Thermal Non-equilibrium Approach Research Articles

Entropy generation minimization of hybrid nanofluid mixed convection flow in lid-driven square enclosure with heat-generating porous layer on inner walls

PurposeThis study aims to numerically scrutinize the entropy generation minimization and mixed convective heat transfer of multi-walled carbon nanotubes–Fe3O4/water hybrid nanofluid flow in a lid-driven square enclosure with heat generation in the presence of a porous layer on inner surfaces, considering local thermal non-equilibrium (LTNE) approach and the non-Darcy flow model.Design/methodology/approachThe dimensionless governing equations for hybrid nanofluid and solid phases are solved by applying the finite volume method and semi-implicit method for pressure-linked equations algorithm.FindingsThe roles of the internal heat generation in the porous layer, LTNE model and nanoparticles volume fraction on mixed convection phenomenon and entropy generation are introduced for lid-driven cavity hybrid nanofluid flow. Based on the investigation of entropy generation and heat transfer, the minimum total entropy generation and average Nusselt numbers are found at 1 ≤ Ri ≤ 10 where the effect of the forced and free convection flow directions being opposite each other is very significant. When considering various nanoparticle volume fractions, it becomes evident that the minimum entropy generation occurs in the case of φ = 0.1%. The outcomes of LTNE number reveal the operating parameters in which thermal equilibrium occurs between hybrid nanofluid and solid phases.Originality/valueThe analysis of entropy generation under various shear and buoyancy forces plays a significant role in the suitable thermal design and optimization of mixed convective heat transfer applications. This research significantly contributes to the optimization of design and the advancement of innovative solutions across diverse engineering disciplines, such as packed-bed thermal energy storage and thermal insulation.

A Local Thermal Non-Equilibrium Approach to an Electromagnetic Hybrid Nanofluid Flow in a Non-Parallel Riga Plate Channel

The fluid flow in a non-parallel configuration exists in the electronic heat removal devices, microchannel heat sinks, and angled confusers/diffusers. The fluids in these applications are prone to flow separation and bifurcations. To deal with such type of problems, a novel idea of a converging or diverging type Riga plate channel is introduced in this study. The Riga plates are utilised to produce the cross-flow magnetic and electric fields which give rise to an exponentially decaying Lorentz force. Also, a porous matrix with variable permeability is considered to fill the Riga plate channel. The thermal equilibrium state between the hybrid nanofluid and porous media is ignored i.e., a local thermal non-equilibrium (LTNE) approach is adopted to model the energy balance equations. The dimension-free form of the guiding equations is tackled by using the Chebyshev pseudospectral quasi-linearization method. The heat transfer rate is respectively incremented by 21.42% and 63.12% in the converging and diverging flow regimes, with the inclusion of a Riga Sheet. The skin friction coefficient is depressed with modified Hartmann number (Ha*) and porosity (ε) for the converging/diverging flow regime. The LTNE state alters to the LTE with Nield number (Ni), thermal conductivity ratio (γ) and ε.

Numerical study of natural convection of fluid with temperature-dependent viscosity inside a porous cube under non-uniform heating using local thermal non-equilibrium approach

Numerical study of natural convection of fluid with temperature-dependent viscosity inside a porous cube under non-uniform heating using local thermal non-equilibrium approach

Computational Modeling of Latent Heat Thermal Energy Storage in a Shell-Tube Unit: Using Neural Networks and Anisotropic Metal Foam

