximum pressure gradient and the non-zero mean velocity sinusoidal flow has the smallest. For larger N_w and St the fully developed velocity profiles in the fully developed flow region have a smaller velocity gradient along the radial direction in the central region. The maximum recirculation length increases for N_w ranging from 0 to 4·2, while this length becomes very small at N_w = 50 and St = 3·98. The deceleration tends to enlarge the recirculation region and this effect appears for N_w ≥ 3 and St ≥ 1·43×10^?2. Linear relations exist between the flow rate and the instantaneous maximum values of velocity, vorticity and shear stress.     

Parametric study of a two degree-of-freedom cylinder subject to vortex-induced vibrations

This numerical work is an attempt to build accurate and continuous response surfaces of two degree-of-freedom vortex-induced vibrations (VIV) of flexibly mounted cylinders for a wide range of transverse and in-line natural frequencies. We consider both the structure and the flow to be two-dimensional. The structure has a low mass damping, with the transverse and in-line mass ratios as well as the transverse and in-line damping coefficients being equal. The goal is to capture the sensitivity of the response to the change in the natural frequencies of the structure. The system is studied for a wide range of transverse natural frequency within the synchronization region. The extent of variation of the in-line natural frequency is chosen to be larger than the one of the transverse natural frequency in order to favor multi-modal responses. No preferred frequencies are emphasized within the intervals of study. The numerical technique uses a multi-element stochastic collocation method coupled to a spectral element based deterministic solver.     

Lamination of anisotropic half-spaces in the presence of contact thermal resistance

Thermoelastic contact of two anisotropic half-planes is studied under conditions leading to their separation by heat flow. The problem is reduced to the solution of a system of singular integrodifferential equations. The dependence of the geometric characteristics of the gap between the half-planes on thermal resistance is studied on the basis of numerical analysis. Ya. S. Podstrigach Institute of Applied Problems of Mechanics and Mathematics, National Academy of Sciences of Ukraine, Lvov, Ukraine. Translated from Prikladnaya Mekhanika, Vol. 35, No. 2, pp. 54–59, February, 1999.