Simultaneous measurement of thermal properties and convective heat transfer coefficient of individual carbon fiber using Raman spectroscopy

For crystalline polymers, especially those in micro/nanoscale, the number of defects per unit volume is significantly lower than that in the bulk. With extended and aligned polymer chains, the resulting polymer fibers possess remarkably enhanced mechanical and thermal properties and approach the inherent properties of the carbon backbones which form the polymer chains. Together with other unique properties of polymers, such as, low density, easy processability, good biocompatibility, and electrical insulation, the crystalline fibers in micro/nano scale can be used in a broad range of applications, for example, heat spreaders in electronics, high strength ropes, and personnel armors. This dissertation studies various schemes of polymer crystallization, especially the stress induced crystallization in fiber drawing process. In this work, a two-stage drawing method is finally adopted to produce individual polyethylene (PE) nanofibers. To demonstrate the PE nanofibers possess more enhanced mechanical properties than the commercially available PE nanofibers, an atomic force microscopy (AFM) based force deflection spectroscopy (FDS) technique is explored to characterize the Young's modulus of the PE nanofibers. By attaching a PE nanofiber onto a specially designed micro trench, and deflecting the nanofiber with an AFM cantilever, we are able to deduce the Young's modulus from the geometry of the trench and the level of deflection on the nanofiber based on Bernoulli's beam equations. The experimentally proved Young's modulus of these nanofibers is 312GPa approaching the theoretical limit of the Young's modulus of PE single crystal. To study thermal properties of a polysilsesquioxane (PSQ) hybrid crystal, we apply a micro device based thermal characterization method. The micro device consists of two suspended SiNx membranes with built-on Pt coils; the two membranes serve as heater and thermometer during the measurements. The PSQ micro beam is placed between the two membranes. Due to the Joule heating on the heating membrane, heat transfers through the sample to the sensing membrane. By analyzing the steady state heat transfer model, we are able to calculate the thermal conductivity of the PSQ beam. The experimentally measured thermal conductivities greatly help us to understand the heat transfer mechanism in the PSQ hybrid crystal which is formed by hydrogen bonding in the longitudinal direction. With the same characterization method as used in PSQ thermal characterization, we also measure the thermal conductivity of PE nanofibers. We discover the thermal contact resistance between the nanofiber and the islands is comparable or even bigger than the intrinsic thermal resistance of the PE nanofibers with an assumed thermal conductivity of 20W/mK at room temperature. Cyanoacrylate based super glue and focus ion beam (FIB) assisted Pt deposition are attempted to reduce the thermal contact resistance, however, as demonstrated by the experiments, super glue is likely to lift the nanofiber above the islands which dramatically increases the thermal resistance. While the FIB assisted Pt deposition introduces great crystal damage on the PE fibers, which results in very low measured thermal conductivities (

