Thermal Exchanges Research Articles

Nonlinear analysis of thermal stresses in mass concrete castings

Temperatures and stress transients due to hydration process in concrete casting for dam constructions and to thermal exchanges between concrete and its surrounding environment are analysed. The problem under investigation is highly nonlinear due to different reasons. Placing technique of concrete is carried out by means of large block castings which are superimposed at different times. Concrete surfaces may be cooled using water at ambient temperature or at a lesser temperature; thermal exchanges between concrete and external ambient are affected by the increasing extension of casting surface, by solar radiation and wind action. Thermal stresses due to the temperature transients are produced and cracks may appear in massive concrete. The numerical analysis of corresponding complex phenomena is carried out according to the following lines. Temperature field is calculated at each time step taking into account the above mentioned nonlinearities, then for each time step equilibrium is imposed for finding displacements and stresses using maturity dependent constitutive relationships. Time integration of the nonlinear equation of heat transport is carried out using a two-level scheme with variable step sizes. Thermal and mechanical fields are then found for the whole time transient.

Thermal exchanges during sleep in anhidrotic ectodermal dysplasia

Anhidrotic ectodermal dysplasia is a congenital syndrome characterized by the absence of sweat glands. A sweating test was performed on such a patient and proved his inability to sweat. Thermal exchanges during night sleep were then measured in this patient and compared with data obtained from a healthy control subject. Ambient conditions were as follows: dry bulb temperature 32.2 degrees C, relative humidity 30%-40%, wind speed 0.7 m.s-1. Polysomnographic recordings showed normal sleep patterns in both subjects, but a "first night effect" in the patient. Rectal (Tre) and mean skin (Tsk) temperatures and loss of mass were monitored continuously throughout the 8-h sleep recording. Loss of mass averaged 34.1 g.h-1 in the patient vs 78.1 g.h-1 in the control subject. No relationship with sleep stages was observed in the patient, in contrast to the control subject who experienced a decrease in evaporation during rapid eye movement sleep. Body temperatures varied little in the patient, but decreased until the 6th h of sleep in the control subject. On two occasions there was a 0.3 degrees C fall in the Tre of the patient during two slow wave sleep (SWS) phases, while Tsk and loss of mass did not change. As thermolytic processes had not varied on these two occasions, it was concluded that the fall in Tre indicated a concomitant decrease in metabolic heat production, in agreement with the assumption that SWS represented a state of energy conservation.

Numerical method for reducing stress level in GaAs crystals

We have developed a numerical method based on the finite element technique for calculating heat transfer in crystal growth furnaces. Our model is global, as it entirely relies upon external control parameters, such as the pulling rate and the power input, while internal heat exchange is calculated on the basis of geometrical and material properties of all constituents of the puller. Radiative exchanges within the furnace are calculated with the assumptions of an axisymmetric geometry and of diffuse-gray surfaces. The viewed and hidden parts of the bodies are taken into account. The influence of the meniscus at the tri-junction is also investigated. It is generally agreed that the presence of structural defects in the grown crystal is related to thermoelastic stresses generated during growth. A finite element prediction of these thermal stresses is coupled to the global heat transfer calculation. The aim of this paper is to show how the thermal stress level in the crystal can be decreased by modifying the thermal exchanges inside the furnace. We study the evolution of the temperature and stress fields during growth and research the optimal pulling rate to obtain a flat solid-liquid interface. Moreover we point out how the temperature gradients and the stress field in the crystal could be affected using a heat shield inside the crucible. Different configurations are studied. Performing quasi-steady simulations at different growth stages, we show that our global numerical model provides a powerful tool for the computer-aided design of crystal growth furnaces.

Design, synthesis and heerotrimetallic complexes of functionalization tweezer ligands

The tweezer molecules pyridine-2,6-[ o-CH 2XC 6H 5(CH 3)(Cr(CO) 3)] 2 and 1,10-phenanthroline-2,9-[ o-CH 2XC 6H 5(CH 3)(Cr(CO) 3)] 2 (X  NH, OCH 2, SCH 2) have been synthesized, and their trinuclear dichromium(0)-rhodium(I) derivatives shown to undergo fast thermal and photochemical carbonyl exchanges.

