Heat pipe cooling for battery thermal management systems (BTMSs) in electric vehicles (EVs) is growing due to its advantages of high cooling efficiency, compact structure and flexible geometry. ...Considering the transient conduction, phase change and uncertain thermal conditions in a heat pipe, it is challenging to obtain the dynamic thermal characteristics accurately in such complex heat and mass transfer process. In this paper, a “segmented” thermal resistance model of a heat pipe is proposed based on thermal circuit method. The equivalent conductivities of different segments, viz. the evaporator and condenser of pipe, are used to determine their own thermal parameters and conditions integrated into the thermal model of battery for a complete three-dimensional (3D) computational fluid dynamics (CFD) simulation. The proposed “segmented” model shows more precise than the “non-segmented” model by the comparison of simulated and experimental temperature distribution and variation of an ultra-thin micro heat pipe (UMHP) battery pack, and has less calculation error to obtain dynamic thermal behavior for exact thermal design, management and control of heat pipe BTMSs. Using the “segmented” model, the cooling effect of the UMHP pack with different natural/forced convection and arrangements is predicted, and the results correspond well to the tests.
•A “segmented” thermal resistance model of a heat pipe is proposed.•Accuracy of “segmented” model is verified by comparing with “non-segmented” model.•Ultra-thin micro heat pipe(UMHP) is compact and effective for EV battery cooling.•The cooling effect of an UMHP pack with natural/forced convection is evaluated.•The thermal performance of an UMHP pack with different arrangements is compared.
•The concept of multi-dimensional conduction-advection thermal resistance in parallel in fluid domain is proposed.•The advection thermal resistance in the fluid domain is closely related to the ...magnitude and direction of temperature gradient and velocity.•"Negative thermal resistance" in the advection thermal resistance is found to be equivalent to a "heat source" or a "heat sink" in the fluid domain.•The capability of analysis methodology of conduction-advection thermal resistance in parallel is illustrated and proved.
Thermo-electric analogy method and thermal resistance are widely used for heat transfer problems to simplify the analysis process of complex heat transfer network. Traditional thermal resistance is usually applicable to one-dimensional steady state heat conduction problem. However, heat convection problem is always involved in engineering applications such as energy power, petrochemical, aerospace and so on. In order to solve this problem, the concept of multi-dimensional conduction-advection thermal resistance in parallel in fluid domain is proposed, and the conduction-advection thermal resistance network with heat capacity is constructed. The advection thermal resistance in the fluid domain is closely related to the magnitude and direction of temperature gradient and velocity. The higher the velocity, the smaller the advection thermal resistance. Simultaneously, there exists a "negative thermal resistance" in the advection thermal resistance, which is equivalent to a "heat source" or a "heat sink" in the fluid domain. A forced convection problem and a natural convection problem are taken as examples to illustrate the practicality of this thermal resistance analysis methodology. The results show that the conduction-advection parallel thermal resistance network is important to analyze the flow and heat transfer processes and guide the regulation and optimization of the heat transfer processes.
Abstract
Epi-side down bonding on a silicon substrate of AlGaAs/GaAs microdisk lasers is presented. A heterostructure with coupled large optical cavities enables location of an InGaAs quantum dot ...active region at a distance of ∼1
µ
m from the heterostructure surface. The thermal resistance was reduced to 0.2 and 0.1 K mW
−1
for disks of 30 and 50
µ
m in diameter, respectively. The maximum continuous-wave power limited by the thermal rollover is more than doubled after bonding.
In the energy distribution networks, the most important and valuable equipment is oil-immersed distribution transformers. Besides, due to the key role of these transformers and their multiplicity, ...their lifetime monitoring is inevitable. The life of a transformer depends on the weakest solid insulation material (i.e. paper insulation). On the other hand, monitoring the transformer insulation status requires accurate information to be available about the oil temperature at every moment. Therefore, it is important to control and predict the oil temperature rise in the transformer. In this study, a new model based on fundamental heat transfer theory is proposed for thermal behaviour prediction of top oil of indoor distribution transformers using the concept of thermal resistance, namely electro-thermal resistance model (E-TRM). In E-TRM, the thermal resistance network is formed by following three-dimensional heat transfer path and assigning thermal resistance to each path. To evaluate the proposed E-TRM, the results of this model are verified with experimental results. Moreover, the E-TRM is used to predict the thermal behaviour of the indoor transformer in the overloading condition. At the end, the transformer loss of life is estimated based on the oil temperature and a normal cyclic overloading strategy is presented for overloading management.
