New Vacuum Blackbody Cavity for Heat Flux Meter Calibration

In the field of thermal radiation measurements, blackbody cavities are commonly used as reference standards for the calibration of heat flux meters. Applying the energy balance equation to the closed system including the cavity and the sensor, it is possible to predict the heat flux density absorbed by the heat flux meter. Calibration procedures developed at Laboratoire National de Métrologie et d’Essais (LNE) in recent years have allowed us to propose practical solutions for heat flux meters working below 100 kW · m⁻². The best relative uncertainties (k = 2) over the range of (10–100) kW · m⁻² vary from 1.7 % to 3 %. During previous studies, three major facilities were constructed, each one with the objective to respond to different technical problems considering the measuring principle of these heat flux sensors. Following this approach, the sensitivity of these meters to radiation, the sensitivity to radiation and convection, and also the influence of the size of the source or of the positioning of the sensor (horizontally, vertically, etc.) have been investigated. As an outcome of this recent experience, a new vacuum blackbody cavity has been set up. As well as the possibility to calibrate at very low irradiance, there are also some substantive improvements in heating, thermal performance, and calibration methodology. After a summary of the state of the art of calibration methods and their limits, the article presents the preliminary results of the characterization obtained with this new facility for which the objective is to reduce the uncertainties by at least a factor of two for heat flux densities lower than 20 kW · m⁻².     

Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

Highly Thermoconductive,Thermostable, and Super-Flexible Film by Engineering 1D Rigid Rod-Like Aramid Nanofiber/2D Boron Nitride Nanosheets

Polymer-based thermal management materials have many irreplaceable advantages not found in metals or ceramics, such as easy processing, low density, and excellent flexibility. However, their limited thermal conductivity and unsatisfactory resistance to elevated temperatures (<200 °C) still prevent effective heat dissipation during applications with high-temperature conditions or powerful operation. Therefore, herein highly thermoconductive and thermostable polymer nanocomposite films prepared by engineering 1D aramid nanofiber (ANF) with worm-like microscopic morphologies into rigid rod-like structures with 2D boron nitride nanosheets (BNNS) are reported. With no coils or entanglements, the rigid polymer chain enables a well-packed crystalline structure resulting in a 20-fold (or greater) increase in axial thermal conductivity. Additionally, strong interfacial interactions between the weaved ANF rod and the stacked BNNS facilitate efficient heat flux through the 1D/2D configuration. Hence, unprecedented in-plane thermal conductivities as high as 46.7 W m⁻¹ K⁻¹ can be achieved at only 30 wt% BNNS loading, a value of 137% greater than that of a worm-like ANF/BNNS counterpart. Moreover, the thermally stable nanocomposite films with light weight (28.9 W m⁻¹ K⁻¹/10³ (kg m⁻³)) and high strength (>100 MPa, 450 °C) enable effective thermal management for microelectrodes operating at temperatures beyond 200 °C.     

Enhancing the thermoelectric performance by defect structures induced in p-type polypyrrole-polyaniline nanocomposite for room-temperature thermoelectric applications

Organic thermoelectric materials mainly conducting polymers are green materials that can convert heat energy into electrical energy and vice versa at room temperature. In the present work, we investigated the thermoelectric properties of polymer nanocomposite of polypyrrole (PPy) and polyaniline (PANI) (PPy/PANI) by varying the pyrrole: aniline monomer ratios (60:40, 50:50, and 40:60). The PPy/PANI composite is prepared by in-situ chemical polymerization of PPy on PANI dispersion. It has been observed that the combination of two conducting polymers has enhanced the electrical and thermal properties in the PPy/PANI composite due to the strong π–π stacking and H-bonding interaction between the conjugated structure of PPy and conjugated structure of PANI. The maximum electrical conductivity of 14.7 S m^?1 was obtained for composite with high pyrrole content, whereas the maximum Seebeck coefficient of 29.5 μV K^?1 was obtained for composite with high aniline content at 366 K. Consequently, the PPy/PANI composite with pyrrole to aniline monomer ratio of 60:40 exhibits the optimal electrical conductivity, Seebeck coefficient, and high power factor. As a result, the maximum power factor of 2.24 nWm^?1 K^?2 was obtained for the PPy/PANI composite at 60:40 pyrrole to aniline monomer ratio, which is 29 times and 65.8 times higher than PPy (0.077 nWm^?1 K^?2) and PANI (0.034 nWm^?1 K^?2), respectively.