Polymer nanofibers have garnered significant attention due to their nanoscale size effects.When their diameter is below ~1 μm, mechanical and thermal properties such as Young’s modulus,tensile strength, and thermal conductivity are enhanced by several times to several orders ofmagnitude. This notable enhancement in the material properties coupled with their intrinsicproperties such as low density, chemical resistance, and biocompatibility open applications intissue engineering, sensors, textiles, composite reinforcements, ballistic armors, thermalmanagement and other areas. The objective of this thesis is to study the thermal, mechanical andthermo-mechanical properties of individual nanofibers and couple the properties with theirmolecular structure.Stress-induced crystallization using two-stage tip drawing technique is known to producehighly crystalline and oriented polymer nanofiber. However, it is time-consuming, low yield andlacks consistency. In this thesis, a local stretching technique is developed to produce highlycrystalline and oriented polyethylene nanofibers consistently. Microstructure characterizationusing a transmission electron microscope (TEM) and micro-Raman analysis verified the evolutionof microstructure from semi-crystalline to highly crystalline from microfiber to nanofiber.The thermal transport in PE microfibers and nanofibers was studied using a previouslydemonstrated suspended micro-thermal device. Temperature-dependent thermal conductivity wasmeasured over a broad temperature range from 20 K to 560 K. PE thermal conductivity increasedfrom the bulk to the microfiber and then to the nanofiber form, consistent with an increase incrystallinity and molecular orientation. The PE nanofiber thermal conductivity increased withincreasing temperature following an unusual ~T1 trend below 100 K, peaked around 130–150 Kreaching a metal-like value of 90 W m-1 K-1, and then decayed as T-1. It was found that thermal transport in aligned PE chain bundles is highly anisotropic and is dominated by the chain backbonesince the inter-chain Van der Waals interactions are much weaker than the covalent bonding alongthe backbone. The thermal contact resistance between a PE nanofiber and the suspended thermaldevice was found to be significant. A capillary-induced van der Waal contact method wasdeveloped to enhance grip and thermal contact. The experimentally measured thermal contactresistance was found to be consistent with the thermal contact resistance predicted using a linecontact model.A fully reversible thermal switching was discovered at 430 K in crystalline PE nanofibersdue to a temperature-induced structural phase transition from the orthorhombic to the hexagonallattice structure. The phase transition introduces segmental rotational disorder along the chain andleads to a switching factor (i.e., the ratio between on-state high and off-state low thermalconductance values) as high as 10 before and after the phase transition, which exceeds anypreviously reported experimental values for solid-solid or solid-liquid phase transition ofmaterials. The phase transformation was found to be thermally stable. A high-performancenanoscale thermal diode was fabricated by creating a heterogeneous amorphous-crystalline PEnanofiber junction. A thermal rectification factor of 25 % was achieved, comparable to the existingsolid-state nanoscale thermal diodes based on carbon nanotubes, boron nitride nanotubes, grapheneand VO2 nanobeams.The tensile strength of individual PE nanofiber was tested in tension using amicroelectromechanical system (MEMS) based device with an on-chip actuator. Since thecrystalline polymer is sensitive to high-energy electron beams, an optical metrology based on subpixelpattern matching was employed. In the tensile tests, PE nanofibers could not be firmlygripped using a variety of adhesives because of the low surface energy of PE. Instead, slip occurred before they were tested to failure. A microscale dog bone shape on a PE nanofiber wasfabricated to provide additional grip by mechanical locking. The tensile strength of 11.4 ± 1.1 GPawas obtained for the nanofiber with a diameter of 85 nm. To our knowledge, this is the highestmeasured tensile strength for any polymer-based fiber including carbon fiber, Zylon, Kevlar andnylon fibers.Polymer nanofibers exhibit viscoelastic behavior which is both dependent on time andtemperature. A variable stress-based creep measurement technique was developed to remove thenecessity of the feedback to keep a creep stress constant. From the temperature-dependent creepcompliance curves, a master curve spanning 30 years was developed for polyacrylonitrile (PAN)nanofibers. A thin nanofiber (150 nm) exhibited an order of magnitude less creep compared to athick fiber (250 nm) after 30 years at room temperature. The reduction in creep compliance for thethin fiber was attributed to the increased orientation within the core molecules. After removing theorientation of core PAN molecules by the exposure to high energy electron beam, higher creepcompliance than that of the oriented sample was obtained. This was because of the globally lesserorientation of the PAN molecules.

Phase change microcapsules can carry large amounts of heat and be dispersed into other mediums either as a solid composite or as slurry fluids without changes to their appearance or fluidity. These two standout features make phase change microcapsules ideal for use in thermal energy applications to enhance the efficiency of energy utilisation. This review paper includes methods used for the encapsulation of phase change materials, especially the method suitable for large scale productions, the trends of phase change microcapsule development and their use in thermal energy applications in static and dynamic conditions. The effect of phase change microcapsules on convective heat transfer through addition to thermal fluids as slurries is critically reviewed. The review highlighted that so far the phase change microcapsules used mainly have polymeric shells, which has very low thermal conductivities. Their enhancement in convective heat transfer was demonstrated in locations where the phase change material experiences phase change. The phase change results in the slurries having higher apparent local specific heat capacities and thus higher local heat transfer coefficients. Out of the phase change region, no enhancement is observed from the solid microcapsule particles due to the low specific heat capacity and thermal conductivity of the phase change microcapsules compared to that of water, which is normally used as slurry media in the test. To further the research in this area, phase change microcapsules with higher specific heat capacity, higher thermal conductivity and better shape stability need to be applied.

