Spatial Temperature Gradients Research Articles

Contact-free monitoring of circulation and perfusion dynamics based on the analysis of thermal imagery.

Acute circulatory disorders are commonly associated with systemic inflammatory response (SIRS) and sepsis. During sepsis, microcirculatory perfusion is compromised leading to tissue hypoperfusion and potentially to multiple organ dysfunction. In the present study, acute lung injury (ALI), one of the major causes leading to SIRS and sepsis, was experimentally induced in six female pigs. To investigate the progress of body temperature distribution, measurements with a long-wave infrared camera were carried out. Temperature centralization was evidenced during ALI owing to impairments of peripheral perfusion. In addition, statistical analysis demonstrated strong correlations between (a) standard deviation of the skin temperature distribution (SD) and shock index (SI) (p<0.0005), (b) SD and mean arterial pressure (MAP) (p<0.0005), (c) ΔT/Δx and SI (p<0.0005), as well as between (d) ΔT/Δx and MAP (p<0.0005). For clarification purposes, ΔT/Δx is a parameter implemented to quantify the spatial temperature gradient. This pioneering study created promising results. It demonstrated the capacity of infrared thermography as well as of the indexes, SD and ΔT/Δx, to detect impairments in both circulation and tissue perfusion.

Numerical study of influence of temperature and matrix cracking on type IV hydrogen high pressure storage vessel behavior

Type IV hydrogen high pressure storage vessels are subjected to severe thermo-mechanical loading due to fast filling. In this study a thermo-mechanical model of this kind of tank is developed, taking into account wind angle and thickness variation in domes, thermal dependencies of mechanical and thermal material properties and damage. Influence of damage and temperature on tank behavior is analyzed with isothermal calculations at different temperatures (−40°C, 25°C and 85°C) before performing a simulation of filling which leads to spatial and temporal temperature gradients in the structure. Influence of this complex thermal loading on the mechanical response of the tank is also studied.

Critical assessment of the 2°c-min−1 threshold for thermal stress weathering

Rock surface temperature changes in excess of 2°C-min−1 have been argued to result in permanent strain in rock. The single threshold value and the supposed importance of a rate of 2°C-min−1 rock temperature change is rejected, based on lack of a sufficient empirical basis, contrasting results from acoustic emission studies, varying strength properties of different rock types, lack of data on actual strain resulting from short‐term temperature variations, and lack of notable long‐term weathering effects in ordic environments. Based on the presented discussion, the role of thermal shock weathering in cold regions is questioned and renewed emphasis is placed on the importance of absolute temperature and measurement of strain, as well as spatial temperature gradients.

Activity and positioning of eurythermal hydrothermal vent sulphide worms in a variable thermal environment

Here we investigate behavioral responses to fine-scale spatial and temporal temperature gradients in a heat tolerant hydrothermal vent worm (Paralvinella sulfincola). While this species has been a model organism for understanding physiological adaptations to extreme environments, lacking are corroborative in situ experiments and characterization of temperature-related behaviors representing the lower thermal tolerance range of this species. To address this knowledge gap, we first quantified the upper thermal limit for this species using a rapid heating protocol executed remotely on the seafloor. Second, we used a combination of in situ observations and shipboard experiments to test for temperature-dependent patterns in activity and behavior. We confirm that P. sulfincola is remarkably eurythermal and demonstrates a thermal niche breadth exceeding 45°C. We further show that the activity and positioning of worms relate to temperatures within its lower preferred range (i.e., between 4 and 20°C). Worms tended to remain closer to their tube openings and held their branchial crown in a consistent location when fluids were relatively warm. By contrast, when fluids were cooler, both the distance worms were observed from their tubes and positioning of their branchial crown were more variable. A Bayesian hidden Markov model classified each worm at each time interval as being in a high or low activity state according to the magnitude of change in their orientation and how far they moved between successive time lapse images. We found that the transition between active and inactive states at any time period is related to fluid temperature. Our observations indicate that the behavior of the worms is temperature-dependent, which may in turn reflect temperature-related variables such as the delivery of food particles, dissolved oxygen concentration, or relative environmental variability. Our findings demonstrate that this species can respond behaviorally to very fine-scale environmental variability in a manner not simply predicted by models of increasing activity with temperature.

