Guarded Heat Research Articles

Thermo-physical and fire properties of sustainable bio composites: Experimental and computational analysis

ABSTRACT Bio-composites were prepared by incorporating broom grass, fishtail palm, sansevieria fibers as reinforcement in unsaturated polyester resin using hand lay-up method. The thermo physical and fire properties of bio-composites has been examined by varying the fiber content (0 to 39 wt. %) and temperature (30–120°C). The thermal insulation and heat storage capability of samples was assessed using guarded heat flow meter and differential scanning calorimeter. The experimental results reveal that, as the weight fraction of fiber increased, the insulation capability of composites increased, whereas it decreased with the temperature. The response of specific heat and thermal diffusivity of composites with temperature was analyzed. Temperature distribution and heat transfer through the composite materials was computed using ANSYS fluent solver. The rate of heat transfer through the fishtail palm fiber composite is 14.02% lesser than glass fiber composite. Cone calorimeter was used to measure fire behavior of the samples and the results reveal that the broom grass fiber composite material possesses better fire resistance characteristics against fire hazard. Morphological study of composites was performed with the help of Scanning Electron Microscopy (SEM) for the visualization of homogeneity of surfaces. Developed bio-composites are the potential materials to replace petrochemical-based products.

Silver (Ag) nanoparticle-decorated expanded graphite (EG) epoxy composite: evaluating thermal and electrical properties

Synthesized expanded graphite (EG) was decorated with Ag nanoparticles using solid-state decomposition by a simple ‘mix and heat’ technique. The epoxy composite was fabricated by a stir casting method with an ultrasonic procedure in order to enhance the thermal conductivity (TC). The morphology of filler (EG–Ag) studied under X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) confirms the attachment of Ag nanoparticles on EG surfaces. The TC of the composite containing 10 wt% of EG–Ag hybrid filler (EG–Ag(10)-Ep(90)) measured using guarded heat flow meter technique was found to be 2.52 W/mK, which is almost 13-fold enhancement as compared to the neat epoxy. The thermo-gravimetric analysis (TGA) of EG–Ag(10)-Ep(90) confirmed thermal stability which enhanced from 289 to 327 °C at 5% weight loss, in N2 atmosphere. The improvement in TC was ascribed to the synergistic effect of micro-nano (EG–Ag) hybrid filler, in which Ag nanoparticles acted as ‘spacer’ between EG threads by reducing interfacial Kapitza resistance as evidenced from fracture surface analysis. After attaining 3D percolation limit, the hybrid composite at 10% filler fraction (EG–Ag) entered the electrical conductivity of the semiconducting range which was enhanced by fourteen orders from neat epoxy.

Influence of Coil Configuration and Operating Conditions on Heat Transfer in Inductively Heated Risers

Electromagnetic (EM) induction heating of open top risers represents an energy-efficient method to preserve mass feeding whilst allowing for the use of a smaller riser and thus higher casting yield. Numerical simulation results for an ill-designed, concentric riser and casting assembly predicting the EM field characteristics and solidification behavior in such systems are presented herein. A vector potential formulation of the EM field was used and solved using a hybrid control volume/integral method, featuring temperature-dependent electrical conductivity. The heat transfer equation with phase change was also solved using the control volume technique. The computed results showed that use of induction heating could shift the hot spot from the casting to the riser. It was also found that use of higher coil frequencies produced a guard heating effect at the periphery of the riser, which was then applied using a new L-shaped coil geometry.

Study on thermal conductive epoxy adhesive based on adopting hexagonal boron nitride/graphite hybrids

The effect of hybrid filler on newly developed thermal conductive epoxy adhesive has been reported in this paper. The hybrid filler containing hexagonal boron nitride (h-BN) and graphite (Gr) with the mass ratio of 1:1 was loaded in to the epoxy resin with different wt% ratio (10:90, 20:80, 30:70, 40:60, 50:50 and 60:40). The corresponding thermal conductivity of adhesives was measured using guarded heat flow meter technique which enhanced up to 2.025 W/mK with the loading of 60 wt% of hybrid filler. Lap shear strength of the adhesive containing 40 wt% of hybrid filler was higher than the pristine epoxy resin thereby confirming reinforcing effect of hybrid filler into the epoxy matrix. Afterward, the shear viscosity of uncured epoxy resin filled with hybrid fillers were analyzed by using parallel plate rheometry. Strong shear thinning behavior was exhibited at higher hybrid filler loading. Further, lower loss factor and higher Tg values were observed with increase in the hybrid filler (h-BN/Gr) loading thereby confirming the higher cross linking density and lower mobility of epoxy chain. Thermal stability of hybrid Gr/BN–epoxy composite adhesives was studied using thermo gravimetric analyzer. Scanning electron microscopy confirmed the uniform deposition of hybrid filler on to the surface of epoxy resin.

