Research Papers: Bio-Heat and Mass Transfer

J. Heat Transfer. 2014;136(7):071101-071101-6. doi:10.1115/1.4027147.

Magnetic interstitial hyperthermia is a hopeful treatment method for tumor. Before treatment, the tumor would be embedded with a number of ferromagnetic seeds, which can produce energy under an alternating magnetic field. The tumor cells would be necrosed once the temperature exceeding to a value. However, the normal tissue around the tumor is expected to be under safety. Hence, temperature simulation is necessary to avoid any mistake treatment planning, meanwhile, the calculation is required as quick as possible. We developed an efficient cellular automata (CA) numerical method to solve the bioheat transfer equation. The CA equation is derived from Lattice Boltzmann equation. As a discrete numerical method in space and time, CA can be used to deal with the complicated boundaries, such as the huge vessels incorporated in the tumor, which were not well treated in traditional methods. The model of ferromagnetic seed, which is critical to the numerical results, is treated with a simple numerical temperature model. In order to evaluate the proposed method, in vitro and in vivo experiments are carried out, respectively. After comparison between the numerical and the experimental results, the proposed method shows perfect calculation precision and high efficiency, which is significant for clinical treatment.

Commentary by Dr. Valentin Fuster

Research Papers: Conduction

J. Heat Transfer. 2014;136(7):071301-071301-6. doi:10.1115/1.4027183.

By using the thermomass-theory approach, the temperature in a thin layer heated by a hot spot is derived in steady states. It is shown that an anomalous temperature profile, which seems to be at odds with the fundamental laws of continuum physics, may occur. The compatibility of this situation with second law of thermodynamics is analyzed in view of the concept of flux limiter.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2014;136(7):071501-071501-9. doi:10.1115/1.4026808.

Numerous investigations have been conducted to extend adiabatic liquid–gas volume-of-fluid (VOF) flow solvers to include condensation phenomena by adding an energy equation and phase-change source terms. Some proposed phase-change models employ empirical rate parameters, or adapt heat-transfer correlations, and thus must be tuned for specific applications. Generally applicable models have also been developed that rigorously resolve the phase-change process, but require interface reconstruction, significantly increasing computational cost, and software complexity. In the present work, a simplified first-principles-based condensation model is developed, which forces interface-containing mesh cells to the equilibrium state. The operation on cells instead of complex interface surfaces enables the use of fast graph algorithms without reconstruction. The model is validated for horizontal film condensation, and converges to exact solutions with increasing mesh resolution. Agreement with established results is demonstrated for smooth and wavy falling-film condensation.

Commentary by Dr. Valentin Fuster

Research Papers: Experimental Techniques

J. Heat Transfer. 2014;136(7):071601-071601-10. doi:10.1115/1.4026908.

An experimental radiation chamber has been developed to nondestructively measure the thermal diffusivity of a combustion chamber deposit (CCD) layer. The accumulation of CCD shifts the operability range of homogeneous charge compression ignition (HCCI) to lower loads where the fuel economy benefit of HCCI over a traditional spark ignition strategy is at a maximum. The formation and burn-off of CCD introduce operational variability, which increases the control system burden of a practical HCCI engine. To fully characterize the impact of CCD on HCCI combustion and develop strategies which limit the CCD imposed variability, the thermal and physical properties of HCCI CCD must be determined without destroying the morphology of the CCD layer. The radiation chamber device provides a controlled, inert atmosphere absent of cyclical pressure oscillations and fuel/air interactions found within an engine. The device exposes temperature probes coated with CCD to controlled heat flux pulses generated by a graphite emitter and a rotating disk. CCD layer thermal diffusivity is then calculated based on the phase delay of the sub-CCD temperature response relative to the response of the temperature probe when clean. This work validates the accuracy of the radiation chamber's diffusivity determination methodology by testing materials of known properties. Wafers of three different materials, whose thermal diffusivities span two orders of magnitude centered on predicted CCD diffusivity values, are installed over the temperature probes to act as CCD surrogates. Multiple thicknesses of each material are tested over a range of heat flux pulse durations. Diffusivity values determined from radiation chamber testing are independent of sample thickness with each of the three calibration materials. The radiation chamber diffusivity values exhibit a slight, but consistent underprediction for all wafers due to residual contact resistance at the wafer–probe interface. Overall, the validation studies establish the radiation chamber as an effective device for the nondestructive thermal diffusivity determination of thin insulative coatings. The similarity of expected CCD diffusivity values to those of the validation specimens instills confidence that the methodology and device presented herein can be successfully utilized in the characterization of HCCI CCD layers.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2014;136(7):071701-071701-9. doi:10.1115/1.4027250.

