Processing of Cellular Polyurethane by Ultrasonic Excitation

1992 ◽  
Vol 114 (3) ◽  
pp. 323-328 ◽  
Author(s):  
H. Park ◽  
J. R. Youn
Keyword(s):  
Size Distribution ◽  
Bubble Size ◽  
Negative Pressure ◽  
Bubble Nucleation ◽  
High Rate ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Foam Structure ◽  
Pressure Equilibrium ◽  
Theoretical Results

Processing of cellular polyurethane was investigated experimentally and theoretically to examine the possibility of ultrasonic foaming. Polyol resin was supersaturated with nitrogen and ultrasonic excitation was applied for copious bubble nucleation. The ultrasonic excitation resulted in good foam structure whose size distribution was uniform and less than 100 μm. The ultrasonic excitation was modelled by utilizing the classical nucleation theory to predict the rate of nucleation. Theoretical results suggest that a high rate of nucleation will be obtained if the ultrasonic excitation generates large enough negative pressure. Final bubble size was calculated by considering the pressure equilibrium between inside and outside of the bubble.

Ultrasonic Bubble Nucleation in Reaction Injection Moulding of Polyurethane

1994 ◽  
Vol 208 (2) ◽  
pp. 121-128 ◽  
Author(s):  
W J Cho ◽  
H Park ◽  
J R Youn
Keyword(s):  
Injection Moulding ◽  
Saturation Pressure ◽  
Bubble Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Ultrasonic Activation ◽  
Cluster Theory ◽  
Foam Structure ◽  
Ultrasonic Effect

Ultrasonic foam processing of polyurethane for reaction injection moulding (RIM) was studied experimentally to investigate feasibility of ultrasonic bubble nucleation in polyurethane. Bubble nucleation was also studied theoretically to predict the rate of nucleation. Classical nucleation theory and cluster theory have been employed for explanation of the nucleation phenomena. A polyol resin was saturated with nitrogen at various pressures and the pressure was released slowly in order to generate supersaturated resin. Other components of the selected polyurethane system were added to the supersaturated resin and ultrasonic disruption was applied to the system producing enhanced nucleation. The ultrasonic excitation created a good foam structure even at a low saturation pressure around 0.15 MPa (1.5 atm). The effect of the ultrasonic activation on the bubble nucleation was considered and included in the nucleation theories. The cluster nucleation theory along with consideration of the ultrasonic effect predicted a higher rate of nucleation than the classical nucleation theory for the same condition.

Analytical Investigation on the Homogeneous Nucleation in a Mono-Component and Bi-Component Droplet

2019 ◽  
Author(s):  
Xi Xi ◽  
Hong Liu ◽  
Chang Cai ◽  
Ming Jia ◽  
Weilong Zhang
Keyword(s):  
Nucleation Rate ◽  
Homogeneous Nucleation ◽  
Ambient Pressure ◽  
Analytical Investigation ◽  
Bubble Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Liquid Temperature ◽  
Wide Range ◽  
Good Agreement

Abstract The work attempts to analyze the performance of homogeneous nucleation by using the non-equilibrium thermodynamics theory and the classical nucleation theory. A nucleation rate graph was constructed under a wide range of operating temperature conditions. The results indicate that the superheat limit temperature (SLT) estimated by the modified homogeneous nucleation sub-model is in good agreement with the experimental results. The nucleation rate increases exponentially with the liquid temperature rise when the liquid temperature exceeds the SLT under atmospheric pressure. The superheated temperature needed to trigger the bubble nucleation decreases with the elevated ambient pressure.

Numerical Simulation of Subcooled Flow Boiling Using a Bubble Tracking Method

2016 ◽  
Author(s):  
Tomio Okawa ◽  
Naoki Miyano ◽  
Kazuhiro Kaiho ◽  
Koji Enoki
Keyword(s):  
Numerical Simulation ◽  
Size Distribution ◽  
Void Fraction ◽  
Bubble Size ◽  
Flow Boiling ◽  
Bubble Nucleation ◽  
Bubble Size Distribution ◽  
Subcooled Flow Boiling ◽  
Tracking Method ◽  
Subcooled Flow

The process of bubble nucleation in subcooled flow boiling was visualized using a high speed camera to show that the bubble size can be significantly different between the nucleation sites. However, the bubble size is usually assumed constant in the numerical simulation of subcooled flow boiling. To explore the effect of the bubble size distribution on the void fraction in subcooled flow boiling, numerical simulations were performed using a bubble tracking method in which the size and position of each bubble are calculated individually using a Lagrangian coordinates. In the present simulation, the void fraction was greater when the bubble size distribution was taken into consideration. Since the bubble tracking method requires many correlations, further improvement is necessary. The present numerical results however indicate that the bubble size distribution should be taken in to consideration to evaluate the void fraction in subcooled flow boiling accurately.

