Natural Convection Heat Transfer From a Hot Rectangular and a Square Corrugated Plate to a Cold Flat Plate

IntroductionReduction of heat loss from the absorber plate of asolar collector through the cover plates improvescollector efficiency. Therefore, the natllral convectionheat loss across air layers bounded by tWo parallel platesis of special interest to the designers of solar collectors.Most of the investigations on heat transfer in aconfined space have been cAned out with parallel platesin horizontal and inclined positions. Hollands, et al.[l]experimentally investigated the heat trallsferchara…     

Study of thermal conductivity,permeability, and adsorption performance of consolidated composite activated carbon adsorbent for refrigeration

Composite adsorbents, comprising activated carbon and expanded natural graphite, have been developed, and their thermal conductivity, permeability and adsorption performance were tested. The thermal conductivity varied with the ratio of activated carbon to expanded natural graphite. Thermal conductivity increased as the ratio of expanded graphite increased. Considering that the density of activated carbon for the composite adsorbent should not be lower than 200 kg/m³, otherwise the volumetric cooling capacity would be unacceptably low, the highest thermal conductivity obtained from experiments was 2.47 W m^?1 K^?1. The permeability was also measured, and the best result obtained was 4.378 × 10^?12 m². In order to evaluate the influence of heat and mass transfer on adsorption performance, the adsorption rate was tested using a Rubotherm magnetic suspension balance, and results showed that for the freezing conditions lower than ?10 °C the performance of granular activated carbon was better than that of solidified adsorbent because of the reduced mass transfer of ammonia at low saturated pressure. The adsorption performance of consolidated adsorbents increased rapidly when the evaporating temperature was higher than ?10 °C. When the evaporating temperature was 8 °C, the adsorption rate of consolidated adsorbent was improved by 29% if compared with that of granular adsorbent.     

Properties of Ag-doped Bi-Sb alloys as thermoelectric conversion materials for solid state refrigeration

The energy conversion properties of Bi-Sb system thermoelectric materials doped by Ag was investigated. Bi₈₅Sb_{15 − x} Ag _x (x = 0, 1, 2, 3, 4) alloys with Ag substitution for Sb were synthesized by mechanical alloying and then pressed under 5 GPa at 523 K for 30 min. The phase structure of the alloys was characterized by Xray diffraction. The electric conductivities and the Seebeck coefficients were measured at the temperature range of 80–300 K. The results reveal that the electric conductivities of the Ag-doped Bi-Sb alloys are highly improved. The power factor of Bi₈₅Sb₁₄Ag₁ reaches a maximum value of 2.98×10⁻³ W/(K²·m) at 255 K, which is about three times that of the un-doped sample Bi₈₅Sb₁₅ at the same temperature.     

Development of low thermal expansion nickel base superalloy for steam turbine applications

Abstract

The target operating temperature of ultrasupercritical power plants is increasing and is planned to reach 700°C. Austenitic superalloys are promising materials for these applications to replace ferritic heat resistant steels, because of their high strength at 650–700°C. In general, austenitic nickel base superalloys show higher creep rupture strength than ferritic heat resistant steels; however, they have higher coefficients of thermal expansion, lower creep rupture ductilities, and higher costs. The effect of the Mo and Co content, amount of γ' phase, and Al/Ti ratio in the γ' phase on the thermal expansion behaviour of a Mo containing superalloy has been investigated by use of the conventional Mo containing Alloy 252 as a reference. Tensile and creep rupture properties were also measured. Following a modified heat treatment, the Co free superalloy developed on the basis of these tests showed higher creep rupture ductility than Alloy 252, while retaining comparable low thermal expansion and high creep rupture strength. Creep rupture properties at 700°C for up to 20 000 h were satisfactory, suggesting that the alloy is suitable for long term applications. Initial assessments of the weldability and mechanical properties of weld joints at 750°C are encouraging for boiler tube applications.     

