Time response characteristics of microcapsulated liquid-crystal particles

Microcapsulated liquid-crystal particles are widely used as temperature sensors in the field of heat transfer engineering. Conventionally, these particles are painted on a surface of a heated plate for temperature measurements. The temperature is measured by tracing the color changes of the microcapsulated liquid-crystal particle through the color digital image processing technique. Recently, these particles are often suspended in thermal fluid flows as temperature tracers of the flows. For the use of a microcapsulated liquid-crystal particle itself as a temperature sensor in the suspending method, the heat capacity of the capsule covering the liquid crystal should be regarded as an important factor in predicting the time response of the microcapsulated liquid-crystal particle which is directly injected into a thermal fluid flow. The heat capacity of the liquid crystal and the capsule can produce a delayed time response for the temperature change of the outside fluids and eventually produce erroneous measurement data. Without a new temperature sensor smaller than the particle, it is very difficult to measure the time response of the microcapsulated liquid-crystal particle since the particles move with the working thermal fluid in different flow conditions. Therefore, a numerical simulation for the time response of the particle is made and its usable limit is discussed in detail for the measurement of turbulent thermal flows. Responses for a temperature step change, fluctuating temperature changes, and the thermal inertia of the working fluid temperature are considered. The response time of the microcapsulated liquid-crystal particle has been evaluated to be as much as 150 ms time delay for a step change of the working fluid temperature, which means the physical properties of the particle itself must be considered for outside temperature changes. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 390–398, 1998     

Feasibility study of pulse shaping for a solar furnace

The study of the effect on materials of the heat flash emitted by the fireball of a nuclear weapon requires a thermal source that will produce a shaped thermal pulse similar to that from a fire-ball. This paper deals with the basic characteristics necessary in a thermal pulse for experimental work, which are the changes in intensity and spectral distribution with time. The changes required in the solar spectrum to match this pulse are detailed. A possible mechanism for the shutter of the DCBRL solar furnace to provide the required characteristics is suggested.     

On impulsively started convection: The case of stagnation point flow

Transient convection in incompressible planar and axisymmetric point flow is analyzed numerically in this work, and the thermal boundary layer response to surface sudden heating and cooling in the two settings is presented and compared over a range of Prandtl number between 0.5 and 100. A comparison between surface sudden cooling and heating is performed and different criteria are established as to when surface sudden heating and cooling are equivalent in terms of the transition time. With no initial thermal boundary layer (surface and fluid are at the same temperature), the transition time from the initial steady state to the final steady state upon surface sudden cooling or heating is found to be a constant regardless of the surface heating or cooling extent above or below the initial surface temperature, and is dependent only on the Prandtl number. With the existence of an initial thermal boundary layer, the transition time is dependent upon the heating or cooling extent, the initial surface temperature, the Prandtl number and whether heating/cooling is towards building-up or demolishing the thermal boundary later. It takes longer time when surface sudden heating or cooling is towards demolishing the thermal boundary layer than building it up. With symmetric surface sudden cooling or heating above or below the far-field fluid temperature, the transition time is independent on the surface cooling or heating extent and is a function of only the Prandtl number. A considerable difference in the thermal boundary layer response in the two settings is found. The transition time from the initial to the final steady state in axisymmetric stagnation point flow is less than that in plane stagnation flow under the same conditions.     

Effects of pulse width and mass flux on microscale flow boiling under pulse heating

A Pt microheater (140 × 100 μm²) is fabricated on a glass wafer and enclosed in a silicon microchannel of trapezoidal cross section by MEMS technology. With the aid of a high-speed CCD and data acquisition system, subcooled flow boiling phenomena and temperature response on the surface of the microheater under pulse heating are observed and recorded. Experiments are conducted for six pulse widths (50 μs, 100 μs, 200 μs, 600 μs, 1 ms, and 2 ms) under different mass and heat fluxes. With increasing heat flux at a fixed pulse width and different mass fluxes, four flow regimes including single phase, nucleate boiling, film boiling and dry out are identified. Since flow boiling regimes are relatively independent of mass flux, correlation equations based on experimental data for the transitional heat flux of different flow boiling regimes are obtained in terms of pulse width only. It is also found that pulse width and mass flux have little influence on boiling inception time, and the classical analytical solution for the nucleation inception time in terms of heat flux is verified experimentally.     

