Guard Heater Research Articles

Experimental Evaluation of Thermoelectric Generator Performance under Different Heat Conduction Boundary Conditions

Heat conduction boundary conditions play a crucial role in the performance of thermoelectric generators (TEG). The TEG output voltage and power were measured under constant temperature boundary and heat flux conditions to evaluate the TEG performance under different heat conduction boundary conditions. External loading pressure and thermal interface material (TIM) were applied to reduce the interfacial thermal contact resistance. In our measurement setup, a fast-response electronic load was used for the rapid current-voltage scan, which can eliminate the thermal drift caused by the Peltier effect. A guard heater arrangement is used to minimize heat loss. In constant temperature boundary conditions, reducing the thermal contact resistance can increase the effective temperature drop across the TEG module and significantly improve the output voltage and power. But in the constant heat flux conditions, since the heat flux flow through the TEG is unchanged, the temperature drop across the TEG was unaffected by the thermal contact resistance. As a result, the TEG performance was lightly influenced by the thermal contact resistance.

Conjugate turbulent natural convection with thermal radiation in a vertical and inclined wall with heat sources

This work presents an experimental and numerical study of turbulent conjugate natural convection heat transfer in a plate for vertical and inclined positions containing protruding heat sources mounted on a conductive substrate, with thermal radiation effects. Experiments were performed with seven aluminum protruding heat sources uniformly mounted over an upper substrate fiberglass plate. Insulation and a guard heater were used at the back of the substrate plate. The temperature was measured for each power input and downward inclination angle. Numerical simulations were performed to explore the effects of conductivity, upward and downward inclinations, and heater emissivity. The conjugate problem was numerically addressed for a two-dimensional model, steady-state and incompressible flow under turbulent flow conditions. The main effort was to investigate the effects of power dissipation, conductivity plate-fluid ratio, plate inclination angle, and emissivity of the heater on temperature and Nusselt number distribution. The range of the modified Rayleigh number covers the values between 9 × 105 and 1.3 × 1010. The results found that heat loss by radiation was greater than 35%, the heat transfer became less efficient as the downward angle increased, and a jump in the average Nusselt number was found for upward cases due to plume detachment. A correlation for the Nusselt number was also developed.

An accurate steady-state approach for characterizing the thermal conductivity of Additively manufactured polymer composites

This study investigates the accuracy of the steady-state meter bar implementation of ASTM D5470, popularly used to measure the thermal conductivity of 3D-printed polymers and polymer composites manufactured using fused filament fabrication (FFF). We show that a meter bar approach can result in significant error in thermal conductivity measurement, depending on the meter bar design and the thermal conductivity and thickness of the tested samples. This is because a fraction of the thermal power measured by the meter bar bypasses the sample through the surrounding insulation. To address this issue of heat bypass, a modified version of the steady-state guarded hot plate method (ASTM C177) is proposed; this version features a one-sided, cup-shaped guard heater that greatly simplifies the standard implementations of ASTM C177 and C1044. This approach also does not require the calibrated heat flux meter bars described by popular implementations of ASTM D5470 or E1225. Simulations of this approach for typical FFF composite samples shows less than 2% measurement error. A detailed design of this apparatus is presented and employed to measure the thermal conductivity of several 3D-printed PLA polymer composites which showed a significant degree of anisotropy.

Temperature Regulation in Colored Infrared-Transparent Polyethylene Textiles

Summary Effectively regulating heat flow between the human body and its environment not only increases thermal comfort but also presents a novel and potentially cost-effective approach to reducing building energy consumption. Infrared property-engineered textiles have been shown to passively regulate radiative heat dissipation for effective cooling and warming of the human body. However, a lack of dyes that can tune the textile color without compromising the infrared properties remains a major impediment to textile commercialization. Here, we report a new strategy utilizing inorganic nanoparticles as a coloring component for scalable brightly colored, infrared-transparent textiles. The as-fabricated composite textiles not only show a high infrared transparency of ∼80% and a passive cooling effect of ∼1.6°C–1.8°C but also exhibit intense visible colors with good stability against washing. This facile coloration approach will promote the commercialization of radiative cooling textiles in temperature-regulating wearable applications for effective energy savings.

