Heat Conductivity Coefficient Research Articles

Experimental investigation of a novel thermal storage solar air heater (TSSAH) based on flat micro-heat pipe arrays

A novel thermal storage solar air heater (TSSAH) is proposed in this study. This TSSAH is composed of a vacuum glass tube, flat micro-heat pipe arrays (FMHPA), and a thermal storage material (i.e., paraffin) with a phase change temperature of 58 °C. The FMHPA acts as the core heat transfer component. Considering the low heat conductivity coefficient of paraffin, louver fins are utilized to enhance heat transfer. Thermal performance is analyzed and discussed. During thermal collection, natural convection evidently contributes to enhancing heat transfer. Increasing solar radiation intensity can improve collection efficiency. Thermal collection efficiency increases from 70.22% to 77.28% when solar radiation is increased from 675 W/m2 to 835 W/m2 under an ambient temperature range of 20.8 °C–23.3 °C. A high ambient temperature results in high collection efficiency that can reach 80.59%. During thermal discharge, enhancing the air volumetric flow rate can evidently increase the amount of useful energy and shorten discharge time. From 80 m3/h to 170 m3/h, the amount of useful energy is increased from 335 W to 550 W, and discharge time is shortened from 309 min to 195 min under inlet temperatures ranging from 24.1 °C to 24.2 °C. A high inlet temperature generates a high outlet temperature but decreases the amount of useful energy. From 19.7 °C to 32.0 °C, outlet temperature is increased from 30.4 °C to 36.6 °C, and the amount of useful energy is reduced from 692 W to 287 W at an air volumetric flow rate of 200 m3/h. During thermal discharge, the average amount of useful energy can reach 692 W at an inlet temperature of 19.7 °C and an air volumetric flow rate of 200 m3/h, indicating that the proposed TSSAH can release heat quickly. In the experiment scale, the discharged heat is 6150–6450 kJ at a phase change material temperature scale of 85 °C–35 °C.

Comparison of the Thermal Properties of Geopolymer and Modified Gypsum.

The paper presents the results of research concerning the influence of micromaterials on the heat conductivity coefficient λ, specifically heat Cp and thermal diffusivity a of modified gypsum and geopolymer. Microspheres, hydroxyethyl methylcellulose (HEMC) polymer, and aerogel were used as the gypsum’s modifying materials. The study also investigated an alkali potassium-activated methakaolin-based geopolymer with the addition of aluminium dust. During the measurements of thermal parameters, the nonstationary method was chosen, and an Isomet device—which recorded the required physical quantities—was used. When compared to the reference sample, a decrease in the thermal conductivity and diffusivity of the hardened gypsum— and a simultaneous increase in specific heat—was observed with the addition of micromaterials. The geopolymer sample was characterized by the lowest value of thermal conductivity, equal to 0.1141 W/(m·K). It was over 62% lower than the reference sample containing only gypsum. The experimental values of the thermal conductivity of the gypsum samples with the addition of HEMC, aerogel and microspheres were, respectively, over 23%, 6%, and 8% lower than those of the unmodified gypsum samples. The lowest values of thermal conductivity were observed in the case of the gypsum samples modified with polymer; this resulted from the fact that the polymer caused the greatest change in the structure of the gypsum’s composite, which were expressed by the lowest density and highest porosity.

A decoupled wavelet approach for multiple physical flow fields of binary nanofluid in double-diffusive convection

The paper aims at applications of a decoupled wavelet method for investigating multiple physical steady flow fields of binary nanofluids in double-diffusive mixed convection. The Buongiorno’s mathematical model of nanofluids is further perfected in the presence of Dufour and Soret effects, incorporating with linear and nonlinear diffusiophoresis effects based on experiments [Chemical Engineering Science 176 (2018): 632–640]. Nonhomogeneous thermal boundaries corresponding to heat flux on vertical walls and convection heat transfer at the bottom along with moving top lid are effectively approximated by interpolated Coiflet-type wavelet. Highly coupled and nonlinear governing equations for the complex fields of temperature, nanoparticles volume fraction and solute concentration have been formulated and decomposed into linear differential ones by homotopy transformation. Numerical wavelet solutions with larger range of physical parameters are finally obtained and validated by solving a set of iterative algebra equations applying Galerkin method, which are difficult to be given by traditional numerical methods. The results reveal that nanoparticles and double-diffusive buoyancy parameters, the thermo-nanofluid and thermo-solutal Lewis numbers, the heat conductivity coefficient, the periodical heat flux with different phase differences, the diffusiophoresis parameters, the nanoparticles and solute Dufour parameters, the solute Soret parameter are of great significance on characteristics of heat and mass transfer in the complex flow.

