Transient Boundary Conditions Research Articles

The use of pulsating jet impingement to minimize temperature oscillations caused by fluctuating heat-flux boundary conditions

Thermal cycling poses a significant challenge in modern applications employing semiconductor devices such as power electronics, owing to the dynamic heat dissipation encountered across diverse operational conditions. The oscillatory variation between peak and low temperatures can induce thermal fatigue, thereby compromising long-term reliability. To address this issue, an innovative approach involving intermittent pulsation of jets has emerged as a promising solution to mitigate peak temperatures and attenuate temperature fluctuations within acceptable limits. This paper presents a comprehensive numerical investigation into the efficacy of intermittent jet impingement in minimizing temperature fluctuations arising from transient heat-flux boundary conditions across a frequency spectrum ranging from 1 to 25 Hz. Comparative analysis against steady jets reveals that intermittent jet pulsation enables superior control over maximum temperature thresholds. An enhancement factor is introduced to quantitatively assess the effectiveness of intermittent jet pulsation, elucidating that up to 8.2 Hz, the reduction in temperature oscillation is notably enhanced compared to higher frequencies. A reduced model is developed to facilitate estimation of transient heat-transfer coefficients during the off period of the jet, shedding light on the underlying mechanisms driving enhanced heat transfer under intermittent jet conditions due to vorticity rings.

An Experimental Derivation of Transient Nonuniform External Boundary Conditions for the Solidification Process Modeling of Equiaxed Investment Castings

An Experimental Derivation of Transient Nonuniform External Boundary Conditions for the Solidification Process Modeling of Equiaxed Investment Castings

The role of pit lake thermal dynamics on the thermal performance of ground heat exchangers

The addition of ground heat exchangers (GHEs) to a pit lake’s basin has the potential for abundant, clean and renewable geothermal energy extraction using shallow geothermal systems. Basin-embedded GHEs avoid direct interaction with mine water, which has been shown to impact efficiency and longevity in mine open-loop geothermal systems negatively. The now accelerated closure of open-pit coal mines presents itself as an opportunity to use this technology. However, no guidelines currently exist for designing or operating GHEs embedded in the sediment of water bodies. Furthermore, the two-way coupling between the complex annual thermal fluid dynamics that lakes are naturally subjected to and heat fluxes on the sediments and the GHE system has not been explored. In this study, we develop and validate finite element models to assess the relevance of lake thermal stratification in the performance of a geothermal system embedded in water bodies basins, e.g., on open-pit mine closures, under temperate residential thermal loads. The results show that the pit lake’s role as a thermal sink improves significantly when the lake’s thermal dynamics are accounted for, with an increase of up to 292% in the lake’s available energy budget. A minor variation in energy budget (~8%) was found whether the lake is modelled explicitly or simplified as a transient Dirichlet temperature boundary condition. This small difference vanishes if horizontal circulation along the lake is considered, highlighting the lake’s thermal energy potential. Finally, the impact on the GHE Coefficient of Performance (COP) is evaluated, with a maximum of ~15% difference among all cases.

Parametric analysis of system performance and cost of heating systems with heat pump and latent thermal energy storage

This paper presents a parametric analysis examining the impact of phase change material (PCM) melting temperature, latent thermal energy storage (LTES) volume, and solar collectors’ surface area on the performance and costs of heating systems integrating a heat pump and LTES by using dynamic system simulations. The computational models, implemented in TRNSYS software, and the numerical procedure were validated by comparing the numerical results with experimental measurements. The computational models represent the entire heating systems and thus consider the mutual interaction of heat production, distribution and automatic regulation system as well as a building which represent the heat load, in transient boundary conditions. Two main system designs with different LTES roles were considered – a system in which the role of the LTES is to provide more favorable operating conditions and a system in which the LTES role is to store the heat on a sufficiently high temperature to be used directly for heating. System performance was analyzed in terms of the share of renewable energy in total delivered energy, power consumption, stored energy in LTES, seasonal performance factor, average solar collector efficiency and total annual costs. The results showed that, regarding the system in which LTES was used as a heat source for the heat pump, a better overall performance and lower costs were observed in systems with PCMs with melting temperatures in range between 19 and 29 °C, compared to systems with PCMs with melting temperatures outside this range. For system in which the heat from the LTES was used directly for heating, the highest values of system performance criteria and the lowest cost were achieved with the PCM of 54 °C melting temperature. Almost all of the considered system performance criteria and total costs were higher when the LTES volume and solar collector surface area were larger as well.