     

Silica-based aerogel composites reinforced with different aramid fibres for thermal insulation in Space environments

Silica aerogel composites reinforced with different aramid fibres have been synthesized and compared considering their potential use in thermal protection systems of Space devices. These composites were prepared from tetraethoxysilane and vinyltrimethoxysilane and the network was strengthened with aramid fibres. The results showed that the physical and chemical properties of the fibres were relevant, leading to composites with different properties/performance. In general, the obtained values for bulk density were low, down to 150 kg m^?3. Very good thermal properties were achieved, reaching thermal conductivities bellow 30 mW m^?1 K^?1, and thermal stability up to 550 °C in all cases. Short length fibres produce stiffer composites with lower thermal conductivities, while among longer fibres, meta-aramid-containing fibres lead to nanocomposites with best insulation performance. Standard tests for Space materials qualification, as thermal cycling and outgassing, were conducted to assess the compliance with Space conditions, confirming the suitability of these aerogel composites for this application.

Graphical abstract

     

A Through-Thickness Arrayed Carbon Fibers Elastomer with Horizontal Segregated Magnetic Network for Highly Efficient Thermal Management and Electromagnetic Wave Absorption

Multifunctional thermal management materials with highly efficient electromagnetic wave (EMW) absorption performance are urgently required to tackle the heat dissipation and electromagnetic interference issues of high integrated electronics. However, the high thermal conductivity (λ) and outstanding EMW absorption performance are often incompatible with each other in a single material. Herein, a through-thickness arrayed NiCo₂O₄/graphene oxide/carbon fibers (NiCO@CFs) elastomer with integrated functionalities of high thermal conductivity, highly efficient EMW absorption, and excellent compressibility is reported. The NiCO@CFs elastomer realizes a high out-of-plane thermal conductivity of 15.55 W m⁻¹ K⁻¹, due to the through-thickness vertically aligned CFs framework. Moreover, the unique horizontal segregated magnetic network effectively reduces the electrical contact between the CFs, which significantly enhances impedance matching of NiCO@CFs elastomer. As a result, the vertically arrayed NiCO@CFs elastomer synchronously exhibits ultrabroad effective absorption bandwidth of 8.25 GHz (9.75–18 GHz) at a thickness of 2.4 mm, good impedance matching, and a minimum reflection loss (RL_min) of −55.15 dB. Given these outstanding findings, the multifunctional arrayed NiCO@CFs elastomer opens an avenue for applications in EMW absorption and thermal management. This strategy of constructing thermal/electrical/mechanical pathways provides a promising way for the high-performance multifunctional materials in electronic devices.     

The application of biomass-based carbon materials in flexible all-solid supercapacitors

Four different kinds of carbon materials were synthesized successfully from saussurea involucrata, cotton stalk and cellulose by two activation methods (KOH-chemical activation method and mixed molten-salt synthesis). Carbon structures, electrochemical and flexible properties are studied. The saussurea involucrata-based flexible symmetric all-solid states supercapacitor exhibited high electrochemical performances, including high specific capacitance (129 F g^?1 at 2 mV s^?1) and excellent cycle stability (～ 85% capacitance retention even after 10,000 cycles). More importantly, it also displays excellent bending endurance, the specific capacitance is almost unchanged after 100 bends. This study shows promising materials in symmetric all-solid state supercapacitor applications.

     

Fire retardant evaluation of carbon nanofiber/graphite nanoplatelets nanopaper-based coating under different heat fluxes

Three types of carbon nanofiber based nanopapers, namely, 1Clay/5CNF/9APP, 1xGnP/5CNF/9APP, and 3xGnP/1CNF/9APP were made and their flame retardant efficiency was compared with thermogravimetric analysis and cone calorimeter test with 50 kW/m² of heat flux. The nanopaper of 3xGnP/1CNF/9APP was selected for experimental study because of its relatively good bonding with underlying structure and flame resistance performance. The fire response of glass fiber reinforced polyester composites with and without the selected nanopaper coating was thoroughly examined with cone calorimeter test using varied heat fluxes. It was found that at higher heat flux, the nanopaper demonstrated better flame retardant efficiency. Specifically, at 100 kW/m² of heat flux, the 1st and 2nd PHRR of the nanopaper-coated sample were more than 32% and 47% lower than the PHRR of control sample, respectively. In order to gain an insight into the pyrolysis process and flame retardation mechanism, the temperature profiles at the middle and back of the samples subjected to different heat fluxes were recorded. At 100 kW/m² of heat flux, the final temperature within the nanopaper-coated sample was roughly 280 ^oC, which is lower than control sample. The degradation rates in flexural moduli of the samples with coupon shape were determined using three-point bending. The three-point bending test results showed when the sample was exposed to 25 kW/m² heat flux for 240 seconds, the flexural modulus of control sample almost reduced to zero, whereas the nanopaper-coated sample still retained a half of its original flexural modulus. Finally, flame retardation mechanism was proposed for the nanopaper-coated composites.     

