Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

Critical flow rate of anode fuel exhaust in a PEM fuel cell system

A manual purge line was added into the exterior fuel exhaust stream of a Ballard PEM stack in a Nexa™ power module. With the addition of manual exhaust purge, high levels of inert gases were intentionally added to the anode feed without changing normal operational procedures. A new method of determining the critical minimum flow rate in the anode exhaust stream was given by an anode mass balance. This type of operation makes dual use of membranes in the MEAs as both gas purifiers and as solid electrolytes. The PEM stack was successfully operated with up to ca. 7% nitrogen or carbon dioxide in the absence of a palladium-based hydrogen separator at ca. 200 W power level. Nitrogen in the anode stream was concentrated from 7.5% to 91.6%. The system maintained a fuel efficiency of 99% at a manual purge rate of 2.22 ml s⁻¹ and no auto purge. The fuel cell stack efficiency was 64% and the stack output efficiency was 75%. The overall system efficiency was 39%. After troublesome CO and H₂S poisons were removed, a hydrocarbon reformate containing high levels of CO₂ and H₂O was further used in the Nexa™ stack. The size and complexity of the fuel processing system may be reduced at a specified power level by using this operational method.     

A state-space dynamic model for vapor compression refrigeration system based on moving-boundary formulation

A transient response model for vapor compression refrigeration system has been developed in the paper. The system model contains four sub-models representing condenser, evaporator, compressor and electronic expansion valve (EEV). The condenser and the evaporator are developed based on the moving-boundary formulation. Through linearization, these dynamic models are transformed into state-space representation which is expressed in the form of matrix. The compressor and EEV adopt steady models because their thermal inertia is much smaller compared with the heat exchangers (i.e., condenser and the evaporator). The system model has been validated by experiment in terms of step change of EEV opening degree and heat load as well as ramp increase of inlet temperature of coolant oil of condenser. The results show that the model simulations have a good agreement with the experimental data. The simulation errors compared with the experimental data are mostly less than 10%. Since the model proposed in this study is expressed in the form of state-space matrix, they are featured by strong portability and high computation efficiency. It allows us to investigate the thermal dynamic characteristics of a refrigeration system under any complicated conditions and develop excellent control schemes.     

First and second law analysis of pulse tube refrigerator

The first and second law of thermodynamics was used to analyze the orifice type and the double-inlet type of pulse tube refrigerator (PTR). Detailed dynamic characteristics of the thermodynamics, flow and heat transfer processes in the PTR were revealed, including the dynamic pressure variations, transient gas temperature, mass flow rate in the PTR. The exergy loss method was used to analyze each component in the PTR for the first time, and the performance coefficients of all components of PTR have been obtained. It was found that the performance coefficient of the double-inlet PTR was 0.108, 9% higher than that of the orifice PTR. The analysis also showed that the exergy efficiency of the double-inlet PTR was 29.95%, significantly higher than that of the orifice PTR (25.04%). In addition, it was found that the exergy losses in the regenerator and orifice were substantially larger than in other components of the PTR system. The optimal design of these two key components is, therefore, essential for the further improvement of the PTR performance.     

Study of Heat Transfer in Ice-storage Tank

IntroductionThe use of ice-storage for aircondihoulng systeinscan shift some or all of the sPace-cooling load to offpeak period. It follows that icestorage systeths havetbree advantaes in Practice. First, lOad eq is usefulto decrase the peak load of power netWork because aircondihonin systems are major factor brining about theelectrical peak load in sununer Second, ice-storagesystems can reduce the cOInPrssor size tO mect peakcooling load. Thir, load shiftin often resuhs inoPeIating cost …     

Development and test of CFD–DEM model for complex geometry: A coupling algorithm for Fluent and DEM

CFD–Discrete Element Method (DEM) model is an effective approach for studying dense gas–solid flow in fluidized beds. In this study, a CFD–DEM model for complex geometries is developed, where DEM code is coupled with ANSYS/Fluent software through its User Defined Function. The Fluent Eulerian multiphase model is employed to couple with DEM, whose secondary phase acts as a ghost phase but just an image copy of DEM field. The proposed procedure preserves phase conservation and ensures the Fluent phase-coupled SIMPLE solver work stable. The model is used to simulate four typical fluidization cases, respectively, a single pulsed jet fluidized bed, fluidized bed with an immersed tube, fluidization regime transition from bubbling to fast, and a simplified two-dimensional circulating fluidized bed loop. The simulation results are satisfactory. The present approach provides an easily implemented and reliable method for CFD–DEM model on complex geometries.     

Catalytic gasification of lignin with Ni/Al2O3–SiO2 in sub/supercritical water

Lignin has been gasified with a Ni/Al₂O₃–SiO₂ catalyst in sub/supercritical water (SCW) to produce gaseous fuels. XRD pattern at 6θ angle shows characteristic peaks of crystalline NiO, NiSi, and AlNi₃, suggesting that Al₂O₃–SiO₂ not only offers high surface area (122 m² g) for Ni, but also changes the crystal morphology of the metal. 9 mmol/g of H₂ and 3.5 mmol/g of CH₄ were produced at the conditions that 5.0 wt% alkaline lignin plus 1 g/g Ni/Al₂O₃–SiO₂ operating for 30 min at 550 °C. A kinetic model was also developed, and the activation energies of gas and char formation were calculated to be 36.68 ± 0.22 and 9.0 ± 2.4 kJ/mol, respectively. Although the loss of activity surface area during reuse caused slight activity reduction in Ni/Al₂O₃–SiO₂, the catalyst system still possessed high catalytic activity in generating H₂ and CH₄. It is noted that sulfur linkage could be hydrolyzed to hydrogen sulfide in the gasification process of alkaline lignin. The stable chemical states of Ni/Al₂O₃–SiO₂ grants its insensitivity to sulfur, suggesting that Ni/Al₂O₃–SiO₂ should be economically promising for sub/supercritical water gasification of biomass in the presence of sulfur.     

