Hydrothermal preparation and electrochemical properties of Gd3+ and Bi3+, Sm3+, La3+, and Nd3+ codoped ceria-based electrolytes for intermediate temperature-solid oxide fuel cell

The structure, the thermal expansion coefficient, electrical conductivities of Ce_0.8Gd_0.2?xM_xO_2?δ (for M: Bi, x = 0–0.1, and for M: Sm, La, and Nd, x = 0.02) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (28–59 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300–1400 °C. The maximum conductivity, σ_700 °C around 4.46 × 10^?2 S cm^?1 with E_a = 0.52 eV, is found at x = 0.1 for Bi-co-doping. Among various metal-co-dopings, for x = 0.02, the maximum conductivity, σ_700 °C around 2.88 × 10^?2 S cm^?1 with E_a = 0.67 eV, is found for Sm-co-doping. The electrolytic domain boundary (EDB) of Ce_0.8Gd_0.1Bi_0.1O_2?δ is found to be 1.2 × 10^?19 atm, which is relatively lower than that of the singly doped samples. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.6 × 10^?6 K^?1 for the CeO₂, 12.1 × 10^?6 K^?1 for Ce_0.8Gd_0.2O_2?δ, and increase with co-doping to 14.2 × 10^?6 K^?1 for Ce_0.8Gd_0.18Bi_0.02O_2?δ. The maximum power densities for the single cell based on the codoped samples are higher than that of the singly doped sample. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes.     

Isolation and characterization of two soil derived yeasts for bioethanol production on Cassava starch

Two ethanol-producing yeast strains, CHY1011 and CHFY0901 were isolated from soil in South Korea using an enrichment technique in a yeast peptone dextrose medium supplemented with 5% (w v^?1) ethanol at 30 °C. The phenotypic and physiological characteristics, as well as molecular phylogenetic analysis based on the D1/D2 domains of the large subunit (26S) rRNA gene and the internally transcribed spacer (ITS) 1 + 2 regions suggested that they were novel strains of Saccharomyces cerevisiae. During shaking flask cultivation, the highest ethanol productivity and theoretical yield of S. cerevisiae CHY1011 in YPD media containing 9.5% total sugars was 1.06 ± 0.02 g l^?1 h^?1 and 95.5 ± 1.2%, respectively, while those for S. cerevisiae CHFY0901 were 0.97 ± 0.03 g l^?1 h^?1 and 91.81 ± 2.2%, respectively. Simultaneous saccharification and fermentation for ethanol production was carried out using liquefied cassava (Manihot esculenta) starch in a 5 l lab-scale jar fermenter at 32 °C for 66 h with an agitation speed of 2 Hz. Under these conditions, S. cerevisiae CHY1011 and CHFY0901 yielded a final ethanol concentration of 89.1 ± 0.87 g l^?1 and 83.8 ± 1.11 g l^?1, a maximum ethanol productivity of 2.10 ± 0.02 g l^?1 h^?1 and 1.88 ± 0.01 g l^?1 h^?1, and a theoretical yield of 93.5 ± 1.4% and 91.3 ± 1.1%, respectively. These results suggest that S. cerevisiae CHY1011 and CHFY0901 have potential use in industrial bioethanol fermentation processes.     

Heat transfer on a power law stretching sheet subjected to free stream pressure gradient

The present work deals with the numerical study of temperature distribution in the laminar boundary layer driven by the stretching boundary surface subjected to pressure gradient. The similarity transformation obeying the same power law based on composite reference velocity (union of velocities of the stretching boundary and free stream) has been employed that leads to a single set of equations, irrespective of the condition whether U_w > U_∞ or U_w < U_∞, containing three parameters: β measuring the stretch rate of the moving boundary, ε is the ratio of free stream velocity to composite reference velocity and Pr is the Prandtl number of the ambient fluid. The numerical solutions of the thermal boundary layer equations are obtained for three Prandtl numbers 0.7, 1.0 and 10 for 0 ? ε ? 1 and for 0 ? β ? 2. The heat transfer coefficient show appreciable dependence on the ratio of free stream velocity to union of velocities of the stretching surface boundary and free stream.     

