Use of Temporal Irreversibility of Symbolic Time Series for Early Detection of Extinction in Thermal Pulse Combustors

A nonlinear fourth-order dynamic model of a thermal pulse combustor has been developed. In this work, the time series data generated by solution of the fourth order system is converted into a set of symbols based on the values of pressure variables. The key step to symbolization involves transformation of the original values to a stream of discretised symbols by partitioning the range of observed values into a finite number of regions and then assigning a symbol to each measurement based on the region in which it falls. Once all the measured values are symbolized, a symbol sequence vector consisting of L successive temporal observations is defined and its relative frequency is determined. In this work, the relative frequencies of different symbol sequences are computed by scanning the time series data in forward and reverse directions. The difference between the relative frequencies obtained in forward and reverse scanning is termed as "irreversibility" of the process. It is observed that for given alphabet and word sizes, the "irreversibility" increases as the system approaches extinction. The effects of different choices of alphabet and word sizes are also considered.

Numerical Simulation of Welding Induced Residual Stresses in a Circular Hollow Section T-Joint

Heavily welded circular hollow cross sections (CHS) are a common feature in civil structures such as draglines used in the mining industry and other off-shore structures. The sheer mass of the weldment and the application of intense heat generated during the welding process give birth to significant residual stresses in the structure. Often, residual stresses are high enough to act to accelerate factors such as corrosion, crack growth and fatigue. The objective of this research investigation was to predict welding generated residual stresses in a typical CHS T-Joint using Sysweld+, a welding Finite Element Analysis software. The T-joint is the first of the four lacings welded on to the main chord of a BE 1370 mining dragline cluster (designated All) of a type which is often used in the mining industry in Australia. This work examines a massive 3-dimensional geometry, which is on a much larger scale than those examined in existing studies. The paper presents the results of the simulation of residual stresses generated during the welding process in a single weld pass and compares them with the approach used in the commonly used document R6-Revision 4, Assessment of the Integrity of Structures Containing Defects.

Modeling of CO2 Gas Excitation Under CO2 Laser Irradiation

Lasers especially multiple laser beams demonstrate unique advantages as energy sources in diamond synthesis. However, the fundamental mechanisms involved in the laser-assisted processes are not Well understood. In a reported amazingly-fast multiple laser coating technique, CO2 gas is claimed as the sole precursor or secondary precursor, which remains poorly understood and unverified. The absorption coefficient changes under the irradiation of the multiple lasers are one of the keys to resolve the mysteries of multiple laser beam coating processes. This study investigates the optical absorption in CO2 gas at the CO2 laser wavelength. This resonance absorption process is modeled as an inverse process of the lasing transitions of CO2 lasers. The well-established CO2 vibrational-rotational energy structures are used as the basis for the calculations with the Boltzmann distribution for equilibrium states and the three-temperature model for non-equilibrium states. Based on the population distribution, our predictions of CO2 absorption coefficient changes as the function of temperature are in agreement with the published data.

Image-Based Modeling for Assessing Thermal Conductivity of Thermal Spray Coatings at Ambient and High Temperature

Thermal spray coatings are used extensively for protection of engineering components and structures in a variety of applications. Due to the nature of thermal spraying process, the coating thermal, mechanical, and electrical properties depend strongly on the coating microstructure, which consists of many individual splats, interfaces between the splats, defects and voids. The coating microstructure, in turn, is determined by the thermal spray process parameters. In order to relate coating process parameters to the final coating performance, then, it is desirable to relate coating microstructure to coating properties. In this work, thermal conductivity is used as the physical parameter of interest. Thermal conductivity of thermal spray coatings is studied by using an image analysis-based approach of typical coating cross sections. Three coating systems, yttria stabilized zirconia (YSZ), molybdenum, and Ni-5wt.%Al are explored in this work. For each material, thermal conductivity is simulated by using a microstructure image-based finite element analysis model. The model is then applied to high temperature conditions (up to 1200 °C) with a hot stage-equipped scanning electron microscope imaging technique to assess thermal conductivity at high temperatures. The coating thermal conductivity of metallic coatings is also experimentally measured by using a high-temperature laser flash technique.

An Investigation of Particle Injection and Resulting In-Flight Particle Behavior During Air Plasma Spraying

In this article we present our studies on the role of particle injection on the in-flight particle characteristics in an external orthogonally injected air plasma spray system. The influence of carrier gas on the in-flight particle state has been investigated, experimentally and using simulation, for Yttria Stabilized Zirconia (YSZ) thermal spray powder processed in an Ar-H2 plasma. Diagnostic tools such as IPP and SPT have been used to measure the plume characteristics and ensemble temperature while DPV-2000 has been used to measure the distributions of individual particle characteristics such as temperature, velocity and size, at the point of the maximum particle flux and at various points (square grid) in the plume cross-section. Three-dimensional simulations have been performed for the cases presented in the experiments. Specifically, the effects of carrier gas flow rate on the in-flight particle characteristics were studied at multiple stand-off distances. Simulation results agree well with the experimental observation that the particle velocity and temperature will increase with the plume angle and then decrease after reaching a maxima for a given process parameter combination and stand-off distance. This maxima has been observed at the same plume angle for different process parameter combinations. The results of this study are currently being used to 'optimize' the particle injection and trajectory, which enables better understanding of the influence of plasma forming and stabilizing parameters (gas flows and arc current) on the in-flight particle behavior.

