Energy loss at bends in the pneumatic conveying of fly ash

An accurate estimation of the total pressure drop of a pipeline is important to the reliable design of a pneumatic conveying system. The present paper presents results from an investigation into the modelling of the pressure drop at a bend in the pneumatic conveying of fly ash. Seven existing bend models were used (in conjunction with solids friction models for horizontal and vertical straight pipes, and initial acceleration losses) to predict the total pipeline pressure drop in conveying fly ash (median particle diameter: 30 μm; particle density: 2300 kg/m³; loose-poured bulk density: 700 kg/m³) in three test rigs (pipelines with dimensions of 69 mm inner diameter (I.D.) × 168 m length; 105 mm I.D. × 168 m length; 69 mm I.D. × 554 m length). A comparison of the pneumatic conveying characteristics (PCC) predicted using the seven bend models and experimental results shows that the predicted total pipeline PCC and trends depend on the choice of bend model. While some models predict trends that agree with the experimental results, other models predicted greater bend pressure drops for the dense phase of fly ash than for the dilute phase. Models of Pan, R. (1992). Improving scale-up procedures for the design of pneumatic conveying systems. Doctoral dissertation, University of Wollongong, Australia, Pan, R., & Wypych, P.W. (1998). Dilute and dense phase pneumatic conveying of fly ash. In Proceedings of the sixth International Conference on Bulk Materials Storage and Transportation (pp. 183–189), Wollongong, NSW, Australia and Chambers, A.J., & Marcus, R.D. (1986). Pneumatic conveying calculations. In Proceedings of the second International Conference on Bulk Materials Storage and Transportation (pp. 49–52), Wollongong, Australia reliably predicted the bend losses for systems conveying fly ash over a large range of air flows.     

Effect of temperature on the effective thermal conductivity of n-tetradecane-based nanofluids containing copper nanoparticles

Nanofluids were prepared by dispersing Cu nanoparticles (∼20 nm) in n-tetradecane by a two-step method. The effective thermal conductivity was measured for various nanoparticle volume fractions (0.0001–0.02) and temperatures (306.22–452.66 K). The experimental data compares well with the Jang and Choi model. The thermal conductivity enhancement was lower above 391.06 K than for that between 306.22 and 360.77 K. The interfacial thermal resistance increased with increasing temperature. The effective thermal conductivity enhancement was greater than that obtained with a more viscous fluid as the base media at 452.66 K because of nanoconvection induced by nanoparticle Brownian motion at high temperature.     

Separation performance of fine low-rank coal by vibrated gas–solid fluidized bed for dry coal beneficiation

Vibrational energy was introduced to a dense medium gas–solid fluidized bed to improve the separation performance of 1–6 mm fine low-rank coal. The setup was termed a vibrated gas–solid fluidized bed and could provide a stable fluidization state and uniform density distribution for dry coal beneficiation by the transfer of vibrational energy and the interaction between vibrations and the gas phase. Favorable segregation of the ash content of the 1–6-mm-sized lignite samples is achieved under suitable operating conditions. Higher yields of cleaning coal were acquired when the ash content was reduced. The probable error values were 0.065 and 0.055 at separating densities of 1.68 and 1.75 g/cm³ for the 1–3- and 3–6-mm-sized lignite samples, respectively. Effective beneficiation of 1–6-mm-sized fine lignite could be achieved using the vibrated gas–solid fluidized bed, which provides an alternative technique for the separation of fine low-rank coal in arid areas.     

Set up of an experimental apparatus for the study of fragmentation of solid fuels upon severe heating

An experimental apparatus has been developed in order to perform tests of primary fragmentation of solid fuels under severe heating conditions. The device is a modified heated strip reactor, capable to reach 2000 °C in less than 0.2 s. Particles are laid on the strip and pyrolysed under inert or moderately oxidizing conditions. The char particles and their fragments, generated upon pyrolysis, can be recovered and analysed to assess the fragmentation propensity of the fuel.Some preliminary experiments have been carried out on two biomass samples in order to assess the time-temperature history of particles in the experimental apparatus. In particular biomass particles of approximately 2–3 mm have been used. The temperature of the heated strip reactor in such preliminary tests was varied between 1000 and 1600 °C, while the strip nominal heating rate was kept at 10⁴ °C/s and the holding time was set at the value of 10 s. A near infrared fast camera (38,000 frames/s) has been used to measure the temperature of the heated strip and of the particles during the tests. A heat up model was developed and validated against experimental results. The model was then used to estimate the temperature gradients across particles of biomass and of coal as well.Results show that the strip of the reactor reaches the set temperature in less than 0.2 s. When particles are laid on the strip, their bottom surface, which is in physical contact with the strip, immediately reaches the set temperature value. For 1 mm coal particles the upper surface can be considered at the same temperature as well. Under the most severe conditions tested (strip temperature of 1600 °C , biomass particles of 2 mm thickness) the temperature difference between the bottom and the upper face is 200 °C after 3 s and drops to 100 °C after 10 s. On the whole the experimental apparatus simulates uniform heating of the particles with reasonable approximation. In the next future the apparatus will be further upgraded to operate at pressures up to 20 bars.     

