Plate Gap Research Articles

Viscous fingering and heat transfer during boiling in a Hele–Shaw cell

Nucleate boiling in a Hele–Shaw cell was experimentally investigated considering the effects of plate gap, heat flux level, and heating area size. The fluid was pure and de-mineralized water at atmospheric pressure. The highly confined bubble dynamics were visualized by means of a high speed camera mounted perpendicular to the surface. Viscous fingering was observed in case of special combinations of plate spacing, heat flux, and sub cooling. That viscous fingering led to a small-scale roughening of the fast evaporating front. The interface velocity and the gap size were identified as major parameters for the onset of that instability, and the observed dynamics was in a reasonable agreement with theoretical considerations. The boiling heat transfer coefficients were determined, too. It was found that the spacing was of major importance for the heat transfer level that was rather high for special combinations of heat flux level and plate spacing. This behavior was explained as result of the micro-layer evaporation and the refilling of the Hele–Shaw cell after a computer-based image processing of the high speed cinematography.

Refiner optimization and control Part I: Fiber residence time and major dynamic fluctuations in TMP refining processes

The study focuses on dynamic considerations in a mill-wide perspective showing that mill economy is closely linked to optimization of TMP and power plant. Special attention is given to stability issues in the refining processes. Describing the dynamics in such nonlinear processes is difficult and tedious and requires significant knowledge about where in the operating window to run the refiners. Results show the dynamics in the internal states, temperature profiles and the plate gap sensors responses and how they correspond to the refiner motor load. Disturbances in production affect the plate gap sensors and the temperature profiles in the same direction, while deliberate changes in the plate gap result in a dynamic situation where the motor load and temperature profile change in opposite directions. It is shown that the total residence time and the ratio between the residence time before and after the maximum temperature will be important when formulating specifications for energy efficient refining. As an example of how the refining process affects the mill economy, the start-up procedures for two different types of refiners will be penetrated. It is shown that the start-ups are most often associated with an over shoot in the motor load, which is easy to overcome.

The Influence of Plate Gap on the Fatigue Properties of Friction Stir Welded AA5182-H111 and a Comparison with MIG Welding

In industrial applications tight control during weld set-up equates to increased manufacturing cost. Solid state welding processes, particularly friction sir welding, do not generally make use of filler metals and hence a weld gap will have an influence on joint quality. In the current study the influence of a weld gap of 20% of the plate thickness (i.e. 1mm) in friction stir welded (FSW) joints was compared dynamically to welds made without a gap. These results were benchmarked against samples joined through MIG welding, a widely used method to join aluminium plates in industry. Tests revealed that the introduction of a 1mm gap caused a steeper slope of the stress-life curve of the friction stir welded joints when compared to flaw free welds made without a weld gap. Crack initiation in the 1mm gap FSW samples occurred at the advancing side weld edge where the tool shoulder interacts with the material, while the flaw free ‘zero gap’ welds failed in the parent material from the marks induced during oxide removal. Intermittent root flaws were, however, present in the welds made without a weld gap which decreased the life of the samples as the area of the flaw was increased. MIG welded samples always failed from the weld toe and the fatigue life of these samples were considerably lower than that of the 1mm gap friction stir welded samples. In the friction stir welded samples containing root flaws, a flaw across the entire sample width was required to reduce the life of the friction stir welds to that reached by the MIG welded samples.

Studies on the effect of electrode plate position and feed temperature on the tribo-electrostatic separation of high ash Indian coking coal

Investigations were carried out to understand the design aspects of tribo-electrostatic separator to separate ash from high ash Indian coking coal. Initial studies were aimed to evaluate the different physical, chemical and electrical properties of quartz, kaolinite, and carbon particles. The significant design variables such as plate angle and plate gap were evaluated to effect the optimum separation. The absolute charge acquired by quartz, kaolinite, and carbon were observed to be different. The results indicate that the magnitude of charge increases with the increase in the time of tribo-charging using copper tribo-charging medium. A mathematical model was formulated based on design and operating parameters of the experimental set up to simulate the particle trajectories. The particle trajectories were simulated using measured physical and electrical properties of mineral and carbon particles at experimental design and operating conditions. The simulated trajectories were validated with experimental data. The results of plate position and feed particle temperature indicated that there were optimum conditions to achieve the desired performance. The simulation and experimental results were in good agreement. The optimum separation was achieved at plate inclination of 5°. It was possible to reduce ash content by 10% at 61% yield. Better quality clean coal at 33% ash was achieved from 53% ash feed coal with lower yield.

