Thermal Exchanges Research Articles

Experimental performance of a direct evaporative cooler operating during summer in a Brazilian city

This paper presents the basic principles of the evaporative cooling process for human thermal comfort, the principles of operation for the direct evaporative cooling system and the mathematical development of the equations of thermal exchanges, allowing the determination of the effectiveness of saturation. It also presents the results of experimental tests in a direct evaporative cooler that take place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, and the experimental results are used to determinate the convective heat transfer co-efficient and to compare with the mathematical model.

Guided elastic waves in porous materials saturated by air under Lamb conditions

The propagation of guided elastic waves in porous materials saturated by air under Lamb conditions is studied theoretically and experimentally. The modes are derived from expressing the boundary conditions on the normal and tangential stresses and the displacements at the interfaces between the porous layer and the surrounding fluid. The stresses and the fluid pressure inside the porous medium are obtained from Biot’s equations of poroelasticity. Symmetrical and antisymmetrical modes are found when the porous layer is loaded by the same fluid on both sides. Damping mechanisms include viscous and thermal exchanges between the solid and the fluid, in addition to the classical structural damping. Using an experimental setup based on the generation of standing waves in the layer and taking the spatial Fourier transform of the displacement profile, the phase velocities of three modes were measured for two porous materials in a frequency range between 80 Hz and 4 kHz. The measurements confirm the theoretical predictions and provide information on the shear modulus of a sound-absorbing material in a wide frequency range.

Impact of the anomalous thawing in the Tibetan Plateau on summer precipitation in China and its mechanism

The impact of the anomalous thawing of frozen soil in the late spring on the summer precipitation in China and its possible mechanism are analyzed in the context of the frozen soil thawing date data of the 50 meteorological stations in the Tibetan Plateau, and the NCEP/NCAR monthly average reanalysis data. Results show that the thawing dates of the Tibetan Plateau gradually become earlier from 1980 to 1999, which is consistent with the trend of global warming in the 20th century. Because differences in the thermal capacity and conductivity between frozen and unfrozen soils are larger, changes in the freezing/thawing process of soil may change the physical properties of the underlying surface, thus affecting exchanges of sensible and latent heat between the ground surface and air. The thermal state change of the plateau ground surface must lead to the thermal anomalies of the atmosphere over and around the plateau, and then further to the anomalies of the general atmospheric circulation. A possible mechanism for the impact of the thawing of the plateau on summer (July) precipitation may be as follows. When the frozen soil thaws early (late) in the plateau, the thermal capacity of the ground surface is large (small), and the thermal conductivity is small (large), therefore, the thermal exchanges between the ground surface and the air are weak (strong). The small (large) ground surface sensible and latent heat fluxes lead to a weak (strong) South Asian high, a weak (strong) West Pacific subtropical high and a little to south (north) of its normal position. Correspondingly, the ascending motion is strengthened (weakened) and precipitation increases (decreases) in South China, while in the middle and lower reaches of the Changjiang River, the ascending motion and precipitation show the opposite trend.

On heat transfer in gas–solid pipe flows: Effects of collision induced alterations of the flow dynamics

An Eulerian–Lagrangian approach is used to model turbulent gas–solid flows in heated pipes. The key problems of dispersion, turbulence modulation and particle collisions are addressed by means of a complete four-way description. In particular, special care has been brought to the effects of both particle–particle and particle–wall collisions, which are known to be important on the dynamical point of view. Dilute or moderately dense suspensions are considered (loading ratio up to 10). Comparison of the numerical results with available experimental data about suspension Nusselt numbers leads to satisfactory agreement in vertical pipe flows and qualitative accordance in the horizontal case. It is confirmed that the collision treatment is a very crucial and sensitive aspect of the global model, in so far that it strongly affects the flow dynamics. The collision induced alteration of the velocity and concentration profiles leads to significant changes in the heat exchange rate. Since the question may arise of possible effect of conductive heat transfer during collisions, this point has been examined too, confirming that the direct thermal exchanges during solid–solid contacts are negligible under the present conditions.

Numerical Simulation of Gas-Jet Wiping in Steel Strip Galvanizing Process

We have investigated the film thickness and thermal exchange occurring during the coating and cooling of a liquid film on a substrate. The aim of our study is to demonstrate the interest of Direct Numerical Simulation tools for Interfacial flows to understand, on the one hand, the mechanisms involved in the deposition of the film on the moving substrate, and on the other, to measure characteristic parameters of the process. The direct numerical simulation is presented here in two-dimensions based on the Navier-Stokes equations, generalized to free surface flows. They are approximated by VOF (Volume Of Fluid) methods. A Large Eddy Simulation (LES) model is used to take the turbulence into account. We attempt to define the governing parameters of the process and propose a description of the flow. The characteristic parameters of the wiping of the film and the associated thermal exchanges are presented in terms of wiping thickness, transfer coefficient, cooling rate and temperature gradient along the film.

