The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow is numerically studied in order to improve the understanding of the complex heat transfer and optimum design of the combustor. The heat transfer performance of a porous media combustor strongly depends on the thermophysical properties of the porous material. In order to explore how the material properties influence reciprocating superadiabatic combustion of premixed gases in porous media (short for RSCP), a two‐dimensional mathematical model of a simplified RSCP combustor is developed based on the hypothesis of local thermal non‐equilibrium between the solid and the gas phases by solving separate energy equations for these two phases. The porous media is assumed to emit, absorb, and isotropically scatter radiation. The finite‐volume method is used for computing radiation heat transfer processes. The flow and temperature fields are calculated by solving the mass, moment, gas and solid energy, and species conservation equations with a finite difference/control volume approach. Since the mass fraction conservation equations are stiff, an operator splitting method is used to solve them. The results show that the volumetric convective heat transfer coefficient and extinction coefficient of the porous media obviously affect the temperature distributions of the combustion chamber and burning speed of the gases, but thermal conductivity does not have an obvious effect. It indicates that convective heat transfer and heat radiation are the dominating ways of heat transfer, while heat conduction is a little less important. The specific heat of the porous media also has a remarkable impact on temperature distribution of gases and heat release rate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 336–350, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20120     

Comparative analysis of thermophysical properties of mixed nitrates

熔融盐作为传蓄热介质已经广泛应用于太阳能光热发电中,硝酸盐以其熔点低、成本和腐蚀性小等优点成功应用于商业电站。采用差示扫描量热法、热重法、DIN法、激光闪射法、阿基米德原理和旋转法对Solar salt（60% NaNO₃+40% KNO₃）、Hitec（7% NaNO₃+53% KNO₃+40% NaNO₂）、Hitec XL[7% NaNO₃+45% KNO₃+48% Ca（NO₃）₂]以及本课题组自主研制的四元混合硝酸盐[16.67% Ca（NO₃）₂·4H₂O+44.17% KNO₃+5.83% NaNO₃+33.33% NaNO₂]的熔点、分解温度、比热容、热导率、密度及黏度进行测量和对比研究,分析4种混合硝酸盐热物性优缺点,为工程应用提供基础性数据。结果显示,在四种混合硝酸盐中,四元盐熔点最低、分解温度最高、平均比热容和热导率均高于其他三种混合硝酸盐,Hitec XL密度最大,Solar salt黏度最低。     

Inverse analysis of forced convection in micro-channels with slip flow via integral transforms and Bayesian inference

The present work addresses the direct and inverse problems for convective heat transfer with incompressible laminar gas flow in micro-channels, within the range of validity of the slip-flow regime. The direct problem analysis combines the classical integral transform method and the generalized integral transform technique (GITT), by analytically solving the two-dimensional steady-state convection problem and finding a hybrid numerical-analytical solution for the required eigenvalue problem. The inverse problem analysis makes use of the accuracy and robustness of the direct problem solution and focus on the simultaneous identification of the momentum and thermal accommodation coefficients, related to gas flow and heat transfer within micro-channels, besides the usually unknown boundary condition parameters, here represented by the external Biot number. The inverse analysis is based on the availability solely of temperature measurements at the channel external wall, along its length, as obtained for instance via infrared camera thermography. A Bayesian inference approach is adopted in the solution of the identification problem based on the Monte Carlo Markov Chain method (MCMC) and the Metropolis-Hastings sampling algorithm. A typical example of slip flow in parallel-plates micro-channel is selected to illustrate both the direct and inverse problems solution approaches.     

Inverse radiation analysis of simultaneous estimation of temperature field and radiative properties in a two-dimensional participating medium

An inverse radiation analysis is presented for simultaneous estimation of temperature field and radiative properties including absorption and scattering coefficients in a two-dimensional rectangular, absorbing, emitting and scattering gray medium from the knowledge of the exit radiative energy received by charge-coupled device (CCD) cameras at boundary surfaces. The backward Monte Carlo method was introduced to describe the radiative heat transfer for its efficiency. The inverse problem is formulated as an optimization problem and solved by the least-square QR decomposition (LSQR) method. The effects of measurement errors, optical thickness and search step length on the accuracy of the estimation were investigated and the results show that the temperature field and radiative properties can be reconstructed accurately for the exact and noisy data.     