Experimental investigation of a pulsating heat pipe for hybrid vehicle applications

This paper deals with the experimental results of an unlooped pulsating heat pipe (PHP) developed and tested in an electronic thermal management field with hybrid vehicle applications in mind. The 2.5 mm inner tube diameter device was cooled by an air heat exchanger to replicate the environment of a vehicle.In order to characterize this pulsating heat pipe, four working fluids have been tested. They are acetone, methanol, water, and n-pentane, with applied thermal power ranging from 25 W to 550 W, air temperature ranging from 10 °C to 60 °C and air velocity ranging from 0.25 m s^?1 to 2 m s^?1. Three inclinations have also been tested according to their horizontal positions: +45° (condenser above the evaporator), 0° and ?45° (condenser below the evaporator).Among the different results, some of the most revelatory were obtained with regard to unfavourable inclination (?45°), for which the performances were very interesting considering a terrestrial application. On the other hand, one also observed low temperature limitations for water as a working fluid and degradation of performances for n-pentane tested at 60 °C air temperature. On an overall basis, however, it should be noted that the PHP functioned with high reliability and reproducibility and without any failure during the start-up or working stage.     

Developing rapid heating and cooling systems using pyrolytic graphite

Rapid heating and cooling is desired in many applications. A new type of rapid heating and cooling system using pyrolytic graphite (PG) was investigated. Due to a large degree of anisotropy, a factor of 200 in thermal conductivity difference and a factor of 1200 in electrical resistivity difference, PG is suitable for rapid surface heating. The high electrical resistivity and the low thermal conductivity in the thickness direction provide the electrical and thermal insulation between the surface and the bulk base. The low thermal expansion coefficient about 0.5×10⁻⁶ cm/cm/°C in the direction parallel to the surface helps maintain mechanical integrity during rapid thermal cycling. Heating and cooling characteristics of PG were studied in both simulation and experiments. The actually constructed heating and cooling system based on PG was able to raise the surface temperature from 50 to 250 °C in 2 s and cool to 50 °C in 8 s.     

Electrochemical and thermal properties of SmBa0.5Sr0.5Co2O5+δ cathode impregnated with Ce0.8Sm0.2O1.9 nanoparticles for intermediate-temperature solid oxide fuel cells

SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC55) impregnated with nano-sized Ce_0.8Sm_0.2O_1.9 (SDC) powder has been investigated as a candidate cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The cathode chemical compatibility with electrolyte, thermal expansion behavior, and electrochemical performance are investigated. For compatibility, a good chemical compatibility between SBSC55 and SDC electrolyte is still kept at 1100 °C in air. For thermal dilation curve, it could be divided into two regions, one is the low temperature region (100–265 °C); the other is the high temperature region (265–850 °C). In the low temperature region (100–265 °C), a TEC value is about 17.0 × 10^?6 K^?1 and an increase in slope in the higher temperatures region (265–800 °C), in which a TEC value is around 21.1 × 10^?6 K^?1. There is an inflection region ranged from 225 to 330 °C in the curve of d(δL/L)/dT vs. temperature. The peak inflection point located about 265 °C is associated to the initial temperature for the loss of lattice oxygen and the formation of oxygen vacancies. For electrochemical properties, the polarization resistances (R_p) significantly reduced from 4.17 Ω cm² of pure SBSC55 to 1.28 Ω cm² of 0.65 mg cm^?2 of SDC-impregnated SBSC55 at 600 °C. The single cell performance of SBSC55∣SDC∣Ni-SDC loaded with 0.65 mg cm^?2 SDC exhibited the optimum power density of 823 mW cm^?2 at operating temperature of 800 °C. Based on above-mentioned properties, SBSC55 impregnated with an appropriate SDC is a potential cathode for IT-SOFCs.     

Approximate evaluation of operating conditions and effectiveness of solar impurity thermovoltaic element

The effectiveness of solar impurity thermovoltaic elements based on semiconductors with forbidden gap width 0.3 eV with thin highly doped p- and n-layers with concentration ∼5 × 10¹⁹ cm⁻³, containing in addition a deep impurity with concentration ∼4 × 10¹⁸ cm⁻³ and ionization energy 0.15 eV, is shown to reach 30–40% with solar radiation intensity 0.1 W/cm².     

