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### Research Papers

J. Thermal Sci. Eng. Appl. 2018;10(5):051001-051001-13. doi:10.1115/1.4039422.

In this paper, affecting parameters of porous medium to improve the rate of convective heat transfer in a two-dimensional porous gas heat exchanger (PGHE) for two arrangements (symmetric and asymmetric) of barriers are numerically investigated. Two barriers have been located on the top and bottom walls and one obstacle was placed in the central zone of the PGHE. In the present study, solving the momentum and energy equations has been done by Lattice–Boltzmann method with multiple-relaxation-time (LBM-MRT). The boundary conditions in both arrangements include the left and right walls which are kept at the cold constant temperature and both top and bottom walls are insulated. There is a volumetric heat source within the PGHE. The temperature of barriers and fixed obstacle are kept at hot temperature. In this study, impact of effective parameters in porous medium and heat transfer including dimensionless number of Darcy, porosity, and Rayleigh number on the flow and temperature fields has been investigated. According to the numerical results, it has been shown that the porous medium and barriers cause increase and improvement in the heat transfer within PGHE in both symmetrical and asymmetrical arrangements. The results also demonstrate that as dimensionless Darcy number increases, more convection occurs within the chamber. Examining arrangement of barriers shows that in asymmetrical arrangement, more space appears in chamber and convective heat transfer is done better.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051002-051002-14. doi:10.1115/1.4039544.

In the present framework, a model is constituted to explore the peristalsis of magnetohydrodynamics (MHD) viscoelastic (second grade) fluid with wall properties. The study is beneficial in understanding blood flow dynamics through microchannels. The mechanisms of heat and mass transfer are also modeled in the existence of viscous dissipation and Soret effects. The conducting second grade fluid is permeated by a vertical magnetic field. Perturbation technique is opted to present series solutions by assuming that the wavelength of the sinusoidal wave is small in comparison to the half-width of the channel. The solution profiles are computed and elucidated for a certain range of embedded parameters. Moreover, plots of heat transfer coefficient against the axial coordinate are also portrayed and deliberated. The main outcome of the current research is that both viscoelasticity and slip effect considerably alter the flow fields. Moreover, an increasing trend in solute concentration is anticipated for increasing the Soret effect strength.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051003-051003-9. doi:10.1115/1.4039355.

Present paper conducts an extensive numerical study on entropy analysis of mixed convective condensation inside a vertical parallel plate channel. A new approach is proposed to separate pump velocity component/Reynolds number from inlet mixed convection velocity. Influence of inlet governing parameters on condensation heat and mass transfer at different inlet pressure, velocity, channel length, and width are widely studied. The central focus of this paper is to study entropy generation under mixed convective condensation. Variation of local as well as overall entropy generation and second law efficiency for different geometric and environmental conditions are presented. For effective condenser design, present study provides two important correlations of overall volumetric entropy generation due to thermal transport and overall volumetric entropy generation due to mass transport.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051004-051004-12. doi:10.1115/1.4039460.

This paper establishes a multiscale design evaluation framework that integrates performance models for a thermal energy storage (TES) unit and a subsystem heat exchanger (HX). The modeling facilitates the analysis of transient input and extraction processes for the TES device which uses solid–liquid phase change to store thermal energy. We investigate sensible and latent heat transfer through the unit's matrix structure which contains phase change material (PCM) in the interstitial spacing. The heat transfer is driven by a temperature difference between fluid flow passages and the PCM matrix which experiences sensible heat transfer until it reaches the PCM fusion point; then it undergoes melting or solidification in order to receive, or reject, energy. To capture these physics, we establish a dimensionless framework to model heat transfer in the storage device much like effectiveness-number of transfer units (NTU) analysis methods for compact HX. Solution of the nondimensional governing equations is subsequently used to predict the effectiveness of the transient energy input and extraction processes. The TES is examined within the context of a larger subsystem to illustrate how a high efficiency design target can be established for specified operating conditions that correspond to a variety of applications. The general applicability of the model framework is discussed and example performance calculations are presented for the enhancement of a Rankine power plant via asynchronous cooling.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051005-051005-11. doi:10.1115/1.4039922.

