Transient Boundary Conditions Research Articles

Analytical solutions of advection-dispersion-reaction equation with first decay under constant and time-dependent boundary conditions: Mass transfer shape factor effects

To describe the combined dispersion and convection mechanisms of solute transport through porous media, one may typically refer to the advection-dispersion equation (ADE). In this study, a generalized analytical solution to the ADE combined with first order decay term is derived for the problem of solute transport through porous media in one-dimensional finite region considering transient boundary conditions. First, the addressed convection-dispersion-reaction equation is simplified into a diffusion equation. Subsequently, an analytical solution is generated for a general fractured porous medium for a case where a fracture is viewed as the transient boundary condition. To verify the proposed analytical solution, experimental data acquired from literature as well as numerical techniques of finite difference and element are utilized. Previous studies have utilized fixed values for mass transfer shaper factor which is deployed in dual porosity modeling approach of fractured reservoirs, while in reality this parameter changes over time. In this regard, an attempt is made to attain mass transfer shape factor while considering the impacts of important parameters such as Péclet number, and decay coefficient during both pseudo steady state (PSS) and transient periods. Moreover, for two circumstances of linearly ascending and constant concentration boundary conditions, a correlation is developed to compute PSS mass transfer shape factor as a function of Péclet number, dimensionless time, and decay coefficient. The results illustrated the dependency of PSS shape factor on the decay coefficient, Péclet number, and time while transient shape factor solely depended on time. In addition, the findings showed that the larger the Péclet number, the larger the PSS matrix-fracture mass transfer shape factor. The obtained analytical solution is helpful when validating the numerical simulation results and for performing fast sensitivity analysis. The developed mass transfer shape factor can be applied in the dual porosity modeling part of the commercial reservoir simulation software packages for more accurately modeling the solute transport in the fractured reservoirs.

High-resolution heat transfer measurements on a rotating turbine endwall with infrared thermography

Computational thermo-fluid dynamics in the field of turbo machinery research tend to account for transient interaction mechanisms to predict the convective heat transfer within the hot gas path. In this context, the rotor hub side endwall region of the high pressure turbine depicts an object of interest as the near wall flow field may be dominated by rotating flow structures emerging from the disc space cavities. The validation of the applied numerical tools rely on experimental heat transfer setups reproducing such transient boundary conditions. This paper describes an experimental approach to quantify the heat transfer coefficient and the adiabatic wall temperature on the rotating endwall of a large scale test turbine. The wall heat flux distribution in a thin film isolator coated to a well conducting support structure is quantified for a series of quasi-isothermal boundary conditions. A high-resolution infrared camera is used to capture triggered thermograms of the rotating surface. Distributed thermocouples in the base body serve as reference points for camera calibration and to deduce the temperature distribution at the interface to the isolator. The calibration is in-situ and includes the pixel-wise quantification of uncertainties in the surface temperatures. An advanced linear fit approach is applied to derive the unknown adiabatic quantities and their uncertainties. For the examined operating point with a rim seal purge flow rate of 1% the random part of the relative measurement uncertainty is clearly below 10% for the heat transfer coefficient and below 5% for the adiabatic wall temperature. As the evaluation algorithm is designed to consider covariances between the thermocouple and infrared readings, the surface wall heat flux can be evaluated for every single infrared image.

Effect of a new synthetic bubble model on forces in simulations of two-phase flows in tube bundles

In this paper, the influence of the phase distribution imposed as an inlet boundary condition in a two-phase flow simulation is investigated. Firstly, a new inlet model is proposed which defines a transient inlet boundary condition to be applied in a subsequent Eulerian simulation, more specifically using the Volume-Of-Fluid method. This inlet model is aimed at generating large gas bubbles, making a Lagrangian approach less applicable. The new model, which is coined the “Synthetic Bubble Model” (SBM), generates a transient inlet condition with large gas bubbles such that there is no need for bubble creation through break-up of a continuous gas jet in a long inlet region upstream of the region of interest, which results in a smaller computational mesh. This model is applied on a 5-by-3 tube bundle subjected to an axially flowing air/water mixture and compared to a simulation with a precursor-domain in which bubbles are created by break-up of air jets imposed at the inlet. Compared to the precursor domain simulations, the SBM is both faster and more or equally accurate. Secondly, a tube bundle subjected to two-phase cross-flow is modelled. It is shown that the SBM yields better correspondence with the experimental force data than a steady inlet condition, in this case without addition of a precursor-domain (yielding a similar computational time for both simulations).