Transport of particles by a thermally induced gradient of the order parameter in nematic liquid crystals

We demonstrate manipulation and transport of microparticles and even fluorescent molecules by the thermally induced gradient of the order parameter in the nematic liquid crystal. We use IR light absorption of the tightly focused beam of laser tweezers to heat locally a thin layer of the nematic liquid crystal by several degrees. This creates a spatial gradient of temperature of the nematic liquid crystal over separations of several tens of micrometers. We show that a dipolar colloidal particle is attracted into the hot spot of the laser tweezers. The depth of the trapping potential scales linearly with particle radius, indicating that the trapping mechanism is due to elastic self-energy of the distorted nematic liquid crystal around the particle and softening of the elasticity with increased temperature of the liquid crystal. We also demonstrate that this thermal trapping mechanism is efficient down to the nanoscale, as fluorescent molecules are also transported into hotter regions of the liquid crystal. This effect is absent in the isotropic phase, which calls into question particle transport due to the Soret effect.

A spatially resolved physical model for transient system analysis of high temperature fuel cells

This work builds upon previous high temperature fuel cell (HT-FC) modeling studies, capturing both steady state performance and transient behavior of HT-FC stacks by merging simplified dimensional aspects of a planar fuel cell stack with first principles physical modeling. Dynamic simulations are developed that spatially resolve fluctuations in temperature, pressure and concentration distributions during transient operation. A significant portion of the heat transfer occurs prior to and after the air passes over the electrochemically active portions of the cell, justifying additional heat transfer pathways from the stack to the air in order to accurately characterize the thermal transients and temperature distributions in the HT-FC stack. The highly configurable MatLab-Simulink® model developed can simulate both solid oxide and molten carbonate fuel cells utilizing either direct or indirect internal reforming. The perturbation response characteristics of the dynamic model to load, fuel flow, air flow and composition perturbations are discussed, and control strategies are introduced that minimize temperature fluctuations. Analysis indicates air flow and inlet temperature controls are sufficient to control average temperature and average internal temperature gradients. Internal heat transfer dynamics substantially change the spatial temperature distribution and local temperature gradients during typical operating conditions and perturbations.

Modeling passive power generation in a temporally-varying temperature environment via thermoelectrics

This paper presents a model to predict the power generation of a thermoelectric generator in a temporally-varying temperature environment. The model employs a thermoelectric plate sandwiched between two different heat exchangers to convert a temporal temperature gradient in the environment to a spatial temperature gradient within the device suitable for thermoelectric power generation. The two heat exchangers are designed such that their temperatures respond to a change in the environment's temperature at different rates which sets up a temperature differential across the thermoelectric and results in power generation. In this model, radiative and convective heat transfer between the device and its surroundings, and heat flow between the two heat exchangers across the thermoelectric plate are considered. The model is simulated for power generation in Death Valley, CA during the summer using the diurnal variation of air temperature and radiative exchange with the sun and night sky as heat sources and sinks. The optimization of power generation via scaling the device size is discussed. Additional applications of this device are considered.

Connectivity Planning to Address Climate Change

As the climate changes, human land use may impede species from tracking areas with suitable climates. Maintaining connectivity between areas of different temperatures could allow organisms to move along temperature gradients and allow species to continue to occupy the same temperature space as the climate warms. We used a coarse-filter approach to identify broad corridors for movement between areas where human influence is low while simultaneously routing the corridors along present-day spatial gradients of temperature. We modified a cost-distance algorithm to model these corridors and tested the model with data on current land-use and climate patterns in the Pacific Northwest of the United States. The resulting maps identified a network of patches and corridors across which species may move as climates change. The corridors are likely to be robust to uncertainty in the magnitude and direction of future climate change because they are derived from gradients and land-use patterns. The assumptions we applied in our model simplified the stability of temperature gradients and species responses to climate change and land use, but the model is flexible enough to be tailored to specific regions by incorporating other climate variables or movement costs. When used at appropriate resolutions, our approach may be of value to local, regional, and continental conservation initiatives seeking to promote species movements in a changing climate. Planificación de Conectividad para Atender el Cambio Climático.

