0

### Guest Editorial

J. Heat Transfer. 2019;141(5):050301-050301-2. doi:10.1115/1.4043128.
FREE TO VIEW

Dr. P. S. Ayyaswamy, or “Ayya” to his friends and colleagues, came to Columbia University to pursue his graduate studies after receiving a Bachelor's degree from the University of Mysore in 1962. He worked with Professor Harold G. Elrod, Jr., on his Master's thesis on Supercritical Heat Exchangers. After receiving his Master's degree from Columbia, Dr. Ayyaswamy moved to UCLA to pursue his Ph.D. He worked with Professor Ivan Catton on pioneering work in natural convection in rectangular enclosures tilted at arbitrary angles with respect to the gravity vector which is now considered classical in the field. His elegant scaling of Nusselt number for a vertical configuration to various tilted geometries is now commonly employed in industrial practice [1]. After receiving his Ph.D. in 1971, he continued to work at UCLA as a postdoctoral scholar for three years, first with Professor Friedrich H. Busse at the Geophysics and Planetary Physics Institute (Turbulent Flows) and then with Professors David Okrent (Nuclear Reactor Safety) and Donald K. Edwards (Capillary Flows). In 1974, Dr. Ayyaswamy joined the faculty of the University of Pennsylvania where he was appointed as the Asa Whitney Professor of Dynamical Engineering in 1996. In his 44 year career at Penn, he made seminal contributions in as diverse fields as plasma arc heat transfer to biotechnology to nanoscale transport to phase change processes.

Commentary by Dr. Valentin Fuster

### Review Article

J. Heat Transfer. 2019;141(5):050801-050801-11. doi:10.1115/1.4042297.

In relation to intravitreal drug delivery, predictive mathematical models for drug transport are being developed, and to effectively implement these for retinal delivery, the information on biophysical properties of various ocular tissues is fundamentally important. It is therefore necessary to accurately measure the diffusion coefficient of drugs and drug surrogates in the vitreous humor. In this review, we present the studies conducted by various researchers on such measurements over the last several decades. These include imaging techniques (fluorescence and magnetic resonance imaging (MRI)) that make use of introducing a contrast agent or a labeled drug into the vitreous and tracking its diffusive movement at various time points. A predictive model for the same initial conditions when matched with the experimental measurements provides the diffusion coefficient, leading to results for various molecules ranging in size from approximately 0.1 to 160 kDa. For real drugs, the effectiveness of this system depends on the successful labeling of the drugs with suitable contrast agents such as fluorescein and gadolinium or manganese so that fluorescence or MR imagining could be conducted. Besides this technique, some work has been carried out using the diffusion apparatus for measuring permeation of a drug across an excised vitreous body from a donor chamber to the receptor by sampling assays from the chambers at various time intervals. This has the advantage of not requiring labeling but is otherwise more disruptive to the vitreous. Some success with nanoparticles has been achieved using dynamic light scattering (DLS), and presently, radioactive labeling is being explored.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(5):050802-050802-8. doi:10.1115/1.4043282.

In this work, the effectiveness of the numerical simulations in advancing fundamental understanding of bubble dynamics and nucleate pool boiling heat transfer is discussed. The results of numerical simulations are validated with experiments on ground, in parabolic flights and on the International Space Station (ISS). As such validation is carried out when the level of gravity is varied over seven orders of magnitude. It is shown that reduced gravity stretches the length and time scales of the process and generally leads to degradation of rate of heat transfer associated with nucleate boiling.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Heat Transfer. 2019;141(5):051001-051001-10. doi:10.1115/1.4042487.

The present investigation proposes an innovative convergent double pipe heat exchanger (C-DPHE). A two-dimensional (2D) axisymmetric heat transfer model with counterflow is employed to analyze the thermal and hydraulic performance of this configuration numerically. The impact of convergence in the flow direction, using a wide range of contraction ratio ($Cr$), is explored. The effect of Reynolds and Prandtl numbers on the flow and heat transfer is addressed, as well. The model results were validated with available data from the literature, and an excellent agreement has been confirmed. In general, the findings of the present study indicate that increasing the contraction ratio increases heat transfer and pressure drop in the C-DPHE. Moreover, this configuration has a prominent and sustainable performance, compared to a conventional double pipe heat exchanger (DPHE), with an enhancement in heat transfer rate up to 32% and performance factor ($PF$) higher than one. Another appealing merit for the C-DPHE is that it is quite effective and functional at low Reynolds and high Prandtl numbers, respectively, since no high-operating pumping power is required. Further, the optimal operating conditions can be established utilizing the comprehensive information provided in this work.