Fire retardant evaluation of carbon nanofiber/graphite nanoplatelets nanopaper-based coating under different heat fluxes

Three types of carbon nanofiber based nanopapers, namely, 1Clay/5CNF/9APP, 1xGnP/5CNF/9APP, and 3xGnP/1CNF/9APP were made and their flame retardant efficiency was compared with thermogravimetric analysis and cone calorimeter test with 50 kW/m² of heat flux. The nanopaper of 3xGnP/1CNF/9APP was selected for experimental study because of its relatively good bonding with underlying structure and flame resistance performance. The fire response of glass fiber reinforced polyester composites with and without the selected nanopaper coating was thoroughly examined with cone calorimeter test using varied heat fluxes. It was found that at higher heat flux, the nanopaper demonstrated better flame retardant efficiency. Specifically, at 100 kW/m² of heat flux, the 1st and 2nd PHRR of the nanopaper-coated sample were more than 32% and 47% lower than the PHRR of control sample, respectively. In order to gain an insight into the pyrolysis process and flame retardation mechanism, the temperature profiles at the middle and back of the samples subjected to different heat fluxes were recorded. At 100 kW/m² of heat flux, the final temperature within the nanopaper-coated sample was roughly 280 ^oC, which is lower than control sample. The degradation rates in flexural moduli of the samples with coupon shape were determined using three-point bending. The three-point bending test results showed when the sample was exposed to 25 kW/m² heat flux for 240 seconds, the flexural modulus of control sample almost reduced to zero, whereas the nanopaper-coated sample still retained a half of its original flexural modulus. Finally, flame retardation mechanism was proposed for the nanopaper-coated composites.     

Evaporative heat transfer and pressure drop of R410A in microchannels

Convective boiling heat transfer coefficients and two-phase pressure drops of R410A are investigated in rectangular microchannels whose hydraulic diameters are 1.36 and 1.44 mm. The mass flux was varied from 200 to 400 kg/m²s, heat flux from 10 to 20 kW/m², as the saturation temperatures were maintained at 0, 5 and 10 °C. A direct heating method was used to provide heat flux into the fluid. The boiling heat transfer coefficients of R410A in the microchannels were much different with those in single tubes, and the test conditions only slightly affected the heat transfer coefficients before dryout vapor quality. The present heat transfer correlation for microchannels, which was developed by introducing non-dimensional parameters of Bo, We_l, and Re_l used in the existing heat transfer correlations for large diameter tubes, yielded satisfactory predictions of the present data with a mean deviation of 18%. The pressure drops of R410A in the microchannels showed very similar trends with those in large diameter tubes. The existing two-phase pressure drop correlations for R410A in microchannels satisfactorily predicted the present data.     

Phase-field simulation of directional solidification of a binary alloy under different boundary heat flux conditions

Complicated morphologies of directional solidification structures attract a lot of theoretical studies and commercial uses. As known, the boundary heat flux has an important significance to the microstructures of directional solidification. In this article, the interface evolution of directional solidification with different boundary heat fluxes is discussed. In this study, only one interface has heat flow, and Neumann boundary conditions are imposed at the other three interfaces. From the calculated results, it is found that different heat fluxes cause different microstructures in the directional solidification. When the heat flux equal to 18 W/cm², the growth of lengthways side branches is accelerated and the growth of transverse side branches is restrained. At the same time, there is dendritic remelting in the calculating domain. When the heat flux equal to 36 W/cm², the growth of the transverse side branches and the growth of the lengthways side branches compete with each other. When the heat flux equal to 90 or 180 W/cm², the growth of transverse side branches absolutely dominates. The temperature field of dendritic growth is also analyzed and the relation between heat flux and temperature field is found.     

