Viscosity and thermal conductivity of dispersions of sub-micron TiO2 particles in water prepared by stirred bead milling and ultrasonication

Experiments were carried out on the preparation of dispersions of sub-micron TiO₂ particles in water by stirred bead milling, for potential use as coolants. The prepared dispersions were characterized through the measurement of particle size distribution, zeta potential, viscosity and thermal conductivity. The effects of particle concentration (0.27–1.39 vol%), ultrasonication time (0–7 h) on viscosity and thermal conductivity have been studied. The effect of temperature (29–55 °C) on viscosity has also been investigated. The results indicate that the ultrasonication can be utilized to tailor the transport properties of the sub-micron dispersions produced by stirred bead milling. The entire particle size distribution data has been utilized to develop correlations for prediction of relative viscosity and thermal conductivity ratio of these dispersions. These dispersions possess higher thermal conductivity than water and can also be utilized as coolants.     

Viscosity and thermal conductivity comparative study for hybrid nanofluid in binary base fluids

Thermal conductivity and viscosity analysis of Al₂O ₃/CuO (50/50) hybrid nanofluid in various mass fractions of ethylene glycol (EG) and propylene glycol (PG) binary base fluid have been investigated in the present work. Hybrid nanofluid with vol. fraction range limited to 1.5% and within the higher temperature range of 50°C to 70°C is considered for thermal conductivity and viscosity analysis. Impact on viscosity and conductivity models with various shape nanoparticles, i.e, spherical, cylindrical, brick, platelets, and blades have been discussed and were compared in EG and PG binary base fluids. Also, the analysis extends to the prediction for the stability with zeta potential and synthesis of spherical shape Al₂O₃/CuO hybrid nanofluid with X‐ray diffraction (XRD) and scanning electron microscope (SEM). The theoretical analysis revealed that thermal conductivity of Al₂O₃/CuO hybrid nanofluid in EG binary base fluid is lower compared to in PG binary base fluid. The thermal conductivity is observed to be higher in spherical and cylindrical shape nanoparticle compared to bricks, blades, and platelets shape nanoparticles. Optimum viscosity of Al₂O₃/CuO hybrid nanofluid is observed at 50%EG and 30%PG of the binary base fluid. Hybrid nanofluid in 30% of PG as binary base fluid results 16.2% higher dynamic viscosity compared to pure PG base fluid for a volume concentration of 2%. Zeta potential measurement results in the stability of spherical Al₂O₃‐CuO/ (50/50) EG/W hybrid nanofluid, and it may be considered as a heat transfer fluid.     

Heterogeneous electrolytes: Variables for and uncertainty in conductivity measurements

This paper discusses variables and uncertainty associated with the measurement of ionic conductivity of heterogeneous solids. The conductivity data of heterogeneous solids of diverse chemistries have been analyzed. All these solids exhibit space charge and blocking effects. The coexistence of the two effects may lead to significant scatter and uncertainty in the measured values if the variables are not isolated and controlled.     

Viscosity correlations for jojoba oil blends with biodiesel and petroleum diesel

In this study, the effect of temperature and mixture composition on viscosity of Jojoba oil-Biodiesel (JO-BD) and Jojoba oil-Diesel (JO-PD) blends are investigated. Moreover, the relationship between the viscosity and the specific gravity of the blends is studied. Experimental viscosity data for the temperature range between 20°C and 80°C are used. The results show that the viscosity–temperature dependency can be well correlated by Vogel model for the viscosity–temperature relation. Also, a method that could estimate the blends viscosity from the specific gravity data is established.     

Electrical conductivity measurements of aqueous and immobilized potassium hydroxide