Commentary by Dr. Valentin Fuster

### Research Papers: Electronic Cooling

J. Heat Transfer. 2019;141(5):051401-051401-12. doi:10.1115/1.4042328.

We develop a vapor chamber integrated with a microelectronic packaging substrate and characterize its heat transfer performance. A prototype of vapor chamber integrated printed circuit board (PCB) is fabricated through successful completion of the following tasks: patterning copper micropillar wick structures on PCB, mechanical design and fabrication of condenser, device sealing, and device vacuuming and charging with working fluid. Two prototype vapor chambers with distinct micropillar array designs are fabricated, and their thermal performance tested under various heat inputs supplied from a 2 mm × 2 mm heat source. Thermal performance of the device improves with heat inputs, with the maximum performance of ∼20% over copper plated PCB with the same thickness. A three-dimensional computational fluid dynamics/heat transfer (CFD/HT) numerical model of the vapor chamber, coupled with the conduction model of the packaging substrate is developed, and the results are compared with test data.

Commentary by Dr. Valentin Fuster

### Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2019;141(5):051501-051501-12. doi:10.1115/1.4042702.

Pool boiling is postulated as a single-phase heat transfer process with nucleating bubbles providing a liquid pumping mechanism over the heater surface. This results in three fluid streams at the heater surface—outgoing vapor and liquid streams, and an incoming liquid stream. Heat transfer during periodic replacement of the liquid in the influence region around a nucleating bubble is well described by transient conduction (TC) and microconvection (MiC) mechanisms. Beyond this region, free convection (FC) or macroconvection (MaC) contributes to heating of the liquid. A bubble growing on the heater surface derives its latent heat from the surrounding superheated liquid and from the microlayer providing a direct heat conduction path. Secondary evaporation occurs in the bubbles rising in the bulk after departure, and at the free surface. This secondary evaporation does not directly contribute to the heat transfer at the heater surface but provides a means of dissipating liquid superheat. A sonic limit-based model is then presented for estimating the theoretical upper limit for pool boiling heat transfer by considering the three fluid streams to approach their respective sonic velocities. Maximum heat transfer rates are also estimated using this model with two realistic velocities of 1 and 5 m/s for the individual streams and are found to be in general agreement with available experimental results. It is postulated that small bubbles departing at high velocity along with high liquid stream velocities are beneficial for heat transfer. Based on these concepts, future research directions for enhancing pool boiling heat transfer are presented.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(5):051502-051502-10. doi:10.1115/1.4042699.

Nucleate pool boiling heat transfer and its ebullient dynamics in polymeric solutions at atmospheric pressure saturated conditions are experimentally investigated. Three grades of hydroxyethyl cellulose (HEC) are used, which have intrinsic viscosity in the range 5.29 ≤ [η] ≤ 10.31 [dl/g]. Their aqueous solutions in different concentrations, with zero-shear viscosity in the range 0.0021 ≤ η0 ≤ 0.0118 [N⋅s/m2], exhibit shear-thinning rheology in varying degrees, as well as gas–liquid interfacial tension relaxation and wetting. Boiling heat transfer in solutions with constant molar concentrations of each additive, which are greater than their respective critical polymer concentration C*, is seen to have anomalous characteristics. There is degradation in the heat transfer at low heat fluxes, relative to that in the solvent, where the postnucleation bubble dynamics in the partial boiling regime is dominated by viscous resistance of the polymeric solutions. At higher heat fluxes, however, there is enhancement of boiling heat transfer due to a complex interplay of pseudoplasticity and dynamic surface tension effects. The higher frequency vapor bubbling train with high interfacial shear rates in this fully developed boiling regime tends to be influenced by increasing shear-thinning and time-dependent differential interfacial tension relaxation at the dynamic gas–liquid interfaces.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(5):051503-051503-9. doi:10.1115/1.4043129.

Pool boiling heat transfer with the use of femtosecond laser surface processing (FLSP) on copper surfaces has been studied. FLSP creates a self-organized micro/nanostructured surface. In the previous pool boiling heat transfer studies with stainless steel FLSP surfaces, enhancements in critical heat flux (CHF) and heat transfer coefficients (HTCs) were observed compared to the polished reference surface. However, this study shows that copper FLSP surfaces exhibit reductions in both CHF and HTCs consistently. This reduction in heat transfer performance is a result of an oxide layer that covers the surface of the microstructures and acts as an insulator due to its low thermal conductivity. The oxide layer was observed and measured with the use of a focused ion beam milling process and found to have thickness of a few microns. The thickness of this oxide layer was found to be related to the laser fluence parameter. As the fluence increased, the oxide layer thickness increased and the heat transfer performance decreased. For a specific test surface, the oxide layer was selectively removed by a chemical etching process. The removal of the oxide layer resulted in an enhancement in the HTC compared to the polished reference surface. Although the original FLSP copper surfaces were unable to outperform the polished reference curve, this experiment illustrates how an oxide layer can significantly affect heat transfer results and dominate other surface characteristics (such as increased surface area and wicking) that typically lead to heat transfer enhancement.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2019;141(5):052001-052001-8. doi:10.1115/1.4042186.