Design assessment of a 150 kWt CFBC Test Unit

For clean and efficient energy generation from coal, the most suitable technology known to date is ‘Fluidized Bed Combustion’ technology. Applications of circulating fluidized bed (CFB) combustion technology have been steadily increasing in both capacity and number over the past decade. Designs of these units have been based on the combustion tests carried out in pilot scale facilities to determine the combustion and desulfurization characteristics of coal and limestone reserves in CFB conditions. Similarly, utilization of Turkish lignites in CFB boilers necessitates adaptation of CFB combustion technology to these resources. However, the design of these test units are not based on firing coals with high ash, volatile matter and sulfur contents like Turkish lignites. For this purpose, a 150 kWt CFB combustor test unit is designed and constructed in Chemical Engineering Department of Middle East Technical University, based on the extensive experience acquired at the existing 0.3 MWt Bubbling Atmospheric Fluidized Bed Combustor (AFBC) Test Rig. Following the commissioning tests, a combustion test is carried out for investigation of combustion characteristics of Çan lignite in CFB conditions and for assessment of the design of test unit. Comparison of the design outputs with experimental results reveals that most of the predictions and assumptions have acceptable agreement with the operating conditions. In conclusion, the performance of 150 kWt CFBC Test Unit is found to be satisfactory to be utilized for the long term research studies on combustion and desulfurization characteristics of indigenous lignite reserves in circulating fluidized bed combustors.     

Start-up investigation on a dual-evaporator MPCL system

This paper describes the start-up process of a space activate thermal control system, two-phase mechanically pumped cooling loop (MPCL) with two evaporators, in ground-based testing. Each evaporator has an outer diameter of 3 mm and a length of 10 m and the total loop of the system is about 40 m. In this paper, the system design and work principle as well as the test setup of an MPCL are presented and the start-up processes of the MPCL are studied. The experiments on the start-up processes under different evaporative temperatures were carried out. Tests attention has been paid to the system performance characteristics such as differential pressure, absolute pressure, mass flow rate, main components temperatures and so on. During the start-up processes, the system presents a good stability and each part of the system performs a reasonable temperature wave, except some superheat phenomena in the evaporator which cause a pressure shock to the system. The superheat is mainly related to evaporative temperature and the initial liquid distribution in the evaporator. In general, the lower the evaporative temperature is the higher superheat occurs. When set-point evaporative temperature is ?15 °C, the differential pressure shock can reach 6.23 bar which is as 7.5 times as the stable state. In conclusion, the MPCL with dual-evaporators can be started up successfully and is an effective kind of thermal control technology for future space applications.     

The influence of pipe length on thermal statistics computed from DNS of turbulent heat transfer

We present results from direct numerical simulation of turbulent heat transfer in pipe flow at a bulk flow Reynolds number of 5000 and Prandtl numbers ranging from 0.025 to 2.0 in order to examine the effect of streamwise pipe length (πδ ≡ πD/2 ⩽ L ⩽ 12πδ) on the convergence of thermal turbulence statistics. Various lower and higher order thermal statistics such as mean temperature, rms of fluctuating temperature, turbulent heat fluxes, two-point auto and cross-correlations, skewness and flatness were computed and it is found that the value of L required for convergence of the statistics depends on the Prandtl number: larger Prandtl numbers requires comparatively shorter pipe length for convergence of most of the thermal statistics.     