Forecast of the thermal regime of an underground storage facility for heat-generating materials under mixed convection conditions

The paper presents the results of a study based on numerical simulation methods of the thermal regime of an underground facility for long-term spent nuclear fuel storage in the version of a built-in reinforced concrete structure. A multiphysical computer model was constructed in a two-dimensional setting by means of the COMSOL software. The mathematical model was based on the continuity equations, Navier-Stokes equations and the general heat transfer equation. The conditions of mixed convection were taken into account in the ‘incompressible ideal gas’ approximation, in which the thermophysical properties of air were a function of temperature only. For two parameters of the model, the following values were taken: the air flow rates providing the velocity at the inflow boundary = 0.01, 0.03 and 0.05 m/s, and the effective heat conductivity coefficients of the material of the built-in structure = 1.0 and 2.0 W/(m×K). Numerical experiments were performed for a period of up to 5 years of fuel storage. Special emphasis was given to the fundamental difference between the non-stationary structure of the velocity fields forecasted in the model of an ‘incompressible ideal gas’ and the ‘frozen’ picture of aerodynamic parameters in the model of an incompressible fluid. An analysis was made of the dynamics of spatial temperature field distributions in different areas of the model. It was shown that the criterion temperature control requirements were met when the facility was operated under conservative ventilation conditions in terms of the air flow rate and the heat conductivity coefficient of the built-in structure material. The dynamics of heat flows directed into the rock mass through the base and from the surface of the built-in structure into the air was analyzed. The heat flow dominance from the structure surface was also noted. Finally, the influence of the effective heat conductivity coefficient of the built-in structure material and the air flow rate on the values of heat flows directed into the air and rock mass was demonstrated.

Simultaneous thermal analysis on the dynamical oxygen-lean combustion behaviors of coal in a O2/N2/CO2 atmosphere

This study aims at study the dynamical oxygen-lean combustion behaviors of coal in a multi-gas concomitant atmosphere. Two coal samples from Bulianta and Yuwu collieries were collected for simultaneous thermal analysis in O2/N2 (21 vol%/79 vol%) and O2/N2/CO2 (15 vol%/80 vol%/5 vol%) atmospheres. Based on the TG-DTG curves, five evolution stages were analyzed. A delay phenomenon occurred for coal lean-oxygen combustion in O2/N2/CO2, which was obvious at IV stage with a high staged temperature and a wide temperature range. Furthermore, the kinetic parameters were obtained after determining the most probable mechanism function, showing an increased apparent activation energy at III stage and a decreased one at IV stage in O2/N2/CO2 compared with in O2/N2, which indicated that the low heat conduction coefficient of CO2 played a dominant role on these changes. The study also showed the relationship between the thermal formation rate and reaction rate constant.

Physical and mechanical properties of insulation from rubber leave

This research aims to study the insulation sheet of rubber leaves. By testing physical properties and mechanical property at the ratio of rubber leave : rubber latex were 20:80, 30:70, 40:60, 50:50, 60:40, 70:30 and 80:20. Use rubber latex as a binder to allow the material to be bonded. The insulating sheets produced are smooth 16×16×2 cm3. From the results showed that the moisture content of the sample were 6.722 - 14.747%. The tensile strength were 0.0002 – 0.0288 MPa. The fire spread rate were run of 0 - 6.640 mm/min and the ratio of 70:30 and 80:20 were no fire spread rate. Then finally, the heat conductivity coefficient were 0.0283 - 0.0442 W/m·K. Moreover, samples of ratio 80:20 have a better conductivity coefficient than commercial insulation and confirmed with the SEM-EDX analysis.

The deviation of thermal conductivity under different operating power is analyzed by linear heat source superposition method

Heat conductivity and multiple correlation coefficients of various drill holes under high and low-power levels were acquired through the field heat response test, so as to capture heat exchanging performances of the ground heat pump system. Moreover, by analyzing the changing characteristics of heat conductivity with IGSHPA linear heat source model, significant deviations were found between test results of heat conductivity when the high and low-power tests are conducted at short intervals, which was resulted from the influence of the residual temperature field of the first test on the second test, that is, the temperature is not returned to the initial temperature yet. Deviation caused by short test intervals between high and low-power test results can be eliminated by the line-source superposition method. In addition, by introducing the time function, the exact value of heat conductivity obtained by the second test can be solved with the utilization of linear relationship between the time function and the average temperature of the fluid. The dependence of fitting the linear heat source was verified. By calculating the linear regression of important parameters in the linear heat source superposition method, the linear fitting upon processing is characterized by good degree of fitting, small temperature deviation, and significant linear correlation. The linear heat source superposition method was proven to be reliable and accurate. Also, the analysis result shows that there is no need to process data using the linear heat source superposition method when two tests are conducted at long intervals.

