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International Journal of Thermal Sciences

International Journal of Thermal Sciences

Archives Papers: 1,443
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Tunable nonreciprocal thermal emitter based on metal grating and graphene
Jun Wu; Feng Wu; Tiancheng Zhao; Xiaohu Wu;
Keywords:Grating;Graphene;Nonreciprocal emitter;Kirchhoff's law
Abstracts:Various nonreciprocal thermal emitters, which provide opportunities for higher energy conversion efficiency, have been proposed to completely violate the traditional Kirchhoff's law. However, the tunable nonreciprocal thermal emitters remain barely investigated. In this paper, by sandwiching a graphene monolayer between a top metallic grating and a bottom magneto-optical film backed with a metallic mirror, we achieve tunable nonreciprocal radiation effect. It is shown that strong nonreciprocal radiation for graphene under initial state is realized at wavelength around 14.845 μm when the incident angle is 30° and the external magnetic field is 3 T. The physical origin behind such phenomenon is disclosed by investigating both the distribution of magnetic field at the resonant peak and the coupled-mode theory. Besides, the nonreciprocal radiation performance remains perfectly within certain range of structure parameters, which is benefit for practical fabrication and applications. More importantly, the strong nonreciprocal radiation of the proposed scheme can be flexibly tuned by the gate voltage of graphene without redesigning and refabricating the structure. We believe that this work will provide new approaches for the design of novel energy harvesting systems and nonreciprocal thermal emitters.
Modal and absolute thermal instability in a vertical porous layer
A. Barletta; M. Celli;
Keywords:Porous medium;Linear stability;Convection;Wavepackets;Permeable boundaries;Vertical layer;Modal instability;Absolute instability
Abstracts:The conduction regime in a vertical porous layer subject to a horizontal temperature gradient is studied. The boundaries are considered as isothermal, with different temperatures, and permeable to the external environment. The linear stability of this basic flow state is analysed by testing the dynamics of the normal modes of perturbation. The numerical solution of the stability eigenvalue problem leads to the determination of the neutral stability condition. Then, the evolution in time of localised wavepacket perturbations is investigated leading to the determination of the threshold to absolute instability.
Quantitative validation of an in-flow water condensation model for 3D-CFD simulations of three-way junctions using indirect condensation measurements
J. Galindo; R. Navarro; D. Tarí; F. Moya;
Keywords:Condensation measurements;Computational fluid dynamics;Mixing streams;Model validation;Low-pressure EGR;Enthalpy balance
Abstracts:The usage of three way junctions to merge fluid streams is widely extended. For certain applications, such as refrigeration systems or internal combustion engines, the mixing of humid gaseous flow leads to bulk condensation, which compromises the integrity of the downstream elements. In this work, a test bench is adapted to manage the mixing of wet streams and a novel experimental technique is developed to measure condensation indirectly. Well-resolved temperature distributions are measured by means of a rotating array of thermocouples at experiments with and without humidity. Enthalpy balances using temperature distributions of both cases allow to infer the condensation mass fraction field. 3D CFD simulations with an in-flow condensation sub-model are compared with these measurements for two junction geometries and two operating conditions, with an average agreement of 11% in terms of condensation mass flow rate. The three-way junction design and its ability to reduce mixing is found to be of paramount importance to reduce bulk condensation. This validated model is therefore suitable for optimizing the junction geometry in terms of condensation reduction. With limited water condensation, NOx, CO2 and particulate matter emissions can be strongly abated for internal combustion engines by extending the usage of low-pressure exhaust gas recirculation to cold conditions.
Optimization of convective drying performance of multiple porous moist objects in a 3D channel
Fatih Selimefendigil; Seda Ozcan Coban; Hakan F. Öztop;
Keywords:Convective drying;Three dimensional;Porous moist;Optimization;Unsteady CFD;Neural networks
Abstracts:In this study, a procedure for optimizing the convective drying performance of multi porous moist objects in a three dimensional channel is proposed. The numerical simulation is performed by using the finite element method and COBYLA optimization algorithm is used to find the optimum spacing between the objects without mass transfer in the first stage. Then, heat and mass transfer equations for the porous moist objects are coupled with the channel flow equations at the optimum spacing which delivers the best convective drying performance. It is observed that the flow recirculation and flow reversal in the inter-spacing with various distances between the objects resulted in thermal gradient variations along the multi object surfaces. The average Nusselt number rises for second block while it shows non-monotonic behavior for the first block when the distance between the first and second group objects are varied. Distance between the second and third objects also affected the average Nu variation for all of the objects. The lateral distance between first and second group objects resulted in up to 50% variation in the average Nu for the second block. The optimum spacing between the objects for the maximum Nusselt number of the objects are obtained as d 1=5.93h c, d 2=7h c and d 3=0.584h c. The moisture reduction amounts for each of the object at the optimums spacing are found higher as compared to parametric variation of unsteady simulation results. The computational cost for the parametric unsteady coupled heat and mass transport equations in the channel and in the porous moist objects is 75 h 12 min while the optimization assisted simulation results reduced the computational cost to 2 h 33 minutes. Also, artificial neural networks are utilized to obtain the dynamic feature of convective drying at the optimum spacing considering various values of hot dry air temperature which delivers fast and accurate prediction results when compared to high fidelity computational fluid dynamics simulation results.