Predicting range shifts under global change: the balance between local adaptation and dispersal

Bioclimate envelope models (BEMs) have often been criticized as being too simplistic due to e.g. not incorporating effects of biotic interactions or evolutionary adaptation. However, BEMs are widely applied and have proven to be often useful. Here we investigate, under which conditions evolution of dispersal, local adaptation or interspecific competition may be of minor importance for forecasting future range shifts. Therefore we use individual‐based simulations of metapopulations under climate change living in spatial temperature gradients. Scenarios incorporate single‐species systems or systems with competing species, respectively. Dispersal rate is evolving and adaptation to local conditions may also evolve in some scenarios. Results show that in single‐species scenarios excluding evolutionary adaptation, species either follow optimal habitat conditions or go extinct if habitat connectivity is too low. These simulations are in close accordance to predictions from BEMs. Including evolutionary adaptation qualitatively changes these results. In the absence of competing species the species either completely invades the world or goes extinct. With competitors, results strongly depend on habitat fragmentation. For highly connected habitats the range border may shift as predicted by BEMs, for intermediate connectivity it will lag behind, while species will go extinct if fragmentation is too high. Our results indicate that (simple) BEMs may work well if habitats are well connected and species will not encounter many difficulties in dispersing to new sites. Selection in this case may promote evolution of even higher dispersal activities. We thus show that the presence of biotic interactions may be ignored for predictions of range shifts when high dispersal can be expected.

Temperature Gradient Driven Lasing and Stimulated Cooling

A laser can be understood as a thermodynamic engine converting heat to a coherent single mode field close to Carnot efficiency. To achieve lasing, spectral shaping of the excitation light is used to generate a higher effective temperature on the pump than on the gain transition. Here, using a toy model of a quantum well structure with two suitably designed tunnel-coupled wells kept at different temperatures, we predict that lasing can also occur on an actual spatial temperature gradient between the pump and gain regions. Gain and narrow band laser emission require a sufficiently large temperature gradient and resonator quality. Lasing appears concurrent with amplified heat flow between the reservoirs and points to a new form of stimulated solid state cooling. In addition, such a mechanism could reduce intrinsic heating and thus extend the operating regime of quantum cascade lasers by substituting phonon emission driven injection by a phonon absorption step.

Nanosecond‐scale time‐resolved electron imaging during laser crystallization of GeTe

AbstractLaser crystallization of amorphous phase change materials (PCMs) occurs on the nanosecond scale, during which it is challenging to probe the details of microstructural development experimentally. In this study, in situ laser‐induced crystallization of amorphous GeTe thin films was imaged with transmission electron microscopy with 15‐ns time resolution. These data provide insight into the crystallization process of PCMs during laser crystallization. The initial results allow us to begin to develop a detailed picture of the crystallization process when high spatial and temporal temperature gradients exist. Differences in the nucleation rate may be distinguished within the laser‐affected area and changes in the microstructure before crystallization suggest the occurrence of melting of the amorphous phase at the laser energy used. The amorphous melting temperature is estimated based on the thermodynamic data available.

Pyroelectric energy harvesting using liquid-based switchable thermal interfaces

The pyroelectric effect offers an intriguing solid-state approach for harvesting ambient thermal energy to power distributed networks of sensors and actuators that are remotely located or otherwise difficult to access. There have been, however, few device-level demonstrations due to challenges in converting spatial temperature gradients into temporal temperature oscillations necessary for pyroelectric energy harvesting. We demonstrate the feasibility of a device concept that uses liquid-based thermal interfaces for rapid switching of the thermal conductance between a pyroelectric material and a heat source/sink and can thereby deliver high output power density. Using a thin film of a pyroelectric co-polymer together with a macroscale mechanical actuator, we operate pyroelectric thermal energy harvesting cycles at frequencies close to 1Hz. Film-level power densities as high as 110mW/cm3 were achieved, limited by slow heat diffusion across a glass substrate. When combined with a laterally interdigitated electrode array and a MEMS actuator, the present design offers an attractive option for compact high-power density thermal energy harvesters.

Phosphor thermometry: A comparison of the luminescence lifetime and the intensity ratio approach

In the context of phosphor thermometry, the most common measurement strategies, namely the lifetime and the intensity ratio approach, have been compared. Using the phosphor Mg4FGeO6:Mn and the identical experimental setup, both techniques have been characterized in terms of precision and accuracy. Therefore, two-dimensional calibration measurements have been performed to evaluate temperature-lifetime and temperature-ratio characteristics as well as shot-to-shot and pixel-to-pixel standard deviations. In a second experiment, different parameters were varied at constant temperatures to quantify the accuracy of both measurement strategies. Finally, both methods were compared monitoring a spatial temperature gradient.

