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.

Case Study of Impeller Profile Considering Additional Parameters

In the previous paper, the optimum meridian profile of impeller was obtained for various specific speed by means of five shape factors. In this paper, the optimum meridian profile of impeller is obtained by means of eight shape factors. The basic five shape factors are inlet relative flow angle β1, turning angle Δβ, axial velocity ratio kc = Cm2/Cm1 impeller diameter ratio kd = D1c/D2c and outlet hub-tip ratio ν2 (β1 and Δβ are in mid span stream surface). The additional three parameters are three stream lines solidity (tip solidity σt, mid span solidity σc, and hub solidity σh). The blade length of impeller meridian profile is able to obtain by additional three parameters. The method of optimization is the calculation of hydraulic efficiency and suction specific speed in all combinations of eight shape parameters. The number of five shape factors are expressed by Nβ1, NΔβ, Nkc, Nkd, Nν2. The number of calculations is expressed by Nβ1 × NΔβ × Nkc × Nkd × Nν2. For example, Nβ1 = NΔβ = Nkc = Nkd = Nν2 = 40, the number of calculations is about 100000000. The calculation time is about 2 hours. The best parameters are selected in 100000000 cases. In case of eight shape factors, the number of calculation is Nβ1 × NΔβ × Nkc × Nkd × Nν2 × Nσt × Nσc × Nσh. Nβ1 = NΔβ = Nkc = Nkd = Nν2 = Nσt = Nσc = Nσh = 10, the number of calculation is 100000000. In this case, the calculation time of eight shape factors is as same as that of five shape factors. By means of this method, the more detailed optimum mixed flow impeller meridian shape is obtained. In case study, the best 1000 optimum meridian profiles and the best design parameter are selected for few kinds of specific speed using eight dimensional optimum method. In the previous paper, the mixed flow angle on tip meridian stream line isn’t able to be decided by this optimization using diffusion factor. But, in this paper, the mixed flow angle is able to be decided by the number of blade and optimum solidity. As the best solidity of three stream lines is obtained, the axial coordinates of impeller inlet and outlet are obtained. The more detailed optimum mixed flow impeller meridian shape is drawn.

CHF in Vertical Round Tubes With Uniform Heat Flux

In recent years it is well known that models based on the local condition hypothesis give significant correlations for the prediction of CHF (Critical Heat Flux), using only few local variables. In this work, a study was carried out to develop a generalized CHF correlation in vertical round tubes with uniform heat flux. For this analysis, a CHF database that composed of over 10,000 CHF data points, which were collected from 12 different sources, was used. The actual data used in the development of this correlation, after the elimination of some questionable data, consisted of 8,951 data points with the following parameter ranges: 0.101 ≤ P (pressure) ≤ 20.679 MPa, 9.92 ≤ G (mass flux) ≤ 18,619.39 kg/m2s, 0.00102 ≤ D (diameter) ≤ 0.04468 m, 0.03 ≤ L (length) ≤ 4.97 m, 0.11 ≤ qc (CHF) ≤ 21.42 MW/m2, and −0.87 ≤ Xe (exit qualities) ≤ 1.58. The result of this work showed that regardless of various flow patterns and regimes that exist in the wide flow conditions, the prediction of CHF can be made accurately with few major local variables: the system pressure (P), tube diameter (D), mass flux of water (G), and true mass flux of vapor (GXt). The new correlation was compared with 5 well-known CHF correlations published in world literature. The new correlation can predict CHF within the root mean square error of 13.44% using the heat balance method with average error of −1.34%.

Experimental Investigations Concerning Erosive Aggressiveness of Cavitation at Different Test Configurations

The experimental results, which will be presented in this paper, demonstrate the significant influence of the flow velocity, respectively the rotational speed, on the erosive aggressiveness of cavitating flows. On two of the three investigated test objects, cavitation erosion can only be observed in the initial stage by the so-called pit-count evaluation method. Developed erosion with mass loss is impossible to measure because of the very long duration until mass loss appears. The third test rig generates a very aggressive type of cavitation, so that mass loss, depending on the tested material, will appear after relatively short durations. In addition, the initial stage of cavitation erosion can be observed. Three different techniques were applied to investigate cavitation erosion in the initial and developed stage. Thereby, the capability of methods to quantify erosive effects in dependence of influencing operating parameters has been proven.

Cross-Sectional Imaging of the Liquid Film in Horizontal Two-Phase Annular Flow

Planar laser induced fluorescence (PLIF) was applied to horizontal air/water two-phase annular flow in order to clearly image the liquid film and interfacial wave behavior at the top, side and bottom of the tube. The visualization section was fabricated from FEP, which has nearly the same refractive index as water at room temperature. This index-matched test section was used to allow imaging of the water to within approximately 10 microns of the 15.1 mm I.D. tube wall. A small amount of dye was added to the water with a peak excitation wavelength near that of a pulsed Nd:YAG laser (532 nm). The laser system generated an approximately 5 ns pulsed light sheet at 30 Hz. Images of the liquid film were captured using a digital video camera with a macro lens for a resolution of about 8.2 microns/pixel. Cross-sectional data at 68 annular flow conditions were obtained. The observations of the liquid film between waves indicated that the film thickness was relatively insensitive to both gas and liquid flow in the annular regime, confirming film thickness measurements reported elsewhere. In addition, the structure of waves changes significantly from wavy-annular, where peaked or cresting waves dominate, to fully annular, where the waves are much more turbulent and unstructured. The wave height decreases with increased gas flow and is relatively insensitive to increased liquid flow in the annular regime. The entrainment of gas in the liquid by the waves is very apparent from these images. Although the precise entrainment mechanisms are not entirely clear, a viable folding action mechanism is proposed. The visualization results will be discussed in relation to both conceptual and computational annular flow modeling.

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.