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Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2016;138(5):051501-051501-7. doi:10.1115/1.4032291.

An experimental investigation of the effects of droplet diameters and fluid properties on the Leidenfrost temperature of polished and nano/microstructured surfaces has been carried out. Leidenfrost experiments were conducted on a stainless steel 304 polished surface and a stainless steel surface which was processed by a femtosecond laser to form above surface growth (ASG) nano/microstructures. Surface preparation resulted in a root mean square roughness (Rrms) of 4.8 μm and 0.04 μm on the laser processed and polished surfaces, respectively. To determine the Leidenfrost temperatures, the droplet lifetime method was employed using deionized (DI) water and HFE 7300DL. A precision dropper was used to vary the size of DI water droplets from 1.5 to 4 mm. The Leidenfrost temperature was shown to display increases as high as 100 °C on the processed surface over the range of droplet sizes, as opposed to a 40 °C increase on the polished surface. Average increases of the Leidenfrost temperature between polished and processed samples were as high as 200 °C. The experiment was repeated with HFE 7300DL; however, with no noticeable changes of the Leidenfrost temperatures with droplet size whether on the polished or the processed surface. The difference in the Leidenfrost behavior between DI water and HFE 7300DL and among the various droplet sizes can be attributed to the nature of the force balance and flow hydrodynamics at a temperature slightly below the Leidenfrost point (LFP).

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(5):051502-051502-9. doi:10.1115/1.4032358.

A new response surface methodology (RSM) based neural network (NN) modeling method is proposed for finned-tube evaporator performance evaluation under dry and wet conditions. Two RSM designs, Box–Behnken design (BBD) and central composite design (CCD), are applied to collect a small but well-designed dataset for NN training, respectively. Compared with additional 7000 sets of test data, for all the evaporator performance including total cooling capacity, sensible heat ratio and pressure drops on both refrigerant and air sides, the standard deviation (SD) and coefficient of determination of trained NNs are less than 2% and higher than 0.998, respectively, under dry conditions while those are less than 4% and greater than 0.974, respectively, under wet conditions. Classic quadratic polynomial response surface models were also included for reference. By comparison, the proposed model achieves higher accuracy. Finally, parametric study based on the trained NNs is conducted. This new method can remarkably downsize the training dataset and mitigate the over-fitting risk of NN.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(5):051503-051503-16. doi:10.1115/1.4032294.

Heat transfer during condensation of refrigerant blends R404A and R410A flowing through horizontal tubes with 0.76 ≤ D ≤ 9.4 mm at nominal Pr = 0.8–0.9 was investigated. Local heat transfer coefficients were measured for the mass flux range 200 < G < 800 kg m−2 s−1 in small quality increments over the entire vapor–liquid region. Heat transfer coefficients increased with quality and mass flux, while the effect of reduced pressure was not very significant within this range of pressures. The heat transfer coefficients increased with a decrease in diameter. Correlations from the literature were not able to predict the condensation heat transfer coefficient for these fluids at these near-critical pressures over the wide range of tube diameters under consideration. A new flow-regime based model for heat transfer in the wavy, annular, and annular/wavy transition regimes, which predicts 91% of the data within ±25%, is proposed.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2016;138(5):051701-051701-13. doi:10.1115/1.4032287.

This paper presents a study of the conjugate heat transfer, involving conduction through a solid slab and turbulent convection in fluid, for a combined turbulent wall jet and offset jet flow using unsteady Reynolds averaged Navier–Stokes (URANS) equations. The conduction equation for the solid slab and convection equation for the fluid region are solved simultaneously satisfying the equality of temperature and heat flux at the solid–fluid interface. The fluid flow is complex because of the existence of periodically unsteady interaction between the two jets for the chosen ratio of jets separation distance to the jet width (i.e., d/w = 1). The heat transfer characteristics at the solid–fluid interface have been investigated by varying various important parameters within a feasible range: Reynolds number (Re = 10,000–20,000), Prandtl number (Pr = 1–4), solid-to-fluid thermal conductivity ratio (ks/kf = 1000–4000), and nondimensional solid slab thickness (s/w = 1–10). The bottom surface of the solid slab has been maintained at a constant temperature. The mean conjugate heat transfer characteristics indicate that the mean local Nusselt number along the interface is a function of flow (Re) as well as fluid (Pr) properties but is independent of solid properties (ks and s). However, the mean interface temperature and mean local heat flux along the interface always depend on all the aforementioned properties.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(5):051702-051702-10. doi:10.1115/1.4032329.

