Effects Of Thermal Losses Research Articles

Heat Transfer Effect on Micro Gas Turbine Performance for Solar Power Applications

This paper presents an experimentally validated computational study of heat transfer within a compact recuperated Brayton cycle microturbine. Compact microturbine designs are necessary for certain applications, such as solar dish concentrated power systems, to ensure a robust rotodynamic behaviour over the wide operating envelope. This study aims at studying the heat transfer within a 6 kWe micro gas turbine to provide a better understanding of the effect of heat transfer on its components’ performance. This paper also investigates the effect of thermal losses on the gas turbine performance as a part of a solar dish micro gas turbine system and its implications on increasing the size and the cost of such system. Steady-state conjugate heat transfer analyses were performed at different speeds and expansion ratios to include a wide range of operating conditions. The analyses were extended to examine the effects of insulating the microturbine on its thermodynamic cycle efficiency and rated power output. The results show that insulating the microturbine reduces the thermal losses from the turbine side by approximately 11% without affecting the compressor’s performance. Nonetheless, the heat losses still impose a significant impact on the microturbine performance, where these losses lead to an efficiency drop of 7.1% and a net output power drop of 6.6% at the design point conditions.

Numerical optimization of a plate reactor for a metal hydride open cooling system

Open cooling systems based on metal hydrides (MH) are promising as they are purely driven by the on-board available compression work of the compressed hydrogen in a fuel cell vehicle. In this study, the plate reactor is numerically optimized for fast reaction dynamics leading to an increased power density. Experimental results for the thermodynamic characterization of the applied Hydralloy C2 (Ti0.98Zr0.02V0.41Fe0.09Cr0.05Mn1.46) in the temperature range of 0–50 °C and a mathematical expression for the pressure–concentration isotherms are presented. The model is validated with data from the experimental literature for different cooling temperatures (15–25 °C) and electrical fuel cell powers (4–6 kW). As the reactor alternately passes through two temperature levels in its continuous operation, the effects of thermal losses are evaluated in detail. The optimum thickness for the MH bed is close to the channel thickness of a commercially available reactor. A further increase of the cooling power can be obtained by reducing the distance of the hydrogen gas transport, porosity of the MH bed, and fuel cell backpressure.

Modeling and analysis of the effect of thermal losses on thermoelectric generator performance using effective properties

A mathematical model for a thermoelectric generator (TEG) based on constitutive equations has been developed to analyze temperature dependent performance in terms of output power and efficiency. Temperature dependent material properties and thermal losses, which occur as conductive and radiative heat transfer, were considered in the finite element model. Effective material properties were invoked for understanding the influence of temperature dependence of material parameters and related adverse effects on the model TEG. It is shown that analytical equations with effective properties can provide excellent estimation of the performance of a TEG over a broad operating range. The model was simulated, analyzed and validated to examine the effects of different operating conditions and geometry that interact with thermal losses inside the TEG. We believe that this model will further expedite the optimization of TEGs being developed using new material compositions.

Numerical analysis of the beam position monitor pickup for the Iranian light source facility

In this paper, we describe the design of a button type Beam Position Monitor (BPM) for the low emittance storage ring of the Iranian Light Source Facility (ILSF). First, we calculate sensitivities, induced power and intrinsic resolution based on solving Laplace equation numerically by finite element method (FEM), in order to find the potential at each point of BPM's electrode surface. After the optimization of the designed BPM, trapped high order modes (HOM), wakefield and thermal loss effects are calculated. Finally, after fabrication of BPM, it is experimentally tested by using a test-stand. The results depict that the designed BPM has a linear response in the area of 2×4mm2 inside the beam pipe and the sensitivity of 0.080 and 0.087mm−1 in horizontal and vertical directions. Experimental results also depict that they are in a good agreement with numerical analysis.

The effect of thermal losses on traveling waves for in-situ combustion in porous medium

We study a model for the injection of air into an underground porous medium that contains a solid fuel. In our previous works the model was simplified and all wave sequences for the Riemann problem solution were obtained without taking into account thermal losses to the surrounding rock. In this work the first step is made to understand the effect of heat losses, which are important especially in laboratory experiments. In order to simplify the proof of the existence and uniqueness of the traveling wave solution, we disregard diffusion effects and the dependence of gas density on temperature.

