High Effective Thermal Conductivity Research Articles

The design of an extended length helium pulsating heat pipe experiment

Pulsating heat pipes, also known as PHPs, are passive two-phase heat transfer devices capable of moving heat at cryogenic temperatures with high effective thermal conductivities around two orders of magnitude higher than copper. A recent Helium Pulsating Heat Pipe experiment demonstrated a surprising phenomenon, where Helium PHPs of different lengths (300 mm and 1000 mm) displayed the same thermal conductance at equal heat loads. The purpose of this research is to experimentally characterize this apparent length-independence and determine the size limits for Helium PHPs. An experimental approach is developed where three additional Helium PHP experiments will be conducted with the same operating parameters as the original experiment except with extended adiabatic lengths – 1.25 m, 1.5 m, and 1.75 m. All PHPs considered in this study are in the vertical orientation and are bottom-heated. This will give five complete sets of data from which the influence of length on helium pulsating heat pipes’ performance may be analysed. This paper serves as a work-in-progress report describing the experimental design and fabrication of these Helium PHP experiments.

Effective thermal conductivities of metal powders for additive manufacturing

Effective thermal conductivities (ETCs) of five commonly used metal powders in additive manufacturing have been measured at temperatures up to 750 °C. The ETCs of Al-Mn-Sc and Ti-6Al-4V powders with different particle sizes are also investigated. When the temperature is lower than 300 °C, the ETC of metal powders is insensitive to the temperature and material. In contrast, when the temperature exceeds 400 °C, it increases significantly with increasing temperature and changes markedly dependent on the material. Additionally, in the lower temperature range, the larger size powders have higher ETCs, which is more pronounced for Al-Mn-Sc due to its higher thermal conductivity. In the higher temperature range, the larger Al-Mn-Sc powder shows higher ETCs than the smaller powder. In contrast, the smaller Ti-6Al-4V powder shows higher ETCs when the temperature exceeds 600 °C. The effects of temperature and particle size on ETC are investigated via particle morphology analyses.

Estimation of thermal conductivity of cemented sands using thermal network models

Effective thermal conductivity of soils can be enhanced to achieve higher efficiencies in the operation of shallow geothermal systems. Soil cementation is a ground improvement technique that can increase the interparticle contact area, leading to a high effective thermal conductivity. However, cementation may occur at different locations in the soil matrix, i.e. interparticle contacts, evenly or unevenly around particles, in the pore space or a combination of these. The topology of cementation at the particle scale and its influence on soil response have not been studied in detail to date. Additionally, soils are made of particles with different shapes, but the impact of particle shape on the cementation and the resulting change of effective thermal conductivity require further research. In this work, three kinds of sands with different particle shapes were selected and cementation was formed either evenly around the particles, or along the direction parallel or perpendicular to that of heat transfer. The effective thermal conductivity of each sample was computed using a thermal conductance network model. Results show that dry sand with more irregular particle shape and cemented along the heat transfer direction will lead to a more efficient thermal enhancement of the soil, i.e. a comparatively higher soil effective thermal conductivity.

Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

AbstractHeat pipes and two‐phase thermosyphon systems are passive heat transfer systems that employ a two‐phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (105 W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air‐conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air‐to‐air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m3/h, evaporator inlet‐air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (Ce/Cc) of 1 and 1.5. For HPHE's steady‐state operation, a mathematical model for heat‐transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet‐air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (Ce/Cc) of 1.5. The predicted and experimental values of condenser outlet‐air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet‐air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.

Thermal performance prediction of heat pipe with TiO2 nanofluids using RSM

Heat pipe is a two-phase heat transfer device with high effective thermal conductivity and transfer huge amount of heat with minimum temperature gradient in between evaporator and condenser section. This paper objective is to predict the thermal performance in terms of thermal resistance, R, and heat transfer coefficient, h, of screen mesh wick heat pipe with deionized water-TiO2 as working fluid. The input process parameters of heat pipe such as heat load, Q, tilt angle, ?, and concentration of nanofluid, ?, were modeled and optimized by utilizing response surface methodology with MINITAB-17 software to attain minimum thermal resistance and maximum heat transfer coefficient. The minimum thermal resistance of 0.1764 ?C per W and maximum heat transfer coefficient of 1411.52 W/m2 ?C was obtained under the optimized conditions of 200 W heat load, 57.2? tilt angle, and 0.159 vol.% concentration of nanofluid.