Latent heat storage in a shell-tube is a promising method to store excessive solar heat for later use. The shell-tube unit is filled with a phase change material PCM combined with a high porosity anisotropic copper metal foam (FM) of high thermal conductivity. The PCM-MF composite was modeled as an anisotropic porous medium. Then, a two-heat equation mathematical model, a local thermal non-equilibrium approach LTNE, was adopted to consider the effects of the difference between the thermal conductivities of the PCM and the copper foam. The Darcy–Brinkman–Forchheimer formulation was employed to model the natural convection circulations in the molten PCM region. The thermal conductivity and the permeability of the porous medium were a function of an anisotropic angle. The finite element method was employed to integrate the governing equations. A neural network model was successfully applied to learn the transient physical behavior of the storage unit. The neural network was trained using 4998 sample data. Then, the trained neural network was utilized to map the relationship between control parameters and melting behavior to optimize the storage design. The impact of the anisotropic angle and the inlet pressure of heat transfer fluid (HTF) was addressed on the thermal energy storage of the storage unit. Moreover, an artificial neural network was successfully utilized to learn the transient behavior of the thermal storage unit for various combinations of control parameters and map the storage behavior. The results showed that the anisotropy angle significantly affects the energy storage time. The melting volume fraction MVF was maximum for a zero anisotropic angle where the local thermal conductivity was maximum perpendicular to the heated tube. An optimum storage rate could be obtained for an anisotropic angle smaller than 45°. Compared to a uniform MF, utilizing an optimum anisotropic angle could reduce the melting time by about 7% without impacting the unit’s thermal energy storage capacity or adding weight.

Stability analysis of MHD radiative mixed convective flow in vertical cylindrical annulus: Thermal nonequilibrium approach

AbstractIn the present study, the influence of the induced magnetic field on the MHD mixed convective electrically conducting fluid flow inside the vertical cylindrical annulus is analyzed numerically. The heat transfer is presumed to be due to a combination of mixed convection and radiation. The stability of the flow is examined when the solid and fluid phases are not in local thermal equilibrium. The governing equations are solved numerically by both finite difference and finite element methods. To control the flow formation rate more accurately the induced magnetic field is also considered in this study. As the magnetic Prandtl number (Pm) and Hartmann number (M) get enhanced, the velocity and induced magnetic fields get retarded in the annulus due to the presence of drag‐like force, namely, the Lorentz force. When there is an increase in the mixed convection parameter the induced magnetic field gets enhanced. An increase in radiation parameter tends to decline the fluid temperature and reverse the behavior of the solid temperature. Increment in Pm decreases the wall shear stress near the conducting cylinder. Increasing values of porous, magnetic, and radiation parameters lead to an unstable system with smaller heat transfer coefficient values but the system gets stabilized for larger values of heat transfer coefficient. The results could be used as first‐hand information for comprehending and developing the thermal flow phenomenon in porous media. The obtained numerical results are in good accordance with the existing results. Using an artificial neural network, heat transfer characteristics are analyzed through mean square error and regression analysis.

Forced Convection in a Transpired Passage With a Porous Medium Insert

Abstract Forced convective heat transfer in a passage with a porous medium matrix subjected to uniform blowing has been fully analyzed, using a local thermal nonequilibrium approach, in application of transpiration systems on the basis of porous medium. An exact solution was found for the asymptotic case of infinitely high interstitial Biot number. In order to extend the problem to general local nonthermal equilibrium cases, an analytical solution procedure has been developed, introducing six unknown parameters to describe both fluid and solid temperature profiles, which are then determined by solving six independent differential and integral equations based on the two energy equations for individual phases and their corresponding boundary conditions. The important performance index for the problem, namely, the overall cooling effectiveness is found quite sensitive not only to the blowing ratio, but also to the interstitial Biot number and fluid to solid effective thermal conductivity ratio. It rises with the blowing ratio and effective thermal conductivity ratio, while it drops as increasing the interstitial Biot number toward the local thermal equilibrium. The present analysis reveals that the local nonthermal equilibrium analysis is definitely needed for accurate estimation of the transpiration-cooled wall temperature.

