Total Heat Flux Distribution Research Articles

Numerical analysis of steady-state heat conduction behavior in high-temperature superconductor Roebel cable

In this work, the steady-state heat transfer characteristics of REBCO tape and impregnated Roebel cable are studied numerically. By establishing three-dimensional finite element models of tape and impregnated cable, the temperature-dependent equivalent thermal conductivities of REBCO tapes and impregnated Roebel cables are estimated based on numerical homogenization technique. Effects of different geometric and material parameters are discussed. The steady-state heat transfer characteristics of a transposed tape in Roebel cable are further investigated. The equivalent thermal conductivities of REBCO tape or impregnated cable show strong anisotropy. Various sensitivities to geometric and material parameters are observed in equivalent thermal conductivities. The total heat flux distribution of the transposed tape is closely related to the Roebel angle and the curvature radius at outer corner or inner corner of this region. The corresponding structure and components should be optimized in actual application.

Effects of microencapsulated phase change materials on the thermal behavior of multilayer thermal protective clothing

Microencapsulated phase change materials (MPCMs) are increasingly seen as an adequate tool for heat management. The addition of a MPCM layer into a typical firefighters’ protective clothing can be used to mitigate burn injuries, increasing the time to second degree burns. A numerical approach is introduced to simulate the heat transfer in multilayer fabrics with MPCM. An overview of temperature field and total heat flux distribution for the multilayer fabric system with and without the MPCM is presented. Additionally, the effects of MPCM mass fraction, latent heat, specific heat and thickness are investigated to assess the thermal protective performance. The results demonstrate that the addition of a MPCM layer into the multilayer fabric assembly can be used to decrease the temperature obtained at the comfort liner boundary. Effect of heat intensity and radiation distance between the fabric system and heating source is also analyzed and discussed. The findings obtained in this study can provide some useful information on the beneficial design of clothing containing MPCMs.

Geometric optimization of a radiation–conduction heating device using meshless method

A new optimization design methodology which combines direct collocation meshless method with the conjugate gradient method is presented to conduct geometric optimization for heating devices with considering coupled conductive and radiative heat transfer. To avoid the tedious meshing and remeshing of the variable computational domain in the inverse design process, the direct collocation meshless method is adopted to deal with energy conservation equation including conduction and radiation. The modified conjugate gradient method is used to obtain the optimum geometric shape of the heating device. The design requirement of the pre-specified total heat flux distribution on the pre-appointed region of heated surface is satisfied by adjusting the height of the adiabatic reflection surfaces and the geometric shape of the heating surface. A typical test example is studied to illustrate and verify the optimization design methods. The optimizatized design results show that the desired design requirement could be successfully achieved by using the present inverse design methodology.

Conduction–radiation interaction in 3D irregular enclosures using the finite volume method

Interaction of conduction and radiation in 3D enclosures is carried out with a gray participating media. Application of block structured grid is shown with the finite volume method (FVM). Radiation modeling is performed with the FVM and is coupled with an ‘in-house’ code to solve the set of transport equations. The detailed numerical results are presented for a cubical and a cylindrical enclosure as these results are not available in the literature. The numerical simulation for the cylindrical enclosure is performed using a block-structured ‘O’ grid. Two additional geometries are considered in order to show the applicability of the present work. Results of temperature, radiative heat flux and total heat flux distributions are presented for different optical thicknesses, scattering albedoes, emissivities and conduction–radiation parameters. The 3D results are validated with the available 2D results or results with pure radiation problems as limiting cases.

Inverse radiation–conduction design problem in a participating concentric cylindrical medium

Inverse conduction–radiation problem for design analysis in a two-dimensional concentric cylindrical absorbing, emitting and isotropically scattering medium has been solved, when the desired boundary conditions are available on the design surface. The finite-volume method was adopted to deal with energy conservation equation including conduction and radiation. The radiative transfer equation was also taken into consideration in direct problem, whereas the Levenberg–Marquardt method was used to solve a set of equations in inverse problem, which are expressed by errors between estimated and desired total heat fluxes on the design surface. The automatic differentiation as well as the Broyden combined update was utilized to reduce computational time in calculating the sensitivity matrix. The results have shown that the desired total heat flux distribution on design surface could be successfully estimated with less computational time using the present inverse procedure developed here.

Upward Flame Spread: The Width Effect

Previous work has demonstrated upward flame spread on vertical surfaces to be one of the most hazardous fire scenarios. To assess the risk of this scenario, several models have been developed to predict the flame spread rate, relying on empirical correlations of flame height and heat feedback to unburned surface ahead of pyrolysis region. However, the width effect was not regarded particularly in those models but to influence flame thickness, causing the variation of radiation. Therefore, experiment has been designed to access the width effect. Samples used were 6 and 20 mm thick clear PMMA with height of 1000 mm and widths of 100, 300, 500, 700 and 900 mm. Our data showed that the width effect was significant for samples less than 300 mm wide and not significant for 300 to 900 mm wide samples. In addition, the width effect was slight in total heat flux distribution and not obvious in flame height correlation. As to the radiant heat flux distribution, our measurements were much lower than recognized in previous studies

Upward flame spread along PMMA vertical corner walls Part II: Mechanism of “M” shape pyrolysis front formation

Flame spread behavior and the pyrolysis region spread characteristics along polymethylmethacrylate (PMMA) vertical corner walls were studied in detail with an automated infrared (IR) imaging temperature measurement technique. The technique was recently developed for the measurement of transient pyrolysis temperature on both charring and noncharring materials. Temporal isotherms on large PMMA samples were successfully obtained, from which the progress rate of the pyrolysis front was automatically deduced. It was found that the pyrolysis front shape was always M shaped, i.e., no spread along the corner, and the maximum spread is within a few centimeters of the corner. Understanding of the mechanism of the M-shape formation is important in developing a prediction model of the spread rate. Four possible mechanisms, effect of ignition mode, effect of solid phase conduction heat loss, fire-induced flow cooling, and flame displacement effect, were identified. Four different experiments were designed to test each mechanism, among them the flame displacement effect, which causes a large heat loss in a nonflammable gas layer due to a poor mixing of pyrolysis products and air was found to be the principal mechanism. For an upwardly spreading fire, total heat flux distributions above the M-shape pyrolysis peak were measured by a Gardon-gauge heat flux meter, and visible flame height and pyrolysis front height were respectively measured by a video camera and the IR technique.

Heat-transfer measurements in a cylindricaltest furnace

An oil-cooled, gas-fired cylindrical test furnace was constructed to investigate the radiative and total heat-flux distributions at the circumferential sink wall. The furnace was instrumented such that flow patterns, gas-concentration profiles, temperature distributions, and radiative and total heat-flux distributions could be measured directly. Some typical examples of each of the above measurements are presented and discussed. The radiative heat-flux distribution at the sink wall has been calculated by dividing thefurnace enclosure into a number of uniform elements and simulating the radiative transfer by the Monte Carlo Method. The distributions calculated on the bases of measured temperatures within the furnace, or a measured flow pattern and gas-concentration patterns within the furnace, were found to agree well with the experimental data.