Acoustic Heating Effects in Hartmann Whistle

This paper systematically explores the heating effects in a Hartmann whistle. Effects of thermal losses on the end-wall heating of the whistle are demonstrated by conducting experiments on bare metal cavities, insulated metal cavities, and in wooden cavities. The heating studies are done on wooden cavities of three different configurations, namely, regular cylindrical, stepped and conical ones. The cavity with conical geometry is observed to provide the maximum heating effect at the end-wall. Further, the effects of various parameters such as nozzle pressure ratio, cavity length, and stand-off distance on the end-wall heating of the cavities are also studied. The relationship between acoustic resonance and heating effects are established.

Effects of Thermal Losses on the Heating of a Multifunctional LTCC Module for Atomic Clock Packaging

An innovative multifunctional LTCC module has been designed for miniature atomic clock packaging. Efficient packaging and interconnection of the atomic clock packaging is a critical issue and a precise temperature control is required for some components, such as mini-cell and light source. The great advantage of using LTCC technology for this application is that it allows the integration of different functions, such as heaters and PTCs resistors for temperature measurement and control, and optionally other active elements. In this research, a platform for measuring the thermal conductivity of materials has been developed in order to perform precise thermal studies on the packaging. The relationship between achieved temperature and power dissipated for the heating of the LTCC module has been calculated in different experimental configurations, in order to determine the effects of conduction and convection on the heating and estimate the thermal losses that they introduce into the system.

Texture and viscosity effects in particulate recording media

This paper reports on a study of magnetic texture in a tape that has been fabricated with varying orientation of the long axes of the particles. The results have been correlated to measurements of the physical microstructure made using transmission electron microscopy. Studies of the switching field distribution of the particles in the tape and magnetic viscosity show a strong correlation to the texture with implications for thermal loss effects.

Thermal Energy Loss in the Steam Valves and its Effects

In this study, the effects of thermal losses in steam valves for flow have been analyzed. In a set of experiment built to achieve this, pressure and condensate values have been calculated in different temperatures for insulated and non-insulated cases in three different nominal diameter steam valves. The amount of energy lost from the valve surface in the natural convection conditions has been calculated through application and use of principles of heat transfer and fluid mechanics and equations of mass and energy conservation. Thermal loss coefficient change and the amount of the heat transferred from surface in 150°C steam temperature for different steam velocities have been presented in diagrams. Heat losses have been calculated in two different steam velocities in the cases of five different steam temperatures and their diagrams have been arranged.

Effect of thermal losses on the microscopic hyperbolic heat conduction model

The effects of radiative losses on the thermal behavior of thin metal films, as described by the microscopic two-step hyperbolic heat conduction model, are investigated. Different criteria, which determine the ranges within which thermal radiative losses are significant, are derived. It is found that radiative losses from the electron gas are significant in thin films having ${{C_R \epsilon _e^{{4 \over 3}} T_\infty ^4 } \over {k_e^{{1 \over 3}} L^{{2 \over 3}} G}}\geq 4.6 \times 10^7$ for θo > 4 and τF < 1 and ${{C_R \epsilon _e^{{3 \over 2}} T_\infty ^{{9 \over 2}}} \over {k_e^{{1 \over 2}} L^{{1 \over 2}} G}}\geq 7.4 \times 10^{10}$ for θo 1.

Effect of thermal losses on the microscopic two-step heat conduction model

The effects of radiative and convective thermal losses on the thermal behavior of thin metal films, as described by the microscopic two-step heat conduction model, are investigated. It is found that radiative losses from the electron gas are significant in thin films having ( L/ T e 3)<10 −22, while radiative losses from the solid lattice are significant when ( L/ T e 3)( T e 4/ T l 4)<10 −22. Also, it is found that convective losses from the thin metal film are insignificant in most practical operating conditions.

