Heat Dissipation Process Research Articles

Influence of hydrophobicity on ice accumulation process under sleet and wind conditions

Glaze, the most dangerous ice type in natural environment, forms during sleet weather, which is usually accompanied with wind. The icing performance of hydrophobic coatings under the impact of wind needs further research. This paper studies the influence of hydrophobicity on ice accumulation process under sleet and wind conditions by computer simulations and icing tests. The results indicate that the heat dissipation process of droplets on samples with various hydrophobicity will be accelerated by wind significantly and that a higher hydrophobicity cannot reduce the cooling rate effectively. However, on different hydrophobic surfaces, the ice accumulation process has different characteristics. On a hydrophilic surface, the falling droplets form continuously water film, which will be cooled fast. On superhydrophobic surface, the frozen droplets form ice bulges, which can shield from wind and slow down the heat dissipation process. These ice accumulation characteristics lead to the difference in ice morphology and make a higher hydrophobic surface to have a lower ice mass growth rate in long period icing tests. As a conclusion, superhydrophobic coating remain icephobic under wind and sleet conditions.

Ignition and burning behaviors of automobile oil in engine compartment

Accidental leakage of automobile oils is of great inclination to initiate pool fires in engine compartment, with threats to induce the flashover of other components and flame penetration into the passenger compartment. This paper presents experimental results of the ignition and burning behaviors of a kind of automobile oils (automatic transmission oil) using a cone calorimeter. Measurements of oil temperature, ignition time, mass loss and heat release rate are performed at different external heat fluxes and initial fuel depths. The comparison between experimental and numerical oil temperature evolutions shows that the variations of the ignition time at different experimental conditions depend on the heat dissipation process inside the liquid phase. The steady mass burning rate is nearly independent of initial fuel depth and has a linear relation with external heat fluxes. In addition, the results indicate an increase in peak heat release rate by a large margin initially, followed by a relatively small margin under thicker initial fuel depths, while its variations are proportional to external heat fluxes. Correlations are also developed to determine the peak heat release rate as a function of the initial fuel depth.

A possible role of leaf vascular network in heat dissipation in Vitis vinifera L.

Recent studies showed how the density of leaf vascular system can be involved in the performance of physiological parameters. Major veins are commonly elevated in the lower epidermis of the leaf, and this anatomical feature could play a subsidiary role in increasing heat dispersion in the surrounding environment and may help dissipate excess light energy in the leaves. The aim of this study is to analyse the role of the leaf vein network in the heat dissipation process in Vitis vinifera (L.). Major leaf veins were insulated with liquid paraffin and analysed using thermal imaging. A significantly higher temperature was found on the leaf tissues with insulated veins compared to untreated leaves. Further studies are required to assess the real contribution of the leaf vascular network in thermal dissipation.

Performance evaluation of new modified low-concentrator polycrystalline silicon photovoltaic/thermal systems

A modified polycrystalline silicon solar cell structure is introduced to enhance the heat dissipation process from the cell’s silicon layer. The modification involves two steps. First, the Ethylene-Vinyl Acetate (EVA) layer underneath the silicon wafer of a conventional solar cell is replaced with a nanocomposite layerthat includes an EVA matrix doped with Boron Nitride (BN) nanoparticles at different loading ratios of 20, 40, and 60%. Second, the Tedlar Polyester Tedlar (TPT) layer is substituted with a high thermal conductivity aluminum backing foil layer. To assess the enhancements to the modified solar cell in comparison to the conventional cell, a three-dimensional thermo-fluid model is developed. The model is numerically simulated and the results are validated with the available experimental, numerical, and analytical results. The findings reveal that at a concentration ratio up to 3.5 where no external cooling technique is used, the modified cell attains a slight reduction in solar cell temperature compared to the conventional cell. On the other hand, at a concentration ratio of 20, where the solar cell is integrated with a microchannel heat sink, a significant reduction of cell temperature is observed compared to the conventional cell. It is found that at a concentration ratio of 20, and a coolant mass rate of 100g/min, the maximum temperature of the modified cell with 60% BN and an aluminum back sheet is 66°C, while the conventional solar cell temperature is 108°C. Additionally, of the two cells, the modified solar cell produces the highest net power of 45W, and achieves the highest electrical and thermal efficiency of 17.5%, and 70.8%, respectively. Meanwhile, the conventional solar cell produces 34W, and attains an electrical and thermal efficiency of 13.5%, and 69%, respectively. These findings can guide designers in the industrial field to adopt this type of modified solar cell to improve the performance of low concentrator photovoltaic systems.

