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Review Article

J. Thermal Sci. Eng. Appl. 2017;10(2):020801-020801-12. doi:10.1115/1.4037200.

Ferrofluids, a distinctive class of nanofluid, consists of suspension of magnetic nanoparticles in a nonmagnetic base fluid. Flow and heat transport properties of such a fluid can be manipulated when subjected to external magnetic field and temperature gradient. This unique feature has fascinated researchers across the globe to test its capability as a coolant for miniature electronic devices. The proposed work presents an updated and comprehensive review on ferrofluids with emphasis on heat transfer enhancement of microdevices. Based on the research findings, a number of important variables that have direct bearing on convective heat transport ability of ferrofluid have been recognized. The paper also identifies the key research challenges and opportunities for future research. By critically resolving these challenges, it is anticipated that ferrofluids can make substantial impact as coolant in miniature heat exchangers.

Commentary by Dr. Valentin Fuster

Research Papers

J. Thermal Sci. Eng. Appl. 2017;10(2):021001-021001-14. doi:10.1115/1.4037196.

A 3D computational fluid dynamics (CFD) modeling study has been carried out for the tin bronze (C903) slab of industrial size in a vertical direct chill caster. The melt is delivered from the top across the entire cross section of the caster. An insulated hot-top is considered above the 80-mm mold to control the melt level in the mold. A porous filter is considered in the hot-top region of the mold to arrest the incoming inclusions and homogenize the flow into the mold. The melt flow through the porous filter is modeled on the basis of the Brinkmann–Forchheimer-extended non-Darcy model. Results are obtained for four casting speeds varying from 40 to 100 mm/min. The metal–mold contact region, as well as the convective heat transfer coefficient at the mold wall, is also varied. In addition to the above, the Darcy number for the porous media is also changed. All parametric studies are performed for a fixed inlet melt superheat of 62 °C. The results are presented pictorially in the form of temperature and velocity fields. The sump depth, mushy region thickness, solid shell thickness (ST) at the exit of the mold, and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021002-021002-13. doi:10.1115/1.4037130.

Onboard liquid cooling of electronic devices is demonstrated with liquid delivered externally to the point of heat removal through a conformal encapsulation. The encapsulation creates a flat microgap above the integrated circuit (IC) and delivers a uniform inlet coolant flow over the device. The coolant is Novec™ 7200, and the electronics are simulated with a resistance heater on a 1:1 scale. Thermal performance is demonstrated at power densities of ∼1 kW/cm3 in the microgap. Parameters investigated are pressure drop, average device temperature, heat transfer coefficient, and coefficient of performance (COP). Nusselt numbers for gap sizes of 0.25, 0.5, and 0.75 mm are reduced to a dimensionless correlation. With low coolant inlet subcooling, two-phase heat transfer is seen at all mass flows. Device temperatures reach 95 °C for power dissipation of 50–80 W (0.67–1.08 kW/cm3) depending on coolant flow for a gap of 0.5 mm. Coefficients of performance of ∼100 to 70,000 are determined via measured pressure drop and demonstrate a low pumping penalty at the device level within the range of power and coolant flow considered. The encapsulation with microgap flow boiling provides a means for use of higher power central processing unit and graphics processing unit devices and thereby enables higher computing performance, for example, in embedded airborne computers.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021003-021003-10. doi:10.1115/1.4037199.

In this study, two-dimensional (2D) numerical simulations of liquid slip flows in parallel-plate microchannels have been performed to obtain heat transfer characteristics and entropy generation rate under asymmetric heating conditions. Heat transfer analysis has been conducted along with second-law analysis through utilizing temperature-dependent thermophysical properties. The results indicate that temperature-dependent thermophysical properties have a positive effect on convective heat transfer and entropy generation. Nusselt numbers of the upper and lower plates and global entropy generation rates are significantly affected by slip parameter and heat flux ratio. It is shown that Nusselt number of the lower plate may have very large but finite values at a specific heat flux ratio. This finding resembles to analytical solutions, where singularities leading to an infinite Nusselt number exist.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021004-021004-11. doi:10.1115/1.4037131.