Application of the flash method for the thermal characterisation of woven carbon fibre laminates

An application of a reliable method to measure the thermal diffusivity of woven carbon fibre laminates is presented in this paper. The thickness of the specimens does not exceed 2 mm and the rise in temperature is kept below 2°C. The flash method proposed by Parker has been used, considering that the determination of the thermal diffusivity of materials by means of pulse methods is strongly dependent upon the specimen thickness and energy pulse. Due to the physical and geometrical characteristics of the samples employed, corrections for the effects of thermal losses caused by finite pulse time, radiant energy penetration, flash lamp efficiency, etc., do not need to be considered. The results obtained show very good agreement with those reported by Lurikis et al. [Lurikis W, Desmet C, Glorieux C, Thoen J. J Mater Res 1993; 8(12): 3106] using the Mirage method and those of Navarrete (Navarrete M. Ph.D. thesis, Faculty of Sciences, National University of México, DF México, 1998) for a classical non-steady state method.

Effects of stacking faults on magnetic viscosity in thin film magnetic recording media

There is much interest in crystallographic defects in thin film magnetic recording media and their role in influencing recording performance such as media noise and thermal loss. In this article we report a correlation between the magnetic viscosity in CoPtCr thin film media, which is the origin of thermal loss effects, and the concentration of local fcc-like regions. The concentration of these defects (the type and density of stacking faults) was varied by growth on different underlayers (Cr and CrTa/Cr) and was measured with grazing incidence x-ray scattering using synchrotron radiation. We show that a substantial percentage of local fcc regions in an otherwise hcp cobalt alloy film leads to significant magnetic viscosity effects at quite modest magnetic fields. We find that the activation volume is reduced for a sample with a higher percentage of fcc-like regions and suggest that this can be understood in terms of the effect of weak links acting to stabilize local micromagnetic configurations.

Demagnetising field reduction in keepered media

We have used a comparative study of time decay of magnetisation and thermal loss of signal in keepered and unkeepered recording media to obtain a measurement of the effective reduction in demagnetising field resulting from the keeper. By measuring magnetic viscosity in the recording layer of a CoCrTa media we have determined the loss of magnetisation per decade of time, over a wide range of fields around the coercivity. Measurements of recorded signal thermal loss effects in the same media both with and without a keeper layer exhibit a significant reduction in the thermal loss from 2.8% to 1.1% per decade of time due to the keeper. Comparison with the bulk time dependence data shows that this corresponds to a reduction in the effective demagnetising field from 1786 to 1493 Oe which moves the demagnetising field away from the edge of the switching field distribution onto the flat portion of the hysteresis loop.

A model of acoustic transmission in the respiratory system.

A theoretical model of sound transmission from within the respiratory tract to the chest wall due to the motion of the walls of the large airways was developed. The vocal tract, trachea, and the first five bronchial generations are represented over the frequency range from 100 to 600 Hz by an equivalent acoustic circuit. This circuit allows the estimation of the magnitude of airway wall motion in response to an acoustic perturbation at the mouth. The radiation of sound through the surrounding lung parenchyma is represented as a cylindrical wave in a homogeneous mixture of air bubbles in water. The effect of thermal losses associated with the polytropic compressions and expansions of these bubbles by the acoustic wave is included and the chest wall is represented as a massive boundary to the wave propagation. The model estimates the magnitude of acceleration over the extrathoracic trachea and at three locations on the posterior chest wall in the same vertical plane. The predicted spectral characteristics of transmission are consistent with previous experimental observations. This theoretical approach suggests that the locations of the spectral peaks are a strong function of the geometry and the wall properties of the airways, while the attenuation at higher frequencies is primarily associated with the absorption of sound in the parenchyma.

The necessary conditions for stable combustion of a powder in a half-closed chamber

The low-frequency stability of the steady-state combustion cycle is investigated in linear approximation for a powder in a half-closed chamber, taking account of incompleteness of combustion, heat losses at the walls, and dynamic erosion. The necessary conditions for stable combustion are obtained. Qualitative conclusions are drawn of the incompleteness of the chemical reactions taking place and of the destabilizing effect of thermal losses and erosion on the combustion process. Only two out of three possible steady-state mechanisms are stable. The existence of limits of combustion with respect to pressure is shown.