Capturing the S-Shape of Pump-Turbines by Computational Fluid Dynamics Simulations Using an Anisotropic Turbulence Model

Abstract Pump-turbines cope very well with modern electricity-market demand, having high operational flexibility and storage capabilities. Nevertheless, dynamic operation of these machines can lead to very challenging transient conditions, depending on the shape of the characteristic. Mechanical integrity can be correspondingly affected. Therefore, assessment of the characteristic during the design phase is of crucial importance. In the past years, different attempts to accurately compute the characteristic under steady and transient conditions have been undertaken using Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics. While the k–ω shear stress transport (SST) turbulence model has become the reference for machine design, it often fails for conditions close to or around instabilities. Under unstable conditions, which are characterized by continuous unsteady vortex formation, turbulence isotropy as assumed by linear two equation models is no longer the right choice. Accordingly, a turbulence model able to capture anisotropy, explicit algebraic Reynolds stress model (EARSM), has been implemented in an in-house code and used for the computation of the characteristic of various machines, stable and unstable, to assess the model performance. In this paper, computations of three different dynamic pump-turbine operating conditions are presented. Results using steady boundary conditions (BC) in the unstable region as well as transient BC like load-rejection and runaway are computed with EARSM, showing its superiority compared to linear two equation models. The model's capability to capture anisotropic effects—such as the influence of corners—produces more physical flow structures in the vaneless space, which lead to an overall improvement of the predicted stability characteristics.

Comprehensive experimental study of thermoelectric generators under transient boundary conditions

As the applications of thermoelectric generators (TEGs) have become more extensive in last decades, the transient characteristics of TEGs have gradually attracted attention. The current transient studies on TEG systems, however, mostly rely on simulation learning. Moreover, the small number of designed experiments mostly considered a single heating pulse process. The transient study of output performance of TEG lacks a comprehensive experimental study in a complex and transient input heat source. Therefore, in order to fulfill this lack, an experimental platform is designed in this work to investigate electrical response of TEGs under transient boundary conditions. A classical maximum power point tracking (MPPT) algorithm is employed to apply optimal variable electrical load on TEGs for maximum power generation. Critical design parameters in system level are evaluated for two TEGs with different geometries. Constant, transient but periodic and random heat inputs are applied to the TEG. Further, the initial investment cost of the system and Levelized cost of energy (LCOE) are calculated. The results show that, the system parameters can improve the average output power and conversion efficiency. The MPPT algorithm is proved to be an effective and important tool to enhance the output power of the system especially under random boundary condition. The LCOE of the proposed TEG system is improved 13.2% compared to normal TEG system with steady-state heat input.

Modelling deformations in water retention structures with unsaturated material point method

To conduct landslide risk assessment and assess the consequences of dam and levee failures, it is important to understand the overall failure evolution process that ranges from small-strain initiation to large-strain post-failure runout. To achieve this goal and simulate such landslide events, emerging large-strain numerical approaches, such as the material point method (MPM), show promising potential as tools to model the entire instability mechanism. While successfully incorporating multi-phase interactions in granular porous media. Soil–water interaction in unsaturated soils is critical when dynamic hydraulic loads act on levees, such as when the fluvial upstream water levels oscillate and in episodes of heavy rainfall infiltration. Recently, MPM has been successfully used in many geotechnical engineering applications, which in turn motivated further advancements of multi-phase formulations. This study presents an application of a novel single-point two-phase MPM formulation to model unsaturated soils subjected to transient hydraulic boundary conditions. The potential of this approach is discussed by analysing a large-scale slope failure experiment. Results are compared with the finite-element method and limit equilibrium method. It is concluded that similar results are obtained at small strains, but MPM allows the post-failure behaviour to be estimated, which is essential for risk management.

