Constant Thermal Resistance Research Articles

Thermal-hydraulic characterization of manifold microchannel heat sink with diverging channels and uniform heating

This study presents the design and experimentation of a manifold microchannel heat sink with diverging channels, employing ammonia as the working fluid to evaluate its thermal–hydraulic performance under uniform heating conditions. The impact of heat flux, inlet subcooling, saturation temperature, and outlet thermodynamic quality were investigated. Under single-phase conditions, the heat sink exhibited low sensitivity to variations in heat flux with constant thermal resistance and pressure drop. However, with the onset of boiling, the heat transfer capability and pressure drop rapidly increased. A lower inlet subcooling boosted boiling inside the heat sink, leading to decreased thermal resistance and increased pressure drop. Under near-saturated inlet conditions, a higher saturation temperature enhanced heat transfer owing to smaller bubble nucleation and detachment size. The outlet thermodynamic quality for engineering applications should be moderately low because it provides a balance between high heat transfer capability and low pressure drop. Moreover, the results demonstrated excellent thermal–hydraulic performance, with a heat flux of up to 636.9 W/cm2 efficiently dissipated under uniform heating conditions while maintaining the heating surface temperature and pressure drop below 79.3 °C and 150.3 kPa, respectively.

Carboxylated Poly (p-Phenylene Terephthalamide) Reinforced Polyetherimide for High-Temperature Dielectric Energy Storage.

Dielectric energy storage polymers play a vital role in advanced electronics and electrical systems, due to their high breakdown strength, excellent reliability, and easy fabrication. However, the low dielectric constant and poor thermal resistance of dielectric polymers limit their energy storage density and working temperatures, making them less versatile for broader applications. In this work, a novel carboxylated poly (p-phenylene terephthalamide) (c-PPTA) is synthesized and employed to simultaneously enhance the dielectric constant and thermal resistance of polyetherimide (PEI), leading to a discharged energy density of 6.4 J cm-3 at 150 °C. The introduction of c-PPTA molecules effectively reduces the ΠΠ stacking effect and increases the average chain spacing between polymer molecules, which is conducive to improving the dielectric constant. Additionally, c-PPTA molecules with stronger positive charges and high dipole moments can capture electrons, resulting in reduced conduction loss and enhanced breakdown strength at high temperatures. The coiled capacitor fabricated with the PEI/c-PPTA film exhibits superior capacitance performances and higher working temperatures compared to commercial metalized PP capacitors, demonstrating great potential for dielectric polymers in high-temperature electronic and electrical energy storage systems.

Temperature-dependent thermal resistance of phase change memory

One of the key challenges of phase change memory (PCM) is its high power consumption during the reset operation, when the phase change material (typically Ge2Sb2Te5, i.e., GST) heats up to ∼900 K or more in order to melt. Here, we study the temperature-dependent behavior of PCM devices by probing the reset power at ambient temperatures from 80 to 400 K. We find that different device structures exhibit contrasting temperature-dependent behavior. The reset power in our confined-type PCM is nearly unchanged with ambient temperature, corresponding to a temperature-dependent thermal resistance, whereas results for mushroom-type PCM from the literature show a linear relation between power and temperature, suggesting a more constant thermal resistance. This discrepancy is ascribed to different temperature distributions and thermal properties of the dominant components of the PCM cell thermal resistance, as shown by electro-thermal modeling. In the confined cell, the thermal boundary resistance of the GST and the thermal conductivity of the bottom electrode dominate the thermal resistance, while for the mushroom cell, the GST thermal conductivity plays a greater role. These findings can help to design more power- and energy-efficient PCM devices by better focusing thermal management efforts on the key components of the device.

Stepwise copolymerization of polybenzimidazole for a low dielectric constant and ultrahigh heat resistance.

