Small Heat Transfer Coefficient Research Articles

The influence of evaporation on long-wavelength instabilities in liquid layer with insoluble surfactant

We investigate the long-wavelength Marangoni instability in an evaporating horizontal liquid layer with insoluble surfactant absorbed on the free deformable surface. The insoluble surfactant hinders the evaporation of the liquid into a vapor. The mass flux depends on the difference between the actual surface temperature and the equilibrium one. Different kinetics limited regimes of evaporation are considered. Density, viscosity, and thermal conductivity of the vapor are smaller than those of the liquid, and a one-sided model is applied. The layer is subjected to a heat flux at its bottom. We assume a small heat transfer coefficient at the interface. The linear stability analysis of this system for different values of kinetic resistance parameter is performed. Instability thresholds are determined and critical Marangoni numbers for monotonic, as well as for oscillatory mode, are found.

Theoretical assessment of the combined effects of building thermal mass and earth–air-tube ventilation on the indoor thermal environment

Both earth-tube systems and building thermal mass have the potential to provide desired performance levels for both indoor thermal comfort and energy saving; however, their coupling effects need to be further investigated. A model for evaluation of the combined effects of earth-tube ventilation systems and building thermal mass is proposed. Both the time-averaged indoor air temperatures and periodic temperature fluctuations are given as explicit formulas. Unlike the time lag induced by the separate use of building thermal mass, which is no longer than 6h, the combination with an earth-tube system could extend the time lag of indoor air temperature in an annual fluctuation period by tens of days and that of a daily period by a couple of hours. It is noticeable that the building thermal mass has significant impact on the effectiveness of earth-tube systems; therefore, a small heat transfer coefficient of external envelopes helps increase the annual time lag of indoor air temperature. Using a proper combination of building thermal mass and earth tubes, indoor thermal comfort can be achieved for a building located in a region with both hot summers and cold winters without any additional cooling or heating load.

Performance analysis of organic Rankine cycle based on location of heat transfer pinch point in evaporator

The location of heat transfer pinch point in evaporator is the base of determining operating parameters of organic Rankine cycle (ORC). The physical mathematical model seeking the location of pinch point is established, by which, the temperature variations both of heat source and working fluid with UA can be obtained. Taking heat source with inlet temperature of 160 °C as example, the matching potentials between heat source and working fluid are revealed for subcritical and supercritical cycles with the determined temperature difference of pinch point. Thermal efficiency, exergy efficiency, work output per unit area and maximum work outputs are compared and analyzed based on the locations of heat transfer pinch point either. The results indicate that supercritical ORC has a better performance in thermal efficiency, exergy efficiency and work output while outlet temperature of heat source is low. Otherwise, subcritical performs better. Small heat transfer coefficient results in low value of work output per unit area for supercritical ORC. Introduction of IHX may reduce the optimal evaporating pressure, which has a great influence on heat source outlet temperature and superheat degree. The analysis may benefit the selection of operating parameters and control strategy of ORC.

Influence of thermal performance on design parameters of a He/LiPb dual coolant DEMO concept blanket design

Spanish Breeding Blanket Technology Programme TECNO_FUS is exploring the technological capabilities of a Dual-Coolant He/Pb15.7Li breeding blanket for DEMO and studying new breeding blanket design specifications. The progress of the channel conceptual design is being conducted in parallel with the extension of MHD computational capabilities of CFD tools and the underlying physics of MHD models. A qualification of MHD effects under present blanket design specifications and some approaches to their modelling were proposed by the authors in [1]. The analysis was accomplished with the 2D transient algorithm from Sommeria and Moreau [2] and implemented in the OpenFOAM CFD toolbox [3]. The thermal coupling was implemented by means of the Boussinesq hypothesis. Previous analyses showed the need of improvement of FCI thickness and thermal properties in order to obtain a desirable liquid metal temperature gain of 300°C. In the present study, an assessment through sensitivity and parametric analyses of the required FCI thickness is performed.Numerical simulations have been carried out considering a Robin-type thermal boundary condition which assumes 1D steady state thermal balance across the solid FCI and Eurofer layers. Such boundary condition has been validated with a fluid–solid coupled domain analysis.Results for the studied flow conditions and channel dimensions show that, in order to obtain a liquid metal temperature gain of about 300°C, the required FCI material should have a very small effective heat transfer coefficient ((k/δ)≤1W/m2K) and fluid velocities should be about 0.2m/s or less. Moreover, special attention has to be placed on the temperature difference across the FCI layer. However, for a maximised liquid metal thermal gain, higher velocities would be preferable, what would also imply a reduced temperature difference across the FCI layer.