Experimental investigation on the influence of phase change material on the output performance of thermoelectric generator

Thermoelectric generators (TEGs) can convert heat into electricity directly. However, the thermal boundary conditions are intermittent and high temperature in most TEG systems. Therefore, thermal management of TEG systems is of great significance. It is an efficient technology to use phase change material (PCM) for thermal management of the TEG systems. PCM is applied to hot side of TEG in this study, and the phase change material-thermoelectric generation (PCM-TEG) system is set up. For the purpose of exploring the effect of the PCM on the output performance of TEG, the common TEG system is established to compare with PCM-TEG system. Furthermore, the effect of the heat transfer characteristics of PCM on TEG output performance is investigated and discussed in detail under different pulse transient thermal boundary conditions. Experimental results show that PCM can reduce the temperature fluctuation to protect TEG and extend the temperature difference operation time. The PCM of solid state, solid-liquid coexistence state and liquid state have different effects on reducing the temperature fluctuation. Besides, the PCM-TEG system cannot provide the peak output power provided by the common TEG system, but it is still beneficial to provide a stable output power.

Fire resistance design method for reinforced concrete beams to evaluate fire-resistance rating

In this paper, performance based a fire resistance design method for evaluating the flexural carrying capacity of reinforced concrete (RC) beams has been proposed. The developed method is associated with a simple heat transfer model for RC beams subjected to the unsteady state heat transfer at transient heating boundary condition with three basic heating mechanism, i.e., conduction, convection and radiation. The validity and verification of the fire resistance design method and heat transfer model have been established with the experimentally measured and compiled database. Parametric studies also conducted to investigate the fire-resistance rating of designed RC beams and comparative appraisal of existing codes practices has been examined. Through the results of the fire resistance design method, it reveals that the developed method is capable to predicts fire-resistance rating of RC beams at fire scenarios, nominal cover, type of aggregate and thermal-induced concrete properties.

Computational fluid dynamics of the right atrium: Assessment of modelling criteria for the evaluation of dialysis catheters.

Central venous catheters are widely used in haemodialysis therapy, having to respect design requirements for appropriate performance. These are placed within the right atrium (RA); however, there is no prior computational study assessing different catheter designs while mimicking their native environment. Here, a computational fluid dynamics model of the RA, based on realistic geometry and transient physiological boundary conditions, was developed and validated. Symmetric, split and step catheter designs were virtually placed in the RA and their performance was evaluated by: assessing their interaction with the RA haemodynamic environment through prediction of flow vorticity and wall shear stress (WSS) magnitudes (1); and quantifying recirculation and tip shear stress (2). Haemodynamic predictions from our RA model showed good agreement with the literature. Catheter placement in the RA increased average vorticity, which could indicate alterations of normal blood flow, and altered WSS magnitudes and distribution, which could indicate changes in tissue mechanical properties. All designs had recirculation and elevated shear stress values, which can induce platelet activation and subsequently thrombosis. The symmetric design, however, had the lowest associated values (best performance), while step design catheters working in reverse mode were associated with worsened performance. Different tip placements also impacted on catheter performance. Our findings suggest that using a realistically anatomical RA model to study catheter performance and interaction with the haemodynamic environment is crucial, and that care needs to be given to correct tip placement within the RA for improved recirculation percentages and diminished shear stress values.

