0

### Research Papers

J. Heat Transfer. 2019;141(8):081001-081001-10. doi:10.1115/1.4043946.
FREE TO VIEW

Spiral-coiled tube heat exchangers (SCTHE) have higher heat transfer as compared to the conventional heat transfer devices and are extensively used to extract heat from exhaust gases in the chemical processing industries and also from sunlight for domestic applications. However, no attention has been made to predict heat transfer characteristics considering combined convective and radiative heat transfer in spiral-coiled tubes. In the present study, numerical analysis has been performed to predict fluid flow and heat transfer characteristics by combined forced convection and thermal radiation in spiral-coiled tubes. The P-1 radiation and the renormalized group (RNG) k–ε turbulence models have been used to study the effect of thermal radiation and turbulent convection heat transfer in the spiral-coiled tube, respectively, over a wide range of Reynolds numbers (10,000–100,000) and curvature ratios (0.02–0.05). The emissivity and optical thickness have been varied from 0.0 to 1.0 and 0.0 to 8.0, respectively, to investigate the effect of thermal radiation on heat transfer characteristics in spiral-coiled tubes. For the considered Reynolds number range, it is found that the heat transfer is enhanced by approximately 10% when radiation is taken into account. It is found that the heat capacity increased with an increase in optical thickness and wall emissivity. Further, the effect of optical thickness on fully developed flow is observed weak and the average heat transfer coefficient is influenced by the wall emissivity over the entire flow.

Commentary by Dr. Valentin Fuster

### Research Papers: Experimental Techniques

J. Heat Transfer. 2019;141(8):081601-081601-11. doi:10.1115/1.4043619.

Ångström's method has been used to quantify thermal diffusivity of materials for over 150 years via measurement of thermal waves propagating through a long, thin sample. However, the traditional Ångström's method has some limitations. First, the traditional method is insensitive to potential variability in thermal diffusivity along the length of a sample because only two sensors are used. Second, conventional contact-based sensing techniques such as thermocouples limit the method to samples that are sufficiently large so as to be unaffected by heat loss through the sensors. Here, we develop and validate the infrared microscopy enhanced Ångström's method that overcomes these limitations and enables measurement of microscale samples. This work demonstrates the accuracy and applicability of the technique through measurement of several commercially available polymer monofilaments and films and comparison of the data to published values. This method is particularly robust to uncertainty in emissivity making it attractive for characterization of semitransparent samples.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2019;141(8):081701-081701-9. doi:10.1115/1.4043623.

We consider conjugate forced-convection heat transfer in a rectangular duct. Heat is exchanged through the isothermal base of the duct, i.e., the area comprised of the wetted portion of its base and the roots of its two side walls, which are extended surfaces within which conduction is three-dimensional. The opposite side of the duct is covered by an adiabatic shroud, and the external faces of the side walls are adiabatic. The flow is steady, laminar, and simultaneously developing, and the fluid and extended surfaces have constant thermophysical properties. Prescribed are the width of the wetted portion of the base, the length of the duct, and the thickness of the extended surfaces, all three of them nondimensionalized by the hydraulic diameter of the duct, and, additionally, the Reynolds number of the flow, the Prandtl number of the fluid, and the fluid-to-extended surface thermal conductivity ratio. Our conjugate Nusselt number results provide the local one along the extended surfaces, the local transversely averaged one over the isothermal base of the duct, the average of the latter in the streamwise direction as a function of distance from the inlet of the domain, and the average one over the whole area of the isothermal base. The results show that for prescribed thermal conductivity ratio and Reynolds and Prandtl numbers, there exists an optimal combination of the dimensionless width of the wetted portion of the base, duct length, and extended surface thickness that maximize the heat transfer per unit area from the isothermal base.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Exchangers

J. Heat Transfer. 2019;141(8):081801-081801-8. doi:10.1115/1.4043769.

The present work deals with the study of entropy generation in circular tube fitted with perforated twisted tape (PTT) insert with multiple V cuts. Experimental data pertaining to heat transfer and frictional losses are collected for solid twisted tape (STT), PTT, double V cut PTT, and PTT with multiple V cuts by varying the twist ratio in the range of 2–6 for the Reynolds number range of 2000–25,000. The entropy generation rate of heated tube with insert is found to be less than the smooth tube in most of the cases under similar operating conditions. The minimum value of entropy generation number corresponds to the PTT having twist ratio of 3.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):081802-081802-8. doi:10.1115/1.4043797.