Evaluation of thermophysical properties of refrigerant clathrates with additives

A modeling study is conducted to evaluate the heat transfer properties of novel refrigerant clathrate-based phase change materials (PCMs). Novel PCMs with large specific energy densities are formed by using different additives in the refrigerant clathrates. Refrigerant clathrates of R134a, R1234yf and R32 are investigated at different refrigerant mass percentages with water. Glycols, sodium chloride, magnesium nitrate hexahydrate, nanoparticles of pure aluminum, copper and graphene are used as additives. Empirical correlations are used to predict the liquid-phase thermal conductivities of refrigerant clathrates and the improvement obtained with the addition of different additives. The results show that an increase in refrigerant mass percentage lowers the thermal conductivity of the refrigerant clathrate but not extensively. The addition of salt results in a minor improvement in thermal conductivity while addition of glycols as liquid additives greatly improves the liquid-phase thermal conductivity. The inclusion of nanoparticles significantly improved the thermal conductivity of the phase change material. The liquid-phase specific heat capacity, however, is not generally improved by the nano-particles as it depended on the additive used.     

Qualitative analysis of thermal explosion of sodium droplet with variable thermophysical properties and thermal radiations

There are many coolants frequently used in the industry for controlling not only heat transfer, but also temperature distribution in a confined domain. However, little is known on the thermal properties of sodium droplets. The qualitative analysis of differential equations that model the thermal explosion, nonlinear dynamic of sodium droplet with variable thermophysical properties when thermal radiations are considered as suggested by Cogley et al, Sohrab et al, and P‐1 approximation Sazhin et al is deliberated upon in this study. The governing equations, first‐order nonlinear ordinary differential equations, are nondimensionalized using the appropriate similarity variables. The existence and uniqueness of the solutions, concavity, and convexity of the temperature distribution, and positivity nature of the solutions of the dimensionless governing equations are established. It is concluded that there exists a solution for a certain range of the admissible parameters and when the reduced activation energy is negative and temperature distribution fits concavity. More so, the major criteria to obtain a positive solution are outlined in this study.     

Scaling analysis of hydrogen flow with carbon dioxide cushion gas in subsurface heterogeneous porous media

Subsurface hydrogen (H₂) storage in geological formations is of growing interest for decarbonization. However, there is a knowledge gap in understanding the multiphase flow involved in this process, which can have a significant impact on the recovery performance of H₂. Therefore, a full-compositional modeling study was conducted to analyze potential issues and to understand the fundamental hydrodynamic mechanisms of H₂ storage. We performed a range of 2D vertical simulations at the decametre scale with a very fine cell size (0.1 m) to observe the detailed flow behaviour of H₂ with carbon dioxide (CO₂) as cushion gas in various flow regimes. Issues such as viscous instability, capillary bypassing, gas trapping and gravity segregation are analysed here. To generalize our calculations, we have validated and applied the scaling theory in the context of subsurface H₂ storage. Since this study is focused on the hydrodynamic behaviour, three dimensionless groups, including aspect factor, capillary/viscous ratio and gravity/viscous ratio were identified to correlate recovery performance between various scales in a fixed heterogeneous system. It was found that H₂ could infiltrate the cushion gas in the proximity of the injectors, meaning that CO₂ is not displaced away from the injectors in a piston-like fashion. As a result, the purity of the back produced H₂ is much degraded, particularly in a viscous-dominated scenario. On the other hand, the injected H₂ mostly accumulates at the top forming a highly restricted mixing zone with CO₂ in the gravity-dominated case. The recovery performance is therefore much improved in this case. Although the gas distribution can be significantly altered by capillary forces leading to bypassed zones, the recovery performance of H₂ is hardly influenced. This is because the back-produced H₂ recovery is not dependent on the sweep efficiency of the gas. H₂ can be back produced following the same paths which were formed during injection.     

Hybrid integral transforms analysis of the bioheat equation with variable properties

Pennes’ equation is the most frequently employed model to describe heat transfer processes within living tissues, with numerous applications in clinical diagnostics and thermal treatments. A number of analytical solutions were provided in the literature that represent the temperature distribution across tissue structures, but considering simplifying assumptions such as uniform and linear thermophysical properties and blood perfusion rates. The present work thus advances such analysis path by considering a heterogeneous medium formulation that allows for spatially variable parameters across the tissue thickness. Besides, the eventual variation of blood perfusion rates with temperature is also accounted for in the proposed model. The Generalized Integral Transform Technique (GITT) is employed to yield a hybrid numerical–analytical solution of the bioheat model in heterogeneous media, which reduces to the exact solution obtained via the Classical Integral Transform Method for a linear formulation with uniform coefficients. The open source UNIT code (“UNified Integral Transforms”) is utilized to obtain numerical results for a set of typical values of the governing parameters, in order to illustrate the convergence behavior of the proposed eigenfunction expansions and inspect the importance of accounting for spatially variable properties in predicting the thermal response of living tissues to external stimulus.     

