Intense Heat Loss Research Articles

Extreme heat loss in the Northern Red Sea and associated atmospheric forcing

A regional, high-resolution reanalysis was analyzed to explore extreme heat loss events in the Northern Red Sea (NRS) and their links to specific regional atmospheric circulation patterns. Such events are determinant for the overturning circulation of the Red Sea and occur frequently between November and March, with maximum frequency during December and January. During these events, the most intense heat loss, often with daily-averaged values lower than -1000 W/m2, is found over the southern half of the Gulf of Aqaba and along the western coastline of the open NRS. Analyses of the spatial modes of variability of these events suggest that the majority of them extend over the entire NRS in an almost uniform way; however, secondary, nonuniform patterns related to regional adjustment in the wind field are also identified. The uniform cold outbursts are associated with distinct atmospheric circulation patterns, which favor the transfer of cold air masses from higher latitudes over the eastern Mediterranean Seaviaa strong northwest wind field. Nonuniform events affect considerable parts of the NRS and occur when cold and dry air masses reach the NRS through the Middle East and the northern part of the Arabian Peninsula. The regional sea level pressure drives a clockwise rotation of the wind field that ultimately blows from the northeast/east direction. This rotation of the wind field favors local intensification and lee areas defined by the complex topography and characteristic gaps in the mountain chain along the eastern coastline of the NRS, reflecting the differentiations in the spatial distribution of the heat flux minima.

Numerical study of sporadic combustion waves in straight channels of different diameters

The dynamics of flames propagating in straight channels filled with a stationary low-Lewis-number premixed gas mixture is studied numerically. A method for determining the propagation velocity of a sporadic combustion wave consisted of separate flame spots is proposed. Dependencies of the sporadic combustion wave propagation velocity, the residual fuel concentration and the number of flame spots on the channel size and the value of radiation heat losses are obtained. Analysis of numerical results show that for the channels of diameter exceeding some value the number of separate cup-like fragments constituting sporadic combustion wave is proportional to the channel cross-sectional area. At smaller diameters, the number of flame spots changes insignificantly and is one or two. It is shown that one of the universal characteristics of the sporadic combustion wave depending only on mixture properties but independent on system geometry is the area necessary to accommodate one reacting spot. Flame velocity which is another fundamental combustion characteristic is found to be almost independent on channel size starting from some critical diameter. This diameter, however, depends on mixture properties or radiative heat loss intensity and corresponds to the sporadic flame containing from several to ten reacting spots. Thus, the main properties of sporadic combustion waves in wide channels can be determined by numerical modeling of the flame propagation in the relatively narrow channels in which the flame consists of 1–10 cup-like fragments.

Marine heat waves and tropical cyclones - Two devastating types of coastal hazard in South-eastern Africa

The coast of southeastern Africa is the only part of the continent to experience landfalling tropical cyclones (TCs), the most destructive and hazardous of weather systems. It is also a region which frequently experiences long-lasting marine heat waves (MHWs). Three coastal regions in southeastern Africa with high biodiversity and relatively large populations dependent on local coral reefs and artisanal fishing are analysed in terms of the characteristics of TCs and MHWs. Significantly increasing trends in MHW duration and frequency, TC heat potential and coral bleaching events are found during austral summer in all three coastal areas, with northeastern Madagascar experiencing the highest rate. The co-occurrence of TCs and MHWs in each region is also considered using a parameter-space plot of MHW intensity versus TC wind speed. A high percentage of pre-existing MHWs help strengthen TCs as they pass over the MHW region but an even higher percentage of the TCs tend to weaken or end MHWs after they both co-occur. Of the three coastal zones considered, northeastern Madagascar is more prone to landfalling severe cyclones and long-lasting MHWs. It is suggested that as the TC approaches the MHW region, the rapid increase in surface wind friction velocity, intense heat loss to the atmosphere and entrainment cooling are all important processes that determine the rate at which the MHW weakens towards its demise. TC-MHWs co-occurring together may be considered a compound extreme event. The magnitude of their impacts on the southeast African coast in recent decades highlights the need for increased monitoring and improved real-time forecasting of these devastating coastal hazards.