Electrical and microstructural characterization of spinel phases as potential coatings for SOFC metallic interconnects

Several spinel samples, i.e., Mn_xCr_3−xO₄ (0.5 ≤ x ≤ 2.5), NiCr₂O₄ and CoCr₂O₄, were synthesized and studied in terms of phase analysis, density, electrical resistivity and thermal expansion behaviour. The spinel samples were generally single phase; exceptions included MnCr₂O₄ and Mn_0.5Cr_2.5O₄ with significant amounts of Cr₂O₃ and NiCr₂O₄ with trace amounts of NiO.Porosity, in general, decreased with increasing sintering temperature, except for Mn_0.5Cr_2.5O₄, which showed increasing porosity with increasing sintering temperature. NiCr₂O₄, CoCr₂O₄ and MnCr₂O₄, all had similar thermal expansion behaviour, with thermal expansion coefficients (TEC) ranging from 7.2–7.6 × 10⁻⁶/°C. The TEC difference between the spinels and ferritic stainless steel was larger than the difference between the steel and chromia, which had a TEC of 9.6 × 10⁻⁶/°C. The spinels and chromia exhibited semiconductor-type behaviour, with electrical resistivities decreasing with increasing temperature. Only Mn₂CrO₄ and NiCr₂O₄ had resistivities lower than Cr₂O₃ over the entire temperature range of testing (20–900 °C). For Mn_xCr_3−xO₄, resistivity decreased with increasing Mn content.     

Melting and solidification performance in two horizontal shell-and-tube heat exchangers with different structures

Recently, a critical problem of latent heat thermal energy storage remains the low thermal conductivity of phase change materials (PCMs), which can lead to low heat transfer rate. Structural optimization design is an effective solution for this problem. In this work, two horizontal shell-and-tube heat exchangers (HEs) with one inner tube (n = 1) and four inner tubes (n = 4) are designed keeping the same amount of PCM and water flow rate, and their melting and solidification thermal performance and heat transfer characteristics are compared. The results show that in comparison with one-inner-tube HE, the temperature of detected points are affected by both upper and lower inner tubes for four-inner-tube HE, thus the differences in phase change process appear. In addition, the phase change time reduction rates are 34.1%, 33.39%, 28.82% at T_in (inlet water temperature) = 75°C, 80°C, 85°C during charging process, and 17.2%, 27.69%, 36.67% at T_in = 10°C, 15°C, 20°C during discharging process, respectively. In comparison with the one-inner-tube HE, the theoretical efficiency of four-inner-tube HE is increased from 75.88% to 90.34%. Although more friction loss should be paid by four-inner-tube HE, a lower energy consumption and a higher heat-energy ratio are achieved. Based on the results of this study, the amount of cumulative heat per energy consumption is 1.52 × 10⁸ and 2.88 × 10⁸ for one-inner-tube and four-inner-tube HE, respectively.     

Characterization of metal hydrides for thermal applications in vehicles below 0 °C

Metal hydrides promise great potential for thermal applications in vehicles due to their fast reaction rates even at low temperature. However, almost no detailed data is known in literature about thermochemical equilibria and reaction rates of metal hydrides below 0 °C, which, though, is crucial for the low working temperature levels in vehicle applications.Therefore, this work presents a precise experimental set-up to measure characteristics of metal hydrides in the temperature range of −30 to 200 °C and a pressure range of 0.1 mbar–100 bar. LaNi_4.85Al_0.15 and Hydralloy C5 were characterized. The first pressure concentration-isotherms for both materials below 0 °C are published. LaNi_4.85Al_0.15 shows an equilibrium pressure down to 55 mbar for desorption and 120 mbar for absorption at mid-plateau and −20 °C. C5 reacts between 580 mbar for desorption and 1.6 bar for absorption at −30 °C at mid-plateau.For LaNi_4.85Al_0.15, additionally reaction rate coefficients down to −20 °C were measured and compared to values of LaNi₅ for the effect of Al-substitution. The reaction rate coefficient of LaNi_4.85Al_0.15 at −20 °C is 0.0018 s⁻¹. The obtained data is discussed against the background of preheating applications in fuel cell and conventional vehicles.     