Experimental Study on Optimization of Thermal Properties of Natural Fibre Reinforcement Polymer Composites

In the present work, an experimental approach was used to find out the thermal conductivity of rice husk filled polymer composites using guarded heat flow meter method in accordance with ASTME-1530 standard. The result shows that the incorporation of rice husk reduces the thermal conductivity of polymer resin and improves its insulation capability. At last, an attempt has been made to optimize the thermal properties of rice husk reinforced polymer composites (RHPC) materials using Taguchi technique. In this work, the ANOM results showed that the combination of rice husk particle size of 250 μm with volume fraction of filler material (45%) with vinyl ester as the matrix material was beneficial for minimizing the thermal properties of rice husk particles reinforced polymer composites and the degree of contribution of the parameters to the system were volume fraction > particle size > polymer resin. From ANOVA results, it was found out that the % of filler material has major influence (79.588%) on minimizing thermal conductivity and the particle size has less effect (1.126). Finally, the results revealed that using rice husk particles as reinforcement for polymer matrix could successfully develop beneficial composites and can be used for thermal applications.

Development of a guard-heated thermistor probe for the accurate measurement of surface temperature

It is difficult to accurately measure the absolute value of the surface temperature of a material through ordinary methods such as with a thermocouple or a thermistor. This is due to heat loss along the electrical lead wires of the component, which causes conduction error and can ultimately influence the results. In this paper, we propose a thermistor probe that utilizes a guard heater. The goal of this design is to obtain an accurate measurement of the surface temperature of a material at a higher temperature than the ambient temperature. The probe consists of two thermistors, each with a diameter of 0.43mm. One thermistor was utilized for the temperature sensor, while the other was used with the guard heater to minimize heat loss, and inserted into a fluorinated ethylene propylene (FEP) tube. The guard heater was placed above a half-exposed thermistor, and operated as both a sensor and a heater in order to minimize the temperature difference between the two thermistors. To evaluate the minimization of heat loss along the thermistor’s lead wires in a surface temperature measurement, experiments were conducted with the surface of an aluminum block heated to 35.00°C in two scenarios. Measurements were taken using a guard-heated thermistor probe with and without guard heating. The experimental results showed that the surface temperature was measured as 34.98°C in the scenario where guard heating was utilized, and 34.79°C in the scenario where it was not utilized. Therefore, the results experimentally demonstrated that the guard heater allowed the thermistor probe to provide a more accurate measurement of the surface temperature, regardless of the contact method. In addition, a two-dimensional axisymmetric heat conduction analysis was also conducted. The purpose was to quantitatively evaluate the amount of heat that passes through the lead wires of a thermistor while it is measuring the surface temperature of a heated material. The calculation results confirmed that a guard heater successfully minimized heat loss through the lead wires. The minimized heat loss was −0.016mW, which was one-sixth of the loss measured in the scenario without guard heating.

Techniques for Reducing Thermal Contact Resistance in Steady-State Thermal Conductivity Measurements on Polymer Composites

The National Physical Laboratory (NPL) has developed a new variation on the established guarded hot plate technique for steady-state measurements of thermal conductivity. This new guarded hot plate has been specifically designed for making measurements on specimens with a thickness that is practical for advanced industrial composite materials and applications. During the development of this new guarded hot plate, NPL carried out an experimental investigation into methods for minimising the thermal contact resistance between the test specimen and the plates of the apparatus. This experimental investigation included tests on different thermal interface materials for use in another NPL facility based on a commercial guarded heat flow meter apparatus conforming to standard ASTM E1530-11. The results show the effect of applying different quantities of the type of heat transfer compound suggested in ASTM E1530-11 (clause 10.7.3) and also the effect on thermal resistance of alternative types of thermal interface products. The optimum quantities of two silicone greases were determined, and a silicone grease filled with copper was found to offer the best combination of repeatability, small hysteresis effect and a low thermal contact resistance. However, two products based on a textured indium foil and pyrolytic graphite sheet were found to offer similar or better reductions in thermal contact resistance, but with quicker, easier application and the advantages of protecting the apparatus plates from damage and being useable with specimen materials that would otherwise absorb silicone grease.

