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Abstract This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets. Graphical abstract Twin-jets flow configuration. (a) Perpendicular (b) Divergent (c) Parallel (d) Convergent. Image 1 Highlights • Four types of flow patterns of impinging twin jet are studied by Large Dissipation Simulation. • Nusselt number is enhanced for all types of inclination for increasing Reynolds number. • Averaged Nusselt number of the perpendicular impingement gives better heat transfer than that of the oblique jets. • The poor heat transfer is obtained for the parallel oblique jets. • For the same angle, divergent jets give smallest heat transfer than the convergent jets. [ABSTRACT FROM AUTHOR]

It is difficult to alter the thermal performance of a minichannel heat sink with a fixed structure under constant operating conditions. In particular, if the local heat transfer of the substrate cannot be regulated in real time, the dynamic heat dissipation needs of high-power electronic components cannot be met in the electrical servo systems of flight vehicles. In this paper, a new minichannel heat sink structure based on the classical rectangular channel is proposed and a piezoelectric oscillating fin (POF) is introduced to replace the fixed fin. The thermal performance of the heat sink can then be regulated using the swing of the POF, with supercritical carbon dioxide (S–CO 2) as the coolant. Transient numerical simulations are applied to investigate the influence of various POF operating parameters on the heat transfer of the channel, such as the oscillation amplitude and frequency. The results demonstrate that a channel with a POF can allow for dynamic and rapid adjustment of the thermal performance through control of the oscillation frequency and amplitude of the POF, with a constant channel configuration, coolant, and operating conditions. The heat transfer can be enhanced significantly by the swing of the POF, with the local Nusselt number growing up to 8.3 times and the overall one up to 1.14 times. Both the oscillation frequency and amplitude have a positive correlation with the intensity of heat transfer. The POF also contributes to a greater pressure drop and fluctuation in pumping power. A comprehensive evaluation coefficient shows that the POF is effective at improving the overall thermal performance with a low-frequency swing, while high-frequency operation enhances the local heat transfer and decreases the local temperature near the tail end of the POF. Finally, considering the effects of the frequency and amplitude of the POF, as well as the variable thermophysical properties of the supercritical coolant, a heat transfer correlation based on the Dittus–Boelter equation is established to predict the thermal performance of the channel with a POF. This prediction has a maximum error of ±0.56%. • A minichannel introducing piezoelectric oscillating fin (POF) to replace the fixed fin is proposed. • The thermal performance of the minichannel heat sink is regulated using the POF and S–CO 2. • The influence of oscillation frequency and amplitude of the POF on heat transfer is investigated. • A heat transfer correlation for POF channel based on the Dittus–Boelter equation is established. [ABSTRACT FROM AUTHOR]

This paper deals with the analytical and experimental analysis of heat transfer in the xylem of trees and the determination of stem water content (WC). The aim of this paper was to derive and verify a new approach to WC calculation from measured temperature differences around a continuously heated needle. The configuration of heater and thermocouples of the heat field deformation (HFD) method for sap flow measurements was taken for this purpose. The suggested formula is derived from the heat conduction equation with a continuous linear source of heating. The ability of the HFD method to non-destructively determine the radial distribution of the local conditions of sap flow measurements under zero flow (referred to as K-value) was used to derive the radial distribution of WC. Examples of the application of our method on several contrasting species with different plant hydraulic architecture are given where calculated WC along stem xylem radius was compared with its direct gravimetrical measurements or with relative determination of wetness of cross-sectional stem discs using a modified differential translucence method. The proposed method enables water content measurements during continuous linear heating and is important for information about WC and sap flow together per day or season when there is possible to extrapolate the temperature difference for zero flow condition. [ABSTRACT FROM AUTHOR]