Calculation of non-stationary heat exchange in multi-layer media by means of the theory of Markov chains

Calculation of non-stationary heat exchange in multi-layer dissimilar media is the problem, which is often found in many industries including high temperature metallurgic technologies of production and treatment of metals, alloys and their products. The similar problems arise during the design of buildings and structures. The models used for calculation are mainly based on approximate solutions of classical heat conduction equations. The drawback of such models is a complex computational procedure, associated with the need to solve a large number of equations and to define a large number of identification parameters. At present, the calculation of one mode of heat transfer between the sections of a composite structure requires 6–8 hours of operation of a computer of average power. In this regard, the development of discrete models that require less computer time is of current importance. The proposed model of the process is based on the theory of Markov chains. A multi-layer medium is presented as a chain of small but finite cells. Each of them contains a certain amount of heat that can be transferred to the neighboring cells. The part of the transferred heat is directly proportional to the heat conduction coefficient and in inverse proportion to the material heat capacity, material density and the cell length. The matrix model to describe heat transfer between cell in a multi-layer media based on the theory of Markov chains is developed. Construction of the matrix of transition probabilities is described, evolution of the state vectors i.e. distribution of heat and temperature is carried out, and non-uniformity of the heater temperature is taken into account. Comparison of calculated and experimental data has showed the adequate description of the real process using the model. Analysis of identification parameters has given a satisfactory result.

Влияние зависимости коэффициента теплопроводности от температуры на физические закономерности объемного синтеза интерметаллида

Two-dimensional model of high-temperature synthesis of chemical compounds and alloys in a dynamic thermal explosion mode when a powder compact is heated in a steel cylindrical mold by an induction heat source are proposed. The complex of chemical reactions is described by a total reaction with effective formal kinetic parameters. The kinetic law takes into account strong retardation of the reaction by the layer of the synthesized product that prevents the interaction of the reagents. The model makes it possible to investigate the macroscopic physical laws of the synthesis of a composite with a change in the heating rate and reactor dimensions. It is made a comparison of the results obtained for a constant heat conductivity coefficient and a temperature-dependent heat conductivity coefficient. It is revealed that taking into account the temperature dependence of the heat conductivity coefficient can lead to a numerical change in the ignition delay time and to a qualitatively different temperature distribution in the reactor bulk.

Thermal conduction in two-dimensional complex plasma layers

A simple vibrational model of heat transfer in two-dimensional (2D) fluids relates the heat conductivity coefficient to the longitudinal and transverse sound velocities, specific heat, and mean interatomic separation. This model is demonstrated not to contradict the available experimental and numerical data on heat transfer in 2D complex plasma layers. Additionally, the heat conductivity coefficient of a 2D one-component plasma with a logarithmic interaction is evaluated.

New flame-retardant composition for lowering contribution of expanded poly styrene to the propagation of fire

Despite the major advances in ensuring fire safety of buildings and structures, fire risks still represent the serious issue not only on the territory of the Russian Federation but on the global basis. Presently, while erecting buildings and structures focused attention is given to lowering material consumption and reducing construction time. This is achieved by means of application of lightweight building envelopes with efficient insulators. For the most part, foam poly styrene plates are used as an insulator. Expanded or foam poly styrene has low coefficient of heat conductivity, high acoustic isolation, long-term durability, moisture resistance, and is characterized by ease of installation. However, application of foam poly styrene has constrains due to its high contribution to the propagation of fire.In the present paper, the authors proposed new flame-retardant composition on the base of organo-siloxanes, which inhibits combustion initiating of foam poly styrene and constrains fire propagation when applied on the surface of foam poly styrene plates.

Application of the Protective Coating for Blade’s Thermal Protection

This paper presents an algorithm applied for determining temperature distribution inside the gas turbine blade in which the external surface is coated with a protective layer. Inside the cooling channel, there is a porous material enabling heat to be transferred from the entire volume of the channel. This algorithm solves the nonlinear problem of heat conduction with the known: heat transfer coefficient on the external side of the blade surface, the temperature of gas surrounding the blade, coefficients of heat conduction of the protective coating and of the material the blade is made of as well as of the porous material inside the channel, the volumetric heat transfer coefficient for the porous material and the temperature of the air flowing through the porous material. Based on these data, the distribution of material porosity is determined in such a way that the temperature on the boundary between the protective coating and the material the blade is made of is equal to the assumed distribution To. This paper includes results of calculations for various thicknesses of the protective coating and the given constant values of temperature on the boundary between the protective coating and the material the blade is made of.

