FLOW PATTERNS THROUGH TUBE BUNDLES AND PARTICULATE FOULING ONTO CYLINDERS

The shear-stress distributions around cylinders in tube bundles were measured by means of the limiting current method and the flow inside the bundle was also visualized. In the case of the staggered bundle, the shear-stress distributions had two peaks by the effect of the jet streams generated at the front row spacing and the adjacent cylinder. While in the bundle with in-line arrangement, the front stagnant point was shifted about 30° downward and the traverse streams among the rows were observed. This traverse streams caused asymmetry of the shear-stress distributions with respect to the main flow axis.

These results were discussed in connection with the fouling experiments investigated in dusty air. Particle deposition around cylinders could be qualitatively predicted from the shear-stress distributions, namely particles tend to deposit at positions where are less shear-stress in both tube arrangements. The size of particle deposited on the wake zone was coarser than that near the stagnant point behind the third row in both arrangements.     

Interpretation of flow stress of textured zinc sheet

Abstract

The dependence of the flow stress on the orientation of the stress axis in the plane of textured zinc sheet has been investigated, both experimentally and theoretically. The theoretical investigation consisted of simulating the deformation process by using a generalized Taylor model (based on the minimum-work condition) and the Sachs model (based on the Schmidt shear-stress law). The deformation modes considered were slip and mechanical twinning. Only with the Taylor model was the orientation distribution consistent with the experimentally determined pole figures. The work done and the flow stress for a given plastic strain were therefore calculated for this orientation distribution using the Taylor model. The calculated and measured quantities exhibited the same directional dependence. This anisotropy can be attributed to the different activations for basal and pyramidal slip systems.

MST/80     

MEASUREMENTS OF IN-BED TUBE BUNDLE EROSION AND PARTICLE-TUBE COLLISION FREQUENCY IN A GAS FLUIDIZED BED

Erosion of an in-bed tube bundle by bed materials (ash, coal, and sorbents) impingement has caused serious problems to many fluidized bed combustion (FBC) boilers. Experimental investigation of the tube erosion processes was made by placing the erosion-prone wax cylinders in a bench-scale, cold fluidized bed to simulate the long term erosion effect. This paper discusses the results of systematic measurements of weight losses of an in-bed tube bundle under various test conditions. The specific erosion rate of immersed wax specimens ranges from 25 to 175 µg/ (cm² hr) when impacted by 0.55 mm glass beads at a superficial velocity of 75 cm/s. The tube bundle height and configuration were found to have a prominent influence on the mean solid flow pattern and hence the tube erosion. A comparison of the erosion rates of narrow-pitch and wide-pitch staggered bundles, as well as the in-line bundle are presented. Electrostatic impact probes based on the triboelectric effect of moving particles were developed as the primary standard for measuring the particle-surface collision frequency, which is one of the major parameters characterizing the tube erosion. The peripheral distribution of collision frequency around an embedded tube was measured and correlated with the results from weight loss measurement.     

Microfine Classification of Highly Loaded Suspensions

ABSTRACT

A centrifugal classifier has been designed, consisting of a rectangular rotor with two classification chambers and six rotating joints for connecting water and slurry streams with the rotor. The feed slurry enters the rotor in the middle of the classification chamber. Coarse particles settle to the outside, form a fluidized bed in front of the water inlet, and are sucked out of the rotor. Fine particles are carried to the inner radius with the main water flow. With the flow rates of classification water, feed, underflow, and the number of revolutions, the same cut size is adjusted. This paper presents experimental results with cutsizes between 0.5 and 5μm. The feed slurry was variated up to 30vol%. The performance of the classifier is remarkable, yielding up to 90% recovery of fine panicles.     

Prediction of stress history in carburized and quenched steel cylinders

Abstract

Quenching and residual stresses were evaluated in carburized (0·18% core carbon) steel cylinders. A variety of experimentally generated data including dilatation strains, temperature-dependent elastic-plastic properties, and thermal profiles from oil and water quenching were used as input data for a finite-element program. The program was designed specifically for cylinders and allowed determination of the stress history during quenching and the development of the residual-stress profiles. Special emphasis was given to evaluating the factors that lead to the final residual-stress patterns including the degree of plastic flow and the phase transformations for the carburized and high-carbon steels. To verify the results, residual stresses were evaluated by X-ray diffraction methods and compared with those determined by the finite-element program. The X-ray results corroborated those found with the finite-element program.