Experimental study on the control of temperature oscillation in power electronics via pulsating jet impingement

Thermal oscillations in power electronics due to transient load conditions present a significant challenge. This experimental study investigates the application of intermittent jet impingement, coupled with transient heat flux boundary conditions of 100 W, 200 W, and 300 W, to a thermal test chip at a frequency of 0.5 Hz (70 % duty cycle), to assess its effectiveness in minimizing junction temperature fluctuations. Temperature oscillations were recorded to compare the advantages of intermittent pulsation over steady jet flow. The results demonstrate reduced hot spot temperature oscillations of 10 %, 12.5 %, and 16.7 % at 100 W, 200 W, and 300 W, respectively, at a Reynolds number of 8850. Remarkably, even during the pulsation off-period, the minimum temperature limit was maintained at the inlet temperature of the jet, attributed to the formation of strong vorticity rings at the end of the jet pulse, inducing a cooling effect and reducing the junction temperature. Pulsating jets exhibit superior control over temperature fluctuation compared to steady jets, with the reduction being more pronounced at Reynolds numbers closer to 2000 and diminishing as Reynolds number increases to 17000. Moreover, the heat transfer coefficient and the Nusselt number enhanced by 18 % due to pulsating jet impingement. An enhancement factor is introduced to quantify the benefits of using pulsating jets, revealing a higher reduction in temperature oscillation compared to steady jets at the same time-averaged Reynolds number, valid within the Reynolds number range of 2000–14000. Additionally, a correlation is proposed to calculate the enhancement factor for pulsating jets. Finally, the pulsating jet impingement proves to reduce the exergy destruction by 30 % compared to steady jet.

Unified lattice Boltzmann framework for coupled non-Fourier conduction and thermal radiation based on the C-V model

This research enhances our previously established unified lattice Boltzmann (LB) framework, initially developed for classical Fourier heat conduction and radiation coupling, to now adeptly handle non-Fourier heat conduction and thermal radiation transfer at the micro and nanoscale. It effectively manages the disparities in propagation speeds and complex transient boundary conditions. Validation against one-dimensional benchmarks confirms its accuracy and reliability. The study also examines the influence of critical parameters like the radiative heat transfer parameter Nc, scattering albedo, and optical thickness on thermal dynamics. Significantly, it provides a quantitative error analysis of the steady-state radiation assumption in coupled problems, revealing its limitations under various conditions. This enhanced framework represents a solid improvement in micro and nanoscale multiphysics simulations using the lattice Boltzmann method, ensuring a balance between computational precision and efficiency. It contributes to the development of more refined thermal management strategies in diverse industrial and scientific applications.

Annual performance evaluation of a hybrid concentrated solar–micro gas turbine based on off-design simulation

As the adoption of solar hybrid systems continues to rise due to their potential to compensate for the fluctuation of solar irradiation, it becomes imperative to accurately evaluate their performance, considering the variation of off-design conditions. This paper introduces a detailed analysis method for evaluating the annual performance of a solar-MGT system under transient boundary conditions for a whole-year operation range. A hybrid system of a micro gas turbine, recuperator, and solar dish is considered, and an off-design simulation model is developed and verified with available experimental results. Two different configurations for a recuperated cycle are considered, and simulations are conducted for a test case in Pretoria, SA. The results for Jun.21 and Dec.21 as low and high solar energy days are reported with more details to compare the configurations and demonstrate the effect of ambient temperature on the heat loss of the solar receiver and the overall performance of the system. The alternative configuration reduces heat loss with a lower temperature receiver but has higher fuel consumption compared to the conventional configuration. Operating strategies for different hours of operation from 1 h to 24 h per day are simulated for 365 days, based on real meteorological data, and compared with the operating in solar available hours. It is shown that the whole-year simulation of the system considering the variation of boundary conditions can change the estimation of fuel consumption by 25 %.