Investigations on Thermal Conductivities of Jute and Banana Fiber Reinforced Epoxy Composites

The Jute and Banana fibers are used as reinforcement in epoxy resin matrix for making partially green biodegradable material composite via hand lay-up technique. The thermal conductivity of the jute fiber epoxy composites and banana fiber epoxy composites at different volume fraction of the fiber is determined experimentally by using guarded heat flow meter method. The experimental results had shown that thermal conductivity of the composites decrease with an increase in the fiber content. Experimental results are compared with theoretical models (Series model, Hashin model and Maxwell model) to describe the variation of the thermal conductivity versus the volume fraction of the fiber. Good agreement between theoretical and experimental results is observed. Thermal conductivity of Banana fiber composite is less when compared to that of Jute composite which indicates banana is a good insulator and also the developed composites can be used as insulating materials in building, automotive industry and in steam pipes to save energy by reducing rate of heat transfer.

Vacuum Insulation Panels: Analysis of the Thermal Performance of Both Single Panel and Multilayer Boards

The requirements for improvement in the energy efficiency of buildings, mandatory in many EU countries, entail a high level of thermal insulation of the building envelope. In recent years, super-insulation materials with very low thermal conductivity have been developed. These materials provide satisfactory thermal insulation, but allow the total thickness of the envelope components to be kept below a certain thickness. Nevertheless, in order to penetrate the building construction market, some barriers have to be overcome. One of the main issues is that testing procedures and useful data that are able to give a reliable picture of their performance when applied to real buildings have to be provided. Vacuum Insulation Panels (VIPs) are one of the most promising high performing technologies. The overall, effective, performance of a panel under actual working conditions is influenced by thermal bridging, due to the edge of the panel envelope and to the type of joint. In this paper, a study on the critical issues related to the laboratory measurement of the equivalent thermal conductivity of VIPs and their performance degradation due to vacuum loss has been carried out utilizing guarded heat flux meter apparatus. A numerical analysis has also been developed to study thermal bridging effect when VIP panels are adopted to create multilayer boards for building applications.

An Investigation on Thermal Conductivity of Graphite Filled PA66 Composites

The present study is aimed to find effect of graphite filler on the thermal conductivity of PA66 composites. Thermal conductivity tests were conducted as per ASTM E-1530 using guarded heat flow meter. It was found that graphite reinforcement in PA66 significantly improved the thermal conductivity and thermal diffusivity of the composites. The experimental value of thermal conductivity of the proposed composites is compared with different theoretical models. The results indicate that Graphite reinforcement in PA66 can be a potential composite material for friction components in heat management applications such as gears and bearings.

Steam/oxygen gasification system for the production of clean syngas from switchgrass

A pilot-scale 25kg/h fluidized bed, oxygen/steam blown gasifier and syngas cleaning system was developed to convert switchgrass into clean syngas. The system is rated for operation at gage pressures up to 1bar. The reactor vessel incorporated a novel guard heating system to simulate near-adiabatic operation of large commercial-scale gasifiers, and was effective for gasification temperatures up to 900°C. After removing particulate from the gas stream via cyclones, a warm-gas cleaning operation based on oil scrubbing was used to remove tars. Sulfur compounds were removed via solid-phase adsorption. Ammonia was removed by water scrubbing. Baseline gasification tests with steam and oxygen were conducted at equivalence ratios (ER) between 0.21 and 0.38 using switchgrass as fuel. Measurements on the raw and cleaned syngas included permanent gas composition, C2 hydrocarbons, water, heavy and light tars, gasification residues (char and ash), hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), ammonia (NH3), and the first reported measurements of hydrogen cyanide (HCN) for oxygen/steam blown gasification. Heavy tars were removed with high efficiency by the method employed, although more difficult to remove light tars reduced overall tar removal efficiency to less than 80%. The sulfur scrubbing system demonstrated 99.9% removal efficiency, resulting in less than 200ppb of H2S in the cleaned gas. The NH3 scrubbing system also accomplished greater than 99.9% removal efficiency, resulting in final NH3 concentrations of less than 1ppm.

