Dendrite Growth during Freezing of Millimeter-Scale Eicosane Droplets

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Md Mahamudur Rahman ◽  
Han Hu ◽  
Hamidreza Shabgard ◽  
Philipp Boettcher ◽  
Ying Sun
Keyword(s):  
Phase Change Materials ◽  
Critical Role ◽  
Small Diameter ◽  
Freezing Process ◽  
Vertical Position ◽  
Cold Stage ◽  
Interface Location ◽  
Small Density ◽  
Spray Freezing ◽  
Dry Cooling

The freezing characteristics of small diameter eicosane (Tmelt = 37°C) droplets are studied here for their use in novel dry-cooling strategies based on spray freezing of recirculating phase change materials (PCM). PCM can be used to store thermal energy with relatively small changes in temperature (due to latent heat), as well as volume (due to small density changes). 4.2 mm diameter eicosane droplets are superheated to 40°C, placed on a cold stage at 10°C, and imaged during freezing (a). Similarly, liquid eicosane is enclosed within a custom-built experimental package creating a 5 mm diameter, 100 μm thick disc geometry with a temperature controlled boundary that is rapidly dropped from 40°C to 10°C (b). In both cases the liquid-solid interface is tracked, as well as the formation and growth of long dendrite structures which have been observed to play a critical role in the freezing process. (c) and (d) show the vertical position normalized by the droplet height , y/H, and the radial position (measured inward) normalized by the disc radius, r/R, of both the interface location and the average dendrite tip location. The total freezing time is observed visually, resulting in characteristic Fourier numbers of Fo = 0.55 ± 0.15 (droplet) and Fo = 3.5 ±0.15 (disc) at identical Stefan numbers of St = 0.3 ± 0.03, where the characteristic lengths are taken as the ratio of the eicosane volume to the cooled surface area.

scholarly journals Advantages of the Surface Structuration of KBr Materials for Spectrometry and Sensors

Sensors ◽  
2018 ◽  
Vol 18 (9) ◽  
pp. 3013 ◽  
Author(s):  
Natalia Kamanina ◽  
Svetlana Likhomanova ◽  
Pavel Kuzhakov
Keyword(s):  
Carbon Nanotubes ◽  
Refractive Index ◽  
Electric Fields ◽  
Matrix Material ◽  
Small Diameter ◽  
Main Idea ◽  
Potassium Bromide ◽  
Material Surface ◽  
Vertical Position ◽  
Output Window

A potassium bromide (KBr) material, which has been widely used as the key element in Fourier spectrometers and as the output window of the IR-lasers, was studied via applying carbon nanotubes in order to modify the potassium bromide surface. The laser-oriented deposition method was used to place the carbon nanotubes at the matrix material surface in the vertical position at different electric fields varying from 100 to 600 V × cm−1. The main idea of the improvement of the spectral properties of the potassium bromide structure is connected with the fact that the refractive index of the carbon nanotubes is substantially less than the refractive index of the studied material, and the small diameter of the carbon nanotubes allows one to embed these nano-objects in the voids of the lattice of the model matrix systems. Moreover, the mechanical characteristics and wetting features of potassium bromide structures have been investigated under the condition mentioned above. Analytical and quantum-chemical simulations have supported the experimental results.

Phase Change Process Inside a Small-radii Cylinder Subjected to Cyclic Convective Boundary Conditions: a Numerical Study

2021 ◽  
Author(s):  
Yousef Kanani ◽  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Phase Change ◽  
Phase Change Materials ◽  
Numerical Study ◽  
Convective Boundary Condition ◽  
Freezing Process ◽  
Fourier Number ◽  
Two Phase ◽  
Convective Boundary

Abstract We numerically investigate the melting and solidi?cation behavior of phase change materials encapsulated in a small-radii cylinder subjected to a cyclic convective boundary condition (square wave). Initially, we explore the effect of the Stefan and Biot numbers on the non-dimensionalized time required (i.e. reference Fourier number Tref ) for a PCM initially held at Tcold to melt and reach the cross?ow temperature Thot. The increase in either Stefan or Biot number decreases Tref and can be predicted accurately using a correlation developed in this work. The variations of the PCM melt fraction, surface temperature, and heat transfer rate as a function of Fourier number are reported and analyzed for the above process. We further study the effect of the cyclic Fourier number on the periodic melting and freezing process. The melting or freezing front initiates at the outer periphery of the PCM and propagates towards the center. At higher frequencies, multiple two-phase interfaces are generated (propagating inward), and higher overall heat transfer is achieved as the surface temperature oscillates in the vicinity of the melting temperature, which increases the effective temperature difference driving the convective heat transfer.