总日射表热滞后效应的实验研究—实验装置和结果

介绍了总日射表热滞后效应的测量装置、实验步骤和数据处理方法，并提出总日射表热稳定时间概念。采用微机控制的自动采集系统，测定了常用的六种总日射表的热稳定时间，其值都比铭牌上标明的时间常数长得多。     

Temperature transients in spherical medium irradiated by laser pulse

Temperature wave solution predicted by hyperbolic heat conduction model is developed for a hollow sphere exposed to laser pulse heating. Using the obtained analytical solution, the temperature distribution, the propagation and reflection of the temperature wave due to such heat pulse is investigated for different thermal relaxation time and laser pulse duration. The effect of geometry on the temperature profile is also studied and the results of the hyperbolic and Fourier model are compared.     

CFD analysis of high frequency miniature pulse tube refrigerators for space applications with thermal non-equilibrium model

High frequency, miniature, pulse tube cryocoolers are extensively used in space applications because of their simplicity. Parametric studies of inertance type pulse tube cooler are performed with different length-to-diameter ratios of the pulse tube with the help of the FLUENT^® package. The local thermal non-equilibrium of the gas and the matrix is taken into account for the modeling of porous zones, in addition to the wall thickness of the components. Dynamic characteristics and the actual mechanism of energy transfer in pulse are examined with the help of the pulse tube wall time constant. The heat interaction between pulse tube wall and the oscillating gas, leading to surface heat pumping, is quantified. The axial heat conduction is found to reduce the performance of the pulse tube refrigerator. The thermal non-equilibrium predicts a higher cold heat exchanger temperature compared to thermal equilibrium. The pressure drop through the porous medium has a strong non-linear effect due to the dominating influence of Forchheimer term over that of the linear Darcy term at high operating frequencies. The phase angle relationships among the pressure, temperature and the mass flow rate in the porous zones are also important in determining the performance of pulse tube refrigerator.     

Generalized thermoelasticity of a piezoelectric layer

In the present research, the response of a one-dimensional piezoelectric layer is investigated using the generalized thermoelasticity theory of Lord and Shulman. The layer is subjected to thermal shock on one surface. Three coupled equations, namely, motion equation, energy equation and Maxwell equation in terms of displacement, temperature, and electric potential are established. Using the proper transformation, the mentioned equations are given in a dimensionless form. These equations are discretized by means of the generalized differential quadrature method and traced in time by means of the Newmark time marching scheme. Numerical examples are provided to show the propagation and reflection of thermal, mechanical and electrical waves in a layer. It is shown that under the Lord and Shulman theory, temperature propagates with a finite speed, similar to mechanical displacement wave. However, the electric displacement and potential propagate with infinite speed.     

Effect of dynamic load on the temperature profiles and cooling response time of a proton exchange membrane fuel cell

Polymer Electrolyte Membrane Fuel Cells (PEMFC) is an electrochemical device that generates electrical energy from the reactions between hydrogen and oxygen. An effective thermal management is needed to preserve the fuel cell performance and durability. Cooling by water is a conventional approach for PEMFC. Balance between optimal operating temperature, temperature uniformity and fast cooling response is a continuous issue in the thermal management of PEMFC. Various cooling strategies have been proposed for water-cooled PEMFC and an approach to obtain a fast cooling response was tested by feeding the coolant at a high temperature. In this paper, the operating behaviour was characterized from the perspectives of temperature profiles, mean temperature difference, and cooling response time. A 2.4 kW water-cooled PEMFC was used and the electrical load ranged from 40 A–90 A. The operating coolant temperature was set to 50 °C where the maximum stack operating temperature is 60 °C. The stack temperature profiles, cooling response time, mean temperature difference and cooling rates to the load variation was analysed. The analysis showed that the strategy allowed a fast cooling response especially at high current densities, but it also promotes a large temperature gradient across the stack.     

集热器时间常数测试方法研究

集热器的时间常数反映了集热器对瞬变阶跃输入响应的快慢,如何用实验的方法对它进行简单、准确的测试是集热器热性能测试中的一项重要内容。在已有的几种测试方法的基础上导出了一种更简单的测试方法,且已经过实验验证。     

Modeling of ultrafast phase changes in metal films induced by an ultrashort laser pulse using a semi-classical two-temperature model

A semi-classical two-temperature (2T) model, together with the extended Drude model for temperature-dependent optical properties, is proposed to describe ultrafast thermal transport and phase changes in gold films irradiated by an ultrashort laser pulse. This model reduces to a classical 2T model when the electron momentum equation and the two terms related to electron drifting velocity in the electron energy equation are dropped off. It is found that the significant changes in reflectivity and absorption coefficient due to rapid temperature rise could drastically alter laser energy deposition and in turn, the thermal response in metal films. It is also found that the semi-classical 2T model could predict lower electron and lattice temperature and less severe solid–liquid and liquid–vapor phase change than the classical 2T model. The difference could be more distinct with increase of laser fluence and/or increase of laser pulse duration.     