Thermodynamic analysis of absorption cooling system with LiBr-Al2O3/water nanofluid using solar energy

Together with the developing nanotechnology, nanofluids and nanoparticles are used as working fluid in energy applications. It is foreseen that nanoparticles have high heat conduction coefficient and it will increase system performance by using as a working fluid in energy systems. Many studies in the literature show that nanofluids increase the heat transfer rate by improving heat transfer. In this study, a performance analysis of an absorption cooling system using solar energy was performed as numerically. LiBr-Al2O3-water nanofluid has been used in the cooling system as working fluid. The thermodynamic values and calculations used in the analyses were performed with Engineering Equation Solver program. Heat load necessary for the generator is provided with a flat plate solar collector. For different operation condition, the variation of COP values was determined depend on Al2O3-water nanoparticle concentration ratio. When the Al2O3-water nanoparticle concentrations are changed as 0%, 0.5% and 0.1%, it was determined that the COP values increased. Nanoparticles added to the refrigerant at certain concentration values affects the COP values positively of cooling systems. Maximum COP value is 0.86 for 85 oC generator temperature and 0.1% Al2O3-water nanoparticle concentration. The lowest COP value was obtained for the 75 oC generator temperature. When the Al2O3-water nanoparticle concentration was increased together with the generator temperature, COP values also increased. When the nanoparticle concentration of the working fluid increases, the viscosity of the nanofluid can be increases. Due to, increased viscosity increases the pressure drop in the flow channel and the pump power required for the flow. Thus, minimum viscosity with maximum thermal conductivity optimisation in applications is very important.

Thermal Conductivity of Phase Changing Materials Doped with Carbon Nanotubes

Phase changing materials (PCM) can accumulate and release a great quantity of energy at changing the temperature in result of the phase transition. This permits one to use PCMs as a basis of thermal accumulators storing the thermal energy at elevated temperatures and releasing it at lowering the temperature below the phase transition point. Worldwide usage of PCMs in building technology and other fields is hindered by a rather low thermal conductivity coefficient of the most PCMs which makes the response of the relevant thermal accumulators too slow and limits the possibilities of application of such devices. This drawback can be overcame through doping a PCM with particles having high thermal conduction coefficient. The present article contains results of experimental and theoretical investigations of heat propagation in PCM doped with carbon nanotubes the thermal conduction coefficient of which exceeds that of the most PCM by 4 – 5 orders of magnitude. Paraffin П-2 have been used as PCM. The experiments performed demonstrate a 2 – 3 times enhancement of thermal conductivity and 16 orders of magnitude enhancement of the electric conductivity because of doping paraffin with 10% multi-walled nanotubes. The propagation of both heat and electric current has a percolation character, so that nanotubes form conductive paths at exceeding some concentration of the dopant. The heat propagation process was modelled through the solution of the non-stationary heat conduction equation with taking into account the sorption of heat due to the phase transition. The calculations performed for composite materials with the varied thermal characteristics of a material imply that the characteristic heating time is proportional to the value of the melting enthalpy and inversely proportional to the value of the heat conduction coefficient.

Enhancement of condensation heat transfer on surface with macro-, micro- and nanorelief

Film and drop condensation are important processes in technics. This article includes the features of macroscale heat transfer intensifiers under film condensation, microscale and nanoscale heat transfer intensifiers under drop condensation. It is noted that condensation heat transfer may be influenced by the wall material. The mechanism underlying this influence is the nonuniform distribution of the wall temperature in the case of a low heat conduction coefficient. Based on the research results, a dependence of the wetting angle from the combined coating consisting of 7 alumina oxide and water repellent is obtained.

Global existence of large solutions to the planar magnetohydrodynamic equations with zero magnetic diffusivity

In this paper, we consider the initial-boundary value problem to the planar magnetohydrodynamic (MHD) equations with zero magnetic diffusivity. In the case when the coefficients of viscosity and heat conductivity are positive constants, we establish the global existence and uniqueness of strong solution by the a priori estimates. The results are obtained without any smallness restriction to the initial data and the initial density allows vacuum.

Carbon Composites for Heat and Sound Insulation in Far North and Arctic Conditions

Heat and sound insulating carbon-polymer composites are produced and described. The heat and sound insulation properties, namely, the heat conductivity coefficient and the difference of temperatures between the heater and the opposite side or the layer of the sample of these composites are investigated. The sound insulation properties are characterized by losses of sound pressure during passage through the composite. It is demonstrated that carbon-polymer composites can be used in Far North and Arctic conditions.