Development of a Heat Conduction Model and Investigation on Thermal Conductivity Enhancement of AlN/Epoxy Composites

This work explores on the possibility of enhancement of heat conduction capability of a typical particulate filled polymer composite. A theoretical model for one dimensional heat conduction through such a composite has been developed using the law of minimal thermal resistance and equal law of the specific equivalent thermal conductivity. Based on this model, a mathematical correlation between the effective thermal conductivity of the composite and the filler content is proposed. The proposed model is then validated through experimentation conducted in controlled laboratory conditions. Different samples of aluminum nitride filled epoxy composites are prepared by simple hand layup technique. The content of the conductive filler (AlN) in the composites is varied in the range 0% to 15% by volume. Thermal conductivities of these composite samples are measured according to the ASTM standard E-1530 by using the UnithermTM Model 2022 tester, which operates on the double guarded heat flow principle. Gradual increase in thermal conductivity has been observed, with increase in conductive filler percentage in composite samples. Values obtained using the proposed mathematical correlation are then compared with the measured experimental results and also with values estimated with other established correlations such as Rule-of-Mixture (ROM), Maxwell's model, Bruggeman model and Nielson-Lewis model. This comparison reveals that while none of the above models are correctly predicting the effective thermal conductivity of the composites, the results obtained from the proposed model by incorporating a correction factor are in good agreement with the experimental data.

Effects of coating thickness on thermal conductivities of alumina coatings and alumina/aluminum hybrid materials prepared using plasma electrolytic oxidation

The aim of this paper was to investigate the effects of coating thickness on the thermal conductivities of alumina coatings and alumina-coated aluminum substrates as hybrid materials prepared using plasma electrolytic oxidation (PEO). The coating thermal conductivities were measured with the comparative guarded heat flow method (also called cut-bar method). The guarded insulation system adopted in this method can effectively reduce the radial heat exchange and the axial shunting exchange, thus improving the precise of the measurement. The experimental results show that with the coating thickness increasing from 10μm to 100μm, the thermal conductivity has a slight rise from 1.9W/m·K to 2.5W/m·K. The gradient structure of the coating in grain size was believed to be one of the possible reasons to cause this slight variation. For the alumina-coated substrates (i.e., alumina/aluminum hybrid materials) with a determined substrate thickness, the increase in the coating thickness has more influence on the equivalent thermal conductivity of the hybrid materials than that in the effective thermal conductivity of the coating themselves.

Novel techniques for measurement of thermal conductivity of both highly and lowly conducting solid media

Two novel methods for the measurement of the thermal conductivity of solids are described. One of the methods serves for high-conductivity media (all metals), while the second is appropriate for media of low conductivity (as low as that of air). Both of these methods are distinguished by the unique simplicity of their constituents, the low cost of the measurement apparatus, the easy-to-use operational protocol, and the inherent control of the level of accuracy. The method used for the measurement of high conductivity values is based on a quasi-steady model. Its experimental implementation demonstrated excellent agreement with literature values and indicated that insulation was not necessary to achieve high accuracy. In an enhanced form, it could be applied over a very wide range of temperatures. For low-conductivity media, in contrast to common practice, the experimental approach did not involve guard heating. Synergistic numerical simulation provided the conditions for optimal use of the measurement device. In particular, the accuracy of the measured conductivity values could be predicted without approximation, and guidance was provided for an experimental protocol according to which any desired accuracy could be obtained. Numerical simulation was also used to support the measurement technique for high-thermal-conductivity media. The synergistic use of experimentation and numerical simulation enabled highly accurate values for thermal conductivity to be obtained in the most simple, least costly, and easy-to-use device.

Reduction in emittance of thermal radiator coatings caused by the accumulation of a Martian dust simulant

Measurements were made of the effective emittance of three types of radiator coatings as a Martian dust simulant was added to the radiator surfaces. The apparatus consisted of multiple radiator coupons on which Carbondale Red Clay dust was deposited. The coupon design employed guard heating to achieve the accuracy required for acceptable emittance calculations. The apparatus was contained in a vacuum chamber that featured a liquid-nitrogen cooled shroud that simulated the Martian sky temperature. Three high-emittance radiator coatings were tested: two while silicate paints, Z-93P and NS-43G, and a silver Teflon film. Radiator temperatures ranged from 250 to 350 K with sky temperatures from 185 to 248 K. As dust was added to the radiator surfaces, the effective emittance of all three coatings decreased from initial values near 0.9 to a value near 0.4. A low-emittance control surface, polished aluminum, demonstrated a rise in effective emittance for thin dust layers, and then a decline as the dust layer thickened. This behavior is attributed to the conductive resistance caused by the dust layer.