LASER SHORT-PULSE HEATING: INFLUENCE OF LASER POWER INTENSITY ON TEMPERATURE FIELDS

High-power-intensity and short-pulse laser heating of metallic surfaces results in thermal separation of electron and lattice subsystems. In this case, energy transport between the subsystems is governed mainly by the collisional process. Moreover, electron and lattice subsystems respond differently for different pulse intensities, despite the fact that the laser pulses have the same energy content. Consequently, in the present study, laser step-input pulse heating of gold substrate is considered and the thermal response of electron and lattice subsystems to four different intensity pulses with the same energy content is examined. The electron kinetic theory approach is introduced to model the nonequilibrium energy transport in the substrate material. It is found that electron temperature rises rapidly in the heating cycle while lattice temperature rise is gradual, which is more pronounced for laser short pulse lengths. In the cooling cycle (time after the laser pulse diminishes), electron temperature decay rate differs from the rate of lattice site temperature rise due to the specific heat ratios of electron and lattice sites.     

Numerical Investigation of Opto-Energy Phenomena in Thin Gold Films Irradiated by Femtosecond Pulse Laser Considering Quantum Effects

Nonequilibrium energy transport and optical characteristics in thin gold film structures irradiated by a femtosecond pulse laser are examined numerically. With the use of a two-temperature model, the quantum effect is considered to determine various thermo-optical properties such as electron heat capacity, electron thermal conductivity, collision frequencies, reflectivity, and absorption rates. As a result, estimation on the electron temperature considering the quantum effect is in better agreement with measurements than those without considering quantum effect. During a femtoseond laser pulse, it is found that the electron and lattice are in nonequilibrium, the electron thermal conductivity changes rapidly with time, and reflectivity decreases substantially because of the changes in dielectric function and refractive index of gold films.     

Design and analysis of electrothermal metasurfaces

Electrothermal metasurfaces have garnered considerable attention owing to their ability to dynamically control thermal infrared radiation. Although previous studies were mainly focused on metasurfaces with infinite unit cells, in practice, the finite-size effect can be a critical design factor for developing thermal metasurfaces with fast response and broad temperature uniformity. Here, we study the thermal metasurfaces consisting of gold nanorods with a finite array size, which can achieve a resonance close to that of the infinite case with only several periods. More importantly, such a small footprint due to the finite array size yields response time down to a nanosecond level. Furthermore, the number of the unit cells in the direction perpendicular to the axis of nanorods is found to be insensitive to the resonance and response time; thus, providing a tunable aspect ratio that can boost the temperature uniformity in the sub-Kelvin level.     

Thermal impedance spectroscopy for Li-ion batteries using heat-pulse response analysis

Novel characterization of the thermal properties of batteries have been introduced by defining their frequency-dependent thermal impedance functions. The thermal impedance function can be approximated as a thermal impedance spectrum by analyzing the experimental temperature transient which is related to the thermal impedance function through Laplace transformation.In order to obtain the temperature transient, a process has been devised to generate an external heat pulse with heating wire and to measure the response of the battery. This process is used to study several commercial Li-ion batteries of cylindrical type. Thermal impedance measurements have been performed using a potentiostat/galvanostat controlled by a digital signal processor, which is more commonly available than a flow-meter usually applied for thermal property measurements.Thermal impedance spectra obtained for batteries produced by different manufacturers are found to differ considerably. Comparison of spectra at different states-of-charge indicates an independence of the thermal impedance on the charge state of the battery. It is shown that the thermal impedance spectrum can be used to obtain simultaneously the thermal capacity and the thermal conductivity of the battery by non-linear complex least-squares fit of the spectrum to the thermal impedance model.     

地埋管间断工作进出孔平均温度预测方法研究

按不改变每天总换热量的原则,将单位长度地埋管换热孔在制冷季或制热季中每天的实际释热或取热过程简化为一个矩形释热或取热脉冲,脉冲大小为单位长度地埋管换热孔的设计释热量或设计取热量,时间为每天的等效满负荷释热或取热小时数。采用线热源理论和热流叠加原理,推导若干个矩形脉冲负荷作用后地埋管换热器进出口温度平均值的计算公式,并通过长期现场岩土热响应试验对该公式进行了验证。在已知制冷季或制热季天数和地埋管换热器每天等效满负荷工作小时的基础上,通过设定地埋管在制冷季和制热季传热流体的最高或最低温度,可用该公式计算单位长度地埋管换热孔的设计释热量或取热量。     

High heat flux point source sensitivity and localization analysis for an ultrasonic sensor array