The calculation of the mean radiant temperature of a subject exposed to the solar radiation—a generalised algorithm

The thermal sensation experienced by a subject in a confined environment is significantly affected by the radiative heat exchange between the human body and the surrounding surfaces: it contributes as far as 30% of the whole thermal exchanges of the subject. Besides, the presence of high-intensity radiation sources like, for example, the sun, may appreciably modify the radiant field to which people are exposed. As a consequence, this could alter notably the comfort conditions. In order of properly taking into account this issue, a simple analytical method is introduced in this work, that allows the easy evaluation of the thermal radiant field induced by the presence of the solar radiation. An application to a typical thermal comfort computation is finally presented.

Study on the Characteristic of IR Radiation from the Target of a Shell Containing the Cone

In this paper, based on the analysis of the thermal environment where the space target embeds and the study of thermal exchanges between the environment and the space target, the mathematics model of the temperature field for the shell containing the cone is built by using the thermal network method, the temperature and the illuminance value in the detector of the target is calculated, the characteristic of the distribution of temperature and IR radiation from the shell containing the cone is discussed in detail.

Study on the Characteristic of IR Radiation from the Space Target

In this paper, based on the analysis of the thermal environment where the space target embeds and the study of thermal exchanges between the environment and the space target, the mathematics model of the temperature field for the space target is present by using the thermal network method. The temperature and radiant flux of the target surface are calculated, the characteristic of the distribution of temperature and IR radiation from the space target is discussed in detail.

A MATHEMATICAL MODEL FOR DIRECT EVAPORATIVE COOLING AIR CONDITIONING SYSTEM

Air conditioning systems are responsible for increasing men's work efficiency as well for his comfort, mainly in the warm periods of the year. Currently, the most used system is the mechanical vapor compression system. However, in many cases, evaporative cooling system can be an economical alternative to replace the conventional system, under several conditions, or as a pre-cooler in the conventional systems. It leads to a reduction in the operational cost, comparing with systems using only mechanical refrigeration. Evaporative cooling operates using induced processes of heat and mass transfer, where water and air are the working fluids. It consists in water evaporation, induced by the passage of an air flow, thus decreasing the air temperature. This paper presents the basic principles of the evaporative cooling process for human thermal comfort, the principles of operation for the direct evaporative cooling system and the mathematical development of the equations of thermal exchanges, allowing the determination of the effectiveness of saturation. It also presents some results of experimental tests in a direct evaporative cooler that take place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, and the experimental results are used to determinate the convective heat transfer coefficient and to compare with the mathematical model.

A MATHEMATICAL MODEL FOR DIRECT EVAPORATIVE COOLING AIR CONDITIONING SYSTEM

Air conditioning systems are responsible for increasing men's work efficiency as well for his comfort, mainly in the warm periods of the year. Currently, the most used system is the mechanical vapor compression system. However, in many cases, evaporative cooling system can be an economical alternative to replace the conventional system, under several conditions, or as a pre-cooler in the conventional systems. It leads to a reduction in the operational cost, comparing with systems using only mechanical refrigeration. Evaporative cooling operates using induced processes of heat and mass transfer, where water and air are the working fluids. It consists in water evaporation, induced by the passage of an air flow, thus decreasing the air temperature. This paper presents the basic principles of the evaporative cooling process for human thermal comfort, the principles of operation for the direct evaporative cooling system and the mathematical development of the equations of thermal exchanges, allowing the determination of the effectiveness of saturation. It also presents some results of experimental tests in a direct evaporative cooler that take place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, and the experimental results are used to determinate the convective heat transfer coefficient and to compare with the mathematical model.

Effets thermoacoustiques dans un régénérateur cylindrique contenant un empilement de billes

Linear thermoacoustic theory is applied to study the regenerator of a Stirling machine or a Pulsed gas refrigerator regenerator filled with a stack of beds in a cylindrical envelope. Pressure, velocity and temperature amplitude are calculated, and a formulation of the thermal time constant of the regenerator is proposed. A global equivalent electrical scheme of the regenerator is also deduced. The total axial energy flux expression is demonstrated and the influence of the thermal time constant and of the various impedance is discussed. Axial variations of the energy flux, which correspond to thermal exchanges with the surroundings are expressed through consideration of thermoacoustical phenomena: thermal relaxation of the acoustical energy, viscous losses, thermoacoustical source/sink. Theoretical results are analysed to comment on temperature profiles along the regenerator in a periodical regime.