Fiber energy efficiency Part III: Modeling of bar-to-fiber interaction

This paper presents a model for fluid dynamic studies describing the bar-to-fiber interaction in refining processes. The approach is to introduce and validate assumptions for a deeper understanding of physical phenomena between refining bars in a micro-scale perspective. The model is based on Navier-Stokes equations in 2D and special emphasize is put on the pressure dynamics occurring when changing the plate gap, i.e. the distance between refining segments. It is shown that the model can reproduce pressure fluctuations like those obtained in experimentally validated measurements. It is also shown that the pressure varies considerably dependent on the plate gap as well as the position studied along the bar in tangential direction. This is a consequence of varying shear forces between the bars. Moreover, it is stated that the fiber network density variations will be important to describe in more details. Finally, it is shown that the model for the bar-to-fiber interaction can handle different aspects regarding plate pattern design which have significant impact on the fluid dynamics and thereby the final pulp properties in full-scale refiners.

Fiber energy efficiency Part IV: Multi-scale modeling of refining processes

Models of thermomechanical pulp processes must handle a large number of physical variables and machine specific parameters at different scales. Some variables, measured in the refining zone, will be invaluable when spanning the material and energy balances and thereby they provide essential information to the models. In this paper, measurements of temperature profile and plate gaps from a full-scale CD-refiner are used as model inputs together with process variables such as motor load, production, dilution water, inlet pressure and casing pressure. Two independent models, a micro-model describing fluid mechanics between the refining bars and an extended entropy model (macro-model) describing the material and energy balances for a complete refiner are used to get a realistic estimate of the pulp dynamic viscosity inside the refining zone. The models comprise a multi-scale concept and it is shown that the variability in the estimated pulp dynamic viscosity is directly related to the fiber energy efficiency in a micro-as well as in a macro perspective. To produce homogeneous pulp quality at energy efficient process conditions, it is concluded that large variations in the pressure pulses must be reduced. This can be performed by optimizing refining segments as well as stabilizing conditions by improved control strategies.

NUMERICAL ANALYSIS OF CAVITATION SUSCEPTIBILITY FOR STEAM GENERATOR INTEGRAL PREHEATER TUBES

A new tube degradation mechanism was observed in a recirculating steam generator (SG) with an integral preheater tube at the clearance gap between the tube and the preheater baffle. The general pattern of the damage and material composition in the degraded region suggested that the degradation was cavitation erosion. Cavitation erosion occurs when vapour bubbles exist or form in the flowing liquid and then these bubbles collapse violently in the vicinity of a solid wall. The bubbles collapse when they contact water that is sufficiently subcooled, i.e., below the saturation temperature. In the clearance gap between the tube and the preheater baffle, secondary fluid flow exists due to the pressure difference across the baffle plate. Meanwhile, heat transfer occurs from the primary-side fluid to the secondary-side fluid within this clearance gap, driven by the primary-to-secondary temperature difference. Factors such as the tube position in the baffle hole and fouling may influence the local flow and heat transfer conditions and can cause subcooled boiling that results in cavitation. This paper presents a numerical analysis of flow and heat transfer phenomena to determine the factors contributing to cavitation erosion of tubes in the preheater of a recirculating SG. The analysis used the THIRST code for a 3-dimensional thermalhydraulic simulation of steam generator and the ANSYS Fluent® code for detailed calculations of flow and heat transfer in the clearance gaps. A detailed temperature distribution in the gap was obtained using this analysis to determine the regions where subcooled boiling could occur by comparing the local fluid temperature with its saturation temperature. The susceptibility to cavitation was found to increase with increased inclination (i.e., tilt) and eccentricity (i.e., off-centre) of the tube in the baffle plate gap, and increased fouling on baffle plate surfaces. This methodology could be applied to analyze the cavitation susceptibility for other preheater types with this “tube to baffle plate” gap, as these preheaters might have conditions where boiling followed by the rapid condensation of the steam bubbles are present.