Active Flow Control Utilizing an Adaptive Blade Geometry and an Extremum Seeking Algorithm at Periodically Transient Boundary Conditions

Abstract As a consequence of constant volume combustion in gas turbines, pressure waves propagating upstream the main flow into the compressor system are generated leading to incidence variations. Numerical and experimental investigations of stator vanes have shown that active flow control (AFC) by means of adaptive blade geometries is beneficial when such periodic incidence variations occur. A significant risk reduction in a compressor facing disturbances can thereby be achieved concerning stall or choke. Experimental investigations on such an AFC method with simultaneous application of a closed-loop control are missing in order to demonstrate its potential. This work investigates a linear compressor cascade that is equipped with a 3D-manufactured piezo-adaptive blade structure. The utilized actuators are piezoelectric macro-fiber-composites. A throttling device is positioned downstream the trailing edge plane to emulate an unsteady combustion process. Periodic transient throttling events with a frequency of up to 20 Hz cause incidence changes to the blade’s leading edge. Consequently, pressure fluctuations on the blade’s surface occur, having a significant impact on the pressure recovery downstream of the stator cascade. Experimental results of harmonically actuating the piezo-adaptive blade with the corresponding disturbance frequency show that the impact of disturbances can be reduced to approximately 50%. However, this is only effective if the phase shift of the harmonic actuation is adjusted correctly. Using an inadequate phase shift reverses the positive effects, causing the aforementioned stall, choke, or significant losses. In order to find the optimum phase shift, even under varying, possibly unpredictable operating conditions, an extremum seeking controller is presented. This gradient-based approach is minimizing the pressure variance over time by carefully adjusting the phase shift of the harmonic actuation of the AFC system.

Semi-derivative integral method to transient heat conduction time-dependent heat flux boundary conditions

Transient heat conduction in semi-infinite medium with a time dependent heat flux as boundary condition has been solved by a semi-derivative integral-balance method. Two versions boundary fluxes have been considered: power-law and exponential.

Bubble nucleation on grooved surfaces with hybrid wettability: Molecular dynamics study under a transient temperature boundary condition

Mechanisms of bubble nucleation need a better understanding, especially for heterogeneous surfaces with different wettability. In this study, the bubble nucleation and the heat transfer performance on grooved surfaces with different hybrid wettability under a transient temperature boundary condition was investigated by characterizing the contact-line movement using molecular dynamics simulation. The bubble growth rate on the all-hydrophilic groove is faster than the other grooved surfaces. The grooved surfaces with patterned wettability, especially the BphoSphi (hydrophobic bottom wall and hydrophilic sidewall) surface, can facilitate the nucleation of the bubble. Meanwhile, an apparent enhancement of the maximum heat flux was found for the BphiSpho (hydrophilic bottom wall and hydrophobic sidewall) surface. This work provides insights into the mechanisms of enhancing the boiling heat transfer under irradiation condition by the contact line movement during bubble nucleation on structured surfaces.

Dynamic Behavior of Ropeway String

In this paper a new approach for dynamic behavior of a string is aimed to study lateral vibrations induced in an elastic ropeway string. We are going to deal with the dynamic behavior of a string. In hilly areas we have problem for traveling from one place to another place. There we are using a rope drive system using a string as a rope. A ropeway is a type of maritime lifting gadget used to move light stores and hardware across waterways. Here, multiple ropeways are designed by using solid works and then analyzing with static, transient and dynamic boundary conditions. To suggest optimum materials for optimum ropeway string, 4 materials were chosen (SAE 1020, SAE1137, Al-l7075, Al- 7475) to analyze and study the dynamic behavior of ropeway string by knowing each material results, comparing with each other material results and then showing suitable tables and graphs, finally concluding thesis with optimum material with optimum ropeway string to improve the performance and their durability.

A Computational Fluid Dynamics Analysis of Heat Transfer in an Air-Cooled Proton Exchange Membrane Fuel Cell with Transient Boundary Conditions