Effects of TiO2 and SDC addition on the properties of YSZ electrolyte

In this study, we investigate the effects of adding titanium dioxide (TiO₂) and samarium doped cerium oxide (SDC) on the properties of yttrium-stabilized zirconia (YSZ) electrolyte. The microstructure, mechanical, and electrochemical properties of the electrolyte are investigated. The performance in CO₂ electrolysis is measured by supplying carbon dioxide to Ni-YSZ electrode and nitrogen to LSM electrode. Results show that TiO₂ and SDC addition can reduce the sintering temperature and increase grain size. The ionic conductivity is 0.123 S cm⁻¹ at 1000 °C. In addition, the thermal expansion coefficient at 1000 °C is 8.25 × 10⁻⁶ K⁻¹. The current density of the cell is 439 mA cm⁻² at 1.3 V and 1000 °C in solid oxide electrolysis cell.     

Effects of duct inclination angle on thermal entrance region of laminar and transition mixed convection

Experimental investigation was performed on the mixed convection heat transfer of thermal entrance region in an inclined rectangular duct for laminar and transition flow. Air flowed upwardly and downwardly with inclination angles from ?90° to 90°. The duct was made of duralumin plate and heated with uniform heat flux axially. The experiment was designed for determining the effects of inclination angles on the heat transfer coefficients and friction factors at seven orientations (θ = ? 90°, ?60°, ?30°, 0°, 30°, 60° and 90°), six Reynolds numbers (Re ≈ 420, 840, 1290, 1720, 2190 and 2630) within the range of Grashof numbers from 6.8 × 10³ to 4.1 × 10⁴. The optimum inclination angles that yielded the maximum heat transfer coefficients decreased from 30° to ?30° with the increase of Reynolds numbers from 420 to 1720. The heat transfer coefficients first increased with inclination angles up to a maximum value and then decreased. With further increase in Reynolds numbers, the heat transfer coefficients were nearly independent of inclination angles. The friction factors decreased with the increase of inclination angles from ?90° to 90° when Reynolds numbers ranged from 420 to 1290, and independent of inclination angles with higher Reynolds numbers.     

Dielectric behaviour of UV-crosslinked sulfonated poly (ether ether ketone) with methyl cellulose (SPEEK-MC) as proton exchange membrane

Proton exchange membrane materials based on sulfonated poly ether ether ketone (SPEEK) with Methyl Cellulose (MC) are developed by solution cast technique and exposed to UV radiation with Bezoin Ethyl Ether (BEE) as photoinitiator. The addition of MC into SPEEK polymer enhances the conductivity up to 8.7 × 10^?3 Scm^?1 at 30 °C temperature and 80% relative humidity. This new crosslinked hybrid membrane shows good prospect for the use as proton exchange membrane in fuel cell.     

KNO3/NaNO3 – Graphite materials for thermal energy storage at high temperature: Part I. – Elaboration methods and thermal properties

Composites graphite/salt for thermal energy storage at high temperature (～200 °C) have been developed and tested. As at low temperature in the past, graphite has been used to enhance the thermal conductivity of the eutectic system KNO₃/NaNO₃. A new elaboration method has been proposed as an alternative to graphite foams infiltration. It consists of cold-compression of a physical mixing of expanded natural graphite particles and salt powder. Two different compression routes have been investigated: uni-axial compression and isostatic compression. The first part of the paper has been devoted to the analysis of the thermal properties of these new graphite/salt composites. It is proven that cold-compression is a simple and efficient technique for improving the salt thermal conductivity. For instance, graphite amounts between 15 and 20%wt lead to apparent thermal conductivities close to 20 W/m/K (20 times greater than the thermal conductivity of the salt). Furthermore, some advantages in terms of cost and safety are expected because materials elaboration is carried out at room temperature. The second part of the paper is focused on the analyses of the phase transition properties of these graphite/salt composites materials.     

