Heat Exchange Mechanism Research Articles

ТЕПЛОФИЗИКА ЩЕЛОЧНЫХ ЖИДКИХ МЕТАЛЛОВ. ЧАСТЬ 1: ТЕПЛОГИДРАВЛИКА И БЕЗОПАСНОСТЬ (РЕТРОСПЕКТИВНО-ПЕРСПЕКТИВНЫЙ ВЗГЛЯД)

As a result sixty years of experience in the development of alkaline liquid metals - sodium, eutectic alloys of sodium-potassium, lithium, cesium, the scientific bases of their application in nuclear power were created, the thermohydraulic parameters and highly efficient technological processes were substantiated, devices and systems were developed and implemented that ensured the successful operation of fundamentally new NPPs. The main areas of research were: thermal hydraulics, mechanisms of turbulent heat exchange, boiling and condensation of liquid metals, physical chemistry and liquid metal coolant technology, thermophysical properties of materials of the reactor, coolants, analysis and generalization of thermophysical data, bases of the experimental data, heat pipes, thermophysics of thermionic converters, high-temperature space nuclear power units and thermonuclear installations. The results investigations of hydrodynamics and heat transfer in channels of complex shape were studied, both under normal operating conditions and deformation of a fuel cell grid. The justified reduction of collector hydraulic irregularities in reactors, heat exchangers and steam generators. A channel-by-channel method of thermal-hydraulic calculation of fuel assemblies of the core of fast reactors for nominal and non-nominal operating regimes was developed. A theory of anisotropic porous body has been developed as applied to calculations of complex flows in reactors, heat exchangers and steam generator. Experimental investigations of the circulation and heat transfer in the reactor tank during forced circulation, in transitional and accident regimes with natural circulation showed a stratification and temperature pulsations in the upper mixing chamber and other regimes of the reactor installation. The boiling processes in a large volume, tubes and bundles of rods, the condensation of liquid metals, the occurrence of accident processes during sodium leaks in steam generators and sodium circuits have been experimentally investigated. The tasks of further research are formulated.

A novel method for estimating transient thermal behavior of the wellbore with the drilling string maintaining an eccentric position in deep well operation

An eccentric annulus is very common, caused by a change of hole-deviation angle and azimuth angle with well depth, leading to a difference in the heat transfer mechanism, compared with a concentric annulus. Based on the energy conservation principle, a rigorous transient heat transfer model in each region of wellbore and formation in the eccentric annulus was first established. The numerical model was validated by the field case. Combined with computational fluid dynamics software, the fluid flow regime and heat exchange mechanism in various eccentric annuluses were investigated. The results demonstrate that the annulus temperature gradually decreases with an increase in the eccentricity ratio of the drilling string, and the temperature difference between the concentric annulus and eccentric annulus of 0.9 is around 4 °C. Also, as the eccentricity ratio is elevated, fluid in the narrow gap is slowly heated; fluid in the wide gap is decreased due to a high flow rate carrying more formation heat to the wellhead. Most importantly, the maximal radial temperature difference in the wide gap under the various eccentric conditions is about 0.2 °C, and fluid radial thermal gradient could be neglected in the modeling process during the fluid circulation stage.

Modeling and Simulation of Phase Change Materials: Application to Building with Low Energy Consumption

The use of phase change materials must allow storage / destocking of energy from solar or internal gains. The applications in the case of light constructions will lead to an improvement in the thermal comfort of users and a reduction in energy consumption. The use of phase change materials (PCMs) in the energy-saving walls themselves makes it possible to substitute sensible heat storage for latent heat storage which requires a much lower volume and mass for the same amount of thermal energy. The objective of this work is the study of heat transfer by conduction during a phase change, and aims on the one hand to model and simulate the phase change behavior and on the other hand to approach the mechanism of heat exchange at the solid-liquid interface. The results obtained in 2D show the temporal evolution of the temperature, the position and the speed of the solid-liquid interface.

