Physical Heat Transfer Research Articles

Atmospheric Spray Freeze-Drying: Numerical Modeling and Comparison With Experimental Measurements

Atmospheric spray freeze-drying (ASFD) represents a novel approach to dry thermosensitive solutions via sublimation. Tests conducted with a second-generation ASFD equipment, developed for pharmaceutical applications, have focused initially on producing a light, fine, high-grade powder consistently and reliably. To better understand the heat and mass transfer physics and drying dynamics taking place within the ASFD chamber, 3 analytical models describing the key processes are developed and validated. First, by coupling the dynamics and heat transfer of single droplets sprayed into the chamber, the velocity, temperature, and phase change evolutions of these droplets are estimated for actual operational conditions. This model reveals that, under typical operational conditions, the sprayed droplets require less than 100 ms to freeze. Second, because understanding the heat transfer throughout the entire freeze-drying process is so important, a theoretical model is proposed to predict the time evolution of the chamber gas temperature. Finally, a drying model, calibrated with hygrometer measurements, is used to estimate the total time required to achieve a predefined final moisture content. Results from these models are compared with experimental data.

Methodology for determining the electronic thermal conductivity of metals via direct nonequilibriumab initiomolecular dynamics

Many physical properties of metals can be understood in terms of the free electron model, as proven by the Wiedemann-Franz law. According to this model, electronic thermal conductivity can be inferred from the Boltzmann transport equation (BTE). However, the BTE does not perform well for some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the origin of the thermal energy carried by electrons or how this energy is transported in metals. The charge distribution of conduction electrons in metals is known to reflect the electrostatic potential of the ion cores. Based on this premise, we develop a methodology for evaluating electronic thermal conductivity of metals by combining the free electron model and nonequilibrium ab initio molecular dynamics simulations. We confirm that the kinetic energy of thermally excited electrons originates from the energy of the spatial electrostatic potential oscillation, which is induced by the thermal motion of ion cores. This method directly predicts the electronic thermal conductivity of pure metals with a high degree of accuracy, without explicitly addressing any complicated scattering processes of free electrons. Our methodology offers a route to understand the physics of heat transfer by electrons at the atomistic level. The methodology can be further extended to the study of similar electron-involved problems in materials, such as electron-phonon coupling, which is underway currently.

Mathematical Modeling and Computer Simulations of Nanofluid Flow with Applications to Cooling and Lubrication

There is a growing range of applications of nanoparticle-suspension flows with or without heat transfer. Examples include enhanced cooling of microsystems with low volume-fractions of nanoparticles in liquids, improved tribological performance with lubricants seeded with nanoparticles, optimal nanodrug delivery in the pulmonary as well as the vascular systems to combat cancer, and spray-coating using plasma-jets with seeded nanoparticles. In order to implement theories that explain experimental evidence of nanoparticle-fluid dynamics and predict numerically optimum system performance, a description of the basic math modeling and computer simulation aspects is necessary. Thus, in this review article, the focus is on the fundamental understanding of the physics of nanofluid flow and heat transfer with summaries of microchannel-flow applications related to cooling and lubrication.

Delaying Boundary Layer Separation in a Retarded Potential Flow

The phenomena of convective transport in the boundary layer flow carrying a dilute nanosize suspension of small particles past a stationary wedge is investigated in this communication. The nanofluid modeling is based on non-homogeneous nanoparticle concentration within the boundary layer along with variable thermophysical material properties. It is shown that for the present nanofluid modelling the self-similar solution also exists as it does for the pure fluid case. The self-similar solution is found to exist for the situation when the undisturbed potential flow is of the form \(u_e \left( x \right) =u_1 x^{m}\). A number of qualitatively distinct potential scenarios are predicted concerning the physics of flow and heat transfer. These include boundary layer separation; control or its delay, and heat transfer enhancement. In depth analysis of momentum transport across the boundary layer revealed interesting physics. Worth mentioning effect of nanoparticle is to (slightly) delay the boundary layer separation in the case of retarding potential flow. The values of m are calculated for which the separation starts and are found to depend upon the nanoparticle concentration. Heat transfer augmentation connected directly with the nanoparticle concentration is also an important conclusion of this study.