Thermal adhesives that contain large concentrations of high thermal conductivity filler materials, such as ceramics or metals, are widely used by the electronics industries in a variety of applications. The thermal properties of these materials, such as the thermal contact resistance across a bonded joint and the thermal conductivity of the bulk material, are critical to the selection of the “best” material. A method is presented for the measurement of these thermal properties using a steady-state, guarded heat flux meter test apparatus based on the well-documented and familiar ASTM test standard D-5470. Five different adhesive materials are tested and a linear fit of the resulting resistance versus thickness data are used to determine the bulk thermal conductivity and contact resistance values. Four of the five materials tested had conductivity values of less than 1 W/mK, and the data demonstrates that a small but significant thermal contact resistance exists between the adhesive and the substrate for each of the adhesives.

Test conditions influence on thermal conductivity and contact conductance of sand at transient state

Nanofluids, due to their specific features, have been widely utilized in a wide range of applications such as biomedicine, transportation, food processing and heat transfer. The main feature of nanofluids which makes them a suitable alternative for convectional heat transfer fluids is their augmented thermal conductivity due to dispersion of metallic, carbide or oxide particles in them. The other important feature is the smaller size of particles compared to microfluids which prevents the particles from aggregation/sedimentation, and therefore maintains higher stability of the suspension. In this thesis, heat transfer in two types of nanofluids with special functionalities (ferrofluid and carbon-dots in water) is investigated using an electrically-heated wire. Ferrofluids are colloidal suspensions of magnetic nano-particles while carbon dots are photoluminescent nano-particles of amorphous carbon. The transient temperature rise of the electrically-heated wire immersed in the sample fluid shows the apparent thermal conductivity of the nanofluid, the onset of convection heat transfer and boiling heat transfer phenomena. For the case of the magnetic fluid (Fe3O4 particles in water) “thermomagnetic convection” effects are observed for the first time in the absence of any magnetic field other than that due to the electrical current in the wire. The physics of thermomagnetic convection is a multi-disciplinary area coupling the fluid dynamics and heat transfer with magnetism. The phenomenon is investigated experimentally, analytically and numerically and several aspects of heat transfer including thermomagnetic convection, onset of thermomagnetic convection and boiling heat transfer are studied. The thermal conductivity of the carbon-dot nanofluid (a novel type of nanofluid which can be applied as a tracer fluid) has also been measured for the first time. Concerning the ferrofluid, the objective of the research is to examine heat transfer from an electrically heated wire with a view to establishing the feasibility of using ferrofluid for cooling applications of electrical systems rather than deionized water. A simple and low cost miniaturized set up for the transient hot-wire technique was designed, fabricated and calibrated. A ferrofluid sample with low volume fraction was used to study the thermomagnetic convection for different currents supplied to the wire at various temperatures. Concerning the analytical component of the study, a two-dimensional model has been developed using a scaling analysis to characterize the thermomagnetic convection around the current-carrying wire. Accordingly, a magnetic Grashof number for the induced flow in relation to the applied current was derived. Also the corresponding maximum Nusselt number of the induced convection was analytically correlated to the applied current and experimentally verified. It was observed that using ferrofluid can significantly enhance the heat transfer from the heated wire due to thermomagnetic convection. The critical Fourier number for the onset of thermomagnetic convection was correlated to the magnetic Rayleigh number and the constants used in the correlation were empirically determined. It was shown that magnetic convection will onset earlier than buoyancy-driven convection for large electrical currents applied. Heat transfer of ferrofluid from the hot-wire when the temperature of the wire reaches above the boiling point of water at atmospheric pressure was experimentally studied. It was observed that the boiling heat transfer from the wire deteriorated using ferrofluid as a result of deposition of particles on wire surface. When the wire reaches the boiling temperature, water molecules evaporate leaving behind the particles in the region. Attached particles to the wire form a porous layer with cavities filled with vapor with high thermal resistance which prevents effective heat removal from the wire. The rate of particle deposition on the wire was measured with respect to the current applied, time of boiling and volume fraction of the ferrofluid. In parallel to the experimental examinations, thermomagnetic convection was investigated numerically. A two-dimensional axisymmetric incompressible laminar numerical simulation model established in COMSOL Multiphysics was used to solve the coupled conservation equations. The model was validated against experimental data collected from the heated wire. Taking into account the term for magnetic body force added to the momentum equation, the model was able to show that the observed effects can be explained by thermomagnetic convection. Finally, thermal conductivity measurements for the carbon-dot nanofluid revealed that in contrast to the predictions of empirical correlations, negligible change compared to the thermal conductivity of the base fluid is observed.

