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

J. Heat Transfer. 2015;137(11):111501-111501-12. doi:10.1115/1.4030382.

Heat transfer coefficients in a set of three symmetrically heated narrow gap channels arranged in line are reported at power densities of 1 kW/cm3 and wall heat flux of 3–40 W/cm2. This configuration emulates an electronics system wherein power dissipation can vary across an array of processors, memory chips, or other components. Three pairs of parallel ceramic resistance heaters in a nearly adiabatic housing form the flow passage, and length-to-gap ratios for each pair of heaters are 34 at a gap of 0.36 mm. Novec™ 7200 and 7300 are used as the heat transfer fluids. Nonuniform longitudinal power distributions are designed with the center heater pair at 1.5X and 2X the level of the first and third heater pairs. At all levels of inlet subcooling, single-phase heat transfer dominates over the first two heater pairs, while the third pair exhibits significant increases because of the presence of flow boiling. Reynolds numbers range from 250 to 1200, Weber numbers from 2 to 14, and boiling numbers from O(10−4) to O(10−3). Exit quality can reach 30% in some cases. Overall heat transfer coefficients of 40 kW/m2K are obtained. Pressure drops for both Novec™ heat transfer fluids are approximately equal at a given mass flux, and a high ratio of heat transfer to pumping power (coefficient of performance (COP)) is obtained. With a mass flux of 250 kg/m2s, heater temperatures can exceed 95 °C, which is the acceptable limit of steady operation for contemporary high performance electronics. Thus, an optimal operating point involving power density, power distribution, mass flux, and inlet subcooling is suggested by the data set for this benchmark multiheater configuration.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(11):111502-111502-9. doi:10.1115/1.4030479.

This study presents an enhancement in the heat transfer performance of a glass thermosyphon using graphene–acetone nanofluid with 0.05%, 0.07%, and 0.09% volume concentrations. The heat load is varied between 10 and 50 W in five steps. The effect of heat load, volume concentration, and vapor temperature on thermal resistance, evaporator and condenser heat transfer coefficients, are experimentally investigated. A substantial reduction in thermal resistance of 70.3% is observed for the maximum concentration of 0.09% by volume of graphene–acetone nanofluid. Further, an enhancement in the evaporator heat transfer coefficient of 61.25% is observed for the same concentration. Also from the visualization study the different flow patterns in the evaporator, adiabatic, and condenser regions are obtained for acetone at different heat inputs.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(11):111503-111503-9. doi:10.1115/1.4030884.

This paper quantifies the influence of acoustic excitation of Al2O3 nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a commercial (Turbo-BII-HP) boiling surface. A nanolubricant with 10 nm diameter Al2O3 nanoparticles at a 5.1% volume fraction in the base polyolester lubricant was mixed with R134a at a 1% mass fraction. The study showed that high-frequency ultrasound at 1 MHz can improve R134a/nanolubricant boiling on a reentrant cavity surface by as much as 44%. This maximum enhancement occurred for an applied power level to the fluid of approximately 6 W and a heat flux of approximately 6.9 kW/m2. Applied power levels larger and smaller than 6 W resulted in smaller boiling heat transfer enhancements. In total, five different applied power levels were studied: 0 W, 4 W, 6 W, 8 W, and 12 W. The largest and smallest enhancement averaged over the tested heat flux range were approximately 12% and 2% for the applied power levels of 6 W and 4 W, respectively. In situ insonation at 1 MHz resulted in an improved dispersion of the nanolubricant on the test surface. An existing pool-boiling model for refrigerant/nanolubricant mixtures was modified to include the effect of acoustic excitation. For heat fluxes greater than 25 kW m−2, the model was within 4.5% of the measured heat flux ratios for mixtures, and the average agreement between measurements and predictions was approximately 1% for all power levels.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2015;137(11):112101-112101-9. doi:10.1115/1.4030658.

