Internal Thermal Resistance Research Articles

Experimental investigations on the thermal performance and phase change hysteresis of low-temperature paraffin/MWCNTs/SDBS nanocomposite via dynamic DSC method

Benefitting from high storage density, phase change material (PCM) has been widely utilized in cold chain logistics. However, the phase change process of composites is normally non-isothermal, which will lead to phase change hysteresis (PCH). The exploration of the mechanism and influence factors of PCH remains a huge challenge. In this paper, a nanocomposite PCM (NCPCM) based on paraffin (PA) was prepared to address this issue via the two-step method. Here, the applications of multi-walled carbon nanotubes (MWCNTs) and sodium dodecylbenzene sulfonate (SDBS) were contributed to the enhancement of thermal conductivity and suspension stability. Subsequently, after various characterization techniques, the results showed that thermal conductivity of the NCPCM with 1.0 wt% MWCNTs and a 2:1 mass ratio of MWCNTs and SDBS increased by 1.86 times. Moreover, the NCPCM exhibited high thermal charging-discharging efficiency, thermal stability, and cyclic stability. More importantly, the PCH has been studied via the dynamic differential scanning calorimetry (DSC) method. The effects of temperature range, temperature changing rate, and sample mass on the hysteresis degree were investigated. Furthermore, the PCH was interpreted in terms of energy storage, temperature delay, internal thermal resistance, crystallinity, and interfacial free energy. It was proved that the NCPCM possesses a smaller hysteresis degree when the temperature range was −40–50 °C, temperature changing rate was 10 K min−1, and sample mass was within 10 mg.

Research progress of thermal transport in graphene-based thermal interfacial composite materials

With the rapid increase of the thermal power density of microelectronic devices and circuits, controlling its temperature has become an urgent need for the development and application of the electronic industry. By virtue of the ultrahigh thermal conductivity of graphene, developing high-performance graphene-based composite thermal interface materials has attracted much research attention and become one of hot research topics. The understanding of phonon transport mechanism in graphene-based composites at atomic scale can be helpful to enhance the heat conductive capability of composites systems. In this review, focused on graphene-based thermal interfaces materials, the heat conduction mechanism and the regulating strategy are introduced on both the internal thermal resistance and interfacial thermal resistance. Finally, the reseach progress and opportunities for future studies are also summarized.

EFFECTS OF WALL TEMPERATURE ON THERMOPHYSICAL CHARACTERISTICS OF SPRAY-WALL IMPINGEMENT

For small- and medium-bore diesel engines, the phenomenon of spray-wall impingement is unavoidable, which affects the evaporation of the fuel, thermal efficiency, and emissions. Wall temperature is an important parameter in this process because it has an important influence on fuel evaporation. In this paper, the thermophysical characteristics of spray-wall impingement at different wall temperatures were studied. The results showed that with increase of the initial wall temperature, the impingement spray radius increased, but its rate of increase decreased, while the impingement spray height first increased and then decreased. During this process, the surface heat flux first increased and then decreased with the increase of initial wall temperature, which was due to the change of heat transfer regime on the wall. The transient heat transfer process at different initial wall temperatures was characterized by the Biot and Fourier numbers and the dimensionless surface heat fluxes were highly similar. A dimensionless correlation was developed to quantify the transient heat transfer process, which showed that the ratio of the internal thermal resistance of heat conduction in the wall to the wall surface convective heat transfer resistance changes almost linearly during the process of spray-wall impingement.