Transient simulation of flat-plate solar collectors

A computer model derived from the basic laws governing thermal energy exchanges between surfaces is presented for the transient simulation of flat-plate collectors. The approach of subdivision along the collector was used in the development of energy equations on each of the surfaces in a collector. The model was validated with measured data and a good agreement between the predicted and the measured performance was observed.

Gas turbine modelling using pseudo-bond graphs

A pseudo-bond graph-modelling approach for gas turbines is described. Generally in gas turbines the working medium is a compressible fluid, and there is mass flow as well as mechanical, thermal, and fluid power exchanges. The graph model and the ensuing mathematical model are derived by physical laws. The method presented here offers flexibility and adjustability in dynamic simulations. The technique is applied to the case of a particular gas turbine.

Silicon Temperature and Thermal Gradients Measurements in a Rapid Thermal Processor Operating at Atmospheric Pressure or in Vacuum

ABSTRACTTemperature-time profiles obtained by an optical pyrometer and a mechanically contacted thermocouple are first presented it appears that the thermocouple response is sensitive to the pressure in the processing chamber. The authors suggest that, in vacuum, the thermocouple is thermally isolated from the wafer, until the temperature is high enough for thermal radiation exchanges to occur. Experimental evidence of the influence of thermal history, and of gas pressure and flow, on temperature drop at periphery of the wafer is then given.

Microclimate and weathering of a historical building: The Ducal Palace in Urbino

Among the many historical buildings in Italy, the Ducal Palace of Urbino (XV Century) was chosen in order to study the weathering of stones at a relatively unpolluted site. The main mechanisms of the physical and chemical processes leading to natural weathering could therefore be identified without interference from high pollution levels, such as found in cities, where sulphation dominates. A microclimatic study was carried out in order to determine both the seasonal and diurnal thermal and hygrometric cycles and exchanges of heat and vapour between the stone and the atmosphere. A comparison of environmental forcing cycles over deteriorated and non-deteriorated zones, led to the conclusion that processes induced by micro-climate play a negligible role compared with those induced by liquid water supplied by rainfall. The chemical and physical processes of stone weathering, due both to dissolution and recrystallization of calcium carbonate, and to transformation and crystallization of gypsum inside the stone, were identified. A process of stone weakening due to the internal circulation of rain water is demonstrated. Finally, some suggestions are made for a more effective conservation of artistic heritage.

Temperature-induced frequency shifts in quartz resonators

The thermal exchanges in a quartz crystal resonator are investigated. This leads to a temperature gradient distribution which is in the main plane of the crystal, and therefore does not exhibit any functional dependence (at least at the first order) along the plate thickness. Internal heat source due to the energy dissipation by the internal friction is considered. The corresponding in-plane thermal stresses are calculated in the isotropic approximation, and anisotropy is introduced by means of the stress-strain relations. Applying a perturbation method, the resulting frequency shifts are obtained for both static and dynamic thermal behaviors.

Wetting, deterioration and visual features of stone surfaces in an urban area

Among the many historical buildings in Italy, the Ducal Palace of Urbino (XV Century) was chosen in order to study the weathering of stones at a relatively unpolluted site. The main mechanisms of the physical and chemical processes leading to natural weathering could therefore be identified without interference from high pollution levels, such as found in cities, where sulphation dominates.A microclimatic study was carried out in order to determine both the seasonal and diurnal thermal and hygrometric cycles and exchanges of heat and vapour between the stone and the atmosphere. A comparison of environmental forcing cycles over deteriorated and non-deteriorated zones, led to the conclusion that processes induced by micro-climate play a negligible role compared with those induced by liquid water supplied by rainfall.The chemical and physical processes of stone weathering, due both to dissolution and recrystallization of calcium carbonate, and to transformation and crystallization of gypsum inside the stone, were identified. A process of stone weakening due to the internal circulation of rain water is demonstrated.Finally, some suggestions are made for a more effective conservation of artistic heritage.