In the present study, a three-dimensional transient numerical model was developed to study the temperature field and cutting kerf shape during laser fusion cutting. The finite volume model has been constructed, based on the Navier–Stokes equations and energy conservation equation for the description of momentum and heat transport phenomena, and the volume of fluid (VOF) method for free surface tracking. The Fresnel absorption model is used to handle the absorption of the incident wave by the surface of the liquid metal, and the enthalpy-porosity technique is employed to account for the latent heat during melting and solidification of the material. To model the physical phenomena occurring at the liquid film/gas interface, including momentum/heat transfer, a new approach is proposed which consists of treating friction force, pressure force applied by the gas jet, and the heat absorbed by the cutting front surface as source terms incorporated into the governing equations. All these physics are coupled and solved simultaneously in fluent CFD®. The main objective of using a transient phase change model in the current case is to simulate the dynamics and geometry of a growing laser-cutting generated kerf until it becomes fully developed. The model is used to investigate the effect of some process parameters on temperature fields and the formed kerf geometry.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2015;137(11):112501-112501-11. doi:10.1115/1.4030632.

The immersed boundary method (IBM) was used for three-dimensional numerical simulations, and the results for natural convection in a rectangular channel with an inner hot circular cylinder are presented. This simulation used Rayleigh numbers spanning 3 orders of magnitude, from $1×103$ to $1×106$. The Prandtl number considered in this study was 0.7. We investigated the effects of the inner cylinder's radius on the thermal convection and heat transfer in the space between the cylinder and rectangular channel. A map of the thermal and flow regimes is presented as a function of the cylinder's radius and the Rayleigh number.

Commentary by Dr. Valentin Fuster

### Research Papers: Conduction

J. Heat Transfer. 2015;137(11):111301-111301-7. doi:10.1115/1.4030883.

The problem of determining the steady-state temperature and flux fields in a material containing a plane crack along the bonded interface of two dissimilar functionally graded isotropic materials is considered. The materials exhibit quadratic variation in the coefficients of heat conduction. At large distances from the crack, a uniform heat flux is prescribed. The crack is modeled as an insulated cut. Numerical values for the temperature and flux are obtained for some particular materials. The results demonstrate how the variation in the functional gradation of the thermal parameters in the bonded materials affects the temperature discontinuity across the crack faces and the heat flux flow around the crack tips.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2015;137(11):111901-111901-6. doi:10.1115/1.4030906.

In this paper, we show how a heat-generating domain can be cooled with embedded cooling channels and high-conductivity inserts. The volume of cooling channels and high-conductivity inserts is fixed, so is the volume of the heat-generating domain. The maximum temperature in the domain decreases with high-conductivity inserts even though the coolant volume decreases. The locations and the shapes of high-conductivity inserts corresponding to the smallest peak temperatures for different number of inserts are documented, x = 0.6L and D/B = 0.11 with two rectangular inserts. We also document how the length scales of the inserts should be changed as the volume fraction of the coolant volume over the high-conductivity material volume varies. The high-conductivity inserts should be placed nonequidistantly in order to provide the smallest peak temperature in the heat-generating domain. In addition, increasing the number of the inserts after a limit increases the peak temperature, i.e., this limit is eight number of inserts for the given conditions and assumptions. This paper shows that the overall thermal conductance of a heat-generating domain can be increased by embedding high-conductivity material in the solid domain (inverted fins) when the domain is cooled with forced convection, and the summation of high-conductivity material volume and coolant volume is fixed.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2015;137(11):112001-112001-10. doi:10.1115/1.4030835.

An exploratory study of two-phase physics was undertaken in a slow moving tank containing liquid. This study is under the regime of conjugate heat and mass transfer phenomena. An experiment was designed and performed to estimate the interfacial mass transfer characteristics of a slowly moving tank. The tank was swayed at varying frequencies and constant amplitude. The experiments were conducted for a range of liquid temperatures and filling levels. The experimental setup consisted of a tank partially filled with water at different temperatures, being swayed using a six degrees-of-freedom (DOF) motion actuator. The experiments were conducted for a frequency range of 0.7–1.6 Hz with constant amplitude of 0.025 m. The evaporation of liquid from the interface and the gaseous condensation was quantified by calculating the instantaneous interfacial mass transfer rate of the slow moving tank. The dependence of interfacial mass transfer rate on the liquid–vapor interfacial temperature, the fractional concentration of the evaporating liquid, the surface area of the liquid vapor interface and the filling level of the liquid was established. As sway frequency, filling levels, and liquid temperature increased, the interfacial mass transfer rate also increased. The interfacial mass transfer rate estimated for the swaying tank compared with the interfacial mass transfer rate of stationary tank shows that vibration increases the mass transfer.

Commentary by Dr. Valentin Fuster