Resolving the stratification discrepancy of turbulent natural convection in differentially heated air-filled cavities. Part III: A full convection–conduction–surface radiation coupling

The present study concerns an air-filled differentially heated cavity of 1 m × 0.32 m × 1 m (width × depth × height) subject to a temperature difference of 15 K and is motivated by the need to understand the persistent discrepancy observed between numerical and experimental results on thermal stratification in the cavity core. An improved experiment with enhanced metrology was set up and experimental data have been obtained along with the characteristics of the surfaces and materials used. Experimental temperature distributions on the passive walls have been introduced in numerical simulations in order to provide a faithful prediction of experimental data. By means of DNS using spectral methods, heat conduction in the insulating material is first coupled with natural convection in the cavity. As heat conduction influences only the temperature distribution on the top and bottom surfaces and in the near wall regions, surface radiation is added to the coupling of natural convection with heat conduction. The temperature distribution in the cavity is strongly affected by the polycarbonate front and rear walls of the cavity, which are almost black surfaces for low temperature radiation, and also other low emissivity walls. The thermal stratification is considerably weakened by surface radiation. Good agreement between numerical simulations and experiments is observed on both time-averaged fields and turbulent statistics. Treating the full conduction–convection–radiation coupling allowed to confirm that experimental wall temperatures resulted from the coupled phenomena and this is another way to predict correctly the experimental results in the cavity.     

The influence of pipe length on thermal statistics computed from DNS of turbulent heat transfer

We present results from direct numerical simulation of turbulent heat transfer in pipe flow at a bulk flow Reynolds number of 5000 and Prandtl numbers ranging from 0.025 to 2.0 in order to examine the effect of streamwise pipe length (πδ ≡ πD/2 ? L ? 12πδ) on the convergence of thermal turbulence statistics. Various lower and higher order thermal statistics such as mean temperature, rms of fluctuating temperature, turbulent heat fluxes, two-point auto and cross-correlations, skewness and flatness were computed and it is found that the value of L required for convergence of the statistics depends on the Prandtl number: larger Prandtl numbers requires comparatively shorter pipe length for convergence of most of the thermal statistics.     

Thermal performance of a carbon fiber composite material heat sink in an FC-72 thermosyphon

This study analyzes the use of a carbon fiber epoxy heat sink for evaporator surface enhancement in a FC-72 thermosyphon. The pin-fin heat sink features 945 small-cross-section (1.27 mm by 0.965 mm) fins fabricated with an integral base plate. These fins have a high thermal conductivity (500 W/m K) along the length of the fin. The influence of heat load, thermosyphon fill volume, and condenser operating temperature on the overall thermal performance is examined. The results of this experiment provide significant insight into the possible implementation and potential benefits of carbon-fiber heat sink technology in two-phase flow leading to significant improvements in thermal management strategies for advanced electronics.     

Flow behavior of high-temperature flue gas in the heat transfer chamber of a pilot-scale coal-water slurry combustion furnace

The flow characteristics of high-temperature flue gas are important in the heat transfer of coal-water slurry (CWS) combustion furnaces. The flow field of a 250 kg/h vertical-type slag tap cyclone furnace was non-intrusively investigated, using two-dimensional particle-image velocimetry (2D PIV). The method was verified using traceable fly ash particles in high-temperature flue gas. The flow field of the flue gas was analyzed with a time-averaged method, based on which the effects of excess air ratio and loading were investigated. The flue gas separated by a gas separator maintained good rigidity near the furnace wall, rather than eroding the heating surface. Numerical simulations validated the reliability of PIV under the actual circumstances within the furnace. This study provides guidelines for applying 2D PIV in analyzing flue gas in thermal test boilers.     

Plasma emission spectroscopy of solids irradiated by intense XUV pulses from a free electron laser

The FLASH XUV-free electron laser has been used to irradiate solid samples at intensities of the order 10¹⁶ W cm^?2 at a wavelength of 13.5 nm. The subsequent time integrated XUV emission was observed with a grating spectrometer. The electron temperature inferred from plasma line ratios was in the range 5–8 eV with electron density in the range 10²¹–10²² cm^?3. These results are consistent with the saturation of absorption through bleaching of the L-edge by intense photo-absorption reported in an earlier publication.     

