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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

### 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