Poromechanics Modeling Forecasts Production, Analyzes Productivity Decline

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 212200, “Accurate Production Forecasting and Productivity Decline Analysis Using Coupled Full-Field and Near-Wellbore Poromechanics Modeling,” by Yan Li, Bin Wang, and Jiehao Wang, Chevron, et al. The paper has not been peer reviewed. _ Productivity index (PI) decline is caused by different mechanisms in both the wellbore region and the far field. Damage in the wellbore region can be simulated by detailed wellbore modeling. A newly developed full-field and near-wellbore poromechanics coupling scheme is used in the complete paper to model PI degradation against time. Near-wellbore damage and field and well interactions are identified when applying the coupling scheme for a deepwater well. History matching, production forecasting, and safe drawdown limits are derived for operational decisions. Full-Field Reservoir Model and Near-Wellbore Poromechanics Model Coupling Full-field and near-wellbore modeling involves coupling the simulation of a full-field reservoir model with one or more near-wellbore poromechanics models. In the coupled simulation, the full-field reservoir model dictates the changing flow or thermal boundary conditions for the embedded near-wellbore models. Meanwhile, near-wellbore phenomena affect well productivity in the full-field reservoir model, altering the flow and thermal boundary conditions on all near-wellbore models in the same field. While capturing the dynamic interactions between the full-field model and all embedded near-wellbore models is of vital importance, the traditional near-wellbore modeling work flow considers only one-way coupling. This work flow is labor-intensive and lacks the dynamic interplay between the reservoir and near-wellbore models. A novel near-wellbore coupling framework is developed to automate data exchange and capture dynamic interactions between the full-field model and the near-wellbore model. In the full-field reservoir and geomechanics coupling applications, only a reservoir simulator solves reservoir flow and thermal equations and a geomechanics simulator solves solid mechanics equations. The reservoir and geomechanics simulators exchange 3D field data. Because solid mechanics equations are quasistatic, the geomechanics simulator does not take timesteps in full-field coupling. In near-wellbore coupling, however, both reservoir and geomechanics simulators solve reservoir flow and thermal equations. The near-wellbore geomechanics model also may solve solid mechanics or other equations coupled to flow or thermal equations in the near-wellbore model. All physics are included in the near-wellbore model for the coupling. Both field properties (3D) and transient boundary conditions (2D) must be mapped onto the near-wellbore model. It is important to ensure that the full-field and near-wellbore models are consistent. A 3D data-mapping module is developed to map flow and rock properties and initial conditions from the full-field model to the near-wellbore model. During the simulation, the transient pressure/temperature boundary conditions (2D) at the external boundary of the near-wellbore model must be mapped from the full-field model to the near-wellbore model to update transient boundary conditions. The automated data mapping in the coupling scheme eliminates the need for manual mapping of field properties and transient boundary conditions.

Solidification Process Modeling of Equiaxed Investment Castings with Transient Nonuniform Boundary Condition Definition

Solidification Process Modeling of Equiaxed Investment Castings with Transient Nonuniform Boundary Condition Definition

Nonlinear biomechanics of diseased carotid arteries

The aim of this article is to analyse the nonlinear biomechanics of diseased carotid arteries as a potential tool for predicting the onset of cerebral strokes. A two-way coupled three-dimensional (3D) hyperelastic fluid-structure interaction (FSI) analysis of a diseased carotid artery with abnormal luminal projections at the carotid bulb known as carotid web (CaW) is conducted on the geometry of a patient artery, built upon employing CT angiography images. The blood-flow model incorporates non-Newtonian pulsatile turbulent fluid, and the artery wall is considered hyperelastic subject to blood-induced motion. The hemodynamics of artery induced by transient boundary conditions is determined, specifically focusing on shifts in crucial hemodynamic parameters such as wall shear stress (WSS) and alterations in the blood velocity pattern. Structural assessment of the artery wall involves quantifying the von Mises (VM) stress and deformation field. The analysis demonstrates that different CaW models result in different flow patterns for a selection of time steps in a cardiac cycle. The findings reveal that the presence of the web, as the most common disease among younger adults, can significantly influence the hemodynamic parameters and potentially accelerate the formation of thrombus and atherosclerosis. Hemodynamic analysis can potentially predict specific sites prone to plaque formation and rupture within the carotid artery.

