High pressure/high temperature forced and mixed convection experiments have been performed with helium and nitrogen at temperatures and pressures up to 893 K and 64 bar, respectively. The test section had a 16.8 mm ID flow channel in a 108 mm OD graphite column. Flow regimes included turbulent, transitional, and laminar flows with the inlet Reynolds numbers ranging from 1500 to 15,000. Due to strong heating, the local Reynolds number decreased by up to 50% over the 2.7 m test section. In addition, heat transfer degradation and flow laminarization caused by intense heating led to Nusselt numbers 20–50% lower than the values given by the modified Dittus–Boelter and modified Gnielinski correlations. Flow laminarization criteria were considered based on a dimensionless acceleration parameter (K v ) and buoyancy parameter (B o * ). Upward turbulent flows displayed higher wall temperatures than downward flows, due to the impact of flow laminarization which is not expected to affect buoyancy-opposed flows. Laminar Reynolds number flows presented an opposite behavior due to the enhancement of heat transfer for buoyancy-aided flows. At low Reynolds numbers, downward flows displayed higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases. In the entrance region of downward flows, convection heat transfer was reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancy-induced mixing caused higher convection heat transfer and lower wall temperatures.

Heat transfer properties of two expanded polystyrene (EPS) samples of similar density, one without (white) and one with graphite opacifier particles (gray), are compared. Tomographic scans are used to obtain cell sizes of the foams. Using established models for closed-cell polymer foams, the extinction coefficient and the effective thermal conductivity are obtained. The effect of opacifiers is modeled using (1) an effective refractive index for the polystyrene walls within a cell model for the EPS and (2) a superposition of extinction due to a particle cloud upon extinction predicted by the cell model, where particles are modeled as oblate spheroids, or equivalent volume, surface, or hydraulic diameter spheres. Modeled effective conductivities are compared with measurements done on a guarded hot-plate apparatus at sample mean temperatures in the range from 0 °C to 40 °C. Typically, cells of the gray EPS are about 40% larger than those of the white EPS and the cell walls in the gray EPS are thicker. The refractive index mixing model and the model with graphite opacifier particles as oblate spheroids overpredict extinction, however, the mean error in the effective conductivity predicted by the oblate spheroids model is only 2.7%. Equivalent volume/surface sphere models underpredict extinction, but still yield a low mean error in effective conductivity of around 4%. While the oblate spheroids model has a lower mean error, the computationally less expensive equivalent volume or equivalent surface models can also be recommended to model the inclusions.

Hot wire anemometer (HWA) measurements of turbulent gas flow have been performed in upward forced convection experiments at pressures ranging from 0.6 MPa to 6.3 MPa and fluid temperatures ranging from 293 K to 673 K. The results are relevant to deteriorated turbulent heat transfer (DTHT) and flow laminarization in strongly heated gas flows which could occur in gas-cooled very high temperature reactors (VHTRs). 2 The HWA signals were analyzed to directly confirm the occurrence of flow laminarization phenomenon due to strong heating. An X-probe was used to collect radial and axial velocity fluctuation data for pressurized air and pure nitrogen flowing through a circular 16.8 mm diameter flow channel in a 2.7 m long graphite test section for local Reynolds numbers varying from 500 to 22,000. Analyses of the Reynolds stresses and turbulence frequency spectra were carried out and used as indicators of laminar, transition, or fully turbulent flow conditions. Low Reynolds stresses indicated the existence of laminar or transitional flow until the local Reynolds number reached a large value, ∼11,000 to 16,000, much higher than the conventional Re = 4000–5000 for transition to fully turbulent flow encountered in pipe flows. The critical Reynolds number indicating the completion of transition approximately doubled as the pressure was increased from 0.6 MPa to 2.8 MPa.