Heat transfer correlations for open-cellular porous materials

A general correlation for volumetric heat transfer coefficient between stream of air and open-cellular porous materials was derived utilizing experimental data obtained by several researchers. The derived correlation is written in form of h_v = (A/D_s²⁻ⁿ)uⁿ. Here, h_v denotes the volumetric heat transfer coefficient, A is the constant, n is the velocity exponent, u is the mean fluid velocity and D_s is the equivalent strut diameter of Dul'nev's unit cell for open-cell foam. The parameters, A and n, were determined by a least-square fit of the expression to the above-mentioned experimental data to give A = 13.0 and n = 0.791. Moreover, from the h_v−u correlation thus determined, the following Nusselt vs. Reynolds number heat transfer correlation was proposed: Nu_s = 0.124(Re_sPr)^0.791, where Nu_s represents the Nusselt number defined by h_vD_s²/k_f, Re_s denotes the Reynolds number defined by uD_s/ν_f, Pr is the Prandtl number, k_f is the thermal conductivity of fluid and ν_f is the kinetic viscosity of fluid. It is found that this correlation approximates 78.1% of the available experimental data with an error of less than ± 40%.     

The effect of inclination on impinging jets at small nozzle-to-plate spacing

The effect of inclination on heat transfer characteristics of an impinging slot air jet is experimentally investigated. The effects of inclination angle (0° ? θ ? 40°) and dimensionless pumping power on the Nusselt number are considered. The focus is on cases where the nozzle-to-plate spacing is equal to or less than one nozzle diameter (H/d_h ? 1.0). The results show that the heat transfer characteristics of small nozzle-to-plate spacings are significantly different from those of large nozzle-to-plate spacings. In the cases of fixed flow rate conditions, the impingement point and average Nusselt numbers at small nozzle-to-plate spacing (H/d_h ? 1.0) increase as the inclination angle increases due to an increase in the pumping power, while the impingement point and average Nusselt numbers at large nozzle-to-plate spacing (H/d_h > 1.0) decrease as the inclination angle increases due to momentum loss of the wall jet. In the cases of fixed pumping power conditions, the impingement point and average Nusselt numbers at both of small and large nozzle-to-plate spacings are independent of the inclination angle. Based on the experimental results, correlations for the impingement point and average Nusselt numbers of the impinging jet are suggested as a function of the pumping power alone.     

Novel heat transfer fluids for direct immersion phase change cooling of electronic systems

We describe the use of computer-aided molecular design (CAMD) and figure of merit (FOM) analysis to identify new heat transfer fluids for direct immersion cooling of electronic systems. Thirty-five new fluids, with thermophysical properties in the range 320 K < T_b < 370 K, k > 0.09 W m^?1 K^?1 and H_vap > 35 kJ mol^?1, were identified via a CAMD approach. Further analysis of these 35 fluids led to the selection of 1,1,1-trifluoro-3-methylpentane (C₆H₁₁F₃) for experimental evaluation. C₆H₁₁F₃ was synthesized from commercially available precursors, and its thermophysical properties were measured to verify its FOM. Next, the pool boiling performance of a mixture of 7 wt.% C₆H₁₁F₃ + 93 wt.% HFE 7200 was determined using a 10 mm × 10 mm grooved Si thermal test chip coated with copper. An improvement of 7% in the critical heat flux (CHF) was obtained, suggesting that C₆H₁₁F₃ is worth further examination as a candidate for direct immersion phase change cooling applications.     

Non-Newtonian power-law flow and heat transfer across a pair of side-by-side circular cylinders

Flow and heat transfer of non-Newtonian power-law fluids across a pair of identical circular cylinders in side-by-side arrangement are investigated numerically by solving the continuity, momentum and energy equations along with the appropriate boundary conditions. The numerical calculations are performed in an unconfined computational domain for the following range of physical parameters: Reynolds number, Re = 1–40 and power-law index, n = 0.4–1.8 (covering shear-thinning, n < 1; Newtonian, n = 1 and shear-thickening, n > 1 behaviors) for gap ratio, T/D = 1.5–4.0 at a constant Prandtl number of 50. The global characteristics such as drag coefficients and average Nusselt number, etc. are calculated and the representative streamline and isotherm contours are presented for the above range of conditions. It has been found that the individual and overall drag coefficients decrease and the average Nusselt number increases with Reynolds number for all T/D and n considered here. The heat transfer is found higher in shear-thinning fluids than Newtonian fluids and followed by shear-thickening fluids for 1.5 ? T/D ? 4.0 and 1 ? Re ? 40.     