Experimental evaluation of the thermal properties of two tissue equivalent phantom materials

Tissue equivalent radio frequency (RF) phantoms provide a means for measuring the power deposition of various hyperthermia therapy applicators. Temperature measurements made in phantoms are used to verify the accuracy of various numerical approaches for computing the power and/or temperature distributions. For the numerical simulations to be accurate, the electrical and thermal properties of the materials that form the phantom should be accurately characterized. This paper reports on the experimentally measured thermal properties of two commonly used phantom materials, i.e. a rigid material with the electrical properties of human fat, and a low concentration polymer gel with the electrical properties of human muscle. Particularities of the two samples required the design of alternative measuring techniques for the specific heat and thermal conductivity. For the specific heat, a calorimeter method is used. For the thermal diffusivity, a method derived from the standard guarded comparative-longitudinal heat flow technique was used for both materials. For the 'muscle'-like material, the thermal conductivity, density and specific heat at constant pressure were measured as: k = 0:31 0:001W(mK)-1, rho = 1026 7kgm-3, and c p = 4584 107J(kgK)-1. For the 'fat'-like material, the literature reports on the density and specific heat such that only the thermal conductivity was measured as k = 0:55W(mK)-1.

The Influence of End Conditions on Minimum Film Boiling From a Cylinder

Upon preparing for an experiment in which liquid-solid contacts were to be measured near the minimum film boiling point in flow boiling, it became clear the thermal condition of the heater ends could exercise undue restrictions on the achievable minimum heat flux and wall superheat. For the experiment in question, the heater was to be heated electrically while suspended horizontally across a flow of Freon-11 (R-11). The heater surface was mounted flush with the inside wall of the test section, bringing into play the interaction with the wall as well as potential axial heat loss from the heater during film boiling. Thus the authors set out to investigate the influence of end conditions using a simpler pool boiling apparatus very similar to the flow boiling apparatus. This paper details how modifications of the end conditions for an electrically heated cylinder influence the minimum film boiling point that is achievable in R-11. The modifications consisted of providing guard heating of the ends along with properly designed heater supports so that the heater temperature does not fall off precipitously near the ends. If the end temperature is not maintained high enough, transition boiling begins at the ends while the majority of themore » heater is immersed in a vapor film. Once the film begins to collapse, it becomes unstable and nucleate boiling ensues.« less

New approach to the measurement of the thermal conductivity of electrical conductors

A novel method for measuring the thermal conductivity of an electrical conductor is described. The heat flow is radial, giving important advantages for high-temperature studies. It is an electrical method and is, therefore, most suited to resistive materials which have a high temperature coefficient of resistance, such as semiconductors. Since the outside temperature of the sample need not be determined directly, and the sample is best kept thin, the method holds promise for studies of liquids, especially where convection problems have occurred in the past. In many of these aspects, the proposed method works best where the conventional axial guarded heat flow method fails.

Experimental Determination of the Thermal Conductivity of Molten Lithium from 320° to 830°C

A comparative, axial-heat-flow apparatus with compensating guard heating was developed to determine the thermal conductivity of 99.8+ wt% molten lithium. Contrary to the data for two other alkali metals (sodium and potassium), the conductivity of lithium was found to increase linearly from 0.445 W/cm·°C at 320°C to 0.606 at 830°C. The total uncertainty in the results was conservatively estimated to vary from ±8% to ±15% from the lower to the higher temperatures. Unpublished data from 282° to 521°C by the Naval Research Laboratory average 4% below the present results. The values predicted by two correlations—the equation of Ewing et al. and the Wiedemann—Franz relationship—are uniformly 10% and 6% higher, respectively, than the present data.

MEASUREMENT OF THE THERMAL CONDUCTIVITY OF FIRELAY REFRACTORIES*

AbstractThermal conductivity determinations have been made on superduty fire‐clay, silica, and kaolin refractories, using the Globar‐heated, water calorimeter apparatus previously described. The arrangement, which is satisfactory for insulating firebrick, gives considerable error when firebrick samples are tested. Results obtained by different arrangements of the guard samples and heating elements are shown as well as a final arrangement which gives a satisfactory condition of parallel heat flow.