Analytical modeling of the thermal aspects of metalworking fluids in the milling process

Heat generated in the cutting zone during metal cutting has an undeniable controlling influence on the dimensional accuracy of the workpiece and tool life. The heat dissipation process in the cutting zone is critical to the type of metalworking fluid (MWF) used and the way it is delivered to the cutting zone. Although extended studies have analyzed the thermal aspect of the machining process, few, if any, have incorporated the influence of the delivery method (application) of the MWF on the convective heat transfer process. In this paper, a comprehensive approach analysis was taken to assess the heat generation in the cutting process. Analytical models were developed to estimate the amount of heat generated in the shear zone and the heat partition in the shear plane during the milling process. In addition, the heat transfer coefficients of metalworking fluids during flooding, minimum quantity lubrication (MQL), and through-spindle cooling (TSC) strategies were evaluated. Regarding MQL, the determination of heat transfer coefficients of metalworking fluids involves the use of a homogeneous single-phase flow assumption of the two fluids (air and lubricant). Next, the average temperature change in the workpiece was estimated by considering the conductive and convective heat transfer coefficients of the workpiece and fluids. Finally, the analytical models of the temperature change in the workpiece were experimentally validated against workpiece-embedded thermocouple measurements. It was found that the results of the models show good agreement with the experimental results.

Molecular dynamics simulation of microstructure evolution and heat dissipation of nanoscale friction

The atomic scale interfacial microstructure evolution and heat dissipation process in nanoscale friction are investigated by 3D non-equilibrium molecular dynamics (MD) simulations. Two Ni blocks of different orientations are built to simulate the self-mate friction. The embedded atom (EAM) potentials are employed in these simulations. The microstructure evolution is observed. The temperature and velocity profiles along the height direction, which is perpendicular to the direction of motion, are calculated under sliding velocity. The heat dissipation process is studied. The effect of sliding velocity is also obtained. The results show that extensive plastic deformation and temperature rise occur in the interface. Atomic scale mechanical mixing and generation of mixing layer are observed in the regions near the contact interface. The sliding velocity has great impact on temperature rise. The study of the growth dynamics of mixing layer also sheds light on the formation process of mixing layer.

Thermo-optic effects in on-chip lithium niobate microdisk resonators.

We report the experimental observation and theoretical analysis of thermo-optic effects in high-Q on-chip lithium-niobate (LN) microdisks. We find that the resonance transmission dip of a LN microdisk was broadened or compressed when the wavelength of the input laser was tuned to the shorter or the longer wavelengths at a wavelength sweeping speed of 4.8 pm/s. When the laser wavelength was shifted with a fast rate (4.8 nm/s), the tendencies of the change in the shape of the transmission dip reverse completely. An oscillatory behavior in the transmission spectra was also observed when the laser wavelength was slowly shifted to the shorter wavelengths. The experimental results were successfully explained by using a theoretical mode considering for a fast thermo-optic effect of LN crystal and a slow heat dissipation process from the LN microdisk to the substrate and surroundings that leads to the reduction of the resonance wavelength through the deformation of the LN microdisk.

Validation of Heat Dissipation Processes in Direct Injection Diesel Engines Air Cooled

Small diesel engines with direct injection air cooled a big problem is heat dissipation. Based on bench made ​​with a 295 cc diesel engine with direct injection, air cooled, I set the heat flows through the cylinder head, piston and cylinder. These data are needed to study the possibility of operating at maximum power, knowing that mechanical stresses of thermal origin are the most dangerous. The data obtained in this work will be used in the analysis of temperature and stress fields.