Electrochemical properties of iodine-containing lithium manganese oxide spinel

Iodine-containing, cation-deficient, lithium manganese oxides (ICCD-LMO) are prepared by reaction of MnO₂ with LiI. The MnO₂ is completely transformed into spinel-structured compounds with a nominal composition of Li_1−δMn_2−2δO₄I_x. A sample prepared at 800 °C, viz. Li_0.99Mn_1.98O₄I_0.02, exhibits an initial discharge capacity of 113 mA h g⁻¹ with good cycleability and rate capability in the 4-V region. Iodine-containing, lithium-rich lithium manganese oxides (ICLR-LMO) are also prepared by reaction of LiMn₂O₄ with LiI, which results in a nominal composition of Li_1+xMn_2−xO₄I_x. Li_1.01Mn_1.99O₄I_0.02 shows a discharge capacity of 124 mA h g⁻¹ on the first cycle and 119 mA h g⁻¹ a on the 20th cycle. Both results indicate that a small amount of iodine species helps to maintain cycle performance.     

Effect of aspect ratio on natural convection in power-law liquids from a heated horizontal elliptic cylinder

The influence of aspect ratio and shear-dependent viscosity on free convection heat transfer from a horizontal heated elliptic cylinder in power-law fluids has been investigated. In particular, the coupled momentum and energy equations have been solved numerically over the following ranges of conditions: Grashof number, 10 ? Gr ? 10⁵; Prandtl number, 0.72 ? Pr ? 100; power-law index, 0.3 ? n ? 1.5 and aspect ratio, 0.2 ? E ? 5. The new extensive results demonstrate the influence of the Grashof number (Gr), Prandtl number (Pr), power-law index (n) and aspect ratio (E) on the macroscopic heat and momentum transfer characteristics like local and average values of Nusselt number (Nu) and drag coefficients (C_D). Further insights are developed by examining the structure of the flow and temperature fields adjacent to the cylinder. Broadly speaking, all else being equal, shear- thinning fluid behaviour promotes heat transfer whereas shear-thickening viscosity has a deleterious effect on it with reference to that in Newtonian fluids. Also, the rate of heat transfer gradually increases as the cylinder shape passes from blunt to slender with respect to the direction of gravity. Finally, the present numerical values of the Nusselt number are correlated using a simple analytical form which facilitates interpolation of the present results for the intermediate values of the governing parameters. The paper is concluded by presenting detailed comparisons with the previous numerical and experimental results available in the literature, especially in Newtonian fluids.     

Production of butanol (a biofuel) from agricultural residues: Part I – Use of barley straw hydrolysate

Fermentation of dilute sulfuric acid barley straw hydrolysate (BSH; undiluted/untreated) by Clostridium beijerinckii P260 resulted in the production of 7.09 gL^?1 ABE (acetone butanol ethanol), an ABE yield of 0.33, and productivity of 0.10 gL^?1 h^?1. This level of ABE is much less than that observed in a control experiment (21.06 gL^?1) where glucose (initial concentration 60 gL^?1) was used as a substrate. In the control experiment, an ABE yield of 0.41 and productivity of 0.31 gL^?1 h^?1 were observed. This comparison suggested that BSH is toxic to the culture. To reduce this potential toxicity effect, BSH was treated with lime [Ca(OH)₂] followed by fermentation. The treated BSH resulted in a successful fermentation and ABE concentration of 26.64 gL^?1 was achieved. This was superior to both glucose and untreated BSH (initial sugar 60 gL^?1) fermentations. In this fermentation, an ABE yield of 0.43 and productivity of 0.39 gL^?1 h^?1 (390% of untreated/undiluted BSH) was obtained. It should be noted that using lime treated BSH, a specific productivity of 0.55 h^?1 was obtained as compared to 0.12 h^?1 in the control fermentation suggesting that more carbon was directed to product formation.     

