Showing papers on "Thermal energy published in 1999"

Non-Gravitational Heating in the Hierarchical Formation of X-ray Clusters

Abstract: The strong deviation in the properties of X-ray clusters from simple scaling laws highlights the importance of non-gravitational heating and cooling processes in the evolution of proto-cluster gas. We investigate this from two directions: by finding the amount of `excess energy' required in intra-cluster gas in order to reproduce the observed X-ray cluster properties, and by studying the excess energies obtained from supernovae in a semi-analytic model of galaxy formation. Using the insights obtained from the model, we then critically discuss possible ways of achieving the high excess specific energies required in clusters. These include heating by supernovae and active galactic nuclei, the role of entropy, and the effect of removing gas through radiative cooling. Our model self-consistently follows the production of excess energy as well as its effect on star formation in galaxies. Excess energy is retained in gas as gravitational, kinetic and/or thermal energy. The gas distribution of virialized halos are modelled with a 2-parameter family of gas density profiles; gas profiles are then chosen according to the total energy of the gas. We investigate four contrasting ways of modifying gas density profiles in the presence of excess energy, in order to study the sensitivity of results to the model. In addition, we estimate the minimum excess specific energy required when all available gas profiles are considered, using a fiducial cluster with a temperature of around 2 keV. We find that the excess specific energies required in clusters lie roughly in the range 1--3 keV/particle. (abridged)

Heat transfer catheter apparatus and method of making and using same

Abstract: Heat transfer catheter apparatus and methods of making and using same are disclosed wherein a fluid connection is provided between the distal portions of two adjacent, thin-walled, high strength fluid lumens to define a closed loop fluid circulation system capable of controlled delivery of thermal energy to or withdrawal of thermal energy from remote internal body locations.

Active Brownian particles with energy depots modeling animal mobility.

Abstract: In the model of active motion studied here, Brownian particles have the ability to take up energy from the environment to store it in an internal depot and to convert internal energy into kinetic energy. Considering also internal dissipation, we derive a simplified model of active biological motion. For the take-up of energy two different examples are discussed: (i) a spatially homogeneous supply of energy, and (ii) the supply of energy at spatially localized sources (food centers). The motion of the particles is described by a Langevin equation which includes an acceleration term resulting from the conversion of energy. Dependent on the energy sources, we found different forms of periodic motion (limit cycles), i.e. periodic motion between ‘nest’ and ‘food’. An analytic approximation allows the description of the stationary motion and the calculation of critical parameters for the take-up of energy. Finally, we derive an analytic expression for the efficiency ratio of energy conversion, which considers the take-up of energy, compared to (internal and external) dissipation.

The Viability of Thermal Energy Storage

Abstract: With rising energy costs and an increasing demand for renewable energy sources, thermal energy storage (TES) systems are becoming an interesting option. TES is a key component of any successful thermal system and a good TES should allow minimum thermal energy losses. In this study, various ways of thermal conservation are outlined and discussed, both theoretical and experimental. In this respect, the TES systems and their practical applications and some selection criteria have also been given.

Thermoelectric cooling apparatus and method for maximizing energy transport

Abstract: Apparatus and method for sub-ambient cooling using thermoelectric dynamics in conjunction with novel configuration schemes to maximize energy transport to thereby increase the efficiency of thermoelectric cooling. In one form, a junction maximizes energy transport being positioned between and coupled to thermoelectric elements having minimal spacing to provide efficient thermoelectric cooling. Preferable implementations provide thermoelectric elements configured such that thermal energy is transferred away from the junction and dissipated by thermal sinks coupled to thermoelectric elements

Development of energy concepts in introductory physics courses

Abstract: The work-energy theorem, derived from Newton’s second law, applies to the displacement of a particle or the center of mass of an extended body treated as a particle. Because work, as a quantity of energy transferred in accordance with the First Law of Thermodynamics, cannot be calculated in general as an applied force times the displacement of center of mass, the work-energy theorem is not a valid statement about energy transformations when work is done against a frictional force or actions on or by deformable bodies. To use work in conservation of energy calculations, work must be calculated as the sum of the products of forces and their corresponding displacements at locations where the forces are applied at the periphery of the system under consideration. Failure to make this conceptual distinction results in various errors and misleading statements widely prevalent in textbooks, thus reinforcing confusion about energy transformations associated with the action in everyday experience of zero-work forces such as those present in walking, running, jumping, or accelerating a car. Without a thermodynamically valid definition of work, it is also impossible to give a correct description of the connection between mechanical and thermal energy changes and of dissipative effects. The situation can be simply corrected and student understanding of the energy concepts greatly enhanced by introducing and using the concept of internal energy, that is, articulating the First Law of Thermodynamics in a simple, phenomenological form without unnecessary mathematical encumbrances.