The growing electrification of transportation systems is dramatically increasing the waste heat that must be dissipated from high-density power electronics. Transformative embedded heat spreading technologies must be developed in next-generation systems to enable air cooling of power semiconductors with heat fluxes exceeding 500 W/cm2 over large hotspot areas up to 1 cm2. In this study, vapor chamber heat spreaders, or thermal ground planes (TGPs), with customized wick structures are investigated as one possibility. A 10 cm × 10 cm TGP with hybrid wick, which is a blend of a biporous wick with a standard monoporous wick, was designed. The TGP was tested in combination with a straight pin fin heat sink under air jet impingement and a 1 cm2 size heat source. The experimental performance of the hybrid wick TGP was compared under the same air-cooled conditions with an off-the-shelf TGP of the same size from a commercial vendor and a TGP with a biporous wick only. The customized hybrid wick TGP exhibits ∼28% lower thermal resistance compared with a traditional commercial TGP, and the capillary limit heat flux is measured as 450 W/cm2. Technical challenges in extending this capillary limit heat flux value and TGP integration into packaged electronics are described.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2019;141(8):081901-081901-11. doi:10.1115/1.4043835.

In order to ensure flight safety in cold winter, aircraft ground deicing is crucial and necessary. In Chinese deicing fluid heating system, the helically coiled tube is paramount exchanger to heat deicing fluid. The deicing fluid is ethylene-glycol-based mixture with high viscosity. Aiming at heat transfer enhancement of deicing fluid, ring rib is formed by an embossed tube wall toward the internal of the tube; thus, transversely corrugated helically coiled tube (TCHC) is achieved. Depth and width are two key geometrical parameters of ring rib. Based on field synergy principle, the influence of depth–diameter ratio (H/D) and width-diameter ratio (w/D) is investigated through numerical simulation. The results show that outlet temperature, mean convection heat transfer coefficient, and Nusselt number have similar trends, which first increase and then decrease nonlinearly. The variation of flow resistance coefficient is inversely proportional to Reynolds number. Especially, the effect of H/D is more significant than that of w/D. Field synergy angle and velocity field are also analyzed to reveal the mechanism of heat transfer. TCHC performs better than the original tube. Orthogonal experiment calculates the outlet temperature of TCHC when H/D and w/D change. The combination of $H/D=0.075$ and $w/D=0.5$ is best solution. TCHC effectively enhances heat transfer of deicing fluid. Therefore, TCHC is beneficial to improve the deicing efficiency and ensure the flight punctuality.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2019;141(8):082001-082001-5. doi:10.1115/1.4043852.

Radiative and conductive heat transfer is fairly important in the nuclear pebble bed. A continuum model is proposed here to derive the effective thermal conductivity (ETC) of pebble bed. It is a physics-based equation determined by the temperature, number density, heat transfer coefficient, and the radial distribution function (RDF). Based on a concept of continuum, this model considers the conduction and thermal radiation in nuclear pebble bed through a uniform framework and the results are in good agreement with the existing model and correlations. It indicates that the local temperature in the radiation case without internal heat sources is determined by all possible surrounding pebbles weighted by a radiative kernel function. The discrete element method (DEM) packing results are in good agreement with the solution of the continuum model. Both the conductive and radiative continuum models converge to the heat conduction in continuum mechanics at size factor μ ≪ 1.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):082002-082002-10. doi:10.1115/1.4044009.

Viscoelastic materials are a kind of representative passive vibration control materials with many applications in civil engineering for earthquake mitigation in building structures, and these materials often serve in a thermo-elastic coupling environment. In this work, a one-dimensional magneto-thermoviscoelastic problem of a single-layer viscoelastic plate is investigated with memory-dependent derivative and nonlocal effect in the context of generalized thermo-elasticity. The plate is placed in a magnetic field, and the upper surface is subjected to a thermal shock. The governing equations for the single-layer plate are formulated considering the time delay and the kernel function of the memory-dependent derivative, nonlocal effect, temperature-dependent properties, and magnetic field. The Laplace transform and its numerical inversion are employed to solve this problem. The nondimensional temperature, displacement, and stress are calculated and presented graphically. Based on the numerical results, the influence of time delay and kernel function of the memory-dependent derivative, nonlocal effect parameters, temperature-dependent properties, and magnetic field parameters on the distributions of the nondimensional temperature, displacement, and stress are discussed.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2019;141(8):082101-082101-12. doi:10.1115/1.4043895.