Diel aspects of the thermal structure and energy budget of a small English lake

SUMMARY. The did response of the thermal structure of Esthwaite Water to prevailing meteorological conditions was analysed for seven selected days, covering different phases of stratification. Meteorological measurements were combined with published empirical relations to compute 24‐h thermal energy budgets for 2 days. Amplitudes of diel changes were controlled by the seasonal phase of stratification and the magnitude of incident solar radiation. Heat storage usually followed a sinusoidal curve with a minimum during the night and early morning and a maximum during the late afternoon. Wind‐induced turbulent mixing, and vertical circulation induced by nocturnal cooling of the surface water through evaporative and conductive processes, also modified depth‐time trends in thermal structure.The budget accounted for almost all the thermal energy exchanges of the lake. Possible sources of a slight imbalance are considered. Although there was a positive daily surplus of net radiation, it may not always imply heat gain by the lake as it may be counter‐balanced by heat losses through fluxes of latent and sensible heat, Net back‐radiation was a major component of daily heat loss. The combined latent and sensible heat fluxes also accounted fora high proportion of heat loss to the atmosphere. Net flow of sensible heat to the lake was small but not always inappreciable. Net daily heat storage represented 26% (9 May) and 13% (17 June) of solar radiation Input. Advected energy due to lake inflow and outflow was negligible. Qualitative comparisons are made with long‐term (seasonal and annual) energy budgets and heat fluxes.

Analysis and design of a differential sunshine recorder

A solar drying system designed on the principles of convective heat flow was constructed from local materials (wood, metals and glass sheets) and used to dry food crops (cassava, pepper, okro, groundnuts, etc.). The solar collector could transfer 118 W m−2 thermal power to the drying air. The thermal exchanges within the dryer were determined from a psychometric chart. Ambient air at 32°C and 80% relative humidity (RH) could be heated to 45°C at 40%. The crops were dried to a final moisture content of <14% and were preserved for a period of one year without deterioration. The low-temperature drying system ensured the viability of the seeds for planting. The drying process can be represented by an empirical equation of the form M(t)=MOexp (−kt) or dM/dt=−kM, where MO is the initial moisture content, M(t) is the moisture content at time t, and k is the drying constant. Under identical conditions, a high value of k was correlated with a shorter drying period. The drying process takes place in two phases: constant rate and falling rate periods, and the drying equation was solved to predict the total drying time. The mechanisms for the dehydration are the removal of unbound “free” water in the cell cavities and of “bound” water (water films) trapped within cells or chemically bound with solids as water of crystallization.

Planck’s formula for classical oscillators with stochasticity thresholds

In this paper a further contribution is given to the attempt of understanding the consequences that occur in statistical mechanics from the hypothesis that there exist stochasticity thresholds in the dynamics of an oscillator weakly coupled to a conservative system of oscillators, following an old idea of Nernst. By taking a point of view due to Boltzmann, and identifying the thermodynamic energy with that fraction of mechanical energy which is highly unstable and thus available to thermal exchanges, it is shown how one is thus led to Planck’s formula.

INTERPRETATION OF THERMAL PROSPECTION ON BARE SOILS

Thermal prospection, by which we measure the surface temperature of the ground, has interesting characteristics for archaeological research, especially in surveying large areas, since a thermography corresponds to several square kilometers. Like other surveying techniques, it has its limitations and a more complex interpretation of the results than electrical or magnetic prospecting. In addition, with organisational data processing and cost problems, the difficulties encountered in interpretation explain why this method has been of little use until now. Temperature can be measured by direct contact as well as from the radiation transmitted by the ground surface: but only a radiometer can be operational in the field, because in order to compare the points of the surface it is necessary that the temperature should not vary during the time of the survey, which is limited to a few minutes. In the presence of vegetation one does not measure the ground surface temperature but that of the plants, which is controlled by plant transpiration. Archaeological features may be very well detected by plant temperature, as in the case of Villeneuve la Guyard on the 4th May 1976 (Tabbagh 1979), but it is a non-direct detection and the interpretation needs knowledge about plant metabolism and soil conditions on which it depends. This knowledge is necessary in order to determine at which stage of plant development it will be interesting to make a survey. The interpretation of results for bare soils is a direct application of the physical laws of thermal exchanges. In previous papers several rules have been established (Tabbagh 1973 and 1977a): (1) soil temperature depends on solar radiation and exchanges with the atmosphere, (2) the diurnal variation of the flux of heat in the ground has a too low penetration depth to give a measurable anomaly linked to features lying below the cultivated superficial layer, (3) slow transient variations of the flux may induce anomalies revealing archaeological features.

