Non-uniform Thermal Boundary Conditions Research Articles

Free Convection in a Square Ternary Hybrid Nanoliquid Chamber with Linearly Heating Adjacent Walls.

In this study, mathematical modeling of the energy transfer and flow characteristics of ternary nanoliquid in a square enclosure is performed. In the cavity considered, the left and bottom borders are warmed uniformly or non-uniformly when the rest of the borders are cooled. The robust finite element method with quads and triangles as elements is used to work out the control equations of the problem. The current study is validated against previously published works, and good agreement is shown. The isolines are investigated for various Rayleigh numbers at uniform and non-uniform thermal boundary conditions. The impact of ternary hybrid nanofluids on the mean Nusselt number at hot borders is explored in dependence on the Rayleigh number and nanoparticle concentration. A comparative study of different fluids for the mean Nusselt number at heated borders is also conducted and analyzed with appropriate graphs and tables. It has been shown that ternary nanofluids can be more effective compared to mono- and hybrid nanofluids, with a more essential growth of the energy transport rate with nanoadditives concentration.

Thermal analysis of thermal barrier coating systems under transient and time harmonic thermal loads

In this study, the temperature fields in a thermal barrier coating system (TBC) under various types of thermal loadings, including steady and transient thermal loads, is investigated. TBCs are exposed to a variety of harsh thermal loadings, including thermal shock and periodic thermal loads. Therefore, the need to achieve the temperature distribution in different positions of the TBCs, especially in the protective coating is very crucial. Non-uniform Thermal boundary conditions are considered for both external temperature and heat flux distributions. The coating and substrate are not stationary at the onset of transient thermal load. Using the solution of steady-state problem as initial condition, the transient temperature fields within the coating-substrate system is obtained. The dual-phase lag (DPL) heat conduction is utilized to scrutinize the transient response of the TBC system. Fourier and Laplace integral transforms have been used to solve the temperature field equations. The effect of concentrated thermal shock and steady harmonic thermal load on the temperature fields of coating and substrate is investigated. The results demonstrate that the steady state and transient temperature fields strongly depend on the type of thermal load, the thickness and thermal properties of the coating, and the loading parameters. The results indicate that the effect of concentrated thermal shock is felt after at least 8.33 s at distance 1.2 cm far from the top surface of ceramic coating. The effect of concentrated thermal shock is eminent adjacent to point (x=0,y=0) of the free boundary of coating, but after a longitudinal distance of 2 cm drastically reduced. The ceramic coating is able to relieve the amplitude of temperature oscillations up to 1280C at a vertical distance of 3mm from the free boundary of coating.

Hydromagnetic Natural Convection Heat Transfer of Copper-Water Nanofluid within a Right-Angled Triangular Cavity

This investigation on free convection flow and temperature transfer within a right-angled triangular cavity loaded uniformly by Cu-H2O nanofluid including heated boundary conditions at horizontal side is performed numerically. The standing side is cooled at low heat while the hypotenuse of the triangular is insulated. The governing non-dimensional highly non-linear partial differential equations are performed by employing Galerkin weighted residual finite element method. The simulated numerical findings are exhibited using streamline contours, isotherm contours and average Nusselt number for the sampling parameters named nanoparticles volume fraction, Rayleigh number, and Hartmann number. The outcome demonstrates temperature transfer value reduces for the enhancement of Hartman number whereas improve significantly for the increase of buoyancy driven parameter Rayleigh number. Also, an excellent average temperature transfer is observed for uniform heated boundary condition (case I) compared to non-uniform thermal boundary conditions (case II & case III).

Quasi-Static Thermal Modeling of Multiscale Sliding Contact for Unlubricated Brush Seal Materials

Prediction of contact temperature between two materials in high-speed rubbing contact is essential to model wear during unlubricated contact. Conventionally, assumptions of either a steady or an annular heat source are used for slow and high speed rotation, respectively. In this paper, a rotating heating source is solved using an in-house finite element method code. This captures the full geometry and rotating speed of the rubbing bodies. Transient heat transfer is modeled quasi-statically, eliminating the need for a transient 3D simulation. This model is shown to be suitable for contact temperature prediction over a wide range of rotating speeds, anisotropic thermal conductivity, and nonuniform thermal boundary conditions. The model calculates heat partition accurately for a thin rotating disk and short pin combination, which cannot be predicted using the existing analytical solutions. The method is validated against ansys mechanical and experimental infrared thermography. Results demonstrate that the annular source assumption significantly underpredicts contact temperature, especially at the rubbing interface. Explicit modeling of a thin disk results in higher heat partition coefficients compared with the commonplace semi-infinite length assumption on both static and rotating components. The thermal anisotropy of tuft-on-disk configurations is evaluated and compared to a uniform pin-on-disk configuration. Despite the effective thermal conductivity (ETC) in the bristle tuft being approximately 1 order of magnitude lower than along the bristle length (treating the bristle pack as a porous medium), its impact on heat partition and contact temperature is shown to be limited.