A review and analysis on influence of temperature and concentration of nanofluids on thermophysical properties,heat transfer and pumping power

The Prandtl number, Reynolds number and Nusselt number are functions of thermophysical properties of nanofluids and these numbers strongly influence the convective heat transfer coefficient. The pressure loss and the required pumping power for a given amount of heat transfer depend on the Reynolds number of flow. The thermophysical properties vary with temperature and volumetric concentration of nanofluids. Therefore, a comprehensive analysis has been performed to evaluate the effects on the performance of nanofluids due to variations of density, specific heat, thermal conductivity and viscosity, which are functions of nanoparticle volume concentration and temperature. Two metallic oxides, aluminum oxide (Al₂O₃), copper oxide (CuO) and one nonmetallic oxide silicon dioxide (SiO₂), dispersed in an ethylene glycol and water mixture (60:40 by weight) as the base fluid have been studied.     

Inverse radiation analysis in two-dimensional gray media using the discrete ordinates method with a multidimensional scheme

This work presents an inverse analysis for temperature field estimation in a two-dimensional gray media; it uses a multidimensional spatial scheme and high order angular quadrature in the discrete ordinates method. The participating media system contains absorbing, emitting, isotropic scattering gray medium. The radiative intensities exiting in some points of boundary surfaces, which simulate data of sensing devises, are known. In this work, the Discrete Ordinates Scheme with Infinitely Small Weight (DOS + ISW) is used to calculate data values. The conjugate gradient method is used to solve the inverse radiation problem for determining the temperature field. The inverse problem is formulated as an optimization problem that minimizes the error between the calculated and the simulated measurement of radiation intensity leaving the media that is sensed at one, two or four points at the boundary of the cavity. The numerical results are obtained by considering simulated data with and without noise. Different arrangements of the position of the sensors at the cavity boundary were analyzed. The temperature field has been estimated with accuracy by using LC11 and Tn6 angular quadratures of DOM when four or two sensors were used and the results for four and two sensors are very close. Also, the effects on the estimations of non-uniform distribution of the absorption coefficient (with random errors of 10%) were analyzed.     

Inverse radiation problem of temperature field in three-dimensional rectangular enclosure containing inhomogeneous,anisotropically scattering media

An inverse radiation analysis is presented for determining the three-dimensional temperature field in an inhomogeneous, absorbing, emitting and anisotropically scattering media of known radiative properties from the knowledge of the exit radiative energy received by charge-coupled device (CCD) cameras at boundary surfaces. The forward Monte Carlo method was employed to describe the radiative energy propagation. The inverse problem was formulated as an ill-posed matrix equation and solved by least square QR decomposition (LSQR) method. The measured data were simulated by adding random errors to the exact solution of the direct problem. The effects of measurement errors, combinations of CCD cameras, concentration distributions of particles, and coefficient fluctuating errors on the accuracy of the inverse problem were investigated. The results show that the three-dimensional temperature field can be estimated accurately, even for the noisy data.     

Convective heat transfer and second law analysis of non-Newtonian fluid flows with variable thermophysical properties in circular channels

This study presents an analytical solution, for fully developed non-Newtonian fluid flows in circular channels under isoflux thermal boundary conditions based on perturbation techniques. Since the physical properties are generally a function of temperature and may not be assumed constant under certain circumstances, the change in viscosity and thermal conductivity with temperature was taken into account. Viscous dissipation term was also included in the performed analysis. In this study, first closed form expressions for velocity, temperature distributions, and Nusselt numbers corresponding to constant thermophysical properties were given in terms of governing parameters. Then, numerical calculation was performed to obtain the values of Nusselt number and global entropy generation for variable thermophysical properties. The results revealed that neglecting the property variation significantly affects heat transfer characteristics and entropy generation, in which the deviation from the constant physical property assumption may reach up to about 32.6%.     

Review of necessary thermophysical properties and their sensivities with temperature and electrolyte mass fractions for alkaline water electrolysis multiphysics modelling

This article presents an exhaustive review of the transport properties necessary for the multiphysics modelling of alkaline water electrolyzer. This article provides experimental data and the correlations needed to calculate thermos-physical properties such as electrical conductivity, density, viscosity, heat capacity, heat and mass transfer diffusion coefficients as a function of temperature and electrolyte mass fraction for two classical alkaline electrolytes (KOH, NaOH). Thus, the different boundary layers growing on the electrodes can be calculated with precision. Different interpolation models from various authors are compared to raw experimental data. The goal of this article is to give to the modeler the correlations needed for the simulation of alkaline water electrolysis.     

Binary eutectics of acetamide with inorganic nitrates: thermophysical properties relevant for heat storage

Binary mixtures of acetamide with the following nitrate salts were studied: potassium nitrate, sodium nitrate, ammonium nitrate and calcium nitrate Eutectic compositions, melting points and enthalpies of melting were defined from DSC thermograms. It was shown that the eutectics may exist in two crystalline forms, differing in melting point and enthalpy of fusion. Specific heat as a function of temperature was also measured by DSC. Latent heat storage density in these eutectics of 160–270 MJ/m³, strongly suggests their consideration as phase change materials (PCMs) in solar heat storage systems.     