A study of thermal instability driven by buoyancy force is carried out in an initially quiescent infinitely extended horizontal rotating fluid layer. The temperature at the boundaries has been taken to be time-periodic, governed by the sinusoidal function. A weakly nonlinear stability analysis has been performed for the oscillatory mode of convection, and heat transport in terms of the Nusselt number, which is governed by the complex form of Ginzburg–Landau equation (CGLE), is calculated. The influence of external controlling parameters such as amplitude and frequency of modulation on heat transfer has been investigated. The dual effect of rotation on the system for the oscillatory mode of convection is found either to stabilize or destabilize the system. The study establishes that heat transport can be controlled effectively by a mechanism that is external to the system. Further, the bifurcation analysis also presented and established that CGLE possesses the supercritical bifurcation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(5):051703-051703-8. doi:10.1115/1.4032453.

This paper describes the application of reduced-order modeling techniques in the simulation of conjugate heat transfer processes. In a reduced-order model (ROM), the dominate features of a system are represented using a limited number of orthonormal basis vectors, which are extracted from a database containing descriptions of the system. Interpolating methods are then used to calculate expansion coefficients that allow representation of the system as linear combinations of the basis vectors. Evidence of the accuracy and computational savings achieved using the reduced-order modeling technique is presented in order to demonstrate its benefits in simulating conjugate heat transfer processes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(5):051704-051704-9. doi:10.1115/1.4032431.

We investigate the entropy generation characteristics of a non-Newtonian fluid in a narrow fluidic channel under electrokinetic forcing, taking the effect of conjugate heat transfer into the analysis. We use power-law model to describe the non-Newtonian fluid rheology, in an effort to capture the essential thermohydrodynamics. We solve the conjugate heat transfer problem in an analytical formalism using the thermal boundary conditions of third kind at the outer surface of the walls. We bring out the alteration in the entropy generation behavior as attributable to the rheology-driven alteration in heat transfer, coupled with nonlinear interactions between viscous dissipation and Joule heating originating from electroosmotic effects. We unveil optimum values of different parameters, including both the geometric as well as thermophysical parameters, which lead to the minimization of the entropy generation rate in the system. We believe that the inferences obtained from the present study may bear far ranging consequences in the design of various cooling and heat removal devices/systems, for potential use in microscale thermal management.

Commentary by Dr. Valentin Fuster

Research Papers: Melting and Solidification

J. Heat Transfer. 2016;138(5):052301-052301-7. doi:10.1115/1.4032310.

Continuous, laser-heated boiling heat transfer experiments with silver nanofluids were conducted to identify the nonequilibrium melting behavior of silver nanoparticles in de-ionized (DI) water. Experimental results with transmission electron microscopy (TEM) and dynamic light scattering (DLS) suggest that surface melting of silver nanoparticles (which have a bulk melting point of 961 °C) can occur at ambient pressure when particles are suspended in saturated, and even subcooled (e.g., <100 °C) water due to the localized (volumetric) heat absorption. These findings are supported by calculating a temperature-dependent Hamaker constant of silver nanofluid—i.e., the interaction between interfaces (Ag-melt-water) at the melting temperature. This finding is significant because of the difficulty to identify the melting of silver nanoparticles in water at present, even though it is important to understand such potential melting to use aqueous silver nanofluids in solar applications.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2016;138(5):052501-052501-8. doi:10.1115/1.4032309.

The instability of steady natural convection of a non-Fourier fluid of the single-phase lagging (SPL) type between two vertical surfaces maintained at different temperatures is studied. SPL fluids possess a relaxation time, which reflects the delay in the response of the heat flux and the temperature gradient. The SPL model is particularly relevant to low-temperature liquids, ultrafast processes, and nanofluids (with a retardation time added in this case). Linear stability analysis is employed to obtain the critical state parameters, such as critical Grashof numbers. For intermediate Prandtl numbers (Pr = 7.5), when non-Fourier level exceeds a certain value, the neutral stability curve comprises a Fourier branch and an oscillatory branch. In this case, oscillatory convection increasingly becomes the mode of preference, compared to both conduction and stationary convection. Critical Grashof number decreases for fluids with higher non-Fourier levels.