Impedance Spectroscopy Analysis of Thermoelectric Modules Fabricated with Metallic Outer External Layers

In recent years, thermoelectric (TE) devices have been used in several refrigeration applications and have gained attention for energy generation. To continue the development of devices with higher efficiency, it is necessary not only to characterize their materials but also to optimize device parameters (e.g., thermal contacts). One attempt to increase the efficiency at the device level consists of the replacement of the typical ceramic layers in TE modules by metallic plates, which have higher thermal conductivity. However, this alternative device design requires the use of a very thin electrical insulating layer between the metallic strips that connect the TE legs and the outer external layers, which introduces an additional thermal resistance. Impedance spectroscopy has been proved to be useful to achieve a detailed characterization of TE modules, being even capable to determine the internal thermal contact resistances of the device. For this reason, we use here the impedance method to analyze the device physics of these TE modules with outer metallic plates. We show for the first time that the impedance technique is able to quantify the thermal contact resistances between the metallic strips and the outer layers, which is very challenging for other techniques. Finally, we discuss from our analysis the prospects of using TE modules with external metallic plates.

Prediction and parametric analysis of 3D borehole and total internal thermal resistance of single U-tube borehole heat exchanger for ground source heat pumps

The borehole and total internal thermal resistance are both significant parameters in evaluating the thermal performance of the ground source heat pump. This study aimed to obtain the accurate correlation of the 3D borehole and total internal thermal resistance (Rb,3D and Ra,3D) and analyze the impacts of parameters on the Rb,3D and Ra,3D. Firstly, eight parameters affecting the Rb,3D and Ra,3D, including the borehole diameter, pipe diameter, pipe-pipe distance, borehole depth, soil thermal conductivity, grout thermal conductivity, pipe thermal conductivity, and fluid velocity inside the pipe, were considered and an L-54 design matrix was generated. Then, the 3D numerical model, coupling with the four-resistance model, was proposed to calculate Rb,3D and Ra,3D for each case. After that, the response surface methodology was employed to obtain and verify the correlation of Rb,3D and Ra,3D, which were compared with the existing resistance calculation methods. Lastly, analysis of variance was carried out to reveal parameters that have statistically significant impacts on the Rb,3D and Ra,3D. Results show that the rationality and accuracy of the correlation of Rb,3D and Ra,3D can be verified by the determination coefficient and P value of regression model, as well as the P value of lack-of-fit. The existing resistance calculation methods are more or less inaccurate and the discrepancies in some cases can be up to 86.74% and 111.35% for the borehole and total internal thermal resistance. The pipe and grout thermal conductivity, pipe and borehole diameter, and the pipe-pipe distance can be seen as the significant contributory factors to the variation of Rb,3D and Ra,3D.

Comprehensive impedance spectroscopy equivalent circuit of a thermoelectric device which includes the internal thermal contact resistances

Thermoelectric devices are widely used as solid-state refrigerators and have potential energy generation applications. Their characterization is key to develop more efficient devices and monitor their performance. Electrical impedance spectroscopy has been proved to be a useful method for the characterization of thermoelectric modules. However, deviations from current impedance models still exist in experimental results, especially in the high frequency part of the impedance spectrum, which limits its use. Here, we present a new comprehensive impedance model (equivalent circuit) which covers all the key phenomena that affects the module performance, and it is able to explain the observed deviations. The new equivalent circuit includes, as new additions, the thermal influence of the metallic strips (electrodes), combined with the thermal contact resistance between the metallic strips and the outer ceramic layer. Moreover, a new more accurate spreading-constriction impedance element, which considers the variation of the heat flow in the radial direction at the outer ceramic surfaces, is also developed. The comprehensive equivalent circuit was used to perform fittings to impedance spectroscopy measurements of modules fabricated by different manufacturers. From the fittings, it was possible to identify, among other key properties, the internal thermal contact resistances, whose direct determination is very challenging. Thermal contact resistivities at the metallic strips/thermoelectric elements interface in the range 2.20 × 10-6-1.26 × 10-5 m2KW−1 were found. An excellent thermal contact was identified at the metallic strips/ceramic layers. This opens up the possibility of using impedance spectroscopy as a powerful tool to evaluate, monitor, and identify issues in thermoelectric devices.