Exclusion of oscillations in heterogeneous and bi-composite media thermal conduction

Analysis of Fourier heat conduction in heterogeneous and bi-composite media (e.g. porous media, fluid suspensions, etc.) subject to Lack of Local Thermal Equilibrium (LaLotheq) reveals a condition for thermal oscillations and resonance to be possible. This paper shows that this condition cannot be fulfilled because of physical constraints leading to the exclusion of thermal waves and resonance.     

The influence of binary mixture thermophysical properties in the analysis of heat and mass transfer processes in solar distillation systems

Although a substantial amount of research work has already been devoted to various aspects of modeling the convective and mass transport processes in solar distillation systems, it appears that the role of thermophysical and transport properties of the working medium and their effect on the thermal behavior and performance analysis of such systems has been left almost completely unnoticed. The working medium in these systems, which is a binary mixture of water vapor and dry air in equilibrium, appears to exhibit a completely different set of properties than dry air, especially at saturation conditions and at the higher region of the solar still operational temperature range. An analysis is presented aiming to signify the effect of binary mixture thermophysical properties on the transport processes and the associated quantities and evaluate the thermophysical properties of the working medium in these systems, based on contemporary data for dry air and water vapor. The derived results, in the form of convenient algebraic correlations, are employed to investigate the effect of using the appropriate thermophysical properties on the calculation of the convective heat and mass transfer, as well as the distillate mass flow rates. According to the results from the present investigation, although the use of improper dry air data leads to a significant overestimation of the convective heat transfer coefficient, the errors associated with the use of improper dry air properties is a moderate overestimation of distillate output which is estimated to be up to 10% for maximum average still temperatures of 100 °C.     

An integral approach for hydromagnetic natural convection heat and mass transfer from vertical surfaces with power-law variation in wall temperature and concentration in porous media

This work uses the integral method to study the heat and mass transfer by natural convection from vertical plates with variable wall temperature and concentration in porous media saturated with an electrically conducting fluid in the presence of a transverse magnetic field. The surface temperature and concentration are assumed to vary as a power of the axial coordinate measured from the leading edge of the plate. The approximate solutions are found to be in reasonable agreement with the similarity solutions. Results are plotted for the local Nusselt number, the local Sherwood number, and the reciprocal of the ratio of the thermal boundary-layer thickness to the concentration boundary-layer thickness. Increasing the power-law exponents tends to increase the local Nusselt number and the local Sherwood number. Increasing the magnetic parameter decreases the local Nusselt number and the local Sherwood number. Moreover, the ratio of the thermal boundary-layer thickness to the concentration boundary-layer thickness increases with the Lewis number, and it also increases with the buoyancy ratio when the Lewis number is not equal to one.     

Inverse estimation of temperature boundary conditions with irregular shape of gas tank

To broaden the application of inverse estimation, the purpose of this study is to estimate the unknown temperature boundary condition of the complex or irregular shape, like the high pressure gas tank. An inverse algorithm based on the sequential method and the concept of future time combined with the finite-element method is proposed to solve the two dimensional irregular shape heat conduction problems. Special features about the proposed method are that the stiffness matrix of the irregular shape can be solved from the finite-element method and used by the inverse algorithm. The estimated results are quite accurate with the consideration of future time in the different measured errors, the various sensor’s number and the sensor location. These results show that the proposed method is an accurate, sturdy, and efficient method for solving several realistic applications.     

Oscillatory flow and temperature fields in an open tube with temperature difference across the ends

The oscillating flow and temperature field in an open tube subjected to cryogenic temperature at the cold end and ambient temperature at the hot end is studied numerically. The flow is driven by a time-wise sinusoidally varying pressure at the cold end. The conjugate problem takes into account the interaction of oscillatory flow with the heat conduction in the tube wall. The full set of compressible flow equations with axisymmetry assumption are solved with a pressure correction algorithm. Parametric studies are conducted with frequencies of 5–15 Hz, with one end maintained at 100 K and other end at 300 K. The flow and temperature distributions and the cooldown characteristics are obtained. The frequency and pressure amplitude have negligible effect on the time averaged Nusselt number. Pressure amplitude is an important factor determining the enthalpy flow through the solid wall. The frequency of operation has considerable effect on penetration of temperature into the tube. The density variation has strong influence on property profiles during cooldown. The present study is expected to be of interest in applications such as pulse tube refrigerators and other cryocoolers, where oscillatory flows occur in open tubes.