Computational fluid dynamics (CFD) investigation of air flow and temperature distribution in a small scale bread-baking oven

Experimental and computational fluid dynamics (CFD) analyses of the thermal air flow distribution in a 3-zone small scale forced convection bread-baking oven are undertaken. Following industrial bread-making practise, the oven is controlled at different (constant) temperatures within each zone and a CFD model is developed and validated against experimental data collected within the oven. The CFD results demonstrate that careful selection of the flow model, together with implementation of realistic boundary conditions, give accurate temperature predictions throughout the oven. The CFD model is used to predict the flow and thermal fields within the oven and to show how key features, such as regions of recirculating flow, depend on the speeds of the impinging jets.     

An experimental investigation of the dissipation mechanisms in the suction side boundary layer of a turbine blade

The present work is part of an extensive experimental activity carried out by the authors in recent years aimed at investigating the boundary layer transition phenomenon in turbine blades. The large scale of the cascade and the use of advanced LDV instrumentation and precision probe traversing mechanism resulted in high degree of spatial resolution and high accuracy of measurements. The main dissipation mechanism determining the profile losses in turbomachinery blades is the work of deformation of the mean motion within the boundary layer operated by both viscous and turbulent shear stresses. In the present paper, the local viscous and turbulent deformation works have been directly evaluated from the detailed measurements of boundary layer mean velocity and Reynolds shear stress. The results show the distributions and the relative importance of the viscous and turbulent contributions to the loss production, in relation with the boundary layer states occurring along the turbine profile.     

Theoretical and experimental investigation of biomass gasification process in a fixed bed gasifier

This investigation concerns the process of air biomass gasification in a fixed bed gasifier. Theoretical equilibrium calculations and experimental investigation of the composition of syngas were carried out and compared with findings of other researchers. The influence of excess air ratio (λ) and parameters of biomass on the composition of syngas were investigated. A theoretical model is proposed, based on the equilibrium and thermodynamic balance of the gasification zone.The experimental investigation was carried out at a setup that consists of a gasifier connected by a pipe with a water boiler fired with coal (50 kWth). Syngas obtained in the gasifier is supplied into the coal firing zone of the boiler, and co-combusted with coal. The moisture content in biomass and excess air ratio of the gasification process are crucial parameters, determining the composition of syngas. Another important parameter is the kind of applied biomass. Despite similar compositions and dimensions of the two investigated feedstocks (wood pellets and oats husk pellets), compositions of syngas obtained in the case of these fuels were different. On the basis of tests it may be stated that oats husk pellets are not a suitable fuel for the purpose of gasification.     

Comparative experimental investigation of oxyhydrogen (HHO) production rate using dry and wet cells

Hydroxy gas was produced by water electrolysis from dry and wet cells using stainless steel 316L electrode of 136.5 cm² surface area and 4 mm separation. Electrolytes as NaOH and KOH of different concentrations were used. This study investigates the effect of electrolyte concentration, cell connection, electric current, operating time, electrolyte temperature and voltage on HHO productivity of the cells. Different plate configurations were studied. Increases of applied current, electrolyte temperature, electrolyte concentration and voltage led to the increase of gas production. More gas was produced from wet cell as compared to dry cell for the same design. HHO production for the dry cell reaches its maximum values of 866, 985, 1040 and 1090 ml/min at 5, 10, 15 and 20 g/L of NaOH at currents of 14, 18, 20 and 21.3 A and attains stable after about 30 min but the temperatures were increased till 32, 38, 44 and 52 °C, respectively and remained constant after that. The production peak values for wet cell were 975, 1160, 1325 and 1375 ml/min at 5, 10, 15 and 20 g/L of NaOH and flow currents of 17.8, 23.5, 26 and 27 A and remains constant after 90 min. At 10, 15 and 20 g/L NaOH, the temperatures were increased till constant values of 35, 44, 50 and 58 °C, respectively. HHO productivities from dry and wet cells are 866 and 1160 ml/min with electrolyzer efficiency of 72.1 and 69.3% at 14 and 18 A and (5 and 10 gm NaOH/L), respectively.     