Numerical modeling of conduction effects in microscale counterflow heat exchangers

Abstract: This article examines thermal energy transport in a micro counterflow heat exchanger with a numerical model that includes axial conduction. The unique aspect of this work is an analysis that permits end-wall temperature gradients to be determined, thus allowing the conduction heat transfer from the heat exchanger to be assessed. Since there are two unknown initial conditions out of four needed in the set of equations governing the temperature distributions, a two-value shooting approach is needed to solve the problem. Conduction losses are combined with nonunity effectiveness losses to obtain a total normalized heat loss for the system. The results of the study demonstrate the need for using very low thermal conductivity material in the construction of micro counterflow heat exchangers in order to achieve reasonable performance in small devices.

A power–flow analysis based on continuum dynamics

Abstract: Based on the governing equations of continuum mechanics, a power-flow analysis is presented. In developing the mathematical model, the concept of energy-flow density vector is introduced, which uniquely defines the energy transmission between one part of a material body/system and another. This approach allows the energy-flow line, the energy-flow potential and the equipotential surface to be defined. From this model, the local equation of energy-flow balance, the equation of energy exchange between two or many subsystems' and the time-average equations are derived to describe the characteristics of energy flow and energy exchange within the continuum. To demonstrate the applicability of the proposed mathematical model, the energy-flow relation between two simple oscillators is discussed and the concept generalized to sequential and non-sequential multiple systems. Such multiple systems are examined and for non-sequential systems, which are analogous to statically indeterminate structural systems, an approach is developed for the solution of their power flow and energy exchange. It is further shown that the governing equation of energy flow is a first-order partial differential equation which does not directly correspond to the equation describing the flow of thermal energy in a heat-conduction problem.

A two-stage desiccant cooling system using low-temperature heat

Abstract: A two-stage desiccant cooling system using low-temperature heat is described and the computer model is built. The computer calculations show that lower regeneration temperature will be required than for a single-stage desiccant cooling system, which makes low-quality thermal energy utilization possible.

Reducing cold-start emission from internal combustion engines by means of a catalytic converter embedded in a phase-change material

Abstract: Under normal operating conditions, catalytic converters appear to be the most effective means of reducing air pollution from internal combustion (IC) engines. The conversion efficiency, however, declines very steeply for temperatures below about 350 degrees C and is practically zero during the starting and warming-up period. Improving the conversion efficiency under these conditions is important, particularly in large cities, where the number of startings per vehicle per day tends to be high. Among the more successful solutions are preheating of the catalyst electrically, warming up to the catalyst in an external combustion chamber, installation of an auxiliary small-capacity catalytic converter, and employment of an adsorbing unit between two catalysts. Although these methods are quite effective, their disadvantage lies in the fact that they require an external energy source, an additional component (a control unit) or a three-stage catalyst. In the present work an investigation was made of a solution based on the exploitation of thermal capacitance to keep the catalyst temperature high during off-operation periods. A phase-change material (PCM) with a transition temperature of 352.7 degrees C, which is slightly above the light-off temperature of the metallic catalyst, was specially formulated, and a system comprising a catalytic converter embedded in the PCM was designed and tested. Under normal engine operating conditions, some of the thermal energy of the exhaust gases was stored in the PCM. During the time that the vehicle was not in use, the PCM underwent partial solidification, and the latent heat thus produced was exploited to maintain the catalyst temperautre within the desired temperature range for maximum conversion efficiency. (A)

Heat storage system utilizing phase change materials government rights

Abstract: A thermal energy transport and storage system is provided which includes an evaporator containing a mixture of a first phase change material and a silica powder, and a condenser containing a second phase change material. The silica powder/PCM mixture absorbs heat energy from a source such as a solar collector such that the phase change material forms a vapor which is transported from the evaporator to the condenser, where the second phase change material melts and stores the heat energy, then releases the energy to an environmental space via a heat exchanger. The vapor is condensed to a liquid which is transported back to the evaporator. The system allows the repeated transfer of thermal energy using the heat of vaporization and condensation of the phase change material.

Ocean Thermal Energy Conversion (OTEC)

Abstract: Summary The vertical temperature distribution in the open ocean can be simplistically described as consisting of two layers separated by an interface. The upper layer is warmed by the sun and mixed to depths of about 100 m by wave motion. The bottom layer consists of colder water formed at high latitudes. The interface or thermocline is sometimes marked by an abrupt change in temperature but more often the change is gradual. The temperature difference between the upper (warm) and bottom (cold) layers ranges from 10 °C to 25 °C, with the higher values found in equatorial waters. To an engineer this implies that there are two enormous reservoirs providing the heat source and the heat sink required for a heat engine. A practical application is found in a system (heat engine) designed to transform the thermal energy into electricity. This is referred to as OTEC for Ocean Thermal Energy Conversion.