It is important to know the conductivity of the electrolyte of an alkaline electrolysis cell at a given temperature and concentration so as to reduce the ohmic loss during electrolysis through optimal cell and system design. The conductivity of aqueous KOH at elevated temperatures and high concentrations was investigated using the van der Pauw method in combination with electrochemical impedance spectroscopy (EIS). Conductivity values as high as 2.7 S cm⁻¹ for 35 wt%, 2.9 S cm⁻¹ for 45 wt%, and 2.8 S cm⁻¹ for 55 wt% concentrated aqueous solutions were measured at 200 °C. Micro- and nano-porous solid pellets were produced and used to immobilize aqueous KOH solutions. These are intended to operate as ion-conductive diaphragms (electrolytes) in alkaline electrolysis cells, offering high conductivity and corrosion resistance. The conductivity of immobilized KOH has been determined by the same method in the same temperature and concentration range. Conductivity values as high as 0.67 S cm⁻¹ for 35 wt%, 0.84 S cm⁻¹ for 45 wt%, and 0.73 S cm⁻¹ for 55 wt% concentrated immobilized aqueous solutions were determined at 200 °C. Furthermore, phase transition lines between the aqueous and aqueous + gaseous phase fields of the KOH/H₂O system were calculated as a function of temperature, concentration and pressure in the temperature range of 100–350 °C, for concentrations of 0–60 wt% and at pressures between 1 and 100 bar.     

A fractal model for thermal conductivity in a disperse system of even particles

In this paper the geometrical structure of disperse system is described by fractal theory, which is based on the easily measurable parameters, such as the occupation probability, the diameters of disperse pellets, etc. A fractal model of thermal conductivity of a disperse system with even particles has been developed with the aid of the analogy between the heat conduction and percolation. The measurement of the thermal conductivity in the disperse system was carried out by the cutout hot wire method. The model's prediction as to the ratio of thermal conductivity for different particle sizes is reasonably in agreement with the experimental results. © 2000 Scripta Technica, Heat Trans Asian Res, 29(7): 535–544, 2000     

Effect of silicone oil on solid propellant combustion in small motors

The feasibility of reducing troublesome nozzle blockage (by condensation deposits) in laboratory-scale solid rockets by addition of a silicone oil as a propellant ingredient was explored experimentally. An aluminized composite propellant and its counterpart with 1% silicone oil replacing part of the binder were fired in a 63.5 mm diameter, end-burning, all-metal burner. Pressure-time histories were recorded for all of the tests by a Taber gauge mounted at the downstream end of the chamber; temperature-time data at the nozzle throat were obtained in some of the runs by thermocouples having junctions positioned at the wall but insulated from the metal. Deposition of condensables on the nozzle walls causing a progressive increase in the chamber pressure with time was noted. The fraction of firings exhibiting practically no condensation was 59% with silicone and 32% without. On the average, temperature readings at the nozzle throat were higher with the silicone propellants. Although various phenomena may contribute to these findings, the results are not understood completely.     

Distinguishing between apparent and authentic conductivity of protonic ceramic electrolytes using differential resistance analysis applied to conductivity measurements

Differential resistance analysis (DRA) is used for determination of authentic bulk conductivity of yttrium-doped barium cerate proton conductors (BCY) using electrochemical impedance spectroscopy (EIS), by eliminating all electrode and fixture resistance as well as interfacial or surface phenomena. Bulk conductivity determined using DRA in the range 500–800 °C was higher by a factor of 1.5–3 than values obtained for conductivity of the same electrolyte using EIS alone. Application of DRA with EIS provides geometrically independent conductivity values.     

Heat capacity and thermal conductivity considerations for coal particles during the early stages of rapid heating

Heating and cooling transients for a number of individual coal particles in the 100-μm size range were measured under rapid heating conditions (10⁴–10⁵ K/s heating rate). In addition to temperature measurements, each particle was fully characterized with respect to external surface area, volume, mass, and density prior to heating. Measured temperatures were compared with model predictions and a sensitivity analysis was performed to critically evaluate model assumptions regarding particle thermal properties. Simulations using temperature-dependent heat capacity and thermal conductivity correlations routinely applied to coal severely under predicted the particle temperature rise during the early stages of heating. Simulations using constant room temperature values for heat capacity and thermal conductivity showed excellent agreement with measurements during the early stages of heating. Increases in coal heat capacity and thermal conductivity reported in the literature are observed under slow heating conditions and result from bond breaking and structural changes which lead to an increase in vibrational modes of freedom in the coal structure. Results of the present study suggest that under rapid heating conditions the coal structure is frozen and that these vibrational modes only become accessible at higher temperatures or longer soak times. These considerations are important if one desires to accurately model the combustion behavior of coals.     