Experiments and numerical simulations of plate gap model for high energetic materials

Experimental and numerical study for detonation propagation was conducted using the system in which the high energy materials and air gaps alternately stacked. The aim of this simulation is an extraction of information of the EOS for detonation product at arbitrary initial density from the numerical simulation with EOS at TMD (theoretical maximum density). In this report, we described the numerical procedure and an example of the calculation result. On the other hand, the experiment was designed for confirmation of the validity of our reactive flow simulation. The experimental system for this study consists of the pellet explosives and PMMA rings, PMMA pipe and booster explosive part. The pellets and the rings were alternately stacked in the PMMA pipe to make the system. The diameter of the pellet was 20 mm and the thicknesses were 10 or 5 mm. The thickness of the ring was varied to adjust the size of the air gaps between the pellets. The sample explosive was a composition A5 (RDX 98.8 wt%). The relationship between the bulk density which was estimated by the thicknesses of the pellets and the air gaps and the average detonation velocity was compared with the data for RDX. The slopes of those relationships differed mutually. Although the experimental results can be used for confirmation of the validity of the numerical procedure, it does not simulate the detonation wave in the powdered explosive. It may show the interesting process that consists of the shock wave in air, shock to detonation transition and steady detonation.

A study of crumb rubber modified bitumen used in South Africa

In South Africa, an empirical characterisation of crumb rubber modified (CRM) bituminous binders has historically been the only means of predicting their performance in pavement layers, short of constructing pavement test sections. An improved characterisation is provided by means of rheological analysis using a dynamic shear rheometer (DSR). However, the heterogeneous morphology of CRM bitumen makes it a challenge to test using current methods and equipment. DSR testing of CRM bitumen requires a plate gap adjustment to avoid any influence by the rubber particles. This has been done by monitoring the effect of changing the DSR plate gap setting on the measured linear visco-elastic properties of the binder. An adjusted gap was adopted for rheological measurements so as to characterise CRM bitumen properties with ageing. But, the incomplete recovery of CRM binder from asphalt/seals makes it impossible to monitor the rheological properties of the in situ binder within a pavement layer. This has led to indirect methods of investigating relationships between tested properties of the pure CRM bitumen to those of the in situ binder.

Fiber energy efficiency Part I: Extended entropy model

Abstract This is the first in a series of papers presenting the development of a comprehensive multiscale model with focus on fiber energy efficiency in thermo mechanical pulp processes. The fiber energy efficiency is related to the defibration and fibrillation work obtained when fibers and fiber bundles interact with the refining bars. The fiber energy efficiency differs from the total refining energy efficiency which includes the thermodynamical work as well. Extracting defibration and fibrillation work along the radius in the refining zone gives information valuable for fiber development studies.Models for this process must handle physical variables as well as machine specific parameters at different scales. To span the material and energy balances, spatial measurements from the refining zone must be available. In this paper, measurements of temperature profile and plate gaps from a full-scale CD-refiner are considered as model inputs together with a number of process variables. This enables the distributed consistency in the refining zone as well as the split of the total work between the flat zone and the CD-zone to be derived. As the temperature profile and the plate gap are available in the flat zone and the CD-zone at different process conditions it is also shown that the distributed pulp dynamic viscosity can be obtained. This is normally unknown in refining processes but certainly useful for all fluid dynamic models describing the bar-to-fiber interactions. Finally, it is shown that the inclusion of the machine parameters will be vital to get good estimates of the refining conditions and especially the split between the thermodynamical work and the defibration/fibrillation work.

Fiber energy efficiency Part II: Forces acting on the refiner bars

Abstract In this paper it is shown that the defibration and fibrillation work in thermo mechanical pulp refiners can differ significantly dependent on the process conditions. This has a direct impact on the distributed force in the refining zones obtained when the bars hit the fibers or fiber bundles. The distributed force, which is defined as a vector along the surface of the refining segments, is estimated using a model where the total work can be split into reversible and irreversible work. Besides traditional refiner variables such as motor load, dilution water added and inlet- and casing pressures, measurements of temperature profiles in the refining zone and plate gaps from a fullscale CD-refiner are available as inputs. Three data sets are analyzed and it is shown that the shape of the temperature profile and the force distribution vary significantly. This means that the fiber distribution in the refining zone varies as well which affects the final development of the pulp properties. The refining segment pattern and taper play an important role when estimating the force distribution and it is stated that the force distribution close to the contraction part of the flat zone as well as the outer part of the conical section are larger than in other positions. Therefore, specific energy which can be seen as the integral of the total energy distribution along the refining segments cannot be used when describing the variations in the distributed forces.