Critical condition for Rayleigh-Bénard convection of Bingham fluids in rectangular enclosures

Two-dimensional steady-state numerical simulations have been conducted for laminar Rayleigh-Bénard convection of Bingham fluids in rectangular enclosures to analyse the critical Rayleigh number Ra_crit for which convection ceases to influence the thermal transport and thermal conduction becomes the principal heat transfer mechanism. The influences of Bingham number Bn on the critical Rayleigh number Ra_crit have been investigated for different values of aspect ratio (height: length) AR (ranging from 1/4 to 4) and nominal Prandtl number Pr (ranging from 10 to 500) for both constant wall temperature (CWT) and constant wall heat flux (CWHF) boundary conditions for the horizontal walls. It has been found that Ra_crit increases with increasing values of Bn and AR, regardless of the boundary condition. The values of Ra_crit have been found to be greater in the case of CWT boundary condition than in the CWHF configuration for AR ≤ 1, whereas an opposite trend is obtained for AR > 1 for Bingham fluids. Additionally, Ra_crit has been found be insensitive to the change of Pr for Newtonian fluids (i.e. Bn = 0), whereas Ra_crit increases with increasing Pr for Bingham fluids irrespective of the boundary condition. A detailed scaling analysis has also been performed to elucidate the effects of Bn ,Pr , AR on Ra_crit for Bingham fluids. The results of scaling analysis and numerical findings have been utilised to propose a new correlation for Ra_crit for both Newtonian and Bingham fluids in the case of both CWT and CWHF boundary conditions.     

Swirl effects on harmonically excited,premixed flame kinematics

This paper describes the response of a swirling premixed flame with constant burning velocity to non-axisymmetric harmonic excitation. This work extends prior studies of axisymmetric forcing, which have shown that wrinkles are excited on the flame that propagate downstream along the mean flame surface at a speed given by U_o cos ψ, where U_o is the mean flow velocity and ψ is the flame angle. The swirl component in the flow field introduces an azimuthal transport mechanism for disturbances on the flame. As such, the flame response at any given position is a superposition of flame wrinkles excited at earlier times, upstream axial locations, and different azimuthal positions. These swirl transport effects do not arise in problems where axisymmetric flames are subjected to axisymmetric excitation, but enter quite prominently in the presence of non-axisymmetries, such as when the flame is subjected to transverse excitation. The solution characteristics are strongly dependent upon the ratio of angular rotation rate to excitation frequency, denoted by σ = Ω/ω, which describes the fraction of azimuthal rotation a disturbance makes in one acoustic period. When σ ? 1 and σ ? 1, the axial wavelength of flame wrinkles scales with the convective wavelength, λ_c, but becomes much longer for σ ～ O(1). The spatial variation in phase of flame wrinkling is also strongly dependent upon σ. Regardless of swirl number, flame wrinkles propagate in helical spirals along the solution characteristics at a phase speed equal to the local tangential velocity. The axial phase characteristics of flame wrinkling at a fixed azimuthal location, as would be measured by laser sheet imaging, are much more complex. For σ < 1, the wrinkles exhibit the familiar negative roll-off character for the phase with axial downstream distance, indicative of an axially convecting disturbance. The slope of this phase roll-off decreases with increasing σ, however, and becomes zero at σ = 1 for a compact flame. For σ > 1, the wrinkles actually have a positive roll-off character for the phase with axial downstream distance, indicating a flame wrinkle with a negative trace velocity, but whose actual propagation velocity is positive. Finally, these results show that while the flame response to transverse acoustic excitation is quite strong locally, its spatially integrated effect is much smaller for acoustically compact flames. This suggests that the dominant mechanism through which the flame responds globally to transverse excitation is the induced vortical and longitudinal acoustic fluctuations.     