scholarly journals Parameterizing the competition between homogeneous and heterogeneous freezing in ice cloud formation – polydisperse ice nuclei

2009 ◽  
Vol 9 (3) ◽  
pp. 10957-11004 ◽  
Author(s):  
D. Barahona ◽  
A. Nenes
Keyword(s):  
Size Distribution ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Average Error ◽  
Ice Crystal ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Wide Range ◽  
Model Equations

Abstract. This study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from precursor aerosol and ice nuclei with any size distribution and chemical composition. The parameterization provides an analytical solution of the cloud parcel model equations and accounts for the competition effects between homogeneous and heterogeneous freezing, and, between heterogeneous freezing in different modes. The diversity of heterogeneous nuclei is described through a nucleation spectrum function which is allowed to follow any form (i.e., derived from classical nucleation theory or from empirical observations). The parameterization reproduced the predictions of a detailed numerical parcel model over a wide range of conditions, and several expressions for the nucleation spectrum. The average error in ice crystal number concentration was −2.0±8.5% for conditions of pure heterogeneous freezing, and, 4.7±21% when both homogeneous and heterogeneous freezing were active. Apart from its rigor, excellent performance and versatility, the formulation is extremely fast and free from requirements of numerical integration.

Homogeneous Bubble Nucleation Predicted by a Molecular Interaction Model

1991 ◽  
Vol 113 (3) ◽  
pp. 714-721 ◽  
Author(s):  
Ho-Young Kwak ◽  
Sangbum Lee
Keyword(s):  
Bubble Formation ◽  
Interaction Model ◽  
Bubble Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Good Prediction ◽  
Evaporation Time ◽  
Modified Model ◽  
Superheat Limit ◽  
Good Agreement

The homogeneous bubble nucleation of various hydrocarbons was estimated by the modified classical nucleation theory. In this modification, the kinetic formalism of the classical theory is retained while the surface energy needed for the bubble formation is calculated from the interaction energy between molecules. With a nucleation rate value of Jnc =1022 nuclei/cm3s, this modified model gives a very good prediction of the superheat limit of liquids. In another test of the model the complete evaporation time of a butane droplet at its superheat limit is compared with experiments and found to be in good agreement.

Production of Microcellular Polycarbonate Using Carbon Dioxide for Bubble Nucleation

1994 ◽  
Vol 116 (4) ◽  
pp. 413-420 ◽  
Author(s):  
V. Kumar ◽  
J. Weller
Keyword(s):  
Carbon Dioxide ◽  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Bubble Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Homogeneous Microstructure ◽  
Wide Range ◽  
Nucleation Phenomenon

A process to produce a family of novel materials from polycarbonate, having a microcellular structure, is described. The process utilizes the high solubility of carbon dioxide in polycarbonate to nucleate a very large number of bubbles, on the order of 1 to 10 × 109 bubbles/cm3, at temperatures well below the glass transition temperature of the original, unsaturated polycarbonate. Microcellular polycarbonate foams with homogeneous microstructure and a wide range of densities have been produced. In this paper experimental results on solubility, bubble nucleation, and bubble growth in the polycarbonate-carbon dioxide system are presented, and the critical ranges of the key process parameters are established. It is shown that the bubble nucleation phenomenon in polycarbonate near the glass transition temperature is not described by classical nucleation theory.

scholarly journals Classical Nucleation and Lattice Model Unite

2020 ◽  
pp. 32-36
Author(s):  
John H. Jennings
Keyword(s):  
Molecular Weight ◽  
Surface Tension ◽  
Nucleation Rate ◽  
Lattice Model ◽  
Polymer Solutions ◽  
Bubble Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Molecular Weight Polymer ◽  
To Come

Classical nucleation theory predicts the limit of superheat of liquids quite well. To come up with an equation for the limit of superheat of polymer solutions, the lattice model for polymer solutions was used to give the surface tension of polymer solutions. A formula for bubble nucleation in polymer solutions was derived by Jennings with the precursor equation dlnA/dK=1/(6K) where J=AexpK gives the nucleation rate for liquids. The aim of this paper was to show that the precursor equation holds for monomer in the polystyrene-cyclohexane system. Thus, the precursor equation is true for all molecular weight polymer. This happens because the surface tension of polystyrene is significantly more than cyclohexane and the influence of the surface tension dominates.

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