Numerical model for studying axisymmetric thermal energy storages based on granular phase change materials with gaseous heat transfer fluid

A mathematical model and novel numerical method are proposed for calculating flows of gaseous heat transfer fluid in axisymmetric thermal energy storages (TES) based on granular phase change materials (PCM). The developed computational technique is based on finite difference method and uses a combination of explicit and implicit schemes. Using the resolution tests, the detailed convergence analysis of the numerical method is carried out. The efficiency of the developed model is demonstrated by simulating the processes of heating, energy storage and cooling of the TES taking into account the lateral heat losses at various values of heat loss intensity and PCM heat conductivity. A comparison of the solutions obtained by 1D and 2D models is carried out, and the applicability of commonly used 1D approach for simulating processes in the axisymmetric TES with lateral heat losses is analyzed. It is shown that the ability of the 1D model to predict the averaged parameters decreases not only with increasing heat transfer coefficient, but also with decreasing PCM heat conductivity. The use of the 1D approach at large lateral heat losses does not provide guaranteed lower or upper bound for the sought parameters.

Combustion waves in narrow samples of solid energetic material: Chaotic versus spinning dynamics

In this work, we study the conditions determining the appearance of chaotic or spinning dynamic regimes for flames propagating in narrow cylindrical samples of energetic materials. Two models for heat losses from the sample surface are explored. The first model allows for a linear dependence of heat-loss intensity on the surface temperature. Another model assumes radiative heat losses modeled by the fourth degree of temperature. The global stability analysis of steady-state solutions is carried out for both models.It is shown that the main factor determining the wave dynamics is the sample thickness. In sufficiently thin samples, the axisymmetric instability mode prevails and the flame dynamics depends on the intensity of heat-losses. With an increase in their intensity, the auto-oscillatory dynamics first transforms into chaotic through the period doubling Feigenbaum’s cascade and, with a further increase in heat-losses, the dynamical extinction occurs. However if the sample is sufficiently thick, the most unstable mode is the mode with a nonzero azimuthal wave number. In this case, the spinning propagation regime is finally realized with a multi-headed flame structure formed. It is found that, qualitatively, this scenario is independent of the specifics of the heat-loss model.

High-resolution simulations of interactions between surface ocean dynamics and frazil ice

Abstract. Frazil and grease ice forms in the ocean mixed layer (OML) during highly turbulent conditions (strong wind, large waves) accompanied by intense heat loss to the atmosphere. Three main velocity scales that shape the complex, three-dimensional (3D) OML dynamics under those conditions are the friction velocity u* at the ocean–atmosphere interface, the vertical velocity w* associated with convective motion, and the vertical velocity w*,L associated with Langmuir turbulence. The fate of buoyant particles, e.g., frazil crystals, in that dynamic environment depends primarily on their floatability, i.e., the ratio of their rising velocity wt to the characteristic vertical velocity, which is dependent on w* and w*,L. In this work, the dynamics of frazil ice is investigated numerically with the high-resolution, non-hydrostatic hydrodynamic model CROCO (Coastal and Regional Ocean COmmunity Model), extended to account for frazil transport and its interactions with surrounding water. An idealized model setup is used (a square computational domain with periodic lateral boundaries, spatially uniform atmospheric and wave forcing). The model reproduces the main features of buoyancy- and wave-forced OML circulation, including the preferential concentration of frazil particles in elongated patches at the sea surface. Two spatial patterns are identified in the distribution of frazil volume fraction at the surface: one related to individual surface convergence zones, very narrow, and oriented approximately parallel to the wind/wave direction and one in the form of wide streaks with a separation distance of a few hundred meters, oriented obliquely to the direction of the forcing. Several series of simulations are performed, differing in terms of the level of coupling between the frazil and hydrodynamic processes, from a situation when frazil has no influence on hydrodynamics (as in most models of material transport in the OML) to a situation in which frazil modifies the net density, effective viscosity, and transfer coefficients at the ocean–atmosphere interface and exerts a net drag force on the surrounding water. The role of each of those effects in shaping the bulk OML characteristics and frazil transport is assessed, and the density of the ice–water mixture is found to have the strongest influence on those characteristics.