This work focuses on an experimental investigation of convection heat transfer to a gas in a vertical tube under strongly heated conditions at high temperatures and pressures up to 943 K and 65 bar. A unique test facility for convection heat transfer experiments has been constructed, and used to obtain experimental data useful for better understanding and validation of numerical simulation models. This test facility consists of a single flow channel in a 2.7 m long, 0.11 m diameter graphite column with four 2.3 kW heaters placed symmetrically around the 16.8 mm diameter flow channel. Upward flow convection experiments with air and nitrogen were conducted for inlet Reynolds numbers from 1300 to 60,000, thus covering laminar, transition, and fully turbulent flow regimes. Experiments were performed at different flow rates (3.8 × 10 −4 to 1.5 × 10 −2 kg/s) and heater power up to 6 kW. Importantly, the data analysis considered the thermophysical properties of the gas and graphite changing with temperature and pressure. Nusselt number results are further compared to existing correlations. The effect of pressure and heater power on degraded heat transfer is examined. The analyses of the experimental data showed significant reductions in Reynolds number of up to 50% and Nusselt numbers of up to 90% between the gas inlet and outlet.

As a relatively new type of functional material, porous graphite foam exhibits unique thermophysical properties. It possesses the advantages of low density, high specific surface area, and high bulk thermal conductivity and could be used as the core component of compact, lightweight, and efficient heat exchangers. Effective thermal conductivity serves one of the key thermophysical properties of foam-based heat exchangers. The complex three-dimensional topology and interstitial fluids significantly affect the heat conduction in the porous structure, reflecting a topologically based effective thermal conductivity. This paper presents a novel geometric model for representing the microstructure of graphite foams with simplifications and modifications made on the realistic pore structure, where the complex surfaces and tortuous ligaments were converted into a simplified geometry with cylindrical ligaments connected between cuboid nodes. The multiple-layer method was used to divide the proposed geometry into solvable areas, and the series–parallel relation was used to derive the analytical model for the effective thermal conductivity. To explore heat conduction mechanisms at the pore scale, direct numerical simulation was also conducted on the realistic geometric model. Achieving good agreement with experimental data, the simplified geometric model was validated. The numerically simulated conductivity followed the simplified model prediction that the two geometries are equivalent from thermal aspect. It validates further that the simplified model is capable of reflecting the internal microstructure of graphite foam, which would benefit the understandings of the thermophysical mechanisms of pore-scaled heat conduction and microstructures of graphite foam.

During the past decade, research on carbon nanotubes has revealed potential advances in thermal engineering applications. The present study investigates the radiative absorption and reflection of vertically aligned carbon nanotubes (VACNTs) in the broad spectrum from the near-infrared to far-infrared regions. The optical constants of VACNT are modeled based on the dielectric function of graphite and an effective medium approach that treats the CNT film as a homogenized medium. Calculated radiative properties show characteristics of near-unity index matching and high absorptance up to around 20 μm wavelength. The packing density and degree of alignment are shown to affect the predicted radiative properties. The Brewster angle and penetration depth of VACNTs are examined in the infrared spectrum. The radiative properties for VACNT thin films are also evaluated, showing some reduction of absorptance in the near-infrared due to transmission for film thicknesses less than 50 μm. This study provides a better understanding of the infrared behavior of VACNT and may guide the design for its applications in energy harvesting, space-borne detectors, and stealth technology.

Infiltration of phase change materials (PCM) into highly conductive porous structures effectively enhances the thermal conductivity and phase change (solidification and melting) characteristics of the resulting thermal energy storage (TES) composites. However, the infiltration process contributes to formation of voids as micron-size air bubbles within the pores of the porous structure. The presence of voids negatively affects the thermal and phase change performance of TES composites due to the thermophysical properties of air in comparison with PCM and porous structure. This paper investigates the effect of voids on solidification of PCM, infiltrated into the pores of graphite foam as a highly conductive porous medium with interconnected pores. A combination of the volume-of-fluid (VOF) and enthalpy-porosity methods was employed for numerical investigation of solidification. The proposed method takes into account the variation of density with temperature during phase change and is able to predict the volume shrinkage (volume contraction) during the solidification of liquids. Furthermore, the presence of void and the temperature gradient along the liquid–gas interface (the interface between void and PCM) can trigger thermocapillary effects. Thus, Marangoni convection was included during the solidification process and its importance was elucidated by comparing the results among cases with and without thermocapillary effects. The results indicated that the presence of voids within the pores causes a noticeable increase in solidification time, with a sharper increase for cases without thermocapillary convection. For verification purposes, the amount of volume shrinkage during the solidification obtained from numerical simulations was compared against the theoretical volume change due to the variation of density for several liquids with contraction and expansion during the freezing process. The two sets of results exhibited good agreement.

