Creates Temperature Gradients Research Articles

Investigation of the effect of curvature on the local mass flux of evaporating droplets using a phase field method

Droplet evaporation, a fundamental process with wide-ranging applications, involves complex physical mechanisms that significantly influence mass transfer rates and droplet lifetimes. Despite its significance, the influence of droplet deformation induced by capillary forces on local saturation conditions and its subsequent impact on phase change have received limited attention. This study delves into the impact of capillary-induced droplet deformation on local saturation conditions and its subsequent effect on phase change within a quiescent vapor environment. Contrary to most analytical and numerical studies that impose temperature and heat flux conditions at the vapor–liquid interface, our investigation employs direct numerical simulations based on the thermal Navier–Stokes–Korteweg model, capturing the dynamics of the droplet and its interface without relying on ad-hoc interface conditions. Our findings demonstrate that droplet deformation induces free oscillatory motion, which, although not significantly altering droplet lifetime due to viscous damping, leads to non-uniform mass fluxes along the droplet interface. We observe that areas of higher curvature are associated with increased local pressure and reduced temperatures, a discrepancy primarily attributed to evaporative cooling. This effect is further elucidated through an analysis of pressure and energy fluxes, revealing that regions of high curvature experience significant heat consumption, driving evaporative cooling and creating temperature gradients along the interface. Conversely, areas with negative curvature exhibit a reverse trend, indicating the complex interplay of capillarity, deformation, and phase change in droplet evaporation. Our results highlight the intricate relationship between droplet deformation, local pressure, mass flux, and temperature, underscoring the necessity for improved modeling of these factors in future studies on droplet evaporation dynamics.

Thermo-Elastic Topology Optimization For High Temperatures Gradients Using Load Separation

Designing components for thermo-mechanical loads is a challenging process. While mechanical loads like forces or pressure demand a stiff and thick-walled design, thermal loads create temperature gradients, resulting in thermo-mechanical stress from the structure's temperature proportional and, therefore, uneven expansion. In contrast to a pure mechanical load case, an initial design before optimization can already include stress levels beyond the limit of the material. Therefore, common optimization approaches for a preliminary design use exemplary systems with low-temperature gradients, so thermal stresses do not exceed the limit. From there, energy density is used to calculate the topology optimizations sensitivity and therefore decide which elements to remove and which to keep. This paper describes a novel approach for reducing thermo-mechanical stress by following the stress corresponding temperature gradients from the heat source to the sink to calculate a new sensitivity that helps to grow cooling channels. The optimization is exemplarily shown on a piston for internal combustion engines. While handling delta temperatures of 600K, results show a reduction in thermo-mechanical stress while reducing the component's mass. Because the approach reduces critical stress in a component, it allows the initial design (before the topology optimization) to have stress levels way above yield strength.

Ray tracing and finite element modeling of sound propagation in a compartment fire

A compartment fire (a fire in a room or building) creates temperature gradients and inhomogeneous time-varying temperature, density, and flow fields. This work compared experimental measurements of the room acoustic impulse/frequency response in a room with a fire to numerically modeled responses. The fire is modeled using a Fire Dynamics Simulator (FDS). Acoustic modeling was performed using the temperature field computed by FDS. Room acoustics were modeled using two-dimensional ray and finite element modeling. A three-dimensional model was used to simulate an open flame. COMSOLTM Multiphysics was used for finite element acoustic modeling and BELLHOPTM for ray trace acoustics modeling. The results show that the fire causes wave-fronts to arrive earlier (due to the higher sound speed) and with more variation in the delay times (due to the sound speed perturbations). The resonance frequencies of low-frequency modes were shifted upwards. Model results are compared with data and show good agreement in observed trends.

Head-related transfer function measurements in a compartment fire.