Gallium nitride (GaN) is an attractive material for manufacturing light emitting diodes and other electronic devices due to its wide band-gap and superb optoelectronic performance. The quality of GaN thin film determines the reliability and durability of these devices. Metal-organic chemical vapor deposition (MOCVD) is a common technique used to fabricate high-quality GaN thin films. In this paper, GaN growth rate and uniformity in a vertical rotating disk MOCVD reactor are investigated on the basis of a three-dimensional computational fluid dynamics (CFD) model. GaN growth rate is investigated under the influence of reactor pressure, precursor concentration ratio, and composition of the carrier gas mixture. The numerical simulation shows that the carrier gas mixture and the reactor pressure have significant effects on growth rate and uniformity of GaN thin films. It is also found that an appropriate mixture of N2 and H2 may be employed as the carrier gas to improve the flow field characteristic in the reactor. This results in an improved crystal growth of GaN thin films.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):082102-082102-8. doi:10.1115/1.4043674.

This paper addresses heat distribution issues in fused filament fabrication (FFF) process. Three-dimensional (3D) numerical simulations and experimental investigations are performed during additive manufacturing of parts by FFF process. The transient numerical simulations of the filament temperature field are based on the finite difference method. Experimental measurements of the temperature field are carried out using infrared thermography. The proposed model mainly highlights the contribution of heat exchange from the nozzle to the fabricated part and from filament to filament. Optimum adhesion of filaments deposited by FFF requires control of the thermal history. The nozzle radiation is taken into account as a source term in the heat balance equation. The temperature fields of the printed parts computed by numerical simulations are in very good agreement with the temperature fields measured by infrared thermograph. The 3D numerical model provides information on how the nozzle affects the temperature field of the printed part. This source term must be taken into account for the optimization of the FFF process.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):082103-082103-10. doi:10.1115/1.4043894.

In this research paper, the cooling process of an impingement cooled spur gear is examined by means of an analytical model. The process is modeled as a coolant film, which is flung off a rotating gear tooth flank by centrifugal forces. During the process, heat is transferred from the isothermal gear tooth flank to the coolant film. With a numerical solution to the analytical model, a formulation for the transient local Nusselt number is derived. The evaluation of the numerical solution revealed that the heat transfer is dominated by heat conduction in the coolant film. The heat transfer process ends when the thermal capacity of the coolant film is reached. The transient Nusselt number is used to derive a time-averaged and a global heat transfer coefficient. Furthermore, the influence of the initial coolant film height is examined by using a modified version of the analytical model. The global heat transfer coefficient decreases toward smaller initial cooling film heights. The analytical model is then extended to include the temperature dependency of the viscosity of the coolant. A viscosity that decreases with increasing temperature yields a moderate decrease in heat transfer. A discussion is presented regarding the applicability of the analytical model toward impingement cooled spur gears. The effect of the simplifications made in the derivation of the analytical model is outlined and assessed with regard to the heat transfer mechanism.

Commentary by Dr. Valentin Fuster

### Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2019;141(8):082201-082201-12. doi:10.1115/1.4043771.