Thermoelectric energy harvesting using vapour chamber coolers for aerospace applications

Thermoelectric generators (TEGs) have shown great potentials to supply low-power sensor nodes in aerospace applications due to their relatively small size, adequate output power and reliability. TEGs convert waste heat available at aircraft locations into a usable potential difference. However, TEGs’ performance greatly depends on the use of passive cooling systems such as heatsinks to enhance their energy supply. This paper reports the first proof-of-concept use of vapour chambers coupled to traditional circular pin-fin heatsinks to enhance the output power of TEGs. Vapour chambers are compact and small capillary-driven heat spreaders that incorporate a cavity in their volume containing a working fluid to provide a high effective thermal conductivity. Numerical simulations and experimental tests revealed that the use of vapour chambers provided significant increase of the output power, with a maximum produced power of 28.2 W and a relative difference of 6.27 W against conventional energy scavenging configurations. Results demonstrate the high thermal cooling performance of vapour chambers to efficiently support thermal energy harvesting solutions designed for condition and structural health monitoring.

Heat transfer prediction of granular flow in moving bed heat exchanger: Characteristics of heat transfer enhancement and dynamic control

To realize the heat transfer between granular flow and sCO2 cycle, moving bed heat exchangers have been widely applied in Concentrating Solar Power systems. This work developed a prediction model for heat transfer enhancements and dynamic control strategies in a shell-and-plate moving bed heat exchanger. For fast evaluation of heat transfer performances in granular flow, the continuum model was adopted with different thermal resistances. The wall effect of near-wall region was modified by contact thermal resistance, and the equivalent heat diffusion in bulk region was reflected by penetration thermal resistance. The results showed that, the decrement of penetration thermal resistance can improve heat transfer effectively. To weaken the penetration thermal resistance, the thinner thermal boundary layer and the higher effective thermal conductivity are urgently required. The staggered arrangement is a valid way to limit the development of thermal boundary layer. The bank of heat exchanger plates offset horizontally from the previous bank, and the re-evolution of temperature distributions is equal to the equivalent thermal mixing between different banks. Meanwhile, the effective thermal conductivity can be improved by particle self-diffusions perpendicular to the major flow direction. For the particle migrations, the local disturbances can be applied. The relatively decrement of penetration thermal resistance can achieve 45.7% at a disturbance intensity of 5 × 10−7 m2/s. The outlet temperature of sCO2 can be well controlled by the transient temperature–velocity feedback method with relaxation coefficients. The relatively decrement of temperature fluctuation is larger than 80% with a relaxation coefficient of 40.

A channel layout of a micro pulsating heat pipe for an excessively localized heating condition

A channel layout of a flat-plate micro pulsating heat pipe (MPHP) for an excessively localized heating condition is proposed. The excessively localized heating condition is implemented using a small heater, which covers only about 2% of the whole area, in the middle of the bottom part. To simplify the heat transfer characteristics in MPHPs, the channel regions are modeled as a region with the high effective thermal conductivity, and the topology optimization method is employed to determine a distribution of the channel regions of the MPHP. The constraints on the maximum length scale and the minimum solid volume fraction in local regions are imposed to take into account the operational characteristics of MPHPs. In the proposed channel layout, which is inspired by the topology optimization result, the channels are laid out in such a way that some channels are merged to reduce the number of channels. Due to a decrease in the number of channels, all channels are able to overlap the localized heating section without any thermally-isolated channel. To verify the effectiveness of the proposed method, the MPHPs with the channel layout designed by topology optimization and two kinds of channel layouts for the references are fabricated, and their flow characteristics and thermal performances are experimentally compared. In case of the MPHP with the uniform channel layout, only 4 channels out of 18 channels overlap the heating section, and the stagnant liquid slugs in the thermally-isolated channels are observed. On the other hand, in the MPHP with the channel layout designed by topology optimization, all channels overlap the heating section, and the active oscillating motion of the liquid slugs occurs in all channels. This allows the MPHP with the channel layout designed by topology optimization to have about two times higher thermal performance than the MPHP with the uniform channel layout. In this study, the channel layout of the MPHP for the excessively localized heating condition is firstly suggested, and the experimental results show that the proposed channel layout can effectively overcome the limitation associated with the localized heating condition.