BOUNDARY INTEGRAL SOLUTION OF THERMAL RADIATION EXCHANGES IN AXISYMMETRIC FURNACES

A numerical algorithm is presented for predicting thermal radiation exchanges among surface elements in axisymmetric geometries. The algorithm is based on the Galerkin boundary-element formulation, enhanced with the best ideas of computer graphics used for image clipping and image object sorting to detect shadowing effects, and incorporating an adaptive integration scheme to improve the accuracy and efficiency of the computation of the radiation exchange kernels with abrupt discontinuities due to blockages of various complexities. Detailed descriptions of the algorithm and numerical implementation are given. Numerical results are compared with analytical solutions whenever available. The coupling of the surface radiation exchange calculations with those for Marangoni convection and heat conduction, as occurring in materials processing systems, is also discussed.

Realization and simulation of wall shear stress integrated sensors

In this paper we present the impact of hot wire geometry and fluid characteristics on the efficiency of a hot wire to be used as a wall shear stress sensor. Finite differences and finite elements modelling based simulators were used in order to evaluate the thermal performances of hot wire wall shear stress sensor. Several structures were explored including simple conductor or suspended above a cavity (free and sealed with an oxide membrane). It is found out that wire length, wire section and the dimensions of the micro-cavity lying below the hot wire are of fundamental importance on the thermal exchanges occurring between the hot wire and the fluid.

Effects of thermal exchanges on the YBCO transition and the superconductivity recovery

The influence of the current slope (dI/dt) and thermal exchanges on the magneto-thermal transition of textured bulk YBCO are analyzed within the framework of the resistive Fault Current Limiter (FCL). This study was carried out at 90.5 K both in pressurized liquid nitrogen, in vacuum and when the YBCO sample is in contact with alumina plates. The superconductor U(I) response is governed by a power law, depends on dI/dt and is influenced by the thermal surroundings. These phenomena can be justified if the dissipated energy during the transition is concentrated in a small volume of the conductor. The second part of this paper deals with the thermal recovery of bulk materials. Even in the case of film boiling where the exchanges are poor, a thermal gradient due to the weak thermal conductivity of HTc materials appears in the section of the conductor. This point and its consequences are described and discussed by using a finite element software.

A Joint Industry Project To Assess Circulating Temperatures in Deepwater Wells

Summary Drilling oil wells offshore in water depths exceeding 1000 m is not uncommon in many parts of the world. These conditions present a number of challenges for successfully drilling and completing these wells. A major challenge is predicting the temperature of various fluids, such as drilling muds and cements, when they are circulated or placed in the well and during static periods. What sounds like an easy task for the majority of land or offshore wells with shallow water depths turns out to be much more difficult for deepwater wells. This is particularly caused by inverse temperature gradients across the sea and convective thermal exchanges between the sea and the fluids in the riser and/or drillpipe. To better understand the phenomena involved, a series of temperature measurements was made as part of a joint industry project (JIP). The primary objective of these measurements was specifically to monitor the cooling effect of the sea by measuring the fluid temperature at the mudline depth. All temperature data were measured by a sensor deployed inside the bottomhole assembly (BHA) while circulating or drilling. Data were collected in the Gulf of Mexico, Brazil, Indonesia, and west Africa in an average water depth of 1200 m. A summary of the temperature measurements is presented, and comparison is also made with the predictions of a numerical simulator. Detailed interpretation of the data gathered pinpoints the importance of correctly accounting for the exact temperature profile in the sea as well as the velocity of sea currents vs. depth.

FIELD MEASUREMENT OF FLOW VELOCITIES, SUSPENDED SOLIDS CONCENTRATIONS, AND TEMPERATURES IN LAKE OKEECHOBEE1

ABSTRACT: This paper presents a field investigation of collecting hydrodynamic and sediment data in Lake Okeechobee with analyses examining mechanisms affecting sediment resuspension in the lake. Lake Okeechobee is a large subtropical lake located in south central Florida. Three‐dimensional flow velocities, suspended solids concentrations (SSC), and temperatures at four locations were measured from January 18 to March 5, 2000. Analyses of these data indicate that wind is the dominant factor in driving flow velocities and therefore transporting suspended solids. Wind direction also affects the SSC, especially in the north central and west littoral areas of the lake. The surface and bottom velocity components frequently flow in opposite directions, forming a stratification of the water column and preventing suspended solids from settling out. This retention of SSC in the water column may have a strong impact on the water quality of Lake Okeechobee. This study provides valuable storm event data and mechanism analyses, which will improve our understanding of the transport of suspended solids, thermal exchanges, and flow patterns within Lake Okeechobee.

