Magnetic Resonance Thermometry (MRT) is a developing diagnostic technique that leverages advanced medical technologies to accurately measure the temperature of a fluid flow within and around complex geometries. The full three-dimensional temperature field obtained by MRT can be used to analyze heat transfer characteristics and potentially investigate thermal boundary layers near arbitrarily complex surfaces. This technique requires neither optical nor physical accessibility, thereby enabling a wide range of engineering applications. This paper describes the current state of the art for MRT measurement, detailing turbulent water channel tests, materials selection, scanning parameters, data analysis of time-averaged temperature measurements, and uncertainty estimates. The purpose of this work was to evaluate and refine the MRT technique to increase the accuracy of temperature measurements and minimize the error in fully turbulent flow measurements. In the present study, a plate with a vertical cylinder extending from both of its sides was placed between two channels, and a diagonal hole was drilled through the cylinder from one side of the plate to the other. This enabled fluid from one channel to mix with the fluid in the other. This experiment studied the mixing of two fluids at different temperatures. The upstream temperatures of each fluid were measured with thermocouples. Both flows were fully turbulent, and the colder temperature channel had a Reynolds number of 11,800. Tests were run with four different fluid temperatures for calibration and to determine any temperature dependence of measurements. Three-dimensional temperature field measurements are reported and details about data processing and procedures to conduct the experiments are provided. This work resulted in several notable improvements to MRT experimental methods. The test section and water channel were designed to limit the effects of thermal expansion in the stereolithography materials used for manufacturing the complex internal flow geometry. Multiple echo scans were used to minimize the effects of magnetic field drift commonly observed in extended scanning periods in MRI systems. Data analysis techniques were used to quantify expansion effects for both hot and cold flow cases. To quantify measurement uncertainty, the standard deviation of the mean was calculated at each data point across different scan numbers and confidence intervals established using a student t-test. An improved data processing code was used to filter data resulting in increased measurement accuracy and reduced uncertainty to less than 1 °C for most of the domain. Future work will further refine the experimental techniques to improve scanning procedures, employ high conductivity ceramics and larger geometries with relevant applications, and simplify data processing methods to generate full-field flow temperature data.

In this work, the heating process for apple, eggplant, zucchini and potato by means of evaluation of their thermal properties and the Biot number determined in experimental form is presented. The heating process is carried out using a solar cooker box-type as heating device. The thermal experimental properties determined are conductivity (k), density (D), specific heat (C), diffusivity (Dif) and the Biot number (Bi) for each product evaluated. In the experimentation, temperatures for center and surface in each product and water were measured in controlled conditions. For those measures, a device Compact Fieldpoint and thermocouples placed in the points studied were used. By using correlations with temperature as function, k, D and C were calculated, while by using equations in transitory state for the products modeled as sphere and cylinder was possible to estimate the Biot number after calculation of the heat transfer coefficient for each case. Results indicate the higher value for k, C and Dif correspond to zucchini (0.65 W/m °C, 4084.5 J/kg °C, 1.5 × 10−7 m 2 ), while higher value for D correspond to potato (1197.5 kg/m 3 ). The lowest values for k and C were obtained for potato (0.59 W/m °C, 3658.3 J/kg °C) while lowest values for D and Dif, correspond to zucchini (998.2 kg/m 3 ) and potato (1.45 × 10 −7 m 2 /s) respectively. The maximum and minimum values for Bi corresponded to potato (21.4) and zucchini (0.41) in respective way. The results obtained are very useful in applications for solar energy devices, where estimates for properties are very important to generate new results, for example, numerical simulations. Also, results could be used to evaluate the cooking power in solar cookers when the study object is oriented in that direction.