Thin Thermoelectric Generator System for Body Energy Harvesting

Wearable thermoelectric generators (TEGs) harvest thermal energy generated by the body to generate useful electricity. The performance of these systems is limited by (1) the small working temperature differential between the body and ambient, (2) the desire to use natural air convection cooling on the cold side of the generator, and (3) the requirement for thin, lightweight systems that are comfortable for long-term use. Our work has focused on the design of the heat transfer system as part of the overall thermoelectric (TE) system. In particular, the small heat transfer coefficient for natural air convection results in a module thermal impedance that is smaller than that of the heat sink. In this heat-sink-limited regime, the thermal resistance of the generator should be optimized to match that of the heat sink to achieve the best performance. In addition, we have designed flat (1 mm thickness) copper heat spreaders to realize performance surpassing splayed pin heat sinks. Two-dimensional (2-D) heat spreading exploits the large surface area available in a wristband and allows patterned copper to efficiently cool the TE. A direct current (DC)/DC converter is integrated on the wristband. The system generates up to 28.5 μW/cm2 before the converter and 8.6 μW/cm2 after the converter, with 30% efficiency. It generates output of 4.15 V with overall thickness under 5 mm.

Numerical Investigation of a Silicon Six-Wafer Microcombustor Under the Effect of Heat Loss Through the Outer Walls

In this paper, the influences of low heat transfer condition at the outer walls on the microcombustor are investigated due to the fact that a sufficiently small heat transfer coefficient at the outer wall incurs the upstream burning in the recirculation jacket, results in the high wall temperature, and hence possibly damages the microcombustor. Numerical simulation approaches focused on the microcombustor with the flame burning in the recirculation jacket. Combustion characteristics of the combustor were first analyzed based on 2D computational fluid dynamics (CFD), and then the most dangerous locations on the combustor were predicted by means of the 3D finite element analysis method. The study demonstrates the effectiveness of CFD and stress modeling for the design and improvement of the microcombustors.

Effect of rotation on the onset of thermal convection in a sparsely packed porous layer using a thermal non-equilibrium model

Linear and nonlinear stability of a rotating fluid-saturated sparsely packed porous layer heated from below and cooled from above is studied when the fluid and solid phases are not in local thermal equilibrium. The extended Darcy–Brinkman model that includes the time derivative and Coriolis terms is employed as a momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation. The onset criterion for both stationary and oscillatory convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of rotation and thermal diffusion that causes the convection to set in through oscillatory mode rather than stationary. The rotation inhibits the onset of convection in both stationary and oscillatory mode. The Darcy number stabilizes the system towards the oscillatory mode, while it has dual effect on stationary convection. Besides, the effect of porosity modified conductivity ratio, Darcy–Prandtl number and the ratio of diffusivities on the stability of the system is investigated. The nonlinear theory is based on the truncated representation of Fourier series method. The effect of thermal non-equilibrium on heat transfer is brought out. The transient behavior of the Nusselt number is investigated by using the Runge–Kutta method. Some of the convection systems previously reported in the literature is shown to be special cases of the system presented in this study.

Linear and nonlinear double diffusive convection in a rotating sparsely packed porous layer using a thermal non-equilibrium model

Double diffusive convection in a fluid-saturated rotating porous layer is studied when the fluid and solid phases are not in local thermal equilibrium, using both linear and nonlinear stability analyses. The Brinkman model that includes the Coriolis term is employed as the momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for the energy equation. The onset criterion for stationary, oscillatory, and finite amplitude convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal diffusion, solute diffusion, and rotation that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, diffusivity ratio, Vadasz number, and Taylor number on the stability of the system is investigated. The nonlinear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out. Some of the convection systems previously reported in the literature is shown to be special cases of the system presented in this study.

