30,390 research outputs found
Study of plate-fin heat exchanger and cold plate for the active thermal control system of Space Station
Plate-fin heat exchangers will be employed in the Active Thermal Control System of Space Station Freedom. During ground testing of prototypic heat exchangers, certain anomalous behaviors have been observed. Diagnosis has been conducted to determine the cause of the observed behaviors, including a scrutiny of temperature, pressure, and flow rate test data, and verification calculations based on such data and more data collected during the ambient and thermal/vacuum tests participated by the author. The test data of a plate-fin cold plate have been also analyzed. Recommendation was made with regard to further tests providing more useful information of the cold plate performance
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Study of Ceramic Slurries for Investment Casting with Ice Patterns
Ice patterns generated by rapid freeze prototyping or a molding process can be used to
make ceramic investment molds for metal castings. Due to the use of ice, the ceramic slurries
must be poured around the pattern and cured at sub-freezing temperatures. Success of this
process depends greatly on the mold strength after the gelation of the slurries. This paper
describes the experimental results of the mold strength after the gelation of the slurries under
different compositions. The parameters considered include mixing time, alumino-silicate vs.
fused silica ratio, volume of binder, and volume of catalyst. The strength of the gelled slurries is
examined by breaking test bars on a four-point bending apparatus. Weibull modulus for each
trial is calculated based on the breaking strength from four-point bend tests. Analysis of variance
for breaking strength and Weibull analysis is performed to evaluate the significance of the effect
of each parameter. The casting of a bolt is used to demonstrate that metal castings of complex
geometry can be fabricated using investment casting with ice patterns.Mechanical Engineerin
Modeling and Validation of Temperature and Concentration for Rapid Freeze Prototyping
Rapid Freeze Prototyping is a solid freeform fabrication process that uses water as the main build
material in a cold environment to create three-dimensional parts. A eutectic sugar-water solution
(C6H12O6 – H2O) has been used as a sacrificial material in order to create complex 3D parts with
features such as overhangs. A study of the interaction of the build and support materials is
presented in this paper. The temperature of both materials during deposition and subsequent
cooling is modeled using a semi-empirical model and a theoretical model. A concentration
model is used to predict the concentration in the fabricated parts around the interface of the two
materials with predicted temperatures as input. Experiments are conducted to validate both the
temperature and concentration models.Mechanical Engineerin
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Study on Incorporating Support Material in Rapid Freeze Prototyping
Rapid Freeze Prototyping (RFP) is a rapid prototyping method that uses water freezing
into ice to make three-dimensional parts. Each layer of a geometry is deposited and allowed to
freeze before the next layer is added. Using a support material in RFP is a relatively new
addition to the process. Validating the successful use of a support material in conjunction with
the main build material of water is presented in this paper. The support material selected for use
is a eutectic sugar solution. The selection criteria, properties, and characteristics of the support
material are discussed. Of particular interest is the diffusion between the support and main build
material, which must be minimized to an acceptable level for producing good quality,
reproducible, complex parts.Mechanical Engineerin
Shell Cracking In Investment Casting with Laser Stereolithography Patterns
This paper presents an investigation of ceramic shell cracking during the burnout process
in investment casting with internally webbed laser stereolithography patterns. We hypothesize
that shell cracking will occur when the rupture temperature of the ceramic shell is lower than
both the glass transition temperature of the pattern material and the web· link buckling
temperature. The hypothesis is validated by our experimental observations which confirm the
numerical predictions from our fillite element analysis. This provides a basis for design of the
internal web geometry of a lithography pa.tternand evaluation ofthe burnolltprocesswithsuch a
pattern. We show the shell cracking and web link buckling temperatures to be functiollsofthe
pattern geometry (including the cross~sectional dimensions and span length of the web link) and
the shell thickness.Mechanical Engineerin
The Finite-time Ruin Probabilities of a Bidimensional risk model with Constant Interest Force and correlated Brownian Motions
We follow some recent works to study bidimensional perturbed compound Poisson
risk models with constant interest force and correlated Brownian Motions.
Several asymptotic formulae for three different type of ruin probabilities over
a finite-time horizon are established.
Our approach appeals directly to very recent developments in the ruin theory
in the presence of heavy tails of unidimensional risk models and the dependence
theory of stochastic processes and random vectors.Comment: 25page
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