Створення низькотеплопровідних полімерних нанокомпозитів для внутрішніх газовідвідних стволів димових труб котелень

Methods and results of experimental studies of thermophysical, structural and mechanical properties of low-heat-conducting polymer nanocomposites, oriented to use for gas ducts and chimneys of boiler installations, as well as various other gas and water communications are presented. In this work, on the basis of the performed set of methodological studies regarding the analysis of the legitimacy of using different models of heat conductivity for predicting the heat-conducting properties of these composites, the possibility of using for this prediction a number of models of the theory of the effective medium and the theory of percolation is considered. The analysis of thermophysical properties, structural characteristics and Young's modulus of low-heat-conductivity polymer nanocomposites based on polyethylene and polypropylene is carried out. Using these nanocomposites as an example, the achievement of a significant increase in their Young's modulus in comparison with unfilled polymers with a relatively small increase in heat conductivity is demonstrated. To obtain nanocomposites, we used a method based on mixing the components in a polymer melt using an extruder and then shaping the composite into the required shape by hot pressing. The method of differential scanning calorimetry was used to determine Young's modulus. On the basis of the studies carried out, the possibility of obtaining low-heat-conducting polymer nanocomposites with improved mechanical characteristics has been shown. In particular, it was shown that for nanocomposites based on polyethylene or polypropylene filled with CNTs (carbon nanotubes) or nanodispersed aerosil particles, with a mass fraction of the latter up to 2%, the following takes place a relatively insignificant increase in heat conductivity coefficients and a significant increase in the modulus of elasticity in tension. The research data also made it possible to obtain for the developed nanocomposites the temperature dependences of their specific mass heat capacity and, on this basis, to analyze the regularities of changes in the structural characteristics of these materials.

An investigation into the Coulomb logarithm models and their effects on the electron heat transfer in warm dense plasmas

A comparison study of various models in investigating the Coulomb logarithm (CL) for a wide range of densities and temperatures is carried out. Using two known reference parameters, the ion coupling $$ \varGamma $$ and the electron degeneracy $$ \varTheta $$ , the capability of a recently proposed coupled Gericke–Murillo–Schlanges (CGMS) model in evaluating the CL is compared with other previous models—the Gericke–Murillo–Schlanges (GMS) model, the Landau–Spitzer (LS), the Brown–Preston–Singleton (BPS), the classical molecular dynamics (MD), and Khrapak (KH) model. It is observed that the CGMS model shows reliable results in both weakly and strongly coupled regimes. Moreover, the effects of using these models on studying electron heat transfer and conductivity coefficient within the weakly coupled and strongly coupled plasmas are examined.

Comparative experimental study of the dynamic properties and adiabatic shear susceptibility of titanium alloys

Titanium and titanium alloys are prone to adiabatic shear failure when deformed at high strain rates due to their low coefficients of specific heat and thermal conductivity. Comparative study of the adiabatic shear susceptibility of titanium alloys are seldom reported in the literature. To further understand this topic, the mechanical response of five representative titanium alloys, i.e. commercially pure titanium (TA2, α), Ti-6.5Al–2Zr–1Mo–1V (TA15, near α), commercially Ti–6Al–4V (TC4, α+β), Ti–5Al–5Mo–5V–1Cr–1Fe (TC18, near β) and Ti–3Al–8V–6Cr–4Mo–4Zr (TB9, β), is characterized over a wide range of strain rates (ranging from 0.001 to 4000s−1) and temperatures (ranging from 293 to 673K). The findings of this experimental demonstrate that the yield stress of TA2 is the lowest, while it is the highest for TC18. The strain hardening effect of titanium alloys is prominent at low strain rate, while it may be decreased at high strain rates due to the presence of β-phase stabilizers. In addition, the mechanical behavior of titanium alloys is sensitive to strain rate and test temperature. The results also suggest that the formation of ASB is related to the applied strain rate and initial test temperature, and higher test temperature and strain rate promote the occurrence of adiabatic shear localization in dynamic deformation. Moreover, for titanium alloys, a lower strain-hardening coefficient and higher strength does not correspond to high sensitivity to adiabatic shear failure, and the dynamic failure energy, with its constant value, seems to be the best indicator of ASB failure. Furthermore, our results and analysis are important in enhancing the understanding of the dynamic properties and adiabatic shear susceptibility of titanium alloys. Based on this work, engineers can choose the optimal titanium alloy to fabricate components according to the service requirements to fully utilize the potential of the material.