Boiling Heat Transfer Enhancement by Using Micro-pin-finned Surface for Electronics Cooling

For efficiently cooling electronic components with high heat flux, experiments were conducted to study the flow boiling heat transfer performance of FC-72 over square silicon chips with the dimensions of 10 × 10 × 0.5 mm³. Four kinds of micro-pin-fins with the dimensions of 30 × 60, 30 × 120, 50 × 60, 50 × 120 μm² (thickness, t × height, h) were fabricated on the chip surfaces by the dry etching technique for enhancing boiling heat transfer. A smooth surface was also tested for comparison. The experiments were made at three different fluid velocities (0.5, 1 and 2 m/s) and three different liquid subcoolings (15, 25 and 35 K). The results were compared with the previous published data of pool boiling. All micro-pin-fined surfaces show a considerable heat transfer enhancement compared with a smooth surface. Flow boiling can remarkably decrease wall superheat compared with pool boiling. At the velocities lower than 1 m/s, the micro-pin-finned surfaces show a sharp increase in heat flux with increasing wall superheat. For all surfaces, the maximum allowable heat flux, q _max , for the normal operation of LSI chips increases with fluid velocity and subcooling. For all micro-pin-finned surfaces, the wall temperature at the critical heat flux (CHF) is less than the upper limit for the reliable operation of LSI chips, 85°C. The largest value of q _max can reach nearly 148 W/cm² for micro-pin-finned chips with the fin height of 120 μm at the fluid velocity of 2 m/s and the liquid subcooling of 35 K. The perspectives for the boiling heat transfer experiment of the prospective micro-pin-finned surfaces, which has been planned to be made in the Drop Tower Beijing/NMLC in the future, are also presented.     

Ablation,mechanical and thermal conductivity properties of vapor grown carbon fiber/phenolic matrix composites

The ablation, mechanical and thermal properties of vapor grown carbon fiber (VGCF) (Pyrograf III™ Applied Sciences, Inc.)/phenolic resin (SC-1008, Borden Chemical, Inc.) composites were evaluated to determine the potential of using this material in solid rocket motor nozzles. Composite specimens with varying VGCF loadings (30–50% wt.) including one sample with ex-rayon carbon fiber plies were prepared and exposed to a plasma torch for 20 s with a heat flux of 16.5 MW/m² at approximately 1650°C. Low erosion rates and little char formation were observed, confirming that these materials were promising for rocket motor nozzle materials. When fiber loadings increased, mechanical properties and ablative properties improved. The VGCF composites had low thermal conductivities (approximately 0.56 W/m-K) indicating they were good insulating materials. If a 65% fiber loading in VGCF composite could be achieved, then ablative properties are projected to be comparable to or better than the composite material currently used on the Space Shuttle Reusable Solid Rocket Motor (RSRM).     

Dielectric and thermal properties of GFs/PTFE composites with hybrid fillers of Al2O3 and hBN for microwave substrate applications

The alumina/hexagonal boron nitride/glass fibers cloth/Polytetrafluoroethylene (Al₂O₃–hBN/GFs/PTFE) composites were prepared by blending-impregnation followed by hot compression method, and the dielectric, thermal properties of the composites with various hBN fillers content (0–20 wt.%) were investigated. The results show that the thermal conductivity of the composites increase significantly, while the coefficient of thermal expansion (CTE) decrease gradually, with the hBN content increasing. Composite substrates with 20 wt.% hBN exhibited high thermal conductivity as 1.05 W m^?1 K^?1, which is 5.3 times that of pure PTFE. Such compositions may be a promising material in high thermal conductivity copper clad laminate.

     

Influence of Spacer Systems on Heat Transfer in Evacuated Glazing

One attractive possibility to essentially improve the insulation properties of glazing is to evacuate the space between the glass panes. This eliminates heat transport due to convection between the glass panes and suppresses the thermal conductivity of the remaining low pressure filling gas atmosphere. The glass panes can be prevented from collapsing by using a matrix of spacers. These spacers, however, increase heat transfer between the glass panes. To quantify this effect, heat transfer through samples of evacuated glazing was experimentally determined. The samples were prepared with different kinds of spacer materials and spacer distances. The measurements were performed with a guarded hot-plate apparatus under steady-state conditions and at room temperature. The measuring chamber of the guarded hot plate was evacuated to < 10⁻² Pa. An external pressure load of 0.1 MPa was applied on the samples to ensure realistic system conditions. Radiative heat transfer was significantly reduced by preparing the samples with a low-ε coating on one of the glass panes. In a first step, measurements without any spacers allowed quantification of the amount of radiative heat transfer. With these data, the measurements with spacers could be corrected to separate the effect of the spacers on thermal heat transfer. The influence of the thermal conductivity of the spacer material, as well as the distance between the spacers and the spacer geometry, was experimentally investigated and showed good agreement with simulation results. For mechanically stable matrices with cylindrical spacers, experimental thermal conductance values ≤0.44W·m⁻² ·K⁻¹ were found. This shows that U _g -values of about 0.5W · m⁻² · K⁻¹ are achievable in evacuated glazing, if highly efficient low-emissivity coatings are used.     