Theoretical study on a novel phase change process

Abstract: This paper mainly deals with a novel homogeneous phase change process in materials (HPCP). The HPCP is analysed in detail and the expressions for one-dimensional HPCPs are derived. It is concluded that, compared with the conventional phase change processes, the complete phase change time of HPCPs can be decreased by 60% for a spherical phase change material (PCM), 50% for a cylindrical PCM and 33% for a flat plate PCM, respectively, and the application of HPCPs to thermal energy storage systems can charge or discharge thermal energy with constant rates. Possible applications of HPCPs to thermal energy storage are simulated and further discussed using composite flat plate PCMs. Copyright © 1999 John Wiley & Sons, Ltd.

A feasibility study of a solar desiccant air-conditioning system. Part II : Transient simulation and economics

Abstract: In this paper, a solid desiccant cooling system with a backup vapour compression system is simulated using TRNSYS and the performance of the system is evaluated in four cities in the United States with different climates. Economic analysis is performed in order to assess the feasibility of these systems and to determine the relevant economic parameters such as life cycle costs, life cycle savings and payback periods. Results show that the system has higher COP values for the locations with more latent loads. The air conditioner was able to meet the cooling demand in all four regions, but it needed more auxiliary energy in the Eastern and Mountain regions than in the Central region, because of the higher solar fraction in the Central region. The simulation also showed that the desiccant cooling system by itself was capable of meeting the cooling demand and hence the requirement of a backup system may be eliminated. Thermal and economic parameters were analysed for varying solar subsystem sizes which proved helpful in optimizing the design of the solar system. Recommendations to minimize the auxiliary energy costs using different methods for supplying the thermal energy for desiccant regeneration are described. Copyright © 1999 John Wiley & Sons, Ltd.

Thermal asperity trends

Abstract: A thermo-mechanical model that predicts the changes in thermal asperities as a function of increasing areal density is described. Conceptually, the problem is divided into two portions: the collision and relative motion of an asperity along the slider, and the subsequent diffusion of thermal energy into the head in the vicinity of the read element. The former is treated using a quasi-static spring model for the various mechanical degrees of freedom; the later with a scaling model for the heat transfer. The thermal model was verified by using a focused fast laser pulse to simulate a thermal transient at the read element. The predictions of the full model were also compared with measured thermal asperities that were produced on a spin-stand. The results support one of the model's main predictions: a significant increase in the amplitude of worst case thermal asperity events can be expected in the next few years.

Prediction of In-Space Durability of Protected Polymers Based on Ground Laboratory Thermal Energy Atomic Oxygen

Abstract: The probability of atomic oxygen reacting with polymeric materials is orders of magnitude lower at thermal energies (< 0.1 eV) than at orbital impact energies (4.5 eV). As a result, absolute atomic oxygen fluxes at thermal energies must be orders of magnitude higher than orbital energy fluxes, to produce the same effective fluxes (or same oxidation rates) for polymers. These differences can cause highly pessimistic durability predictions for protected polymers, and polymers which develop protective metal oxide surfaces as a result of oxidation if one does not make suitable calibrations. A comparison was conducted of undercut cavities below defect sites in protected polyimide Kapton samples flown on the Long Duration Exposure Facility (LDEF) with similar samples exposed in thermal energy oxygen plasma. The results of this comparison were used to quantify predicted material loss in space based on material loss in ground laboratory thermal energy plasma testing. A microindent hardness comparison of surface oxidation of a silicone flown on the Environmental Oxygen Interaction with Materials III (EOIM-III) experiment with samples exposed in thermal energy plasmas was similarly used to calibrate the rate of oxidation of silicone in space relative to samples in thermal energy plasmas exposed to polyimide Kapton effective fluences.

Multi-mode heat transfer using a thermal heat pipe valve

Abstract: An electronic device has a heat pipe containing a heat transfer fluid. The heat pipe has a first section and a second section. Inside the heat pipe is a valve disposed between the first section and second section of the heat pipe. The valve has an actuator that is used to regulate the flow of the heat transfer fluid between the first section and the second section of the heat pipe in response to a changed state detected by a sensor.