Residential Air Conditioning System Integrated with Packed Bed Cool Storage Unit for Promoting Rooftop Solar PV Power Generation

The increase in the share of renewable-based power in the gross power generation in most countries causes significant concerns over the addition of renewable power with the grid, results in stability issues in most developed nations. Energy storage is an emerging technology that is considered the ultimate solution in developing microgrids with distributed renewable power generation. The cool thermal storage plays a vital role in economically promoting renewable power among different storage units. The major objective of the research work is to demonstrate the integration of residential air-conditioning systems with packed bed cool storage units to promote rooftop solar power generation for residential space cooling applications. In order to achieve the said objective, an experimental investigation was made to study the charging/discharging characteristics of a packed bed cool-storage unit combined with a chiller and a cooling coil unit suitable for small capacity air-conditioning applications. The system consists of encapsulated spherical capsules filled with a phase change material blended with distilled water and pseudomonas (nucleating agent) and the heat transfer fluid as a combination of distilled water and Mono-ethylene glycol. A cooling coil unit was connected to the cool-storage tank to transfer cool energy from the storage tank to the space to be cooled when there is a demand. The important parameters, such as instantaneous and cumulative heat transfer during the charging/discharging processes, are presented. The average COP values of the chiller during the charging operation were estimated as 1, 0.93, and 0.89 when the HTF setpoint temperatures were -6°C, -9°C, and -12°C, which shows a decrease in performance as the setpoint temperature decreases. During the discharging process, a cooling load of 2.25 kW is obtained during the first cycle of operation and gradually reduces to 0.3 kW during the sixth cycle of operation. The increase in the HTF temperature during each cycle of operation indicates that the Phase Change Material (PCM) in the balls cannot release the heat as per the demand after a certain period of discharging. Hence, decreasing the internal thermal resistance by suitable measures is essential to achieve uniform heat flux and to operate the system successfully

Influence of environmental boundary conditions on convective heat transfer coefficients of wall internal surface

In this study, convective heat transfer phenomena were investigated by means of a Guarded Hot Box (GHB) apparatus. An experimental setup characterized by air and surface temperature probes, and a hot-wire anemometer was used. Five small fans were installed in the metering chamber to generate a forced air flow characterized by different velocity values. So, the GHB was used for investigating the influence of different air speed values on internal convective coefficients. Considering horizontal heat fluxes, an internal convective coefficient values of 2.5 W/m2K is reported in the Standard ISO 6946. However, no exhaustive description about this value is provided. The aim of this work is to experimentally determine the internal thermal surface resistance, quantifying how the convective heat transfer coefficient varies as air velocity changes.

Fabrication of NbN/SiNx:H/SiO2 membrane structures for study of heat conduction at low temperatures

Here we report on the development of NbN/SiNx:H/SiO2-membrane structures for investigation of the thermal transport at low temperatures. Thin NbN films are known to be in the regime of a strong electron-phonon coupling, and one can assume that the phononic and electronic baths in the NbN are in local equilibrium. In such case, the cooling of the NbN-based devices strongly depends on acoustic matching to the substrate and substrate thermal characteristics. For the insulating membrane much thicker than the NbN film, our preliminary results demonstrate that the membrane serves as an additional channel for the thermal relaxation of the NbN sample. That implies a negligible role of thermal boundary resistance of the NbN-SiNx:H interface in comparison with the internal thermal resistance of the insulating membrane.

Temperature distribution in a multi-layer cylinder with circumferentially-varying convective heat transfer boundary conditions

Theoretical modeling of thermal conduction in a multilayer cylinder has been studied in multiple past papers due to its significance in engineering applications such as nuclear engineering, energy storage and sensing. Most past papers have assumed a constant convective heat transfer coefficient on the outer surface of the cylinder. However, a circumferentially varying heat transfer coefficient may be appropriate in several applications due to the distributed nature of fluid flow around the cylinder. This paper presents a theoretical model for steady-state thermal conduction in a multilayer cylinder with circumferentially-varying convective heat transfer coefficient on the cylinder surface. The theoretical model is presented for both solid and annular cylinders. A series solution is derived for the temperature distribution in each layer by using the convective boundary condition(s) to derive a set of linear algebraic equations for the coefficients of the inner-most layer. Results are shown to be in good agreement with numerical simulations and with closed-form solutions for special cases. The impact of various problem parameters, such as internal heat generation rate and thermal contact resistances on the temperature distribution is analyzed. It is expected that the results presented in this work will improve the theoretical understanding of multilayer heat transfer and be of practical use in multiple engineering applications.