The Personal Alert Safety System (PASS) is an alarm signal device carried by firefighters to help rescuers locate and extricate downed firefighters. A fire creates temperature gradients and inhomogeneous time-varying temperature, density, and flow fields that modify the acoustic properties of a room. To understand the effect of the fire on an alarm signal, experimental measurements of head-related transfer functions (HRTF) in a room with fire are presented in time and frequency domains. The results show that low-frequency (<1000 Hz) modes in the HRTF increase in frequency and higher-frequency modal structure weakens and becomes unstable in time. In the time domain, the time difference of arrival between the ears changes and becomes unstable over time. Both of these effects could impact alarm signal detection and localization. The receive level of narrowband tones is presented that shows that the fire makes the receive level of a source vary by > 10 dB. All of these effects could impact the detection and localization of the PASS alarm and have life safety consequences.

Change in acoustic impulse response of a room due to a fire.

A room fire creates temperature gradients and inhomogeneous time varying temperature, density, and flow fields. Experimental measurements of the room acoustic impulse/frequency response are presented and compared with a ray traced model. The results show that the fire causes wave-fronts to arrive earlier (due to the higher sound speed) and with more variation in the delay times (due to the sound speed perturbations). The frequency response shows that the modes are shifted up in frequency and high frequency (>2500 Hz) modes are significantly attenuated. Model results are compared with data and show good agreement in observed trends.

RETRACTED ARTICLE: Numerical simulation of temperature field and temperature stress of thermal jet for water measurement

ABSTRACT Thermal-jet for water Measurement spallation is a rock removal process that utilizes induced thermal stress to fracture the rock into small fragments before melting of the rock occurs. Because of the low conductivity, thermal stress will be created on the rock surface. When the thermal stress exceeds the strength of the rock, cracks will form and expand. Eventually, rocks disintegrating into small fragments. The simulation result obtained by the Crank–Nicolson method of the temperature field and the distribution of thermal stress has been analyzed. The result indicates that during thermal spallation drilling, the temperature of the surface exposed to the thermal-jet for water Measurement rises rapidly and create temperature gradients in both radial and axial directions. Because of volume expansion, the heated part is subjected to compressive stress in radial direction and shear stress in axial direction, respectively. The result is very helpful to the field application.

Glacial erosion: status and outlook

AbstractGlacier-erosion rates range across orders of magnitude, and much of this variation cannot be attributed to basal sliding rates. Subglacial till acts as lubricating ‘fault gouge’ or ‘sawdust’, and must be removed for rapid subglacial bedrock erosion. Such erosion occurs especially where and when moulin-fed streams access the bed and are unconstrained by supercooling or other processes. Streams also may directly erode bedrock, likely with strong time-evolution. Erosion is primarily by quarrying, aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes. Debris-bed friction significantly affects abrasion, quarrying and general glacier flow. Frost heave drives cirque headwall erosion as winter cold air enters bergschrunds, creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock, and the glacier removes the resulting blocks. Recent subglacial bedrock erosion and sediment flux are in many cases much higher than long-term averages. Over glacial cycles, evolution of glacial-valley form feeds back strongly on erosion and deposition. Most of this is poorly quantified, with parts open to argument. Glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes, motivating vigorous research.

Heat transfer in human hair.

In the process of daily grooming, human hair often undergoes heat treatments. These treatments create temperature gradients inside individual hair fibres or bundles of hair which can lead to hair damage. Mathematical models were built for heat transfer in a single hair as well as thin tresses of hair during blow drying. Mathematical analysis led to temperature profiles suggestive of fast transfer of heat in a single hair mainly through conduction. However, in a hair assembly, the process involved both conduction and convection leading to a slower rate of heat transfer. Penetration of heat to the centre of the hair assembly took minutes rather than seconds. This model is useful in understanding hair damage potential in styling hair with high temperature devices such as flat irons and processes which involve deforming hair at relatively high temperatures.