Bio-based polymer materials having great potential due to the depletion of fossil-fuel resources have been applied as single-use and medicinal materials but their low thermomechanical resistance have limited wider applications. Here, ultrahigh thermoresistant bio-based terpolymers with a low dielectric constant, comprising polybenzimidazole and poly(benzoxazole-random-aramid), were prepared by a method involving stepwise polycondensation of three monomers, 3,4-diaminobenzoic acid for benzimidazoles, 3-amino-4-hydroxylbenzoic acid for benzoxazoles, and 4-aminobenzoic acid for aramids. For optimized monomer compositions, the obtained terpolymers exhibited dielectric constants lower than 3, and a 10% mass loss at approximately 760 °C which is a temperature higher than that for any other polymer material reported so far. The high thermal degradation temperatures of the prepared terpolymers were a result of the high interaction enthalpies of hydrogen bonding between imidazole rings in the polymer chains, which were obtained from density functional theory calculations using trimer models. Furthermore, the applicability of the prepared terpolymers as a wire-coating material for a simple motor insulation was demonstrated, indicating that it has significant potential to be used as a thermostable material with a low dielectric constant (k).

Thermo-mechanical analysis of a nuclear spent fuel dry cask using CFD results in a FE code

As the pools in nuclear power plants are reaching their full capacity, the use of dry cask systems to store the spent fuel is growing. This leads to an increasing need of a better understanding of nuclear spent fuel behavior during its storage phase in dry casks, ultimately, ensuring that the safety limits are not compromised. Previous computational fluid dynamic (CFD) studies have found that the gap size between the canister and the basket have a remarkable impact on the fuel peak cladding temperature. However, because the exact gap size is usually unknown, this is routinely modelled as a constant thermal resistance between the components or it is not considered directly. On the other hand, the non-uniform temperature distributions in the components are expected to affect differently the gaps, through the non-uniform thermal displacements of the casks components. In turn, the possible reduction or increase of the gaps will affect the thermo-hydraulic performance of casks.In this paper, a thermo-mechanical analysis of a spent fuel dry cask is presented. The temperatures of the cask’s components obtained in a previous CFD simulation with STAR-CCM+ code are employed in the finite element code ABAQUS with the aim to study the overall behavior of the cask components. Additionally, a detailed analysis of the gaps is performed to help improving future CFD studies. The results presented in the paper confirm that the thermal displacements and stresses are relatively low, as compared to safety limits. The non-uniform relative displacements of the components obtained in the analysis of the gaps have facilitated explaining the differences in fuel temperatures between CFD simulations and experiments.

Non-linear thermal resistance model for the simulation of high power GaN-based devices

We report on the modeling of self-heating in GaN-based devices. While a constant thermal resistance is able to account for the self-heating effects at low power, the decrease of the thermal conductance of semiconductors when the lattice temperature increases, makes necessary the use of temperature dependent thermal resistance models. Moreover, in order to correctly account for the steep increase of the thermal resistance of GaN devices at high temperature, where commonly used models fail, we propose a non-linear model which, included in an electro-thermal Monte Carlo simulator, is able to reproduce the strongly non-linear behavior of the thermal resistance observed in experiments at high DC power levels. The accuracy of the proposed non-linear thermal resistance model has been confirmed by means of the comparison with pulsed and DC measurements made in devices specifically fabricated on doped GaN, able to reach DC power levels above 150 W mm−1 at biases below 30 V.

Soft robotic fabric design, fabrication, and thermoregulation evaluation

Usually, traditional insulation materials have a constant thermal resistance value that cannot change within the ambient temperature and will decrease as ambient humidity or external stress increases. Humans heavily rely on heating, ventilation, and air conditioning (HVAC) systems to meet the thermal comfort requirements of their bodies, giving rise to energy waste and global warming. As an infinitely available natural resource, air is one of the most efficient thermal retaining substances known to science. Inspired by soft pneumatic robotics, we propose an architecture for air-driven thermoregulation fabrics called soft robotic fabrics (SRF). By changing the thickness of trapped air layer in fabric system through SRF, wearers could modify garments’ thermal insulation performance. A fabrication method is introduced to rapidly manufacture low-cost pneumatic structures using various types of construction and dimensions. With excellent ductility, elasticity, and compression resistance, the thickness of SRF increases by 12 times or more after inflation, and the fabric even can lift an object 270 times heavier than its weight. The excellent deformability can effectively increase stable air layer between clothing and skin. Based on the Predicted Mean Vote–Predicted Percentage of Dissatisfied model, the thermoregulation capability of SRF helps HVAC expand the temperature setpoint range by 3–8 times when compared with traditional fabrics, and has far-reaching significance in saving energy.