Thermal Properties Characterization of Glass Fiber Hybrid Polymer Composite Materials

In the current work characterization of thermal properties are find out to the prepared specimens of silicon filler hybrid composite materials (silicon filler glass – fiber chop strand). The specimens were prepared by hand layup followed by compression molding machine by non-heating molding technique. Thermal conductivity (K), Coefficient thermal expansion (CTE) and Thermal gravimetric analysis (TGA) are found by composite slab method and by thermal muffler oven in a laboratory. The guard heater is used to supply heat which is measured by voltmeter and ammeter. Thermocouples are placed between the interface of the copper plates and the specimen of silicon filled hybrid polymer composite material (HPC), to read the temperatures. By the experimental readings it is found that the K of silicon filler hybrid composite material directly proportional to the % of silicon fillers for the different trails. The CTE inversely varies with % of silicon fillers and in thermal gravimetric analysis the failure of material takes place at 300°C for a time of 20 minutes and also reduction in mass of silicon inserted hybrid composite material. From the results it has been concluded that the considerable enhance in thermal conductivity with negligible decrease in CTE and increase in thermal resistivity of hybrid composite materials. Â

An improved wall shear stress measurement technique using sandwiched hot-film sensors

ABSTRACT In this letter we present a novel wall shear stress measurement technique for a turbulent boundary layer using sandwiched hot-film sensors. Under certain conditions, satisfactory results can be obtained using only the heat generated by one of the hot-film and a calibration of the sensors is not required. Two thin Nickel films with the same size were used in this study, separated by an electrical insulating layer. The upper film served as a sensor and the bottom one served as a guard heater. The two Nickel films were operated at a same temperature, so that the Joule heat flux generated by the sensor film transferred to the air with a minimum loss or gain depending on the uncertainties in the film temperature measurements. Analytical solution of the shear stress based on the aforementioned heat flux was obtained. The preliminary results were promising and the estimated wall shear stresses agreed reasonably well with the directly measured values (with errors less than 20%) in a fully developed turbulent pipe flow. The proposed technique can be improved to further increase precisions.

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.

Exothermicity Characteristics of Wolf Lake Heavy Oil, Athabasca Tar Sand and Clair Medium Heavy Oil

Abstract High pressure accelerating rate calorimetry (ARC) tests have been performed on three different crude oils at their respective reservoir pressures. The experiments on medium heavy Clair oil and heavy Wolf Lake oil used clean silica sand, incorporating 3% kaolinite, to represent the reservoir matrix, whereas Athabasca Tar Sand was used in its preserved, virgin state. The Clair oil and Wolf Lake oil tests involved high initial water saturations in the reservoir, representative of a post-waterflooded or a post-steam injection state. A second test on Athabasca Tar Sand used a lower oil (bitumen) saturation, but without an adjustment of the original brine saturation. Although the medium Clair oil exhibited high exothermicity throughout the temperature range, the temperature detected for the onset of low temperature oxidation (LTO) was much higher than that for Athabasca Tar Sand. The calculated activation energies also indicate that Athabasca Tar Sand is very reactive in the LTO region. Furthermore, the overall continuity exhibited by the measured exotherms indicates that all three oils are potentially good candidates for in situ combustion as an oil recovery method. Introduction The most important factor governing the selection of any improved oil recovery (IOR) process is, ultimately, the availability of a suitable fluid to inject into the reservoir. For an increasing number of reservoir situations, air injection is gaining increasing attention for both heavy and light oil recovery. The most recent heavy oil projects are in the Cambay Basin, India(1), using conventional ISC, and the first field pilot at Christina Lake, using the advanced THAI (Toe-to-Heel Air Injection) process(2). There have also been significant developments in high pressure air injection into light oil reservoirs, as evidenced by economically viable air injection projects in the Williston Basin, and project feasibility studies for Barrancas, Argentina(3) and Cantun, Mexico(4). There are many economic and technical benefits derived from air injection – excellent displacement efficiency, mobilization of extra oil ahead of the combustion zone, reservoir pressurization, oil swelling from produced CO2, flue gas stripping, injection gas substitution and, for high pressure hot reservoirs, additional enhancing factors such as spontaneous ignition and near miscibility effects. The accelerating rate calorimeter (ARC) was first advocated as a technique for screening reservoir oil candidates for air injection by Yannimaras et al.(5) The technique was developed as a method for studying the reaction kinetics by following the reactions adiabatically, especially at high pressure. Moreover, the method can provide insights to explain the occurrence of low temperature oxidation (LTO) and high temperature oxidation (HTO). To date, investigations using the ARC technique have focused mainly on light oil applications. In this paper, results are presented for heavy crude oils and also Athabasca Tar Sand, to see if the measures of oxidation reactivity obtained are relevant when considering the application of air injection processes using conventional ISC or THAI. Experimental ARC Equipment The main apparatus consists of a pressure shell, inside which the calorimeter ‘bomb’, or test cell, is placed (Figure 1). The cylindrical test cell is surrounded by a top guard heater, as well as side and bottom guard heaters.

