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IN THIS ISSUE

### Research Papers

J. Thermal Sci. Eng. Appl. 2017;9(4):041001-041001-14. doi:10.1115/1.4035970.

A computational analysis of conjugate mixed convection heat transfer from shrouded vertical nonisothermal heat sink on a horizontal base is performed. The overall Nusselt number and the product of friction factor (f) and Reynolds number (Re) are found to vary significantly with the spacing of heat sink as well as with the clearance between shroud and heat sink. By increasing the fin conductance by 200%, an enhancement of Nusselt number is noted to be around 58%, while the same Nusselt number enhancement is 134% for isothermal fin, within the range of parametric studies. The fRe value for smaller fin spacing shows a maximum with clearances, while the same for higher fin spacing remains the same or increases with clearances. Finally, overall Nusselt number and friction factor are well correlated with the governing parameters of the problem.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041002-041002-12. doi:10.1115/1.4036012.

The present work is aimed to examine how the heat transfer coefficient (HTC) and main three-dimensional (3D) passage aerodynamic features may be affected by a nonadiabatic wall temperature condition. A systematic computational study has been first carried out for a 3D nozzle guide vane (NGV) passage. The impacts of wall temperature on the secondary flows, trailing edge shock waves, and the passage flow capacity are discussed, underlining the connection and interactions between the wall temperature and the external aerodynamics of the 3D passage. The local discrepancies in HTC in these 3D flow regions can be as high as 30–40% when comparing low and high temperature ratio cases. The effort is then directed to a new three-point nonlinear correction method. The benefit of the three-point method in reducing errors in HTC is clearly demonstrated. A further study illustrates that the new method also offers much enhanced robustness in the wall heat flux scaling, particularly relevant when the wall thermal condition is also shown to influence the laminar–turbulent transition exhibited by two well-established transition models adopted in the present work.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041003-041003-11. doi:10.1115/1.4036202.

In this present work, bubble dynamics of subcooled flow boiling in water–ethanol mixture is investigated through visualization using a high-speed camera in horizontal rectangular channels. The heat transfer coefficient of water–ethanol mixture during subcooled flow boiling is determined for various parameters like heat flux, mass flux, and channel inlet temperature. The effect of bubble departure diameter on heat transfer coefficient is discussed. A correlation is developed for subcooled flow boiling Nusselt number of water–ethanol mixture. The parameters considered for correlation are grouped as dimensionless numbers by Buckingham $π$-theorem. The present correlation is compared with the experimental data. The mean absolute error (MAE) of Nusselt number of water–ethanol mixture calculated from the experimental data and those predicted from the present correlation is 10.39%. The present correlation is also compared with the available literature correlations developed for water. The MAE of Nusselt number of water predicted from the present correlation and those predicted with Papel, Badiuzzaman, Moles–Shaw, and Baburajan correlations is 41%, 19.61%, 29.9%, and 43.1%, respectively.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041004-041004-9. doi:10.1115/1.4036203.

Unsteady conjugate natural convection in a semicircular cavity with a solid shell of finite thickness filled with a hybrid water-based suspension of Al2O3 and Cu nanoparticles (hybrid nanofluid) has been analyzed numerically. The governing equations for this investigation are formulated in terms of the dimensionless stream function, vorticity, and temperature and have been solved by the finite difference method of the second-order accuracy. The effects of the dimensionless time, Rayleigh number, thermal conductivity ratio, and the nanoparticles volume fraction on the flow patterns and heat transfer have been studied. The obtained results have revealed essential heat transfer enhancement at solid–fluid interface with addition of nanoparticles. In addition, a comparison of the heat transfer enhancement level due to the suspension of various nanoparticles materials (Al2O3 and Cu) in water as regular nanofluids (Al2O3/water and Cu/water) and as a hybrid Al2O3–Cu/water nanofluid is reported.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041005-041005-8. doi:10.1115/1.4036204.

An innovative relationship between the effectiveness $(ε)$ and number of transfer unit (NTU) was presented in this work for indirect evaporative cooler (IEC). This relationship is featured by its simplicity in use and has noniterative procedure to be implemented as the traditional one in the literature. The new model can be implemented in sizing and rating design of the IEC at different Lewis numbers with a reasonable accuracy. General integral equation, which is similar to that of Merkel equation, is developed in this model. The new relationship was verified through comparison with experimental and numerical work reported in the available literature for closed or indirect cooling tower (ICT), as an example of IEC. Additionally, the predicted results of the present model were compared to those obtained from the traditional numerical models at different Lewis numbers. The simulated results from the new model show a satisfactory agreement with those obtained from the experimental work of less than 10%. The new correlations can be implemented easily in predicting the thermal design and performance of IEC in any simulation program or in real site.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041006-041006-9. doi:10.1115/1.4035972.