Enhanced Effective Thermal Conductivity of Composite Materials by Incorporating Constructal Fillers

It is a feasible way to use constructal filler to increase the effective thermal conductivity of polymer. However, the combined effect of filler constructal-shape, contact, anisotropy and interface thermal resistance on the effective thermal conductivity of the composites has not been considered, and the heat transfer mechanisms remain unclear. In this study, these effects are evaluated at the same time. The results show that the filler contact is a key factor in increasing the effective thermal conductivity of composite. The constructal fillers are easy to contact and form heat conduction network than common filler. The filler content of 5 % is the percolation threshold of constructal filler. It is lower than common filler and previous reports. The enhanced heat transfer performance of constructal filler is insensitive to directional angle and has no significant anisotropy. The composite with constructal filler has high effective thermal conductivity in different directions. Series numerical models are proposed to predict the effective thermal conductivity of composite. These models have high accuracy and reliability, the deviations with experiment results is in an acceptable range. The interface thermal resistance should be considered in predicting the effective thermal conductivity of composite with nano-fillers.

A study of the natural convection of water-AA7075 nanoliquids in low-porosity cylindrical annuli using a local thermal non-equilibrium model

Natural convection in nanoliquid-saturated porous cylindrical annuli due to uniform heat and mass influxes from the solid cylinder and effluxes from the outer hollow cylinder is investigated analytically. The Darcy model and the modified version of the Buongiorno two-phase model are used, and local thermal non-equilibrium between the phases is assumed. A nanoliquid-saturated porous medium made up of glass balls with a dilute concentration of AA7075 alloy nanoparticles well-dispersed in water is considered. Out of three types of annuli considered, shallow annuli provide the best heat transport and tall annuli show the worst performance. The presence of a dilute concentration of nanoparticles significantly enhances the heat transport in the system. Of nine nanoparticle shapes considered, lamina-shaped nanoparticles enhance heat transport the most. Heat transport is enhanced in the case of heat-and-mass-driven convection compared to the case of purely heat-driven convection. The results for a rectangular enclosure are obtained as a particular case of the present study. Two asymptotic routes that take us to the results of thermal equilibrium are shown. The vanishing limit of the concentration Rayleigh number yields the result for a single-phase model. Results for the base-liquid-saturated porous medium form a limiting case of the present study. We conclude that a shallow cylindrical annulus saturated with water-AA7075 lamina-shaped alloy nanoparticles is best suited for heat transfer due to its high effective thermal conductivity in comparison with that of other shaped nanoparticles and a tall rectangular enclosure saturated by water is best suited for heat storage applications.

Impact of fission gas bubbles on thermal conductivity of UO[formula omitted] fuels with high thermal conductivity additives

High thermal conductivity additives are being considered to create composite nuclear fuels with a higher effective thermal conductivity (ETC) to reduce the peak fuel temperatures during reactor operation. However, the benefits of the additive may be reduced, or possibly eliminated, when placed into a reactor environment. In this work, we investigate the impact of fission gas bubbles on the ETC of composite fuels. We create 2D simulations representing a small portion of a composite UO2 fuel containing a single additive particle and calculate the ETC with various volume fractions of fission gas bubbles. Our results show that fission gas bubbles, depending on the volume fraction and contact angle at the additive interface, can completely remove any benefit of the additive and even result in a lower thermal conductivity than in UO2 without the additive. We also expose a trend that relates the fraction of additive that is screened by bubbles to the ETC. We then propose a model which predicts the ETC of a composite fuel with fission gas bubbles to better estimate the benefits of a novel fuel design.

Investigation on the effect of thermal properties by changing geometry of a heat pipe using simulation

A heat pipe is a two-phase device with a very high effective thermal conductivity and heat transfer rate. It consists of a phase change material, wick structure and an envelope. Usually, the heat pipes are circular in shape. In this paper, we analyze different shapes of the heat pipe and how it affects the performance of the pipe without changing its internal volume and central thickness. Flow property of the phase change material changes with geometry. The fluid velocity is minimum at the pipe surface and maximum at the pipe centre because of the no-slip condition. We have selected one circular and two flat geometries with a different number of edges. The advantage of flat geometry is an increased surface area of contact. ANSYS simulation softwareis used to simulate the working of the heat pipe. Several performance parameters including pressure, velocity, temperature and effective thermal conductivity were calculated and compared. The effect of an increase in the number of edges on different performance aspects of the heat pipe is studied. From the results of simulation software, it is observed that the change in geometry affects the thermal properties significantly.