The guarded torus: numerical model of a novel transient method for thermal conductivity measurements

Numerous transient methods have been described and applied to measure the thermal conductivity, but none has reached the popularity of the line heat source (LHS) technique, also known as the transient hot wire method. One of the many advantages of the LHS is that it allows us to measure the thermal conductivity directly and almost independently of the otherwise influencing density and specific heat. In some cases, the LHS method cannot be applied because the probe cannot be made thin enough or long enough to regard the heat source as infinitely long. We study the transient behaviour of torus-shaped heaters numerically, and we propose a probe design that brings back the advantages of a LHS, at the expense of a moderate complexity added to the set-up; guard heaters above and below the heated torus minimize vertical heat flux and create a transient behaviour of the probe that is remarkably similar to that of a LHS. The remaining errors made in the determination of the thermal conductivity are ≈5 to 10% and are tolerable in many applications, and can be further reduced to less than 1% by modelling.

Contact Resistance Measurement and Its Effect on the Thermal Conductivity of Packed Sphere Systems

There has been a growing interest in porous systems with a smaller length-scale modeling requirement on the order of each particle, where the existing tools tend to be inadequate. To address this, a Discrete Conduction Model was recently proposed to allow for the transient temperature calculation of 3D random packed-sphere systems for various microstructures. Since many of the motivating applications involve contacting spheres and since there has been a limited number of contact-resistance studies on spheres undergoing elastic deformation, the objective of this study is to obtain measurements of the contact resistances between metallic spheres in elastic contact, as well as to quantify their influence on the effective thermal conductivity. To accomplish this, an experiment was constructed utilizing air and interfacial resistance to replace the functions of the guard heater and vacuum chamber, and in so doing, enabled transient observations. The overall uncertainty was estimated to be ±6%, and the results were benchmarked against available data. A correlation was obtained relating the contact resistance with the contact radius, and results showed the contact resistance to have minimal transient behavior. The results also showed that the neglect of contact resistance could incur an error in the effective thermal conductivity calculation as large as 800%, and a guideline was presented under which the effect of the contact resistance may be ignored. A correlation accounting for the effect of contact resistance on the effective thermal conductivity was also presented.

A Quasi-Steady State Technique to Measure the Thermal Conductivity

A new method is developed for the measurement of thermal conductivity. It combines characteristic advantages of steady-state and transient techniques but avoids major drawbacks of both these classes of methods. On the basis of a simple transient hot wire (THW) or transient hot-strip (THS) arrangement, a direct indicating thermal-conductivity meter is realized by adding only one temperature sensor. After a short settling time during which all transients die out, the instrument operates under quasi-steady state conditions. No guard heaters are required because outer boundaries are free to change with time. The instrument's uncertainty is provisionally estimated to be 3%.

Heat sinks with fluted and wavy plate fins in natural and low-velocity forced convection

The effect on heat transfer of geometrically rearranging the surface area of a finned heat sink is investigated. Novel heat sinks with fluted and wavy plate fin configurations are designed and fabricated together with conventional longitudinal-plate and pin fin heat sinks. The experimental apparatus, consisting of the guard heater assembly, isolation chamber, wind tunnel, and data acquisition instrumentation, is described. The thermal performance of the novel and conventional heat sinks is measured and compared for the horizontal and vertical base plate orientations under natural and low-velocity forced convection conditions. Results, presented as the Nusselt number against the Rayleigh or Reynolds numbers, reveal that the pin fin heat sink generally outperforms the other heat sinks, when the heat sink surface area is held constant. A significant effect on heat transfer of the orientation of the forced flow with respect to the buoyancy flow is observed. Overall, the novel heat sink designs do not yield significantly better thermal performance than an optimized conventional longitudinal-plate heat sink.