Winter Breeding in Emperor Penguins: A Consequence of the Summer Heat?

During their Antarctic breeding season Emperor Penguins (Aptenodytes forsteri) fast as long as 115 days and walk as far as 200 km from the sea to their rookeries and back. The breeding males stand on the ice incubating a single egg for about 62 days. We examined the thermal exchanges between Emperor Penguins and their environment in an attempt to answer the question: Why do these penguins breed during the winter? Existing physiological data for the temperature regulation of Emperor Penguins were used in conjunction with meteorological data from the Antarctic to model thermal interactions between the animal and its environment. Computer simulations for the energy budget of Emperor Penguins in a variety of microclimatic conditions indicate that the heat load imposed on these birds during the Antarctic summer does not limit their choice of breeding season. We conclude that the physiological and morphological characteristics associated with heat exchange do not constrain Emperor Penguins to winter breeding.

Frequency variations in quartz crystal resonators due to internal dissipation

The amplitude-frequency effect in quartz crystal resonators is a consequence of the nonlinear elastic behavior of the crystal. However, dissipation due to internal losses can also induce frequency changes caused by amplitude variations, but in this case with a large time constant. Nonuniform heating introduces thermal stresses and density changes which act on a propagating wave by nonlinear coupling through the third-order elastic constants. Both the heat and wave equations are solved for the quartz crystal, and thermal exchanges with the surrounding medium are considered for several types of mounting. The resonator frequency change due to variation of the driving level, which appears as the indirect amplitude-frequency effect, is calculated. It is shown that the large time constants experimentally observed can be mainly attributed to thermal diffusion through the connection wires.

Influence des echanges thermiques entre le capteur et ses supports sur la mesure des fluctuations de temperature dans un ecoulement turbulent

The transfer function for a platinum wire used as a temperature sensor has been calculated in order to determine the influence of conduction effects on fluctuations measurement in a turbulent airflow. This calculation shows that thermal exchanges between the sensor and its supports can appreciably act upon measurements of variance and spectrum of temperature fluctuations. The influence of these conduction effects on measurements performed behind a heated grid is pointed out.

A Comparison of Two Optical Methods for Measuring Line Averages of Thermal Exchanges Above Warm Water Surfaces

Abstract Line averages of the vertical transports of sensible and latent heats in the surface boundary layer are determined above a warm water surface by the use of two optical, line-of-sight, remote-sensing techniques. In one method, the amount of atmospherically-induced blurring of images is observed visually with the aid of a small portable astronomical telescope. The other method utilizes unaided observations of the formation of inferior mirages, in particular the heights of distant objects that appear to be partially hidden behind the mirage. In the present application above industrial cooling ponds, lines of sight are within 1 m of the water surface and 0.4 to 1.5 km in length. The evaluation of the heat fluxes also requires estimates of the surface friction velocity and the ratio of the sensible to latent heat fluxes, each of which can be obtained with sufficient accuracy over a warm water surface from relatively simple measurements of temperatures and wind speeds. The direct visual measurements ar...

Étude thermique de la vitesse de l'interface solide-liquide au cours d'une solidification bridgman

A model of thermal exchanges applied to Bridgman solidifications is presented. It gives an analytic expression relating the solidification rate to geometrical and thermal parameters: length and radius on one side, temperatures, conductivity, and exchange coefficients on the other side. Series of curves permit to choose the values of parameters for which the interface rate is close to the translation rate of the crucible with respect to the furnace.

Control of Heat Exchanges: An Alternate Concept for Temperature Regulation in Man

A model of the thermoregulatory mechanisms of man subjected to a heat load is presented. It is based on the hypothesis that the thermoregulatory system controls mainly the level of body heat storage but not the deep body temperature. The thermoregulatory system would thus appear to be a “feedback control system” of the thermal exchanges rather than a “regulator” of the deep body temperature.