Investigation of the internal structure, combustion, and thermal resistance of the rigid polyurethane materials reinforced with vermiculite

In this study, 5%, 10%, and 15% vermiculite was added to the rigid polyurethane (PU) structure as a fire retardant. The scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), limiting oxygen index, thermal gravimetric analysis, differential scanning calorimetric (DSC) curves, and combustion behavior of the composite materials were investigated. When SEM/EDS analyses were examined, it was observed that surface morphology was affected by the amount of additive material. When XRD analyses were examined, peaks appeared in three different groups. Thermogravimetric analysis was used to determine the thermal stability of PU material with the addition of vermiculite. When the DSC analysis was examined, four temperature zones were obtained. The combustion behaviors of the vermiculite added PU samples with single-flame source and the flammability classes were determined with TS EN ISO 11925-2 standard. It can be said that the usage of inorganic mineral, which is harmless to the environment, can enhance the thermal stability of the material and also reduce the cost of production.

INFLUENCE OF THERMAL RESISTANCE OF AIR CONDENSER TUBES ON STEAM COOLING EFFICIENCY


 
 
 The research is carried out using a mathematical model of conditions and features of condensation processes with the influence of changes in internal and external thermal resistances of working bodies, which occur during contamination of outside and inside metal pipes of heat exchange surfaces of air condenser. capacitor. Particular attention is paid to the selection, detailing and determination of more than twenty basic parameters that characterize the operation of the direct cooling unit of the condensing unit for the summer, the conditions of heat transfer processes between the working bodies taking into account the finned outer surface of elliptical condenser tubes. The results of experiments on the mathematical model are analyzed and the influence of the incoming air velocity and ambient temperature on the output steam pressure in the condenser direct air cooling system within the change of internal and external thermal resistances in the range 0-0.001(m2·K)/W due to cooling tube contamination is determined. air condenser steam turbine installation. Conditions, character and features of influence of thermal resistance of pollution in cooling tubes on steam pressure at an exit from them are defined, the basic factors defining steam pressure at an exit, necessity of the organization of control of thermal resistance of pollution in a pipe during unit operation at variable operating conditions and expediency is substantiated. conducting test studies of operating modes while taking into account the influence of thermal resistance of external and internal pollution on the thermal efficiency of the cooling unit.
 
 
 
 Studies have shown that at a fixed value of the heat load of the exhaust steam, the pressure of the steam outlet increases with increasing ambient temperature and decreasing the speed of the incoming air.
 
 
 
 
 

The Study on Thermal Analysis Method of PCB

This essay makes the reliable analysis and modelling on printed circuit boards(PCB) by software of Flotherm and get conclusions of three points through simulation analysis:1) there is an important impact on heat transfer of the distribution of copper foil on PCB, and the actual condition should be selected for different precision simulation modules; 2) the heating device with simple structure and small thermal resistance can be dealt with as the simple square; 3) in the case of knowing the internal thermal resistance, integrated chip adopts double thermal resistance model as much as possible. In accordance with above inference, Flotherm modelling and simulation is carried out for certain printing circuit board, and computing results are compared with actual testing data to verify the correction of analysis methods.