Large-eddy simulation of thermal fatigue in a mixing tee

Temperature fluctuations occur due to thermal mixing of hot and cold streams in the T-junctions of the piping system in nuclear power plants, which may cause thermal fatigue of piping system. In this paper, three-dimensional, unsteady numerical simulations of coolant temperature fluctuations at a mixing T-junction of equal diameter pipes were performed using the large eddy simulation (LES) turbulent model. The experiments used in this paper to benchmark the simulations were performed by Hitachi Ltd. The calculated normalized mean temperatures and fluctuating temperatures are in good agreement with the measurements. The influence of the time-step ranging from 100 Hz to 1000 Hz on the numerical simulation results was explored. The simulation results indicate that all the results with different frequencies agree well with the experimental data. Finally, the attenuation of fluctuation of fluid temperature was also investigated. It is found that, drastic fluctuation occurs within the range of less than L/D = 4.0; the fluctuation of fluid temperature does not always attenuate from the pipe center to the wall due to the continuous generation of vortexes. At the top wall, the position of L/D = 1.5 has a minimum normalized mean temperature and a peak value of root-mean square temperature, whereas at the bottom wall, the position having the same characteristics is L/D = 2.0.     

Buckling behavior of Kelvin open-cell foams under [0 0 1], [0 1 1] and [1 1 1] compressive loads

This paper describes buckling modes and stresses of elastic Kelvin open-cell foams subjected to [0 0 1], [0 1 1] and [1 1 1] uniaxial compressions. Cubic unit cells and cell aggregates in model foams are analyzed using a homogenization theory of the updated Lagrangian type. The analysis is performed on the assumption that the struts in foams have a non-uniform distribution of cross-sectional areas as observed experimentally. The relative density is changed to range from 0.005 to 0.05. It is thus found that long wavelength buckling and macroscopic instability primarily occur under [0 0 1] and [0 1 1] compressions, with only short wavelength buckling under [1 1 1] compression. The primary buckling stresses under the three compressions are fairly close to one another and almost satisfy the Gibson–Ashby relation established to fit experiments. By also performing the analysis based on the uniformity of strut cross-sectional areas, it is shown that the non-uniformity of cross-sectional areas is an important factor for the buckling behavior of open-cell foams.     

Experimental investigation on thermal properties of silver nanofluids

This paper reports on an experimental investigation of the thermal properties behavior of 0.5 wt% silver nanoparticle-based nanofluids (NF) containing oleic acid (OA) and potassium oleate surfactant (OAK⁺) with concentrations of 0.5, 1, and 1.5 wt% respectively. The experiments were conducted from 20 °C to 80 °C. It was shown that the NF with 1 wt% OAK⁺ yielded the highest thermal behavior enhancement of about 28% at 80 °C compared to deionized water. The thermal performance had higher than the base fluid/nanofluids at approximately 80%. Moreover, the NF containing OAK⁺ showed higher thermal conductivity and dynamics of specific heat capacity than deionized water in all of the experimental conditions in this study. The rheological experiment showed that viscosity of NF was significantly dependant on temperature. As shear rate increased, the shear stress of the NF increased; however, the viscosity of the nanofluids decreased first and then stabilized. It was further found that NF containing OAK⁺ at a range of operating temperatures produced Newtonian behavior.     

Numerical and experimental studies of temperature field characteristic inside an irregularly-shaped cavity of a space environment simulator system

Numerical and experimental methods were used to explore temperature and pressure distributions inside an irregularly-shaped cavity of a novel three-dimensional space environment simulator (SES) system. In order to obtain better temperature and pressure distributions, a plenum chamber and airflow diffusion perforated plate were adopted. Three-dimensional heat and mass transfer characteristics were analyzed using the Standard k–ε turbulence model. Simulation results revealed that the temperature and pressure distributions were greatly improved with improved diffusion configuration design, the temperature gradient decreased from 5 K to 1 K, and the pressure gradient decreased to 0.5% of the former value. Based on the simulation results, an improved experimental system for simulating space environment was set up. This experiment system could supply airflow with temperature ranging from 193 K to 353 K for simulating the real space environment. Experimental results showed that the temperature and pressure fields had smaller gradients across the surface and the inner cavity, which agreed considerably with the numerical results. The results of this study present useful information for the design of similar cavity structure.     