Experimental investigation on heat transfer mechanism of air–water mist jet impingement cooling on a heated cylinder is presented. The target cylinder was electrically heated and was maintained under the boiling temperature of water. Parametric studies were carried out for four different values of mist loading fractions, Reynolds numbers, and nozzle-to-surface spacings. Reynolds number, Rehyd, defined based on the hydraulic diameter, was varied from 8820 to 17,106; mist loading fraction, f ranges from 0.25% to 1.0%; and nozzle-to-surface spacing, H/d was varied from 30 to 60. The increment in the heat transfer coefficient with respect to air-jet impingement is presented along with variation in the heat transfer coefficient along the axial and circumferential direction. It is observed that the increase in mist loading greatly increases the heat transfer rate. Increment in the heat transfer coefficient at the stagnation point is found to be 185%, 234%, 272%, and 312% for mist loading fraction 0.25%, 0.50%, 0.75%, and 1.0%, respectively. Experimental study shows identical increment in stagnation point heat transfer coefficient with increasing Reynolds number, with lowest Reynolds number yielding highest increment. Stagnation point heat transfer coefficient increased 263%, 259%, 241%, and 241% as compared to air-jet impingement for Reynolds number 8820, 11,493, 14,166, and 17,106, respectively. The increment in the heat transfer coefficient is observed with a decrease in nozzle-to-surface spacing. Stagnation point heat transfer coefficient increased 282%, 248%, 239%, and 232% as compared to air-jet impingement for nozzle-to-surface spacing of 30, 40, 50, and 60, respectively, is obtained from the experimental analysis. Based on the experimental results, a correlation for stagnation point heat transfer coefficient increment is also proposed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):082202-082202-14. doi:10.1115/1.4043893.

In this study, effects of extended jet holes to heat transfer and flow characteristics of jet impingement cooling were numerically investigated. Cross-flow in the impinging jet cooling adversely affects the heat transfer on the target surface. The main purpose of this study is to reduce the negative effect of cross-flow on heat transfer by extending jet holes toward the target surface with nozzles. This study has been conducted under turbulent flow condition (15,000 ≤ Re  ≤  45,000). The surface of the turbine blade, which is the target surface, has been modeled as a flat plate. The effect of the ribs, placed on the target surface, on the heat transfer has been also investigated, and the results were compared with the flat surface. The parameters such as average and local Nusselt numbers on the target surface, flow characteristics, and compressor power have been examined in detail. It was obtained from the numerical results that the average Nusselt number increases with decreasing the gap between the target surface and the nozzle. In addition, the higher average Nusselt number was obtained on the flat surface than the ribbed surface. The lowest compressor power was achieved in the 5Dj nozzle gap for the flat surface and in the 4Dj nozzle gap for the ribbed surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):082203-082203-12. doi:10.1115/1.4044008.

Thermal management has a key role in the development of advanced electronic devices to keep the device temperature below a maximum operating temperature. Jet impingement and high conductive porous inserts can provide a high efficiency cooling and temperature control for a variety of applications including electronics cooling. In this work, advanced heat management devices are designed and numerically studied employing single and multijet impingement through porous-filled channels with inclined walls. The base of these porous-filled nonuniform heat exchanging channels will be in contact with the devices to be cooled; as such the base is subject to a high heat flux leaving the devices. The coolant enters the heat exchanging device through single or multijet impingement normal to the base, moves through the porous field and leaves through horizontal exit channels. For numerical modeling, local thermal nonequilibrium model in porous media is employed in which volume averaging over each of the solid and fluid phase results in two energy equations, one for solid phase and one for fluid phase. The cooling performance of more than 30 single and multijet impingement designs are analyzed and compared to achieve advantageous designs with low or uniform base temperature profiles and high thermal effectiveness. The effects of porosity value and employment of 5% titanium dioxide (TiO2) in water in multijet impingement cases are also investigated.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2019;141(8):082401-082401-11. doi:10.1115/1.4043819.

Energy and exergy performances of natural circulation loop (NCL) with various water-based hybrid nanofluids (Al2O3 + TiO2, Al2O3 + CNT, Al2O3 + Ag, Al2O3 + Cu, Al2O3 + CuO, Al2O3 + graphene) with 1% volumetric concentration are compared in this study. New thermophysical property models have been proposed for hybrid nanofluids with different particle shapes and mixture ratio. Effects of power input, loop diameter, loop height, loop inclination and heater/cooler inclination on steady-state mass flow rate, effectiveness, and entropy generation are discussed as well. Results show that both the steady-state mass flow rate and energy–exergy performance are enhanced by using the hybrid nanofluids, except Al2O3 + graphene, which shows the performance decrement within the studied power range. Al2O3 + Ag hybrid nanofluid shows highest enhancement in mass flow rate of 4.8% compared to water. The shape of nanoparticle has shown a significant effect on steady-state performance; hybrid nanofluid having cylindrical and platelet shape nanoparticles yields lower mass flow rate than that of spherical shape. Mass flow rate increases with the increasing loop diameter and height, whereas decreases with the increasing loop and heater/cooler inclinations. Both effectiveness and entropy generation increase with the decreasing loop diameter and height, whereas increasing the loop and heater/cooler inclinations. This study reveals that the particle shape has a significant effect on the performance of hybrid nanofluids in NCL, and the use of hybrid nanofluid is more effective for higher power.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2019;141(8):082701-082701-7. doi:10.1115/1.4043772.