Numerical simulation on thermal performance of vacuum insulation panels with fiber /powder porous media based on CFD method
Lei Ni; Zhaofeng Chen; Phalguni Mukhopadhyaya; Xinyang Zhang; Qiong Wu; Qianhua Yu; Ganhua Miu;
Keywords:Vacuum insulation panel;Fiber/powder porous media;Effective thermal conductivity;Numerical simulation
Abstracts:Up to now, the fiber/powder porous media is getting more and more attention as a new development direction in the field of vacuum insulation panels. In this paper, the random generation-growth method is used to generate the structure of the fiber/powder porous media. The numerical simulation method is used to simulate the thermal conductivity of the fiber/powder porous media based on Fourier's law to study the correlation between the effective thermal conductivity and the microstructure of the fiber/powder porous media under vacuum conditions. The model was verified with experimental and theoretical data. The effect of different microstructure parameters such as powder diameter and fiber orientation angle on the effective thermal conductivity of the fiber/powder porous media is introduced in detail. The results show that the finer the diameter of the powder leads to the smaller the pore size of the fiber/powder porous media and has the stronger the ability to maintain a lower effective thermal conductivity under higher pressure. In addition, the more inconsistent between the fiber axial direction and the heat transfer direction, the better the thermal insulation performance. The results are very important for the optimization of the core material of the vacuum insulation panel.
Numerical investigation and implementation of the Taguchi based entropy-ROV method for optimization of the operating and geometrical parameters during natural convection of hybrid nanofluid in annuli
Suhail Ahmad Khan; Mohammad Altamush Siddiqui; Zahid A. Khan; Mohammad Asjad; Shahid Husain;
Keywords:Annuli;Hybrid nanofluid;Design of experiments;Entropy method;ROV method;Multi-response optimization
Abstracts:Numerical analyses have been carried out to investigate the effect of geometrical and operating parameters on heat and fluid flow of Cu–Al2O3/water Hybrid nanofluid inside annuli. The simulations are performed over a range of aspect ratio (200–500), orientation angle (30–90°), heat flux (2500W/m2-10,000W/m2), and volume fraction of nanoparticles (0–0.045) as controllable parameters. As a novelty, the study is further extended through Multi-response optimization using an entropy-based Range of Value (ROV) method to obtain optimum values of controllable parameters. The four output responses viz. heat transfer coefficient (h), Nusselt number (Nu), rate of mass flow (ṁ), and Reynolds number (Re) were computed and recorded using Taguchi's L16 orthogonal array. The obtained results illustrate that the heat transfer coefficient and mass flow rate increases with aspect ratio, inclination, heat flux, and nanoparticle concentration. The optimum grouping of the controllable factors which maximizes output responses are A4B4C4D4 (i.e.: Aspect ratio = 500, inclination = 90°, Heat flux = 10,000W/m2 and volume fraction of nanoparticles = 0.045). It is also inferred that the inclination has the maximum influence on the multi-performance characteristics followed by heat flux, aspect ratio, and volume fraction.
Transport processes for a bubble entrapment during horizontal solidification
P.S. Wei; P.Y. Lin;
Keywords:Thermal convection;Solute convection;Pore shape;Bubble entrapment;Phase field modelling;Horizontal solidification;Porous materials
Abstracts:Transport processes coupling with shape development of a bubble captured by a solidification front advancing in a horizontal direction are numerically studied. Porosity in solid plays important roles on microstructure of materials and functional materials in biology, tissue, MEM, micro- or nano-engineering, foods, and geophysics, etc. In view of significant force torque, pore shape development during horizontal solidification is more significant than that during vertical solidification. In this work, transport equations of fluid flow, energy and concentration with sources governing interfacial balance of momentum, energy and concentration in the presence of a bubble entrapped by the solidification front were solved by the commercial COMSOL computer code. The results find the pore shape development affected by velocity, pressure and concentration fields for different gravity forces and flow boundary conditions. Prediction of contact angle agrees with that from Abel's equation during solidification, previously confirmed by experimental data. Controlling pore formation in solid via selecting different gravitational acceleration or ambient pressure and flow boundary conditions can therefore be achieved.
Estimating thermal properties of phase change material from heat flux measurements
Elodie Courtois; Patrick Glouannec; Anthony Magueresse; Tahar Loulou;
Keywords:Latent heat;Effective specific heat;PCM material;Numerical simulation;Experimental analysis;Thermal conductivity;Inverse analysis
Abstracts:An inverse analysis is applied to estimate the effective heat capacity and thermal conductivity of phase change material ( PCM) as function of temperature. A sequential work in two distinct parts was adopted here which consists in the estimation of the thermal properties of the PCM in the solid and liquid states in a first step and completed by the characterization of the phase change in a second step. The effective heat capacity is judiciously parameterized as temperature dependent function to take into account the phase change phenomenon and its two solid and liquid phases. An experimental setup was built to collect the heat fluxes and temperatures histories around and inside a one dimensional sample of PCM. First the experimental data were used to estimate thermal conductivity and specific heat both at solid and liquid state and later combined with the developed inverse analysis to estimate the specific heat function over the phase change transition. Obtained results are compared to those obtained with the DSC facilities and an acceptable agreement between the two approaches is observed. The experiment was found to be well designed and the collected measurements trustworthy and complementary to handle this important estimation problem.
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