Efficiency of Different Heat Exchange Mechanisms for Ignition of Coal–Water Compositions

An optimal self-ignition of different fuels requires certain conditions according to the fuel properties. It is clear that the main factor is the surrounding temperature, but in fact, the chosen mechanism of heat transfer has a definitive influence on the ignition delay time. We have shown that ignition of the fuel composition containing 35–40 wt % water has very different dependencies of ignition delay versus temperature when radiative and convective heating are used. The estimation of a generalized activation energy of the ignition process at radiative heating shows that it is 3 times higher than that at convective heating. Together with this, the changes of the fuel composition up to 5 wt % do not change the activation energy more than 30%. The water evaporation decreases the efficiency of convective heat transfer and makes the growth of the furnace temperature almost useless for fuel ignition.

African lakes Nios and Mone — indicators of unique carbon element-deep respiration of the Earth

As a result of the analysis of space, geological and tectonic information, it was established that the true causes of natural ecolymnological disasters in Cameroon in 1984 and 1986 were modern fault-block tectonic movements, which are closely associated with seismic and geodynamics in one of the sections of the “living” Adamawa Mountains. The main cause of catastrophes must be considered the activation of endogenous processes occurring in the mantle and the tectonosphere of the Earth. Cosmo- and rotogenesis of the planet Earth, in the near-surface parts of the Earth’s crust of the Adamava mountain segment, led to intensive mountain-building and heat exchange processes, the causes and mechanism of which are closely associated with the rise of abnormally hot magmatic material and gas-liquid fluids containing CO2 from the mantle. Favorable transport routes for heat and mass transfer in the Earth’s lithosphere are volcanic channels, as well as the orthogonal and diagonal network of deep faults. At the same time, volcanic channels should be considered as unique drain pipes of our planet. The lethal carbon dioxide ejected from the depths of Lakes Nyos and Monun is mainly a differentiate of igneous melts, and the latter, in turn, have mantle “roots” extending to a depth of 200–300 km. The volcano-crater lakes Nios and Monun are confined to the nodes of the intersection of “living” deep faults, revealing the deep horizons of the planet, where in magmatic foci CO2 is predominant as products of differentiation. The author proposed a mechanism for the formation of a solid gas hydrate shell, a relatively tightly sealed volcanic crater. This giant gas hydrate plug prevented the gradual-passive circulation, i. e. outflow of CO2 into the hydrosphere and atmosphere coming from deep and intermediate magmatic foci. So, under the gas hydrate shell of the lakes Nios and Monun, a large amount of CO2 accumulated. Explosive emissions of significant amounts of lethal gas could appear only with the geodynamic activation of the earth’s crust, where these unique volcano-crater lakes are located. Seismotectonic processes contributed to the destruction of the gas hydrate shell and the breakthrough of CO2 through fractures, cracks and through the water membrane to the surface. Emissions of gases on the volcano-crater lakes Nyos and Monun are the brightest example (indicator) of the Earth’s carbon dioxide-deep degassing.

Development mechanisms for Mediterranean tropical‐like cyclones (medicanes)

Midlatitude cyclones with characteristics similar to tropical cyclones (also known as Tropical‐Like Cyclones, TLCs, or medicanes) are sometimes observed in the Mediterranean region. The Wind Induced Surface Heat Exchange (WISHE) mechanism has been considered responsible for their development, in analogy with tropical‐cyclone theory. However, some recent papers have proposed a different explanation, suggesting that the deep warm core in the TLC is mainly an effect of the seclusion of warm air in the cyclone core. To investigate the latter hypothesis, two case‐studies of Mediterranean TLCs are analysed here by means of high‐resolution numerical experiments. The evolution of the near‐surface equivalent potential temperature is followed along back‐trajectories around the cyclone centre, showing for both cases a strong heating when the parcel moves from the outer part of the cyclone to its inner, warmer core. Sensitivity experiments clarify the mechanism of cyclone intensification and the way the warm‐core structure is generated, showing that sea‐surface fluxes and/or condensation latent heating are fundamental to explain the intensification of the cyclones. However, the importance of air–sea interaction processes is case dependent. For the first cyclone, the intense sea‐surface fluxes, associated with tramontane and cierzo winds over the western Mediterranean Sea, transfer a large amount of energy from the ocean to the atmosphere in the area where the cyclone developed, so that the vortex is able to sustain itself in a barotropic environment and reach a tropical‐like structure at a later stage in its lifetime. For the second cyclone, the cyclone never develops a fully tropical‐like structure, evolving in the baroclinic environment associated with the potential vorticity streamer in which the cyclone formed. Based on the distinction emerging in this and other articles, a classification of medicanes in three different categories is proposed.