Coat color and morphological hair traits influence on the mechanisms related to the heat tolerance in hair sheep.

The principal focus of this study was to describe how the coat characteristics could affect the heat exchanges in animals managed in a hot environment. The Morada Nova ewes were monitored once a month, during 10 consecutive months, in three commercial flocks. Initially, an analysis was performed to measure the differences regarding the coat color in the thermoregulation mechanisms. The animals were grouped into 4 different groups according to coat tonality, as follows: dark red animals (group 1, N = 23), intermediate red color (group 2, N = 27), light red animals (group 3, N = 30), and white-coated animals (group 4, N = 30). The data were collected from 1100 to 1400 h, after the animals were exposed to 30 min of direct sunlight. The cluster analysis was performed considering the hair structural characteristics such as coat thickness (CT, cm), hair length (HL, mm), hair diameter (HD, m), and number of hairs (NH, hairs per unit area), after that these clusters were compared in relation to thermoregulatory mechanisms that include rectal temperature (RT, °C), respiratory rate (RR, breaths min-1), cutaneous evaporation (CE, °C), and respiratory evaporation (RE, W m-2). The groups were characterized and compared using mean and standard deviation, and the differences between the clusters were compared using the Tukey test with a 5% probability of error. In relation to coat color, no differences were found in groups 1, 2, and 3 regarding the activation of the thermoregulation mechanisms. The most different was observed in the totally white coat that presented different thermoregulatory responses as the highest sweating rate. White-coated animals showed a non-pigmented epidermis, and the hair structure is responsible to promote skin protection as necessary, such as a dense coat (1242.7 hair cm-2), long hair (14.2 mm), and thicker coat (7.38 mm). In red-coated animals, the hair structure favored heat loss to the environment, such as short hairs, less thick coat, and less hairs per square centimeter. All evaluated animals showed the rectal temperature within the reference limits for the ovine species, regardless of the coat color. In the analysis of clusters related to the physical structure of hair, it was possible to observe that the animals with thick hair, short hair, and less dense coat tended to have a higher capacity to eliminate heat through their respiratory rate and showed less intense heat loss by cutaneous evaporation. We verified that coat color presents a direct influence on the hair structure and the activation of mechanisms related to thermoregulation.

Self-Organised Approach to Designing Building Thermal Insulation

Traditionally, the uniform application of thermal insulation is practised within the built environment sector to achieve desired building regulation standards for energy efficiency. However, that approach does not follow the building heat loss field, and it is therefore poorly matched to the actual heat loss from the building, thus achieving sub-optimum energy performance. This research aims to visualise building heat loss field in three dimensions and to create self-organised thermal insulation patterns that are proportional in thickness to the intensity of heat loss. This is achieved using a 3D agent-based model, in which each agent that represents a miniature object of thermal insulation moves up the gradient of the heat loss representation and competes for its position with the neighbouring thermal insulation components, depending upon the gradient intensity. This creates a self-organised thermal insulation pattern through the competition between the thermal insulation components and through overcrowding in the areas with higher heat loss intensity. This helps to visualise the heat loss field and create a representation of thermal insulation that is ideally matched to it. The result is assessed for its energy performance using a conventional energy performance analysis. That analysis shows that this approach leads to reductions in energy consumption and carbon emissions in comparison with the conventional approach that uses the same amount of thermal insulation material. The overall result increases our understanding of 3D heat loss and introduces a new approach for designing building thermal insulation.

Dynamics of combustion waves in narrow samples of solid energetic material: Impact of radiative heat losses on chaotic behavior and dynamical extinction phenomenon

The dynamics of combustion waves propagating in a solid reacting material is investigated in the presence of radiative heat losses within the one-step Arrhenius reaction mechanism and the narrow sample approximation. As expected, a C-shaped response curve caused by the competition of heat release and heat dissipation is found where the lower branch of the response curve is unstable. Along the upper branch, the period doubling Feigenbaum’s cascade occurs with increasing values of heat-loss intensity leading to chaotic dynamics. Finally, for the further increase of the heat-loss parameter, the persistent chaotic dynamics is replaced by the sudden extinction. It must be emphasized that this extinction occurs for significantly lower heat losses than the steady extinction limit associated with the turning point of the C-shaped response curve. The most unexpected finding is that the temporal interval of chaotic behavior preceding the sudden extinction event may be extremely long with apparently unpredictable duration. These results may be relevant for combustion processes in solid energetic materials or other similar systems.