•A multiscale method for cementitious composite thermal conductivity is proposed.•Thermal cracks caused by high-temperature load generate crack thermal resistances.•Crack thermal resistances induce ...significant reduction in thermal conductivity.•Interfacial thermal resistance (ITR) effect dominates crack thermal resistance effect.•Synergistic effect of heat bridge and ITR is a main mechanism for conductivity reduction.
The mesoscopic thermal cracking behavior in steel fiber reinforced concrete (SFRC) caused by high-temperature load can result in a substantial reduction of effective thermal conductivity (ETC) at macroscale level. In this study, a multiscale homogenization method for identifying the ETC of thermally damaged SFRC is proposed. This new method is characterized by the consideration of crack thermal resistance effect rather than using temperature-dependent thermal conductivities of components in cementitious composite. A series of experiments on thermal conductivity of mortar, plain concrete, and SFRC heated at various temperatures is performed. The method predictions satisfy the experimental results extremely well when the interfacial thermal resistance (ITR) coefficient is assumed to increase linearly with temperature. The numerical and experimental results show that the ITR effect makes approximately 50%, 36%, and 7%–12% contributions to the overall thermal conductivity reductions in the mortar, plain concrete, and SFRC heated up to 600 ∘C, respectively. Moreover, it is found that the trend and magnitude of ETC reduction caused by heating increase with the increase in particle content and particle-matrix thermal conductivity ratio due to the synergistic effect of heat bridge and ITR.
With the growing challenges of modern electronics in heat dissipation, developing thermal management materials with high thermal conductivity and electrical insulation property remains an important ...issue for electronics. In this work, a novel three-dimensional network (3D) of boron nitride/reduced graphene oxide (BN/rGO) with covalent bond connections were fabricated by using the surface modification and ice-templated methods. The as-prepared boron nitride/reduced graphene oxide/nature rubber composites (BN/rGO/NR) possessed an enhanced through-plane thermal conductivity of 1.28 W m−1 K−1 and satisfactory electrical insulation at a low filler loading of 4.9 vol%. The results demonstrated that the covalent bond connections and three-dimensional networks of fillers greatly reduced the interfacial thermal resistance as well as phonon scatterings at the filler/filler and filler/matrix interface simultaneously. More importantly, this strategy provided a creative insight to the design of advanced thermal management materials and also presented a bright application prospect for next-generation electronic packing.
BN and rGO were connected by the covalent bonds through using the surface modified method. BN/rGO/NR composites were fabricated by the ice-templated and vacuum-assisted method. Display omitted
•The thermal resistance of typical double U-tube BHEs is studied numerically.•Simulation results are validated by applying Zeng’s analytical model.•For high shank spacing the 2D thermal resistance ...Rb,2D depends on ground conductivity.•The 3D effective thermal resistance can be 30% higher than Rb,2D.•The 3D thermal resistance defined through real mean temperatures is equal to Rb,2D.
The effects of the surface temperature distribution on the thermal resistance of a double U-tube Borehole Heat Exchanger (BHE) are studied by finite element simulations. It is shown that the thermal resistance of a BHE cross section is not influenced by the bulk-temperature difference between pairs of tubes, but is influenced by the thermal conductivity of the ground when the shank spacing is high. Then it is shown that, if the real mean values of the fluid bulk temperature and of the BHE external surface are considered, the 3D thermal resistance of the BHE coincides with the thermal resistance of a BHE cross section, provided that the latter is invariant along the BHE. Finally the difference between the BHE thermal resistance and the effective BHE thermal resistance, defined by replacing the real mean temperature of the fluid with the average of inlet and outlet temperature, is evaluated in some relevant cases.
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