Theoretical evaluation of bio-thermal response in human tissue subjected to pulse-laser induced hyperthermia therapy for cancer treatment

This paper aims to develop a theoretical estimation of thermal response in human tissues appearing in a hyperthermia treatment under the influence of laser-induced external heat sources. In general, to develop a theoretical investigation on bioheat transfer, the fundamental aspect is to adopt an appropriate thermal model for predicting the correct thermal response. Among the existing bioheat models, the two-temperature model is considered to be the most feasible one, as it takes into account the arterio-vascular architectural interaction in the analysis. This model is established based on the local thermal non-equilibrium (LTNE) approach. The exact solution of the two-temperature bioheat model has been established by the implementation of the combined scheme of Duhamel's theorem and the Finite integral transform method. A pulse laser heating applied externally as a therapeutic heat source is introduced newly in the area of the LTNE bioheat modelling whereas the existing research works were developed by utilizing scattering laser heat sources. The thermal response of the tissue is analyzed for different laser variables such as laser intensity, optical penetration depth, pulse laser time, and laser irradiation radius. A parametric study has been made to set a theoretical treatment protocol for hyperthermia therapy and correspondingly, the thermal field has been developed within the prescribed peak temperature spectrum for the hyperthermia treatment. The thermal field has also been investigated for various biological parameters, viz. blood perfusion rate, porosity, and pore diameter of the blood vessel. The present research outcome depicts that the hyperthermia temperature can be achieved at the optical penetration depth of 0.0005 m with the laser irradiation radius in the range of 0.004 m–0.008 m. From the present work, it can be highlighted that the health monitoring of infected tissues or the organ of the human body is necessary before processing therapeutic heating as the thermal field depends on the porosity of tissue significantly. A qualitative investigation between pulse heating and scattering laser heating depicts that pulse laser heating is a better option to choose for the thermal therapy for the curtailment of the possibility of thermally induced damage of healthy tissues. A successful verification and authentication study has been carried out between the present work and the published experimental and theoretical works.

Natural convection inside nanofluid superposed wavy porous layers using LTNE model

This work describes the natural convection and thermodynamic irreversibility of alumina-water nanofluid filled inside a layered cavity. The local thermal non-equilibrium (LTNE) approach is taken into consideration. The cavity bed is considered as a wavy hot plate of a very thin thickness, while its roof is thermally insulated. The heat is dissipated from the cold vertical walls. A porous layer fills the cavity's lower half and is assumed to obey the Darcy-Forchheimer model. The finite-element method is adopted to compile the numerical results. The relevant parameters assessed in this paper are the Darcy number, waviness of the hot bottom wall, porosity, volume fraction of alumina nanoparticles and the modified ratio of thermal conductivity. The obtained numerical results show that the LTNE condition expands as the Darcy number augments, while a quasi-local thermal equilibrium is attained with a higher modified ratio of thermal conductivity. The waviness of the hot wall raises the Nusselt number of both the fluid and the solid phases but produces more entropy. It is quoted that the augmentations in the fluid and solid phases of the Nusselt numbers and the increase in the maximum entropy production are 42.25%, 69% and 55.5%, respectively.

Entropy generation of natural heat exchange into a porous medium accumulated by a hybrid nanoliquid applying LTNE model

PurposeThis study aims to use the thermal non-equilibrium approach to inquire the entropy production and conjugate natural heat exchange in a porous medium. Entropy generation is studied separately for the solid matrix and the hybrid nanoliquid.Design/methodology/approachThe characteristic equations are unraveled by applying the finite element method. Mathematical relations are used to calculate the generated entropy for the hybrid nanoliquid and matrix structure.FindingsBased on the results, the produced entropy and the viscous friction term associated with the hybrid nanoliquid phase are not affected by increasing the thermal conductivity ratio of the rigid wall to nanoliquid. Moreover, a higher amount of entropy is generated by the thermal gradients in the hybrid nanoliquid phase compared to the solid matrix.Originality/valueNo investigation in the literature has been reported in this context.