Measurement of Thermal Conductivity of Fibrous Insulation Using Guarded Hot Plate Method and Cyclic Heat Method

A thermal conductivity measuring apparatus using both the Guarded Hot Plate (GHP) method and the Cyclic Heat method was designed, and thermal conductivity of a fibrous insulations were measured in atmospheric conditions and in evacuated conditions at temperature range from 100 to 1300°C. When the apparatus using the GHP method was designed, the sizes of the meter and guard heater are determined using the theory by William Woodside. In the Cyclic heat method apparatus, the error magnitude due to difference in period of heat wave was examined, and the time lag method was compared with the amplitude decay method.The thermal conductivities of the fibrous insulations (aluminum-silica) of both the GHP method and the Cyclic Heat method are in good agreement. This result indicates that values of measurements by the nonsteady state method (Cyclic heat method) is in agreement with the steady state method (GHP method) on proper conditions.As applied measurement, I compared thermal conductivity of the brick with the fibrous insulation in atmospheric conditions at temperature range from 100 to 1300°C, and examined the relationship between heat flow and different types of fibrous fleeces in a fibrous insulation in evacuated conditions.

低温アルミニウムの全半球ふく射率の高精度測定法

To measure total hemispherical emissivities eTH(T) of metals at low temperatures, a new transient calorimetric technique is developed to improve its accuracy. The accuracy of this technique mainly depends upon heat loss Qt through a thermocouple (TC) which suspends a specimen and measures its temperature. To reduce Qt, an Iron. Constantan TC with diameter of about 20μm is used, and the temperature profile of TC is controlled by a guard heater attached to the other sides of TC. eTH(T) of aluminum specimens which shaped wire (99.9994% purity) and plate (99.988%) are measured in the temperature range from 125 to 350K. The specimens are mechanically polished. The roughness of the plate surface is measured. Qt, evaluated by a combined radiation and conduction heat transfer analysis, is less than 1% of the total power loss of the specimen. eTH(T) for wire and annealed plate are in good agreement with each other, and close to the values extrapolated from the date obtained above room temperature. An increase in eTH(T) of the plate without annealing may be due to the Beilby/damaged surfaces formed by mechanical polishing.

A Large Block Heater Test for High Level Nuclear Waste Management,

AbstractThe radioactive decay heat from nuclear waste packages may, depending on the thermal load, create coupled thermal-mechanical-hydrological-chemical (TMHC) processes in the near-field environment of a repository. A group of tests on a large block (LBT) are planned to provide a timely opportunity to test and calibrate some of the TMHC model concepts. The LBT is advantageous for testing and verifying model concepts because the boundary conditions are controlled, and the block can be characterized before and after the experiment. A block of Topopah Spring tuff of about 3 × 3 × 4.5 m was sawed and isolated at Fran Ridge, Nevada Test Site. Small blocks of the rock adjacent to the large block were collected for laboratory testing of some individual thermal-mechanical, hydrological, and chemical processes. A constant load of about 4 MPa will be applied to the top and sides of the large block. The sides will be sealed with moisture and thermal barriers. The large block will be heated by heaters within and guard heaters on the sides so that a dry-out zone and a condensate zone will exist simultaneously. Temperature, moisture content, pore pressure, chemical composition, stress, and displacement will be measured throughout the block during the heating and cool-down phases. The results from the experiments on small blocks and the tests on the large block will provide a better understanding of some concepts of the coupled TMHC processes. The progress of the project is presented in this paper.

The thermal behavior of vertically-operated near-adiabatic in-situ combustion tubes