This paper discusses the approach of cooling design optimization of a high-pressure turbine (HPT) endwall with applied 3D conjugate heat transfer (CHT) computational fluid dynamics (CFD). This study involved the optimization of the spacing of impingement jet array and the exit width of shaped holes, which are different for each cooling cavity. The optimization objectives were to reduce the wall-temperature level and to increase the aerodynamic performance. The optimization methodology consisted of an in-house parametric design and CFD mesh generation tool, a CHT CFD solver, a database of CFD results, a metamodel, and an algorithm for multi-objective optimization. The CFD tool was validated against experimental data of an endwall at CHT conditions. The metamodel, which could efficiently estimate the optimization objectives of new individuals without CFD runs, was developed and coupled with nondominated sorting genetic algorithm II (NSGA II) to accelerate the optimization process. Through the optimization search, the Pareto front of the problem was found in each iteration. The accuracy of metamodel with more iterations was improved by enriching database. But optimal designs found by the last iteration are almost identical with those of the first iteration. Through analyzing extra CFD results, it was demonstrated that the design variables in the Pareto front successfully reached the optimal values. The optimal pitches of impingement arrays could be decided accommodating the local thermal load while avoiding jet lift-off of film coolant. It was also suggested that cylindrical film holes near throat should be beneficial to both aerodynamic and cooling performances.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021005-021005-7. doi:10.1115/1.4037209.

In this paper, numerical solutions to thermally radiating Marangoni convection of dusty fluid flow along a vertical wavy surface are established. The results are obtained with the understanding that the dust particles are of uniform size and dispersed in optically thick fluid. The numerical solutions of the dimensionless transformed equations are obtained through straightforward implicit finite difference scheme. In order to analyze the influence of various controlling parameters, results are displayed in the form of rate of heat transfer, skin friction coefficient, velocity and temperature profiles, streamlines, and isotherms. It is observed that the variation in thermal radiation parameter significantly alters the corresponding particle pattern and extensively promotes the heat transfer rate.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021006-021006-8. doi:10.1115/1.4037449.

A cryogenic propellant submitted to heat load during long duration space missions tends to vaporize to such an extent that the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS), one of the possible control strategies, has been investigated using on-ground experiments with NOVEC1230 as substitution fluid. Results obtained for self-pressurization (SP) and TVS control phases have been reported in a previous work. The unexpected inverse thermal stratification observed during these experiments is analyzed in the present work and related to the influence of noncondensable gases. Noncondensable gases, present inside the tank in the form of nitrogen—ten times lighter than the substitution fluid vapor—generate a concentration stratification in the ullage. Assuming the NOVEC1230 remains at saturation in the whole ullage, the density stratification which results from this concentration stratification can explain the observed inverse thermal stratification.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021007-021007-9. doi:10.1115/1.4037493.

Measurements of heat transfer from an array of vertical heater rods to the walls of a square, helium-filled enclosure are performed for a range of enclosure temperatures, helium pressures, and rod heat generation rates. This configuration is relevant to a used nuclear fuel assembly within a dry storage canister. The measurements are used to assess the accuracy of computational fluid dynamics (CFD)/radiation simulations in the same configuration. The simulations employ the measured enclosure temperatures as boundary conditions and predict the temperature difference between the rods and enclosure. These temperature differences are as large as 72 °C for some experiments. The measured temperature of rods near the periphery of the array is sensitive to small, uncontrolled variations in their location. As a result, those temperatures are not as useful for validating the simulations as measurements from rods near the array center. The simulated rod temperatures exhibit random differences from the measurements that are as large as 5.7 °C, but the systematic (average) error is 1 °C or less. The random difference between the simulated and measured maximum array temperature is 2.1 °C, which is less than 3% of the maximum rod-to-wall temperature difference.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021008-021008-10. doi:10.1115/1.4037543.

