Numerical Simulations and Experimental Study of Hot Core Distortion Phenomenon in Aluminum Casting

This paper presents the results of experimental and numerical studies of hot distortion phenomenon in the phenolic urethane cold box systems. Dual Pushrod Dilatometer has been used to measure a thermal expansion/contraction of phenolic urethane cold box sand core specimens at temperature range from 25° C to 800° C. The high temperature tensile tests showed that the tensile strength of the phenolic urethane cold box silica sand cores is significantly affected by the bench life, temperature and binders level. High temperature hot distortion furnace tests on cylindrical cores showed that some aluminum coatings increase the temperature limit when distortion starts, but can’t prevent it. The hot distortion test aluminum castings showed that regardless of the application of coating, the type of coating, and anti-veining additives, all cores (silica sand) with density less than the density of the molten metal (aluminum alloy) were significantly distorted. Numerical simulations of the liquid metal flow around the cylindrical sand core and analysis of dynamic forces acting on the core during fill process showed that a buoyancy force is the major contributor to the hot distortion. It is concluded that the one of the solutions in preventing the hot distortion of sand cores is increasing their weigh, which will balance the buoyancy force and will bring the resultant force to the minimum. The hot distortion test castings using zircon sand cores (both coated and non-coated) with density almost equal to the density of the molten aluminum proved our predictions, and hot distortion has been prevented.

Modelling of Slurry-Handling Piping Element Under Interference Conditions

The paper presents a new approach to predict the two-phase performance of jet-pumps under interference conditions. We limit our study mainly to diffuser and transport regions of the jet pump. The five essential pre-requisites which form the backbone of our approach are a fairly generalized and accurate approach to (i) solid-fluid interaction, (ii) particle diffusion under generalized flow field, (iii) friction factor-Reynolds number equation, (iv) solid-fluid flow through ducts and (v) mixing of primary and secondary jets using the approach of Wang et al. [1] based on boundary layer concept. The extensive experimental data of several research workers along with fresh data generated on specially designed test-rig support the new approach.

Numerical Study on Substrate Remelting, Flow, and Resolidification in Microcasting

A large and highly superheated molten droplet impacting onto the substrate during the microcasting was studied numerically. In this study, same material for both the droplet and the substrate was considered. Numerical model including the complex fluid dynamics of droplet, interfacial thermal contact resistance, and substrate remelting, as well as the flow in the substrate has been developed. Numerical simulations of a microcasting experiment were conducted with the different thermal contact resistances. The results of simulations show that the spreading factor and substrate remelting agreed well with the experimental data under the assumption of an appropriate thermal contact resistance. It is also found that the thermal contact resistance plays an important role not only in droplet spreading arrest but also in the determination of substrate remelting volume and remelting front shape. The effects of droplet impacting velocity, superheat and substrate temperature were also investigated.

Mass Transfer During Fluid Sphere Dissolution in an Alternating Electric Field

In this study we considered mass transfer in a binary system comprising a stationary fluid dielectric sphere embedded into an immiscible dielectric liquid under the influence of an alternating electric field. Fluid sphere is assumed to be solvent-saturated so that an internal resistance to mass transfer can be neglected. Mass flux is directed from a fluid sphere to a host medium, and the applied electric field causes a creeping flow around the sphere. Droplet deformation under the influence of the electric field is neglected. The problem is solved in the approximations of a thin concentration boundary layer and finite dilution of a solute in the solvent. The thermodynamic parameters of a system are assumed constant. The nonlinear partial parabolic differential equation of convective diffusion is solved by means of a generalized similarity transformation, and the solution is obtained in a closed analytical form for all frequencies of the applied electric field. The rates of mass transfer are calculated for both directions of fluid motion — from the poles to equator and from the equator to the poles. Numerical calculations show essential (by a factor of 2–3) enhancement of the rate of mass transfer in water droplet–benzonitrile and droplet of carbontetrachloride–glycerol systems under the influence of electric field for a stagnant droplet. The asymptotics of the obtained solutions are discussed.

An Investigation of Droplet Diffusion in Isotropic Turbulence With Digital Holographic PIV

In-line digital holography is utilized to measure the Lagrangian trajectory of droplets in locally isotropic turbulence. The objective of these measurements is to determine the diffusion rate of these droplets as a function of density ratio between the continuous and dispersed phases, Stokes number and turbulence level relative to the quiescent settling/rise velocity of the droplets. The present experiments are conducted using diesel fuel with diameters of 0.5–2 mm, specific gravity of 0.85 and Stokes number in the 0.2 to 5 range. The droplets are injected into a 50 × 50 × 50 mm sample volume located in the center of a 160 1 tank. The turbulence is generated by four spinning grids, located symmetrically in the corners of the tank. Planar PIV has been used to characterize the turbulence prior to the experiments. A time series of in-line digital holograms is recorded at 2000 frames per second using a 1000×1000 pixel digital camera by back illuminating the sample volume with a collimated laser beam. Numerical reconstruction generates a time series of high-resolution images of the droplets and tracer particles throughout the sample volume. Subsequent analysis is used to obtain the velocity along the droplet trajectory. Lagrangian correlations can then be used for calculating the diffusion rate of these droplets. In a smaller sample volume, with a 15×15 mm cross section, and by using localized seeding, we can also simultaneously measure the droplet velocity along with the velocity of the fluid in the vicinity of this droplet. The results provide statistics on the correlations between the droplet and fluid velocities.