An experimental radiation chamber has been developed to nondestructively measure the thermal diffusivity of a combustion chamber deposit (CCD) layer. The accumulation of CCD shifts the operability range of homogeneous charge compression ignition (HCCI) to lower loads where the fuel economy benefit of HCCI over a traditional spark ignition strategy is at a maximum. The formation and burn-off of CCD introduce operational variability, which increases the control system burden of a practical HCCI engine. To fully characterize the impact of CCD on HCCI combustion and develop strategies which limit the CCD imposed variability, the thermal and physical properties of HCCI CCD must be determined without destroying the morphology of the CCD layer. The radiation chamber device provides a controlled, inert atmosphere absent of cyclical pressure oscillations and fuel/air interactions found within an engine. The device exposes temperature probes coated with CCD to controlled heat flux pulses generated by a graphite emitter and a rotating disk. CCD layer thermal diffusivity is then calculated based on the phase delay of the sub-CCD temperature response relative to the response of the temperature probe when clean. This work validates the accuracy of the radiation chamber's diffusivity determination methodology by testing materials of known properties. Wafers of three different materials, whose thermal diffusivities span two orders of magnitude centered on predicted CCD diffusivity values, are installed over the temperature probes to act as CCD surrogates. Multiple thicknesses of each material are tested over a range of heat flux pulse durations. Diffusivity values determined from radiation chamber testing are independent of sample thickness with each of the three calibration materials. The radiation chamber diffusivity values exhibit a slight, but consistent underprediction for all wafers due to residual contact resistance at the wafer–probe interface. Overall, the validation studies establish the radiation chamber as an effective device for the nondestructive thermal diffusivity determination of thin insulative coatings. The similarity of expected CCD diffusivity values to those of the validation specimens instills confidence that the methodology and device presented herein can be successfully utilized in the characterization of HCCI CCD layers.

Measured values of heat transfer and pressure loss are presented for a variety of porous graphite foams in subsonic turbulent airflow. These foams were developed over the last decade to find combinations of high conductivity, porosity, strength, and low density suitable for application to rapid cooling of electronics and to corrosionless heat-exchangers. Measured maxima in the thermal performance that is the ratio of heat transfer to pressure loss, were correlated the pore structure obtained from scanning electron microscopy, to show a linear dependence of thermal performance on the average diameter of interpore windows representative of the cross-sectional area through which cooling air flows. For the same heat transfer, measured pressure losses were reduced by over two orders of magnitude by increasing pore and window diameters. However, the best thermal performance of porous graphite foams that were strong enough for industrial application, had measured pressure losses that were more than an order of magnitude greater than losses in conventional finned heat exchangers.

Paraffin phase change material (PCM) is enhanced with suspended graphite nanofibers at high loading levels. The loading levels reach in excess of 10% by weight. The thermal effects of the nanofiber loading level, the PCM module design, and the applied power density on the transient thermal response of the system are examined. A strong effect of nanofiber loading level on thermal performance is found, including a suppression of Rayleigh-Benard convection currents at high loading levels. Increases in nanofiber loading level also result in lowered heating rates and greater thermal control of the heated base. Increases in power density are found to result in higher heating rates, and increases in mass lead to lower operating temperatures. The design of the module is found to have a strong effect on thermal performance.