A growing interest has developed in the use of carbon nanotube (CNT) arrays as thermal interface materials (TIMs). However, theoretical modeling of CNT TIMs has largely been limited to semi-empirical methods without detailed consideration of array microstructure, primarily due to the inherent randomness of the microstructure and the computational complexity involved in full atomistic modeling of CNTs. In this work, we report combined thermo-mechanical simulation of CNT arrays with a coarse-grain approach for the mechanics modeling and a thermal network approach for the heat transfer modeling. Parametric studies on the effects of CNT height on the Young’s modulus and buckling load of CNT arrays are reported. The thermal network model is used to estimate the pressure dependence of diffusive and tip contact resistances of CNT arrays; the predictions are compared with thermal resistance measurements using the photoacoustic method. The resulting simulation framework enables a particularly rich and broad thermo-mechanical data set. Selected parametric variations are computed to assess the stress-strain behavior, effective conductivity within the CNT array, and aspects of the contact topologies of the CNT-substrate interface.

This paper reports a numerical study on the discharge of carbon dioxide using a non-thermal dielectric barrier discharge (DBD) plasma reactor at ambient conditions. DBD plasma reactors have been used for various applications due to their ease of production, process control, operation at different conditions. The applications of DBD plasma reactors include discharge of gases. Carbon dioxide is a greenhouse gas formed as a byproduct of fossil fuel combustion. Use of DBD non-thermal plasma reactor can be a promising technology for carbon dioxide mitigation due to its operation at low temperatures, lack of need for catalysts, and flexibility in controlling the products generated. In this study, a tubular DBD non-thermal plasma reactor was modeled with different electrode materials separated by different dielectric materials. The aim was to provide guidelines for the design and material selection for optimizing DBD plasma reactors for CO 2 discharge. A parametric set of simulations was performed using a finite element solver to investigate how electrode and dielectric materials affect the discharge volume of CO 2 and power requirement of the non-thermal plasma discharge of CO 2 . The results showed that electrode material did not affect the discharge or the power requirement. However, dielectric material with higher permittivity or lower conductivity increased the gas discharge and power requirement. Among the analyzed materials, aluminum electrode and mica tube were suggested based on the simulation results for the maximum gas discharge and low power requirement.

Void formation is encountered in the form of air pockets during preparation of composite thermal energy storage systems, consisting of phase change materials (PCM) infiltrated into a high-conductivity porous structure. The presence of voids within the pores of a porous structure degrades the thermal and phase change behavior of such composites. Recent work devoted to multiphase modeling of the infiltration of PCM in liquid state into porous media and formation of voids showed that among the various contributing driving forces (i.e. gravity, pressure gradient and interfacial forces), the interfacial forces (resulting from surface tension and contact angle) play a significant role at the pore level. Additionally, modeling the solidification and melting of PCM within the pores in presence of a void revealed that there is a temperature gradient along the interface between the PCM and void. Considering the surface tension as the major driving force at the pore level, this temperature gradient is large enough to give rise to a gradient in surface tension that then triggers the Marangoni convection at the interface. Thus, as a convection mechanism, it affects the phase change process as well as the interface shape. Therefore, in this paper, the effects of the Marangoni convection on PCM solidification time and shape of the interface was investigated at the pore level. A numerical approach was employed for solidification of a PCM based on the combination of the Volume-of-fluid (VOF) and enthalpy-porosity methods, including the variation of the surface tension with temperature, i.e. Marangoni effects. A two-dimensional model of a pore was developed based on the average geometric features of the pores in a porous structure with interconnecting pores. Following the grid independence study, the transient simulation of solidification was performed, whereas the PCM within the pore and the air within the void were treated as incompressible liquid and compressible gas, respectively. The liquid density change during the solidification was included to explicate the formation of shrinkage void and its distribution within the pores. The PCM solidification time and shape of the final interface between the PCM and air pocket (representing the amount and distribution of the shrinkage void evolving during the solidification) were extracted and compared between the cases with and without Marangoni convection. For verification purposes, the volume of the predicted infiltration void is in agreement with experimental measurements and the volume of the shrinkage void shows a good agreement with theoretical volume change. The final shape of the interface was justified and turned out to be in agreement with the prevailing Marangoni convection pattern.