Momentum and heat transfer characteristics of a semi-circular cylinder immersed in power-law fluids in the steady flow regime

The continuity, momentum and energy equations describing the flow and heat transfer of power-law fluids over a semi-circular cylinder have been solved numerically in the two-dimensional steady flow regime. The influence of the Reynolds number (Re), Prandtl number (Pr) and power-law index (n) on the local and global flow and heat characteristics have been studied over wide ranges of conditions as follows: 0.01 ? Re ? 30, 1 ? Pr ? 100 and 0.2 ? n ? 1.8. The variation of drag coefficient and Nusselt number with the Reynolds number, Prandtl number and power-law index is shown over the aforementioned ranges of conditions. In addition, streamline and isotherm profiles along with the recirculation length and distribution of pressure coefficient and Nusselt number over the surface of the semi-circular cylinder are also presented to gain further insights into the nature of the underlying kinematics. The wake size (recirculation length) shows almost linear dependence on the Reynolds number (Re ? 1) for all values of power-law index studied herein. The drag values show the classical inverse variation with the Reynolds number, especially for shear-thinning fluids at low Reynolds numbers. The point of maximum pressure coefficient is found slightly displaced from the front stagnation point for highly shear-thinning fluids, whereas for shear-thickening and Newtonian fluids, it coincides with the front stagnation point. For fixed values of the Prandtl number and Reynolds number, the rate of heat transfer decreases with the gradual increase in power-law index; this effect is particularly striking at high Prandtl numbers due to the thinning of the thermal boundary layer. Conversely, as expected, shear-thinning behavior facilitates heat transfer and shear-thickening impedes it. The effect of power-law index on both momentum and heat-transfer characteristics is seen to be appreciable at low Reynolds numbers and it gradually diminishes with the increasing Reynolds number.     

Boundary layer instability of the natural convection flow on a uniformly heated vertical plate

Direct numerical simulation is employed to investigate the two-dimensional boundary layer instability of a natural convection flow on a uniformly heated vertical plate submerged in a homogeneous quiescent environment. A Boussinesq fluid with Prandtl numbers of Pr = 0.733 (air) and 6.7 (water), in the local Rayleigh number range 0 ? Ra_x ? 2.4 × 10¹⁰, is studied. Controlled low amplitude numerical disturbances introduced into the base flow excite unstable travelling waves, with the resulting waves tracked and analyzed as they travel up the boundary layer. The numerical simulation readily reproduced what is predicted by the parallel linear stability theory for the two dimensional mode relatively short wave spectrum, but not for some parts of the long wave spectrum. Critical Rayleigh numbers have been obtained separately for both the temperature and velocity signals using the numerical results, and shown to be in good agreement with each other provided the data is renormalized using the boundary layer scalings of Sparrow and Greg [1]. It has been shown that the disturbance behavior depends on the Prandtl and Rayleigh numbers, the excitation frequency and to a lesser extent the prescribed thermal coupling at the plate.     

Preparation and characterization of activated carbon from rubber-seed shell by physical activation with steam

The use of rubber-seed shell as a raw material for the production of activated carbon with physical activation was investigated. The produced activated carbons were characterized by Nitrogen adsorption isotherms, Scanning electron microscope, Thermo-gravimetric and Differential scanning calorimetric in order to understand the rubber-seed shell activated carbon. The results showed that rubber-seed shell is a good precursor for activated carbon. The optimal activation condition is: temperature 880 °C, steam flow 6 kg h^?1, residence time 60 min. Characteristics of activated carbon with a high yield (30.5%) are: specific surface area (S_BET) 948 m² g^?1, total volume 0.988 m³ kg^?1, iodine number of adsorbent (q_iodine) 1.326 g g^?1, amount of methylene blue adsorption of adsorbent (q_mb) 265 mg g^?1, hardness 94.7%. It is demonstrated that rubber-seed shell is an attractive source of raw material for producing high capacity activated carbon by physical activation with steam.     

Development of heat and mass transfer model for condensation

Divergence and the interfacial temperature deviation are the two main problems in condensation simulation with the Lee model. Based on the heat transfer analysis at the vapor-liquid interface, a correlation is revealed describing the relationship between interfacial temperature deviation and the model parameters, q_i ≈ (T_sat − T_i)(Ak_v)^0.5 where A = h_fgrρ_v / T_sat. With this correlation, the determination of the condensation frequency r is no longer empirical. Furthermore, the correlation indicates that the thermal conductivity of vapor plays an important role. Accordingly, an improved model is proposed amplifying the thermal conductivity of vapor in the phase interaction region. The model is verified with the Nusselt problem and the impacts of the model parameters are discussed and compared with the original Lee model. It is shown that the interfacial temperature deviation is reduced by the amplified thermal conductivity of vapor. The convergence is maintained by increasing both A and k_v synchronously. Verification is also obtained on the forced convection condensation of R134a. The correlation predicts a temperature deviation at 0.1 K and the numerical result successfully reaches 0.12 K.     