For a hydrocarbon burning with oxygen, the resulting exhaust stream is composed mainly of carbon dioxide and water vapor. This exhaust allows for easier carbon capture and sequestration since the water can be condensed out. Another advantage is the significant reduction of NOx since much of the nitrogen found in air-fired systems is eliminated. Although beneficial, many of the exhaust gas products' radiative heat transfer characteristics are unknown. Motivated by this, this paper focuses on the spectral radiation measurement of premixed oxy-methane combustion flames. This is important for combustion system designers since radiative heat from the flame is significant for oxy-flames. This study is conducted by varying equivalence ratio, firing input, and CO2 recirculation ratio. The spectral radiation of premixed oxy-methane flames is collected from 1.2 μm to 5 μm wavelengths. During the experimental study, it is found that the water vapor emits at 1.4 μm, 1.85 μm, and 2.5 μm wavelengths. A short band of carbon dioxide emission is detected at 1.96 μm. Three other carbon dioxide radiation maxima are observed at the proximity of 2.71 μm, 2.85 μm, and 4.38 μm. The study revealed that the spectral intensity of CO2 and H2O for oxy-methane combustion increases almost five times compared to the air-methane combustion at stochiometric condition. It is also found that the spectral intensity decreases as the equivalence ratio increases. The spectral radiative emission intensity increases as the firing input increases. Another observation includes the fact that spectral intensity increases up to five times when 60% CO2 is recirculated as a diluent in the flame.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(8):082702-082702-8. doi:10.1115/1.4044010.

A method to simultaneously measure two-dimensional temperature and emissivity distributions on high-temperature diffuse surfaces is developed using an auxiliary light source. The high-temperature diffuse surface is irradiated from the hemispherical space with the auxiliary light source switched “on” or “off.” Two images of the effective radiation intensity are obtained in quick succession for the two states to determine the temperature and emissivity distributions. The measurement method and uncertainty models show that the effect of the unknown emissivity on the accuracy of the temperature field measurement can be eliminated. The optical pyrometer is a color charge coupled device (CCD) sensor with a quartz lamp array used as the auxiliary light source to illustrate the measurement method. An oxidized W–Ni–Fe alloy sample is heated at high temperatures of 600–1000 °C by a 700 W induction-heating device. The distributions of the effective radiation intensities from the sample surface during the “on” and “off” states of the lamp array are measured in the three color channels (R, G, and B channels) to calculate the temperature and emissivity distributions. The temperature measurement uncertainties are less than 4 °C for a temperature range of 600–900 °C. The temperature measurements are experimentally validated by the thermocouple method only with a small temperature difference. The emissivities calculated from the three color channels are very close with a range of 0.855–0.957. The relative uncertainties in the emissivities for channels R and G are less than 2.0%, while the relative uncertainty for channel B data was higher at 2.8% and 7.5% due to lower measurement signals in channel B. This analysis may provide a useful method for measuring the temperatures of high-temperature diffuse surfaces by successfully compensating for the effects of unknown or changing emissivities.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2019;141(8):084501-084501-7. doi:10.1115/1.4043729.

This paper aims to determine the flow characteristics and thermal performance of plate heat exchangers. The study is divided into two parts. In the first part, four different shapes of corrugated boundaries have been recommended, rectangular, trapezoidal, triangular, and sinusoidal shapes. In addition, an artificial roughness has been introduced to improve heat transfer within corrugated channel. In the second part, a corrugated wall was used at the inlet channel. Numerical results are presented as Nusselt number (Nu) and friction factor (Cf) using the commercial software ansys-fluent where the Reynolds number is ranged between 3000 and 12,000. The results of this investigation reveal that the overall thermal performance improves greatly by 50% due to the use of the sinusoidal artificial roughness and added undulations in the inlet channel. It is also observed that the latter case with the ratio A″/λ″ = 0.05 is the optimal design for the plate heat exchanger.

Commentary by Dr. Valentin Fuster