Development of 1D Model of Constant-Volume Combustor and Numerical Analysis of the Exhaust Nozzle

Development of 1D Model of Constant-Volume Combustor and Numerical Analysis of the Exhaust Nozzle

Root-induced changes in the hydraulic properties of a fine slope cover

Recently, the soil-vegetation-atmosphere (SVA) interaction is becoming a topic of intense scientific research within the geotechnical community, because it was recognized to potentially induce significant pore pressure variations in slopes, at both shallow and larger depths, being then responsible for weather-induced landsliding. The processes within such interaction are of different nature, and result in a chemo-thermo-hydro-mechanical transient boundary condition at the ground level, causing exchanges of liquid, gas and energy within the soil cover layer (from ground level to 3-4 metres depths). Hence, both the soil cover material and its thermo-hydro-mechanical constitutive properties, as well as the vegetation properties, become of key relevance within such processes. In this study, with reference to a full scale in-situ test where deep-rooted crop spices were seeded and farmed, the hydraulic properties of rooted clayey soil were investigated in the laboratory as well as in-situ in terms of both saturated permeability and retention properties. As for the saturated permeability, both laboratory and in-situ tests were conducted, the latter being also back-analysed. The retention states of the material were investigated by means of the filter paper technique in the laboratory and were of use for the numerical back-analyses of the Guelph-induced wetting process in the soil cover. The measurements show that roots within the vegetated zone determine both an increase of the permeability and a reduction of the retention properties of the composite material if compared to the soil in bare conditions.

Computational analysis of patient-specific pulsatile blood flow: The influence of non-Newtonian models on wall shear stress assessment

Blood is a sophisticated biological fluid with components like erythrocytes that give it non-Newtonian behavior. Hemodynamic factors such as velocity magnitude, pressure, and wall shear stress descriptors are the most important factors in the development of atherosclerosis. The wall shear stress descriptors are regulated not only by flow geometry but also by blood rheological properties. In the current study, we carried out a numerical analysis of the non-Newtonian pulsatile blood flow while taking into account a patient-specific geometry and transient boundary conditions. Non-Newtonian blood flow is modeled using the four non-Newtonian models: the power-law model, the Carreau model, the Casson model, and the Quemada model, and compared with the Newtonian model. Streamline analysis vividly illustrates velocity patterns, revealing the presence of recirculation zones near sinus regions. The study suggests the significance of selecting appropriate viscosity models for accurate assessments, particularly in regions with low time-average wall shear stress values, such as those associated with atherosclerotic plaques. The differences in the time-averaged wall shear stress between the four non-Newtonian models were found to be the highest in the Quemada model. The study concluded that the non-Newtonian model is required when the focus is on the low-time-averaged wall shear stress area.

Control of flow separation over an aerofoil by external acoustic excitation at a high Reynolds number

The effectiveness of acoustic excitation as a means of flow control at high Reynolds number turbulent flows is investigated numerically by using improved delayed detached eddy simulations (IDDES). Previous studies on low Reynolds number laminar flows have shown that acoustic excitation can substantially suppress flow separation for specific effective frequency and amplitude ranges. However, the effect of acoustic excitation on higher Reynolds number turbulent flow separation has not yet been explored due to limitations on appropriate fidelity computational methods or experimental facility constraints. Therefore, this paper addresses this research gap. A NACA0015 airfoil profile at 1 × 106 Reynolds number based on the airfoil chord length is used for the investigations. Acoustic excitation is applied to the baseline flow field in the form of transient boundary conditions at the computational domain inlet. A parametric study revealed that the effective sound frequency range shows a Gaussian distribution around the frequency of the dominant disturbances in the baseline flow. A maximum of ∼43% increase in lift-to-drag ratio is observed for the most effective excitation frequency F+=1.0 at a constant excitation amplitude of Am=1.8%. The effect of excitation amplitude follows an asymptotic trend with a maximum effective excitation amplitude above which the gains are not significant. A fully reattached flow is observed for the highest excitation level considered (Am=10%) that results in ∼120% rise in airfoil lift-to-drag coefficient. Overall, the findings of the current work demonstrate the higher Reynolds number effectiveness of acoustic excitation on separated turbulent flows, thereby paving the way for application in realistic flow scenarios observed in aircraft and gas turbine engine flow fields.

