A Parametric Numerical Study of Radiative Transfer in Thermotropic Materials

A thermotropic material is modeled as an absorbing, thin slab containing anisotropic scattering, monodisperse, spherical particles. Monte Carlo ray tracing is used to solve the governing equation of radiative transfer. Predicted results are validated by comparison to the measured normal-hemispherical reflectance and transmittance of samples with various volume fraction and relative index of refraction. A parametric study elucidates the effects of particle size parameter, scattering albedo, and optical thickness on the normal-hemispherical transmittance, reflectance, and absorptance. The results are interpreted for a thermotropic material used for overheat protection of a polymer solar absorber. For the preferred particle size parameter of 2, the optical thickness should be less than 0.3 to ensure high transmittance in the clear state. To significantly reduce the transmittance and increase the reflectance in the translucent state, the optical thickness should be greater than 2.5 and the scattering albedo should be greater than 0.995. For optical thickness greater than 5, the reflectance is asymptotic and any further reduction in transmittance is through increased absorptance. A case study is used to illustrate how the parametric study can be used to guide the design of thermotropic materials. Low molecular weighted polyethylene in poly(methyl methacrylate) is identified as a potential thermotropic material. For this material and a particle radius of 200 nm, it is determined that the volume fraction and thickness should equal 10% and 1 mm, respectively.

Analysis of Li-Ion Battery Characteristics and Thermal Behavior

Development of electric and hybrid electric vehicles is of great interest to the transportation industry due to increased demand and cost of imported fuel, uncertainty in the steady supply of oil, and increased standards for reduced emissions. Lithium-ion batteries are considered as one of the leading types for the battery systems to be employed in electric vehicles (EVs) or hybrid electric vehicles (HEVs). Using a regenerative braking system and storing it in battery stacks and using it later for propulsion and acceleration can improve the overall efficiency and reduction of fuel consumption. The objective of this study is to evaluate experimentally the battery performance considering different discharge and charge rates, and investigate the thermal behavior and thermal management requirements of the batteries under a variety of environmental conditions. An experimental test facility has been developed to evaluate thermal performance during charging and discharging modes. Environmental temperatures were varied in environmental chamber to analyze their effects on the charging and discharging patterns of the battery by using the CADEX battery analyzer in order to find the temperature range for optimum battery performance. The batteries were monitored with thermal sensors and a thermal imaging camera while they were run through different load scenarios. In the present study, lithium-ion batteries have been tested and battery performance in terms of polarization curves and discharge capacity were measured using a computerized battery analyzer system for different discharge and charge rates, and over a range of ambient temperatures. Results indicate that at higher discharge and charge rates battery performance decreases due to increased polarization losses, which results in increased internal heat generation and temperature of the battery. Battery performance also depends strongly on the ambient temperature conditions.

Research on Measurement of Spectral Emissivity Based on Fourier Infrared Spectrometer

As one of the basic parameters characterizing the radiation heat transfer of material surface, the emissivity is of important significance to perform non-contact thermometry research. Comparing with the traditional measurement method, measurement method of spectral emissivity based on the Fourier spectrometer has many advantages such as high accuracy and fast measurement. However, the measurement accuracy is subject to the influence of the radiant energy and the spectrometer electromagnetic radiation noise resulted from the environment. In this study, the geometric factor of the sample was defined and the reflectance of the background radiation in the surface of the sample was applied to accurately determine the energy of the radiation received on the detector. An emissivity measurement model was established and a mathematical formula was derived in this study to eliminate the influence of the background radiation noise. To improve the measurement accuracy of the surface temperature of samples, a heat conduction model is established so that the radiation heat transfer of the sample surface can be calculated and the surface temperature of the sample was obtained by equilibrium calculation. Moreover, we conducted emissivity measurement of black paint samples with high emissivity using the Fourier spectrometer and the proposed model is proven valid. Comparing the experimental results modified by the eliminating calculation formula with the experimental data obtained by the monochromator, it was found that there was good qualitative agreement between two sets of results.

Natural Convection in an Enclosure With Blend of Micro Encapsulated Phase Change Materials

This numerical study investigates the effect of using a blend of micro-encapsulated phase change materials (MEPCMs) on the heat transfer characteristics of a liquid in a rectangular enclosure driven by natural convection. A comparison has been made between the cases of using single component MEPCM slurry and a blend of two-component MEPCM slurry. The natural convection is generated by the temperature difference between two vertical walls of the enclosure maintained at constant temperatures. Each of the two phase change materials store latent heat at a specific range of temperatures. During phase change of the PCM, the effective density of the slurry varies. This results in thermal expansion and hence a buoyancy driven flow. The effects of MEPCM concentration in the slurry and changes in the operating conditions such as the wall temperatures compared to that of pure water have been studied. The MEPCM latent heat and the increased volumetric thermal expansion coefficient during phase change of the MEPCM play a major role in this heat transfer augmentation.