Evaluation of Stored Energy in Ultrafine Aluminum Powder Produced by Plasma Explosion

Abstract: Ultrae ne aluminum powder produced by plasma explosion (ALEX) exhibits burn behavior unlike that of ordinary aluminum powders. Others have previously suggested that the source of this unique behavior might be stored internal energy. The objective of this study is to evaluate theoretically the feasibility of energy being stored in ALEX as a result of work performed by means of the compression of the liquid portion of an ALEX particle by its shrinking solid shell during rapid solidie cation of the particle. A theoretical model of this process of energy storage was developed. This model was then formulated and numerically solved on a computer for a variety of conditions. Results show that the postulated mechanism results in measurable stored energy for only unrealistically high cooling rates. It is concluded that, although the postulated mechanism could store energy, the amount of energy stored is realistically negligible. This theoretical e nding is in agreement with recent experiments that show no observable stored energy in ALEX particles. Nomenclature A = surface area, m 2 B = bulk modulus, N/m 2 Cp = specie c heat, J/kg ¢K D = diameter, m Est = thermal energy stored during solidie cation, J H f = latent heat of fusion, J/kg h = heat transfer coefe cient, W/m 2 ¢K k = thermal conductivity, W/m ¢K m = mass, kg N = surface node P = pressure, Pa r = radius,m T = temperature, K Ti = initial temperature, K V = volume, m 3 W = work, J ¯ = thermal coefe cient of expansion, 1/K 1t = time-step increment, s Ω = density, kg/m 3

Air conditioning, especially for electric vehicle passenger compartment, involves meeting demand if compatible with thermal store's charge state, driving state, heat exchanger circuit parameters

Abstract: The method involves determining the instantaneous charge state of a thermal energy store (3) in a heat exchanger circuit (4) carrying a thermal bearer liquid and regulating the heating or cooling power extracted from the store accordingly. A user air conditioning demand is fulfilled if it is compatible with the thermal energy store's state of charge, the vehicle's driving state and the heat exchanger circuit parameters, otherwise reduced heating or cooling power is output. An Independent claim is also included for an arrangement for air conditioning a useful space in a motor vehicle.

Solar collector system

Abstract: A solar collector system for converting solar radiation to thermal energy and electricity has an upper cover with a material that is transparent to solar radiation. A solar energy absorbing structure is disposed under the upper cover and has a heat conducting material such as metal. In addition, a first heat transfer system is disposed in contact with the solar energy absorbing structure and has a material that transfers heat from the solar energy absorbing structure to a first substance flowing within the first heat transfer system. The solar collector system also has electric cells that absorb solar radiation and convert it into electricity. The cells are configured and disposed on at least a portion of the upper cover.

SIMULATION OF HYDRATION/DEHYDRATION OF CaO/Ca(OH)2 CHEMICAL HEAT PUMP REACTOR FOR COLD/HOT HEAT GENERATION

Abstract: A chemical heat pump (CHP) utilizes reversible reactions involving significant endothermic and exothermic heats of reaction in order to develop a heat pump effect by storing and releasing energy while transforming it from chemical to thermal energy and vice versa. In this paper, we present a mathematical model and its numerical solution for the heat and mass transport phenomena occurring in the reactant particle bed of the CHP for heat storage and cold/hot heat generation based on the CaO/Ca(OH)2 reversible hydration/dehydration reaction Transient conservation equations of mass and energy transport including chemical kinetics are solved numerically subject to appropriate boundary and initial conditions to examine the influence of the mass transfer resistance on the overall performance of this CHP configuration. These results are presented and discussed with the aim of enhancing the CHP performance in next generation reactor designs.

Monte Carlo description of gas flow from laser-evaporated silver

Abstract: In the present work, we have studied ablation of a silver metal surface with a Nd:YAG laser (355 nm, 0.8 J/cm2, 6 ns) on the basis of measured data. We have solved the nonlinear heat conduction equation for the laser heating of the system and calculated the varying surface temperature and evaporation rates. These realistic experimental input parameters are further combined with a direct simulation Monte Carlo (DSMC) description of collisions in the gas flow of ablated surface atoms. With this method, new data of plume development and collision processes in the beginning of the ablation process can be extracted. It also allows us to identify important processes by comparing the computational results with experimental ones, such as density, energy, and angular distributions. Our main results deviate only slightly from an earlier study with constant surface temperature and evaporation rate at times t≫τlaser, and this demonstrates that at these later times, the collisions in the plume efficiently smear out the characteristics of the varying temperature at the surface during ablation. The physical properties of the gas flow are determined by the mean thermal energy in the initial plume as well as the number of monolayers emitted.

One-dimensional arrays of oscillators: Energy localization in thermal equilibrium

Abstract: All systems in thermal equilibrium exhibit a spatially variable energy landscape due to thermal fluctuations. Thus at any instant there is naturally a thermodynamically driven localization of energy in parts of the system relative to other parts of the system. The specific characteristics of the spatial landscape such as, for example, the energy variance, depend on the thermodynamic properties of the system and vary from one system to another. The temporal persistence of a given energy landscape, that is, the way in which energy fluctuations (high or low) decay toward the thermal mean, depends on the dynamical features of the system. We discuss the spatial and temporal characteristics of spontaneous energy localization in 1D anharmonic chains in thermal equilibrium.