Soumya S. Patnaik

Modeling of thermal transport in pillared-graphene architectures.

Carbon nanotubes (CNT) and graphene are considered as potential future candidates for many nano/microscale integrated devices due to their superior thermal properties. Both systems, however, exhibit significant anisotropy in their thermal conduction, limiting their performance as three-dimensional thermal transport materials. From thermal management perspective, one way to tailor this anisotropy is to consider designing alternative carbon-based architectures. This paper investigates the thermal transport in one such novel architecture-a pillared-graphene (PG) network nanostructure which combines graphene sheets and carbon nanotubes to create a three-dimensional network. Nonequilibrium molecular dynamics simulations have been carried out using the AIREBO potential to calculate the thermal conductivity of pillared-graphene structures along parallel (in-plane) as well as perpendicular (out-of-plane) directions with respect to the graphene plane. The resulting thermal conductivity values for PG systems are discussed and compared with simulated values for pure CNT and graphite. Our results show that in these PG structures, the thermal transport is governed by the minimum interpillar distance and the CNT-pillar length. This is primarily attributed to scattering of phonons occurring at the CNT-graphene junctions in these nanostructures. We foresee that such architecture could potentially be used as a template for designing future structurally stable microscale systems with tailorable in-plane and out-of-plane thermal transport.

Anchoring characteristics and interfacial interactions in a polymer dispersed liquid crystal : a molecular dynamics study

Abstract Molecular dynamics (MD) simulations were carried out to gain an understanding of the interfacial interactions and the phase separation process in a polymer dispersed liquid crystalline (PDLC) system. The most important components of the PDLC system of interest are: E7 (an eutectic mixture of cyanobiphenyl liquid crystals), a crosslinked polyacrylate, and octanoic acid (OA). The miscibility of the different components was investigated and the anchoring characteristics of 4- n -pentyl-4′-cyanobiphenyl (5CB) molecules on the penta-acrylate polymer surface were analyzed. The effects of introducing a surfactant on the miscibility and the anchoring strengths were also studied. Bilayers of polymer and liquid crystal (LC) of different alignments, show that an amorphous 5CB structure on the polymer surface was energetically most favorable, whereas homeotropic alignment had the next higher energy and planar alignment was the least favorable. Calculated solubility parameters indicate that prior to polymerization, the prepolymer and LC are miscible and OA is equally immiscible with both of them. Upon polymerization, the polymer is no longer miscible with the LC; phase separation occurs and OA acts as a surfactant forming a layer between the polymer and the LC. Anchoring energies calculated from the interfacial tensions indicate that the LC and polymer interface is strong. On addition of the surfactant, the anchoring energy at the interface of the LC and the surfactant becomes weaker. This decrease in anchoring strength may be one of the major factors responsible for a reduction in droplet size and also a lowering of critical field for switching, both of which are observed experimentally in volume holograms made of these PDLC materials.

A molecular simulations study of the miscibility in binary mixtures of polymers and low molecular weight molecules

Abstract The miscibility behavior of binary mixtures of polymeric and low molecular weight molecules was studied using a combination of modified Flory–Huggins theory and molecular simulation techniques. Three different atomistic approaches were used to investigate the phase behavior and χ parameters of binary mixtures consisting of polymethyl methacrylate (PMMA) and 4- n -pentyl-4′-cyanobiphenyl (5CB). Binary mixtures of methyl methacrylate monomer/5CB and methyl methacrylate oligomer/5CB were also studied. As a first approach, a fast method that calculates the local interaction between a fragment of the polymer and the organic molecule and then extends it to determine the energy of mixing using an estimated coordination number was used. By using modified coordination numbers, we were able to extend this method to include cases where the polymer segment and the small molecules are slightly dissimilar in size. More detailed studies which take into account bulk effects were also carried out where the cohesive energies of the pure compounds were derived from molecular dynamics simulations and the interaction parameters were determined from the differences in the cohesive energies. The concentration and temperature dependence of the χ parameters was evaluated by calculating the energy of mixing from the differences in the cohesive energy densities of the mixed and demixed systems. The present study provides a detailed understanding of the miscibility of PMMA and 5CB as PMMA polymerizes from its monomer, and the results indicate that although methyl methacrylate and 5CB are completely miscible, 5CB is not miscible in PMMA even in small quantities.

A Dynamic Modeling Toolbox for Air Vehicle Vapor Cycle Systems

Abstract : Modern air vehicles face increasing internal heat loads that must be appropriately understood in design and managed in operation. This paper examines one solution to creating more efficient and effective thermal management systems (TMSs): vapor cycle systems (VCSs). VCSs are increasingly being investigated by aerospace government and industry as a means to provide much greater efficiency in moving thermal energy from one physical location to another. In this work, we develop the Air Force Research Laboratory Transient Thermal Modeling and Optimization (ATTMO) toolbox: a modeling and simulation tool based in Matlab/Simulink that is suitable for understanding, predicting, and designing a VCS. The ATTMO toolbox also provides capability for understanding the VCS as part of a larger air vehicle system. The toolbox is presented in a modular fashion whereby the individual components are presented along with the framework for interconnecting them. The modularity allows for easy user re-configurability as well as the ability to scale from simple to full vehicle systems. A computational environment is discussed that allows for simulations running many times faster than real time. Simulation results are presented for a laboratory scale test stand system consisting of both single and multiple evaporators. The simulations are verified against experimental results demonstrating the potential of the tool.