Transport properties in molten carbonates: self-diffusion and conductivity measurements at high temperature

A better understanding and use of molten carbonate fuel cells (MCFCs) requires more detailed consideration on transport properties in melt. The combination of different methodological developments can be one solution to improve our comprehension. Here we present ²³Na and ⁷Li self-diffusion coefficients measured by pulsed field gradient (PFG) NMR technique combined with electrical conductivity obtained by a 4-electrode set up, in the eutectic mixture Li₂CO₃–Na₂CO₃ (52:48 %mol) at high temperature (up to 1050 K), and under pure CO₂ atmosphere. The results were compared with known experimental data from literature obtained by radiotracers techniques and 2-electrode set up and also with some calculations of the transport properties.     

Combustion of nanofluid fuels with the addition of boron and iron particles at dilute and dense concentrations

The combustion characteristics of nanofluid fuels containing additions of boron and iron particles were investigated. The effects of particle materials, loading rate, and type of base fuel on suspension quality and combustion behavior were determined. The burning behaviors of dilute and dense suspensions were compared, and the results for dense nanosuspensions showed that most particles were burned as a large agglomerate at a later stage when all the liquid fuel had been consumed. Sometimes this agglomerate may not burn if the energy provided by the droplet flame is insufficient. For dilute suspensions, the burning characteristics were characterized by a simultaneous burning of both the droplet and the particles, which integrated into one stage. The fundamental mechanism responsible for bringing the particles out of the droplet, which is a prerequisite condition for them to burn, is different for n-decane- and ethanol-based fuels. For the former, the particles are brought out of the droplet by a disruptive behavior of the primary droplet, which was characterized by multiple-time disruptions and with strong intensity. This was caused by the different boiling points between n-decane and the surfactant. For ethanol-based fuels having no added surfactant, the particles are also brought out of the droplet by disruptive behavior, but characterized by continuous disruptions of mild intensity. This was very likely caused by a continuous water absorption by the ethanol droplet during its burning process.     

Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form-stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.     

Thermal conductivity measurements of magnesium hydride powder beds under operating conditions for heat storage applications

One of the major issues of the change in energy politics is the storage of renewable energy in order to facilitate a continuous energy supply to the grid. An efficient way to store energy (heat) is provided by the usage of Thermochemical Energy Storage (TES) in metal hydrides. Energy is stored in dehydrogenated metal hydrides and can be released by hydrogenation for consumption. One prominent candidate for high temperature (400 °C) heat storage is magnesium hydride. It is a well-known and investigated material which shows high cycling stability over hundreds of cycles. It is an abundant material, non-toxic and easy to prepare in bigger scales. One of the major drawbacks for heat storage applications is the low heat transfer capability of packed beds of magnesium hydrides. In this work we present results of effective thermal conductivity (ETC) which were measured under hydrogen pressure up to 25 bar and temperatures up to 410 °C in order to meet the operating conditions of magnesium hydride as a thermochemical heat storage material. We could show that the effective thermal conductivity of a magnesium hydride – hydrogen system at 410 °C and 25 bar hydrogen increases by 10% from 1.0 W m⁻¹ K⁻¹ to 1.1 W m⁻¹ K⁻¹ after 18 discharging and charging cycles. In dehydrogenated magnesium hydride this increase of the thermal conductivity was found to be at 50% from 1.20 W m⁻¹ K⁻¹ to 1.80 W m⁻¹ K⁻¹ at 21 bar hydrogen. These data are very important for the design and construction of heat storage tanks based on high temperature metal hydrides in the future.     

Effect of silicone oil additive on swelling stress alleviation in the metal hydride reactor

Considerable swelling stress associated with hydrogen absorption process caused by metal hydride expansion is extensively observed in a metal hydride tank which is usually designed for hydrogen storage application. Such swelling stress being applied to tank wall may cause potential safety issue such as tank failure. In the present investigation, silicone oil is selected as an additive incorporating into MlNi_4.5Cr_0.45Mn_0.05 alloy in an attempt to alleviate the swelling stress. The results obtained by a self-built direct swelling stress testing apparatus show that the addition of silicone oil can significantly reduce alloy particle swelling stress. The addition of 3 wt% silicone oil is appropriate to acquire efficient swelling stress alleviation. During cycling the maximum swelling stress increases with charging pressure. The formation of silicone oil thin film on the surface of alloy particles, acting as a “cushion” among alloy particles, would reduce particle agglomeration and enhance particle movement during hydrogen absorption and desorption cycling. This is the reason for the observed swelling stress alleviation by silicone oil.     

