Hendrik K. Versteeg

A Review of Research and an Experimental Study on the Pulsation of Buoyant Diffusion Flames and Pool Fires

This paper reviews the past research, experimental techniques and scaling relationships used in the studies of oscillatory buoyant diffusion flames and reports an experimental investigation conducted to determine the pulsating characteristics of such flames. The experimental data were obtained by using three techniques, namely, pressure fluctuation measurements, thermal imaging and high-speed video photography. Present findings are compared with data sets reported in the literature and correlations for pulsation frequency suggested by previous studies are independently verified. Analysis of the experimental data on frequency of pulsations in different burners shows that for a fixed-diameter flame the pulsation frequency is almost independent of fuel flow rate. The equation f=1.68D−0.5 gives the best approximation for the relationship between pulsating frequency and diameter over a wide range of data. An alternative way of expressing the relationship between the key variables is St=0.52*(1/Fr)0.505. This proves to be a better way of expressing the relationship since it can include the effect of the fuel flow rate. Slight modifications to this expression allows prediction of flame oscillations under elevated/reduced gravity and isothermal buoyant plumes. This relationship and the observations of the present study confirm the hydrodynamic nature of flame puffing: interplay of buoyancy and fluid motion.

Improvement of the efficiency of energy transfer in the hydro-entanglement process

Hydro-entanglement is a versatile process for bonding non-woven fabrics by the use of fine, closely-spaced, high-velocity jets of water to rearrange and entangle loose arrays of fibres. The cost of the process mainly depends on the amount of energy consumed. Therefore, the economy of the process is highly affected by optimisation of the energy required. In this paper a parameter called critical pressure is introduced which is indicative of the energy level requirement. The results of extensive experimental work are reported and analysed to give a clear understanding of the effect of the web and fibre properties on the critical pressure in the hydro-entanglement process. Furthermore, different energy-transfer distribution schemes are tested on various fabrics. The optimum scheme which involves the lowest energy consumption and the best fabric properties is identified.

Effect of nozzle geometry on the flow characteristics of hydroentangling jets

Cone-capillary nozzles with varying cone angles from 10° to 120° and a capillary diameter of 120μ are experimentally investigated for their application in the hydroen tanglement process. Cone-up and cone-down configurations in a range of water pressures of 30-120 bar are tested. The effects of the cone angle on flow parameters such as discharge and velocity coefficients and intact length are studied. Flow visualization techniques are used to recognize the flow regimes and characteristics and to inspect and compare the intact length and appearance of the jets. Cone-down nozzles with more consistent flow properties, lower discharges, and higher velocity coefficients are more suitable for the hydroentanglement process. Single-cone nozzles without capillaries and with varying cone angles are also tested. The flow properties of the jets from the single-cone nozzles are compared with the cone-capillary nozzles of the same cone angle to study the effect of the capillary section. The effect of the interaction of adjacent nozzles on the flow from multi-hole nozzles is studied, and the characteristics of the jets from the multi-hole nozzles are compared with the single-hole nozzles.

A validated model of GAG deposition, cell distribution, and growth of tissue engineered cartilage cultured in a rotating bioreactor.