Thermal and mechanical analysis of a sodium-cooled solar receiver operating under a novel heliostat aiming point strategy

The nature in which a solar receiver in a concentrated solar power plant interacts with an accompanying heliostat field plays a significant role in plant performance and economics. An appropriate heat flux distribution should help deliver maximum receiver thermal performance, while minimising mechanical damage – thereby maximising power production and reducing costs. The current work presents an investigation into the thermal performance and mechanical reliability of a sodium-cooled solar receiver operating under heat flux profiles generated by a novel heliostat aiming strategy. A modification of the HFLCAL model is used to generate heat flux profiles for individual heliostats in a representative plant, and simulated annealing optimisation techniques are used to produce a novel heliostat aiming strategy. The importance of giving consideration to receiver limitations under non-uniform thermal boundary conditions in the development of a heliostat aiming strategy is demonstrated in this study, with mathematical optical, thermal, and mechanical models used to complete the analysis. An investigation has been conducted for a point-in-time resulting in maximum thermal loading conditions, with theoretical modelling techniques used to calculate receiver tube temperatures for aiming strategy yielded heat flux profiles, thereby allowing for the determination of heat losses and mechanical reliability through creep-fatigue damage. Results show that the simulated annealing algorithm can significantly improve heat flux homogeneity on the receiver, potentially reducing peak heat flux to less than 10% that of a single aiming point strategy, given an appropriate spillage allowance and aiming point grid size. A satisfactory configuration of spillage allowance and aiming grid size exists so as to supply maximum power to the receiver, while uniformly distributing the incident heat flux in order to meet mechanical reliability requirements. Based on the receiver design and conditions simulated in the analysis, a grid constructed of more than 81 aiming points (receiver area coverage of 32.7%), and an additional spillage allowance of 10% allows the receiver to deliver maximum power output while retaining mechanical durability through a 30 year plant life cycle.

Natural Convection in a Non-Uniformly Heated Vertical Annular Cavity

Natural convection from the linearly heated inner and/or outer walls of a vertical annular cavity has been numerically investigated. The bottom wall is uniformly heated and top cylindrical wall is thermally insulated. In this analysis, we considered two different thermal boundary conditions, namely case (I) and case (II) to understand the effect of non-uniform heating of inner and/or outer walls on the convective flow and subsequently the local and global heat transfer rate. For case (I), the inner and outer walls are heated linearly, while the linearly heated inner wall and cooled outer wall is considered in case (II). An implicit finite difference scheme is applied to solve the model equations of the problem. The numerical simulations in terms of streamlines and isotherms, local and global Nusselt numbers are presented to illustrate the effects of Rayleigh number and non-uniform thermal boundary conditions for a fixed Prandtl number of Pr = 0.7.

The Effects of Nonuniform Thermal Boundary Condition on Thermal Stress Calculation of Water-Cooled W/Cu Divertors

The thermal boundary condition has very important effects on the accuracy of thermal stress calculation of a water-cooled W/Cu divertor. In this paper, phase-change heat transfer was simulated based on the Euler homogeneous phase model, and local differences of liquid physical properties were considered under one-sided high heating conditions. The steady-state temperature field and thermal stress field under nonuniform thermal boundary conditions were obtained through numerical calculation. By comparison with the case of traditional uniform thermal boundary conditions, the results show that the distribution of thermal stress under nonuniform thermal boundary conditions exhibits the same trend as that under uniform thermal boundary conditions, but is larger in value. The maximum difference of maximum von Mises stress is up to 42% under the highest heating conditions. These results provide a valuable reference for the thermal stress calculation of water-cooled W/Cu divertors.

The effect of thermal gradients on the performance of lithium-ion batteries

An experimental apparatus is described, in which Peltier elements are used for thermal control of lithium-ion cells under isothermal and non-isothermal conditions, i.e. to induce and maintain thermal gradients. Lithium-ion battery packs for automotive applications consist of hundreds of cells, and depending on the pack architecture, individual cells may experience non-uniform thermal boundary conditions. This paper presents the first study of the impact of artificially induced thermal gradients on cell performance.The charge transfer resistance of a 4.8 Ah is verified to have a strong temperature dependence following the Arrhenius law. Thermal cycling of the cell, combined with slow rate cyclic voltammetry, allows to rapidly identify phase transitions in electrodes, due to the thermal effect of entropy changes. A cell with a temperature gradient maintained across is found to have a lower impedance than one held at the theoretical average temperature. This feature is attributed to details of the inner structure of the cell, and to the non-linear temperature dependence of the charge transfer resistance.