(Invited) Phonon Dispersion Engineering and Thermal Transport in Si Membranes

Present silicon technology provides single crystal films and membranes with thicknesses on the order of 10 nm and below. Along the years, the thermal studies on such structures have shown a reduction of the thermal conductivity consistent with the decrease of the characteristic size. Although the lowering of thermal conductivity is detrimental for heat dissipation processes in nanoelectronic devices, it becomes advantageous to increase the figure of merit (ZT) of Si and turning it as a promising material for thermoelectric applications. It is widely accepted that, for submicrometer thicknesses and down to 20 nm, the reduction of the in-plane thermal conductivity is mainly determined by the shortening of the phonon mean free path due to the diffusive scattering of phonons at the boundaries. In this case an analytical model that adopts the phonon bulk properties and includes the boundary scattering by a conductivity reduction factor is shown to match the experimental trend of systematical reduction of the thermal conductivity as the thickness of the film or membrane decreases. In this talk we will report on the modification of the phonon properties in Si membranes, such as the dispersion relation, and its dependence on the thickness. The thermal conductivity in Si results from the cumulative contribution of the transport of phonons with a broad range of wavevectors and mainly from long-wavevector phonons. Thus, heat transport at room temperature can be influenced by the reduction of the membrane thickness as the dispersion relation of the short-wave-phonons starts to be affected by emerging new vibrational modes arising from the boundary conditions at the membrane surface. Our measurements of the thermal conductivity in ultra-thin sub-20 nm Si membranes have demonstrated this extreme and, in fact, we have been able to disentangle the effects of phonon confinement from those related to surface morphology and chemical composition. Finally, we will discuss the prospects of employing the increased modification of phonon dispersion relation achieved in phononic crystals in controlling heat transport. While some applications require the existence of absolute band gaps, the approach to reduce the thermal conductivity is more linked to the decrease of phonon’s group velocity and phonon-phonon interactions in an extended range of frequencies.

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior (107 – 109 cycles) of 304L austenitic stainless steel was studied with ultrasonic fatigue testing system (20 kHz). The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation. It was found that micro-plasticity (slip markings) could be observed on the specimen surface even at very low stress amplitudes. The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure. By detailed investigation using a scanning electron microscope and an infrared camera, slip markings appeared at the large grains where the fatigue crack initiation site was located. The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack. Finally, the coupling relationship among the fatigue crack propagation, appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.

Alternative nanostructures for thermophones.

Thermophones are highly promising for applications such as high-power SONAR arrays, flexible loudspeakers, and noise cancellation devices. So far, freestanding carbon nanotube aerogel sheets provide the most attractive performance as a thermoacoustic heat source. However, the limited accessibility of large-size freestanding carbon nanotube aerogel sheets and other even more exotic materials recently investigated hampers the field. We describe alternative materials for a thermoacoustic heat source with high-energy conversion efficiency, additional functionalities, environmentally friendly, and cost-effective production technologies. We discuss the thermoacoustic performance of alternative nanostructured materials and compare their spectral and power dependencies of sound pressure in air. We demonstrate that the heat capacity of aerogel-like nanostructures can be extracted by a thorough analysis of the sound pressure spectra. The study presented here focuses on engineering thermal gradients in the vicinity of nanostructures and subsequent heat dissipation processes from the interior of encapsulated thermoacoustic projectors. Applications of thermoacoustic projectors for high-power SONAR arrays, sound cancellation, and optimal thermal design, regarding enhanced energy conversion efficiency, are discussed.