Solidification of Phase Change Materials Infiltrated in Porous Media in Presence of Voids

2014 ◽  
Vol 136 (11) ◽  
Author(s):  
Mahmoud Moeini Sedeh ◽  
J. M. Khodadadi
Keyword(s):  
Phase Change ◽  
Porous Structure ◽  
Phase Change Materials ◽  
Solidification Process ◽  
Volume Shrinkage ◽  
Freezing Process ◽  
Infiltration Process ◽  
Noticeable Increase ◽  
Melting Characteristics ◽  
Micron Size

Infiltration of phase change materials (PCM) into highly conductive porous structures effectively enhances the thermal conductivity and phase change (solidification and melting) characteristics of the resulting thermal energy storage (TES) composites. However, the infiltration process contributes to formation of voids as micron-size air bubbles within the pores of the porous structure. The presence of voids negatively affects the thermal and phase change performance of TES composites due to the thermophysical properties of air in comparison with PCM and porous structure. This paper investigates the effect of voids on solidification of PCM, infiltrated into the pores of graphite foam as a highly conductive porous medium with interconnected pores. A combination of the volume-of-fluid (VOF) and enthalpy-porosity methods was employed for numerical investigation of solidification. The proposed method takes into account the variation of density with temperature during phase change and is able to predict the volume shrinkage (volume contraction) during the solidification of liquids. Furthermore, the presence of void and the temperature gradient along the liquid–gas interface (the interface between void and PCM) can trigger thermocapillary effects. Thus, Marangoni convection was included during the solidification process and its importance was elucidated by comparing the results among cases with and without thermocapillary effects. The results indicated that the presence of voids within the pores causes a noticeable increase in solidification time, with a sharper increase for cases without thermocapillary convection. For verification purposes, the amount of volume shrinkage during the solidification obtained from numerical simulations was compared against the theoretical volume change due to the variation of density for several liquids with contraction and expansion during the freezing process. The two sets of results exhibited good agreement.

Toward Small-Diameter Carbon Nanotubes Synthesized from Captured Carbon Dioxide: Critical Role of Catalyst Coarsening

2018 ◽  
Vol 10 (22) ◽  
pp. 19010-19018 ◽  
Author(s):  
Anna Douglas ◽  
Rachel Carter ◽  
Mengya Li ◽  
Cary L. Pint
Keyword(s):  
Carbon Dioxide ◽  
Carbon Nanotubes ◽  
Critical Role ◽  
Small Diameter

scholarly journals Numerical Solution of Heat Transfer Process in PCM Storage Using Tau Method

2015 ◽  
Vol 2015 ◽  
pp. 1-9 ◽  
Author(s):  
B. Heydari ◽  
F. Talati
Keyword(s):  
Phase Change ◽  
Phase Change Materials ◽  
Numerical Approach ◽  
Heat Transfer Process ◽  
Liquid Interface ◽  
Tau Method ◽  
Two Dimensional ◽  
Interface Location ◽  
Solid Liquid Interface ◽  
Solid Liquid

Thermal energy storage units that utilize phase change materials have been widely employed to balance temporary temperature alternations and store energy in many engineering systems. In the present paper, an operational approach is proposed to the Tau method with standard polynomial bases to simulate the phase change problems in latent heat thermal storage systems, that is, the two-dimensional solidification process in rectangular finned storage with a constant end-wall temperature. In order to illustrate the efficiency and accuracy of the present method, the solid-liquid interface location and the temperature distribution of the fin for three test cases with different geometries are obtained and compared to simplified analytical results in the published literature. The results indicate that using a two-dimensional numerical approach can predict the solid-liquid interface location more accurately than the simplified analytical model in all cases, especially at the corners.

Endothelial Cell Transplantation

1995 ◽  
Vol 4 (4) ◽  
pp. 401-410 ◽  
Author(s):  
Stuart K. Williams
Keyword(s):  
Endothelial Cells ◽  
Endothelial Cell ◽  
Cell Transplantation ◽  
Vascular System ◽  
Clinical Performance ◽  
Healing Process ◽  
Critical Role ◽  
Vascular Grafts ◽  
Graft Function ◽  
Small Diameter

Endothelial cells line the lumenal surface of al) elements of the vascular system. These cells exhibit numerous metabolic functions necessary for the maintenance of homeostasis. The critical role of endothelium in maintaining normal blood vessel function is exemplified by the poor clinical performance of small diameter polymeric vascular grafts which fail due, in part, to the lack of a functional endothelium on the lumenal surface. Extensive research has explored the potentiality of transplanting endothelial cells onto polymeric vascular grafts to improve graft function. Several critical issues have been explored including the source of endothelial cells for transplantation, the interaction of endothelium with polymers and the healing process of endothelial cell transplanted grafts. The future of endothelial cell transplantation will also include the use of these cells as vehicles for genetic engineering.