Variable thermal resistance model of GaN-on-SiC with substrate scalability

A drain current model for an AlGaN/GaN high-electron-mobility transistor (HEMT) with variable thermal resistance is developed. For the first time, a variable thermal resistance in terms of the substrate thickness $$(t_{\text{SiC}} )$$ is included rather than a constant thermal resistance. One of the distinguishing features of this model is that it is scalable with substrate thickness. The compact drain current model is compared with simulated characteristics for different substrate thicknesses (tSiC = 100 μm, tSiC = 200 μm, and tSiC = 300 μm). The proposed model clearly captures the self-heating effect well in the saturation region of the drain current at an ambient temperature of 300 K. The transfer and transconductance characteristics of the model show good correlations with both experimental and simulation data under a wide range of bias conditions. Furthermore, the smoothness of the model is confirmed by Gummel symmetry and continuity tests. Hence, the proposed model is compact and considered to be a promising candidate for GaN-HEMT circuit simulation.

The effect of interfacial thermal resistance on interface crack subjected to remote heat flux

We consider the thermoelastic problem of an interface crack between two dissimilar semi-infinite isotropic materials under a uniform remote heat flux in plane deformation. The crack face is assumed to be partially thermopermeable (defined by a partial insulation coefficient of the crack), while the interface is assumed to be perfectly bonded except that a constant thermal resistance is introduced into the interfacial region near the tips of the crack. By using the integral transform method, we obtain the analytic solution for the thermoelastic field in the entire bi-material system. Numerical examples are presented to study the influence of interfacial thermal resistance on the thermal stress intensity factors and the crack opening/sliding displacements. It is shown that the magnitudes of the mode I and mode II TSIFs, as well as the crack opening displacements, increase with the increasing interfacial thermal resistance, while the crack sliding displacement is insensitive to the change of interfacial thermal resistance.

A sub-interface thermal crack problem for bonded dissimilar plates with interfacial thermal resistance

The present work aims to investigate the effect of interfacial thermal resistance on thermal fracture behavior of bonded and composite materials. We consider a sub-interface crack parallel to the interface between two semi-infinite dissimilar plates subjected to remote heat flux thermal loading. A constant thermal resistance is assumed to exist along the interface. The temperature distribution along the crack, the thermal stress intensity factors (TSIFs), and the crack opening/sliding displacements (COD/CSD) are obtained using an integral transform/superposition method. The numerical results for Al2O3/Si3N4 bimaterial systems show that the magnitude of the mode I TSIF generally decreases with increasing thermal resistance of the interface but increases with increasing thermal resistance for cracks that are very close to the interface. On the other hand, the model II TSIF increases with increasing thermal resistance if the crack is in the Al2O3 semi-infinite plate, and decreases if the crack is in the Si3N4 semi-infinite plate. The COD/CSD are also significantly influenced by the thermal resistance of the interface.

