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### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2016;139(1):012101-012101-8. doi:10.1115/1.4034337.

Low-order thermal models of electrical machines are fundamental for the design and management of electric powertrains since they allow evaluation of multiple drive cycles in a very short simulation time and implementation of model-based control schemes. A common technique to obtain these models involves homogenization of the electrical winding geometry and thermal properties. However, incorrect estimation of homogenized parameters has a significant impact on the accuracy of the model. Since the experimental estimation of these parameters is both costly and time-consuming, authors usually prefer to rely either on simple analytical formulae or complex numerical calculations. In this paper, we derive a low-order homogenized model using the method of multiple-scales (MS) and show that this gives an accurate steady-state and transient prediction of hot-spot temperature within the windings. The accuracy of the proposed method is shown by comparing the results with both high-order numerical simulations and experimental measurements from the literature.

Commentary by Dr. Valentin Fuster

### Research Papers: Porous Media

J. Heat Transfer. 2016;139(1):012601-012601-10. doi:10.1115/1.4034181.

The objective of the current investigation is to investigate the entropy generation inside porous media utilizing a pore scale modeling approach. The current investigation improves the thermodynamics performance of the recent analysis (Int. J. Heat Mass Transfer, 2016, 99, pp. 303–316) by considering different cross-sectional configurations and analyzing the thermal system for various Reynolds numbers, porosities, and a comparison between the previous and current investigation. The Nusselt number, the dimensionless volume-averaged entropy generation rate, Bejan number, and performance evaluation criterion (PEC) are all presented and discussed. The dimensionless volume-averaged entropy generation rate was found to increase with increasing Reynolds number, with the increase being higher for lower porosity medium. A slight variation of the dimensionless volume-averaged entropy generation rate is observed for higher Reynolds numbers which is confirmed for both cross-sectional configurations. Examination of the Bejan number demonstrates heat transfer irreversibility (HTI) dominance for most of the Reynolds number ranges examined. The results indicate that the longitudinal elliptical cross-sectional configuration with porosity equals to 0.53 provides superior performance when applying the performance evaluation criterion utilized.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2016;139(1):012701-012701-7. doi:10.1115/1.4034310.

This study investigates the bulk radiative properties of absorbing and scattering fibers. This type of problem can be applied to a group of geometrically similar applications including thermal radiators, insulation, and tube-and-shell heat exchangers. The specific application studied here is an ordered array of carbon fibers acting as a radiating fin for a space-based heat rejection system. High total emissivity is beneficial for this application, so this study focuses on how geometric factors affect the effective emissivity of an array of fibers. Photon scattering among fibers in an array can result in an effective emissivity greater than the emissivity of the fiber surfaces themselves.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):012702-012702-11. doi:10.1115/1.4034485.

The multiscale multigroup full-spectrum k-distribution (MSMGFSK) model was improved to adapt to radiation heat transfer calculations of combustion gas flow field with large temperature and pressure gradient. The improvements in calculation accuracy resulting from new sorting strategy of the spectral absorption coefficients were validated using a series of semi-1D problem in which strong temperature, pressure, and mole fraction inhomogeneities were present. A simpler method to attain compatibility between the MSMGFSK model and the gray-wall radiation emission has been established and validated. Finally, estimates are given for the calculation of wall radiation heat transfer characteristics and thermal emission imaging of the exhaust system of the parallel turbine-based combined cycle (TBCC) engine, using finite volume method (FVM) and ray trace method (RT), respectively.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2016;139(1):012001-012001-12. doi:10.1115/1.4034064.

This work deals with numerical simulation of a hyperthermia treatment of skin cancer as a state estimation problem, where uncertainties in the evolution and measurement models, as well as in the measured data, are accounted for. A reduced model is adopted, based on a coarse mesh for the solution of the partial differential equations that describe the physical problem, in order to expedite the solution of the state estimation problem with a particle filter algorithm within the Bayesian framework of statistics. The so-called approximation error model (AEM) is used in order to statistically compensate for model reduction effects. The Liu and West algorithm of the particle filter, together with the AEM, is shown to provide accurate estimates for the temperature and model parameters in a multilayered region containing a tumor loaded with nanoparticles. Simulated transient temperature measurements from one sensor are used in the analysis.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2016;139(1):011501-011501-9. doi:10.1115/1.4034552.