In this work a new phenomenological model of growth of cartilage tissue cultured in a rotating bioreactor is developed. It represents an advancement of a previously derived model of deposition of glycosaminoglycan (GAG) in engineered cartilage by (i) introduction of physiological mechanisms of proteoglycan accumulation in the extracellular matrix (ECM) as well as by correlating (ii) local cell densities and (iii) tissue growth to the ECM composition. In particular, previously established predictions and correlations of local oxygen concentrations and GAG synthesis rates are extended to distinguish cell secreted proteoglycan monomers free to diffuse in cell surroundings and outside from the engineered construct, from large aggrecan molecules, which are constrained within the ECM and practically immovable. The model includes kinetics of aggregation, that is, transformation of mobile GAG species into immobile aggregates as well as maintenance of the normal ECM composition after the physiological GAG concentration is reached by incorporation of a product inhibition term. The model also includes mechanisms of the temporal evolution of cell density distributions and tissue growth under in vitro conditions. After a short initial proliferation phase the total cell number in the construct remains constant, but the local cell distribution is leveled out by GAG accumulation and repulsion due to negative molecular charges. Furthermore, strong repulsive forces result in expansion of the local tissue elements observed macroscopically as tissue growth (i.e., construct enlargement). The model is validated by comparison with experimental data of (i) GAG distribution and leakage, (ii) spatial‐temporal distributions of cells, and (iii) tissue growth reported in previous works. Validation of the model predictive capability—against a selection of measured data that were not used to construct the model—suggests that the model successfully describes the interplay of several simultaneous processes carried out during in vitro cartilage tissue regeneration and indicates that this approach could also be attractive for application in other tissue engineering systems. Biotechnol. Bioeng. 2010. 105: 842–853.

The mechanism of the air-jet texturing: the role of wetting, spin finish and friction in forming and fixing loops

A comprehensive review of the roles played by the airflow, wetting and spin finish on the air-jet texturing process is given. The results of an experimental investigation of the air-jet texturing process using residual spin finish, yarn-to-yarn static and kinetic friction, filament strength, filament diameter, and on-line tension measurements and high-speed cine-photography are reported. Filament yarn motion in different regions of the texturing nozzle during dry and wet texturing was analyzed. During the study it was found that water acted as lubricant to reduce friction between the filaments in the wet texturing process as the filament yarn traveled through the nozzle enabling easier relative motion of the filaments resulting in enhanced entanglement. Wet texturing also reduced spin finish on the yarn surface, which in turn, caused an increase in static friction between the filaments of the textured yarn resulting in better fixing of the loops and consequently superior yarns.

EFFECTS OF GEOMETRY ON THE FLOW CHARACTERISTICS AND TEXTURING PERFORMANCE OF AIR-JET TEXTURING NOZZLES

Air-jet texturing is a versatile process for producing a range of synthetic yarns with a spun-like appearance, which are widely used for apparel and furnishing fabrics and industrial textiles. There is no universal nozzle capable of processing any supply yam of any linear density. The role played by nozzle geometry is still not fully understood. The experimental study presented here seeks to compare air flow characteristics and texturing data for nine nozzles under realistic texturing conditions as a basis for an improved understanding of the effect of nozzle geometry. Compressed air consumption results show that the nozzle flow is choked at air inlets; thus the nozzles behave as converging-diverging passages. The exit flow distribution is approximately axisym metric in all cases. Nozzle exit flow characteristics are typical of underexpanded jets with a ratio of jet exit plane static pressure to ambient pressure smaller than or equal to approximately 2. Textured yams with varying visual appearance were produced by different nozzles under identical processing conditions. Nevertheless, the strength properties of the yams were broadly the same, as was their increase in linear density. Of all test variables, the tension in the stabilizing zone was the only quantity to show some promise as a correlating parameter with texturing quality. Neither the presence of shock waves in the exit region nor the magnitude of the exit zone velocity correlated with texturing effectiveness. The texturing results of these trials highlight the fact that the current descriptions of air-jet texturing are not fully capable of explaining the subtle effects due to nozzle geometry and can at best be described as incomplete.