MST/18     

Validity of the cylindrical localized approximation for arbitrary shaped beams in generalized Lorenz-Mie theory for circular cylinders

Abstract

A so-called cylindrical localized approximation, allowing one to speed up the evaluation of beam shape distributions in generalized Lorenz-Mie theory for circular infinitely long cylinders, has been previously introduced and, in the case of Gaussian beams, rigorously justified. In this paper, we examine and demonstrate the validity of this approximation for arbitrary shaped beams.     

Experimental investigation on the thermodynamic performance of double-row liquid–vapor separation microchannel condenser

A double-row liquid–vapor separation microchannel condenser (D-LSMC) was presented, and its tube pass scheme was optimized using the theoretical method. A series of experiments were conducted to investigate the heat load, average heat transfer coefficient (AHTC), and pressure drop of the optimal D-LSMC. Experimental results were compared with an optimal common double-row parallel-flow microchannel condenser (D-PFMC). The findings showed that, at the inlet mass flux of 585 kgm⁻² s⁻¹ to 874 kgm⁻² s⁻¹, the AHTC of the D-LSMC was 3.3%–14.4% higher than that of the D-PFMC. However, the pressure drop of the D-LSMC was only 43.4%–52.1% of that of the D-PFMC. The heat exchange capacity of the back row was weaker by almost half of that of the front row. In addition, the tube wall temperature of the back row decreased faster than that of the front row, which indicated that the back row had a larger pressure drop. The minimum entropy generation number (Ns) was used to evaluate the D-LSMC and the D-PFMC, which indicated the greater thermodynamic performance of the D-LSMC.     

A novel broadband terahertz filter for photonic crystal

Abstract

The influence of the change of the radius of point defect cylinders, scattering dielectric cylinders, dielectric cylinders on both sides of line waveguide on S parameter is studied. According to the resonant coupling principle between micro-cavity and waveguide, a novel broadband terahertz filter is designed. The novel filter is formed by introducing scattering dielectric cylinders into the resonant microcavity, and the point defect cylinders are composed of HgTe material and adjusting the radius of dielectric cylinders on both sides of line waveguide. Results show that the 3 dB bandwidth reaches 74.2 GHz, the return loss is less than ?12.02 dB, the maximum insertion loss in-band reaches 0.35 dB and its drop efficiency is up to 96.79%. The novel terahertz filter has flat passband, sharp rejections at out-bands and its central frequency is 0.338THz. The good performances show that it can meet the requirements of high speed and broadband in terahertz atmosphere communication I window.     

单侧受限管排自然对流的数值模拟

给出了具有 5,10,15,20 根管的管排在 103相似文献   

Aerodynamic forces on two stationary and oscillating square prisms in tandem and side by side arrangements

Abstract

In order to gain further understanding of aerodynamic forces and their effects on groups of high‐rise buildings, this study used wind‐tunnel experiments. Two square prisms were arranged both in tandem and side‐by‐side arrangement with different spacings in between. Similar experiments were carried out to study the interactions of aerodynamics between the two prisms when both were stationary, when only one prism oscillated, and finally, when both prisms oscillated.

The results showed that the aerodynamic responses were either enhanced or suppressed by the spacing ratios, the oscillating frequencies, and the mutual influences of the two square prisms in various arrangements. The aerodynamics also changed due to the occurrences of different flow patterns, such as channel flow, deflected flow, pulsating flow, and so on. Obviously, the aerodynamics of the flow patterns of the two square prisms in tandem and side‐by‐side arrangements proved to be more complex than those of a single square prism.     

Optimising the palletisation of cylinders in cases

Summary This paper is concerned with the analysis of the packing arrangements of cylindrical objects into a rectangular case with respect to the palletisation efficiency of the resultant case. A set of regular arrangements are considered. The size and shape of the surrounding case depends not only on the number of rows and the number of cylinders per row but also on the angle between the centres of cylinders in adjacent rows. Previous theoretical results on the palletisation of rectangular boxes are used to develop an algorithm for determining the optimal angle for any arrangement. Arrangements with angles of 0° or 30° are often used in practice as they are easy to load. The expected optimality of this practice is investigated by an empirical study.     