An investigation of refrigerant R410A condensation on a shell and tube heat exchanger simulation is conducted. Tests are on the outside of a horizontal smooth tube, a herringbone tube, and a newly developed three-dimensional-enhanced tube, called the enhanced tube (EHT) tube, all of the same outer diameter. Experiments were conducted at a constant saturation temperature of 45 °C, a constant inlet vapor quality of 0.8, a constant outlet vapor quality of 0.1, and mass fluxes ranging from 5 kg/(m2 s) to 50 kg/(m2 s). At low-mass velocities, the smooth tube shows superior performance over the herringbone tube and the EHT tube. The cause might lie in surface tension effects that result in liquid inundation at the lower portion of the tube, thickening the film on the tube and deteriorating the heat transfer performance. Analyses were conducted to find a suitable correlation of the experimental data.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):011502-011502-9. doi:10.1115/1.4034495.

The effect of temperature difference (Tsat − Tcoolant) on condensation heat transfer coefficients inside horizontal tubes is investigated in detail. Condensation experiments are conducted on propane inside a 7.75 mm horizontal tube at four temperature differences between the test fluid and coolant at three mass fluxes and four saturation temperatures. The heat transfer coefficient is shown to increase with temperature difference, with this effect diminishing with larger temperature differences, and being most significant at higher saturation temperatures. Heat transfer coefficients at the low-reduced pressures (Pr = 0.25) corresponding to lower saturation temperatures (30 °C) are mostly unaffected by the temperature difference. Subcooling of the condensate is expected to increase heat transfer coefficients at the larger temperature differences. Flow visualization studies are used to explain the inadequacy of the Nusselt film theory for the conditions investigated. The underlying mechanisms are also used to explain why the correlations from the literature do not predict the observed trend, and a new correlation to account for the effect of temperature difference is developed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):011503-011503-8. doi:10.1115/1.4034486.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2016;139(1):012401-012401-14. doi:10.1115/1.4033698.

A linear and nonlinear stability analysis of a viscoelastic fluid in a porous medium layer saturated by a nanofluid with thermal conductivity and viscosity dependent on the nanoparticle volume fraction is studied. To simulate the momentum equation in porous media, a modified Darcy model has been used. To describe the rheological behavior of viscoelastic nanofluids, an Oldroyd-B type constitutive equation has been used. The onset criterion for stationary and oscillatory convection is derived analytically. The nonlinear theory based on the truncated representation of Fourier series method is used to find the transient heat and mass transfer.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):012402-012402-8. doi:10.1115/1.4034484.

Rayleigh–Bénard convection in liquids with nanoparticles is studied in the paper considering a two-phase model for nanoliquids with thermophysical properties determined from phenomenological laws and mixture theory. In the absence of nanoparticle-modified thermophysical properties as used in the paper, the problem is essentially binary liquid convection with Soret effect. The base liquids chosen for investigation are water, ethylene glycol, engine oil, and glycerine, and the nanoparticles chosen are copper, copper oxide, silver, alumina, and titania. Using data on these 20 nanoliquids, our theoretical model clearly explains advanced onset of convection in nanoliquids in comparison with that in the base liquid without nanoparticles. The paper sets to rest the tentativeness regarding the boundary condition to be chosen in the study of Rayleigh–Bénard convection in nanoliquids. The effect of thermophoresis is to destabilize the system and so is the effect of other parameters arising due to nanoparticles. However, Brownian motion effect does not have a say on onset of convection. In the case of nonlinear theory, the five-mode Lorenz model is derived under the assumptions of Boussinesq approximation and small-scale convective motions, and using it enhancement of heat transport due to the presence of nanoparticles is clearly explained for steady-state motions. Subcritical motion is shown to be possible in all 20 nanoliquids.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2016;139(1):012501-012501-8. doi:10.1115/1.4034546.