Direct measurement of through-plane thermal conductivity and contact resistance in fuel cell materials

This article analyzes the detailed natural convection phenomena for a rhombic enclosure with differential heating and Rayleigh-Bѐnard configuration. A bi-quadratic element has been used to calculate the differential fluxes in the Navier-Stokes equations. A fourth order artificial viscosity was used to stabilize the numerical residue. The present numerical solution is performed over a wide range of parameters; 10^3 ≤ Ra ≤ 10^8, 15^0 ≤ ϕ ≤ 165^0 for differential heating and 10^3 ≤ Ra ≤ 10^6, 15^0 ≤ ϕ ≤ 90^0 for Rayleigh-Bѐnard configuration. The analysis of the net convective heat transfer across the mid-passage or the mid-height is used to identify the contribution of vortex motion from conduction dominate to convection dominate. The overshoots or undershoots of this net convective contribution is highly related to inclination angle of the rhombic enclosure and also the thermal boundary conditions. The compressibility effect slightly alters the overall performance and overshoots or undershoots of the net heat transfer by less than 1% value. Average Nusselt number distributions show that heat transfer rate is maximum for ϕ=90o in differential heating case, while for Rayleigh-Bѐnard convection, the heat transfer rate is maximum for ϕ=75o except for ϕ=15o at Rayleigh number 10^3 where conduction heat transfer is dominate. There is a linearity between the average Nusselt number and log(Ra) for all the inclination angles for both cases. Results of the study shows that the slope of the linearity is steeper for smaller or wider inclination angles when convective heat transfer is dominate i.e., at larger Rayleigh number.

The investigation into the laser irradiation effects of composites is important to the development of laser application. An axisymmetric model was proposed to predict the thermal responses of carbon fiber resin composite(CFRC) to laser irradiation in time and space domain. The finite element method was used to solve the governing equation. In this model, the boundary conditions were considered, including laser heat flow, heat convection and heat radiation. The progressive change in thermo-physical properties including decomposition degree, mass transfer, specific heat capacity, and thermal conductivity was determined. A code was developed to predict the transient temperature and thermal decomposing area of composites matrix under laser irradiation, and the results were compared to experimental data. The computed temperatures of composites consist with the experimental measurements well. It was found that when laser intensity was low, the thermal response pattern of CFRC composite was matrix decomposing.

Thermal contact analysis of Flip-Chip package considering microscopic contacts of double-layer thermal interface materials

A two-dimensional heat conduction problem with phase-change is solved by using a linear perturbation method. Temperature at the outer surface of the mold is approximately uniform, but contains a small sinusoidal perturbation in one space dimension. The problem is solved analytically with the assumption of infinitely large diffusivities of the solidified shell and mold materials. Results are obtained for the solid/melt interface as a function of time and for the temperature distributions in the shell and mold. The inverse problem, in which the solid/melt boundary is prescribed and the mold outer surface temperature is to be determined, is also discussed. The effect of process parameters such as the mold thickness, the thermal contact resistance between the shell and mold, and the thermal conductivity ratios between the shell and mold materials on the growth of the perturbation in the shell thickness and the outer temperature of the mold is investigated in detail.

Comprehensive study and optimization of concentrated photovoltaic-thermoelectric considering all contact resistances

Thermal conductivity and contact resistance of mesoporous silica gel adsorbents bound with polyvinylpyrrolidone in contact with a metallic substrate for adsorption cooling system applications