Commentary by Dr. Valentin Fuster

Research Papers: Porous Media

J. Heat Transfer. 2016;138(5):052601-052601-4. doi:10.1115/1.4032313.

An analytical investigation of the onset on convection in a vertical porous layer saturated by a nanofluid is presented. The Darcy model is used for the vertical porous layer and a linear stability analysis is used to determine the convection threshold in terms of the key parameters for the nanofluid. This study reveals that the Taylor number and gravity effects are passive, and that the most critical mode is roll cells aligned with the vertical axis of rotation. The critical Rayleigh number is presented in terms of the nanofluid parameters for both stationary and oscillatory convection.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2016;138(5):051901-051901-5. doi:10.1115/1.4030903.

In this study, we report enhancement in heat capacity of SiO2 nanoparticle dispersions in a molten ternary nitrate salt (NaNO3, KNO3, and Ca(NO3)2) to obtain an effective heat transfer fluid (HTF)/thermal energy storage (TES) for concentrated solar power (CSP) application. The enhanced heat capacity of the molten salt mixture is expected to greatly increase thermal storage density of HTF/TES, and thus the cost of electricity produced by CSP plant can be significantly reduced. A custom two-step method was used to synthesize SiO2 nanoparticle/ternary salt mixture. The nanoparticle concentration of the mixture was fixed at 1% by weight for comparison with the previous studies. A modulated differential scanning calorimeter (MDSC) was employed to measure heat capacity of the mixture, and it was found to be enhanced by ∼19% compared to the pure ternary salt.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(5):051902-051902-13. doi:10.1115/1.4032331.

In an attempt to investigate the acoustic resonance effect of serpentine passages on internal convection heat transfer, the present work examines a typical high pressure turbine (HPT) blade internal cooling system, based on the geometry of the NASA E3 engine. In order to identify the associated dominant acoustic characteristics, a numerical finite-element method (FEM) simulation (two-step frequency domain analysis) is conducted to solve the Helmholtz equation with and without source terms. Mode shapes of the relevant identified eigenfrequencies (in the 0–20 kHz range) are studied with respect to induced standing sound wave patterns and the local node/antinode distributions. It is observed that despite the complexity of engine geometries, the predominant resonance behavior can be modeled by a same-ended straight duct. Therefore, capturing the physics observed in a generic geometry, the heat transfer ramifications are experimentally investigated in a scaled wind tunnel facility at a representative resonance condition. Focusing on the straight cooling channel's longitudinal eigenmode in the presence of an isolated rib element, the impact of standing sound waves on convective heat transfer and aerodynamic losses are demonstrated by liquid crystal thermometry, local static pressure and sound level measurements. The findings indicate a pronounced heat transfer influence in the rib wake separation region, without a higher pressure drop penalty. This highlights the potential of modulating the aerothermal performance of the system via acoustic resonance mode excitations.

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2016;138(5):052201-052201-10. doi:10.1115/1.4032311.

An experimental approach has been used to investigate the influence of a thin layer of carbon nanotubes (CNTs) on the convective heat transfer performance under impinging flow conditions. A successful synthesis of CNT layers was achieved using a thermal catalytic vapor deposition process (TCVD) on silicon sample substrates. Three different structural arrangements, with fully covered, inline, and staggered patterned layers of CNTs, were used to evaluate their heat transfer potential. Systematic surface characterizations were made using scanning electron microscope (SEM) and confocal microscopy. The external surface area ratio of fully covered, staggered, and inline arrangement was obtained to be 4.57, 2.80, and 2.89, respectively. The surface roughness of the fully covered, staggered, and inline arrangement was measured to be (Sa = 0.365 μm, Sq = 0.48 μm), (Sa = 0.969 μm, Sq = 1.291 μm), and (Sa = 1.668 μm, Sq = 1.957 μm), respectively. On average, heat transfer enhancements of 1.4% and − 2.1% were obtained for staggered and inline arrangement of the CNTs layer. This is attributed to the negligible improvement on the effective thermal resistance due to the small area coverage of the CNT layer. In contrast, the fully covered samples enhanced the heat transfer up to 20%. The deposited CNT layer plays a significant role in reducing the effective thermal resistance of the sample, which contributes to the enhancement of heat transfer.

Commentary by Dr. Valentin Fuster

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