Using phase lags to evaluate model biases in simulating the diurnal cycle of evapotranspiration: a case study in Luxembourg

Abstract. While modeling approaches of evapotranspiration (λE) perform reasonably well when evaluated at daily or monthly timescales, they can show systematic deviations at the sub-daily timescale, which results in potential biases in modeled λE to global climate change. Here we decompose the diurnal variation of heat fluxes and meteorological variables into their direct response to incoming solar radiation (Rsd) and a phase shift to Rsd. We analyze data from an eddy-covariance (EC) station at a temperate grassland site, which experienced a pronounced summer drought. We employ three structurally different modeling approaches of λE, which are used in remote sensing retrievals, and quantify how well these models represent the observed diurnal cycle under clear-sky conditions. We find that energy balance residual approaches, which use the surface-to-air temperature gradient as input, are able to reproduce the reduction of the phase lag from wet to dry conditions. However, approaches which use the vapor pressure deficit (Da) as the driving gradient (Penman–Monteith) show significant deviations from the observed phase lags, which is found to depend on the parameterization of surface conductance to water vapor. This is due to the typically strong phase lag of 2–3 h of Da, while the observed phase lag of λE is only on the order of 15 min. In contrast, the temperature gradient shows phase differences in agreement with the sensible heat flux and represents the wet–dry difference rather well. We conclude that phase lags contain important information on the different mechanisms of diurnal heat storage and exchange and, thus, allow a process-based insight to improve the representation of land–atmosphere (L–A) interactions in models.

The Role of WISHE in Secondary Eyewall Formation

Abstract Numerical simulations are conducted to examine the role of the wind-induced surface heat exchange (WISHE) mechanism in secondary eyewall formation (SEF). The control experiment exhibits a coherent secondary eyewall structure as quantified by various parameters (e.g., the azimuthal-mean tangential wind and vertical velocity). Prior to SEF, an area characterized by increasing diabatic heating, enhanced inertial stability, and increasing supergradient winds at the top of the boundary layer is observed outside the eyewall. While these characteristics offer the possibility of both balanced and unbalanced dynamical pathways to SEF, the focus of this study is to evaluate the impact of WISHE. To examine the sensitivity of SEF to WISHE, the surface wind used for the calculation of surface heat fluxes is capped at several designated values and at different radial intervals. When the heat fluxes are moderately suppressed around and outside the SEF region observed in the control experiment, sensitivity experiments show that the formation of the outer eyewall is delayed, and the intensity of both eyewalls is weaker. When the heat fluxes are strongly suppressed in the same region, SEF does not occur. In contrast, suppressing the surface heat fluxes in the storm’s inner-core region has limited effect on the evolution of the outer eyewall. This study provides important physical insight into SEF, indicating that WISHE plays a crucial role in SEF and tropical cyclone evolution.

The approximation between thermal sensation votes (TSV) and predicted mean vote (PMV): A comparative analysis

The Fanger's predicted mean vote (PMV) model is used to evaluate thermal comfort. However, when PMV is compared to people's real thermal sensations, collected in field studies, some discrepancies are verified. One of the components for the calculation of PMV is clothing surface temperature (tcl), which can be a factor that contributes towards these discrepancies. The aim of this study was to propose alternative methods for predicted mean vote, seeking to reduce these discrepancies. The mathematical Newton's method was applied to obtaining tcl values. The PMV1 was determined by replacing the tcl values in the traditional equation of PMV as described by ISO 7730 (2005). The second model of thermal prediction, named as PMV2, was obtained by a multiple linear regression considering the thermal sensation votes, the metabolic rate and the six heat exchange mechanisms. Two groups (welders and army officers) were used to verify the accuracy of the methods used in this research. The results show that both methods were able to describe the thermal sensation votes. For the welder group, both PMV1 and PMV2 overestimated the results: when people voted TSV = 0, PMV1 = 0.64 and PMV2 = 0.23. In the case of the army officers group, applying PMV1, when TSV = 0, PMV1 = 1.47. The application of the multiple regression increased the potential of PMV2 to obtain responses closer to those provided by the occupants of the thermal environment studied: when TSV = 0, PMV2 = 0.0068, demonstrating a greater effectiveness of this method.