Conjugate turbulent heat transfer in a square cavity with a solar control coating deposited to a vertical semitransparent wall

This paper presents a numerical study of the conjugate heat transfer (natural convection, surface thermal radiation and conduction) in a square cavity with turbulent flow. The cavity has one vertical isothermal wall, two horizontal adiabatic walls and one vertical semitransparent wall with a selective coating applied to the inner side to control the solar radiation transmission. Later on the semitransparent wall is replaced with another one without the selective coating. The mathematical model for the turbulent flow in the cavity was solved using the finite volume method. The system had the following conditions: the uniform temperature in the isothermal wall was 21 °C, the external ambient temperature was fixed at 35 °C and on the semitransparent wall the direct normal solar irradiation of 750 W/m² was considered constant. The Rayleigh number was varied in the range of 10⁹ ? Ra ? 10¹² by changing the lengths of the cavity from 0.70 m to 6.98 m, respectively. The results show that, even though the air temperature of the cavity with the solar control film coating semitransparent wall (case A) is higher compared with the one without solar film coating (case B), the total amount of heat going through the cavity is lower compared to the one going through the cavity without solar control film. The total amount of energy transferred to the air in cavity for the case A was 41.98% less than for the case B. A set of correlations for the Nusselt number was obtained for both cases considering the conjugate heat transfer.     

Size and shape effects on magnetic properties of Ni nanoparticles

Pure Ni nanoparticles ranging in size from 24 to 200 nm are prepared via thermal decomposition of nickel acetylacetonate in oleylamine. The as-prepared Ni particles change from spherical to dendritic or starlike with increasing precursor concentration. The particles are stable because the organic coating occurs in situ. Magnetic measurement reveals that all the Ni nanoparticles are ferromagnetic and show ferromagnetic–paramagnetic transitions at their Curie points. The saturation magnetization M_s is size-dependent, with a maximum value of 52.01 and 82.31 emu/g at room temperature and 5 K, respectively. The coercivity decreases at first and then increases with increasing particle size, which is attributed to the competition between size effect and shape anisotropy. The Curie temperature T_c is 593, 612, 622, 626 and 627 K for the 24, 50, 96, 165 and 200 nm Ni nanoparticles, respectively. A theoretical model is proposed to explain the size-dependence of Ni nanoparticle Curie temperature.     

The Effects of duct height on heat transfer enhancement of a co-rotating type rectangular finned surface in duct

Experiments were performed to investigate the effect of duct height on heat transfer enhancement of a surface affixed with arrays (7 × 7) of short rectangular plate fins of a co-rotating type pattern in the duct. An infrared imaging system is used to measure detailed distributions of the heat transfer at the endwall along with the fin base. An infrared camera of TVS 8000 with 160 × 120 point In–Sb sensor was used to measure the temperature distributions in order to calculate the local heat transfer coefficients of the representative fin regions. Pressure drop and heat transfer experiments were performed for a co-rotating fin pattern varying the duct height from 20?50 mm. The friction factor calculated from the pressure drop shows that comparatively larger friction occurs for the smaller duct cases and the friction factor slowly decreases with increasing Reynolds number. The effect of duct height on the area-averaged heat transfer results show that heat transfer initially increases with duct height and then finally decreases with increasing the duct height. Detailed heat transfer analysis and iso-heat transfer coefficient contour gives a clear picture of heat transfer characteristics of the overall surface. The relative performance graph indicates that a 25 mm duct is the optimum duct height for the highest thermal performance. In addition, a significant thermal enhancement, 2.8?3.8 times the smooth surface, can be achieved at lower Reynolds number with a co-rotating fin pattern in the duct.     

Study on premixed combustion in cylindrical micro combustors: Transient flame behavior and wall heat flux

The micro combustor is a key component of the micro thermophotovoltaic (TPV) system. Improving the wall temperature of the micro combustor is an effective way to elevate the system efficiency. An experimental study on the wall temperature and radiation heat flux of a series of cylindrical micro combustors (with a backward-facing step) was carried out. For the micro combustors with d = 2 mm, the regime of successful ignition (under the cold wall condition) was identified for different combustor lengths. Acoustic emission was detected for some cases and the emitted sound was recorded and analyzed. Under the steady-state condition, the effects of the combustor diameter (d), combustor length (L), flow velocity (u₀) and fuel–air equivalence ratio (Ф) on the wall temperature distribution were investigated by measuring the detailed wall temperature profiles. In the case that the micro combustor is working as an emitter, the optimum efficiency was found at Ф ≈ 0.8, independent of the combustor dimensions (d and L) and the flow velocity. Under the experimental conditions employed in the present study, the positions of the peak wall temperature were found to be about 8–11 mm and 4–6 mm from the step for the d = 3 mm and d = 2 mm micro combustors, respectively, which are 8–11 and 8–12 times of their respective step heights. This result suggests that the backward-facing step employed in the combustor design is effective in stabilizing the flame position.