Size effect on boundary condition at solid-liquid interface in microchannel

The heat transfer in microchannel has attracted considerable attention due to many important applications in biology, chemistry, physics and engineering. When the fluid size shrinks to nanoscale, the energy transport of micro-system is significantly different from the conventional case. It is of great significance to study the size effect on heat transfer in a micro-system. However, there is a large size gap between existing molecular dynamics simulation and experimental measurement, in which the size effect on solid-liquid interfacial thermal resistance is rarely involved. Non-equilibrium molecular dynamics simulation is performed to investigate the heat transfer through the solid-liquid interface. Simple Lennard-Jones (LJ) fluid is simulated as the ultra-thin liquid film in a non-equilibrium simulation system. The liquid film is confined in a nanochannel composed of two solid surfaces. The potential function between solid and liquid atom is represented by a modified LJ function to control the solid-liquid interfaces of different surface wettabilities. We examine the size effect on temperature jump and thermal resistance at the solid-liquid interface. The fluid number density and temperature distribution in the perpendicular direction of solid wall are evaluated. It is found that the liquid atoms near wall are arranged as a solid-like structure. Particularly in the small channel, liquid atoms confined in the channel are affected by two solid walls. However, with the increase of channel height, the liquid atoms in the middle channel move freely, leading to the decrease of the size effect. The simulation results show that the dependence of thermal resistance on microchannel height exhibits two regimes: (i) monotonically increasing dependence for the small channel and (ii) keeping constant thermal resistance for the large channel. These two distinct trends can be explained by phonon vibrational density of states (VDOS) of solid wall and liquid. For the small channel, a stronger confinement of liquid leads to a weaker mismatch in VDOS of solid wall and liquid, thus resulting in a smaller thermal resistance. Whereas, for the large channel, the vibrational coupling between the solid and the liquid atom remains unchanged and the size effect is negligible. The size thresholds of the two regimes of the thermal resistance are both sensitive to the liquid-solid interaction strength, which decreases with solid-liquid interaction increasing. Furthermore, with the increase of the microchannel height, the temperature jump at the solid-liquid interface monotonically decreases and eventually approaches to the non-jump temperature boundary on a macroscopic scale. These findings may help to understand the mechanism of temperature boundary conditions on a microscopic scale and a macroscopic scale and provide a theoretical support for manufacturing new nano-devices.

An F82H steel pressurized tube creep capsule for irradiation in HFIR

A novel capsule design has been developed for measurement of irradiation creep in pressurized tubes and is being used to irradiate reduced activation ferritic/martensitic F82H steel creep test specimens. These tests are being conducted in the flux trap of the High Flux Isotope Reactor at the Oak Ridge National Laboratory. The capsule design uses a tight-fitting corrugated aluminum foil placed in the center of a vanadium alloy holder to conduct heat from the centrally located test specimen. The foil acts as a compressible thermal interface between the pressurized tube and holder, maintaining a constant thermal resistance (and thus a constant tube temperature gradient) during irradiation, regardless of differential thermal expansion, creep, and swelling in the test specimen. Mechanical interference with creep deformation of the specimen tube is minimized by using a thin (0.05 mm) foil with sufficient room to crush. Finite element analysis of the contact pressure between the specimen and foil, combined with thermal creep in the foil, showed little interference with specimen stress conditions. Specimens were designed to experience hoop stresses of 380, 300, 150, and 0 MPa at a temperature of 300 °C while being irradiated to a dose of 3.7 dpa. Passive SiC thermometry is located within the pressurized tube and the holder material for confirmation of experiment irradiation target temperatures. This work discusses aspects of the capsule's fabrication and design, including thermal models of the capsule during irradiation.

Fouling formation and thermal performance of aqueous carbon nanotube nanofluid in a heat sink with rectangular parallel microchannel

Experimental investigation is performed on the thermal performance of a copper-made heat sink with rectangular microchannel. Carbon nano tube aqueous nanofluid is used as a coolant inside the microchannel at mass concentrations of 0.05–0.1%. Influence of different operating parameters including applied heat flux, fluid flow rate and mass concentration of nanofluid on the local and average heat transfer coefficients, fouling thermal resistance, overall thermal resistance and local (axial) temperature profile is investigated. Results showed the higher heat transfer coefficient and lower temperature profile inside the heat sink in comparison with the base fluid (water). Fluid flow rate and mass concentrations were found to increase the heat transfer coefficient significantly, while slight improvement was seen when higher heat fluxes were applied into the microchannel heat sink. Fouling thermal resistance was found to asymptotically increase with an increase in operating time and strongly depends on the concentration of nanofluid such that the required operating time to reach a constant value was different. The higher the mass concentration, the lower operating time is required to reach the constant fouling thermal resistance. However, overall thermal resistance of the microchannel (without considering the fouling effect) was found to linearly decrease with the mass concentration of nanofluid.