Consideration of the effectiveness of cryotherapy for different thicknesses of the skin using numerical modelling

In this paper, a method for measuring the temperature of the integumentary tissues of a person during a general cryotherapy treatment (WBC) is considered. The authors investigated the physical processes in the device for cryotherapeutic effects on the human body (cryosauna). Also we constructed the computer model of the object of study, using the finite element method. The mathematical model allows you to calculate the temperature of the integumentary tissue during the WBC procedure and at its end. The dependence of the intensity of heat loss on the thickness of the covering tissues was studied. The Bioheat Transfer module was used, which allowed taking into account thermal effects. Recommendations on the choice of cryosauna modes are formulated. Research aims to improve the efficacy and safety of WBC due to the individual characteristics of patients.

Propagation of premixed isobaric flames in narrow channels with heat-losses: The asymptotic analysis revised and reliance on the flame-sheet model

A novel comprehensive analysis based on the matched asymptotic expansions method is curried out for a problem of flame propagation in narrow channels with heat-losses using the activation energy as a large parameter. The novelty consists in presentation of the results, namely the dependence of the flame velocity versus the heat-loss intensity, in a parametric form. Comparison with the numerical calculations reveals that the asymptotic results becomes in close agreement at relatively low values of the expansion parameter in such a manner extending a practical implementation of the method. The analysis may be of utility to flame-sheet model applications increasing its accuracy in general combustion studies.

Volume, Heat, and Freshwater Divergences in the Subpolar North Atlantic Suggest the Nordic Seas as Key to the State of the Meridional Overturning Circulation

AbstractThe meridional overturning circulation (MOC) decreases rapidly in subpolar and Nordic regions where the warm upper layer loses its buoyancy due to intense heat loss, sinks, and flows south. The major volume loss of the upper limb of the MOC, ~9.6 Sv out of 18.4 ± 3.4 Sv, occurs as subduction across the Iceland Basin and Irminger Sea while the major heat loss, 273 TW out of 395 ± 74 TW is associated with the MOC branch that continues into the Nordic Seas where North Atlantic deep overflow water is produced. The 122 ± 79 TW heat flux convergence in the subpolar gyre appears to be significantly larger than various estimates of heat loss to the atmosphere. Much of the 0.09 ± 0.02 Sv freshwater divergence is presumably balanced by runoff from the Greenland shelf. These estimates suggest that the Nordic Seas, not the Labrador Sea, are key to the state of the MOC.

Draining and Upwelling of Greenland Sea Deep Waters

AbstractSince the beginning of the 1980s, deep convection has ceased in the Greenland Sea. The effects of this halt on the intense warming and salinification of Greenland Sea deep waters and on the “dome collapse” (sinking of the dome) have been intensively studied. However, their causes and the potential outflow of Greenland Sea deep and bottom waters toward southern adjacent basins have remained less investigated and are the focus of this work. Combining oceanic and atmospheric in situ data, together with satellite and reanalysis data, it is found that there is not a unique factor responsible for the halt of deep convection in the Greenland Sea. Changes in the Ekman pumping seem to only explain one tenth of the observed deepening of isopycnals of 1,300 m in the central Greenland Sea (dome collapse). Since the early 1980s, in addition, a decrease in the mean net heat loss from the ocean to the atmosphere and in the intensity and number of intense heat loss events would have contributed to the decrease in both the intensity of convective mixing and the generation of dense deep waters. The generation of waters not sufficiently dense to contribute to the reservoir of water below the Greenland Sea dome would have led to its emptying and sinking. More importantly, it is found that in situ evidence of a sustained bottom‐water upwelling driven by a secondary circulation cell whose major role in ventilating and draining the Greenland Sea deep and bottom waters has been largely underestimated.