Three-dimensional thermocapillary–buoyancy flow of silicone oil in a differentially heated annular pool

In order to understand the characteristics of thermocapillary–buoyancy flow, we conducted a series of unsteady three-dimensional numerical simulations of thermocapillary–buoyancy flow of 0.65cSt silicone oil (Prandtl number Pr = 6.7) in an annular pool with different depth (d = 1–11 mm) heated from the outer wall (radius r_o = 40 mm) and cooled at the inner cylinder (r_i = 20 mm) with an adiabatic solid bottom and adiabatic free surface. Simulation conditions correspond to those in the experiments of Schwabe [D. Schwabe, Buoyant–thermocapillary and pure thermocapillary convective instabilities in Czochralski systems, J. Crystal Growth 237–239 (2002) 1849–1853]. Simulation results with large Marangoni number predict three types three-dimensional flow patterns. In the shallow thin pool (d = 1 mm), the hydrothermal wave characterized by curved spokes is dominant. In the deep pools (d ⩾ 5 mm) the three-dimensional stationary flow appears and this flow pattern corresponds to the Rayleigh-Benard instability, which consists of pairs of counter-rotating longitudinal rolls. When 2 mm ⩽ d ⩽ 4 mm, the hydrothermal wave and three-dimensional oscillatory flow coexist in the pool and travel along the same azimuthal direction with the same angular velocity. The critical conditions for the onset of three-dimensional flows were determined and compared with the experimental results. The characteristics of three-dimensional flows were discussed.     

Scattering of thermal waves and non-steady effective thermal conductivity of composites with coated particles

In this study, thermal wave method is proposed to predict the non-steady effective thermal conductivity of composites with coated particles, and the analytical solution of this problem is obtained. The Fourier heat conduction law is introduced to analyze the propagation of thermal waves in the particular composite. The scattering and refraction of thermal waves by a coated particle in the matrix are analyzed, and the results of the single scattering problem are applied to the composite medium. The wave fields in different material zones are expanded by using the spherical wave functions and Legendre polynomial, and the expanded mode coefficients are determined by satisfying the boundary conditions of the coating layer. The theory of Waterman and Truell is employed to obtain the effective propagating wave number and the non-steady effective thermal conductivity of composites. As an example, the effects of the material properties of the particles, coating and matrix on the effective thermal conductivity of composites under different wave frequencies are graphically illustrated and analyzed. Analysis shows that the non-steady effective thermal conductivity under higher frequencies is quite different from the effective thermal conductivity under lower frequencies. In the region of lower frequency, the effect of the properties of the coating on the effective thermal conductivity is greater. Comparisons with the steady effective thermal conductivity obtained from other methods are also presented.     

Modelling and measurements of heat transfer in charcoal from pyrolysis of large wood particles

The pyrolysis rate limiting heat transfer properties of charcoal from large wood particles are studied by comparing experiments and simulations of transient heat conduction in large charcoal samples. The interior temperatures in cylindrical charcoal samples of 20±2 mm radius were measured during heating from room temperature to 700°C in an inert atmosphere. Simulations are performed for two cases of constant material properties and for two cases of temperature dependent specific heat and/or effective thermal conductivity. The material properties of charcoal used in the simulations are found in literature related to modelling of wood pyrolysis. The simulations show that a constant thermal diffusivity of approximately 0.7 mm²/s agrees better with measured data than the assumption of temperature dependent material properties. Constant material properties are preferred due to simplicity, although the correct interpretation is that the increase in specific heat and effective thermal conductivity with temperature cancel each other.     

Investigation of exfoliated graphite nanoplatelets (xGnP) in improving thermal conductivity of paraffin wax-based phase change material

Composite phase change materials (PCM) for latent heat thermal energy storage were made by mixing two different kinds of exfoliated graphite nanoplatelets (xGnP-1 and xGnP-15) into paraffin wax. Direct casting and two roll milling were used to prepare samples. The investigation on the thermal and electrical conductivity of nanocomposites with these two nanoplatelets was performed. Higher thermal conductivity of composite PCM can be achieved with nanofillers of larger aspect ratio, better orientation and lower interface density. The thermal physical properties of the nanocomposites were investigated by differential scanning calorimetry and thermal gravimetric analysis. It was found that the latent heat of the nanocomposites was not adversely affected by the presence of xGnP nanoplatelets and the thermal stability improved.