An investigation on post-fire behavior of hybrid nanocomposites under bending loads

This work focuses on residual bending properties of hybrid nanocomposites after intense heat conditions. Carbon fiber/epoxy-nanoclay and carbon fiber/epoxy-graphene nanosheets were manufactured. The nanoparticles employed were Cloisite 30B nanoclay and surface modified graphene nanosheets. The epoxy system was RemLam M/HY956. For short beam samples exposed to 800 KW/m² heat flux, for a various period of time up to 120 s, the addition of nanoparticles (nanoclay and graphene nanosheets) increased the unburned thickness from 0.16 mm (original) to 2.63 mm and 2.74 mm, respectively. When the two-dimensional (plates) samples were tested, the improvement on heat performance was reduced. The unburned thickness improved close to 10% with the presence of nanoclay. The addition of graphene nanosheets leads to a decrease in unburned thickness of 12.8%. This result can be due to the good thermal protection properties of the graphene nanosheets. Using SEM analysis, it was observed that when the hybrid nanocomposites were subjected to a large heat flux, nanoparticles remained trapped inside the char layers. Finally, the proposed model seems to overestimate the residual bending response by 8%.     

Experimental study on thermal characteristics of ammonia flow boiling in a plate evaporator at low mass flux

Thermal characteristics of a plate evaporator using ammonia are experimentally investigated. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient of the evaporator are discussed. The experiments are conducted for mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is obtained as a function of quality for all experimental conditions. The characteristics of heat transfer coefficient are discussed and compared with those of earlier works. All experimental results are compiled by using Lockhart–Martinelli parameter. The developed empirical correlation predicts 85% of the experimental data within ±30% range.     

Hot-Ball Method for Measuring Thermal Conductivity

This article deals with the theory and performance of a sensor for measuring thermal conductivity. The sensor, in the form of a small ball, generates heat and simultaneously measures its temperature response. An ideal model of the hollow sphere in an infinite medium furnishes a working equation of the hot-ball method. A constant heat flux through the surface of the ball generates the temperature field. The thermal conductivity of the surrounding medium is to be determined by the stabilized value of the temperature response, i.e., when the steady-state regime is attained. Error components of the sensor are discussed due to analysis of the deviations of the real hot-ball construction from the ideal model. The functionality of a set of hot balls has been tested, and the calibration for a limited range of thermal conductivities was performed. A working range of thermal conductivities of tested materials has been estimated to be from 0.06 W· m⁻¹ · K⁻¹ up to 1 W· m⁻¹ · K⁻¹.     

Flow Boiling Heat Transfer in Two-Phase Micro Channel Heat Sink at Low Water Mass Flux

Boiling heat transfer at water flow with low mass flux in heat sink which contained rectangular microchannels was studied. The stainless steel heat sink contained ten parallel microchannels with a size of 640 × 2050 μm in cross-section with typical wall roughness of 10–15 μm. The local flow boiling heat transfer coefficients were measured at mass velocity of 17 and 51 kg/m²s, heat flux on 30 to 150 kW/m² and vapor quality of up to 0.8 at pressure in the channels closed to atmospheric one. It was observed that Kandlikar nucleate boiling correlation is in good agreement with the experimental data at mass flow velocity of 85 kg/m²s. At smaller mass flux the Kandlikar model and Zhang, Hibiki and Mishima model demonstrate incorrect trend of heat transfer coefficients variation with vapor quality.     

Survival of actively cooled microvascular polymer matrix composites under sustained thermomechanical loading

Exposure to high heat can cause polymer matrix composites (PMC) to fail under mechanical loads easily sustained at room temperature. However, heat is removed and temperature reduced in PMCs by active cooling through an internal vascular network. Here we compare structural survival of PMCs under thermomechanical loading with and without active cooling. Microchannels are incorporated into autoclave-cured carbon fiber/epoxy composites using sacrificial fibers. Time-to-failure, material temperature, and heat removal rates are measured during simultaneous heating on one face (5–75 kW/m²) and compressive loading (100–250 MPa). The effects of applied compressive load, heat flux, channel spacing, coolant flow rate, and channel distance from the heated surface are examined. Actively cooled composites containing 0.33% channel volume fraction survive without structural failure for longer than 30 min under 200 MPa compressive loading and 60 kW/m² heat flux. In dramatic comparison, non-cooled composites fail in less than a minute under the same loading conditions.