Thermal design of a hydrogen storage system using La(Ce)Ni5

Hydrogen storage within a metal hydride involves exothermic and endothermic processes for hydrogen absorption and desorption, respectively. In addition, the thermal conductivity of the particulate metal hydride (i.e., powder) after repeated absorption processes is extremely low compared to its bulk phase. Low heat conduction through the metal hydride powder makes the hydrogen charging slow; thus, appropriate thermal management is necessary to achieve the fast charging time with the maximum energy density. In this work, we propose a thermal design of a portable hydrogen storage system made of a 300-mL vessel by balancing the internal and external thermal resistances. A copper-mesh structure is employed inside the vessel for enhancing the effective thermal conductivity of metal hydride powder (i.e., reducing the internal thermal resistance). On the other hand, a compact fan is used for enhancing the forced convection heat transfer from the vessel (i.e., reducing the external thermal resistance). Consequently, a copper-mesh structure sacrificing 4.3% of the internal vessel volume was manufactured by following the thermal design. In addition, the effect of the proposed thermal design was confirmed by actual hydrogen-charging experiments that showed 73.5% reduction of the charging time.

New technological and technical solutions in dietary pear compote production

Perfection of technological processes, both in preliminary preparation of raw materials and during the final mandatory step of pasteurization, plays a key role in ensuring the quality of finished products, which is important in the production of canned dietary products. The aim of the research was to develop a more efficient way of blanching raw materials with its hardware and soft pasteurization modes, which will allow the production of high quality and competitive compotes for functional nutrition. We have developed and proposed a method of pulse-steam blanching of raw materials directly in glass jars with saturated water vapor, instead of the traditional method using hot water. The essence of the method is as follows. Fruits stacked in jars are pulse heated for 100–160 seconds (depending on the volume of the container) with saturated water vapor with temperature of 105–110 °C, and then fed into jars with cycles of 10 and 10 seconds respectively. The use of pulsed supply of saturated water vapor contributes to achieving more even heating of the fruit, which are characterized by a relatively large internal thermal resistance, causing overheating of the surface layers, and also provides an increase in the temperature of the product, which allows to pour into the jars syrup at relatively high temperature (97–98 °C), while the traditional technology accounts for the temperature of only 80–85 °C. Implementation of this method ensures the temperature level of the product entering the pasteurization stage being 78–80 °C, as opposed to the traditional method, where the temperature of the product is 45–48 °C. After that, the jars will be filled with syrup with a temperature of 97–98 °C, sealed and sent for pasteurization on accelerated modes. To implement the new method of blanching, the design of the device for pulse-steam blanching of fruits in glass jars has been developed. New thermal sterilization regimes have been developed, taking into account the increased temperature of the product after sealing and improved technology for the production of pear compote. The results of physical and chemical testing confirm the high quality of the finished product.

Allotrope carbon materials in thermal interface materials and fuel cell applications: A review

The performance of allotrope carbon materials has been explored because of their superior properties in energy system applications. This review provides an understanding of the current work focusing on the applications of selected carbon materials in important energy systems, focus on thermal interface materials (TIMs), and fuel cell applications. This article begins with the introduction of TIMs and fuel cell in general working principle and presents details on carbon materials. The discussion focuses on updates from the latest research work and addresses current challenges and opportunities for research toward TIMs and fuel cell applications. The optimum performance of TIMs was seen when thermal conductivity achieved at a maximum of 3000 W (m K)−1 by using vertically aligned carbon nanotubes (CNTs) and a minimum internal thermal resistance of 0.3 mm2 K W−1. Meanwhile for fuel cell, the platinum/CNTs catalyst applied proton exchange membrane fuel cell achieved high power density of 661 mW cm−2 in the presence of Nafion electrolyte membrane. This review provides insights for scientists about the use of carbon materials, especially in energy system applications.

Experimental Lumped Analysis of Different Solid Geometries

Present work describes the experimental analysis of unsteady state heat transfer of different solid geometries. Heat transfer process in considered as a function of time only i.e. solid body behave as ‘Lumped’ means there is uniform temperature with in the body and having negligible internal thermal conduction resistance. The lumped analysis is done with cylindrical, conical and mixed geometry of cylinder and cone of two different materials. Specimens are heated in water ant cooled in air to estimate the heat transfer response of geometries in same ambient conditions MATLAB programming is done for calculation to obtain good and accurate results and to estimate heat transfer response of different geometries used in many engineering application.