Gastrointestinal Transit Time:— An in Vitro Analysis of the Various Parameters Controlling the Critical Flow Velocity of Particles in a Horizontal Tube

Abstract

The factors which affect the critical flow velocity (V_c) of particles of barium sulphate, bismuth subcarbonate, kaolin, sulphadiazine and latex particles has been determined in a horizontal tube. These factors were particle size, particle density, fluid viscosity and tube diameter. V_c was found to increase with increasing particle diameter, particle density and tube diameter but decreases as viscosity of the flowing fluid. The results obtained were found to fit the models of Wicks, Durand and Wasp for the flow conditions of settled beds.     

Out of roundness distortion of bearing rings owing to internal stresses from tube bending

Abstract

The out of roundness distortion of bearing rings owing to relaxation of internal stresses was investigated experimentally and numerically. Internal stresses in the rings were owing to bending of the tubes from which the rings were produced. This three point bending operation can be regarded as an idealised tube straightening operation. Bending is a typical deformation mode in real tube straightening. The experiments and simulations included the following steps: three point bending of tube, ring turning, and soft annealing heat treatment (also representing the heating in a hardening operation). The ring distortion was measured after ring turning and after soft annealing. A considerable out of roundness was found in both steps but the major contribution was owing to the elastic deformation of the ring at the final stage of turning when the ring was cut off from the tube.     

Buoyancy effects on the laminar boundary layer heat transfer along vertically moving cylinders

Abstract

The effects of buoyancy forces on the laminar boundary layer flow and heat transfer along vertically moving cylinders are analyzed for the cases of prescribed surface temperature and prescribed wall heat flux in power of streamwise distance. Local similarity solutions are obtained to show the effects of buoyancy parameters and the transverse curvature of the cylinder on the surface friction and heat transfer rate.     

STOKES FLOW THROUGH A SYSTEM OF PARALLEL INFINITE CYLINDERS WITH AXES ORIENTED AT AN ANGLE TO THE DIRECTION Of MEAN FLOW

Abstract

Theoretical models based on Stokes flow of air through a fibrous filter predict a significantly higher pressure drop than experimentally measured values. This discrepancy persists even when the interaction of the flow between) neighboring fibers is accounted for. Various authors have attributed this discrepancy to the inhomogeneity of the fiber distribution within the filter and to the possibility that some fibers are partially orientation in the directon of mean flow. It has been shown that fiber density inhomogeneity does indeed contribute to this discrepancy

In this paper, the effect on the flow and subsequent pressure drop when the fibers are oriented at an angle to the directon of mean flow is studied. The solution of the three dimensional equation for creeping, incompressible flow in a doubly periodic, infinite lattice of infinite circular cylinders when there is a constant mean flow whose direction makes an acute angle with the axes of the cylinders is given. If the volume fraction of fibers is small, the periodic boundary conditions can be replaced by requiring zero vorticity at the outer boundary of an imagined cylindrical cell of fluid surrounding one of the cylinders. The resulting parallel and transverse problems have known solutions and give an approximate solution to the flow through the periodic lattice. The resulting drag is used to compute the dimensionless pressure drop across a filter for several values of the volume fraction of fiber and is compared to the experimentally determined formula of Davies. It is shown that the average drag over a uniform distribution of fiber orientations yields a pressure drop which is significantly closer to the experimental values of Davies than that resulting from strictly transverse flow.     

Entropy generation analysis of eccentric cylinders pair sources on nanofluid natural convection with non-Boussinesq state

In this study, the natural convection heat transfer and entropy generation in horizontal eccentric cylinders with different arrangements of two constant temperature sources are investigated numerically. The distance between eccentric cylinders was filled with pure fluid and Cu_ water nanofluid. The sources with constant temperature T_h and T_c were located on the inner and outer cylinders and the other walls were assumed to be insulated. Governing equations were formulated by using Boussinesq approximation and non-Boussinesq state (density inversion) and were solved on a non-uniform mesh in eccentric cylinders by using the finite volume method. The numerical calculation was carried out for Rayleigh number (${10}^4?\text{Ra}?5\times {10}^5$), volume fraction of nanoparticles ($0?\mathrm{\Phi }?0.08$) and different arrangements of heat sources with different angles in Pr = 13.31 and constant eccentricity (e_v = 0.7). The results were compared with concentric cylinders and presented from streamlines and isotherms flow field, local and average Nusselt number, local and total entropy generation. The results showed that eccentricity, different arrangements, discrete constant temperature sources and non-Boussinesq state affected the best state of heat transfer. In addition, increasing Rayleigh number and volume fractions of nanoparticles caused an increase in the rate of heat transfer and total entropy generation. It was concluded that Boussinesq approximation and eccentric cylinders had higher rate of heat transfer and entropy generation than non-Boussinesq state and concentric cylinders, respectively. The results indicated which arrangements and kinds of cylinders were optimum and applicable to use in industry and heat exchanger.     