Applications of coupling thermosyphons with phase change materials: A review

As energy demands and environmental contaminants are facing significant challenges, advanced techniques and high-efficient thermal systems are extensively explored to protect the environment and save energy. Thermosyphons (TSs) are widely conducted as outstanding passive thermal transport devices, which can have high effective thermal conductivities. Nowadays, thermosyphons have been recognized in conjunction with phase change materials (PCMs) to improve the performance of the latent heat thermal energy storage systems. The primary purpose of the integration is to embed the high-efficient thermal conductor into the thermal storage unit to transfer and reserve a significant value of heat efficiently. This review documents applications of coupling TSs and PCMs, general design procedures, and numerical analysis of heat transfer for the systems based on the thermal network approach. Various applications are sorted and compared to reveal the potential advantages of the integration of TSs with PCMs. Moreover, this review also discusses the opportunities and challenges based on a comprehensive framework for the benefit of designers and engineers to implement the combination of TSs and PCMs.

Relating the thermal properties of a micro pulsating heat pipe to the internal flow characteristics via experiments, image recognition of flow patterns and heat transfer simulations

We investigate the relationship between the thermal properties of a micro pulsating heat pipe (MPHP) and the internal flow characteristics via measurements of effective thermal conductivities, flow visualization followed by image recognition of the flow patterns, and heat transfer simulations employing the extracted flow patterns. The MPHP consists of an eleven-turn closed-loop of a meandering square microchannel with a hydraulic diameter of 350 μm engraved on a silicon substrate, which is covered with a transparent glass plate to allow internal flow visualization. The MPHP charged with Fluorinert FC-72 tends to exhibit higher effective thermal conductivities for the coolant temperature of Tc = 40 °C compared to Tc = 20 °C, and provides the highest effective thermal conductivity of about 700 W/(m K) for Tc = 40 °C and a filling ratio of 48%. Interestingly, we observe two different self-oscillation modes having different thermal conductivities, even for identical heat input rates. This tendency indicates a hysteresis of the effective thermal conductivity, which originates from the difference in the heat input rates at which the MPHP falls into and recovers from dryout. Subsequently, semantic segmentation-based image recognition is applied to the recorded flow images to identify the flow characteristics, successfully extracting four different flow patterns involving liquid slugs, liquid films, dry walls, and rapid-boiling regions. The image recognition results indicate that high effective thermal conductivities of the MPHP relate to stable self-oscillations with large amplitudes and high frequencies, along with long and thin liquid films beneficial for latent heat transfer. Finally, we perform numerical simulations of latent/sensible heat transfer via vapor plugs and of sensible heat transfer via liquid slugs using the extracted flow patterns and measured channel temperatures as inputs. We find that latent heat transfer via liquid films accounts for a considerable portion of the overall heat transfer, while the sensible heat transfer via liquid slugs is much less significant.

Energy Transfer Enhancement Inside an Annulus Using Gradient Porous Ribs and Nanofluids

Abstract Porous media and nanofluid utilization are two passive heat transfer improvement tools, which have been employed extensively in recent years. Porous media with gradient properties result in both a higher effective thermal conductivity and better local convective heat transfer because of conducting the flow to the desired regions. In this study, distinct porous ribs are located on the internal border of an annulus. Four different conditions are considered for permeability change of ribs, including the minimum and maximum Darcy numbers and linearly increasing or decreasing variation in the radial direction, called LIV and LVD, respectively. In the first step, effects of porous rib relative height, porous rib porosity, and flow Reynolds number on the thermal efficiency and pressure drop are investigated. The results show that the configuration with Da = LVD and W/Rh = 0.25 has the maximum performance number PN = 2, that is the Nusselt improvement over pressure drop increment. Porous ribs arrangement with W/Rh = 0.25 and the minimum porosity (ɛ = 0.9) give the best PN. In the next step, the effects of nanoparticle addition with different volume fractions to the base fluid in different Reynolds numbers are investigated. In this step, porous rib relative height is set to W/Rh = 0.25. The results show that the maximum volume fraction has the highest heat transfer enhancement (about 2–2.5 times) but the lower volume fractions have higher PNs (PN ≈ 2.5 at ϕ = 1% and Re = 500).