Seasonal and Interannual Mixed‐Layer Heat Budget Variability in the Western Tropical Atlantic From Argo Floats (2007–2012)

AbstractOceanic and atmospheric processes were investigated in order to explore the causes of seasonal and interannual variability of sea surface temperatures (SST) in the western tropical Atlantic (WTA; 20°S–20°N, 15°W–60°W). A mixed‐layer (ML) heat budget was performed by using Argo profiles and supplementary data sets based on satellite and atmospheric products during the period 2007–2012. The WTA is divided into four boxes which represent the main temporal and spatial heterogeneities of this region. An analysis of error of each term pointed out that the mean net surface heat fluxes are systematically underestimated by 20 W m−2. A correction of this term provides realistic estimates of the vertical mixing which was obtained as residual term. In agreement with previous studies, the results show that surface flux is the most important process that governs the seasonal cycle of the heat content. Changes in shortwave radiation and latent heat fluxes dictate the oceanic response to the meridional migration of the ITCZ. Along the equator, surface fluxes modulate the annual cycle of ML temperature, but are strongly balanced by horizontal advection. The entrainment term proves a small contribution to the cooling of the ML. On an interannual time scale, the strong positive (negative) SST anomalies observed in 2010 (2012) were generated during the previous winter in both years, mainly north of 10°N, during which the wind anomalies were at the origin of intense heat loss anomalies. Horizontal advection may contributes to the maintaining of these SST anomalies in the equatorial zone and south Atlantic.

Dual-pulse laser ignition of ethylene-air mixtures in a supersonic combustor.

To reduce the energy of an individual laser pulse, dual-pulse laser ignitions (LIs) at various pulse intervals were investigated in a Mach 2.92 scramjet engine fueled with ethylene. For comparison, experiments on a single-pulse LI were also performed. Schlieren visualization and high-speed photography were employed to observe the ignition processes simultaneously. The results indicate that the energy of an individual laser pulse can be reduced by half via a dual-pulse LI method as compared with a single-pulse LI with the same total energy. The reduction of the individual laser pulse energy degrades the requirements on the laser source and the beam delivery system, which facilitates the practical application of LI in hypersonic vehicles. A pulse interval shorter than 40 μs is suggested for dual-pulse LI in the present study. Because of the intense heat loss and radical dissipation in high-speed flows, the pulse interval for dual-pulse LI should be short enough to narrow the spatial distribution of the initial flame kernel.

Propagation of a reaction front in a narrow sample of energetic material with heat losses: Chaotic regimes, extinction and intermittency

The influence of heat-losses on the flame dynamics in narrow samples of energetic material is investigated numerically. The model is reduced to a one-dimensional form with the flame-sheet approximation applied for the reaction rate. Both the steady-state solutions and its linear stability analysis are treated analytically. A typical C-shaped response curve is found for the dependence of the flame-propagation velocity on the heat-loss parameter, with solutions along the lower branch of slower flames being always unstable. It is found that a part of the upper branch of the C-shaped response curve is also unstable and the Poincaré–Andronov–Hopf bifurcation takes place at a certain value of heat-loss intensity even if the steady state solution is stable under the corresponding adiabatic conditions.The numerical simulations show that an increase in heat-losses induces, for sufficiently high Zel’dovich numbers, the Feigenbaum’s cascade of period doubling bifurcations after which a chaotic dynamics is setting in. The chaotic dynamics precedes the flame extinction occurring for the further increase of the heat-loss parameter which, nevertheless, remains significantly lower than the steady extinction limit dictated by the C-shaped response curve. Apparently, the parametric dependence of the extinction time in these cases is also irregular with appreciable disparities in magnitude. Finally, the intermittency effect is detected slightly below the extinction limit with irregular dynamics alternating by apparently periodic stages. These results may be important for the flammability limits theory and practical fire safety applications.