This study examines experimentally the cooling performance of narrow impingement channels as could be cast-in in modern turbine airfoils. Full surface heat transfer coefficients are evaluated for the target plate and the sidewalls of the channels using the transient liquid crystal technique. Several narrow impingement channel geometries, consisting of a single row of five cooling holes, have been investigated composing a test matrix of nine different models. The experimental data are analyzed by means of various post-processing procedures aiming to clarify and quantify the effect of cooling hole offset position from the channel centerline on the local and average heat transfer coefficients and over a range of Reynolds numbers (11,100–86,000). The results indicated a noticeable effect of the jet pattern on the distribution of convection coefficients as well as similarities with conventional multi-jet impingement cooling systems.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):071702-071702-10. doi:10.1115/1.4027195.

In the present work constructal design is employed to optimize the geometry of a convective, Y-shaped cavity that intrudes into a solid conducting wall. The main purpose is to investigate the influence of the dimensionless heat transfer parameter a over the optimal geometries of the cavity, i.e., the ones that minimize the maximum excess of temperature (or reduce the thermal resistance of the solid domain). The search for the best geometry has been performed with the help of a genetic algorithm (GA). For square solids (H/L = 1.0) the results obtained with an exhaustive search (which is based on solution of all possible geometries) were adopted to validate the GA method, while for H/L ≠ 1.0 GA is used to find the best geometry for all degrees of freedom investigated here: H/L, t1/t0, L1/L0, and α (four times optimized). The results demonstrate that there is no universal optimal shape that minimizes the thermal field for all values of a investigated. Moreover, the temperature distribution along the solid domain becomes more homogeneous with an increase of a, until a limit where the configuration of “optimal distribution of imperfections” is achieved and the shape tends to remain fixed. Finally, it has been highlighted that the GA method proved to be very effective in the search for the best shapes with the number of required simulations much lower (8 times for the most difficult situation) than that necessary for exhaustive search.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Exchangers

J. Heat Transfer. 2014;136(7):071801-071801-10. doi:10.1115/1.4027072.

Recuperator is one of the key components in high temperature gas cooled reactors. Although cross-corrugated plates have been used to increase the thermal performance of the recuperators, the fundamental mechanisms of fluid flow and heat transfer are generally not clear. Fluid dynamics simulations and experiments are hence carried out to study the performance of the recuperators. A periodic cell is employed as the control volume. The flow field and heat transfer in sine-wave crossed-corrugated channels are investigated based on the Navier–Stokes and energy equations in the laminar flow regime between Re = 84 and 1168. The numerical results of the heat transfer factors and friction factors in different operating conditions show a fairly good agreement with the experimental measurements. The influence factors on the heat transfer and the hydraulic performance are also discussed in the paper. It is found that the heat transfer factor j and friction factor f decrease with the increase of the pitch-height ratio for a given Reynolds number.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2014;136(7):071901-071901-9. doi:10.1115/1.4027184.