Demonstration of a Compliant Microspring Array as a Thermal Interface Material for Pluggable Optoelectronic Transceiver Modules

Abstract Pluggable optoelectronic transceiver modules are widely used in the fiber-optic communication infrastructure. It is essential to mitigate thermal contact resistance between the high-power optical module and its riding heat sink in order to maintain the required operation temperature. The pluggable nature of the modules requires dry contact thermal interfaces that permit repeated insertion–disconnect cycles under low compression pressures (∼10 to 100 kPa). Conventional wet thermal interface materials (TIM), such as greases, or those that require high compression pressures, are not suitable for pluggable operation. Here, we demonstrate the use of compliant microstructured TIM to enhance the thermal contact conductance between an optical module and its riding heat sink under a low compression pressure (20 kPa). The metallized and polymer-coated structures are able to accommodate the surface nonflatness and microscale roughness of the mating surface while maintaining a high effective thermal conductance across the thickness. This dry contact TIM is demonstrated to maintain reliable thermal performance after 100 plug-in and plug-out cycles while under compression.

Characterization and thermal modeling of a miniature silicon vapor chamber for die-level heat redistribution

Vapor chambers are passive heat spreaders that can improve system level temperature uniformity through efficient heat transport in a high effective thermal conductivity vapor core. Fabricating a vapor chamber out of silicon is highly appealing due to the potential for direct integration schemes with existing semiconductor devices, but may be impractical from a cost perspective if the size of the vapor chamber must be much larger than the die. We investigate the potential benefit to using a miniature silicon vapor chamber with an active vapor transport region of 1 × 1 cm2 for the purpose of die-level heat redistribution. Due to the high amount of liquid charging precision required for working with such small-scale vapor chambers, a reduced order thermo-fluidic model is developed to predict the effect of both heat flux and liquid charge on the overall device thermal performance. The model incorporates wick microstructure effects and is validated against experimental results from a prototype device to agree within ±25%. The thermal performance of the vapor chamber is benchmarked against simulation results for solid silicon spreaders of comparable dimensions and is found to improve the hotspot temperature uniformity at heat fluxes above 60 W/cm2.

On the operating temperature of heat pipes

The relationship between the operating temperature of heat pipes and the maximum heat transport capacity posed by the heat pipe capillary limit is often overlooked. It is demonstrated through heat pipe experiments that for a given heat input, there exists a minimum temperature for the heat pipe system to operate. This phenomenon occurs due to the temperature dependence of the thermo-physical properties of the working fluid in the heat pipes and the working temperature range of the heat pipe system can thus be discerned by the capillary limit equation in conjunction with the heat pipe transient equation obtained by energy conservation. It may sometimes seem counterintuitive in the sense that if a heat pipe system is aided by a fan (and therefore increase the heat transfer coefficient), then the heat pipes break down and reduce the effectiveness of the thermal management system. This is due to the fact that heat pipes have excessively high effective thermal conductivity and their breakdown leads to heat transfer only through their constituent materials, whose thermal conductivities are lesser by at least an order of magnitude. Heat pipes in a thermal management system must therefore be meticulously designed for precise temperature ranges.

Experimental Set up of Two Closed Loop Pulsating Heat Pipe (CLPHP) with Water base Fluids

Heat pipes are deliberated to be effective heat dissipation devices compared to other types of heat sinks due to their high effective thermal conductivity. Because of the flexibility in the design and layout of heat pipe turns along the heat source, pulsating heat pipes have gained popularity. One of the parameters that have the mainimpact on the presentation of CLPHP is the thermo physical properties of the working fluid. The properties of the working fluid affect the temperature difference between the evaporator and the condenser which in turn affect the thermal resistance of the CLPHP. In this connection, the influence of different working fluids is experimentally investigated on a two loop CLPHP, varying the evaporator heat flux. Pure fluids, viz., water, acetone, benzene and binary mixture, viz., Acetone-water and Benzene-water are utilized on working fluids. The heat input considered at the evaporator is 32W, 48W and 60W. The filling ratio is kept as 50 %. The results show that among the working fluids considered for the study, acetone exhibits least thermal resistance among the pure fluids at all heat fluxes considered in the analysis, while Acetone-water mixture has exhibited least thermal resistance among the water based mixtures.

An Experimentally Validated Mesoscale Model of Thermal Conductivity of a UO2 and BeO Composite Nuclear Fuel

An experimentally validated mesoscale model was developed to simulate the effective thermal conductivity of UO2–BeO nuclear fuel with different microstructural features. The effects of the second phase (BeO) fraction and morphology, temperature, and interface thermal resistance were investigated. The model predicts that the continuous second phase configuration has a higher effective thermal conductivity than the dispersed second phase configuration for the same volume fraction and temperature. Companion experiments were conducted to validate the model predictions. It was demonstrated that accounting for the interface (Kapitza) thermal resistance is necessary to improve the model predictions. The largest difference between the model calculations and experimental results was about 5%, which is within the precision of the experimental measurements.