Failure analysis on a primary superheater tube of a power plant

In this paper failure analysis on the SA213-T12 superheater tube by visual inspection, in situ measurements of hardness and finite element analyses is presented. A primary superheater tube has failed with a wide open burst after running at around 28,194 h. Heavy clinkers were found to almost entirely cover the primary superheater region. In situ hardness measurements were carried out on the selected primary superheater first row tubes at the middle region between furnace rear screen tube and primary superheater blower flow path. Hardness measurements are also taken on the as-received failed tube. Finite element analyses on possible features prior to failure are also conducted in order to illustrate and deduce the failure mechanism and failure root cause. Localized short-term overheating of the tube due to localized and concentrated flue gas flow resulted in a failure of the primary superheater tube.     

Experimental study and modelling of heat transfer during condensation of pure fluid and binary mixture on a bundle of horizontal finned tubes

An experimental investigation was conducted to measure the local heat transfer coefficient for each row in a trapezoidal finned horizontal tube bundle during condensation of both pure fluid (HFC 134a) and several compositions of the non-azeotropic binary mixture HFC 23/HFC 134a. The test section is a 13×3 (rows × columns) tube bundle and the heat transfer coefficient is measured using the modified Wilson plot method. The inlet vapour temperature is fixed at 40 °C and the water flow rate in each active row ranges from 170 to 600 l/h. The test series cover five different finned tubes all commercially available, K11 (11 fins/inch), K19 (19 fins/inch), K26 (26 fins/inch), K32 (32 fins/inch), K40 (40 fins/inch) and their performances were compared. The experimental results were checked against available models predicting the heat transfer coefficient during condensation of pure fluids on banks of finned tubes. Modelling of heat exchange during condensation of binary mixtures on bundles of finned tubes based on the curve condensation model is presented.     

Finite element solutions for laminar flow and heat transfer around a square prism

Abstract

The finite element solutions of Navier‐Stokes and energy equations for steady laminar flow and heat transfer around square prisms, with attack angles of 0° and 45° have been obtained for a gas of Pr=0.7. The variations of surface shear stress, local pressure and Nusselt number are obtained over the entire prism surface including the zone beyond the point of separation. The predicted values of drag coefficients, the location of. separation, the average Nusselt number and the plots of velocity flow fields and isotherms are also presented. The trend of the present numerical results seems reasonable.     

Effect of Air Flow and Acceleration on Particle Size of Dry Powder Aerosols determined by Time-of-Flight Aerosol Beam Spectrometry

ABSTRACT

The effect of air flow and acceleration on the particle size distributions of two Turbuhalers containing drug loads of 0.5 and 1.3 mg per dose was determined with a time-of-flight aerosol beam spectrometer (Aerosizer®). While the particle size of both inhalers decreased with increasing flow and acceleration, the distributions became narrower and variability was reduced. Simultaneously, a decrease in the number and mass of particles measured was observed which was more pronounced for the 1.3 mg dose. Additionally the count rate for the 1.3 mg dose was lower than for 0.5 mg. These observations were accompanied by a remarkably fine particle size distribution for the high dose Turbuhaler®. It was concluded that the quantity of particles in the DPI aerosols exceeded the resolution of the Aerosizer, erroneously shifting the size distributions towards smaller diameters.

In addition, the DPI aerosols were analyzed with a Four Stage Impinger to evaluate the results. The particle size estimates obtained using the impinger were in the same range as those obtained at an acceleration of 5 L/s2 using the Aerosizer to study the 0.5 mg Turbuhaler. The Aerosizer produced significantly smaller particle sizes than the impinger in all cases evaluating the 1.3 mg Turbuhaler, supporting the theory of distorted particle size distributions due to particle overload.