This paper examines the augmentation of heat and mass transfer due to dual clearances on cylindrical pin fins, relative to a channel between parallel plates, in mini/microchannel reactors at low Reynolds numbers. In this work, diffusion limitations to heat and mass transfer in smooth-walled mini/microchannel reactors were minimized by the implementation of microcylinder pin fins with dual clearances that, (1) promote the production of instabilities in the wakes that enhance mixing and (2) reduce the viscosity dominated regions at pin-wall interfaces. A smooth catalyst coating is assumed on all exposed surfaces of the microchannel interior walls and pin fins. Due to the analogy of heat and mass transfer, augmentation of the Nusselt number is equivalent to the augmentation of the Sherwood number. Heat transfer augmentation is investigated in air (Pr = 0.705) at dual clearances ranging from 0 to 0.4 of the channel height and Reynolds numbers from 10 to 600. The pin fins and the clearance augmented the heat transfer coefficient by a factor of 4.0. The combination of the augmentation of the heat transfer coefficient and the increase in the surface area, by the clearances, results in an increase in the conductance over a plane channel, by a factor of 7.1. The results are extendable to overcoming laminar diffusion with laminar periodic wakes of fuel vapors such as methanol vapor in air where Scfuel ∼ Prair. For turbulent wakes impinging upon downstream pins, the results can be extended to fuel vapors with (Scfuel)turb ∼ (Prair)turb. A large eddy simulation (LES) approach was used in this study.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2014;136(7):072001-072001-8. doi:10.1115/1.4027152.

Silicon is the primary material of integrated circuit (IC) manufacturing in microelectronic industry. It has high thermal conductivity and superior thermomechanical properties compatible to most semiconductors. These characteristics make it an ideal material for fabricating micro/mini heat pipes and their wick structures. In this article, silicon wick structures, composed of cylindrical pillars 320 μm in height and 30–100 μm in diameter, are developed for studies of phase change capability. Fabrication of the silicon wick structures utilizes the standard microelectromechanical systems (MEMS) approach, which allows the precise definition on the wick dimensions, as well as the heated wick area. On these bases, experimental characterizations of temperature variations versus input heat fluxes, associated with simultaneous visualization on the liquid transport and the dryout, are performed to investigate the wick dimensional effects on the maximum phase change capability. On the wick structure with the pillar diameter/pores of 100 μm and a heated wick area of 2 mm × 2 mm, the phase change reached a maximum heat flux of 1130 W/cm2. Despite of the liquid bottom-feed approach, interactions between liquid and vapor phases enables the heated wick structure absorb liquid from its surrounding wick area, including from its top side with a longer liquid transport path. In contrast, a wick structure with fine pillars (10 μm in diameter) inhibited the generation of nucleate boiling. Evaporation on the meniscus interface becomes the major phase change mechanism. A large heated wick area (4 mm × 4 mm) increases the viscous loss in transporting liquid to wet the entire wick, advancing the dryout at 135 W/cm2. Mass transfer analysis, as well as discussion of the experimental results, indicates that a dimensional ratio r/l (pillar diameter/characteristic length of the heated wick area) is a key parameter in determining the maximum phase change capability. A low r/l ratio enhances heat and mass transport capability, as well as heat transfer coefficient.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2014;136(7):072101-072101-11. doi:10.1115/1.4027151.

Amorphous fiber, ribbon, or film is produced through melt spinning. In this manufacturing process, a continuous delivery of amorphous material is simultaneously dependent on the wheel spinning rate, metallic liquid viscosity, surface tension force, heat transfer inside the melt pool and along the substrate, and other parameters. An analysis of a free-jet melt spinning for fiber manufacture has been performed to relate the process control parameters with amorphous formation. We present a numerical simulation of transient impingement of a free melt jet with a rapidly rotating wheel, along with theoretical estimates of melt ribbon thickness, to investigate dynamical characteristics of the flow in melt pool. The nucleation temperature and the critical cooling rate are predicted in the paper for alloy Fe75–Si10–B15 (at. %). Thermal conduction is found to dominate undercooling in melt spinning by comparing the temperature and velocity measurements with our numerical simulation and the analytical solutions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):072102-072102-10. doi:10.1115/1.4027182.