Transient thermal behavior of clay walls integrated with phase change materials

There are several advantages of using phase change materials (PCMs) in building walls for creating energy-efficient structures. Incorporating PCMs into clay walls with the hope of improving their thermal characteristics under transient boundary conditions is the major goal of this study. Numerical simulations are performed utilizing the finite element method and the apparent heat capacity formulation to predict the melting and solidification behavior of PCM-integrated walls. By introducing a 2 cm thick layer of PCM with a typical melting temperature of 32 °C into the middle of a clay wall, the results demonstrate a significant reduction of approximately 57 % in heat flow on the inner surface. Furthermore, compared to a standard clay wall without PCM, this integration extended the time lag by about 5.5 h.

Thermal performance and corrosion resistance analysis of inorganic eutectic phase change material with one dimensional carbon nanomaterial

The inherent thermal characteristics, supercooling phenomenon, and corrosion issues associated with salt hydrate phase change materials (PCMs) limit their practical applications. In this research work, we report a newly formulated eutectic salt hydrate PCM using a) sodium sulphate decahydrate (SSD) & b) sodium phosphate dibasic dodecahydrate (SPDD); with a focus on customizing its properties to enhance its suitability for low temperature thermal regulation (achieving a melting point of 27.8 °C and a high heat storage capacity of 215 J/g). Additionally, we have successfully reduced the degree of supercooling and introduced corrosion resistant properties to this PCM. To enhance both the thermal energy transfer rate and optical absorbance of the eutectic PCM, we have incorporated one-dimensional (1D) multiwall carbon nanotube (MWCNT) at various weight fractions, extending up to 0.9 %, utilizing a two-step method. The dispersion and chemical stability of SSD/SPDD + MWCNT nanocomposite are verified through the morphological visual and spectral peaks obtained in Fourier transfer infrared spectroscopy. Additionally, studies evaluating the optical and thermal property reveal a substantial 500 % increase in absorbance, a notable 77.9 % reduction in transmissibility, a thermal conductivity increase from 0.464 W/m⋅K to 0.742 W/m⋅K (reflecting a 59.9 % increment), and the retention of a consistent melting enthalpy of 218.6 J/g. This stability is attributed to the intermolecular interaction with MWCNT. Similary, the degree of supercooling (ΔTs) for the SSD/SPDD EPCM containing MWCNT decreased to 2.2 °C from 16.5 °C, marking an 86 % reduction compared to the pure eutectic salt solution. Furthermore, this composite demonstrated enhanced thermal and chemical stability throughout 200 thermal cycles. Auxiliary ANSYS simulation, with transient boundary condition, are provided to analyze the heat transfer interactions between the thermic fluid and the newly developed PCM when integrated into a thermal regulation system. Subsequently, a corrosion analysis of the developed eutectic PCM and the nanocomposite eutectic PCM exhibits a corrosion rate of 0.018 mpy, well below the permissible level (<5mpy). The insights gained from the development of this nanocomposite PCM offer valuable guidance for the design and creation of tailored eutectic PCM for low-temperature thermal regulation systems, resulting in significant energy savings.

An approximate analytical model for unconventional reservoir considering variable matrix blocks and simultaneous matrix depletion

In regard to unconventional oil reservoirs, the transient dual-porosity and triple-porosity models have been adopted to describe the fluid flow in the complex fracture network. It has been proven to cause inaccurate production evaluations because of the absence of matrix–macrofracture communication. In addition, most of the existing models are solved analytically based on Laplace transform and numerical inversion. Hence, an approximate analytical solution is derived directly in real-time space considering variable matrix blocks and simultaneous matrix depletion.To simplify the derivation, the simultaneous matrix depletion is divided into two parts: one part feeding the macrofractures and the other part feeding the microfractures. Then, a series of partial differential equations (PDEs) describing the transient flow and boundary conditions are constructed and solved analytically by integration. Finally, a relationship between oil rate and production time in real-time space is obtained.The new model is verified against classical analytical models. When the microfracture system and matrix–macrofracture communication is neglected, the result of the new model agrees with those obtained with the dual-porosity and triple-porosity model, respectively. Certainly, the new model also has an excellent agreement with the numerical model. The model is then applied to two actual tight oil wells completed in western Canada sedimentary basin. After identifying the flow regime, the solution suitably matches the field production data, and the model parameters are determined. Through these output parameters, we can accurately forecast the production and even estimate the petrophysical properties.