Graphite Foam Infused with Pentaglycerine for Solid-State Thermal Energy Storage

The use of a solid-state phase change material, pentaglycerine, in thermal energy storage was investigated. The motivation for exploring a thermal energy storage system that relies on a solid-state...

An Integrated Chemical Reactor-heat Exchanger based on Ammonium Carbamate

Abstract : In this work we present our recent effort in developing a novel heat exchanger based on endothermic chemical reaction (HEX reactor). The proposed HEX reactor is designed to provide additional heat sink capability for aircraft thermal management systems. Ammonium carbamate (AC) which has a decomposition enthalpy of 1.8 MJ/kg is suspended in propylene glycol and used as the heat exchanger working fluid. The decomposition temperature of AC is pressure dependent (60 ?C at 1 atmosphere; lower temperatures at lower pressures) and as the heat load on the HEX increases and the glycol temperature reaches AC decomposition temperature, AC decomposes and isothermally absorbs energy from the glycol. The reaction, and therefore the heat transfer rate, is controlled by regulating the pressure within the reactor side of the heat exchanger. The experiment is designed to demonstrate continuous replenishment of AC. This requires recovering the depleted glycol while expelling waste gases, dispersing and suspending fresh AC, and injecting the mixture into the heat exchanger. A gasketed plate heat exchanger is used as the reactor for this experiment, and heated water is used to provide the thermal load. The performance of the HEX reactor is characterized as a function of water flow rate and temperature, AC/glycol mixture flow rate, and AC concentration. Varying these parameters permits mapping the performance of the system under different conditions.

Atomistic modelling of nematic liquid crystals: a study of structure-property relationships in steroidal mesogen based liquid crystals

Abstract In this study we report molecular mechanics calculations designed to predict and interpret structure property relationships in nematic liquid crystals. A family of liquid crystals with steroidal mesogens were studied and the results were compared with available X-ray data. Low energy conformations of dimers were analysed to provide quantitative information about the local intermolecular interactions and their anisotropic nature. Important contributions to the molecular packing could be identified and the geometry of the dimers and the extent of their positional correlation was successfully related to their observed packing behaviour. By monitoring the relative orientation of the two molecules, a qualitative study of liquid crystalline phase stability was accomplished. Simulations were also carried out with a modified energy function which includes a nematic contribution representing the cumulative intermolecular interactions owing to long range orientational order present in liquid crystals. Along with providing a systematic study of the relative importance of the various competing forces (steric repulsion, attractive forces, long-range electrostatic interactions) in the formation of liquid crystalline phases, this method can also be expected to be useful in predicting mesophase behaviour.

Molecular Dynamics Study of Thermal Transport Phenomena in Cross-linked Polymer: Epon 862 with Curing Agent W (DETDA)

In this article, thermal behavior of a cross-linked epoxy network along with its un-cross-linked counterpart has been discussed using atomistic molecular dynamics simulations. The thermal conductivity of EPON 862 resin monomer, cross-linking agent DETDA and the cured epoxy polymer was calculated using equilibrium molecular dynamics (EMD) as well as nonequilibrium molecular dynamics simulations (NEMD) based on Green-Kubo and Fourier law formalism, respectively. The simulations are performed using Consistent Valence Force Field (CVFF) and the results were found to be in good agreement with experimental findings.

A Simulink Pathway for Model-Based Control of Vapor Compression Cycles

Current and next generation tactical aircraft face daunting thermal challenges that involve reliably maintaining thermal constraints despite large transient loads. Model-based control synthesis has the potential to improve the performance of a vapor compression cycle system during its transient operating condition, driven by intermittent and dynamic thermal loads, when compared to the current heuristic control design technique. However, the excessive labor and expertise necessary to develop models amenable to model-based control design techniques has been an impediment to widespread deployment. This paper demonstrates a Simulink pathway for model-based design via the AFRL Transient Thermal Modeling and Optimization (ATTMO) toolbox. An effective, simple linear quadratic gaussian control design is demonstrated and opens the door for widespread deployment of many advanced control techniques.Copyright

Suppression of vortex-induced vibration of a circular cylinder using thermal effects

Transverse vortex-induced vibration (VIV) of a cylinder with various body-to-fluid density ratio and stiffness is studied. The cylinder is elastically mounted and heated, and the flow direction is aligned with the direction of the thermal induced buoyancy force. Amplitude of VIV can be reduced as the thermal control parameter Richardson number (Ri) increases, or even be fully suppressed when Ri is above a critical value. This critical Richardson number depends on both body-to-fluid density and structural stiffness. A higher critical Richardson is required to fully suppress the VIV of a structure with smaller density ratio. With the same density or mass, a structure with intermediate stiffness vibrating in lock-in regime needs higher critical Ri to suppress VIV than either rigid or flexible structures. Drag experienced by the body is also studied. It is found that for a flexible body, drag gradually increases with the Richardson number. For a body with intermediate stiffness, both drag and amplitude of VIV...