Numerical Evaluation of the Transient Performance of Rock-Pile Seasonal Thermal Energy Storage Systems Coupled with Exhaust Heat Recovery

This study seeks to investigate the concept of using large waste rocks from mining operations as waste-heat thermal energy storage for remote arctic communities, both commercial and residential. It holds its novelty in analyzing such systems with an experimentally validated transient three-dimensional computational fluid dynamics and heat transfer model that accounts for interphase energy balance using a local thermal non-equilibrium approach. The system performance is evaluated for a wide range of distinct parameters, such as porosity between 0.2 and 0.5, fluid velocity from 0.01 to 0.07 m/s, and the aspect ratio of the bed between 1 and 1.35. It is demonstrated that the mass flow rate of the heat transfer fluid does not expressively impact the total energy storage capacity of the rock mass, but it does significantly affect the charge/discharge times. Finally, it is shown that porosity has the greatest impact on both fluid flow and heat transfer. The evaluations show that about 540 GJ can be stored on the bed with a porosity of 0.2, and about 350 GJ on the one with 0.35, while the intermediate porosity leads to a total of 450 GJ. Additionally, thermal capacity is deemed to be the most important thermophysical factor in thermal energy storage performance.

Three-dimensional thermal assessment in cancerous tumors based on local thermal non-equilibrium approach for hyperthermia treatment

An appropriate evaluation of thermal characteristics in biological bodies under the influence of external heat source during the hyperthermia therapy is inherently difficult, since living tissues have a non-uniform and non-homogeneous structure composed of solid-liquid interfaces. In general, biological bodies are very sensitive to temperature alteration as it leads to a drastic change of protein structures, which in turn causes a permanent thermal injury. Therefore, it can always be desirable to use a suitable model for determining the temperature response in therapeutic domains accurately. Recently, the local thermal non-equilibrium (LTNE) model develops based on the energy exchange in the vascular geometry confined with arteriol-venule systems. The present study establishes a hybrid analytical scheme by applying the LTNE bioheat approach for the three-dimensional (3-D) temperature response in a cancer domain under the hyperthermia treatment. To take care of non-uniformity and porous media approaches of living tissues, this study suggests a pertinent spatially dependent initial condition along with the appropriate boundary conditions in the physical domain for the accurate temperature prediction. A comparative study of temperature distribution between LTNE and local thermal equilibrium (LTE) models has been performed which justifies the necessity to include the porous media approach in the analysis. In order to understand a 3-D heat flow in the hyperthermia domain, thermal contours may give such information and thus 3-D heat transfer analysis with two different time lags always predicts the exact thermo-biological behaviour as a function of the therapeutic exposure time. Finally, the research output validated with the available experimental hyperthermia therapeutic data and the present 3-D thermal response is in good agreement with the existing experimental values, which indicates the present model is appropriate to determine the thermal behaviour of living tissues accurately.

Free convection of a suspension containing nano-encapsulated phase change material in a porous cavity; local thermal non-equilibrium model

Due to the instinctive temperature-dependent heat capacity of the Nano-Encapsulated Phase Change Material (NEPCM), there is a growing interest in the potential applications of such materials in heat transfer. As such, steady-state natural convection in a porous enclosure saturated with nanofluid using NEPCMs has been investigated in this study. The cavity is assumed to have constant hot and cold temperatures at the left and right vertical boundaries, respectively, and fully insulated from the bottom and top walls. Considering the Local Thermal Non-equilibrium (LTNE) approach for the porous structure, the governing equations are first non-dimensionalized and then solved by employing the finite element Galerkin method. The impact of different parameters, such as porous thermal conductivity (ks), solid-fluid interface heat transfer (10 ≤ H ≤ 105), Stefan number (0.2 ≤ Ste ≤ 1), and volume fraction of nanoparticles (0.0 ≤ φ ≤ 0.05) on the patterns of the fluid and solid isotherms, streamlines and the contours of the heat capacity ratio, fusion temperature (0.05 ≤ θf ≤ 1), local and average Nusselt numbers, and overall heat transfer ratio has been studied. It is shown that improving the porous thermal conductivity not only leads to an increase in the rate of heat transfer but also augments the fluid flow inside the cavity. For low values of the Ste, the rate of heat, transferred in the porous enclosure, is intensified. However, regardless of the amount of the Stefan number, the maximum rate of heat transfer is achievable when the non-dimensional fusion temperature is approximately 0.5. Employing NEPCMs in a highly conductive porous structure is more efficacious only when the phases are in the state of local thermal equilibrium. Nonetheless, the rate of heat transfer is higher when the Local thermal non-equilibrium is validated between the phases. Besides, for poor thermal conductivity of the porous medium like glass balls (LTE condition), adding 5% of the nano-encapsulated phase change materials to pure water can boost the rate of heat transfer up to 47% (for Ste = 0.2 and θf = 0.5). This thermal investigation of NEPCMs shows in detail how advantageous are these nanoparticles in heat transfer and opens up an avenue for further application-based studies.