In recent studies, it was found that the current density and power density in air-cooled proton exchange membrane fuel cells is limited by heat transfer. The air that serves as both reactant and coolant has a low thermal conductivity which leads to an early onset of membrane overheating at current densities that are typically in the range of 0.3 - 0.4 A/cm^2. In addition, the stoichiometric air flow ratios are in the regime that lead to laminar flow while it is known that turbulent flow leads to higher heat transfer rates and more efficient cooling. It has therefore been suggested to place a turbulence grid before the entrance of such an air-cooled fuel cell, and first experiments have shown that both the current density and power density can be increased while keeping the fan power constant.In order to further improve the heat transfer, it has been suggested to induce pulsating flow to the air stream by sinusoidally varying the fan power as function of time. In this numerical study, a turbulence grid was placed before the entrance of a single air-cooled fuel cell channel with varying distances, and the velocity inlet of the air was varied periodically as function of time. One finding is that the choice of the numerical turbulence model is crucial in this analysis. Results indicate that the best distance between the grid and the cathode inlet is the closest at 2.5 mm which is in good agreement with prior experiments. The transient effects are small but show an additional cooling effect. Figure 1

Brine Formation and Mobilization in Submarine Hydrothermal Systems: Insights from a Novel Multiphase Hydrothermal Flow Model in the System H2O\u2013NaCl

Numerical models have become indispensable tools for investigating submarine hydrothermal systems and for relating seafloor observations to physicochemical processes at depth. Particularly useful are multiphase models that account for phase separation phenomena, so that model predictions can be compared to observed variations in vent fluid salinity. Yet, the numerics of multiphase flow remain a challenge. Here we present a novel hydrothermal flow model for the system H2O–NaCl able to resolve multiphase flow over the full range of pressure, temperature, and salinity variations that are relevant to submarine hydrothermal systems. The method is based on a 2-D finite volume scheme that uses a Newton–Raphson algorithm to couple the governing conservation equations and to treat the non-linearity of the fluid properties. The method uses pressure, specific fluid enthalpy, and bulk fluid salt content as primary variables, is not bounded to the Courant time step size, and allows for a direct control of how accurately mass and energy conservation is ensured. In a first application of this new model, we investigate brine formation and mobilization in hydrothermal systems driven by a transient basal temperature boundary condition—analogue to seawater circulation systems found at mid-ocean ridges. We find that basal heating results in the rapid formation of a stable brine layer that thermally insulates the driving heat source. While this brine layer is stable under steady-state conditions, it can be mobilized as a consequence of variations in heat input leading to brine entrainment and the venting of highly saline fluids.

Heat pipe air-cooled thermal management system for lithium-ion batteries: High power applications

Thermal management of lithium-ion (Li-ion) batteries in Electrical Vehicles (EVs) is important due to extreme heat generation during fast charging/discharging. In the current study, a sandwiched configuration of the heat pipes cooling system (SHCS) is suggested for the high current discharging of lithium-titanate (LTO) battery cell. The temperature of the LTO cell is experimentally evaluated in the 8C discharging rate by different cooling strategies. Results indicate that the maximum cell temperature in natural convection reaches 56.8 °C. In addition, maximum cell temperature embedded with SCHS for the cooling strategy using natural convection, forced convection for SHCS, and forced convection for cell and SHCS reach 49 °C, 38.8 °C, and 37.8 °C which can reduce the cell temperature by up to 13.7%, 31.6%, and 33.4% respectively. A computational fluid dynamic (CFD) model using COMSOL Multiphysics® is developed and comprehensively validated with experimental results. This model is then employed to investigate the thermal performance of the SHCS under different transient boundary conditions.

Industrialization and Thermal Performance of a New Unitized Water Flow Glazing Facade

New light envelopes for buildings need a holistic vision based on the integration of architectural design, building simulation, energy management, and the curtain wall industry. Water flow glazing (WFG)-unitized facades work as transparent and translucent facades with new features, such as heat absorption and renewable energy production. The main objective of this paper was to assess the performance of a new WFG-unitized facade as a high-performance envelope with dynamic thermal properties. Outdoor temperature, variable mass flow rate, and solar radiation were considered as transient boundary conditions at the simulation stage. The thermal performance of different WFGs was carried out using simulation tools and real data. The test facility included temperature sensors and pyranometers to validate simulation results. The dynamic thermal transmittance ranged from 1 W/m2K when the mass flow rate is stopped to 0.06 W/m2K when the mass flow rate is above 2 L/min m2. Selecting the right glazing in each orientation had an impact on energy savings, renewable energy production, and CO2 emissions. Energy savings ranged from 5.43 to 6.46 KWh/m2 day in non-renewable energy consumption, whereas the renewable primary energy production ranged from 3 to 3.42 KWh/m2 day. The CO2 emissions were reduced at a rate of 1 Kg/m2 day. The disadvantages of WFG are the high up-front cost and more demanding assembly process.