Modeling microwave heating of frozen mashed potato in a domestic oven incorporating electromagnetic frequency spectrum

Domestic microwave oven magnetrons produce microwaves in a frequency range of 2.45 ± 0.05 GHz. Most microwave heat transfer simulations simplify that the magnetron produces a monochromatic electromagnetic wave of frequency of 2.45 GHz to reduce the computational complexity. This study assumes that the magnetron produces a frequency spectrum defined by a Gaussian distribution of frequencies with a central frequency of 2.45 GHz and investigates the effect of Gaussian distribution variance of (0.05 GHz)2, (0.025 GHz)2, (0.017 GHz)2 on prediction accuracy when compared to using monochromatic frequency of 2.45 GHz. A three-dimensional finite element model coupling electromagnetic and heat transfer physics was developed to simulate heating of 550 g of frozen mashed potato for 6 min. The model was validated in a 1250 W rated microwave oven with the mashed potato tray placed at the center of the stationary turntable. The electromagnetic power densities were determined separately at five different frequencies equidistant between 2.4 and 2.5 GHz. They were then weighted averaged, based on the selected Gaussian distribution. Simulated temperature profiles of the models using the monochromatic frequency of 2.45 GHz and Gaussian frequency spectrum with different variances were compared with experimental temperature profiles obtained using a thermal imaging camera at the end of cooking and five fiber-optic thermocouples during cooking. The model results showed that predicted spatial surface temperature pattern by the model using frequency spectrum with the largest variance (0.05 GHz)2 had better agreement with the experimental temperature pattern when compared to that using a monochromatic frequency of 2.45 GHz. In the transient temperature profile measurement, the average RMSE value of five locations was 7.5 and 13.1 °C for simulations using frequency spectrum and monochromatic frequency of 2.45 GHz, respectively. When compared to using the monochromatic frequency of 2.45 GHz, the frequency spectrum with an assumption of having a Gaussian distribution with mean of 2.45 GHz and variance of (0.05 GHz)2 improved the accuracy of temperature field pattern and transient temperature profile.

A Computational and Experimental Investigation of Synthetic Jets for Cooling of Electronics

Seamless advancements in electronics industry resulted in high performance computing. These innovations lead to smaller electronics systems with higher heat fluxes than ever. However, shrinking nature of real estate for thermal management has created a need for more effective and compact cooling solutions. Novel cooling techniques have been of interest to solve the demand. One such technology that functions with the principle of creating vortex rings is called synthetic jets. These jets are mesoscale devices operating as zero-net-mass-flux principle by ingesting and ejection of high velocity working fluid from a single opening. These devices produce periodic jet streams, which may have peak velocities over 20 times greater than conventional, comparable size fan velocities. These jets enhance heat transfer in both natural and forced convection significantly over bare and extended surfaces. Recognizing the heat transfer physics over surfaces require a fundamental understanding of the flow physics caused by microfluid motion. A comprehensive computational and experimental study has been performed to understand the flow physics of a synthetic jet. Computational study has been performed via FLUENT commercial software, while the experimental study has been performed by using laser Doppler anemometry (LDA). Since synthetic jets are typical sine-wave excited between 20 and 60 V range, they have an orifice peak velocity of over 60 m/s, resulting in a Reynolds number of over 2000. Computational fluid dynamics (CFD) predictions on the vortex dipole location fall within 10% of the experimental measurement uncertainty band.

Heat transfer and flow analysis of jet impingement on concave surfaces

The current study evaluates the performance of three turbulence models in predicting the heat transfer and flow physics of jet impingement on concave surfaces. Two of the applied models are zero–equation subgrid-scale (SGS) models which belong to large eddy simulation (LES), namely the RAST and dynamic Smagorinsky model (DSM), and the third one is RNG k-∈ Reynolds Averaged Navier–Stokes (RANS) model. These models are utilized to analyze the heat transfer for two cases: (1) jet impingement on a curved surface with different jet–to–surface distances (2) jet impingement on a heated circular cylinder with varying nozzle–to–surface distances at two different Reynolds numbers. The predicted results are compared with the available experimental data in the literature. The findings revealed that RAST and DSM predictions are in better agreement with experiments than RNG k-∈ model. It is also concluded that at higher jet–to–surface ratios, all three models produced almost similar results, proving that the heat transfer distribution and the flow are more affected by the jet–to–surface distance than the magnitude of Reynolds number.

Numerical Modelling of Debris Bed Water Quenching

Debris beds may be formed during a nuclear reactor severe accident and coolability of these beds is important to avoid release of radioactive materials into the environment. However, debris bed water quenching is challenging to understand, and model, because of the complex multi-phase flow and heat transfer physics involved which may include boiling. This paper develops a modelling method for boiling and demonstrates its abilities with some applications. The model is based on a multi-fluid approach, in which one phase represents the liquid, the second phase the gas phase the third phase represents the solid debris bed. In each fluid phase a set of conservation equations for mass, energy and momentum are solved with appropriate inter-phase exchange terms coupling the fluid phases.