The paper deals with the study of the heat transfer phenomena within a vertical enclosure containing PCM integrated inside a test cell with the dimension of (0.5, 0.5, 0.5 m 3 ). An experimental framework was conceived and installed in the Thermal Processes Laboratory in Borj Cedria, Tunisia. The experimental investigation was carried out by measuring the temperature and the heat fluxes exchanged throughout the PCM vertical enclosure. It was found that during the heating phase the temperature inside the test cell equipped with the PCM vertical enclosure is about 28 °C. It was also found that once the PCM vertical enclosure is omitted the cell temperature inside the test cell ranges between 29 and 40 °C. A complete model was also formulated by taking into account various modes of heat transfer inside the PCM vertical enclosure. The numerical model was performed by following the temperature changes at the bottom position and at the high position of the PCM vertical enclosure. The numerical model followed also the evolution of the melting front during the melting process. The model was then exploited for the evaluation of the melting front evolution during the charging and the discharging processes. Another TRNSYS simulation was achieved to evaluate the feasibility of integrating the PCM vertical enclosure in a real building envelop. The results of the TRNSYS simulation showed that the integration of a well dimensioned PCM vertical enclosure enhances the thermal comfort of the occupant by reducing the thermal fluctuation and by improving the thermal inertia of the walls. [ABSTRACT FROM AUTHOR]

Current heat transfer systems in the industry may result in problems such as low efficiency and environmental pollution. In this paper, the influence of heater power, speed and direction of rotation cylinders, and underfloor heating power are investigated on the Nusselt number in a water-filled enclosure with Pr = 5.4 numerically and experimentally based on the Arduino control unit. The enclosure is under constant heat flux from the bottom wall. Using the mixed-Taguchi method and according to different considered cases, 36-state experiments were designed in the range of 2 × 10 8 < R a < 8 × 10 8 and R e < 4000. The problem was simulated by using the Realizable k-ε turbulence model, in 3D steady-state mode. Isotherms and streamlines were reported for each case and analysis revealed that the speed of rotation and the heater power has a more important role than other parameters on the heat transfer rate. In the best situation with a maximum speed of cylinders (S = S (max)), maximum floor heating system ( q f ˙ = q f ˙ (max)) , and maximum power of the heater ( q r ˙ = q r ˙ (max)) the Nusselt number is 110.5 with an almost 22% increase in heat transfer rate. A polynomial surf fit for Nusselt number according to Re, Ra proposed and there are three parts to the variation range of Nusselt numbers based on Richardson numbers. [Display omitted] [ABSTRACT FROM AUTHOR]

Despite a variety of techniques available for thermal conductivity measurement, there is a lack of methods to directly measure the radial thermal conductivity of a cylinder. This presents a critical gap in heat transfer metrology, particularly in presence of anisotropic thermal conduction, such as in a Li-ion cell, where radial thermal conductivity plays a key role in determining performance and safety. This paper reports a technique for measurement of radial thermal conductivity of a cylinder by accounting for variable heat flux into the cylinder when heated on the outside. It is shown that heat flux into the cylinder from a thin heater wrapped around its surface varies significantly with time. This variation, which was neglected in past work, is accounted for by developing a variable heat flux model for experimental data analysis. For two different materials, measurements of radial thermal conductivity using this approach are shown to be in close agreement with standard thermal conductivity measurement using the transient plane source method. Radial thermal conductivity of a 26650 Li-ion cell is measured to be 0.39 W/mK. Besides contributing a new approach for thermal metrology in cylindrical systems, this work also improves the understanding of thermal phenomena in Li-ion cells. [ABSTRACT FROM AUTHOR]