The intracellular production and transport of energetic substrate adenosine triphosphate (ATP) produced by mitochondria is dependent on multiple factors. These include local metabolic demand, mitochondrial motility and intracellular location, mitochondrial intermembrane potential, bioenergy substrate diffusion within the cell cytosol, and energy transport to the cell nucleus, which itself does not contain any mitochondria. Herein, we demonstrate via cell-based experiment and scaling argument that intracellular bioenergy transport is readily compartmentalized into perinuclear and peripheral regions of the cell. We draw on direct fluorescence-based measurement of quantum dot tracking, high-resolution respirometry, mitochondrial dynamics, and intermembrane potential to assess intracellular quantum dot diffusion to define the intracellular milieu for small molecule transport, and chemical perturbations which challenge cells by altering bioenergetics states. We identify a heterogeneous environment for intracellular bioenergy transport, with a dominant feature being present: the intracellular bioenergy distribution in response to pharmacologically induced cell challenge is determined to be preservation of perinuclear mitochondrial ATP-linked respiration in order to preserve, maintain, or otherwise support bioenergy delivery to meet the metabolic requirements of the cell nucleus whereas there is a decrement in bioenergetic capacity in the cell periphery. This dynamic effect of motile intracellular bioenergy production yields efficient transport of ATP in the maintenance of cellular health.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(5):052002-052002-9. doi:10.1115/1.4042700.

The growth dynamics of isolated gas bubbles from a submerged capillary-tube orifice in a pool of an aqueous surfactant (sodium dodecyl sulfate or SDS) solution is computationally investigated. The governing equations for surfactant mass transport in the bulk liquid and interfacial adsorption–desorption are solved simultaneously with the Navier–Stokes equations, employing the volume-of-fluid (VOF) technique to track the deforming liquid–air interface. The VOF method tends to spread the liquid–air interface over two to three computational cells, creating an interface region with finite thickness. A new numerical treatment is developed to determine the surfactant transport and adsorption/desorption in the interface region. From the variation of the surfactant interfacial concentration, the spatio-temporal variation in interfacial tension is determined and the shape of the growing bubble is predicted. To validate the numerical model, experimental measurements of bubble shape and size are carried out using high speed videography. Because of the decrease in surface tension with surface age, bubble departure diameters in SDS–water solutions are smaller than those obtained in pure water, and they are a function of bubble frequency. At higher air-flow rates (smaller surface age), the bubble departure diameters tend toward those in pure water, whereas at low flow rates (larger surface age), they are significantly smaller than those in water and are closer in size to those in a pure liquid having surface tension equal to the equilibrium value in SDS solution. Furthermore, the nonuniform surfactant adsorption–desorption at the evolving interface results in variation in interfacial tension around the bubbles, and thus their shapes in surfactant solution are different from those in a pure liquid.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2019;141(5):052401-052401-6. doi:10.1115/1.4043014.

Describing the hydrodynamics of nanoparticles in fluid media poses interesting challenges due to the coupling between the Brownian and hydrodynamic forces at the nanoscale. We focus on multiscale formulations of Brownian motion and hydrodynamic interactions (HI) of a single flexible polymeric nanoparticle in confining flows using the Brownian Dynamics method. The nanoparticle is modeled as a self-avoiding freely jointed polymer chain that is subject to Brownian forces, hydrodynamics forces, and repulsive interactions with the confining wall. To accommodate the effect of the wall, the hydrodynamic lift due to the wall is included in the mobility of a bead of the polymer chain which depends on its proximity to the wall. Using the example of a flexible polymeric nanoparticle, we illustrate temporal dynamics pertaining to the colloidal scale as well as the nanoscale.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2019;141(5):052701-052701-9. doi:10.1115/1.4042158.
OPEN ACCESS

The radiation fractional function is the fraction of black body radiation below a given value of λT. Edwards and others have distinguished between the traditional, or “external,” radiation fractional function and an “internal” radiation fractional function. The latter is used for linearization of net radiation from a nongray surface when the temperature of an effectively black environment is not far from the surface's temperature, without calculating a separate total absorptivity. This paper examines the analytical approximation involved in the internal fractional function, with results given in terms of the incomplete zeta function. A rigorous upper bound on the difference between the external and internal emissivity is obtained. Calculations using the internal emissivity are compared to exact calculations for several models and materials. A new approach to calculating the internal emissivity is developed, yielding vastly improved accuracy over a wide range of temperature differences. The internal fractional function should be used for evaluating radiation thermal resistances, in particular.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(5):052702-052702-8. doi:10.1115/1.4042698.

Despite the abundant theoretical studies of magnetic polaritons (MPs) in tailoring the radiative properties of nanostructures, experimental investigation of MPs in deep metal gratings is still lacking. This work experimentally demonstrates the excitation of MP from several microfabricated aluminum gratings in the mid-infrared region by measuring the specular reflectance (zeroth-order diffraction) of the specimen using a Fourier-transform infrared (FTIR) spectrometer. The rigorous coupled-wave analysis (RCWA) and an LC-circuit model are employed to elucidate the mechanism of various resonant modes and their coupling effect. The influence of incidence angle, plane of incidence, polarization, and the trench depth on the spectral reflectance is also discussed. Moreover, the MP dispersion for off-plane layout has been investigated and demonstrated for the first time. The insight gained from this work may facilitate future design and applications of subwavelength periodic structures with desired radiative properties.

Commentary by Dr. Valentin Fuster