The effect of nozzle geometry on the flow characteristics of small water jets

Abstract A wide variety of processes make use of plain orifice nozzles. Fuel injectors for internal combustion engines incorporate these nozzles to generate finely atomized sprays. Processes such as jet cutting, jet cleaning, and hydroentanglement, on the other hand, use similar nozzles, but require coherent jets. The spray or jet characteristics depend on the stability of the flow emerging from the orifice. This problem has been extensively researched for nozzles with diameters above 300 μm. Much less is known about the characteristics of jets produced by nozzles with smaller diameters, where viscous effects and small geometric variations due to manufacturing tolerances are likely to play an increasing role. Results are presented of a wide-ranging investigation of geometry effects on the flow parameters and jet characteristics of nozzles with diameters between 120 and 170 μm. Nozzles with circular cross-section and conical, cone-capillary and capillary axial designs were investigated. For conical and cone-capillary nozzles, the effect of cone angle and effects due to interactions between adjacent nozzles in the multi-hole cone-capillary nozzles were studied. For capillary nozzles, the effects of diameter variations and inlet edge roundness for capillary nozzles were considered. Furthermore, the effect of varying the aspect ratio (ratio of major and minor axes) of elliptical nozzles was studied. Flowrate and jet impact force measurements were carried out to determine the discharge coefficient Cd, velocity coefficient Cv, and contraction coefficient Cc of the nozzles for supply pressures between 3 and 12 MPa. Visualizations of the jet flow were carried out in the vicinity of the nozzle exit in order to identify near-nozzle flow regimes and to study jet coherence. The relationship between nozzle geometry, discharge characteristics, and jet coherence is examined.

EFFECT OF NOZZLE GEOMETRY ON AIR-JET TEXTURING PERFORMANCE

This paper systematically investigates the effect of a number of geometric parameters on the texturing performance of air-jet texturing nozzles. In order to facilitate the research, an air-jet texturing nozzle with a rectangular cross section has been developed. The texturing performance of the nozzles is assessed by means of process observations and on-line measurement of stabilizing zone tension, and also by measuring the in creased linear density of the yams on textured yarn samples. Furthermore, instability, elongation at break, and tenacity are measured, and texturing quality is judged by visual inspections and examination of scanning electron microscopy images of the textured yarns. Tension in the stabilizing zone, increase in linear density, and to a somewhat lesser extent instability are reliable measures of texturing quality. The best texturing comes from nozzles with a slightly diverging main channel and a single air inlet hole located far from the nozzle exit. A curved diverging exit profile is essential for successful texturing. The results of the tests to determine the effect of air inlet angk are inconclusive and require further investigation.

Prediction of electromagnetic flowmeter characteristics

The active fluid volume in an electromagnetic flowmeter is taken as a finite circular cylinder. The magnetic potential and virtual-current potential in this volume are expanded in Fourier series in the variables theta (polar angle) and z (axial coordinate). Any magnetic field and virtual current is thus represented by the matrices anm and bpq respectively (being the coefficients in the double-Fourier series). Expressions for the matrix elements for a four-pole electromagnet and for one or two pairs of point electrodes are derived. A method is given for obtaining the matrix elements representing a magnetic field in terms of values of the normal component of the flux density over a cylinder coaxial with the tube. The axisymmetric weight function (and hence the fully developed flow sensitivity) can be derived by evaluating a double sum involving the elements anm and bpq and the coefficients Wnm(r) which are independent of magnetic field and virtual current. Calculations of the weight function and sensitivity have been made and examples of results given.

Internal Flow and Near-Orifice Spray Visualisations of a Model Pharmaceutical Pressurised Metered Dose Inhaler

The pressurised Metered Dose Inhaler (pMDI) has become the most prescribed drug delivery system for treating the respiratory diseases. However, the spray generation mechanism of these devices has not been extensively researched and there is very little information regarding the two-phase fluid dynamics associated with pre-atomisation inside the valve stem. The aim of the work presented in this paper is to provide high-quality, time-resolved imaging of the internal flow structures of pMDIs in an attempt to link the characteristics of the internal flow to external spray atomization processes. Visualisations of the aerosols in the near-orifice region findings from previous studies of commercial pMDIs and showed the following characteristics: (i) start-up transient, (ii) fully developed spray with slow spray density variations and (iii) rapid spray density pulsations with large droplet production. The results clearly highlighted the potential of optical diagnostics in the development of improved accounts of the state of the flow inside a pMDI valve and its relationship with drop formation.