A Computational Fluid Dynamics Analysis of Heat Transfer in an Air-Cooled Proton Exchange Membrane Fuel Cell with Transient Boundary Conditions

Turbulence-inducing grids can enhance heat transfer in air-cooled proton exchange membrane fuel cells and help to increase the fuel cell performance. In this computational fluid dynamics analysis, the distance between the turbulence grid and the cathode inlet was first varied, and then a transient sinusoidal inlet velocity around the previously investigated steady-state inlet velocity was applied because the heat transfer coefficient is known to increase in pulsating flow. Results indicate that the best distance between the grid and the cathode inlet is the closest at 2.5 mm, which is in good agreement with prior experiments. The transient effects are small but show an additional cooling effect.

A 3-D Strong-Coupled Electromagnetic-Thermal Model for HTS Bulk and Its Uses to Study the Dynamic Characteristics of a Linear HTS Maglev Bearing

Thermal effect will greatly affect the engineering performance of high-temperature superconductors (HTSs) due to its strong dependence of electromagnetic parameters upon the local temperature. To advance the understanding of such thermal effects, a validated 3-D strong-coupled electromagnetic-thermal model for HTS bulk was established in commercial finite-element software COMSOL, which ensures the easy access and universality of the model. Jc(B,T) was employed to reflect both magnetic field and thermal field dependences of HTS in this model. In addition, the thermal transient equation and convective boundary condition were employed with experimentally measured HTS thermal conductivity and heat capacity to describe the thermal flux exchange between HTS and cryogenic medium. As an example of application, the established electromagnetic-thermal model was tailored to study the dynamic characteristics of a linear HTS magnetic levitation (maglev) bearing. The methodologies to numerically study the dynamic response of the linear HTS maglev bearing under free vibration state and typical operating excitations, e.g., earthquake, track irregularity, and crosswind, were put forward in this article. The influences of field cooling height, preload, and ambient temperature, were also studied, and promising methods to improve the system stability were put forward according to the obtained conclusions. The above-mentioned results are reasonable and keep in concert with former experimental and theoretical studies. Moreover, some results which are inaccessible in the 2-D models, for instance, the thermal field distribution inside HTS bulk, can also be obtained due to the versatility of the 3-D model. To conclude, the established HTS electromagnetic-thermal model could serve as a flexible and extensible simulation tool to study various applications of HTS bulk. Besides the application in linear HTS maglev bearing, which was systematically studied in this article, other potential applications, such as thermal analysis of HTS bulk in pulse magnetization process and HTS bulk-based electrical machines, can also be expected in future work.

New Semi-Analytical Solutions for Advection–Dispersion Equations in Multilayer Porous Media

A new semi-analytical solution to the advection-dispersion-reaction equation for modelling solute transport in layered porous media is derived using the Laplace transform. Our solution approach involves introducing unknown functions representing the dispersive flux at the interfaces between adjacent layers, allowing the multilayer problem to be solved separately on each layer in the Laplace domain before being numerically inverted back to the time domain. The derived solution is applicable to the most general form of linear advection-dispersion-reaction equation, a finite medium comprising an arbitrary number of layers, continuity of concentration and dispersive flux at the interfaces between adjacent layers and transient boundary conditions of arbitrary type at the inlet and outlet. The derived semi-analytical solution extends and addresses deficiencies of existing analytical solutions in a layered medium, which consider analogous processes such as diffusion or reaction-diffusion only and/or require the solution of complicated nonlinear transcendental equations to evaluate the solution expressions. Code implementing our semi-analytical solution is supplied and applied to a selection of test cases, with the reported results in excellent agreement with a standard numerical solution and other analytical results available in the literature.