Anomalous Nernst effect and three-dimensional temperature gradients in magnetic tunnel junctions

Localized laser heating creates temperature gradients in all directions leading to three-dimensional electron flux in metallic materials. Temperature gradients in combination with material magnetization generate thermomagnetic voltages. The interplay between these temperature gradients and the magnetization along with their control enable to manipulate the generated voltages in magnetic nanodevices. We present a highly sensitive method to identify the anomalous Nernst effect generated on the nanometer length scale by micrometer-sized temperature gradients in magnetic tunnel junctions with CoFeB electrodes and a MgO tunnel barrier systematically extracted by analyzing the influence of in-plane temperature gradients on the tunnel magneto-Seebeck effect. This method yields an anomalous Nernst effect coefficient of KN ≈ 1.6 × 10−8 V T−1 K−1 for CoFeB. Generally, such investigations are motivated by utilizing otherwise wasted heat in magnetic memory devices for read/write operations. The additionally generated anomalous Nernst effect offers a functionality expansion, opening new application fields such as direction-dependent temperature sensing with downscaling potential.

Rapidly oscillating microbubbles force development of micro- and mesoporous interfaces and composition gradients in solids

Progress in understanding of energy transfer in nature and human being requires novel approaches to the processing of solids on demand, in specially those with composition gradients and those thermodynamically and kinetically inaccessible. We demonstrate that rapidly oscillating microbubbles are useful for materials processing, because they manipulate surface temperature and creates temperature gradients in predictable way. Ultrasonic treatment leads to an increase in the surface area of particles up to 180 m2g−1 and the formation of micropores in metal phase and mesopores in metal oxide phase. The spatially and temporally unique energy dissipation conditions promise new interfaces with higher level of complexity and applications among others in catalysis, energy storage, drug delivery.

Reducing Spatial Heterogeneity of MALDI Samples with Marangoni Flows During Sample Preparation.

This work demonstrates a method to prepare homogeneous distributions of analytes to improve data reproducibility in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). Natural-air drying processes normally result in unwanted heterogeneous spatial distributions of analytes in MALDI crystals and make quantitative analysis difficult. This study demonstrates that inducing Marangoni flows within drying droplets can significantly reduce the heterogeneity problem. The Marangoni flows are accelerated by changing substrate temperatures to create temperature gradients across droplets. Such hydrodynamic flows are analyzed semi-empirically. Using imaging mass spectrometry, changes of heterogeneity of molecules with the change of substrate temperature during drying processes are demonstrated. The observed heterogeneities of the biomolecules reduce as predicted Marangoni velocities increase. In comparison to conventional methods, drying droplets on a 5 °C substrate while keeping the surroundings at ambient conditions typically reduces the heterogeneity of biomolecular ions by 65%-80%. The observation suggests that decreasing substrate temperature during droplet drying processes is a simple and effective means to reduce analyte heterogeneity for quantitative applications. Graphical Abstract ᅟ.

Characterization of Temperature Gradients on MEMS Acceleration Sensors

MEMS acceleration sensors have been systematically analyzed across a wide temperature range between -40°C and +85°C. Failure mechanisms such as offset drift induced by thermal mismatch of different materials are well understood. However, in densely packed electronic devices (e. g., smartphones) distances between neighboring components are very small. Thus, power intensive components like microprocessors can heat up and create temperature gradients in their vicinity. In this paper we introduce a measurement system to systematically investigate the influence of temperature gradients on MEMS sensors. For precise investigation of their offset and sensitivity, the system can be rotated in the earth's gravitational field. Experimental results show that the z-axis offset is linearly dependent on out-of-plane temperature gradients whereas its sensitivity is nearly constant. The in–plane axes are not affected. Several hypothesis are still under investigation but currently a microfluidic effect caused by the temperature distribution of the gas within the sensor cavity is the most likely explanation.