Parameters optimization of borehole and internal thermal resistance for single U-tube ground heat exchangers using Taguchi method

The borehole thermal resistance and internal thermal resistance are both the significant design factors for the ground heat exchanger (GHE). Careful design procedures are required for these two values to decrease GHE’s length and initial cost. In this study, the effects of eight parameters (flow rate, pipe size, borehole diameter, pipe-pipe distance, layered soil, grout thermal conductivity, pipe thermal conductivity, and borehole depth) on the value of borehole thermal resistance and internal thermal resistance are investigated. In order to implement the analysis, L32 orthogonal array has been established. An improved 3D numerical model with the four-resistance model is applied to calculate corresponding thermal resistances in all cases. Furthermore, the Taguchi method is carried out to obtain the optimal scenarios of parameters combination. Results using the optimal parameters set reveal 67.64% increase in the borehole thermal resistance and 148.29% decrease in the internal thermal resistance. The heat transfer performance can be enhanced by 9.63% at least, up to a maximum of 77.07%. The analysis of variance (ANOVA) technique is carried out lastly to figure out the relative importance of tested parameters. The results indicate that the pipe-pipe distance has the most significant impact on the borehole thermal resistance, whereas the flow rate and layered soil almost play no role in it. The pipe size and flow rate are important driving parameters in determining the internal thermal resistance while the layered soil and borehole diameter’s impact can be nearly neglected.

Explicit multipole formulas and thermal network models for calculating thermal resistances of double U-pipe borehole heat exchangers

Double U-pipe boreholes are the second most used type of ground heat exchangers after single U-pipe boreholes. The borehole thermal resistance is a key design and performance parameter for the double U-pipe boreholes. Another parameter that is particularly important for double U-pipe boreholes is the internal thermal resistance between the U-pipes. There is, however, a general lack of methods that can be used to calculate the thermal resistances of the double U-pipe boreholes. This article presents explicit multipole formulas for calculating thermal resistances of double U-pipe boreholes with symmetrically positioned pipes. The presented formulas include zeroth- and first-order expressions for the borehole thermal resistance and the internal thermal resistance. The internal thermal resistance formulas cover various possible flow configurations of the heat carrier fluid in the double U-pipes. The accuracy of the presented zeroth- and first-order formulas is established by comparing them to the original 10th-order multipole method. The article also presents thermal network models for computing effective borehole thermal resistance of double U-pipe boreholes from the multipole resistances. Formulas for calculating effective borehole thermal resistance of double U-pipe boreholes are presented for two idealized cases of uniform heat flux and uniform average wall temperature along the borehole.

Synthesis and characterization of hybrid graphene nanocoolant for heat transfer dissipation in microchannel heat sinks

Abstract An attempt is made to examine the effect of hybrid nanocoolant in microchannel heat sink for computer cooling. Two-hybrid coolants with graphene as one of the prime components are synthesized and images of the particles are shown using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Heat transfer properties like thermal conductivity of the hybrid fluid, specific heat, density, and viscosity are evaluated experimentally and theoretically. The heat transfer characteristics are also studied in heat sink channels of micro level in the processors of personal computers. The parameters like internal heat transfer coefficient, thermal resistances and base temperature representing the processor temperature are examined for the applied heater power of 325 W. The coolant dilution was varied in the range of 0.05 vol%, 0.075 vol% and 0.1 vol% and the base temperature is noted. The recorded lowest base temperature is 310.01 K for the concentration of 0.1 vol% graphene-iron oxide (GFO) system for 0.5 mm fin spacing for the graphene-iron oxide hybrid coolant and for graphene oxide–iron oxide (GOFO) hybrid coolant it is 311.24 K for the same operating conditions.