WISHE-Moisture Mode in an Aquaplanet Simulation.

This study aims to understand the nature of the tropical intraseasonal oscillations (ISOs) in an aquaplanet simulation performed using Geophysical Fluid Dynamics Laboratory's AM2.1 with a uniform sea surface temperature within the deep tropics. The simulated ISO resembles the observed Madden‐Julian Oscillation in that the spectral peak in precipitation appears at zonal wave number 1 and a period of ~60 days. Vertically integrated moist static energy budget of the simulated ISO shows that enhanced latent heat flux to the east of anomalously active convection causes eastward propagation of the ISO mode, which is weakly opposed by horizontal moisture advection. A series of mechanism denial experiments are conducted either by homogenizing select variables—surface wind stress, longwave radiative heating, and surface evaporation—with their zonal means from the control simulation or by suppressing free‐tropospheric moisture variation. Results of the mechanism denial experiments show that the simulated ISO disappears when the interactive surface evaporation is disabled, suggesting that the wind‐induced surface heat exchange (WISHE) mechanism is essential to the simulated ISO. Longwave cloud‐radiation feedbacks and moisture‐convection feedbacks affect horizontal scale and phase speed of the simulated ISO, respectively. Our results strongly suggest that the simulated ISO is the linear WISHE‐moisture mode of Fuchs and Raymond under horizontally uniform boundary conditions.

CONVECTION HEAT TRANSFER PERFORMANCE OF THE FRACTAL TUBE BANK UNDER CROSS FLOW

Convective heat transfer process between fractal tube bank and coolant water was simulated by using the finite element method. Both rectangular and circular tube banks were investigated in detail. For the relationship between Nusselt number and Reynolds number, the fractal circular tube bank shows the same trend as the rectangular tube one. Based on simulation results, the empirical heat transfer correlations for different stage fractals were regressed. The exponents of correlations increase with the fractal stage. Almost linear correlations between Nu and Re for fourth stage fractal were obtained, indicating a good heat transfer enhancement with the increase in fractal stages. Compared to the uniform tube bank, the fractal tube banks show better overall performance in enhanced heat transfer effect. This research would be helpful to understand the heat exchange mechanism of the multi-scale structure.

Heat exchange mechanisms in energy tunnel systems

Ground Source Heat Pump (GSHP) systems are known for their efficiency in space heating and cooling, using the ground as a source of energy. The ground energy can be accessed by the implementation of GSHP systems into subsurface structures such as transport tunnels. This technology comprises the embedment of High-Density Polyethylene (HDPE) pipes into the tunnel lining, converting it into tunnel ground heat exchangers (tunnel GHEs). There are studies available on the thermal interaction between the ground and the tunnel GHEs in the current literature. However, to properly evaluate the thermal performance of tunnel GHEs, the two-way thermal interaction between the tunnel GHEs and both the ground and the tunnel air must be considered. This requires the consideration of the ground conditions and the tunnel air temperature variations. In this work, the tunnel GHEs, the air inside the tunnel and the surrounding ground are modelled using finite element techniques. The models account for groundwater flow in the ground and induced airflow in the tunnel area. Numerical results highlight the significant effect of groundwater flow velocity on the temperature distribution pattern in the ground, the air temperature variations within the tunnel area and the average fluid temperature within the tunnel GHEs. To accurately capture the temperature variations in the tunnel air, it is crucial to account for both conduction and (induced) convection inside the tunnel as numerical results show significantly different temperature distribution inside the tunnel area in comparison to when only pure conduction is considered. In practice, the very common under/over-estimation of GSHP systems performance (including tunnel GHEs) is related – to a certain extent – to the lack of understanding regarding the operational environment of these systems (i.e., groundwater flow and tunnel air interactions with the activated lining). It may also lead to an inadequate air ventilation system design.