FORMATION OF BIDOMAIN STRUCTURE IN SINGLE CRYSTAL LITHIUM NIOBATE QAFERS USING STEADY-STATE EXTERNAL HEATING METHOD

The method of bidomain structure synthesis in lithium niobate single crystal wafers based on the formation of a specific temperature gradient across the sample thickness has been developed. The lithium niobate wafer placed between two silicon wafers was heated due to the absorption of light annealing system radiation by silicon. The work cell design allows one to form and control the power of thermal fluxes entering the ferroelectric wafer thus creating temperature gradients required for a controlled process of formation of two domains with opposite polarization vectors («head to head» domain structure). The efficiency of light absorption for the formation of external thermal sources that allow one to implement symmetric and asymmetric heating, determining the position of the conditional surface with the zero temperature gradient and consequently the position of the domain boundary is experimentally confirmed.In a lithium niobate wafer 1.6 mm in thickness and 60 mm in length, a symmetrical bidomain structure with opposite polarization vectors was formed. The bending strain of cantilevered samples vs applied voltage was investigated in the -300 to +300 V voltage range, the strain amplitude being more than 35 µm. The measurements showed a high linearity and repeatability of the bias voltage vs bending strain curve.

Thermoelectric Device Based on Vertical Silicon Nanowires for On-Chip Integration

A fabricating process of prototype thermoelectric device based on vertical silicon nanowires (SiNWs) for on-chip integration was presented. The SiNWs with diameter of 200 nm and height of 1 μm were fabricated by electron beam lithography and inductively coupled plasma etching. The gaps between the NWs were filled by the spin-on glass, which isolated the top and bottom electrodes. A serpentine platinum resistance thermometer coil was formed on the NWs to create temperature gradient across the NWs and measure the temperature of the top of NWs. I-V characteristics of the vertical device before and after annealing were measured. The nonlinear I-V curves were obtained, but the annealed one demonstrated 1000-fold reduction in resistance than the unannealed one.

Indirect Effects of Impoundment on Migrating Fish: Temperature Gradients in Fish Ladders Slow Dam Passage by Adult Chinook Salmon and Steelhead

Thermal layering in reservoirs upstream from hydroelectric dams can create temperature gradients in fishways used by upstream migrating adults. In the Snake River, Washington, federally-protected adult salmonids (Oncorhynchus spp.) often encounter relatively cool water in dam tailraces and lower ladder sections and warmer water in the upstream portions of ladders. Using radiotelemetry, we examined relationships between fish passage behavior and the temperature difference between the top and bottom of ladders (∆T) at four dams over four years. Some spring Chinook salmon (O. tshawytscha) experienced ∆T ≥ 0.5 °C. Many summer and fall Chinook salmon and summer steelhead (O. mykiss) experienced ∆T ≥ 1.0 °C, and some individuals encountered ΔT > 4.0°C. As ΔT increased, migrants were consistently more likely to move down fish ladders and exit into dam tailraces, resulting in upstream passage delays that ranged from hours to days. Fish body temperatures equilibrated to ladder temperatures and often exceeded 20°C, indicating potential negative physiological and fitness effects. Collectively, the results suggest that gradients in fishway water temperatures present a migration obstacle to many anadromous migrants. Unfavorable temperature gradients may be common at reservoir-fed fish passage facilities, especially those with seasonal thermal layering or stratification. Understanding and managing thermal heterogeneity at such sites may be important for ensuring efficient upstream passage and minimizing stress for migratory, temperature-sensitive species.

ACCRETION THROUGH THE INNER EDGES OF PROTOPLANETARY DISKS BY A GIANT SOLID STATE PUMP

At the inner edge of a protoplanetary disk solids are illuminated by stellar light. This illumination heats the solids and creates temperature gradients along their surfaces. Interactions with ambient gas molecules lead to a radial net gas flow. Every illuminated solid particle within the edge is an individual small gas pump transporting gas inward. In total the inner edge can provide local mass flow rates as high as $\dot{M} = 10^{-5} M_{\odot}$ yr$^{-1}$.