For the development of systems related to renewable energy and specifically for thermo-solar energy, it is essential to study and analyze the different thermal phenomena of heat transfer, in search of the best conditions that allow strengthening the designs. Phenomena such as radiation, convection, and conduction are highly studied, from the field of materials to the field of infrastructure, finding day to day optimal solutions that guarantee a great use of the solar resource available on the earth's surface. This study presented the results of heat transfer analysis on a receiver pipe in a CCP; designed using the theory of branched fractal geometry to improve the heat transfer associated with concentrating solar capture systems. A fractal structure as a branch or arboreal fractal type network, influenced thermal and electrical parameters, as electrical resistance, thermal resistance, and convective heat transfer, so increasing the level branch heat transfer can be enhancement along of the pipe. The use of renewable energies in agriculture benefits the industrial processes that require a continuous power system, due to climatic conditions and geographical location is not a guarantee in food production fields, which reason solar capture systems contribute to food dehydration processes, water heating, refrigeration, and energy production. Keywords: heat transfer, fractal geometry, solar collector, fractality, fractal dimension, parabolic cylindrical concentrator, DOI: 10.25165/j.ijabe.20201305.4733 Citation: Palacios A, Amaya D, Ramos O. Receiver tube heat transfer analysis of a CCP collector designed based on branched fractals geometry. Int J Agric & Biol Eng, 2020; 13(5): 56–62.

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles, requests a good knowledge of the heat transfer and pressure drop properties of substitute fluids. A contribution to this international effort is proposed with the study of two hydrofluorocarbon refrigerants (HFC-134a and the zeotropic mixture HFC-407C) and the study of the natural refrigerant ammonia. The HFCs have been experimentally tested on the first test rig developed in the Laboratory of Industrial Energy Systems (LENI) which was substantially modified to cope with ammonia, both in terms of safety requirements and operational conditions. The experimental test section is composed of two concentric tubes, with in tube evaporation of the refrigerant inside the inner tube of counter-current Annular water heating. A new database of the local heat transfer coefficients for the refrigerants HFC-134a, HFC-407C and ammonia together with pressure drop measurements has been collected and used to define and calibrate a new and more general heat transfer model. The two HFC refrigerants have been tested on the test rig developed by Kattan [36]; due to the chemical and the thermophysical properties of ammonia, a new test rig has been designed for the ammonia tests and a new calculation procedure based on the mean temperature measurement on the tube wall has been used. The experimental database covers evaporation tests on plain and on microfin tubes, and with nominal oil concentrations varying from 0 to 5 [%] (by wt.). Detailed modeling is concentrated on the tests on plain tube without oil. Each of the three refrigerants has been evaporated at 4[°C]. The other experimental conditions for the HFCs refrigerants were an inside tube diameter of 10.92[mm], a heat flux range of 2 – 5 [kW/m2] and three mass velocities of G = 100, 200, 300 [kg/(m2s)] visualized as Stratified-Wavy flows (G = 100[kg/(m2s)]), and mainly Annular for the higher mass velocities (G = 200, 300[kg/(m2s)]). The other global conditions for ammonia were an internal tube diameter of 14.00 [mm], a heat flux range of 5 – 70[kW/m2] and eleven mass velocities of G = 10,20,30,40,45,50,55,60,80,120,140[kg/(m2s)], corresponding to Stratified, Stratified-Wavy, Intermittent and Annular flow patterns; complementary tests, varying the mass velocity at constant vapor qualities (x = 20,50,80[%]), have been made in particular to better characterize boundaries between flow patterns. The flow patterns are visualized through glass sections at both ends of the 3.1 [m] long test tube. An extensive review of void fraction models with sensitivity analysis on the actual available flow pattern map has been made. From the 17 correlations reviewed, the models of Taitel-Dukler [71] and Rouhani [79] have been used. Other improvements to the threshold criteria of the Stratified-Wavy to Annular transition, lead to an accurate diabatic map to predict the flow patterns and their transition for very different families of fluids like HFC's and ammonia. A new approach in the prediction of two-phase flow heat transfer has been proposed through the study of each flow pattern separately, according to a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A function based on a pseudo Biot number allows, from the mean heat flux around the tube periphery, to base the model on two different mean heat fluxes applied respectively to the parts of the perimeter in contact with the liquid and the vapor in stratified types of flow. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. The two-phase flow heat transfer coefficient has finally been obtained as a weighted mean function of the vapor and the liquid heat transfer coefficient with respect to their contact surface with the heated tube. Based on the database of refrigerants HFC-134a and ammonia, the standard deviation of the new heat transfer model is of σ = 27.9[%]. Even if the database showed that the flow conditions were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points were purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation. Finally, a new method to model separated flows is proposed, based on partial hydraulic diameters and a mean interface velocity.