Radioisotope energy conversion using electrostatic vibrationto–to–electricity converters

Wireless sensor nodes deployed at remote and inaccessible locations need long lifetime power sources to prevent cost prohibitive periodic replacement. In this work, we present a radioisotope 63Ni energy converter using radioisotope-powered electrostatic vibration-to-electricity conversion. Free damped vibration happening in a suspended parallel plate structure with a mass enables a variable capacitance, which can be used to realize the generation of electricity energy by an external circuit. The MATLAB/Simulink is used to simulate the vibration and output power, and the Ansys is used to optimize the structure design. The results show that the optimized design structure with a first-order natural frequency of 500 Hz, a plate gap of 75 μm, and an external resistance of 90 kΩ can generate an average output power of 0.416 μW and conversion efficiency of 8.25%.

The rheological properties of magnetic field excited magnetic powders sheared between two parallel plates

The rheological properties of magnetic powders (MPs) excited by magnetic fields under parallel-plate shear were experimentally studied. The results showed that the shear stress increased with increasing magnetic field strength and volume fraction. It was also influenced by the plate gap distance, and was independent of the shear rate. The shear stress could be ascribed to the contributions of magnetic force and friction force, dominated by the normal force, the shear strain and the friction coefficient. The ratio of shear stress to normal stress, a form of friction coefficient, was influenced by volume fraction, magnetic field strength and gap distance. These results provide a better understanding of the mechanisms of magnetic field excited magnetic powders under parallel-plate shear.

Study on the Test System of ERF's Performance

Electrorheological fluid (ERF) is a new kind of smart materials, which has a great deal of market value and broad application prospects. To meet the needs of the ERFs performance testing, design and produce of the ERF's performance test system are based on the relationship of the apparent viscosity, shear stress and electric field strength. Select the NDJ-8S digital viscometer, high voltage DC power supply, liquid container, and other parts to constitute the ERFs performance test system after considering the relationship of the apparent viscosity, shear stress, shear rate and electric field strength. The system has potentials to control voltage, change velocity, change the distance between the parallel panels, and test the yield stress of the ERF, shear stress, viscosity and other parameters. The study introduces the hardware and software device, the related experiments, and obtains the relationship of the viscosity, rotational speed, voltage, and plate gap.

Coupling Effect between Gear Backlash of Friction Plate and Axial Separation Gap in Multi-Friction Pairs

Multi-plate friction pairs are important components in the gearbox of tracked vehicle. When the multi-friction pairs are in separation state, the uniform separation gaps of friction pairs are key factors affecting their operating performance. Most current research focuses on investigation of the failure modes of friction plate, such as, wear. However, the literatures on the vibration excitations from the inner and outer hub which affect the uniform separation gaps of the friction pairs are rare. This paper developed a rigid-flexible coupling dynamic model with different gear backlashes. The vibration characteristics of manipulate components and the separation gap of the friction pairs are studied. The results show that the model can be utilized to investigate the effects of the gear backlashes for manipulation components on the separation gaps of multi-friction pairs and it can be used to study the failure modes of friction plate in brakes. The developed model is expected to be the foundation for further optimization of the gear backlash. Keywords: Multi Friction Pairs; Separation gap; Gear Backlash; Rigid-flexible Coupling Dynamics Model

Sensitive Parameter Predicting Steady-State Pressure Difference of Partially Crosslinked Polyacrylamide Suspension in Core Flow Experiments

Using artificial cores, six different partially crosslinked polyacrylamide (PCPAM) suspensions were used for sandpacked core flow experiments; the results indicated that the migration of the PCPAM particles through porous media was a process of plugging and flooding at the same time. A theoretical model was proposed and the sensitive parameter, G′, was determined by rheological oscillation measurements with a 200 μm plate gap. In addition, the validity of the proposed theoretical model was verified by the results of the core flow experiments and a fitting equation of ΔP 1 with G′ was obtained which can be used for quantifying the steady-state pressure of PCPAM suspensions flowing through porous media in the core flow experiments.