Heat transfer studies of a hot AISI 304 stainless steel plate by water jet impingement with different concentrations of three different types of surfactants have been investigated. The study involves a square plate of 100 mm × 100 mm surface area and 6 mm thickness with three subsurface thermocouples positioned at various locations inside the plate. The influence of jet height has been studied by varying the distance between the nozzle and plate from 200 mm to 600 mm. The results show that the heat transfer rate is found to increase with the jet height up to 400 mm and thereafter decreases due to capillary instability of liquid jet. Based on the maximum surface heat flux obtained for a particular nozzle height of 400 mm and an initial surface temperature of 900 °C, further experiments have been carried out with different types of surfactants. The types of surfactants used in the experimental study are anionic surfactant (sodium dodecyl sulphate, SDS), cationic surfactant (cetyltrimethylammonium bromide, CTAB) and nonionic surfactant (Polyoxyethylene 20 sorbitan monolaurate, Tween 20). During cooling, the transient temperature data measured by thermocouples have been analyzed by inverse heat conduction procedure to calculate surface heat flux and surface temperatures. The increase in surface heat flux has been observed with increasing concentration of surfactants and it has been found to be limited to a particular concentration of surfactant after which further increase in concentration leads to decrease in heat flux. Use of surfactant added water minimizes the surface tension and promotes better spreadability of coolant on the test specimen by reducing the solid–liquid contact angle. The maximum heat transfer rate has been found by using nonionic surfactant additive which can primarily be attributed to its lesser foam formability nature.

Commentary by Dr. Valentin Fuster

Research Papers: Porous Media

J. Heat Transfer. 2014;136(7):072601-072601-10. doi:10.1115/1.4026969.

Porous structures formed by sintering of powders, which involves material-bonding under the application of heat, are commonly employed as capillary wicks in two-phase heat transport devices such as heat pipes. These sintered wicks are often fabricated in an ad hoc manner, and their microstructure is not optimized for fluid and thermal performance. Understanding the role of sintering kinetics—and the resulting microstructural evolution—on wick transport properties is important for fabrication of structures with optimal performance. A cellular automaton model is developed in this work for predicting microstructural evolution during sintering. The model, which determines mass transport during sintering based on curvature gradients in digital images, is first verified against benchmark cases, such as the evolution of a square shape into an area-preserving circle. The model is then employed to predict the sintering dynamics of a side-by-side, two-particle configuration conventionally used for the study of sintering. Results from previously published studies on sintering of cylindrical wires are used for validation. Randomly packed multiparticle configurations are then considered in two and three dimensions. Sintering kinetics are described by the relative change in overall surface area of the compact compared to the initial random packing. The effect of sintering parameters, particle size, and porosity on fundamental transport properties, viz., effective thermal conductivity and permeability, is analyzed. The effective thermal conductivity increases monotonically as either the sintering time or temperature is increased. Permeability is observed to increase with particle size and porosity. As sintering progresses, the slight increase observed in the permeability of the microstructure is attributed to a reduction in the surface area.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):072602-072602-7. doi:10.1115/1.4027146.

This paper is devoted to introduce a numerical simulation using the differential transformation method (DTM) with a theoretical study for the effect of viscous dissipation on the steady flow with heat transfer of Newtonian fluid towards a permeable stretching surface embedded in a porous medium with a second order slip. The governing nonlinear partial differential equations are converted into a system of nonlinear ordinary differential equations (ODEs) by using similarity variables. The resulting ODEs are successfully solved numerically with the help of DTM. Graphic results are shown for nondimensional velocities and temperatures. The effects of the porous parameter, the suction (injection) parameter, Eckert number, first and second order velocity slip parameters and the Prandtl number on the flow and temperature profiles are given. Moreover, the local skin-friction and Nusselt numbers are presented. Comparison of numerical results is made with the earlier published results under limiting cases.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2014;136(7):072701-072701-7. doi:10.1115/1.4026907.

Coal ash inevitably forms deposits as combustion residue on the walls and heat transfer surfaces of coal-fired boilers. Ash deposits decrease the boiler efficiency, reduce the generating capacity, and cause unscheduled outages. The radiative heat transfer is the major heat transfer mechanism in utility boilers; thus, the ash deposit emissivity is critical to boiler efficiency and safety. This paper presents a radiative transfer model to predict the spectral emissivities of coal ash deposits. The model includes the effects of the microstructure, chemical composition, and temperature. Typical ash deposit microstructures are generated using diffusion-limited aggregation (DLA). The radiative properties are then calculated using the generalized multiparticle Mie-solution (GMM). The combined GMM and DLA model predicts spectral emissivity better than the original Mie theory and Tien's dependent scattering theory with the average relative difference between predicted results and experimental data decreasing from 17.8% to 9.1% for sample 1 and from 18.6% to 4.2% for sample 2. Maxwell-Garnett (MG) effective medium theory is used to calculate the ash deposit optical constants based on the chemical compositions to include the effect of chemical composition. Increasing temperatures increase the particle diameters and particle volume fractions and, thus, the spectral emissivities. The spectral emissivity ultimately remains constant and less than one. The homogeneous slab model gives the upper limit of the ash deposit spectral emissivity.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):072702-072702-7. doi:10.1115/1.4026954.