Trapped magnetic flux in superconducting niobium samples

Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with $1200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples.

Experimental Study of the Combustion Regimes Occurring in a Laboratory Combustor

This article examines the combustion regimes occurring in a small-scale laboratory cylindrical combustor, in which the burner and the exhaust port are mounted at the top end of the combustion chamber. Flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging are presented as a function of the air inlet preheat temperature and excess air coefficient, which in the present configuration implies also changes in the air inlet velocity. For three of these combustor operating conditions, detailed in-combustor measurements of temperature and of O2, CO2, CO, unburned hydrocarbons, and NOx concentrations are also reported. The flue-gas data reveal that, for a given air inlet preheat temperature, the air inlet velocity has an important impact on the NOx emissions, which decrease as the air inlet velocity increases. Furthermore, as the air inlet temperature increases, the burner is able to operate with higher excess air coefficients. For a given air inlet temperature, the combustion regime in the present combustor develops from conventional lean combustion at lower air inlet velocities (or lower excess air coefficients) to flameless combustion at higher air inlet velocities (or higher excess air coefficients). The OH* images and both the temperature and the chemistry fields indicate that the reaction zone for the conventional lean combustion regime is well defined and established near the burner with relatively high gradients along the combustion chamber length. In contrast, the flameless combustion regime, which occurs for higher reactants jet momenta, is characterized by a more distributed reaction zone, located far from the burner, as revealed by the chemiluminescence images that show relatively small gradients disperse over a large volume of the combustion chamber. In line with this, the spatial temperature and gas species concentration gradients are much lower in this case than in the case of the conventional lean combustion regime.

Temperature and humidity effects on superhydrophobicity of nanocomposite coatings

This work investigates temperature and humidity effects on the superhydrophobicity of polyurethane/organoclay nanocomposites. Previous reports of superhydrophobic degradation at decreasing surface temperatures for both low and high humidity were generally conducted in open environments. However, the present setup allows a thermally homogeneous environment, i.e., the temperature of the nanocomposite, air and water droplet are equal with no spatial temperature gradients. In such conditions, results showed stable retention of superhydrophobicity for both low humidity (RH &amp;lt; 20%) cool-down and warm-up cycles (20 °C to −3 °C to 20 °C). Similar performance was also observed for a high humidity (RH &amp;gt; 80%) cool-down cycle, though superhydrophobicity degraded during the warm-up cycle, which was attributed to dew condensation.

A Bimetallic Micro Heat Engine for Pyroelectric Energy Conversion

Abstract We present a novel concept of electric power generation from a spatial temperature gradient, achieved by integrating pyroelectric generators (PEGs) into a self-oscillating micro heat engine. The micro heat engine moves the PEGs between a hot and cold reservoir, thus bringing them in an alternating contact with a higher and lower temperature. By their pyroelectric nature, the PEGs convert this periodic temperature variation into electrical energy. The micro heat engine is fabricated from a 280 μm thick bimetallic strip and the PEGs from a 250 μm thick PZT sheet. At an oscillation frequency of 0.19 Hz the device generates 4.4 μW of electrical power from a 96 K temperature difference.

Temporal and spatial variation in ecosystem metabolism and food web carbon transfer in a wet‐dry tropical river