Thermoelectric Generators (TEG) can be used on automobiles to harvest energy from exhaust waste heat. Besides the improvement on material side, the optimization of geometry of the module is also important to maximize the output power density and will be addressed in this paper. A thermal resistance network is established based on heat conduction and radiation from heat source to heat sink. Although the heat transfer model is based on cylindrical exhaust pipe geometry, the thermo-element is approximated as plane geometry because the ceramic layer and thermoelectric layer are much smaller compared with the exhaust pipe diameter. The TE material we proposed to recover waste heat energy is magnesium silicide (Mg 2 Si), which has a reasonable figure of merit in the automobile exhaust temperature range, and the process is thermal spray compatible, which is a mass productive method currently under investigation. Another material that used for comparison is titanium oxide. Based on the Seebeck coefficient, thermal and electrical conductivity of our thermal sprayed samples, the thermoelectric leg length and the area ratio between thermoelectric element and total module area are optimized for maximum power density output. The optimal leg length is around 0.85mm, and the air gap is as small as possible. A parameter sensitivity analysis is conducted to investigate the influence of ceramic layer thickness, exhaust pipe radius, electrical contact resistance, hot and cold side temperature, Seebeck and electrical conductivity on the optimal leg length.

We introduce the modulation-doping strategy in bulk SiGe nanostructures to improve the thermoelectric power factor. By separating charge carriers from their parent atoms via embedding heavily doped nanoparticles inside an intrinsic host matrix, the ionized impurity scattering rate could be largely reduced, resulting in enhanced mobility. By band engineering, the carriers can spill over from nanoparticles into the host matrix, resulting in similar carrier concentrations, Fermi levels and consequently Seebeck coefficients as those of the uniform nanocomposites. In addition, nanoparticles with low thermal conductivities can further reduce the overall thermal conductivity of the sample. Combining the enhanced electrical conductivity, the reduced thermal conductivity and the unaffected Seebeck coefficient, we were able to enhance the thermoelectric properties of Si-rich Si 95 Ge 5 . And therefore were able to fabricate a low-cost sample with a competitive performance as those of the state of the art Si 80 Ge 20 .

Copper nanoparticles were incorporated with Carbon Nanotube (CNT)/polymer nanocomposites in order to enhance the thermopower by enlarging the energy gap between the Fermi level and the mean of differential electrical conductivity. The thermopower was increased as ∼ 2 times with 1 vol% of copper nanoparticles. The effects of copper concentration on the electrical and thermal properties of the composites were studied.

Phase-change materials (PCMs) can absorb large amounts of heat without significant rise of their temperature during the melting process. This effect may be utilized in thermal energy storage and passive thermal management. In order to enhance the rate of heat transfer into PCMs, various techniques have been suggested, like fins, metal and graphite-compound matrices, dispersed high-conductivity particles inside the PCM, and micro-encapsulation. The present work deals with a hybrid PCM-air heat sink. The heat is dissipated on the heat sink base, and may be either absorbed by the PCM stored in compartments with conducting walls, or dissipated to the air using fins, or both. The heat sink is made of aluminum 6061. Eicosane (C 20 H 42 , 96% purity, nominal melting temperature 36.7°C) is used as the PCM. In order to exclude the effect of sensible heating below the melting temperature, a controllable environment is used. The latter is created in a programmable forced-circulation oven. A simplified thermal model is developed for a conservative estimation of temperature growth of the heat sink base. The results of this model are compared to the experimental results. The relative contributions of heat accumulation, both by latent and sensible heat, and of heat removal by air are presented and discussed.

A well-established method for determining the thermal diffusivity of materials is the laser flash method. The work presented here compares two analysis methods for flash heating tests on anisotropic carbon bonded carbon fiber (CBCF). This material exhibits a higher conductivity in the direction in which the fibers are oriented than in the direction perpendicular to the fiber orientation. The two analysis methods being compared in this experiment use different portions of the data in obtaining results. One method utilizes the temperature data from the entire surface of the sample by examining 201 temperature histories simultaneously, with each temperature history originating from an individual pixel within a line across the middle of the sample. The other analysis method utilizes only the temperature history from a single pixel in the center of the sample, similar to the data which is traditionally generated using the classical flash diffusivity method. Both analysis methods include accommodations for modeling the penetration of the laser flash into the porous surface of the CBCF material. Additionally, both models include a parameter which accounts for the non-uniform heating of the sample surface from the flash. Although the sample surface is ostensibly heated uniformly in flash diffusivity experiments, the heating has been found to be somewhat non-uniform, with more energy deposited more heavily in the center of the sample. This affects the analysis results, particularly in tests on anisotropic materials. The results in this work show very little difference between the thermal parameters arising from the two methods. The robustness of the method using the single-pixel temperature history shows that anisotropic thermal diffusivity can be measured using standard flash diffusivity instruments, avoiding the additional complexity associated with a thermal imaging camera.