Effect of Thin Film Condensation on Thermal Performance of Oscillating Heat Pipes

Self-sustainable motions of the slug flow in oscillating heat pipes have been investigated in the paper. Thin film condensation in the capillary channels of the condenser of the oscillating heat pipes was studied. Instability of the thin liquid film on the characteristics of heat pipes was analysed. The extra thermal resistance caused by the thickness of the thin liquid film was taken into account for the numerical simulation of the oscillatory motions of the slug flow in the heat pipes. Saturated temperatures and pressures of the working fluid in the condenser were obtained. Thermoacoustic theory was applied to calculate heat transport through the adiabatic section of the heat pipes. Experimental studies were carried out to understand the heat transfer behaviours of heat pipes. One heat pipe with the working fluid of HFC-134a was evaluated. The heat pipe is made of aluminium plate and has the width of 50 mm and thickness of 1.9 mm. Numerical and experimental results relevant to the heat transport capability of the heat pipe were analysed and compared.

Spot Cooling of Local-High Heat Load by High-Velocity Thin Liquid Flow

Spot cooling of local-high heat load by high-velocity thin liquid flow was examined experimentally. Steady state experiments were conducted using a copper thin-film and rectangular sub-millimeter-channels. The width of the test channel was 2 mm. The heights of the test channel were 0.5 and 0.2 mm. The width and length of a test heater was 2 mm and 2 mm, respectively. The test liquid was degassed pure water. The liquid velocities were 1.5, 5, 10 and 15 m/s. The liquid subcooling was 20 K. Location of the heater in the test channel also was an experimental parameter: the positions of the heater from the exit of the test channel were 30 mm (middle) and 0 mm (exit). Experimental results showed that the maximum heat flux (CHF or cooling limit) during experiment with the heater at exit of the test channel was similar to that with the heater at middle of the test channel: the maximum heat flux was independent of the position of heater in the test channel. The maximum heat flux occurred when bubbles coalesced together or a dry patch appeared on the heater. The coalescence bubble covered over the heater was observed at CHF in condition of low liquid velocity. For condition of high liquid velocity, a dry patch appeared on the heater, and then the dry region extended over the heater to come around the CHF. The maximum heat flux (critical heat flux) was about 8 MW/m2 in a range of present experiments. The CHF for the present sub-millimeter channel was similar to that for conventional channel. Furthermore, models were proposed using heat transfer around a coalesced bubble and at a dry patch on a heater.

Effects of H2-Enrichment on the Propagation Characteristics of CH4-Air Flames

The propagation of H2-enriched CH4-air triple flames in a nonpremixed jet is investigated numerically. The flames are ignited in a nonuniform jet-mixing layer downstream of the burner. A comprehensive, time-dependent computational model is used to simulate the transient ignition and flame propagation phenomena. The model employs a detailed description of methane-air chemistry and transport properties. Following ignition a well-defined flame is formed that propagates upstream towards the burner along the stoichiometric mixture fraction line. As the flame propagates upstream, the flame speed, which is defined as the normal flamefront velocity at the leading edge with respect to the local gas velocity, increases above or decreases below to the corresponding unstretched laminar flame speed of the stoichiometric planar premixed flame. Although the flame curvature varies as a function of axial position, the flame curvature remains nearly constant for a given flame. As hydrogen is added to the fuel stream the flame curvature during flame propagation remains nearly constant. During the flame propagation process, the hydrodynamic stretch dominates over the curvature-induced stretch. Hydrogen increases the heat release and the component of the velocity perpendicular to the flame increases across the surface, whereas the tangential component remains unchanged. This jump in the perpendicular velocity component bends the velocity vector toward the stoichiometric mixture fraction line. This redirection of the flow is accommodated by the divergence of the streamlines ahead of the flame, resulting in the decrease of the velocity and increase in the hydrodynamic stretch.

Visualization of Vertical Bubble Coalescence and Detachment Under the Influence of a Nonuniform Electric Field

The effect of a nonuniform electric field on the formation, coalescence and detachment of air bubbles injected into a stagnant, isothermal liquid through an orifice is studied to identify characteristic bubble behavior patterns. The results of the experimental visualization suggest significant differences in bubble shape and size caused by the electric field. The electric field was applied between a flat, circular and horizontal ground electrode and a spherical, off-axis top electrode. During formation the bubble was tilted towards or away from the upper electrode under the influence of the electric field. The direction of the tilt alternated (even in a single experiment), however, in the majority of the cases the bubble trajectory tilted towards the top electrode. The detachment frequency increased under the influence of the electric field, which indicates decreased bubble volume for lower volume flow rates. The effect of the electric field on vertical bubble coalescence was analyzed and quantified in terms of the detachment time.

Microscale Study of the Boiling Process in Low-Surface-Tension Fluids

A novel MEMS device has been developed to study some of the fundamental issues surrounding the physics of the nucleation process intrinsic to boiling heat transfer. The device generates bubbles from an artificially generated nucleation site centered within a radially distributed sensor array. The array is fabricated within a Silicon/Benzocyclobutene (BCB) composite wall, with the capability to measure surface temperature with an unprecedented radial resolution of 22-40 μm underneath and around the bubble. The temperature data enabled numerical calculation of the surface heat flux with the same spatial resolution as of the temperature data. The temperature of the sensors and the synchronized images of the bubbles were recorded with a sampling frequency of 8 kHz. The unique data determined in this study were used to address some of the unresolved issues regarding the boiling process including 1) dynamics of bubble growth and associated heat transfer processes and 2) the bubble's role in surface heat transfer during the boiling process.