Evolution of plume geometry, dilution and reactive mixing in porous media under highly transient flow fields at the surface water-groundwater interface

Highly transient boundary conditions affect mixing of dissolved solutes in groundwater. An example of these transient boundary conditions occurs at the surface water-groundwater interface, where the water level in rivers can change rapidly due to the operation of hydropower plants, leading to a regime known as hydropeaking. Inspired by this phenomenon, this work studies at laboratory scale the effects of fluctuating surface water bodies on solute transport in aquifers. We performed flow-through experiments at two different flow velocities and under steady and transient flow conditions where a conservative tracer was injected in the system and its concentration measured with optical imaging methods. The experimental results were quantitatively interpreted with numerical simulations implementing a non-linear velocity-dependent dispersive transport model. We estimated plume dilution by computing the dilution index and its evolution as well as two key geometrical metrics of the transient plumes: the perimeter and the area. We further investigated reactive mixing and mixing enhancement considering mixing-controlled bimolecular reactions using different critical mixing ratios. In general, highly transient boundary conditions lead to a larger area, perimeter and plume dilution and the results show greater relative enhancement for the scenarios with low groundwater flow velocity. A linear relationship was observed between the evolution of the area and the dilution index of the plumes for the transient flow scenarios investigated. Considering reactive transport and mixing-limited reactions at the surface water-groundwater interface, we identified different dilution and reaction dominated regimes, characterized, respectively, by increasing and decreasing plume entropies at different mixing ratios of the reactants. Furthermore, reactive mixing was enhanced by transient flows leading to a faster degradation of contaminant plumes compared to corresponding steady flow conditions.

Highlighting Hemodynamic Risks for Bioresorbable Stents in Coronary Arteries

A three-dimensional, transient computational fluid dynamics analysis was conducted on an idealised geometry of a coronary artery fitted with representative geometries of an Absorb bioresorbable vascular scaffold (BVS) or a Xience drug-eluting stent (DES) in order to identify and compare areas of disturbed flow and potential risk sites. A non-Newtonian viscosity model was used with a transient velocity boundary condition programmed with user-defined functions. At-risk areas were quantified in terms of several parameters linked to restenosis: wall shear stress, time-averaged wall shear stress, oscillatory shear index, particle residence time, and shear rate. Results indicated that 71% of the BVS stented surface area had time-averaged wall shear stress values under 0.4 Pa compared to 45% of the DES area. Additionally, high particle residence times were present in 23% and 8% of the BVS and DES areas, respectively, with risk areas identified as being more prominent in close proximity to crowns and link struts. These results suggest an increased risk for thrombosis and neointimal hyperplasia for the BVS compared to the DES, which is in agreement with the outcomes of clinical trials. It is intended that the results of this study may be used as a pre-clinical tool to aid in the design of bioresorbable coronary stents.

Multiphysics analysis of fuel fragmentation, relocation, and dispersal susceptibility–Part 3: Thermal hydraulic evaluation of large break LOCA under high-burnup conditions

Increasing the peak rod average burnup of pressurized water reactor (PWR) fuel beyond 62 GWd/tU may increase fuel fragmentation, relocation, and dispersal (FFRD) susceptibility during a large break loss of coolant accident (LBLOCA). TRACE thermal hydraulic (TH) LBLOCA analyses were performed for a realistic 24-month high-burnup PWR equilibrium cycle, to inform subsequent transient BISON high-burnup FFRD susceptibility evaluations. Realistic LBLOCA systems behavior was first established by configuring to and comparing with the BEMUSE OECD LBLOCA benchmark. Fuel and operating conditions were then applied from high-burnup VERA depletion calculations. LBLOCA simulations were performed for 281 selected high-burnup rods, for which transient TH boundary conditions were collected for later use in BISON. The TRACE results indicated that rod linear heat rate (rather than burnup) is the main predictor of peak cladding temperature (PCT) during the event. PCT typically occurred at a local burnup lower than the rod-average burnup, especially for twice-burned fuel.