A significant fraction of energy is lost during power generation for transportation as a form of waste heat. Approximately over three quarters of waste heat have the temperature range from 200 to 700 ° C. Solid solution materials made of magnesium silicide and magnesium tin have the potential of utilization in thermoelectric (TE) devices in such temperature ranges due to their availability, relatively low density (3.02 g/cm 3 ), non-toxicity compared to classical Te-Pb TE materials and high stability from room temperature (RT) to 600 ° C. The environmentally friendly n-type Mg 2 (Si, Sn) thermoelectric solid solution bulk materials were prepared from powder elements via direct melting and current-assisted hot-press sintering. For the direct melting method, raw element magnesium, tin, and silicon powders were homogeneously mixed and put into a stainless steel container. Then, the sample was heated under a vacuum condition at 500–600 ° C for several hours to obtain crystalline Mg 2 Si, Mg 2 Sn, and Mg 2 Si 0.4 Sn 0.6 ingots respectively. These ingots were ground by using a high energy ball miller to obtain their particles whose sizes are distributed from tens of nanometers to micro-meters. Several doping material with different doping ratio were added to compare doping effect on Mg 2 (Si, Sn) solid solutions. The samples were hot-pressed with electrical current (400–600A) in order to create phonon scattering centers such as nano-size particles and lamellar structures for improving the dimensionless figure of merit, ZT value. Samples were cut into slender pieces flash method. Then, ZT value was calculated from ZT = α 2 Tσ /κ and plotted as a function of temperature. The results were also compared with the results obtained from to test electrical conductivity, by using a four-probe method as well as the Seebeck coefficient,. Thermal conductivity measured as a function of temperature from RT to 600 °C by using a the Harman technique. The nano-structure size, size distribution as well as crystallinity were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray diffraction (XRD). The chemical composition was examined by energy-dispersive x-ray spectroscopy (EDS).

This work analyses the natural convective heat transfer from a horizontal circular cylinder with constant surface heat flux using radial equally spaced exterior low conductivity baffles. The dimensionless governing equations in the stream function–vorticity formulation were solved using the finite difference method. The influence of the system parameters, the number of baffles and the baffle height, on the average Nusselt number is presented.

Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional thermoelectric figure of merit, ZT = (S^2 σT)/k where S, σ, k are the Seebeck coefficient, electrical conductivity and thermal conductivity respectively. These physical properties are interdependent, and hence making ZT of a material high is very challenging work. However, when the size of nanostructure is comparable to the wavelength and mean free path of energy carriers, it is feasible to avoid such interdependence to enhance ZT energy conversion. [1–3]

Carbon nanotube (CNT)-polymer composites were prepared by segregated network approach. CNTs were served as conductive fillers in a polymer matrix to synthesize electrically conducting polymer composites. In the segregated network composites, the thermoelectric properties were further improved by replacing Gum Arabic (GA) with electrically conductive stabilizer PEDOT:PSS doped with dimethyl sulfoxide (DMSO). The electrical and thermal conductivities and Seebeck coefficient were measured to determine the thermoelectric property of the polymer composites. The electrical conductivity of the composites with 9.8wt% of CNT was 3191.8 S/m whereas that of 10wt% CNT composite with GA sample was 400 S/m.