Prediction of evaporation heat transfer coefficient based on gas–liquid two-phase annular flow regime in horizontal microfin tubes

A physical model of gas–liquid two-phase annular flow regime is presented for predicting the enhanced evaporation heat transfer characteristics in horizontal microfin tubes. The model is based on the equivalence of a periodical distortion of the disturbance wave in the substrate layer. Corresponding to the stratified flow model proposed previously by authors, the dimensionless quantity Fr₀ = G/[gd_eρ_v(ρ_l ? ρ_v)]^0.5 may be used as a measure for determining the applicability of the present theoretical model, which was used to restrict the transition boundary between the stratified-wavy flow and the annular/intermittent flows. Comparison of the prediction of the circumferential average heat transfer coefficient with available experimental data for four tubes and three refrigerants reveals that a good agreement is obtained or the trend is better than that of the previously developed stratified flow model for Fr₀ > 4.0 as long as the partial dry out of tube does not occur. Obviously, the developed annular model is applicable and reliable for evaporation in horizontal microfin tubes under the case of high heat flux and high mass flux.     

A detailed comparison between Navier–Stokes and DSMC simulations of multicomponent gaseous flow in microchannels

Numerical results of the Navier–Stokes equations and the DSMC method for H₂/N₂ and H₂/N₂/CO₂ mixtures in planar microchannels are compared in order to verify the accuracy of slip/jump boundary conditions in multicomponent flows. For the solution of the Navier–Stokes and energy equations, a finite-volume method is used together with a complete set of slip/jump boundary conditions derived from the kinetic theory of gases. In order to compare the accuracy of these two methods, different wall temperatures as well as inlet mass flow rates are examined. Isothermal flow is also considered for comparison with available analytical solutions for slip flows. The Knudsen number typically ranged between 0.015 < Kn < 0.09 with a Reynolds number of 5 < Re < 15. The two methods are generally in good agreement under the conditions studied especially at low to moderate Knudsen numbers.     

Forced convection heat transfer from an elliptical cylinder to power-law fluids

Forced convection heat transfer to incompressible power-law fluids from a heated elliptical cylinder in the steady, laminar cross-flow regime has been studied numerically. In particular, the effects of the power-law index (0.2 ? n ? 1.8), Reynolds number (0.01 ? Re ? 40), Prandtl number (1 ? Pr ? 100) and the aspect ratio of the elliptic cylinder (0.2 ? E ? 5) on the average Nusselt number (Nu) have been studied. The average Nusselt number for an elliptic cylinder shows a dependence on the Reynolds and Prandtl numbers and power-law index, which is qualitatively similar to that for a circular cylinder. Thus, heat transfer is facilitated by the shear-thinning tendency of the fluid, while it is generally impeded in shear-thickening fluids. The average Nusselt number values have also been interpreted in terms of the usual Colburn heat transfer factor (j). The functional dependence of the average Nusselt number on the dimensionless parameters (Re, n, Pr, E) has been presented by empirically fitting the numerical results for their easy use in process design calculations.     

Convective diffusion from strip-like micro-probes into colloidal suspensions

Full equations of convective diffusion are solved numerically for a strip-like (2D) electrodiffusion friction probe in a stream of microdisperse liquid, assuming a non-linear near-to-wall velocity profile ranging from simple shear flow (p = 1) to ideal slip (p = 0). The range of generalized Peclet number H from H = 0.01 (almost pure spatial diffusion) to H = 100 (diffusion layer with negligible longitudinal diffusion) covers all cases of possible experimental relevance. The main result is expressed as a relative deviation of actual total diffusion flux N from its diffusion-layer approximation N_DLA, Ψ = N/N_DLA ? 1.