Transversal tube pitch effects on local heat transfer characteristics of the flat tube bank fin mounted vortex generators and their sensitivity to nonuniform temperature of the fin

The tube bank fin is commonly used to increase the area of the heat transfer surface with a small heat transfer coefficient of a heat exchanger. If vortex generators (VGs) are punched on the fin surface, the heat transfer performance of the fin can be improved. This paper focused on the effect of transversal tube pitch on the local heat transfer performance of the three-row flat tube bank fin mounted with VGs. On the fin surface, constructing the flow channel but without mounted VGs, the transversal tube pitch was greater, and the span averaged Nusselt number downstream was larger because fewer interactions of vortices would be generated from different VGs located upstream. When the area goodness factor was used as the criteria on the condition of one tube unit of heat exchanger for commonly used fin materials and fin thickness, the transversal tube pitch has considerable effect on the heat transfer enhancement of VGs. Large transversal tube pitch is more sensitive to fin material than to fin thickness.

The onset of double diffusive convection in a sparsely packed porous layer using a thermal non-equilibrium model

We examine the effect of local thermal non-equilibrium on double diffusive convection in a fluid-saturated sparsely packed porous layer heated from below and cooled from above, using both linear and nonlinear stability analyses. The Brinkman model is employed as the momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for the energy equation. The onset criterion for stationary, oscillatory and finite amplitude convection is derived analytically. It is found that a small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal and solute diffusion that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, ratio of diffusivities, Vadasz number and Darcy number on the stability of the system is investigated. The nonlinear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out.

Linear and non-linear double diffusive convection in a rotating porous layer using a thermal non-equilibrium model

Double diffusive convection in a fluid-saturated rotating porous layer heated from below and cooled from above is studied when the fluid and solid phases are not in local thermal equilibrium, using both linear and non-linear stability analyses. The Darcy model that includes the time derivative and Coriolis terms is employed as momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation. The onset criterion for stationary, oscillatory and finite amplitude convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal and solute diffusions that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, diffusivity ratio, Vadasz number and Taylor number on the stability of the system is investigated. The non-linear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out.

Double diffusive convection in a porous layer using a thermal non-equilibrium model

Double diffusive convection in a fluid-saturated porous layer heated from below and cooled from above is studied when the fluid and solid phases are not in local thermal equilibrium, using both linear and nonlinear stability analyses. The Darcy model with time derivative term is employed as momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation. The onset criterion for stationary, oscillatory and finite amplitude convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal and solute diffusion that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, ratio of diffusivities and Vadasz number on the stability of the system is investigated. The nonlinear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out.

Thermal convection in a rotating porous layer using a thermal nonequilibrium model

Linear stability of a rotating fluid-saturated porous layer heated from below and cooled from above is studied when the fluid and solid phases are not in local thermal equilibrium. The extended Darcy model, which includes the time derivative and Coriolis terms, is employed as a momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation. The onset criterion for both stationary and oscillatory convection is derived analytically. It is found that a small interphase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of rotation and thermal diffusion that causes the convection to set in through the oscillatory mode rather than the stationary one. The rotation inhibits the onset of convection in both stationary and oscillatory mode. In addition, the effect of porosity modified conductivity ratio, Darcy-Prandtl number, and the ratio of diffusivities on the stability of the system is investigated. A weak nonlinear theory based on the truncated representation of Fourier series method is used to find the Nusselt number. The effect of thermal nonequilibrium on heat transfer is brought out. The transient behavior of the Nusselt number is also investigated by solving the finite amplitude equations using Runge-Kutta method.