Arc welding is generally used to join thick metals in many engineering applications. However, poor penetration often occurs due to arc heat diffusion into the base metal. Hence, arc welding of thick metals normally requires grooving and/or preheating of the base metal and sometimes requires multiple passes for very thick metals or metals with high conductivity, such as aluminum alloys. In gas metal arc welding of thick metals with grooves and preheating, complicated melt flow and heat transfer are caused by the combined effect of droplet impingement, gravity, electromagnetic force, surface tension, and plasma arc pressure. Understanding these complicated transport phenomena involved in the welding process is critical in improving the penetration depth and weld quality. In this study, mathematical models and associated numerical techniques have been developed to study the effects of grooves and preheating on melt flow, diffusion of species, and weld penetration in gas metal arc welding of thick metals. Complex melt flow, transient weld pool shape and distributions of temperature and species in the weld pool are calculated. The continuum formation is adopted to handle liquid region, mushy zone and solid region. VOF technique is used to handle transient deformed shape of weld pool surface. The preliminary results show both grooves and preheating have important effects on the melt flow in weld pool and the weld penetration. Computer animations showing the evolutions of temperature; melt flow; and the interaction between droplets and weld pool will be presented.

It was recently found by using the time domain thermal reflectance method that polycrystalline highly preferred orientation WSe 2 and [(W) x (WSe 2 ) y ] z superlattice films possess extremely low cross-plane thermal conductivity, which is desirable for thermal insulation and thermoelectric energy conversion applications. However, it is difficult to obtain the in-plane thermal conductivity by using the laser reflectance or the 3-ω method. Here we employ a suspended microdevice developed for measuring thermoelectric properties of individual nanowires and nanofilms to obtain the in-plane thermal conductivity, electrical conductivity, and Seebeck coefficient of [(W) x (WSe 2 ) y ] z superlattice films. The measurement results show that the in-plane thermal conductivities of these films are much higher than the cross-plane values, making the thermal conductivity of the films highly anisotropic.

Nanofluids have been demonstrated as promising for heat transfer enhancement in forced convection and boiling applications. The addition of carbon, copper, and other high-thermal-conductivity nanoparticles to water, oil, ethylene glycol, and other fluids has been determined to increase the thermal conductivities of these fluids. The increased effective thermal conductivities of these fluids enhance their abilities to dissipate heat in such applications. The use of nanofluids for spray cooling is an extension of the application of nanofluids for enhancement of heat dissipation. In this investigation, experiments were performed to determine the level of heat transfer enhancement with the addition of alumina nanoparticles to the fluid. Using mass percentages of up to 0.5% alumina nanoparticles suspended in water, heat fluxes and surface temperatures were measured and compared. Compressed nitrogen was used to provide constant spray nozzle pressures to produce full-cone sprays in an open loop spray cooling system. Heat fluxes were measured for single-phase and evaporative spray cooling regimes.

The effect of the cathode catalyst layer’s structure and composition on the overall performance of a polymer electrolyte fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of Nafion loading, platinum loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. The studies show that the presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-level mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It was also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance. Cathode tortuosity has a significant effect on fuel cell performance at low values of agglomerate radius.

The use of composite material in modern commercial aircraft has increased significantly in recent years. The very low conductivity relative to Aluminium of composite materials means that the thermal environment experienced in an aircraft, during flight and on the apron, are significantly altered. The heat transfer mechanism is complex: natural and mixed convection flows established in compartments. This paper presents the thermally induced flow structures under representative conditions for a rectangular cavity representative of wing boxes and horizontal tail planes. The paper highlights the sensitivity to boundary conditions, the effect of structural stiffeners. The results indicate it may be possible to incorporate the effect of stringers and heating from above into existing correlations.

In this investigation, a Loop Heat Pipe (LHP) evaporator has been studied using a borescope inserted through the compensation chamber into the liquid core. This minimally intrusive technique allows liquid/vapor interactions to be observed throughout the liquid core and compensation chamber. A low conductivity ceramic was used for the wick and ammonia as the working fluid. Results indicate that buoyancy driven flows, both two-phase and single-phase, play essential roles in evacuating excess heat from the core, which explains the several differences in performance between horizontal and vertical orientations of the evaporator. This study also found no discernable effect of the pre-start fill level of the compensation chamber on thermal performance during startup at moderate and high heat loads.