This paper reports the results of an experimental investigation of the heat transfer performance of a phase change material (PCM) based composite 72 pin fin heat sink subjected to individual heat loading of 4 discrete heaters of equal area. The investigation is then followed by multi objective optimization of the individual heat inputs, wherein the total power of the heaters is fixed. The PCM used is n-eicosane and the heat sink is made of aluminium. First, in house experiments are conducted for different combinations of power. The hotspots imposed by spatially non uniform heat flux are seen to have a considerable effect on the melting and solidification cycle and hence on the thermal performance of the heat sink. The diversity in the performance of heat sink motivates us to perform a multi objective optimization using Non-dominated sorting genetic algorithm (NSGA-II) to determine the optimum combination of the discrete power levels, that stretches the charging period and minimizes the discharging period of the heat sink simultaneously. The solutions obtained thus obtained are validated by conducting in house experiments. This study establishes the fact that discrete heating scenarios have to be considered in the future optimization of the heat sinks. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical study of heat transfer from spheres settling under gravity in a box filled with liquid. Three-dimensional Lattice Boltzmann Method is applied to simulate fluid-particle interaction. Firstly the developed numerical model is validated in comparison with experimental and numerical data for a falling sphere in a box filled with liquid. Then the effects of Reynolds, Prandtl and Grashof numbers (Re, Pr, Gr) are investigated for a settling particle at fixed/varied temperature. The time variations of velocity, height and Nusselt number of the settling particle are also investigated. The results depict that the maximum settling velocity of the particle at varied surface temperature is higher at lower Reynolds numbers. As the Reynolds number increases, the settling velocity of the particle for both cases of constant and varied temperature is the same. However, the Nusselt number of the particle at varied temperature is lower compared with that for the particle at constant surface temperature. Increasing the Grashof number leads to slower settling and larger average Nusselt number. Finally sedimentation of 30 hot spherical particles in an enclosure and their hydraulic and heat transfer interactions with the surrounding fluid are studied. The results depict how the presence of heat transfer phenomena can significantly alter the behavior of settling particles. [Copyright &y& Elsevier]

Abstract: Fluid flow with phase change heat transfer in a three-dimensional porous channel with asymmetrically heating from one side is numerically studied in this paper. The “modified” Kirchhoff method is used to deal with the spatial discontinuity in the thermal diffusion coefficient in the energy equation. The velocity and temperature fields, as well as the liquid saturation field on the heated section of the wall with different Peclet and Rayleigh numbers are investigated. The results show that the liquid flow bypasses the two-phase zone, while the vapor flows primarily to the interface between the sub-cooled liquid zone and the two-phase zone. An increase in the Peclet number decreases the two-phase region while an increase in the Rayleigh number helps to spread the heat to a larger region of the domain. The distribution of the liquid saturation on the heated section of the wall indicates that the minimum liquid saturation increases with the increase of both the Peclet and Rayleigh numbers. [ABSTRACT FROM AUTHOR]

Abstract: The transient behavior of two-phase flow and heat transfer in a channel filled with porous media was numerically studied in this paper. Based on the two-phase mixture model, numerical solutions were obtained using the Finite-Volume Method (FVM). Two methods to treat the discontinuous diffusion coefficient in the energy equation, i.e. the harmonic mean method and the “modified” Kirchhoff method were compared. It was found that the “modified” Kirchhoff method was better in dealing with the rapid change in the diffusion coefficient. Three different cases, with discrete heat flux applied at (1) the upper wall, (2) lower wall and (3) both the upper and lower walls were studied. The velocity and temperature fields for these cases were discussed. The results show that the liquid and vapor flow fields, as well as the temperature and liquid saturation fields have distinctly different features with the change in heating location. An analysis of the vapor volume fraction indicates that the largest amount of vapor with the highest vapor generation rate was for the case in which the heat flux is applied from the lower wall. [Copyright &y& Elsevier]

New heating technologies are constantly being developed worldwide, specially the electrical ones that take advantage of renewable energy. In this paper, the Basic Cell of Energy Transference (BCET) is proposed as an innovative fluid heater, carrying a microwave-fed heat transfer plate for thermal contact. A fully-dimensional thermo-fluid analysis was implemented and validated to determine the key design parameters and operation features for heat transfer to temperature-sensitive working fluids. Circulation patterns were observed, when using certain fluids, in turn causing strong temperature non-uniformities. As fluid treatment in the heater relies on the thermal contact at its active plate, the model was used to ascertain the undesired excess/lack temperature range for quality/safety treatments, with reference to a final effective process temperature. Therefore, a geometry optimization by means of internal baffles was carried out which ensured variation to fluid pattern and more uniform active plate temperature. In a base case, the new design allowed to limit the uncontrolled temperature excess by almost 30%, while favouring the pressure drop reduction across the flow device by more than 10%. [ABSTRACT FROM AUTHOR]