Estimation of spatially varying thermal contact resistance from finite element solutions of boundary inverse heat conduction problems split along material interface

This paper presents a method for estimating spatially varying thermal contact resistance from a computational point of view. The method starts by splitting the computational domain along material interface to yield two boundary inverse heat conduction problems. Temperatures computed from analytical solutions are specified at only three interior points of each material instead of temperatures available from experiments. A number of equations constructed from modified cubic spline specified along the interface are incorporated into finite element equations during the solution process. After extracting temperatures and heat transfer rates from both finite element solutions, which are solved separately, thermal contact resistance at each pair of coincident nodes on the interface are calculated.Constant, sinusoidal, and triangle functions are selected as spatial functions of thermal contact resistance along material interface to test the method. Constant thermal contact resistance is estimated accurately. Shapes of sinusoidal and triangle functions are fairly captured, locations of their maximums are correctly predicted. Thermal load is suggested to be applied on the boundary of material that has lower thermal conductivity to let the method works properly. Quantifying the uncertainty of estimated thermal contact resistance is demonstrated by adding bias error based on accuracy of sensors to the temperatures specified on the interior points of both materials.

Comparison of Steady-state and In-situ Testing of High Thermal Resistance Walls Incorporating Vacuum Insulation Panels

Two standard test methods exist to experimentally determine the effective thermal resistance of a wall: in-situ testing to calibrate a computer simulation and steady-state testing in a guarded hot box. This paper compares results obtained using both methods on a wall assembly incorporating vacuum insulation panels (VIPs) with an experimentally determined effective R-Value of8.21 m2K/W using a guarded hot box. The result using a guarded hot box was lower than that using in-situ measurements paired with computer simulation as a result of the more accurate representation of thermal bridging between VIP panels using this test method. When testing in a guarded hot box, all thermal bridges, including between panels is accounted for. By comparison, an experimentally obtained thermal resistance measured at the center of a VIP panel and assumed as a constant thermal resistance across the complete VIP layer in the computer model does not account for the non-homogenous nature of a VIP layer. As a result, it was concluded that wall assemblies that contain numerous small thermal bridges like those incorporating VIP panels should be experimentally evaluated using a guarded box.

The influence of longitudinal vibrations on the heat transfer performance of inclined heat pipes

This study focused on investigating the influence of longitudinal vibrations, the condensation section temperature, and the inclination angles on the heat transfer performance of grooved cylindrical copper heat pipes with lengths of 600 and 150 mm and an outer diameter of 8 mm. The inclination angles of the tested heat pipes were 0°, ±45°, and ±90°. Longitudinal vibrations with frequencies of 3, 4, 5, 6, and 9 Hz and amplitudes of 2.8, 5, 10, 15, 20, and 25 mm, which resulted in accelerations between 0.1 and 1.01 g, were experimentally tested. The condensation section temperatures were set at 20°C, 30°C, and 40°C. A heating jacket and a cooling sleeve were installed at the evaporation and condensation sections of the test cell to simulate a constant heat flux and a constant temperature boundary, respectively. The results showed that with the heat pipe placed with the condensation section on top and the evaporation section on bottom, a fairly low and constant thermal resistance (approximately 0.25 K/W for the 600-mm heat pipe and 0.75–1.2 K/W for the 150-mm heat pipe) was obtained, both with and without heat pipe vibration and regardless of the condensation section temperature.

Optimal design method for plate fin heat sinks subject to natural convection

In this study, an optimal design method for minimizing the size of plate fin heat sinks subject to natural convection is proposed. An analytic model for predicting the thermal resistance of a fin array, as well as the heat sink substrate, is developed. In order to validate the analytic results, we performed numerical study and experimental studies. The results indicate that the proposed analytic model predicts the numerical and the experimental results well, within a maximum error of 5%. Based on the validated analytic results, we suggest an algorithm for minimizing the size of a natural convective plate fin heat sink subject to a constant thermal resistance. As the substrates thickness increases, the substrates conduction thermal resistance decreases while the convection thermal resistance decreases as the fin height increases. The minimum heat sink size is determined by a balance between those two competing effects. The minimum heat sink size is shown to increase as the heat source localization intensity increases.