Monte Carlo simulations of radiative heat exchange in a street canyon with trees

Land surface energy balance in a built environment is widely modelled using urban canopy models with representation of building arrays as big street canyons. Modification of this simplified geometric representation, however, leads to challenging numerical difficulties in improving physical parameterization schemes that are deterministic in nature. In this paper, we develop a stochastic algorithm to estimate view factors between canyon facets in the presence of shade trees based on Monte Carlo simulation, where an analytical formulation is inhibited by the complex geometry. The model is validated against analytical solutions of benchmark radiative problems as well as field measurements in real street canyons. In conjunction with the matrix method resolving infinite number of reflections, the proposed model is capable of predicting the radiative exchange inside the street canyon with good accuracy. Modeling of transient evolution of thermal filed inside the street canyon using the proposed method demonstrate the potential of shade trees in mitigating canyon surface temperatures as well as saving of building energy use. This new numerical framework also deepens our insight into the fundamental physics of radiative heat transfer and surface energy balance for urban climate modeling.

Heat Transfer Enhancement of Backward-Facing Step Flow by Using Nano-Encapsulated Phase Change Material Slurry

Laminar forced convection of nano-encapsulated phase change material (NEPCM) slurry over a 2D horizontal backward-facing step is numerically investigated using a finite volume method based on a collocated grid. The slurry consists of water as base fluid and n-octadecane NEPCM particles with an average diameter of 100 nm. Uniform heat flux boundary condition is imposed to the downstream wall while the step and upstream walls are subjected to adiabatic boundary condition. The effects of Reynolds number ranging from 20 to 80, volume fractions of nanoparticles ranging from 0% to 30%, as well as heat flux ranging from 500 to 2,500 W/m2 are studied. In order to understand the physics of flow and heat transfer of slurry over the backward-facing step, the streamlines and isotherms of the flow were studied. An enhancement in heat transfer coefficient up to 67% using slurry as working fluid compared with pure water can be observed. However, because of the higher viscosity of mixture compared with pure water, the slurry can cause a higher pressure drop in the system. Furthermore, as wall heat flux and Reynolds number increase, the heat transfer coefficient of the bottom wall increases until a critical heat flux is reached and heat transfer performance becomes independent of heat flux.

Aerodynamics Research of Thermal Fluid Circulation in a Heated Cavity

In this paper, numerical simulation of natural convection in a closure square cavity with heating source on the basement is carried out using thermal lattice Boltzmann method. The purpose of this work is to analysis the physics of mass and heat transfer in such a condition. For different Rayleigh numbers, the relative heating area ratio is changed in the range of 0.1-1.0, correspondingly. It is found that the flow pattern is symmetrical for increases from 103to 106, and the boundary layers at low temperature become thinner, also the circulation rate is higher. For a special Rayleigh number, is as a monotone increment function of relative heating area ratio, and the value of which almost increases linearly when the area is below 0.8, and then reach to its maximum exponentially.

Effect of decoupling electromagnetics from heat transfer analysis on prediction accuracy and computation time in modeling microwave heating of frozen and fresh mashed potato

Microwave heating models incorporating physics of electromagnetics and heat transfer are computationally intensive, which limits their use in the industry. The electromagnetic power density changes within the food during microwave heating were characterized. The effect of decoupling electromagnetics from heat transfer analysis on accuracy and computation time was evaluated for microwave heating of mashed potato at various initial states (frozen and fresh). Coupled models used updated heat source term based on temperature-dependent dielectric properties for every rotation; while decoupled models used a constant heat source term based on dielectric properties at room temperature. The simulation and validation results showed that the simplification of using the decoupled approach did not affect the predicted temperatures considerably, while reducing the computation time by 93%. When compared to the experiment, the averaged RMSE values of six transient point temperatures of room-temperature and frozen decoupled-models were 5.2 and 6.6°C, respectively, which were close to those of the coupled models. The decoupled model can be used for screening food-package designs in a short computation time. While this study demonstrated the feasibility of using the decoupled approach for a homogeneous food at various initial temperatures, further study needs to be conducted to evaluate this for heterogeneous foods.