Three-dimensional study on steady thermohydraulics characteristics in secondary side of steam generator

Steam generator (SG), as the primary-to-secondary heat exchanger and pressure boundary of primary loop, should be integrated and perform well in heat transfer ability. Flow characteristics of the secondary side fluid of SG are essential to analyze U-tube wastage caused by the flow-induced vibration and thermal stress. In this paper, secondary side two-phase flow was simulated based on the porous media model. Additional momentum and energy source terms were appended to the momentum and energy equations for porous media region, respectively. The additional momentum source contained the resistances of downcomer, tube bundle, support plate and separator. The additional energy source included the heat transfer from primary side to secondary side fluid. Solving the governing equations by ANSYS FLUENT solver yielded the distributions of velocity, temperature, pressure, density and quality, which can be used in the analysis of flow-induced vibration and separators. The thermal-hydraulic characteristics of hot side differed from these of cold side considerably. The minimum flow quality of cold side was 0.07, while the maximum one of hot side was 0.71; the average flow quality of outlet was 0.272. The flow rate in the gap of the hot side was 1.02 times of that of the cold side.     

Understanding the influence of acidic components (Si,Al, and Ti) on ash flow temperature of South African coal sources

Ash flow temperature is one property that specifically gives more information on the suitability of a coal source for combustion or gasification purposes. Therefore the chemistry and mineral interaction have to be understood in order to determine the suitability for fixed bed gasification purposes with regards to ash flow properties. Various authors ([Seggiani, M., 1999. Empirical correlations of the ash flow temperatures and temperature of critical viscosity for coal and biomass ashes. Fuel 78, 1121; Alpern, B., Nahuys, J., Martinez, L., 1984. Mineral matter in ashy and non-washable coals—its influence on chemical properties. Commun. Serv. Geol. Portugal 70 (2), 299]) have expressed the fusibility of coal ash as a function of the content of the principal oxides frequently found in coal ash, i.e. SiO₂, Al₂O₃, TiO₂, Fe₂O₃, CaO, MgO, Na₂O and K₂O. However, coal ash fusibility characteristics are difficult to determine precisely, partly because coal ash contains many components with different chemical behaviours, and may very from coal source to coal source.The purpose of this study is primarily to understand the effect and chemistry of the acidic components (Si, Al and Ti) of South African coal sources, as well as the manipulation or addition of these components to the coal sources, with the view to understand their effect on the ash flow properties.A representative coal blend as it is currently used for gasification purposes, as well as coal mixtures with the addition of pure SiO₂, Al₂O₃ and TiO₂, respectively, were prepared. The variation of the acidic components SiO₂, Al₂O₃ and TiO₂ varied from 1 mass% to 50 mass% to the coal blend. The particle size of the samples were representatively prepared to <1 mm and analyzed for ash flow temperature and ash composition as dependant variables according to ASTM D1857-87 and ASTM D2795-95, respectively. The raw data was then statistically evaluated by means of regression models.Results of the statistical evaluation of ash flow temperature and the ash elemental composition indicated that based on the 95% confidence interval Al₂O₃, Fe₂O₃, CaO, MgO, P₂O₅, as well as the SiO₂–Al₂O₃ ratio have a statistical significant effect on ash flow temperature. Regression trends of the coal and Al₂O₃, SiO₂ and TiO₂ mixtures indicated that Al₂O₃ has the biggest effect on the ash flow temperature. A shift towards increasing the Al₂O₃ concentration has a significant increasing effect on the ash flow temperature in the three-component system Al₂O₃–SiO₂–TiO₂ phase diagram. The ash flow temperature is the highest at the point where the Al₂O₃ concentration is maximized. The shifts in frequency of the absorption band associated with the 6s–6p electron transaction which relates to the basicity of a glass or slag also relates to the different effect of the individual acidic components on ash flow temperature. The shift can be considered as a measure of the electron donor power and is usually expressed in terms of the optical (Λ) basicity. The ion–oxygen attraction and the stronger (more positive) value obtained for Al₂O₃ in comparison with TiO₂ and SiO₂ could be seen; and possibly explain why Al₂O₃ has a bigger influence and effect on increasing the ash flow temperature.It can be concluded from this experimental work with the emphasis on flow properties, that the acidic components Al₂O₃, SiO₂ and TiO₂ all have an increasing effect on ash flow temperature when added to the coal blends currently used for gasification. In addition to this general conclusion, it has also been confirmed that Al₂O₃ addition to the coal blend has the most significant effect towards increasing the ash flow temperature when compared to SiO₂ and TiO₂.     

Controlled synthesis of anatase TiO2 octahedra with enhanced photocatalytic activity

Titanium dioxide (TiO₂) nanocrystals with specific exposed crystal facet have attracted considerable interest due to their promising applications in the fields of energy and environment. In this paper, we report on a simple solvothermal approach for the synthesis of anatase TiO₂ octahedra with high yield, using titanium(IV) sulfate and hydrazine hydrate as the starting materials. The formation mechanism of anatase TiO₂ octahedra is suggested. The samples were characterized with XRD, Raman, SEM, TEM, FTIR, XPS, and UV/vis techniques, and further tested as a candidate in photocatalysis to decompose methyl orange in aqueous solution at room temperature. The results show that SO₄²⁻ ions not only benefit the formation of octahedral nanocrystals, but also inhibit nitrogen doping into TiO₂ matrix. More importantly, it is found that the octahedral TiO₂ nanocrystals show enhanced photocatalytic activity compared to TiO₂ P25 and anatase TiO₂ counterparts.