Analytical solution for combined heat and mass transfer in laminar falling film absorption with uniform film velocity - diabatic wall boundary

Abstract In a previous study the Laplace transform has been introduced in order to solve the combined heat and mass transfer problem in absorbing laminar falling films with constant film velocity for an isothermal as well as an adiabatic wall boundary condition. It has been stated, that the Laplace transform basically allows to apply arbitrary wall boundary conditions in contrast to the Fourier method. Therefore, in the present study a diabatic wall boundary condition is applied, which, as limiting cases, includes both the isothermal wall if the thermal resistance of the wall is zero as well as the adiabatic wall condition for an infinite thermal resistance of the wall. Temperature and mass fraction profiles across the film as well as the evolution of the absorbed mass flux with increasing flow length are presented for two different thermal resistances of the wall and modified Stefan numbers. The results are compared to the limiting cases of the isothermal and the adiabatic wall boundary condition. The present study offers an analytical solution with a more realistic boundary condition of a constant thermal resistance of the wall including the isothermal and the adiabatic wall boundary condition.

A model-based approach for analysis of working fluids in heat pipes

Heat pipes (HP) are efficient heat transfer devices, utilizing a working fluid to transfer heat based on evaporation and condensation. To make HPs even more efficient, one viable approach is to modify the utilized working fluid thermophysical properties. In this paper, a simple yet effective method based on dimensional analysis, leading to a reduced-order model, is proposed to analyze and modify working fluid's thermophysical properties and to predict the impact of working fluid modification on thermal resistance of a trapezoidal micro-grooved heat pipe. A validated one dimensional mathematical model based on a semi-analytical hydrodynamic approach is used to estimate the reduced-order model parameters. Alternatively, experimental data can be used for estimation of the reduced-order model parameters. The reduced model is further used to analyze and quantify the contribution of each of the thermophysical properties of working fluid on heat pipe thermal resistance. The simplified reduced-order model yielded a dimensionally reduced form of the numerical model which revealed constant thermal resistance contours. In addition, the reduced-order model successfully predicted the performance of the heat pipe when filled with different working fluids. It is also found that high thermal conductivity, low surface tension, low latent heat of evaporation, high viscosity, and low liquid density are the most favourable thermophysical properties of the working fluid leading to improvement of heat pipe thermal resistance, respectively. The proposed method for working fluid modification is generic and thus applicable to a range of situations associated with the selection and/or design of heat pipe working fluids and mixtures thereof.

Transient Electro-Thermal Modeling of Bipolar Power Semiconductor Devices

This book presents physics-based electro-thermal models of bipolar power semiconductor devices including their packages, and describes their implementation in MATLAB and Simulink. It is a continuation of our first book Modeling of Bipolar Power Semiconductor Devices. The device electrical models are developed by subdividing the devices into different regions and the operations in each region, along with the interactions at the interfaces, are analyzed using the basic semiconductor physics equations that govern device behavior. The Fourier series solution is used to solve the ambipolar diffusion equation in the lightly doped drift region of the devices. In addition to the external electrical characteristics, internal physical and electrical information, such as junction voltages and carrier distribution in different regions of the device, can be obtained using the models. The instantaneous dissipated power, calculated using the electrical device models, serves as input to the thermal model (RC network with constant and nonconstant thermal resistance and thermal heat capacity, or Fourier thermal model) of the entire module or package, which computes the junction temperature of the device. Once an updated junction temperature is calculated, the temperature-dependent semiconductor material parameters are re-calculated and used with the device electrical model in the next time-step of the simulation. The physics-based electro-thermal models can be used for optimizing device and package design and also for validating extracted parameters of the devices. The thermal model can be used alone for monitoring the junction temperature of a power semiconductor device, and the resulting simulation results used as an indicator of the health and reliability of the semiconductor power device.