Thermal exchanges and transition of bulk YBCO conductors

The influence of heat transfer during the transition of bulk YBCO was studied within the framework of the resistive fault current limiter. The aim of this work is to determine whether the cryogenic surroundings has an influence on the limitation ability and process. Samples are meanders (section: 4 mm 2) cut in YBCO single domain pellets. All the electrical measurements were carried out above 90 K. Firstly, the sample is introduced into a vacuum container. The magnetic penetration (AC losses signal), the transition (power law) and the thermal runaway are successively observed. The current slope influence on the V( I) characteristic during the transition is significant. The thermal runaway occurs for a small energy of 25 mJ cm −3. Secondly, the sample was plunged directly in a pressurised liquid nitrogen and its transition was compared to the vacuum experiment. It was observed that the transition depends on heat transfer conditions. The transition in a liquid is finally analysed and its influence on the current distribution is discussed.

Transient natural 2D convection in a cylindrical cavity with the upper face cooled by thermoelectric Peltier effect following an exponential law

The present study is based on a numerical calculation involving the finite volume method. The transient 2D natural convection in vertical cylindrical cavities is addressed. Two cases have been examined. First we study the case of an adiabatic air-filled cavity whose upper face is submitted to a cooling following an exponential law observed experimentally, imposed by a Peltier’s module. Furthermore, we study the convection in an insuline-filled cylindrical cavity whose partition has a very weak thermal resistance. Space–time evolutions of temperature and velocity are presented, with the influence of the main representative parameters of the investigation. Local and mean thermal exchanges have been calculated, and we propose a Nusselt–Rayleigh type relation. The aim of this study is to determine the heat transfer in a long autonomy isothermal cavity designed for the conservation of insulin cartridges or any other product.

Effect of uniform and non-uniform skin temperature on thermal exchanges in water in humans

We investigated the effect of uniform (UST) and non-uniform (NUST) skin temperature on thermal exchanges during a 3-h water immersion in five male subjects wearing (NUST) or not wearing (UST) a water-perfused garment. UST was achieved by immersing the nude subject in water up to the neck. For each subject, the water temperature was adjusted to the critical temperature ( T(cw), 31.4 +/- 0.9 degrees C) or 3 degrees C below T(cw) ( T(cw) - 3). NUST was achieved by perfusing different segments of the perfused garment with water of different temperatures. The water temperature of the segment was independently adjusted according to the skin temperature distribution in cold air, the mean skin temperature being the same as the UST. At T(cw) and T(cw) - 3, changes in esophageal and mean skin temperatures were identical in UST and NUST conditions, but the skin temperature of the trunk was higher and that of the limb was lower in the NUST condition. Heat production and the overall skin heat flux at T(cw) were identical in the two conditions, but those at T(cw) - 3 were about 25% lower ( P < 0.05) in NUST than in UST conditions. At T(cw) - 3, the overall tissue insulation was 36% higher ( P < 0.05) in NUST than in UST conditions, mainly because of higher limb insulation. Thermogenesis due to shivering was lower by 62% ( P < 0.05) in NUST than in UST. We conclude that the NUST condition increased tissue insulation and suppressed shivering. This suggests that a high skin temperature of the trunk attenuates shivering in cold water and increases the ability to defend body temperature more economically in cold water.

Thermal Control Simulation in High Pressure Treatment of Foods

High hydrostatic pressure (HHP) has become in the last few years a promising technology for food processing and preservation. Pressure treatment of foods always results in a temperature increase due to the work of compression. After compression, heat loss through the metal wall of the high-pressure vessel causes temperature gradients in the processed product. So, it is absolutely necessary to know how thermal exchanges in high-pressure treatments are produced and at what rate in order to establish the real conditions at which a given process is realised. In this paper, a modelling/simulation of the thermal exchanges taking place in a high-pressure pilot unit during different processes of pressurisation and depressurisation is presented. Good agreement between simulated and experimental values is found. This work involves an important advance in optimisation and regulation of high-pressure processes in food industry.