State estimation procedures using the extended Kalman filter are investigated for a transient heat transfer problem in which a high heat flux point source is applied on one side of a thin plate and ultrasonic pulse time of flight is measured between spatially separated transducers on the opposite side of the plate. This work is an integral part of an effort to develop a system capable of locating the boundary layer transition region on a hypersonic vehicle aeroshell. Results from thermal conduction experiments involving one-way ultrasonic pulse time of flight measurements are presented. Uncertainties in the experiments and sensitivity to heating source location are discussed. One key finding is that sensitivity to heating source location is greater in the direction normal to the ultrasonic pulse propagation path. Scaled sensitivities to boundary conditions and thermal conductivity are presented and analyzed for all possible source locations using a square sensor grid. While sensitivity to the primary heat flux was determined to be the highest, sensitivity to the other parameters is either on the same order of magnitude or one order of magnitude less. Two different measurement models are compared for heating source localization: (1) directly using the one-way ultrasonic pulse time of flight as the measurement vector and (2) indirectly obtaining distance from the one-way ultrasonic pulse time of flight and then using these obtained distances as the measurement vector in the extended Kalman filter. Heating source localization results and convergence behavior are compared for the two measurement models. Two areas of sensitivity analyses are presented: (1) heat source location relative to sensor array position, and (2) sensor noise. The direct measurement model produced the best results when considering accuracy of converged solution, ability to converge to the correct solution given different initial guesses, and smoothness of convergence behavior.     

Analysis of effective thermal conductivity stability of geothermal heat exchanger according to ambient environmental conditions

At present, geothermal energy is a promising research area but still has numerous areas to be explored. The thermal performance verification of U‐shaped closed geothermal heat exchange was proposed to improve the effective thermal conductivity value applying initial ignoring time (IIT). To verify the thermal performance of the underground heat exchanger in the rock layer according to the surrounding environment and groundwater conditions, verifications were conducted for the amount of circulating water, amount of heat input, and groundwater conditions. The circulating fluid flow rate of the system, supply of water into the ground, and influence of the change of supplied heat source were analyzed. The effective heat conduction characteristics were analyzed using the linear heat source model and initial exclusion time. Therefore, in the thermal response test, the main error occurs according to the determined initial removal time, and the reliability of the effective thermal conductivity value is increased by applying IIT to reduce this error.     

Multiscale Investigation of Femtosecond Laser Pulses Processing Aluminum in Burst Mode

Megahertz is the highest femtosecond laser repetition rate that the state-of-the art technology can achieve. In this article, a single femtosecond laser pulse is burst into multiple femtosecond laser pulses to process aluminum. The temporal gap between two consecutive burst pulses is 2 picoseconds, which is much shorter than the temporal gap between two consecutive pulses at the repetition rate of megahertz. By taking the thermophysical scenarios of femtosecond laser induced of electron thermalization, electron heat conduction, electron–phonon-coupled heat transfer and atomic motion into account, a multiscale framework integrating ab initio quantum mechanical calculation, molecular dynamics and two-temperature model are constructed. The effect of femtosecond laser pulse number on the incubation phenomenon is studied. Comparing with the single pulse-processing aluminum film, the femtosecond laser in burst mode leads to smaller thermal stress, which is favorable to reduce the thermal mechanical damage of the material beneath the laser-irradiated surface. Appreciable differences among the simulation results by using electron thermophysical parameters from ab initio quantum mechanical calculation and those from experimental measurement, empirical estimation and calculation are found, indicating the essentials to precisely model the electron thermal response subject to femtosecond laser excitation.     

The growth and collapse of a micro-bubble under pulse heating

The possibility of using a micro-thermal bubble, generated by a micro-heater under pulse heating, as an actuator for applications in micro-bio-analytical systems is investigated in this paper. The perturbation force, generated when the micro-thermal bubble is formed instantaneously, can be used to promote such actions as mixing in the solution of a micro-reactor. Under pulse heating, a specially designed non-uniform width micro-heater (10 × 3 μm²) can induce highly localized near-homogeneous nucleation and results in periodic generation of stable single bubbles in DI water. The single bubble appears precisely on the narrow part of the micro-heater with size restricted within the superheated region in the fluid. The growth and collapse of the bubble, recorded by a high-speed CCD, is shown to be asymmetric with time if the pulse width is at milliseconds in time scale. This asymmetric behavior is very much different from those in thermal ink-jet printers. The bubble behavior under different heating duration, ranging from microseconds to milliseconds, is experimentally studied. A transient 3-D heat conduction numerical simulation is carried out to study the temperature field of the fluid before the nucleation process. To evaluate the perturbation area of the micro-bubble, submicron particles with diameter of 0.96 μm were placed in the fluid and their dynamic response during the transient bubble formation is recorded.