Theoretical investigation of solvent effects on the selective hydrogenation of furfural over Pt(111)

It was well known that solvent effect plays a very important role in the catalytic reaction. There are many theoretical studies on the solvent effect in homogeneous catalysis while there are few theoretical studies on the solvent effect in the heterogeneous catalytic reaction and there has been no work to investigate the solvent effect on furfural transformation in heterogeneous catalysis. In the present work, both the density functional calculations and the microkinetic analysis were performed to study the selective hydrogenation of furfural over Pt(111) in the presence of methanol as well as toluene and compared with that in the gas condition. The present results indicated that the methanol can enhance the adsorption strength of furfural and other oxygen-containing reaction species due to its relatively strong polarity properties and this can be a main reason for solvent-induced high activity and selectivity. Another reason is that reaction paths study showed that the presence of methanol solvent makes the dehydrogenation of furfural less thermochemical due to the fact that furfural is more stabilized than that of dehydrogenation species, and methanol also has an inhibition effect on the dehydrogenation of furfural in the kinetic aspect, and further energetic span theory proves highest activity and selectivity for hydrogenation in methanol solvent of vapor, methanol and toluene. Moreover, microkinetic model simulation demonstrated that the activity and selectivity of hydrogenation in methanol is both higher than that in vapor and toluene. The much higher activity in methanol is due to the stabilized adsorbed reactants in the surface, which leads to a higher surface coverage of furfural. It might be proposed based on the present work that a solvent with relatively strong polarity may be favorable for the high selective hydrogenation of furfural.     

Theoretical calculation and experimental investigation on acoustic characteristics of a regenerator in thermoacoustic apparatus

An acoustic‐driven thermoacoustic device, which is used to investigate acoustic characteristics of a regenerator, was designed and manufactured. A model of the acoustic characteristics of the regenerator is discussed. The acoustic characteristics of the regenerator, such as acoustic impedance Ẑ_n, reflection coefficient , transmission loss TL, and phase angle between incident and reflected wave at x=0, were obtained by processing the experimental results with the correlation‐spectra analysis (the auto‐ and cross‐spectra) methods theoretically. Comparisons of acoustic characteristics between two cases, A (regenerator) and B (regenerator and two additional heat exchangers), are discussed. Different heating power influence on acoustic characteristics is also investigated. The results obtained will be helpful in further investigations on the regenerator model. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 539–546, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20093     

Characteristics and structural change of layered polysilane (Si6H6) anode for lithium ion batteries

Layered polysilane (Si₆H₆) has a graphite-like structure with higher capacity than crystalline silicon. The rate of increase of the thickness of a layered polysilane electrode after 10 charge-discharge cycles was smaller than that for a Si powder electrode, although the layered polysilane electrode has higher capacity. The structural changes of electrochemically lithiated and delithiated layered polysilane at room temperature were studied using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Layered polysilane became amorphous by insertion of lithium to 0 V, whereas insertion of lithium into crystalline silicon produces Li₁₅Si_4. Layered polysilane maintained an amorphous state during lithium insertion and deinsertion, whereas silicon changed between Li₁₅Si₄ and amorphous Li_xSi, which explains the smaller volume change of a layered polysilane electrode compared with a Si powder electrode.     

Theoretical and experimental investigation of total equivalent temperature difference (TETD) values for building walls and flat roofs in Turkey

The aim of this study is to find time lag (TL), decrement factor (DF) and total equivalent temperature difference (TETD) values for multilayer walls and flat roofs of buildings using experimental and theoretical methods, and to compare the experimental results with theoretical ones. The TETD is a method for calculating cooling load due to heat gain from the walls or flat roofs, and it can be obtained using values of inside and outside air temperatures, solar radiation, TL and DF. The TL and DF depend on the highest and the lowest temperatures at the inner and outer surfaces of the walls or flat roofs, and the time periods involved in reaching these temperatures. Hence, two testing rooms each consisting of four multilayered walls and a flat roof, air conditioner, measuring elements are built to measure all required temperatures. The required temperatures, which are hourly inside and outside air temperatures, and surface temperatures of each structure layer, are measured in every minute during testing periods of the 2007 summer season of Gaziantep, Turkey. Hourly solar radiation values on the walls are computed using hourly measured solar radiation on a horizontal surface. The TL, DF and TETD values of eight different walls and two different flat roofs commonly used in Turkey are computed utilizing the measured temperature and solar radiation values. The computed values for the TL, DF and TETD are compared with theoretical results obtained numerically using periodic solution of one dimensional transient heat transfer problem for the same structures.     

On the improvement of chemical conversion in a surface-wave microwave plasma reactor for CO2 reduction with hydrogen (The Reverse Water-Gas Shift reaction)

A novel surface-wave microwave discharge reactor configuration to generate syngas via gaseous CO₂ reduction with H₂ (non-catalytic Reverse Water-Gas Shift reaction) is studied in the context of power-to-chemicals concept. Improvement of CO₂ conversion to maximize CO production is explored by adding an external cylindrical waveguide downstream of the plasma generation system. A 2D self-consistent argon model shows that power absorption and plasma uniformity are improved in the presence of the waveguide. We show experimentally that CO₂ conversion is increased by 50% (from 40% to 60%) at the stoichiometric feed ratio H₂:CO₂ equal to 1 when using the waveguide. At higher H₂:CO₂ ratios, the effect of the waveguide on the reactor performance is nearly negligible. Optical emission spectroscopy reveals that the waveguide causes significant increase in the concentration of O atoms at a ratio H₂:CO₂ = 1. The effects of the operating pressure and cooling rate are also investigated. A minimum CO₂ conversion is found at 75 mbar and ratio H₂:CO₂ = 1, which is in the transition zone where plasma evolves from diffusive to combined operation regime. The cooling rates have significant impact on CO₂ conversion, which points out the importance of carefully designing the cooling system, among other components of the process, to optimize the plasma effectiveness.     

Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hill

This study presents a three‐dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn‐shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn‐shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high‐turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20261     

Three Dimensional Separation with Spiral—Focus in a Decelerating Duct Flow （Effect of Asymmetric Inlet Boundary Layer Thickness）

An experimental apparatus was developed to study the three dimensional separated flow with spiral-foci. The internal decelerating flow was generated by the air suction from a side wall to produce the separation on an opposite-side wall. The relation between the upstream boundary layer and the generation of spiral-foci in the separation region was observed by a tuft method. As a result, it was clarified that the spiral-focus type separation could be produced on the side wall and its behavior was closely related to the vortices supplied into the separation region from the boundary layer developing along top wall or bottom one.     

Solving of boundary‐layer equations with transpiration effects,governance on a vertical permeable cylinder using modified differential transform method

The suction and injection effects on the free convection boundary‐layer flow over a vertical cylinder are studied. The main stream velocity and wall temperature are proportional to the axial distance along the surface of the cylinder. Both analytic and numerical solutions of the arising mathematical problem are obtained. An analytic solution is derived by a new analytical method (DTM‐Padé) and numerical solutions have been performed by using a fourth‐order Runge–Kutta and shooting methods. Velocity and temperature profiles are shown graphically. It is shown that the differential transform method (DTM) solutions are only valid for small values of the independent variable but the obtained results by DTM‐Padé are valid for the whole solution domain with high accuracy. These methods can be easily extended to other linear and nonlinear equations and so can be found widely applicable in engineering and sciences. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20366     

An experimental investigation of sewage sludge gasification in near and super-critical water using a batch reactor

The gasification of sewage sludge in near and super-critical water was investigated in a batch reactor. Results showed that the formation of gaseous products could be intensively affected by temperature. In order to understand the effect of temperature on the development of reaction process and the formation of gaseous products better, the detailed characteristics of solid and liquid products were analyzed by SEM, N₂ adsorption–desorption technique, FTIR, TOC, Ammonia–nitrogen analysis and SPE-GC/MS. The changes in the yield distribution of products and the characteristics of solid and liquid products indicated that organic matters in sewage sludge were almost completely dissolved and hydrolyzed in water at 425 °C. The dissolution and hydrolysis products were gasified by reforming and other reactions. The polymerization and dehydrogenation also occurred in dissolution and hydrolysis products, and the Diels–Alder reaction mechanism could be used to explain the phenomenon.     

Energy and exergy analyses in drying process of non-hygroscopic porous packed bed using a combined multi-feed microwave-convective air and continuous belt system (CMCB)

This paper is concerned with the energy and exergy analyses in the drying process of non-hygroscopic porous packed bed by combined multi-feed microwave-convective air and continuous belt system (CMCB). Most importantly, this work focused on the investigation of drying phenomena under industrialized microwave processing. In this analysis, the effects of the drying time, hot-air temperature, porous structure (F-Bed and C-Bed) and location of magnetron on overall drying kinetics and energy utilization ratio (EUR) were evaluated in detail. The results showed that using the continuous microwave application technique had several advantages over the conventional method such as shorter processing times, volumetric dissipation of energy throughout a product with higher energy utilization and less exergy efficiency in drying process. The results presented here provided fundamental understanding for drying process using CMCB in industrial size.     

Photocatalytic property of partially substituted Pt-intercalated layered perovskite, ASr2TaxNb3−xO10 (A=K, H; x=0, 1, 1.5, 2 and 3)

A series of layered perovskite photocatalysts, ASr₂Ta_xNb_3−xO₁₀ (A=K, H; x=0, 1, 1.5, 2 and 3), were synthesized by conventional solid-state reaction followed by an ion-exchange reaction. Pt was incorporated in the interlayer of HSr₂Ta_xNb_3−xO₁₀ by the stepwise intercalation reaction. The HSr₂Ta_xNb_3−xO₁₀ showed hydrogen production activity and the activities were greatly enhanced by Pt co-incorporating. The x value in HSr₂Ta_xNb_3−xO₁₀ had an important effect on the photocatalytic activity of the catalyst. When the x=1, the HSr₂TaNb₂O₁₀/Pt photocatalyst showed a photocatalytic activity of 208 cm³ g⁻¹ h⁻¹ hydrogen evolution rate in 10 vol% methanol solution under irradiation with wavelength more than 290 nm from a 100-W mercury lamp at 333 K. The HSr₂TaNb₂O₁₀/Pt photocatalyst exhibited much higher photocatalytic activity than the well-known TiO₂/Pt photocatalyst under the same conditions.     