Carbon nanotube- or/and graphite-filled polymer composites were synthesized by using simple mixing and drying methods, and their thermal conductivities and structures were characterized by using a steady-state method (ASTM D5470) and scanning and transmission electron microscopies. In order to investigate the influence of synthesis conditions on the thermal conductivity of composites, various concentrations of multiwall carbon nanotubes, graphites, surfactants, and polymer matrix materials as well as two different nanoparticles and solvents were tested. Our composites containing both nanotubes (25 wt. %) and graphites (25 wt. %) with sodium dodecyl benzene sulfonate (SDBS) as a dispersant showed the highest thermal conductivity, ∼1.8 W/m-K at room temperature. The highest conductivity from nanotube/graphite mixtures would be from good adhesion and less voids between nanotubes and polymers as well as excellent thermal conduction from graphite sheets. The thermal conductivities of the composites have been calculated as a function of carbon nanotube concentrations by using a model based on the Maxwell’s effective medium theory, and the most effective method of improving thermal conductivity was suggested.

To simulate the heat transfer performance of devices incorporating high-conductivity porous materials, it is necessary to determine the relevant effective properties to close the volume-averaged momentum and energy equations. In this work, we determine these effective properties by conducting direct simulations in an idealized spherical void phase geometry and use the results to establish closure relations to be employed in a volume-averaged framework. To close the volume-averaged momentum equation, we determine the permeability as defined by Darcy’s law as well as a non-Darcy term, which characterizes the departure from Darcy’s law at higher Reynolds numbers. Results indicate that the non-Darcy term is nonlinearly related to Reynolds number, not only confirming previous evidence regarding such behavior in the weak inertia flow regime, but demonstrating that this is generally true at higher Reynolds numbers as well. The volume-averaged energy equation in the fluid phase is closed by the thermal dispersion conductivity tensor, the convecting velocity, and the interfacial Nusselt number. Overall, it has been found that many existing correlations for the effective thermal properties of graphite foams are oversimplified. In particular, it has been found that the dispersion conductivity is not well characterized using the Péclet number alone, rather the Reynolds and Prandtl numbers must be considered as separate influences. Additionally, the convecting velocity modification, which is not typically considered, was found to be significant, while the interfacial Nusselt number was found to exhibit a nonzero asymptote at low Péclet numbers. Finally, simulations using the closed volume-averaged equations reveal significant differences in heat transfer when employing the present dispersion model in comparison to a simpler dispersion model commonly used for metallic foams, particularly at high Péclet numbers and for thicker foam blocks.

Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate.

Highly conductive porous media have recently been considered for enhanced cooling applications due to their large internal contact surface area, which promotes convection at the pore level. In this paper, graphite foams that possess high thermal conductivity but low permeability are investigated for convection heat transfer enhancement using air as coolant. Two novel heat sink structures are designed to reduce the fluid pressure drop. Both experimental and numerical approaches are adopted in the study. The experimental data show that the designed structures significantly reduce flow resistance in graphite foams while maintaining relatively good heat removal performance. The numerical results obtained based on the local thermal nonequilibrium model are validated by experimental data and show that the inlet air flow partially penetrates the structured foam walls, while the remaining air flows tortuously through slots in the structure. Flow mixing, which is absent in the block graphite foam, is observed in the freestream area inside the designed structure. It can be concluded that graphite foams with appropriately designed structures can be applied as air-cooled heat sinks in thermal management applications.

This paper introduces a novel heat transfer enhancement surface, referred to as metal-graphite composite surface. It is comprised of high thermal conductivity graphite microfibers interspersed within a metal matrix (copper or aluminum) to enhance the bubble formation at the nucleation sites, and significantly improve the nucleate boiling heat transfer. Experiments revealed that its boiling heat transfer enhancement is comparable or in some respect even superior to the commercially available boiling heat transfer enhancement surfaces such as porous boiling surface and integral roughness surface. In addition, it does not result in any extra pressure loss and it minimizes surface fouling. Macro- to microscale heat transfer phenomena of the composite surfaces is treated. Discussions include characteristics of the surface, enhancement mechanisms, critical heat flux, boiling thermal hysteresis, bubble generation, growth and departure, and applications in electronic cooling, and under reduced gravity conditions.