The apparent thermal conductivity of packed copper wire test specimens was measured parallel and perpendicular to the axis of the wire using laser flash, transient plane source, and transmittance test methods. Approximately 50% wire packing efficiency was produced in the specimens using either 670- or 925-μm-diameter copper wires that both had an insulation coating thickness of 37 μm. The interstices were filled with a conventional varnish material and also contained some remnant porosity. The apparent thermal conductivity perpendicular to the wire axis was about 0.5–1 W/mK, whereas it was over 200 W/mK in the parallel direction. The Kanzaki model and an finite element analysis (FEA) model were found to reasonably predict the apparent thermal conductivity perpendicular to the wires but thermal conductivity percolation from nonideal wire-packing may result in their underestimation of it.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041007-041007-6. doi:10.1115/1.4035973.

Steady nanofluid convective flow in a porous cavity is investigated. Darcy and Koo–Kleinstreuer–Li (KKL) models are considered for porous media and nanofluid, respectively. The solutions of final equations are obtained by control volume-based finite element method (CVFEM). Effective parameters are CuO–water volume fraction, number of undulations, and Rayleigh and Hartmann numbers for porous medium. A correlation for Nuave is presented. Results depicted that heat transfer improvement reduces with the rise of buoyancy forces. Influence of adding nanoparticle augments with augment of Lorentz forces. Increasing Hartmann number leads to decrease in temperature gradient.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041008-041008-7. doi:10.1115/1.4036366.

Electric thermal storage (ETS) devices can be used for grid demand load-leveling and off-peak domestic space heating (DSH). A high-resolution three-dimensional finite element model of a forced air ETS heater core is developed and employed to create a general charge/discharge model. The effects of thermal gradients, air flow characteristics, material properties, and core geometry are simulated. A simplified general stove discharge model with a single time constant is presented based on the results of the numerical simulations. This simplified model may be used to stimulate economic/performance case studies for cold climate communities interested in distributed thermal energy storage.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):041009-041009-13. doi:10.1115/1.4036303.

The present study focuses on oil cooling for electric motors. A 40 kW test machine in which oil was introduced at each side of the machine to directly cool the stator coil end-windings was previously implemented. The lumped system analysis is used to model the thermal behavior of this test electric machine. An inverse method is applied to interpret the data obtained by the experimental setup. The inverse method leads to interior convection coefficients that help describe the heat transfer mechanisms.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Thermal Sci. Eng. Appl. 2017;9(4):044501-044501-4. doi:10.1115/1.4036009.

Effects of server/rack locations and server loading configurations on the thermal performance of data center racks' array are experimentally investigated using a scaled physical model simulating real data. Front and rear rack temperatures profiles, server temperatures, and performance indices supply/return heat index (SHI/RHI) are used to evaluate the thermal management of the racks' array. The results showed that (i) servers located in high level rack cabinet have the worst thermal performance, (ii) middle racks of the rack row showed optimum thermal performance and energy efficiency, and (iii) locating the servers of high power densities in the middle of the racks row improves the thermal performance and energy efficiency of the racks array.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(4):044502-044502-6. doi:10.1115/1.4035971.

In this paper, we consider the heat transfer problems associated with a periodic array of triangular, longitudinal, axisymmetric, and pin fins. The problems are modeled as a wall where the flat side is isothermal and the other side, which has extended surfaces/fins, is subjected to convection with a uniform heat transfer coefficient. Hence, our analysis differs from the classical approach because (i) we consider multidimensional heat conduction and (ii) the wall on which the fins are attached is included in the analysis. The latter results in a nonisothermal temperature distribution along the base of the fin. The Biot number ($Bi=ht/k$) characterizing the heat transfer process is defined with respect to the thickness/diameter of the fins (t). Numerical results demonstrate that the fins would enhance the heat transfer rate only if the Biot number is less than a critical value, which, in general, depends on the geometrical parameters, i.e., the thickness of the wall, the length of the fins, and the period. For pin fins, similar to rectangular fins, the critical Biot number is independent of the geometry and is approximately equal to 3.1. The physical argument is that, under strong convection, a thick fin introduces an additional resistance to heat conduction.

Commentary by Dr. Valentin Fuster

### Discussion

J. Thermal Sci. Eng. Appl. 2017;9(4):045501-045501-1. doi:10.1115/1.4036013.

In the above paper, the energy equation (see Eq. (3) in Ref. [1]) is as follows: Display Formula

(1)$∂T∂t+u∂T∂x+v∂T∂y=1ρcp∂∂y(κ∂T∂y)+Φρcp$

Commentary by Dr. Valentin Fuster