Effect of porous material properties on thermal efficiencies of a thermocline storage tank

Thermocline energy storage systems can be adapted to store energy in a tank where the hot fluid is pumped in during a charging cycle and cold fluid during a discharging cycle, substituting the need for two storage tanks. Here, a numerical investigation of a hot air flow transferring thermal energy to a solid porous bed is performed to evaluate the influence of the thermal conductivity and capacity ratios on the efficiency of a charging cycle of a thermocline storage tank. The numerical model considers the mixed convection turbulent flow (k-ε model) through clear and porous media using the Local Thermal Non-Equilibrium approach to provide solutions for the solid and fluid phases separately. Macroscopic equations are obtained by the method of volume averaging and numerically processed by finite volume method. The system was modelled as a vented axisymmetric cavity, partially filled with a porous medium, with hot fluid inflow from the top and outflow at the bottom. The investigation included changing Reynolds number (Ret from 8.3 × 103 to 5 × 104), thermal conductivity ratios (ks/kf from 3.5 to 1062) and thermal capacity ratio (ρscps/ρfcpf from 1483 to 7415). It was found that lower ks/kf ratios decrease the heat loss throughout the charging cycle which allow for higher temperatures in the later stages of the cycle and thus improve charging efficiency. However, this effect decreases in importance as the system undergoes higher Ret number flows. On the other hand, increasing the ρscps/ρfcpf ratio affects the entire cycle, increasing the temperature difference between phases, lowering the velocity in which the solid raises its temperature but storing heat more efficiently. Finally, a design consideration is highlighted as the importance of the ks/kf ratio on the thermal efficiency is predominant at lower Ret flows, whereas as Ret increases, ρscps/ρfcpf becomes the dominant parameter providing efficiency gains.

Natural convection of a hybrid nanofluid affected by an inclined periodic magnetic field within a porous medium

Heat transfer enhancement for various engineering systems can be achieved by the inclusion of metal nanoparticles inside the heat transfer liquid. Such an effect can be improved by considering the hybrid nanofluid when nanoparticles of different materials are added to the base fluid. The present study is devoted to computational analysis of thermal gravitational convection within a porous chamber under the impact of tilted periodic magnetic force. Governing equations formulated employing the single-phase nanoliquid approach, Brinkman-extended Darcy model for transport processes within the porous layer with the local thermal non-equilibrium approach, Boussinesq approximation for the description of the buoyancy force and magnetic field, have been resolved using the Galerkin finite element method. Impacts of the Darcy number, Hartmann number, Rayleigh number, periodicity of the magnetic field, magnetic field inclination angle, thermal conductivity ratio, and medium porosity on flow and thermal patterns have been examined. It has been found that parameters of the periodic magnetic field have the non-monotonic influence of the heat transfer performance.

Effect of Radiation on Hydromagnetic Mixed Convective Flow in a Vertical Channel Filled With Porous Media: A Thermal Nonequilibrium Approach

AbstractThis work addresses the magnetic and radiation effects on the fully developed mixed convective flow in a vertical channel occupied by a porous medium with the thermal nonequilibrium state. The assumption that the fluid is electrically conducted is taken into account and permitted by a uniform transversal magnetic field while the temperature of the wall is changing linearly with the direction of the fluid flow. The spectral collocation technique is used for the numerical solution, whereas the analytical solution is governed for the special case when the drag force F* and the ratio of porosity-scaled thermal conductivity γ are zero. It is observed that, in the buoyancy assisted case, the fluid flow for Ra<102, (Nuf) increased near the wall with increasing the Hartmann number (M). Beyond this when Ra≥102, (Nuf) is decreased with increasing M. It is also perceived that there exists an interval [0,H0] in which (Nuf) increases with increasing M as well as increasing radiation parameter Rd, furthermore beyond the value of H0, Nuf decreasing asymptotically. While for the buoyancy opposed case, the flow separation and inflection point appear in the velocity profile for different values of M, further both the flow separation and inflection point are dying out as M increases. Overall, for the both cases, the magnetic and radiation parameters are stabilizing the flow in the system.