Structural and electrochemical characterization of YBa(Fe,Co,Cu)2O5+δ layered perovskites as cathode materials for solid oxide fuel cells

Single- and double-doped YBa(Fe,Co,Cu)₂O_5+δ layered perovskites are prepared by solid state reaction method and their structural characteristics, thermal expansion coefficient, oxygen nonstoichiometry, electrical conductivity, and electrochemical performance are comparatively studied. The substitution of Co by Fe or/and Cu significantly improves thermal expansion properties as compared to undoped YBaCo₂O_5+δ. Electrochemical tests demonstrate the promising performance of synthesized materials as cathode materials at intermediate temperatures. Single doped YBaCuCoO_5+δ cathodes reveal the lowest polarization resistance equal to 0.24 and 0.78 Ω cm² at P(O₂) P^?1 = 0.2 at temperature of 800 and 700 °C, respectively.     

Compatibility and conductivity of La2Ni1−xFexO4+δ and LaNi0·6Fe0·4O3−δ with GDC electrolyte

Abstract

The compatibilities and conductivities of K₂NiF₄ typed La₂Ni_0·9Fe_0·1O_4+δ (L₂NF91) and LaNi_0·6Fe_0·4O_3?δ (LNF64) perovskites, promising cathode materials for solid oxide fuel cell, with Gd_0·1Ce_0·9O_1·95 (GDC) electrolyte were investigated. L₂NF91 and LNF64 were synthesised using citrate and modified citrate methods with the calcination temperature of 1000°C for 5 h. The single phased oxides with the average particle sizes of L₂NF91 and LNF64 ～0·2 μm were obtained. The thermal expansion coefficients of L₂NF91 and LNF64 were 12·7×10^?6 and 13·2×10^?6 K^?1 respectively. The mixtures of cathode materials and the electrolytes were heated between 800 and 1200°C to observe the formation of secondary phases at the operation temperatures of solid oxide fuel cell. The X-ray diffraction and scanning electron microscopy–energy dispersive X-ray results indicated that L₂NF91 and LNF64 had good chemical compatibility with GDC from room temperature up to 900°C. Both L₂NF91 and LNF64 showed higher conductivities when in contact with GDC electrolyte than with Zr_0·92Y_0·08O_1·96 electrolyte.     

Three－Dimensional Numerical Simulation of Natural Convection Heat Transfer in an Inclined Cylindrical Annulus

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Effects of residual stress in creep crack growth analysis of cold bent tubes under internal pressure

In this paper, the effects of residual stresses are considered in crack growth of cold bent and longitudinally cracked carbon–manganese tubes and pipes tested under pressure at 360°C, using the fracture mechanics parameters reference stress, σ_ref, K and C¹. Residual stress measurements using the X-Ray diffraction technique and a successive layer removal method have been performed through the thickness of the pipe extrados. These data have been used in finite element analyses to model effects of the secondary stresses acting on the crack tip. Creep crack growth rates versus C¹ at 360°C, in cold bent tubes, were shown to be faster by a factor of ∼50 at constant C¹ compared to cracking in fracture mechanics CT test specimens. This difference is due to geometry as well as the method of C¹ analysis. In cold-bend tubes after 1000 h thermal soaking at 650°C, there was an increase in incubation time and steady state cracking rate reduced by a factor of 10. Furthermore, it was found that residual stress measurements performed after a thermal treatment at 360°C for 50 h indicated a reduction of 40–50%. However, the calculations of C¹ using a combination of primary plus secondary stresses showed only an increase by a factor of ∼2 which did not fully explain the difference in cracking rate in the tube specimens.     

Inverse analyses of diffusion processes in type 13X zeolite particles

To determine effective surface self-diffusion coefficients of CO₂ within type 13X zeolite particles, at various temperatures (25–70 °C), inverse analyses of observed CO₂-uptake curves were successfully performed. The obtained effective surface self-diffusion coefficients increase with both the amount adsorbed and the temperature and ranged from 7.8 × 10⁻¹⁰ to 1.95 × 10⁻⁹ m²/s under the present experimental conditions.