The direct utilization of solar radiation has been considered a promising energy source because of its abundance, sustainability, and cleanness. The conversion of solar radiation into usable heat largely depends on the absorption characteristics of a solar thermal collector. In the present study, we conducted design analysis of a wavelength-selective absorber composed of a two-dimensional Nickel grating, a thin SiO2 film, and a Nickel substrate. Dimensions of the two-dimensional grating were determined with the Taguchi method, which optimized the spectral absorptance for both polarizations. The spectral absorptance demonstrated a broad-band plateau within the visible and the near-infrared spectral region, but it was significantly suppressed at longer wavelengths. Moreover, the absorptance plateau was nearly insensitive to the incident orientation of solar radiation. Physical mechanisms of the absorption enhancement were elucidated with the local magnetic field distribution.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):072703-072703-12. doi:10.1115/1.4027153.

A Monte Carlo model is applied to determinate the steady state, solar-weighted optical properties of potential thermotropic composite materials for overheat protection of polymer solar absorbers. The key results are dimensionless plots of normal-hemispherical transmittance, reflectance and absorptance as a function of particle size parameter, scattering albedo, and overall optical thickness. The optical behavior of thermotropic materials at different temperatures is represented by a change in the relative refractive index which affects the scattering albedo and optical thickness. At low temperatures where overheat protection is not required, referred to as the clear state, the overall optical thickness should be less than 0.3 to ensure high transmittance for the preferred particle size parameter of 2. At higher temperatures where overheat protection is required, referred to as the translucent state, the overall optical thickness should be greater than 10 and the scattering albedo should be greater than 0.995 to achieve 50% reflectance. A case study of low molecular weighted polyethylene in poly(methyl methacrylate) is presented to illustrate use of the results to guide the design of thermotropic materials.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2014;136(7):074501-074501-4. doi:10.1115/1.4026091.

The H1 and H2 forced convection heat transfer in rectangular ducts are studied using an accurate, analytic method. It is confirmed that, as the aspect ratio tends to infinity, the Nusselt number for the H2 case approaches 2.9162, much lower than the parallel plate value of 8.2353 attained by the H1 case. The controversy about the H2 limit is thus settled. An explanation of the behavior is suggested.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):074502-074502-4. doi:10.1115/1.4027251.

An algorithm for choosing the number of quadrature nodes before calculation of a view factor is proposed. Simple criterion is introduced that allows one to estimate the error in the computed view factor. The algorithm allows one to save much computation time by always using the minimum number of nodes for each pair of surface zones and insures a desired accuracy. The algorithm is applied for model of a continuous furnace and is compared with a standard method which uses predefined number of nodes at each surface. The proposed algorithm is many times faster and also more accurate than the standard one.

Topics: Errors , Algorithms , Furnaces
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(7):074503-074503-4. doi:10.1115/1.4027205.

Switchable thermal interfaces allow controlled modulation of thermal conductance and are a key enabler of microdevices and systems that require reconfigurable heat transfer paths. We report a solid-liquid hybrid thermal interface for reliable low-contact pressure (<1 kPa) switching with on-state thermal contact resistance <15 × 10−6 m2K/W. Reduction in the thermal resistance of hybrid interfaces created through electroplating was evaluated using transient pulsed heating measurements and thermal time constant characterization. Compared with pure liquid-mediated interfaces and direct solid-solid contacts reported previously, the hybrid interface shows superior thermal performance under the same loading pressure while avoiding the use of liquid metals. The hybrid interface may be readily used with low-power electrostatic or Lorenz force-based actuators as part of integrated thermal microdevices.

Commentary by Dr. Valentin Fuster

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