Numerical simulation of heat dissipation processes in underground power cable system situated in thermal backfill and buried in a multilayered soil

This paper presents the thermal analysis of the underground transmission line, planned to be installed in one of the Polish power plants. The computations are performed by using the Finite Element Method (FEM) code, developed by the authors. The paper considers a system of three power cables arranged in flat (in-line) formation. The cable line is buried in the multilayered soil. The soil layers characteristic and thermal properties are determined from geological measurements. Different conditions of cable bedding are analyzed including power cables placement in the FTB or direct burial in a mother ground. The cable line burial depth, measured from the ground level, varies from 1m to 2.5m. Additionally, to include the effect of dry zones formation on the temperature distribution in cable line and surroundings, soil and FTB thermal conductivities are considered as a temperature-dependent. The proposed approach for determining the temperature-dependent thermal conductivity of soil layers is discussed in detail. The FEM simulation results are also compared with the results of the simulation that consider soil layers as homogeneous materials. Therefore, thermal conductivity is assumed to be constant for each layer. The results obtained by using the FEM code, developed by the authors, are compared with the results of ANSYS simulations, and a good agreement was found.

Study of heat dissipation process from heat sink using lensless Fourier transform digital holographic interferometry.

This paper presents the results of experimental investigations about the heat dissipation process of plate fin heat sink using digital holographic interferometry. Visual inspection of reconstructed phase difference maps of the air field around the heat sink with and without electric power in the load resistor provides qualitative information about the variation of temperature and the heat dissipation process. Quantitative information about the temperature distribution is obtained from the relationship between the digitally reconstructed phase difference map of ambient air and heated air. Experimental results are presented for different current and voltage in the load resistor to investigate the heat dissipation process. The effect of fin spacing on the heat dissipation performance of the heat sink is also investigated in the case of natural heat convection. From experimental data, heat transfer parameters, such as local heat flux and convective heat transfer coefficients, are also calculated.

AtPHT4;4 is a chloroplast-localized ascorbate transporter in Arabidopsis.

Ascorbate is an antioxidant and coenzyme for various metabolic reactions in vivo. In plant chloroplasts, high ascorbate levels are required to overcome photoinhibition caused by strong light. However, ascorbate is synthesized in the mitochondria and the molecular mechanisms underlying ascorbate transport into chloroplasts are unknown. Here we show that AtPHT4;4, a member of the phosphate transporter 4 family of Arabidopsis thaliana, functions as an ascorbate transporter. In vitro analysis shows that proteoliposomes containing the purified AtPHT4;4 protein exhibit membrane potential- and Cl−-dependent ascorbate uptake. The AtPHT4;4 protein is abundantly expressed in the chloroplast envelope membrane. Knockout of AtPHT4;4 results in decreased levels of the reduced form of ascorbate in the leaves and the heat dissipation process of excessive energy during photosynthesis is compromised. Taken together, these observations indicate that the AtPHT4;4 protein is an ascorbate transporter at the chloroplast envelope membrane, which may be required for tolerance to strong light stress.

Thermal management of thermoacoustic sound projectors using a free-standing carbon nanotube aerogel sheet as a heat source

Carbon nanotube (CNT) aerogel sheets produce smooth-spectra sound over a wide frequency range (1–105 Hz) by means of thermoacoustic (TA) sound generation. Protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features for the TA sound projector and attractive performance at needed low frequencies. Energy conversion efficiencies in air of 2% and 10% underwater, which can be enhanced by further increasing the modulation temperature. Using a developed method for accurate temperature measurements for the thin aerogel CNT sheets, heat dissipation processes, failure mechanisms, and associated power densities are investigated for encapsulated multilayered CNT TA heaters and related to the thermal diffusivity distance when sheet layers are separated. Resulting thermal management methods for high applied power are discussed and deployed to construct efficient and tunable underwater sound projector for operation at relatively low frequencies, 10 Hz–10 kHz. The optimal design of these TA projectors for high-power SONAR arrays is discussed.

Improvement of thermal efficiency in computer heat sink using functionally graded materials

This article presents a new method for improvement of thermal efficiency in the heat dissipation process from electrical components by replacing the conventional heat sink material with functionally graded materials. For physical properties of functionally graded material, the linear and power-law functions are used. An approximate analytical approach based on the mean value theorem is applied for analyzing the thermal behavior of heat sink. The obtained results in the present study reveal that replacing the heat sink fin material with functionally graded material leads to a significant energy saving in heat sinks since the heat transfer enhances between the fin surfaces and surrounding fluid. Alternatively, the rate of energy saving varies as a function of in-homogeneity index of fin material. It is hoped that the obtained results in the present study arouse interest among the thermal designers and heat sink industries.