Visualization of convective boiling heat transfer in single microchannels with different shaped cross-sections

Convective boiling in transparent single microchannels with similar hydraulic diameters but different shaped cross-sections was visualized, along with simultaneous measurement of the local heat transfer coefficient. Two types of microchannels were tested: a circular Pyrex glass microtube (210 μm inner diameter) and a square Pyrex glass microchannel (214 μm hydraulic diameter). A 100-nm-thick semi-transparent ITO/Ag thin film sputtered on the outer wall of the microchannel was used for direct joule heating of the microchannel. The flow field visualization showed semi-periodic variation in the flow patterns in both the square and circular microchannels. Such variation was because the confined space limited the bubble growth in the radial direction. In the square microchannel, both the number of nucleation bubbles and the local heat transfer coefficient increased with decreasing vapor quality. The corners acted as active nucleation cavities, leading to the higher local heat transfer coefficient. In contrast, lack of cavities in the smooth glass circular microchannel yielded a relatively smaller heat transfer coefficient at lower vapor quality. Finally, the heat transfer coefficient was higher for the square microchannel because corners in the square microchannel acted as effective active nucleation sites.

5506 微小領域における温度勾配生成デバイスの創製(J19-1 マイクロメカトロニクス(1),J19 マイクロメカトロニクス)

A precise control of various fields is required on local field. The thermal field, that is one of the most general fields, is commonly used, but it is difficult to precise control of the thermal field in local field, because of very small thermal capacity of the micro device. Then, we propose the device for larger thermal gradient and fast response, which has great heat liberation and small heat transfer coefficient. Using MEMS process, this device has TaN heat wear on 6μm SiO_2 membrane. In the experiment, our device achieved the fast response unless 0.2s and thermal gradient over 20℃ between the interval of 250μm.

Determination of thermal conductivity and convective heat transfer coefficient during deep fat frying of “Kroštula” dough

In this study, the influence of oil temperature, water migration and surface temperature of “Krostula” dough on convective heat transfer coefficient was investigated. The convective heat transfer coefficient during deep fat frying was determined at temperatures 160, 170, 180 and 190±1 °C. Heat transfer coefficient was the highest at the start of deep fat frying process; 579.12±2.46, 583.88±1.81, 597.05±1.10 and 657.91±0.95 W/m2 K for 160, 170, 180 and 190 °C of oil temperature, respectively. The smallest heat transfer coefficient was in the case of setting up a uniform period of water migration from sample, which corresponded to surface temperatures slightly higher than 100 °C; 26.53±0.63, 14.42±0.56, 56.78±0.49 and 37.52±0.54 W/m2 K for 160, 170, 180 and 190 °C of oil temperature, respectively. Higher oil temperature for deep fat frying increased values of heat transfer coefficient. A steady-state method was used to determine thermal conductivity of “Krostula” dough in temperature range of 40–70±1 °C. The thermal conductivity first increased with temperature and then after reaching maximum values decreased. The maximum value 0.5985±0.0196 W/(m K) was determined at 47.5 °C. The minimal value 0.4723±0.0192 W/(m K) was determined at 65 °C.

A numerical simulation of temperature field in plasma-arc forming of sheet metal

Flexible forming using plasma arc (FFUPA) is a newly developed method of sheet metal forming. It makes the forming by means of thermal stress and thermal strain without mould and die, and is recognized as a promising forming method in developing new products. In this paper, the mechanism and features of FFUPA have been studied. Two basic forms of flexible forming, bending towards and away from plasma arc, have been analyzed and discussed. In order to provide the basis of simulation analysis of thermal stress and thermal strain during FFUPA, a mathematical model for heat transfer of FFUPA has been developed. Computational analysis has been performed for thermo-physical parameters of the sheet metal, scanning speeds, arc powers and cooling conditions. The influence of these factors on temperature field distribution was calculated and analyzed. The results indicate that thermo-physical parameters influence the distribution of temperature field primarily and the sheet metal with small heat transfer coefficient is easy to be formed. Within the given surface temperature of sheet metal, the higher scanning speed can perform the better forming effect. Under specified arc power, material and scanning speed, the shapeable sheet metal has a thickness limit. With the cooling system, distribution of temperature field can be adjusted and hence final shape can be controlled.