Summary1. High light availability and stable base flow during the dry season promote primary production in perennial rivers of the wet–dry tropics, in contrast to production during the wet season which is often limited by turbidity and scouring. The Mitchell River of northern Queensland (Australia) was studied to understand controls on aquatic production and respiration in the dry season in relation to spatial and temporal gradients of light and temperature.2. At three sites along the river, whole‐ecosystem gross primary production (GPP) and respiration (ER) were measured from diel changes of dissolved oxygen using the open‐channel single station method. Using stable carbon and nitrogen isotope analysis, aquatic consumers and their potential basal food resources were also assessed to determine food web relationships at the beginning and end of the dry season.3. Nutrient limitation of aquatic net primary production was implied from the oligotrophic conditions and high algal C:N ratios. Rates of GPP were comparable with other tropical and temperate rivers and were regulated by light availability.4. Respiration rates were high and similar to other tropical and subtropical rivers. Up to 52% of temporal variation of ER was explained by temperature, while P/R was lowest at the downstream site.5. Benthic algae were the major carbon source for primary and secondary benthic consumers (insects) in the dry season but not for higher consumers (fish and crustaceans). Despite high rates of ER, which were probably supported by decaying terrestrial C3 plant material, this carbon source was not identified as contributing to animal consumer biomass.6. While benthic algal production in the dry season sustained benthic invertebrates, the importance of external subsidies of carbon along the river, probably from the floodplain, was emphasised for fish and large invertebrates, which evidently were feeding on carbon sources not present in channel waterholes during the dry season.

Theoretical interpretation of X-rays photo-absorption in medium- Z elements plasmas measured at LULI2000 facility

We analyzed the spectra of X-ray transmission through radiatively heated medium- Z plasma (Fe, Ni, Cu and Ge) measured at LULI2000 facility in the wavelength range of 2 p– nd transitions. The analysis was performed using the statistical superconfiguration code SCO, two line-by-line opacity codes based on the HULLAC and FAC packages and a new hybrid statistical-detailed code SCORCG. The temperature and mass density of the samples were estimated from hydrodynamic simulations based on the cavity radiative temperature measurements. The theory–experiment agreement is relatively good in the wavelength range corresponding to the 2 p–3 d transitions except in the germanium case. In the wavelength range of the 2 p–2 d, n > 3 transitions a relatively good theory–experiment agreement was found in the copper case. As predicted by calculations the separation of the characteristic spin-orbit-split 2 p–3 d structures, absent in the iron measured spectrum, appears in the nickel spectrum and is visible in the copper and germanium spectra. Comparisons of the experimental transmission with calculations confirm the importance of the relativistic configuration interaction. The absorption strength of the measured germanium 2 p–3 d transition is much larger than that obtained from the codes. Spatial temperature and density gradients, relatively high in the germanium sample, may be at the origin of this discrepancy.

Rapid PCR Thermocycling using Microscale Thermal Convection

Many molecular biology assays depend in some way on the polymerase chain reaction (PCR) to amplify an initially dilute target DNA sample to a detectable concentration level. But the design of conventional PCR thermocycling hardware, predominantly based on massive metal heating blocks whose temperature is regulated by thermoelectric heaters, severely limits the achievable reaction speed1. Considerable electrical power is also required to repeatedly heat and cool the reagent mixture, limiting the ability to deploy these instruments in a portable format.Thermal convection has emerged as a promising alternative thermocycling approach that has the potential to overcome these limitations2-9. Convective flows are an everyday occurrence in a diverse array of settings ranging from the Earth's atmosphere, oceans, and interior, to decorative and colorful lava lamps. Fluid motion is initiated in the same way in each case: a buoyancy driven instability arises when a confined volume of fluid is subjected to a spatial temperature gradient. These same phenomena offer an attractive way to perform PCR thermocycling. By applying a static temperature gradient across an appropriately designed reactor geometry, a continuous circulatory flow can be established that will repeatedly transport PCR reagents through temperature zones associated with the denaturing, annealing, and extension stages of the reaction (Figure 1). Thermocycling can therefore be actuated in a pseudo-isothermal manner by simply holding two opposing surfaces at fixed temperatures, completely eliminating the need to repeatedly heat and cool the instrument.One of the main challenges facing design of convective thermocyclers is the need to precisely control the spatial velocity and temperature distributions within the reactor to ensure that the reagents sequentially occupy the correct temperature zones for a sufficient period of time10,11. Here we describe results of our efforts to probe the full 3-D velocity and temperature distributions in microscale convective thermocyclers12. Unexpectedly, we have discovered a subset of complex flow trajectories that are highly favorable for PCR due to a synergistic combination of (1) continuous exchange among flow paths that provides an enhanced opportunity for reagents to sample the full range of optimal temperature profiles, and (2) increased time spent within the extension temperature zone the rate limiting step of PCR. Extremely rapid DNA amplification times (under 10 min) are achievable in reactors designed to generate these flows.