Experimental Analysis of an Air-to-Air Heat Exchanger for Use in a Refrigeration Brayton Cycle

The refrigeration Brayton cycle, which has been used extensively in various industries, has an excellent potential for use in automotive air conditioning applications. However, the air-cycle system has a couple of drawbacks including fog generation and low cycle efficiency. In this research project, an air-to-air heat exchanger called a ‘mixer’ is designed and used at the outlet of a refrigeration Brayton cycle. The primary function of the mixer is to remove moisture from the secondary warm airflow into the system. Successful moisture removal from the secondary airflow results in achieving the second function of fog dissipation from the primary cold airflow. In order for the system to perform appropriately, the moisture removal rate must be kept at the highest possible rate. The experimental results from this research project reveal that to enhance moisture removal rate, one may either increase the primary cold airflow rate, decrease the secondary warm airflow rate, or the combination of the above airflow adjustments. Furthermore, based on experimental results, one may speculate that there is an optimum point in decreasing the secondary airflow rate. However, in increasing the primary airflow rate, one must be aware of the pressure drop through the cold side of the mixer as the higher pressure drop results in higher power consumption for the Brayton cycle. It is important to point out that appropriate levels of the primary and secondary airflows impacts the mixer effectiveness, and that for a constant cold airflow rate, decreasing the warm airflow rate below the cold airflow rate results in higher effectiveness.

Nucleation and Growth of Bubble With the Effects of Solute Gas

Bubble nucleation and growth of formed nuclei are investigated by molecular dynamics simulation in Lennard-Jones liquid with gas impurities. For the onset of nucleation from bulk, it has been found that a dissolved gas whose interaction is very weak and whose diameter is larger than that of solvent molecules makes the action to cause composition fluctuation or local phase separation so strong that the nucleation probability predicted from pressure change becomes qualitatively wrong. It has been confirmed that this wrong prediction is generally explained by introducing the superheat ratio nondimensionalized by saturation pressure and spinodal pressure. For the growth stage of formed bubble nuclei, it is observed that the coalescence of nuclei occurs when a weak-interaction gas is dissolved at a high concentration while the competition between neighbor nuclei is dominant in the case of pure liquid.

Transport Phenomena and Their Effect on Weld Quality in GMA Welding of Aluminum Alloys

Gas metal arc welding (GMAW) of aluminum alloys has recently become popular in the auto industry to increase fuel efficiency of a vehicle. In many situations, the weld is short (say, less than two inches) and the “end effects” become very critical in determining the strength of the weld. At the beginning stage of the welding, when the metal is still “cold”, which is frequently called cold weld, limited weld penetration occurs. On the other hand, at the ending stage of the welding, a “crater” is formed involving micro-cracks and micro-pores. Both the cold weld and the crater can significantly decrease the strength of the weld and are more severe for aluminum alloys as compared to steels. Hence, there are strong needs to improve the GMAW process in order to reduce or eliminate the aforementioned end effects. In this paper, both mathematical modeling and experiments have been conducted to study the beginning stage, ending stage, as well as the quasi-steady-state stage of GMA welding of aluminum alloys. In the modeling, a three-dimensional model using the volume-of-fluid (VOF) method is employed to handle the free surfaces associated with the impingement of droplets into the weld pool and the weld pool dynamics. Transient weld pool shapes and the distributions of temperature and velocity in the weld pool are calculated. The predicted solidified weld bead shapes, including weld penetration and/or reinforcement, are in agreement with experimental results for welds in the aforementioned three stages. It was found that the thickness of the molten weld pool is smaller and there is no vortex developed, as compared to steel welding. The lack of penetration in cold weld is due to the lack of pre-heating by the welding arc. Three techniques are proposed and validated numerically to improve weld penetration by increasing the energy input at the beginning stage of the welding. The crater formation is caused by rapid solidification of the weld pool when the welding arc is terminated. By reducing welding current and reversing the welding direction before terminating the arc, the weld pool is maintained “hot” for a longer time allowing melt flow to fill-up the crater. This method is validated experimentally and numerically to be able to eliminate the formation of the crater and the associated micro-cracks.

Effect of Fluid Loading on the Performance of Wicked Heat Pipes

A study was performed to experimentally characterize the effect of fluid loading on the heat transport performance of wicked heat pipes. In particular, experiments were performed to characterize the performance of heat pipes with insufficient fluid to saturate the wick and excess fluid for a variety of orientations. It was found that excess working fluid in the heat pipe increased the thermal resistance of the heat pipe, but increased maximum heat flux through the pipe in a horizontal orientation. The thermal performance of the heat pipe was reduced when the amount of working fluid was less than required to saturate the wick, but the maximum heat flux through the heat pipe was significantly reduced at all orientations. It was also found in this case the performance of this heat pipe deteriorated once dry-out occurred.

Unsteady Cavitating Flow Around an Inclined Rectangular Cylinder

Unsteady characteristics of cavitating flow around an inclined rectangular cylinder with a width to height ratio of 8.0 were experimentally investigated for various angles of attack and cavitation numbers. Measurements of fluid dynamic force and surface pressure were made and the cavity configuration was observed with a camera. Especially considered are the self-oscillating unstable flow characteristics along with the time variation of cavity configuration. It is found that a severe vibration occurs at some cavitation number, in which the attached cavity is formed in the separation bubble. As the cavitation number further decreases, the low frequency fluctuation of flow occurs.