Impact of side injection on heat removal from truncated conical heat-generating porous bed: thermal non-equilibrium approach

Effective removal of heat from the heat-generating porous bed, particularly in a confined space, is essential due to its implications on thermal management and system safety. The safety aspect becomes particularly important during post-accident heat removal from decay heat-generating debris in nuclear reactors, as well as thermal management of self-igniting coal stockpiles. In spite of detailed research on thermal convection through porous media, only a handful of studies have considered mixed convective heat transport involving heat-generating porous bed along with external fluid injection. The situation, however, demands an in-depth analysis due to the associated practical implications. The present work addresses one cooling method providing side injection of cold fluid and assuming a typical conical heat-generating porous bed located centrally within a fluid-filled cylindrical enclosure. The study is carried out in a general way to suit other applications. The dimensionless governing equations, along with the boundary conditions in a two-dimensional coordinate system, are solved numerically assuming laminar and incompressible flow along with the Boussinesq approximation. The analysis is carried utilizing the local thermal non-equilibrium model within the porous bed. Injection of cold fluid can markedly affect the convective heat transport rate. Porous media permeability considerably influences the flow mechanism. The major findings of this study can be very useful in improving the management of thermal energy removal from self-igniting coal stockpiles, grain storages, porous debris, etc. Heat transport intensification from heat-generating porous bed is analyzed by injecting coolant through the sidewall and considering liquid water as the working medium. The impacts of pertinent parameters on the thermal convection characteristics are illustrated using the average Nusselt number and energy flux vectors.

Possible effect of flow velocity on thawing rock-water-ice systems under local thermal non-equilibrium conditions

Climate change causes thawing of many permafrost regions, increasing the rockfall and landslide hazard when the cementing ice melts and slopes become unstable. The thermo-hydro-mechanical processes are strongly coupled and complex to describe in those intrinsic multi-phase systems. So far, thermal models often utilize the local thermal equilibrium approach, assuming that all three phases (solid rock or soil, liquid water and ice) are in thermal equilibrium. While this approach is very tempting due to its easy implementation, there are situations especially during thawing in which temperature gradients exist between the involved phases. In such cases, the local thermal equilibrium approach has to be replaced by the more complex local thermal non-equilibrium approach in which the thermodynamic state of each phase is described by its own temperature. In those models, the heat transfer between the phases is described explicitly in dependence of the heat transfer coefficient and the contact area. Parameterization of the heat transfer coefficient is a challenging task and mostly relies on empirical values and relationships. Most models using the local thermal non-equilibrium approach utilize a constant heat transfer coefficient. However, experiments under geothermal temperature conditions prove that the heat transfer coefficient depends, among others, on flow velocity. This is of special interest as strong dynamics are characteristic for thawing system which impose a unique challenge to heat transfer models. For example, the volume fraction of the liquid domain increases significantly during thawing of rock-ice systems due to the melting of the ice phase. This again, directly influences flow velocity. In this work, the possible effect of a velocity dependent heat transfer coefficient on thawing rock-water-ice systems is investigated. With higher flow velocity, advection effects of the intruding fluid become more significant but also heat transfer is increased. When the intruding liquid water is colder than the surrounding rock, the melting rate is maximum for a specific flow velocity, as advection of cold water and heating from the rock counteract. While this is a purely synthetic study and no experimental data yet exists for validation of the proposed scheme, this study indicates the existence of a way more complex interaction between liquid fluid flow and melting processes as previously expected and way beyond state-of-the-art modeling.