In certain contemporary technologies, porous media with high solid-phase conductivity are impregnated with low-conductivity fluids, e.g., metal and graphite foam cooled by air. For such cases, an approximate analytical model for the developing heat transfer inside a two-dimensional rectangular porous medium subjected to constant heat flux is presented. The model neglects conduction in the fluid and assumes plug flow. The resulting nonthermal-equilibrium equations are solved for the solid and fluid temperatures by separation of variables. The temperatures decay exponentially as the distance from the heated base increases. The effects of the Biot and Peclet numbers are presented. Fully developed heat-transfer conditions are achieved at an axial distance equal to five times the height of the porous medium, with a constant Nusselt number equal to 3.

To improve the thermal performance of phase change materials (PCMs), graphite nanofibers were embedded into a paraffin PCM. The thermal effects of graphite fiber loading levels ( 0 – 5 wt % ) and graphite fiber type (herringbone, ribbon, or platelet) during the melting process were examined for a 131 cm 3 volume system with power loads between 3 W and 7 W ( 1160 – 2710 W ∕ m 2 ) . It was found that the maximum system temperature decreased as graphite fiber loading levels increased and that the results were fiber-structure dependent.

Effects of orientations of porous graphite and smooth copper surfaces, measuring 10 mm × 10 mm , on saturation nucleate boiling and critical heat flux (CHF) of FC-72 dielectric liquid and of liquid subcooling (0, 10, 20, and 30 K ) on nucleate boiling in the upward facing orientation are investigated. Inclination angles ( θ ) considered are 0 deg (upward-facing), 60, 90, 120, 150, and 180 deg (downward facing). The values of nucleate boiling heat flux, nucleate boiling heat transfer coefficient (NBHTC), and CHF are compared with those measured on the smooth copper surface of the same dimensions and CHF values on both copper and porous graphite are compared with those reported by other investigators on the smooth surfaces and microporous coatings. Results demonstrated higher NBHTC and CHF on porous graphite, particularly in the downward-facing orientation ( θ = 180 deg ) . In the upward-facing orientation, NBHTCs on both surfaces decrease with increased subcooling, but increase with increased surface superheat reaching maxima then decrease with further increase in surface superheat. In saturation boiling on copper and both saturation and subcooled boiling on porous graphite these maxima occur at or near the end of the discrete bubble region, and near CHF in subcooled boiling on copper. Maximum saturation NBHTC on porous graphite increases with decreased surface superheat and inclination angle, while that on copper increases with increased surface superheat and decreased surface inclination. At low surface superheats, saturation nucleate boiling heat flux increases with increased inclination, but decreases with increased inclination at high surface superheats, consistent with previously reported data for dielectric and nondielectric liquids. The fractional decreases in saturation CHF with increased θ on smooth copper and microporous coatings are almost identical, but markedly larger than on porous graphite, particularly in the downward-facing orientation. In this orientation, saturation CHF on porous graphite of 16 W ∕ cm 2 is much higher than on copper ( 4.9 W ∕ cm 2 ) and as much as 53% of that in the upward-facing orientation, compared to only ∼ 18 % on copper.

The absorptance of a material at the laser wavelength and as a function of temperature, ranging from room temperature to the removal point, significantly affects the efficiency of the laser machining process. A priori predictions of a laser machining process, using either simplistic or sophisticated models, require knowledge of the material’s absorptance behavior. An experimental apparatus for such measurements is described. The device consists of a specimen mounted inside an integrating sphere, heated rapidly by a CO 2 or a Nd:YAG laser. Reflectances are measured with a small focused probe laser (Nd:YAG or CO 2 ), while specimen surface temperatures are recorded by a high-speed pyrometer. Experimental results have been obtained for wavelengths of 1.06 μm (Nd:YAG) and 10.6 μm (CO 2 ) for graphite, alumina, hot-pressed silicon nitride, sintered α-silicon carbide, as well as two continous-fiber ceramic matrix composites (SiC-based). Data are presented for temperatures between room temperature and the ablation/decomposition points.