Locally adapted Brazilian ewes with different coat colors maintain homeothermy during the year in an equatorial semiarid environment

The present paper aimed to show the thermoregulatory responses of locally adapted Morada Nova sheep with different coat colors that were exposed to direct solar radiation in an equatorial semiarid region. Animals were classified into four groups according to the coat color as follows: (1) dark red, (2) intermediate red, (3) light red, and (4) white coats. Forty Morada Nova ewes were observed in for 7 consecutive months. The experimental variables measured were rectal temperature (RT), skin surface temperature (ST), respiratory rate (RR), skin surface evaporation (CE), respiratory evaporation (RE), and heat exchange by convection (HC) and radiation (R). Data were collected from 1100 to 1400h after the animals were exposed to 30min direct sunlight. The results showed that all groups maintained homeothermy. The RR was higher in the animals of groups 1 to 3, which also showed higher values for ST when compared to the white-coated animals. Sensible heat exchange mechanisms were not important for heat loss, and R was a significant source of heat gain from the environment for the animals. Groups 1, 2, and 3 used RR more intensely than group 4 (P value = 0.001); however, CE was higher for the white-coated animals. It was concluded that Morada Nova sheep are well adapted to the semiarid environment, regardless of coat color.

Impact of shortwave multiple reflections in an urban street canyon on building thermal energy demands

The urban fabric plays a fundamental role in convective and radiative heat exchanges between buildings. The main parameters which influence these heat exchange mechanisms are due to climate conditions such as air temperatures/humidity, wind speed/direction and solar irradiance. These weather data are related on where our cities are located rather than on how they are built. In this work, a building energy simulation tool is exploited to study the impact of multiple shortwave inter-reflections exchanges in an urban environment with the aim of evaluating their influence on the thermal energy demand of a building. These multiple radiative exchanges modify the buildings envelope energy budget influencing space cooling and heating demand. A street canyon model validated in a previous work was used to investigate the effects of the urban radiative trapping. Due to multiple shortwave reflections, the effective solar radiation absorbed by the buildings envelope surfaces is higher than in a street canyon building where only shadowing phenomena due to canyon geometry are considered. A comparison has been performed between these two configurations as a function of several driving parameters such as street canyon aspect ratio, orientation, transparent/opaque surfaces ratio and solar absorption. The goal is to characterize how these parameters influences the inter reflections inside an urban canyon and thus the buildings energy demands. Increases in cooling demand up to 35% and decreases in heating demand up to 7.5% are found.

A versatile computational tool for model-based design, control and diagnosis of a generic Solid Oxide Fuel Cell Integrated Stack Module

This work focuses on the development of a control and diagnosis-oriented model of a Solid Oxide Fuel Cell (SOFC) integrated system module (ISM). Fuel cell researchers and developers are currently investing significant resources on such fuel cell-based energy conversion systems, as a consequence of their adaptability to a variety of applications and power size. In this configuration, heat exchanges among components are difficult to manage and, consequently, reliable model-based tools are required to enhance the design phase. Therefore, the main heat exchange mechanisms (including radiation, here modeled via view-factors-based approach) are evaluated by simplifying 3-D geometries, as well as assuming the outlet temperature from each component as its state variable. The resulting lumped parameters model was proven effective in achieving the required compromise between accuracy and computational time, particularly in view of its real-world deployment for advanced balance of plant (BoP) analyses, as well as within control and diagnostic model-based tasks. Moreover, the entire SOFC ISM model is generic and can be suitably applied to different layouts, through the characterization of main model parameters. Simulation results are in agreement with experimental data collected on a non-conventional micro-CHP system, named HoTBox™, tested under nominal operating conditions within the EU funded project DIAMOND.

Underwater noise emitted during small-scale air entrainment events

Abstract The breaking wave phenomenon significantly takes part in the mechanisms of mass, heat and gas exchange at the air-water boundary and depends on the wind velocity. Some of the energy dissipated during this process is converted into underwater sound emitted by oscillating gas bubbles and bubble plumes. However, the underwater noise accompanying the lowest wind speed conditions has received only little attention. This report describes a study aimed at advancing the knowledge of underwater noise emission from air bubbles injected during small-scale breaking events occurring on the water surface. Results of model experiments performed in a small tank are presented. The object of the research is the relationship between the generated noise and the dissipated potential energy of water poured into a tank filled with water of varying physical water properties. Additionally, the impact of various water properties such as salinity, surface tension or microscale gas bubbles was examined. The experiment revealed that noise spectra are affected by different water properties and most likely reflect the varying efficiency of bubble formation and bubble size spectra.