Sensitivity of Thermal Transport and Phase-Change in Thin Porous Layers to the Distribution of the Solid Matrix

Understanding the water transport in the Porous Transport Layer (PTL) is important to improve the operational performance of polymer electrolyte membrane fuel cells (PEMFC). High water content in the PTL and flow channel decreases the transport of the gas reactants to the polymer electrolyte membrane. Dry operating conditions result in increased ohmic resistance of the polymer electrolyte membrane. Both cases result in decreased fuel cell performance. Multi-phase flow in the PTL of the fuel cell is simulated as a network of pores surrounded by the solid material. The pore-phase and the solid-phase of the PTL are generated by varying the parameters of the Weibull distribution function. In the network model, the mass transfer takes place in the pore-phase and the bulk heat transfer takes place in the both the solid-phase and liquid phase of the PTL. Previous studies have looked at the thermal and mass transport in the porous media considering the pore size distribution. In the present study, the sensitivity of the thermal and mass transport to the different arrangements of the solid-phase is carried out and the effect of different solid-phase distributions on the thermal and liquid transport in PTL of PEM fuel cell are discussed.

Energy Conservation Measures of a Primary Data Center on an Academic Campus

Energy Conservation Measures (ECMs) of the primary data center at the University of Maryland are developed. Measurement and simulation are performed to validate the developed ECMs. Three ECMs — 1) Increase in the return temperature at Computer Room Air Conditionings (CRACs) 2) Cold aisle containment 3) Elimination of unnecessary CRACs — are suggested to reduce energy consumption by optimizing the thermo-fluid flow in the data center. Power savings of 12.7 kW – 17.4 kW and 14.1 kW are obtained by increasing the return air temperatures at the CRACs and performing the cold aisle containment, respectively. In addition, a power saving of 11.2 kW is obtained by turning off CRACs 3 and 8 which have an adverse effect on the data center cooling.

Computational Evaluation of the Effects of Voids on a Thermal Energy Storage System Using Molten Silicon as the Phase Change Material

Latent heat energy storage is one of the most efficient ways to store solar thermal energy. A system capable of receiving, absorbing, and collecting solar energy and storing it within a high temperature phase change material has been designed as part of a power system to be used on a low Earth orbit satellite. The system employs silicon as the phase change material and thermophotovoltaic cells for the conversion of stored heat energy into electrical energy. The effect of a void, in the phase change material, on system temperature and the associated thermophotovoltaic power production is determined through computational evaluation.

Enhancement of Forced Convection Heat Transfer Using Twisted Heater With Exponential Heat Inputs

Forced convection transient heat transfer coefficients have been measured for nitrogen gas flowing over a twisted heater due to exponentially increasing heat inputs (Q0exp(t/τ)). And then, the effect of heater configuration on transient heat transfer by a twisted heater has been investigated comparing to that of the plate heater. In the experiment, the platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as a test heater. For heat transfer enhancements in single-phase flow, it was twisted at the central part of the heater with an angle of 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q0exp(t/τ). The gas flow velocity ranged from 1 to 4 m/s for the gas temperatures of 313K. The periods of heat generation rate ranged from 46 ms to 17 s. The surface temperature difference and heat flux increased exponentially as the heat generation rate increased with the exponential function. The heat transfer coefficients for twisted heater have been compared to those of the plate heater. They were 24 % higher than those of the plate one. The geometric effect (twisted effect) of heater in this study showed an enhancement on the heat transfer coefficient. It was considered that the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. Finally, the empirical correlations for quasi-steady-state heat transfer and transient one have been obtained based on the experimental data.

Transient Internal Forced Convection Under Arbitrary Time-Dependent Heat Flux

A new all-time model is developed to predict transient laminar forced convection heat transfer inside a circular tube under arbitrary time-dependent heat flux. Slug flow condition is assumed for the velocity profile inside the tube. The solution to the time-dependent energy equation for a step heat flux boundary condition is generalized for arbitrary time variations in surface heat flux using a Duhamel’s integral technique. A cyclic time-dependent heat flux is considered and new compact closed-form relationships are proposed to predict: i) fluid temperature distribution inside the tube ii) fluid bulk temperature and iii) the Nusselt number. A new definition, cyclic fully-developed Nusselt number, is introduced and it is shown that in the thermally fully-developed region the Nusselt number is not a function of axial location, but it varies with time and the angular frequency of the imposed heat flux. Optimum conditions are found which maximize the heat transfer rate of the unsteady laminar forced-convective tube flow. We also performed an independent numerical simulation using ANSYS to validate the present analytical model. The comparison between the numerical and the present analytical model shows great agreement; a maximum relative difference less than 5.3%.