Heat Induced Desorption of Moisture in Timber Joints with Fastener During Charring

Connections are often the weakest link in timber structures so that their failure during fire exposure may result in local or total collapse of a building. The failure modes are complex and poorly understood due to wood’s orthotropicity, the temperature and moisture dependency of wood’s properties, and the presence of metal fasteners that complicates the stress distribution and changes the process of heat transfer into the timber element. In this study, we investigated the coupled physics of heat and mass transfer in a test specimen made from spruce, with a steel fastener inserted in a pre-drilled hole. It imitates a central slice of a timber-to-timber connection exposed to the elevated temperature of 523 K from one side. The heat induced redistribution of the hygroscopic moisture in wood was quantified and visualized by means of neutron radiography. The temperature gradient was measured inside the specimen, at different distances from the heated surface. As the temperature increased in wood, moisture was displaced downstream of the heat source, resulting in a zone with increased moisture content ahead of the drying front and beneath the steel fastener. This wet front occurred where the temperature was below the water evaporation point. It was shown that the steel fastener affected the transport of heat and moisture within the test specimen. The occurring phenomena during the transient state may play a critical role in the embedment strength of wood and therefore influence the load-carrying capacity of timber connections in fire.

Thermoelectric Cool-Film Shear Stress Sensor

Hot-wire anemometers, being a robust and highly sensitive method for measuring flow properties, can be limited in sensitivity where locally increasing the temperature may induce measurement inaccuracy such as when used in near-boiling fluids. In this environment, locally decreasing the temperature allows for a larger temperature difference between the sensor and the ambient environment, thereby increasing device sensitivity while maintaining single-phase convection heat transfer physics. In this letter, we present the new capability of using thermoelectrically cooled sensors to measure wall shear stress. The power required to maintain a constant sensor temperature was increased as the wall shear stress in the channel was increased, providing proof of concept.

Particle-Resolved Direct Numerical Simulation for Gas-Solid Flow Model Development

Gas-solid flows in nature and industrial applications are characterized by multiscale and nonlinear interactions that manifest as rich flow physics and pose unique modeling challenges. In this article, we review particle-resolved direct numerical simulation (PR-DNS) of the microscale governing equations for understanding gas-solid flow physics and obtaining quantitative information for model development. A clear connection between a microscale realization and meso/macroscale representation is necessary for PR-DNS to be used effectively for model development at the meso- and macroscale. Furthermore, the design of PR-DNS must address the computational challenges of parameterizing models in a high-dimensional parameter space and obtaining accurate statistics of flow properties from a finite number of realizations at acceptable grid resolution. This review also summarizes selected recent insights into the physics of momentum, kinetic energy, and heat transfer in gas-solid flows obtained from PR-DNS. Promising future applications of PR-DNS include the study of the effect of number fluctuations on hydrodynamics, instabilities in gas-solid flow, and wall-bounded flows.

TRANSIENT THERMAL ANALYSIS OF PHASE CHANGE MATERIAL BASED HEAT SINKS

Integrated circuits operate best in a limited range of temperature hence their package must be designe d to remove the excessive heat. As an alternative passive cooling technique means, phase change materials or PCMs have been widely inv estigated for such transient cooling applications considering their advantage su ch as high latent heat of fusion, high specific hea t, controllable temperature stability and small volume change during phase chan ge, etc. This PCM based cooling techniques have gre at potential application in the devices which are not operated continuously ove r a long period, but in intermittently using device s like cellular phones, digital cameras, notebook etc. The PCM absorbs heat from th e electronic component when it operates and melts, the molten PCM needs to be re-solidified by dissipating heat to the surroundin gs while the electronic device are idle, such a coo ling system is applicable only for intermittent use devices and not those in continuou s operation. To achieve effective cooling it is imp ortant to ensure that the operating duration of the electronic device does not exceed tof full melting of PCM. Advanced transient analysis is required for clear understanding of the mechanism behind this method owhile practical implementations are consi dered. Controlled convective cooling techniques can be implemented for continuou s operation such kind of systems. The present work is a numerical study consists of t hermal analysis of various configurations of finned heat sink with PCM. The configurations considered are finned heat sink with out PCM and with PCM, Half-filled PCM towards the f in tip side and cases which includes forced convection for systems with continu ous operation. The unsteady analyses were performed to record the transient nature of problem. The characteristic of PCM and the design of operational time of convective cooling are estimate d. By analyzing these different configurations a vi vid picture of the physics of heat transfer in PCM based heat sink is figured out.