Investigation on the heat loss characteristic of underground household biogas digester using dynamic simulations and experiments

Underground household biogas digesters (UHBD) may achieve higher gas production in cold environments by adopting heating and insulation technologies. Fully understanding the heat loss characteristic of UHBD is the prerequisite to put these technologies into application. Here, a 3-D unsteady heat transfer model coupled with the digester and surrounding soil was developed to more accurately calculate the dynamic heat loss of UHBD. The solar-air temperature (SAT) was taken as the ground surface boundary condition parameter to describe the comprehensive influences of air convection, solar radiation, and long-wave radiation. Field experiments provided the initial and boundary conditions of the model. Furthermore, the changes in the digester heat loss process and the heat response process of the soil temperature under experimental condition were measured to create a more accurate model. The comparison between the numerical and experimental results showed that the errors were minimal when adopting SAT as the boundary condition parameter, in which the maximum error of average heat flux was 5.9%. Therefore, using the SAT boundary model, heat dissipation was calculated under different fermentation temperatures, burial depths, materials, and tectonic forms. The optimal fermentation temperature was 35 °C for maximum net energy production. Heat loss exponentially decreased with increase in burial depth. Insulation could significantly reduce the heat loss of a digester. The heat loss intensity could be decreased by 67.1% when the insulation was increased to 30 mm.

Main processes of the Atlantic cold tongue interannual variability

The interannual variability of the Atlantic cold tongue (ACT) is studied by means of a mixed-layer heat budget analysis. A method to classify extreme cold and warm ACT events is proposed and applied to ten various analysis and reanalysis products. This classification allows 5 cold and 5 warm ACT events to be selected over the period 1982–2007. Cold (warm) ACT events are defined by the presence of negative (positive) sea surface temperature (SST) anomalies at the center of the equatorial Atlantic in late boreal spring, preceded by negative (positive) zonal wind stress anomalies in the western equatorial Atlantic. An ocean general circulation model capable of reconstructing the interannual variability of the ACT correctly is used to demonstrate that cold ACT events develop rapidly from May to June mainly due to intense cooling by vertical mixing and horizontal advection. The simulated cooling at the center of the basin is the result of the combined effects of non-local and local processes. The non-local process is an upwelling associated with an eastward-propagating Kelvin wave, which makes the mixed-layer more shallow and preconditions the upper layers to be cooled by an intense heat loss at the base of the mixed-layer, which is amplified by a stronger local injection of energy from the atmosphere. The early cooling by vertical mixing in March is also shown to be a good predictor of June cooling. In July, horizontal advection starts to warm the mixed-layer abnormally and damps SST anomalies. The advection anomalies, which result from changes in the horizontal temperature gradient, are associated in some cases with the propagation of Rossby waves along the equator. During warm ACT events, processes are reversed, generating positive SST anomalies: a downwelling Kelvin wave triggers stratification anomalies and mixed-layer depth anomalies, amplified by a weaker injection of energy from the atmosphere in May–June. In July, warm ACT events are abnormally cooled due to negative horizontal advection anomalies resulting from processes similar to those that occur during cold ACT events. This additional cooling process extends the period of cooling of the ACT, reducing SST anomalies.

Influence of the heat loss intensity on the characteristics of the filtration combustion of solid organic fuels

The effect of lateral heat loss on the characteristics of the filtration combustion of solid organic fuels is studied experimentally and theoretically. The results show that, for an reaction trailing, an increase in the heat loss intensity leads to a marked reduction in the combustion temperature and an increase in the temperature at which fuel pyrolysis is complete, with the yield of liquid pyrolysis products remaining practically unchanged. For a reaction leading, an increase in the heat transfer coefficient causes a reduction in the combustion temperature and the yield of liquid pyrolysis products.

Gasless combustion of a mixture with an extremely low caloricity under heat loss conditions

The gasless combustion of a mixture with an extremely low caloric value in a cylindrical channel is numerically simulated. Because of an intense heat loss, on the surface of the reaction front emerge waves, which lead to cyclic changes in the chemical reaction rate, which acquire axial symmetry in the steady-state regime. In the ignition stage, a ring of reaction sites arises at the lateral surface of the sample. In the stage of depression, combustion persists on circular protrusions of the front, whereas a site of products is formed is the central part of the sample.