Non-equilibrium natural convection in a differentially-heated nanofluid cavity partially filled with a porous medium

Purpose This study aims to numerically analyze natural convection of alumina-water nanofluid in a differentially-heated square cavity partially filled with a heat-generating porous medium. A single-phase nanofluid model with experimental correlations for the nanofluid viscosity and thermal conductivity has been considered for the description of the nanoparticles transport effect in the present study. Local thermal non-equilibrium approach for the porous layer with the Brinkman-extended Darcy model has been used. Design/methodology/approach Dimensionless governing equations formulated using stream function, vorticity and temperature have been solved by the finite difference method. The effects of the Rayleigh number, Ostrogradsky number, Nield number and nanoparticles volume fraction on nanofluid flow, heat and mass transfer have been analyzed. Findings It has been revealed that the dimensionless heat transfer coefficient at the fluid/solid matrix interface can be a very good control parameter for the convective flow and heat transfer intensity. The present results are original and new for the study of non-equilibrium natural convection in a differentially-heated nanofluid cavity partially filled with a porous medium. Originality/value The results of this paper are new and original with many practical applications of nanofluids in the modern industry.

Transient thermoporoelastic model under local thermal non-equilibrium

Wellbore stability is a key challenge for oil and gas industry since it adds a great deal of additional cost to the industry. Traditional wellbore stability models such as elastic and poroelastic models may not produce reliable stress field around a wellbore since they consider isothermal wellbore condition. During the drilling phase, the drilling fluid temperature is different from the formation temperature due to geothermal gradient and circulation of the fluid inside the wellbore. Therefore, the assumption of isothermal condition will not predict the correct wellbore stability condition, especially for high pressure and high temperature wells. Hence, consideration of a thermoporoelastic model in stress estimation is more appropriate. Although there are several thermoporoelastic models in the literature, they employ the assumption of local thermal equilibrium (LTE) in their modeling. However, thermal properties of phases in a porous medium are rarely similar. This dissimilarity evokes the validation of LTE assumption. The local thermal non-equilibrium assumption ignores additional pore and thermal stresses in the porous medium caused by temperature variations of the fluid and solid phases. Under pure heat conduction, LTE is valid if thermal boundary conditions in a medium is constant or thermal conductivities of phases are similar according to Vadasz (2005) 1. Hence, using a local thermal non-equilibrium (LTNE) approach is required to establish each phase temperature, and update thermoporoelastic model accordingly. In this paper, the effects of induced thermal stresses on wellbore stress resolution and LTNE on thermoporoelastic model around a wellbore during drilling phase are investigated in low and high permeable formations. The LTNE thermoporoelastic model developed herein is a transient model considering conductive and convective heat transfers.

Numerical investigation of rock-pile based waste heat storage for remote communities in cold climates

Remote communities in arctic climates are solely dependent on diesel generators for continuous power supply due to their detached loci from national power gridlines or natural gas pipelines. Moreover, to get along with the harsh, long winters, these communities directly or indirectly depend on fossil-fuel based heating systems. Due to the lower efficiency of the conventional diesel engines, these generators discard a significant amount of heat through the exhaust. Occasionally, during the wintertime, this waste heat from the exhaust is used directly as a heat source after recovery. However, during summer, when the heating demand is lower or nil, this heat is commonly discarded, and its energy potential wasted. A rock-pile based seasonal thermal energy storage is a viable solution that can sustainably resolve this issue. This paper presents a validated numerical study that focuses on the coupling of a waste heat recovery system to a diesel exhaust stream offering a performance assessment of the proposed rock-pile waste heat storage system in a remote, off-grid community located in northern Canada. It examines the impacts of local thermal equilibrium versus non-equilibrium approaches as well as temperature-dependent properties, variation of air mass flow rate, particle size and finally the thermo-physical properties on the proposed model. The presented results show that air should be treated as an ideal gas and that the local thermal non-equilibrium approach should be used for its considerably higher accuracy. Techno-economic assessment of the system resulted in a relatively short payback period of less than six years.