Effect of the Temperature Difference Aspect Ratio on Natural Convection in a Square Cavity for Nonuniform Thermal Boundary Conditions

The present numerical investigation deals with steady natural convection flow in a closed square cavity when the bottom wall is sinusoidal heated and vertical walls are linearly heated, whereas the top wall is well insulated. In the nonuniformly heated bottom wall maximum temperature TH attains at the center of the bottom wall. The sidewalls are linearly heated, maintained at minimum temperature Tc at top edges of the sidewalls and at temperature Th at the bottom edges of the sidewalls, i.e., Tc≤Th≤TH. Nonlinear coupled PDEs governing the flow have been solved by the penalty finite element method with biquadratic rectangular elements. Numerical results are obtained for various values of Prandtl number (Pr)(0.01≤Pr≤10) and temperature difference aspect ratio A=[(Th−Tc)∕(TH−Tc)](0≤A≤1) for higher Raleigh number Ra=105. Results are presented in the form of streamlines, isotherm contours, local Nusselt number, and the average Nusselt number as a function of temperature difference aspect ratio A. The overall heat transfer process is shown to be tuned efficiently with suitable selection of A.

Discrete Green’s Function Measurements in Internal Flows

The discrete Green’s function (DGF) for convective heat transfer was measured in a fully developed, turbulent pipe flow to test a new technique for simple heat transfer measurement. The 10×10 inverse DGF, G−1, was measured with an element length of approximately one pipe diameter and Reynolds numbers from 30,000 to 100,000 and compared to numerical predictions using a parabolic flow solver called STANTUBE. The advantages of using the DGF method over traditional heat transfer coefficients in predicting the thermal response for flows with nonuniform thermal boundary conditions are also demonstrated.

Practical Experience With the Discrete Green’s Function Approach to Convective Heat Transfer

The heat transfer from a short uniform heat flux strip beneath a turbulent boundary layer with and without freestream turbulence was measured using a liquid crystal imaging technique. Freestream turbulence intensities were on the order of 12 percent. Data were taken at momentum thickness Reynolds numbers on the order of 1000 and 2000 for the turbulent and steady freestreams, respectively. Heat transfer enhancement due to the presence of freestream turbulence was quantified in terms of the ratio of the average St’s on the strip: turbulent freestream divided by steady freestream. Compared to the baseline case of a uniformly heated surface upstream of the strip, the heat transfer enhancement decreased by 20 percent. The temperature distribution measured on and downstream of the heated strip represented one column of a discrete Greens function that was used to predict the heat transfer for any arbitrarily specified thermal boundary condition given the same flowfield. Predictions are compared against correlations and numerical predictions as well as data from the literature. The details and practical applications of this approach to handling heat transfer with non-uniform thermal boundary conditions are presented.

What's new in convective heat transfer?

Abstract This paper describes the quest for invariant descriptors of the convective process, and the new requirements those descriptors put on quantitative full-field thermal imaging. There is increasing need, in applications, for heat transfer descriptors which can deal with non-uniform thermal boundary conditions, including those induced by conjugate effects. This paper discusses two approaches which have arisen within the past 10 years: (1) the use of h adiabatic and T adiabatic to describe the convective process and more recently, (2) the emergence of discretized Green's functions for convection. Both of these approaches acknowledge the effects of upstream heat transfer on local behavior but both do so using coefficients which, themselves, are invariant with respect to changes in the thermal boundary conditions. Thus measurements made in the lab can be applied in the field, under different thermal boundary conditions. Both approaches can be used in complex flow fields, such as flows on surfaces with obstructions. To realize the full potential of either approach, the uncertainties in full-field optical data acquistion techniques must be reduced by about a factor of 3.

Forced Convection From a Nonisothermal Cylinder in Crossflow

When nonuniform thermal boundary conditions are imposed on the surface of a circular cylinder in crossflow, the heat transfer characteristics can be quite different compared to what is found for isothermal or constant heat flux boundary conditions. In the present analysis, two kinds of nonuniform boundary condition along the circumference of the cylinder are considered in a uniform stream of air: step changes and linear profiles. To cite only one motivating application, step changes in temperature can arise on the surface of an external, cylindrical, solar central receiver. As the working fluid (water) flows through the vertical tubes that ring the circumference of Solar One (a solar central receiver in Barstow, CA), the solar flux on the receiver heats the water from a liquid to a superheated state. In this process, portions of the receiver panels, and thus portions of the circumference of the cylinder, function as a preheater, boiler, or superheater. Hence the surface temperature can vary significantly around the cylinder. Common engineering practice has been to use an average wall temperature with an isothermal cylinder heat transfer coefficient when estimating the convective loss in these kinds of situation.