Submicron Mapping of Thermal Conductivity of Thermoelectric Thin Films

A tool for evaluating thin-film thermal conductivity to submicron spatial resolution has been developed. The micro-instrumentation utilizes the thermoreflectance (TR) technique to characterize thermal conductivity and material uniformity. The instrument consists of a heating element for creating temperature gradients and an Invar bar with in situ temperature monitoring for heat flux measurements. The thin-film sample is sandwiched between the heater and Invar bar while a microscope is used to direct light onto a cross-section of the sample and reflected light is collected with a camera. By using this technique, we can achieve submicron spatial resolution for thermal conductivity and eliminate contributions from thermal contact resistance, thereby also eliminating the need for sample preparation other than cleaving. The method offers temperature resolution of 10 mK, spatial resolution of 200 nm, and thermal conductivity measurement with 0.01 ± 0.001 W/mK resolution. The thermal conductivity of a 0.6% ErAs:InGaAlAs thermoelectric (TE) element, prepared by molecular beam epitaxy (MBE) growth, obtained with the new instrument is 2.3 W/mK, while the average thermal conductivity obtained with the 3-omega method is 2.5 W/mK. Energy-dispersive x-ray (EDX) spectroscopy is also used to prove that the elemental composition has uniformity consistent with the material variation observed by the TR technique. Moreover, a temperature profile across a 0.6% ErAs:InGaAlAs TE element on InP substrate is imaged. Two different slopes, corresponding to different thermal conductivities, have been observed, showing that the thermal conductivity of the TE element is lower than that of the InP substrate as expected.

Seebeck effect in magnetic tunnel junctions

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, i.e., the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In that respect, it is the analog to the tunneling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configuration are in the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. Experimentally, we realized 8.8 % magneto-Seebeck effect, which results from a voltage change of about -8.7 {\mu}V/K from the antiparallel to the parallel direction close to the predicted value of -12.1 {\mu}V/K.

Characterization of a ferrofluid-based thermomagnetic pump for microfluidic applications

We experimentally characterize the performance of a miniature thermomagnetic pump, where suitably imposed temperature and magnetic field gradients are used to drive ferrofluid in a 2 mm diameter glass capillary tube, without application of any external pressure gradient. Such a pump can operate in a hermetically sealed micro electromechanical system configuration without any moving part, and is thus capable of handling microfluidic samples with little risk of contamination. In the experiment, the ferrofluid in the capillary is exposed to a magnetic field using a solenoid; a small resistive heater wrapped on the tube wall is used to create temperature gradient in such a way that the Kelvin body force in the medium produces a net unbalanced axial component. This causes a thermomagnetic pumping action, transporting the ferrofluid in the capillary tube from the colder end to the warmer end. Performance of the thermomagnetic pump is investigated experimentally to characterize the pump pressure head and discharge under different working conditions, namely, the magnetic field strength, heating power, and ferrofluid properties. A comparison with two other field actuation pumps at comparable length scales is also presented. The pump produces higher output at lower power supplies and magnetic field compared to the other two pumps.

Design and experimental characterization of a membrane-based absorption heat pump

A membrane absorption heat pump uses absorbent and refrigerant (solvent) flows separated by a membrane to create temperature gradients (aka temperature lifts) used for heating or cooling. Compared to vacuum absorption heat pumps, an atmospheric-pressure membrane heat pump provides more compact designs, potentially enabling applications such as energy-efficient cooling for electronics. In addition, storing concentrated absorbent offers unique options for energy storage for solar heating and cooling of buildings. A new membrane heat pump module was built using two sets of rows of hollow fibers with stagnant air between the fibers to reduce conductive heat transfer. Transport coefficients for the complex air-gap geometry were estimated with a three-dimensional finite-volume heat transfer analysis of the air gap region with results fitted to a modified conduction shape factor. A two-dimensional finite-difference model of the entire process shows good agreement with experiments performed over different air-gap widths, flow rates, inlet temperatures, and absorbent concentrations. Temperature lifts up to 9 °C were achieved with 39% (mass) LiCl (aq) feed solution and 35 °C inlet temperatures. Extensions of the modeling to higher-porosity, larger-pore-size membranes suggest that temperature lifts of 14 °C at ambient inlet temperatures are achievable in our module geometry.