Synthesis and performance of layered perovskite-type H-ABi2Ta2O9 (A = Ca,Sr, Ba,K0.5La0.5) for photocatalytic water splitting

Novel photocatalysts, protonated layered perovskite oxides H-ABi₂Ta₂O₉ (A = Ca, Sr, Ba, K_0.5La_0.5) for overall water splitting were synthesized by ion exchange with acid treating. The characterization by XRD, HRTEM indicated that all of ABi₂Ta₂O₉ (A = Ca, Sr, Ba, K_0.5La_0.5) were able to form new single-phase protonated layered oxides. The measurement of photocatalytic activity showed every protonated layered oxide could overall split water into H₂ and O₂, although the ratios of H₂ and O₂ are unstoichiometric. The sequence of photocatalytic H₂ production is HCBT(107.0 μmol/h) < HBBT(119.5 μmol/h) < HSBT(162.7 μmol/h) < HKLBT(189.3 μmol/h). The difference of ionic radius of cations in interlayer influenced the band gaps, and resulted in the distinction of photocatalytic activity. Pt loading enhanced apparently the photocatalytic activity. Among all of photocatalysts in this study, 0.1 wt%Pt/HSBT showed the highest photocatalytic activity for H₂ evolution, reaching 491 μmol/h.     

An experimental investigation of fuel-injection-pressure and engine-speed effects on the performance and emission characteristics of a divided-chamber diesel engine

An experimental study is conducted to investigate the fuel-injection-pressure and engine-speed effects on the performance and exhaust emissions of a naturally aspirated four-stroke indirect-injection (IDI) diesel engine with a swirl combustion chamber. The influence of the injection pressure and the engine rotational speed on fuel consumption, exhaust-gas temperature, exhaust smokiness and exhaust-gas emissions (nitrogen oxides and unburned hydrocarbons) is examined, following a detailed experimental investigation. Empirical easy-to-use correlations are produced, expressing the variation of the various parameters with injection pressure, by applying a regression analysis on the curves fitting the relevant experimental data. Theoretical aspects of diesel fuel spray progress (atomization, evaporation and mixing), combustion and emissions formation are used for the interpretation of the observed engine behaviour.     

Parameter variations and the effects of hydrogen: An experimental investigation in a lean premixed low swirl combustor

With a global focus on the reduction of fossil fuel consumption and harmful pollutant emissions, new technologies have been raised offering reduced emissions with the combustion of alternative and renewable fuels. Low swirl combustion and the addition of highly reactive fuels into the fuel stream are two methods that have been shown to meet these challenges. In the present study, the thermo-acoustic behavior of a lean premixed low swirl combustor is examined by the variation of several parameters: the equivalence ratio, bulk velocity, chamber pressure, and the addition of hydrogen into the fuel mixture. It is reported that the natural modes of the chamber employed shift upwards for both fuel mixtures examined when increasing the equivalence ratio. As additional heat is dumped into the chamber, the increase in acoustic energy is being pumped through these natural modes. An increase in the bulk velocity is found to have opposite effects on these dominant acoustic modes for the two mixtures investigated. The methane mixture shows negative shifts in frequency when increasing the bulk velocity, whereas the hydrogen-methane mixture displays upward-shifting frequencies. Elevating the chamber pressure results in an increase in the acoustic modes for both mixtures, although the trend is more consistently linear for the hydrogen-methane flames.     

Theoretical study of solvent effects on the decomposition of formic acid over a Co(111) surface

Solvent effects on the decomposition of formic acid over a Co(111) surface were studied via density functional theory calculations combined with a continuum implicit solvation model. The solvents used here were water, methanol, and acetone. The adsorption energies of key intermediates, the activation barriers and the rate and equilibrium constants of various elementary reactions in vacuum and in the solvents were obtained. Solvent presences decrease the adsorption energies of species. Formic acid decomposition on the surface goes through HCOO rather than COOH both in vacuum and in the solvents. The most favorable decomposition pathways in vacuum and in acetone are HCOOH → HCOO → HCO → CO. The corresponding rate determining steps are HCOO deoxidation to HCO with activation barriers of 0.78 and 0.76 eV, respectively. In the presences of water and methanol, the preferred pathways are shifted to HCOOH → HCOO → HCOO-m → CO₂ below 750–800 K. Above those temperatures, the path of HCOOH → HCOO → HCO → CO becomes dominant again.