Diurnal and Developmental Changes in Energy Allocation of Absorbed Light at PSII in Field-Grown Rice

The allocation of absorbed light energy in PSII to electron transport and heat dissipation processes in rice grown under waterlogged conditions was estimated with the lake model of energy transfer. With regard to diurnal changes in energy allocation, the peak of the energy flux to electron transport, J(PSII), occurred in the morning and the peak of the energy flux to heat dissipation associated with non-photochemical quenching of Chl fluorescence, J(NPQ), occurred in the afternoon. With regard to seasonal changes in energy allocation, J(PSII) in the rapidly growing phase was greater than that in the ripening phase, even though the leaves of rice receive less light in the growing phase than in the ripening period in Japan. This seasonal decrease in J(PSII) was accompanied by an increase in J(NPQ). One of the reasons for the lower J(PSII) in the ripening phase might be a more sever afternoon suppression of J(PSII). To estimate energy dissipation due to photoinhibition of PSII, J(NPQ) was divided into J(fast), which is associated with fast-recovering NPQ mainly due to qE, and J(slow), which is mainly due to photoinhibition. The integrated daily energy loss by photoinhibiton was calculated to be about 3-8% of light energy absorption in PSII. Strategies for the utilization of light energy adopted by rice are discussed. For example, very efficient photosynthesis under non-saturating light in the rapidly growing phase is proposed.

Visual investigation on the heat dissipation process of a heat sink by using digital holographic interferometry

We present a method for visually and quantitatively investigating the heat dissipation process of plate-fin heat sinks by using digital holographic interferometry. A series of phase change maps reflecting the temperature distribution and variation trend of the air field surrounding heat sink during the heat dissipation process are numerically reconstructed based on double-exposure holographic interferometry. According to the phase unwrapping algorithm and the derived relationship between temperature and phase change of the detection beam, the full-field temperature distributions are quantitatively obtained with a reasonably high measurement accuracy. And then the impact of heat sink's channel width on the heat dissipation performance in the case of natural convection is analyzed. In addition, a comparison between simulation and experiment results is given to verify the reliability of this method. The experiment results certify the feasibility and validity of the presented method in full-field, dynamical, and quantitative measurement of the air field temperature distribution, which provides a basis for analyzing the heat dissipation performance of plate-fin heat sinks.

Heat dissipation energy under fatigue based on infrared data processing

The presented work is devoted to the experimental study of heat dissipation process caused by fatigue crack propagation. To investigate a spatial and time temperature evolution at the crack tip, set of experiments has been carried out using plate titanium specimens with pre-grown centred fatigue crack. An original mathematical algorithm for experimental data treatment has been developed to obtain the power of heat dissipation caused by plastic deformation at the crack tip. The algorithm includes spatial-time filtration and relative motion compensation procedures. The time dependence of the stored energy was calculated as the difference between work caused by plastic deformation near the crack tip and heat dissipation energy obtained from experimental data. As a result, it has been shown that the stored energy has to accumulated during the fatigue test and has to be equal to zero when the crack reaches the critical length corresponding to the failure of sample.

Simulation Analysis of Battery Thermal Management System Using Phase Change Material (PCM)

This article mainly addresses the technology of improving heat dissipation performance of power battery packs using Phase Change Material (PCM). This new technology aims to reduce the temperature gradient among the battery packs. As the thermal properties of the pure paraffin wax are hard to meet the dissipation demand, changes are made to add graphite particles into it. Commercial CFD software ANSYS FLUENT is used to simulate the heat dissipation process during battery discharge using PCM and traditional forced air cooling methods. The simulation results show that both under constant current and pulse current the maximal temperature and temperature gradient by the end of discharging using PCM method are relatively lower than that of using forced air cooling method.