It is becoming often to measure steady-state heat transfer rate from thermal systems with variable speed and volume equipment and hence with fluctuating properties and mass flow rates. However, it is unclear if the conventional heat transfer rate measurement based on averages of temperature and pressure measurement is representative enough to represent the effect of system dynamics and measure their heat transfer rates accurately. This paper studied the issue by comparing its accuracy and uncertainty to that of alternative data-processing methods with theoretically less systematic bias. The comparison was conducted with steady-state data from a variable-speed ductless heat pump (DHP) system with occasional fluctuation of refrigerant flow and properties. The results show that the accuracy improvement brought by one alternative method is statistically significant albeit small in magnitude, and the other method may reduce uncertainty of the heat transfer rate measurement in tests with large periodic changes of measured variables. Nonetheless, both alternative methods are about 100 times more computationally expensive than the conventional averaging method, and averages of temperature and pressure measurement are still appropriate when computational resources are limited.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021009-021009-12. doi:10.1115/1.4037576.

The present work considers the application of the generalized integral transform technique (GITT) in the solution of a class of linear or nonlinear convection–diffusion problems, by fully or partially incorporating the convective effects into the chosen eigenvalue problem that forms the basis of the proposed eigenfunction expansion. The aim is to improve convergence behavior of the eigenfunction expansions, especially in the case of formulations with significant convective effects, by simultaneously accounting for the relative importance of convective and diffusive effects within the eigenfunctions themselves, in comparison against the more traditional GITT solution path, which adopts a purely diffusive eigenvalue problem, and the convective effects are fully incorporated into the problem source term. After identifying a characteristic convective operator, and through a straightforward algebraic transformation of the original convection–diffusion problem, basically by redefining the coefficients associated with the transient and diffusive terms, the characteristic convective term is merged into a generalized diffusion operator with a space-variable diffusion coefficient. The generalized diffusion problem then naturally leads to the eigenvalue problem to be chosen in proposing the eigenfunction expansion for the linear situation, as well as for the appropriate linearized version in the case of a nonlinear application. The resulting eigenvalue problem with space variable coefficients is then solved through the GITT itself, yielding the corresponding algebraic eigenvalue problem, upon selection of a simple auxiliary eigenvalue problem of known analytical solution. The GITT is also employed in the solution of the generalized diffusion problem, and the resulting transformed ordinary differential equations (ODE) system is solved either analytically, for the linear case, or numerically, for the general nonlinear formulation. The developed methodology is illustrated for linear and nonlinear applications, both in one-dimensional (1D) and multidimensional formulations, as represented by test cases based on Burgers' equation.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(2):021010-021010-9. doi:10.1115/1.4037774.

Magnetohydrodynamic (MHD) flow of viscous fluid by curved stretching surface is presented in this paper. Heat and mass transfer analysis is studied with double stratification and thermal radiation effects. Joule heating is also taken into consideration. Basic equations of flow problem are obtained using curvilinear coordinates. The partial differential equations are reduced to the nonlinear ordinary differential equations using suitable transformations. Graphical results are shown and analyzed for the effect of different parameters on fluid characteristics. It is noted that thermal and solutal stratification parameters have opposite effect on temperature and concentration distributions. Magnitude of pressure, skin friction coefficient, and Nusselt number decreases for larger curvature parameter.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Thermal Sci. Eng. Appl. 2017;10(2):024501-024501-5. doi:10.1115/1.4037650.

Two-phase bubbly flows by gas injection had been shown to enhance convective heat transfer in channel flows as compared with that of single-phase flows. The present work explores the effect of gas phase distribution such as inlet air volume fraction and bubble size on the convective heat transfer in upward vertical channel flows numerically. A two-dimensional (2D) channel flow of 10 cm wide × 100 cm high at 0.2 and 1.0 m/s inlet water and air superficial velocities in churn-turbulent flow regime, respectively, is simulated. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS fluent. The bubble size is characterized by the Eötvös number. The inlet air volume fraction is fixed at 10%, whereas the Eötvös number is maintained at 1.0 to perform parametric studies, respectively, in order to investigate the effect of gas phase distribution on average Nusselt number of the two-phase flows. All simulations are compared with a single-phase flow condition. To enhance heat transfer, it is determined that the optimum Eötvös number for the channel with a 10% inlet air volume fraction has an Eötvös number of 0.2, which is equivalent to a bubble diameter of 1.219 mm. Likewise, it is determined that the optimum volume fraction peaks at 30% inlet air volume fraction using an Eötvös number of 1.0.

Commentary by Dr. Valentin Fuster

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