HEAT AND MASS TRANSFER AND RESISTANCE CHARACTERISTICS OF EVAPORATIVE AIR COOLER WITH HIGH FINNED-TUBE BUNDLES

A theoretical model was suggested and experimental analysis of heat and mass exchange mechanism of a high finned-tube evaporative air cooling device was made. Experiments showed that the heat transfer of the upper four rows of tubes was higher than that of the lower four rows. The upper and lower layers of the proportion of water spray experiments were carried out. In the experimental range, the higher the proportion of the upper water spray, the greater the overall heat transfer. Besides, evaporation and heat transfer of water film were not only related to the air flow and spray water intensity, but also were affected by the relative humidity of imported air, with decrease in the relative humidity of imported air decreased, and with evaporation and heat transfer between air and water film strengthened. According to the experiments, the correlations of evaporation loss and heat transfer were fitted by air flow, spray water intensity, and relative humidity, which showed close agreement with the experimental results to within ± 11% and ± 9%, respectively. The correlation of pressure drop in the high finned-tube bundle was related to air flow rate and the intensity of the spray water, and the fitting error was within ± 8%. This study was instructive for the design of wet air cooler with high finned-tube bundle.

About the shortwave multiple reflections in an urban street canyon building related to three different European climates

Energy exchanges between buildings is affected by urban fabric. As a matter of fact, heat exchange between adjacent buildings is due to convective and radiative heat flows. The main parameters which influence these heat exchange mechanisms are due to climate conditions such as air temperatures/humidity, wind speed/direction and solar irradiance. Most building energy simulations are done on an independent single building with typical meteorological year (TMY). These TMY meteorological data cannot represent the state of the urban microclimate and rather ignores the microclimate influence on buildings adjacent to street canyons. However, solar radiation shading and reflection of the environment within the street canyons are important factors affecting the energy consumption of buildings. In this work, a building energy simulation tool is used to study the impact of multiple shortwave inter-reflections in an urban environment. A street canyon model validated in a previous work was modeled in TRNSYS in order to simulate the effects of the urban radiative trapping. An urban canyon with aspect ratio H/W=1 was chosen, with South-North orientation, with transparent/opaque surfaces ratio Atr/Aop=0.5 and 4 values (0.2, 0.4, 0.6, 0.8) of reflectance factor of the envelope surfaces. The goal is to characterize how solar absorption influence the urban energy requirements. The analysis was conducted for 3 cities in different climatic zones: Rome, Palermo and Krakow.

Bugs battle stress from hot blood.

A heat exchange mechanism in the head of kissing bugs helps to prevent stress and regulate their temperature while they feed on warm blood.

A model based on the Pennes bioheat transfer equation is valid in normal brain tissue but not brain tissue suffering focal ischaemia.

Ischaemic stroke is a major public health issue in both developed and developing nations. Hypothermia is believed to be neuroprotective in cerebral ischaemia. Conversely, elevated brain temperature is associated with poor outcome after ischaemic stroke. Mechanisms of heat exchange in normally-perfused brain are relatively well understood, but these mechanisms have not been studied as extensively during focal cerebral ischaemia. A finite element model (FEM) of heat exchange during focal ischaemia in the human brain was developed, based on the Pennes bioheat equation. This model incorporated healthy (normally-perfused) brain tissue, tissue that was mildly hypoperfused but not at risk of cell death (referred to as oligaemia), tissue that was hypoperfused and at risk of death but not dead (referred to as penumbra) and tissue that had died as a result of ischaemia (referred to as infarct core). The results of simulations using this model were found to match previous in-vivo temperature data for normally-perfused brain. However, the results did not match what limited data are available for hypoperfused brain tissue, in particular the penumbra, which is the focus of acute neuroprotective treatments such as hypothermia. These results suggest that the assumptions of the Pennes bioheat equation, while valid in the brain under normal circumstances, are not valid during focal ischaemia. Further investigation into the heat exchange profiles that do occur during focal ischaemia may yield results for clinical trials of therapeutic hypothermia.