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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

### 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