A commonly observed welding defect that characteristically occurs at high welding speeds is the periodic undulation of the weld bead profile, also known as humping. The occurrence of humping limits the range of usable welding speeds in most fusion welding processes and prevents further increases in productivity in a welding operation. At the present time, the physical mechanisms responsible for humping are not well understood. Thus, it is difficult to know how to suppress humping in order to achieve higher welding speeds. The objectives of this study were to identify and experimentally validate the physical mechanisms responsible for the humping phenomenon during high speed gas metal arc (GMA) welding of plain carbon steel. A LaserStrobe video imaging system was used to obtain video images of typical sequences of events during the formation of a hump. Based on these recorded video images, the strong momentum of the backward flow of molten metal in the weld pool that typically occurred during high speed welding was identified as the major factor responsible for the initiation of humping. Experiments with different process variables affecting the backward flow of molten weld metal were used to validate this hypothesis. These process variables included welding speed, welding position and shielding gas composition. The use of downhill welding positions and reactive shielding gases was found to suppress humping and to allow higher welding speeds by reducing the momentum of the backward flow of molten metal in the weld pool. This would suggest that any process variables or welding techniques that can dissipate or reduce the momentum of the backward flow of molten metal in the weld pool will facilitate higher welding speeds and productivity.

No abstract

The flow field created by two impinging liquid jets in a cylindrical chamber is investigated using particle tracing, laser Doppler anemometry, and three-dimensional numerical simulation. The jet Reynolds number (based on the mean jet velocity and diameter) range is from 50–300. Results show that there is a threshold value of the Reynolds number beyond which the flow exhibits oscillations. Computer simulations agree well with the experimental visualizations, for both the overall observed flow pattern and a quantitative measure of the oscillations.

No abstract

he flow field of opposed axisymmetric jets in a confined cavity has been examined for instabilities due to various geometrical and T fluid parameters. Practical examples where such configurations are used are the mix head for reaction injection molding (RIM) and the side dump combustor. RIM involves a rapid mixing process of two or more liquid pre-polymers in a mix chamber that flow into a mold to form a solid polymer part. For most RIM systems, mixing is carried out through directly opposed jet-to-jet impingement of the reactant streams.Stable efficient mixing is paramount to the quality of parts produced with RIM as poorly mixed materials will cause spatial variation in the physical properties and variation in the physical appearance of a finished molded part. Typically, mix chambers are cylindrical and 10 to 15 m m in diameter using nozzles with openings of 0.5 to 3 m m to form the jets near the closed end of the chamber (Figure 1). Fluid issues from the opposed nozzles and may create an impingement region in the mix chamber depending on the geometry and fluid parameters. The closed end is the face of the clean-out piston used to remove the polymer from the nozzle region after completion of the shot. The mixed fluid then leaves the open end of the chamber to the mold. A dimensionless group used to characterize the flow is the Reynolds number, Re,, which is based on the nozzle diameter d, the fluid kinematic viscosity and the volumetric flow rate Q through the nozzle (Q = xd2/4UOvg).Previous visualization studies have shown the existence of an oscillating flow field under some geometric and fluid configurations. They found steady impingement below Re, = 75 (D' = 10.67, D = 25.4 mm, $I = 180"), and a t values higher than this, an instability in the impingement surface began to grow and oscillate. Experimentally, above Red = 150 the jets would not directly impinge, although the computer simulations showed steady oscillations from Re, = 100 to 300. Our interest is in quantifying the range of oscillation parameters and to determine the effect on the stability or performance of the opposed jet mix head configuration. Visualizations of the flow patterns by Sandell et al. (1985) in a mix chamber ( D ' = 5.5, 0 = 30°, H' = 0.73) has shown an oscillating flow above Re, = 150. Generally, the flow became less steady when increasing the Re, or increasing the distance to the head region.In a similar study, Nosseir and Behar (1 986) used a rectangular model with opposed (@ = 180") rectangular jets at an average Re, of 3000 based on nozzle width to simulate the side dump combustor. Oscillations in the flow were detected when the dyed fluid interrupted a laser beam at aThe flow of jets in confining enclosures has significant application in many engineering processes. In particular, the impingement of axisymmetric jets in a confined space has been examined using flow visualization, laser Doppler anemometry, and numerical simulations. Several flow regions were found; stable steady, regular oscillatory, and irregular oscillatory...