On in situ hydrogen sulfide evolution and catalytic scavenging in steam-based oil sands recovery processes

In the SAGD (Steam-Assisted Gravity Drainage) process, high temperature saturated-steam is injected into an oil sands reservoir to lower the viscosity of the bitumen hosted within the reservoir. Typical steam temperatures range from 200 to 260 °C. Under these conditions, the bitumen, in presence of steam–condensate, undergoes aquathermolysis yielding H2S and CO2. Current SAGD simulation models take into account complex spatial heterogeneity of geology and oil composition and the physics of heat transfer, multiphase flow, gas solubility effects, and viscosity variations with temperature; however, few have taken the chemistry of SAGD into account. Here, we have added aquathermolysis reactions to a simulation model to understand H2S reactive zones in SAGD. Given the requirement to constrain sulfur emissions from thermal recovery processes, it is necessary to reduce H2S production to surface. The results demonstrate that injecting small amount of SO2, along with steam, initiates the Claus reaction in the reservoir which converts H2S into liquid sulfur. Thus, sulfur emissions are reduced below that of the original operation without SO2 co-injection. The ability to use in situ Claus reaction-based H2S scavenging offers an elegant way to reduce sulfur emissions from thermal oil sands recovery processes.

Temperature Rise in Electroosmotic Flow of Typical Non-Newtonian Biofluids Through Rectangular Microchannels

Electroosmosis is the main mechanism for flow generation in lab-on-a-chip (LOC) devices. The temperature rise due to the Joule heating phenomenon, associated with the electroosmosis, may be detrimental for samples being considered in LOCs. Hence, a complete understanding of the heat transfer physics associated with the electroosmotic flow is of high importance in design and active control of LOCs. The objective of the present study is to estimate the temperature rise and the thermal entry length in electroosmotic flow through rectangular microchannels, having potential applications in LOC devices. Along this line, the power-law rheological model is used to account for non-Newtonian behavior of the common biofluids encountered in these devices. A mixed type of thermal boundary condition is employed at the channel surface, instead of routinely presumed constant wall heat flux or constant wall temperature conditions. A finite difference-based numerical method is employed for solving the governing equations in dimensionless form. An approximate solution, based on the premise of a uniform temperature field throughout the channel cross section, is also obtained for the bulk mean temperature, which is found to be of high accuracy. This, accompanied by the assessments of the temperature profile, reveals that the temperature variations in the channel cross section are negligible, and as a result, the bulk mean temperature can be used as a very precise estimate of the maximum temperature in an LOC device. Moreover, the evaluation of the entry length shows that a thermally fully developed flow is hardly achieved in practical applications because of small length scales involved. Accordingly, the maximum temperature rise may significantly be smaller than what is calculated based on a thermally fully developed flow assumption.

Sub-picowatt/kelvin resistive thermometry for probing nanoscale thermal transport

Advanced instrumentation in thermometry holds the key for experimentally probing fundamental heat transfer physics. However, instrumentation with simultaneously high thermometry resolution and low parasitic heat conduction is still not available today. Here we report a resistive thermometry scheme with ~50 μK temperature resolution and ~0.25 pW/K thermal conductance resolution, which is achieved through schemes using both modulated heating and common mode noise rejection. The suspended devices used herein have been specifically designed to possess short thermal time constants and minimal attenuation effects associated with the modulated heating current. Furthermore, we have systematically characterized the parasitic background heat conductance, which is shown to be significantly reduced using the new device design and can be effectively eliminated using a "canceling" scheme. Our results pave the way for probing fundamental nanoscale thermal transport processes using a general scheme based on resistive thermometry.

Applications of Heat Transfer Fundamentals to Fire Modeling

The fire industry relies on fire engineers and scientists to develop materials and technologies used to either resist, detect, or suppress fire. While combustion processes are the drivers for what might be considered to be fire phenomena, it is heat transfer physics that mediate how fire spreads. Much of the knowledge of fire phenomena has been encapsulated and exercised in fire modeling software tools. Over the past 30 years, participants in the fire industry have begun to use fire modeling tools to aid in decision making associated with design and analysis. In the rest of this paper we will discuss what the drivers have been for the growth of fire modeling tools; the types of submodels incorporated into such tools; the role of model verification, validation, and uncertainty propagation in these tools; and possible futures for these types of tools to best meet the requirements of the user community. Throughout this discussion, we identify how heat transfer research has supported and aided the advancement of fire modeling.