A commonly used approach for studying in-situ combustion processes has employed vertically-operated adiabatic combustion tubes. The data obtained from such apparatus are sometimes subject to interpretation problems because of convective circulations in the annulus, which are induced by the operation of the guard heaters. The main objective of this study was to systematically investigate the operational domain and impact of these convective heat transfers, and, in general, to provide a comprehensive framework for interpreting such experimental data. A microcomputer-based system was developed to automate the operation of the guard heaters and to provide a record of the power outputs of each heater with time. The automated system was used to gather data from five combustion experiments in which operating pressure, injected oxygen concentrations, water/oxygen ratio, and the type of gas in the annulus were varied. The experimental results showed that thermal energy from the heaters, as they responded to the combustion front, was transported upward by convection in the annulus. This energy elevated temperatures in the core behind the front. The tendency for annular convection to occur was found to increase with operating pressure, the on-time of the heaters, and with the Rayleigh number of the gas used in the annulus. Also, as the severity of convection increased there was (a) a reduction in the thermal efficiency of the heaters, and (b) an overall increase in heat loss from the combustion zone which required higher oxygen fluxes in order to avoid declining peak temperatures and to improve oxygen utilization. An analysis of the thermophysical properties of the annulus gases used explained these experimental observations, and demonstrated that gases of lower thermal conductivity may not necessarily reduce heat loss from the combustion tube.

Analysis and design of experimental systems for heat transfer measurement from constant-temperature surfaces

A design method is presented to minimize experimental uncertainties and errors resulting from heat losses in a common type of constant-wall-temperature convection measurement system. The system consists of one or more thick, individually heated metallic plates that are exposed to convection on the top face, with electric heaters on the bottom surface, and are surrounded on all four sides by thermal insulation and/or guard heaters. A heat balance, which includes thermal losses, is applied to the measurement plate to determine the average heat transfer coefficient over the plate. A generalized analysis was made of the convection and conduction heat transfer in the insulation strips separating heated plates. On the basis of this analysis, the correction for the heat lost from a measurement plate and an estimate of the uncertainty of the measurement are presented. The results are presented in the form of generalized (dimensionless) plots of insulation temperature drop, heat losses, and corrections for losses. The effect of an insulation temperature drop on the isothermal wall temperature boundary condition was also considered. It is shown how the results can be used to minimize the over-all correction for losses and the experimental uncertainty by optimizing the design of the insulation strips between plates. As an example, the analytical and optimization methods are applied to a typical experimental system.

An automated flow calorimeter for heat capacity and enthalpy measurements

An automated flow calorimeter has been developed for the measurement of heat capacity and latent enthalpies of fluids at elevated temperatures (300–700 K) and pressure (<30M Pa) with a design accuracy of 0.1%. The method of measurement is the traditional electrical power input flow calorimeter, utilizing a precision metering pump, which eliminates the need for flow-rate monitoring. The calorimeter cell uses a unique concentric coil design with passive metal radiation shields and active guard heaters to minimize heat leakage, eliminate the traditional constant-temperature bath, and facilitate easy component replacement. An additional feature of the instrument is a complete automation system, greatly simplifying operation of the apparatus. A novel multitasking software scheme allows a single microcomputer simultaneously to control all system temperatures, provide continuous monitoring and updates on system status, and log data. Preliminary results for liquid water mean heat capacities show the equipment to be performing satisfactorily, with data accuracies of better than ±0.3%. Minor equipment modifications and better thermometry are required to reduce systemic errors and to achieve the designed operational range.

Measurement of Total Hemispherical Emissivities of Metal Wires by Using Transient Calorimetric Technique

An application of the transient calorimetric technique was studied for measuring total hemispherical emissivities of metal wires. A unique idea of using guard wires, which have a function similar to usual guard heaters, was incorporated into the emissivity apparatus to reduce conduction heat losses through thermocouple leads suspending a specimen. Emissivity measurements were performed by using a copper wire as a specimen. The heat losses through the leads were analyzed in considering radiation heat transfer from the leads, and the effect of the guard wires on the heat losses was clarified. Temperature distributions in the specimen were also obtained analytically. From these results, it was found that an improved transient calorimetric technique in this work is applicable to the emissivity measurement on metal wires, and that the accuracy in the measurements is adequate.

Transient calorimetric technique for measuring total hemispherical emissivities of metals with rigorous evaluation of heat loss through thermocouple leads

An apparatus for the transient calorimetric technique used for measuring the total hemispherical emissivities of metals has been developed. Measurements have been made on specimens of copper and aluminum in the temperature range of 330–630 K. An exact analysis is given of the conduction heat loss through thermocouple leads suspending the specimen. From the numerical results of the lead-wire heat loss, the significant effect of a guard heater employed in the apparatus is illustrated. By taking account of the heat loss, the emissivities of the specimens are accurately evaluated and are compared with the data of previous investigators.