Experimental Determination of Temperature-Dependent Thermal Conductivity of Solid Eicosane-Based Nanostructure-Enhanced Phase Change Materials

2012 ◽  
Author(s):  
Mahdi Nabil ◽  
J. M. Khodadadi
Keyword(s):  
Thermal Conductivity ◽  
Phase Change Materials ◽  
Small Diameter ◽  
Copper Oxide Nanoparticles ◽  
Water Bath ◽  
Conductivity Data ◽  
Vacuum Oven ◽  
Thermal Conductivity Data ◽  
Ice Water

The effective thermal conductivity of composites of eicosane and copper oxide nanoparticles in the solid state was measured experimentally by using the transient plane source technique. Utilizing a controllable temperature bath, measurements were conducted at various temperatures between 10 and 35°C for the solid samples. In the course of preparation of the solid specimen, liquid samples (0, 1, 2, 5 and 10 wt%) were poured into small diameter molds and were degased within a vacuum oven. The molds were then subjected to either ambient solidification or ice-water bath freezing method. Measured thermal conductivity data of the composites were found to be nearly independent of the measurement temperature for a given loading of CuO nanoparticles regardless of the solidification procedure. Irrespective of the solidification method, as the melting temperature was approached, thermal conductivity data of the solid disks rose sharply for both sets of experiments. The composites prepared using the ice-water bath solidification scheme consistently exhibited lower values of thermal conductivity when compared to the samples which prepared under ambient solidification method. This behavior might be due to the greater void percentage of ice-water bath samples and/or crystal structure deviations due to phase transition method.

Bulk Dynamic Spray Freeze-Drying Part 2: Model-Based Parametric Study for Spray-Freezing Process Characterization

2019 ◽  
Vol 108 (6) ◽  
pp. 2075-2085 ◽  
Author(s):  
Israel B. Sebastião ◽  
Bakul Bhatnagar ◽  
Serguei Tchessalov ◽  
Satoshi Ohtake ◽  
Matthias Plitzko ◽  
...  
Keyword(s):  
Parametric Study ◽  
Freeze Drying ◽  
Freezing Process ◽  
Model Based ◽  
Process Characterization ◽  
Spray Freezing ◽  
Spray Freeze Drying

Expedited Freezing of Nanoparticle-Enhanced Phase Change Materials (NEPCM) Exhibited Through a Simple 1-D Stefan Problem Formulation

2009 ◽  
Author(s):  
J. M. Khodadadi ◽  
Liwu Fan
Keyword(s):  
Stefan Problem ◽  
Phase Change Materials ◽  
Volume Fraction ◽  
Integral Approach ◽  
Thermal Conductivity Ratio ◽  
Freezing Process ◽  
Freezing Time ◽  
Interface Position ◽  
Solid Phases ◽  
Liquid And Solid Phases

An analytic/integral approach is utilized to solve a model 1-dimensional Stefan problem for a nanofluid that undergoes freezing. Initially, the isothermal nanofluid is contained in a finite slab. During the freezing process, the traveling interface separates the liquid and solid phases that possess their respective thermophysical properties. The most favorable feature of this model is that the thermal property jumps between the liquid and solid phases are accounted for. The problem is made dimensionless and is shown to depend on the thermal conductivity ratio, thermal diffusivity ratio, Stefan (Ste) and subcooling numbers. The energy equation within the solid layer is solved exactly and that of the liquid layer is solved using the integral method. The instantaneous interface position and the moving front velocity are obtained and the total freezing time is then determined. Combinations of two base PCM (water and cyclohexane) and four nanoparticles (alumina, copper, copper oxide and titanium oxide) are chosen for demonstration purposes. The thermal properties of the resulting nanofluids as a function of the volume fraction were determined using models proposed in the literature. The results show that the dimensionless freezing time is independent of the nanofluid constituents and only depends on the volume fraction. Keeping everything else the same, the freezing time is shown to decrease as the volume fraction of the nanoparticle is raised.

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