The mould-filling capacity of two casting alloys

The mould-filling capacities of an Au-Ag-Cu alloy and a Ni-Cr-Be alloy for dental use have been studied by measuring the lengths of cast helices of a constant cross-section as a function of the supertemperature of the melt. A vacuum-pressure casting machine was applied in the experiments. Assuming that the conduction of heat through the investment is rate-controlling for the heat flow, the heat of fusion was calculated for the two alloys. These values were found to be close to those obtained by differential thermal analysis measurements. The lengths of the helices are strongly influenced by the deliberated heat of fusion during solidification. The substantially higher mould-filling capacity of the Ni-Cr-Be alloy compared with that of the Au-Ag-Cu alloy can be explained to a large extent by the corresponding difference in their latent heat of fusion. A calculation of the lengths of the helices requires a knowledge of the speeds of the melts. So far there are only indications of a higher speed for the Ni-Cr-Be alloy than for the Au-Ag-Cu alloy with the casting machine employed. The highest slope for the length of helix against supertemperature curve was observed for the Au-Ag-Cu alloy, indicating a smaller heat transfer coefficient for this alloy than for the Ni-Cr-Be alloy.

Influence of cooling characteristics on glass formation of FeB and NiSiB metallic alloys

One common approach to the glass-forming process of rapidly quenched metallic alloys is to prevent the formation and growth of crystalline nuclei in the undercooled melt. Homogeneous nucleation is the dominant mechanism for moderately high cooling rates. In the frequency of a kinetic model, glass-forming ability is defined in terms of a critical cooling rate to produce amorphous foils with quenched-in crystals with volume fractions less than a given value. In a preliminary paper a heat flow model for the formation of amorphous foils by cooling a melt film of given thickness on a heat conducting substrate with constant heat transfer coefficient was presented allowing for a more realistic description of the cooling process as well as spatial differences in the as-quenched state in the amorphous foils. Practically the thermal contact may be limited to a rather short time in certain rapid solidification techniques as in twin-roller experiments or may, according to recent results on single-roller experiments, undergo a sudden change in a smaller heat transfer coefficient during the ribbon-wheel adherence period. Therefore, in this work, the influence of time-dependent foil-substrate heat transfer conditions on the glass formation process and the as-quenched state of the amorphous foils is considered.

Analysis of fatigue behavior of high‐density polyethylene based on dynamic viscoelastic measurements during the fatigue process

AbstractThe fatigue behavior of high‐density polyethylene was studied by measuring the variation in the dynamic viscoelasticity during the fatigue process. Brittle failure was observed under the conditions of small imposed strain amplitude, low ambient temperature, and large heat transfer coefficient to the surroundings. Ductile failure was observed under the conditions of large imposed strain amplitude, high ambient temperature, and small heat transfer coefficient to the surroundings. In the case of brittle failure, absolute value of dynamic complex modulus, ∣E*∣, showed maximum and phase difference, δ, did minimum on approaching the point of failure. In the case of ductile failure, ∣E*∣ decreased and δ increased monotonously from the start of the fatigue testing. The effect of environmental conditions on fatigue behavior was elucidated in terms of the heat transfer coefficient to the surroundings. As both forced convection of air and water enlarge the heat transfer coefficient, temperature rise of the specimen hardly occured and brittle failure took place preferentially. The coefficient, ϰ, was introduced to express the ratio of the actual generated heat to the hysteresis loss. With increase of the magnitude of the strain amplitude, the nonlinear viscoelastic behavior appeared and ϰ became smaller. A positive correlation between ϰ and lifetime was found. As the heat generation rate does not strongly affect lifetime, it was concluded that the hysteresis loss with low efficiency of heat generation would contribute to the fatigue failure. The relationship between average hysteresis loss and lifetime was proposed. The total hysteresis loss up to fatigue failure is constant, being independent of ambient temperature and imposed strain amplitude. The cumulative damage theory of fatigue failure was proposed based on hysteresis loss.