This paper investigates natural convection heat transfer of generalized Oldroyd-B fluid in a porous medium with modified fractional Darcy's law. Nonlinear coupled boundary layer governing equations are formulated with time–space fractional derivatives in the momentum equation. Numerical solutions are obtained by the newly developed finite difference method combined with L1-algorithm. The effects of involved parameters on velocity and temperature fields are presented graphically and analyzed in detail. Results indicate that, different from the classical result that Prandtl number only affects the heat transfer, it has remarkable influence on both the velocity and temperature boundary layers, the average Nusselt number rises dramatically in low Prandtl number, but increases slowly with the augment of Prandtl number. The maximum value of velocity profile and the thickness of momentum boundary layer increases with the augment of porosity and Darcy number. Moreover, the relaxation fractional derivative parameter accelerates the convection flow and weakens the elastic effect significantly, while the retardation fractional derivative parameter slows down the motion and strengthens the elastic effect.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):012502-012502-9. doi:10.1115/1.4034166.

An experimental investigation of transitional natural convection in an air filled cube was conducted in this research. The characteristic dimension of the enclosure is 0.35 m, and data were collected in the middle plane of the cavity. The Rayleigh number range examined is $5.0×107≤Ra≤3.4×108$. This was achieved by varying the temperature on the hot and cold walls. The velocity field in the middle plane is measured using particle image velocimetry (PIV). Temperature measurements in the core of the enclosure indicate a linear profile. The average Nu number is also presented and compared against other correlations in the literature. This study attempts to close the gap of available experimental data in literature and provide experimental benchmark data that can be used to validate computational fluid dynamics (CFD) codes since the estimated error from PIV measurements is within 1–2%.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):012503-012503-10. doi:10.1115/1.4034167.

An experimental investigation of transitional natural convection in an air filled cube was conducted in this research. The characteristic dimension of the enclosure was H = 0.35 m, and data were collected in the middle plane of the cavity. The Rayleigh number range examined was $5.0×107≤Ra≤3.4×108$. In Part I, the authors presented the mean velocity profiles in the enclosure and conducted heat transfer measurements on the hot wall. An expression between Nu and Ra numbers was concluded and compared against other correlations available in literature. In the present work, the authors present a complete description of the flow in the enclosure by quantifying the low turbulence regime developed in the cavity. This was accomplished by estimating Reynolds stresses, turbulent kinetic energy, vorticity, and swirling strength. Proper orthogonal decomposition (POD) was employed to analyze the flow fields obtained from the experimental data and retain the most salient features of the flow field. This study attempts to close the gap of available experimental data in the literature and provide experimental benchmark data that can be used to validate CFD codes since the estimated error from particle image velocimetry (PIV) measurements is within 1–2%.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2016;139(1):014501-014501-3. doi:10.1115/1.4034351.

Many researchers have studied the radiative heat transfer in a viscoelastic boundary layer flow over a stretching sheet after simplifying the complex nature of the radiative heat flux by expanding the fourth power of temperature $(T4)$ in Taylor series about free-stream temperature $(T∞)$ and neglecting the higher-order terms. Similarity solutions obtained by them are found to be valid only in the asymptotic region. This article suggests a modification in the linearization of $T4$ by introducing wall temperature $(Tw)$ as well as freestream temperature $(T∞)$ to capture the realistic nature of the temperature distribution in the boundary layer flows from locally nonsimilar solutions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):014502-014502-6. doi:10.1115/1.4034551.

In the present investigation, problem of heat transfer has been studied during peristaltic motion of a viscous incompressible fluid for two-dimensional nonuniform channel with permeable walls under long wavelength and low Reynolds number approximation. Expressions for pressure, friction force, and temperature are obtained. The effects of different parameters on pressure, friction force, and temperature have been discussed through graphs.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(1):014503-014503-4. doi:10.1115/1.4034487.

Heat pipe technology offers a possible cooling technique for structures exposed to high heat fluxes, as in turbomachinery such as compressors and turbines. However, in its current configuration as single heat pipes, implementation of the technology is limited due to the difficulties in manufacturability and costs. Hence, a study to develop a new radially rotating (RR) heat pipe system was undertaken, which integrates multiple RR heat pipes with a common reservoir and interconnected braches for a more effective and practical solution to turbomachinery cooling. Experimental study has shown that the integration of multiple heat pipe branches with a reservoir at the top is feasible.

Commentary by Dr. Valentin Fuster