Single bubble geometry evolution in micro-scale space

Two types of single vapour bubble growth in micro-scale space are experimentally investigated. Degassed pentane and FC-72 are used to study the bubble confined between superheated parallel plates without liquid flow. Liquid properties which are influential on bubble growth are discussed. Effects of plate superheating and plate gap on bubble growth rate are examined. Bubble growth during flow boiling in high aspect ratio micro-channels is also examined to extensively investigate the single bubble growth in a dynamic configuration. The hydraulic diameters of the tested channel are 571 μm, 762 μm and 1454 μm, with an aspect ratio of 20, 20 and 10 respectively. Transparent heating technique is utilised to provide heating power while ensuring sufficient transparency for visualisation. A three-stage bubble growth mode is presented. Moreover, bubble geometry evolution is found to be strongly related to channel cross-sectional geometry but slightly affected by heat flux or mass flux. Bubble growth critical time is defined. Bubble equivalent radius, bubble aspect ratio and width versus length trends are also discussed.

Automatic Control of Impact Crushers Based On Visual Measurement of Size Distribution

This paper describes a practical implementation of a control scheme for two impact crushers operating in parallel as secondary stage crushers in a large-scale aggregate production plant in the United Kingdom. The control is based on measuring crusher power draw, mass flows and the size distribution of produced rock. The size distribution is estimated using image analysis. The settings, i.e. rotor speed and impact plate gaps, of the impact crushers are then automatically adjusted to produce the desired end products. The system compensates for crusher blow bar wear and changes in feed material properties.This is the first time that such a control scheme has been implemented in a production plant. This paper describes the methods and controls used as well as future developments including an optimizing control based on a second, already installed camera.Unfortunately the plant in question has lately experienced equipment breakages so an analysis of the observed benefits is not available in time for paper preparation deadlines. Such an analysis can be presented at the conference, however.

Buccal bone plate remodeling after immediate implants with or without synthetic bone grafting and flapless surgery: a histomorphometric and fluorescence study in dogs

The aim of this study was to evaluate the buccal bone plate remodeling after immediate implantation using the flapless approach with or without bone graft into the gap between the implant and the buccal bone. Eight dogs had the mandibular bicuspids extracted without flaps, and four implants were installed on each side, totaling eight implants per animal. Randomly, in one side, the implants were positioned at the bone crest level (equicrestal), and on the opposite side, the implants were positioned 2 mm subcrestal. All the implants were positioned 2.0 mm from the buccal bone plate (gap) and associated or not with grafting material. Therefore, the following treatments were performed: implants subcrestal test (SCTG) with bone graft and control (SCCG) without bone graft, and equicrestal test (ECTG) with bone graft and control (ECCG) without bone graft. One week following the surgeries, metallic prostheses were installed. Bone markers were administered 1, 2, 4, and 12 weeks after implant placement for fluorescence analysis. Ground sections were prepared from 12-week healing biopsies, and histomorphometry was performed. The histomorphometric evaluation presents significant better results for the ECTG in the vertical crestal bone resorption, but the other parameters showed better results for the SCCG. The fluorescence evaluation in adjacent areas showed numerically different results between groups with a small decrease at 12 weeks, except for the SCCG, which was higher at this time. The distant area showed a continuous increase in the marked bone. The equicrestally placed implants presented little or no loss of the buccal bone wall. The subcrestally positioned implants presented loss of buccal bone, regardless of the use of bone graft. However, the buccal bone was always coronal to the implant shoulder. Both the equicrestal and subcrestal groups were benefited in the early stages of bone healing as evidenced by the fluorescence analysis.

Tensionless contact of a finite circular plate

A general formulation is developed for the contact behavior of a finite circular plate with a tensionless elastic foundation. The gap distance between the plate and elastic foundation is incorporated as an important parameter. Unlike the previous models with zero gap distance and large/infinite plate radius, which assumes the lift-off/separation of a flexural plate from its supporting elastic foundation, this study shows that lift-off may not occur. The results show how the contact area varies with the plate radius, boundary conditions and gap distance. When the plate radius becomes large enough and the gap distance is reduced to zero, the converged contact radius close to the previous ones is obtained.