Spindles with tilting pad bearings have been widely applied in machine tools due to their high running precision. However, friction power loss will increase dramatically when the bearing runs at a higher speed. So far, little research on the thermal modeling of spindle systems with tilting pad bearings can be found in literature. In this paper, based on the Newtonian law of viscosity, formula that describes the friction power loss of the tilting pad bearing has been derived. The thermodynamic equilibrium equation for the spindle lubrication system has been established. Thermal boundary condition of the spindle system has been obtained using the heat transfer theory. Thermal model of the spindle system with tilting pad bearing has been built with the finite element method in order to calculate its temperature and thermal displacement distribution. Effects of the eccentricity ratio and the lubricant flow rate on thermodynamic behavior of the spindle system have been studied systematically. Finally, experiments have been conducted to verify the proposed thermal model for the spindle system with tilting pad bearing.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051006-051006-12. doi:10.1115/1.4039703.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051007-051007-9. doi:10.1115/1.4039965.

The demand for plastic is eternally growing in urban areas and producing enormous quantity of plastic waste. The management and disposal of plastic waste have become a major concern worldwide. The awareness of waste to energy retrieval is one of the promising modes used for the treatment of the waste plastic. The present investigation evaluates the prospective use of waste plastic oil (WPO) as an alternative fuel for diesel engine. Different blends (WPO0, WPO30, and WPO50) with diesel are prepared on a volume basis and the engine is operated. Experiments are conducted for various injection timings (9 deg, 12 deg, 15 deg, and 18 deg BTDC) and for different exhaust gas recirculation (EGR) rates (0%, 10%, 15%, and 20%) at 100 MPa injection pressure. Combustion, performance, and tail pipe emissions of common rail direct injection (CRDI) engine are studied. The NOx, CO, and Soot emissions for waste plastic oil-diesel blends are found more than neat diesel. To reduce the NOx, EGR is employed, which results in reduction of NOx considerably, whereas other emissions, i.e., CO and Soot, get increased with increase in EGR rates. Soot for WPO-diesel blends is higher because of aromatic compounds present in plastic oils. Brake thermal efficiency (BTE) of blends is found to be higher compared to diesel.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051008-051008-9. doi:10.1115/1.4039926.

A design method for the thermoelectric cooling system is improved in this work based on a graphical approach. It is used to select an appropriate thermoelectric cooler (TEC) and determine the value of optimum input current. Theoretical analysis has been conducted to investigate the cooling performance of the system using the design method. Numerical simulation and experimental tests for the entire cooling system validate the calculation result, which indicates the high reliability of the theoretical design method. The temperature dependence of the heat sink resistance and the contact resistance are the major reasons for the small discrepancy. Research is then conducted based on the design method to investigate how a thermoelectric cooling system under natural convection performs, where the optimization of heat sinks at hot side of TEC is done by using the generalized correlations in the previous studies. Comparison is made between the thermoelectric cooling system and the bare-heat-sink system under natural convection. Results show that the thermal resistance of the heat sink attached to TEC is critical to the cooling performance of the whole system. Besides, TEC under natural convection can perform better than the passive cooling if the heat load is not very high ($qc″≤20,000 W/m2$). The design process and results can provide a useful guidance for other thermal engineers.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051009-051009-7. doi:10.1115/1.4040035.

Heat transfer to two-component gas–liquid mixtures is needed in many industries but there is lack of a well-verified predictive method. A correlation is presented for heat transfer during flow of gas–liquid nonboiling mixtures in horizontal tubes. It has been verified with a wide range of data that includes tube diameters of 4.3–57 mm, pressures from 1 to 4.1 bar, temperatures from 12 to 62 °C, gravity <0.1% to 100% earth gravity, liquid Reynolds number from 9 to 1.2 × 105, and ratio of gas and liquid velocities from 0.24 to 9298. The 946 data points from 18 sources are predicted with mean absolute deviation (MAD) of 19.2%. The same data were compared to five other correlations; they had much larger deviations. Therefore, the new correlation is likely to be helpful in more accurate designs.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051010-051010-6. doi:10.1115/1.4039921.
FREE TO VIEW

A computational study of a metamaterial (MTM)-on-glass composite is presented for the purpose of increasing the energy efficiency of buildings in seasonal or cold climates. A full-spectrum analysis yields the ability to predict optical and thermal transmission properties from ultraviolet through far-infrared frequencies. An opportunity to increase efficiency beyond that of commercial low-emissivity glass is identified through a MTM implementation of Ag and dielectric thin-film structures. Three-dimensional finite difference time-domain (FDTD) simulations predict selective nonlinear absorption of near-infrared energy, providing the means to capture a substantial portion of solar energy during cold periods, while retaining high visible transmission and high reflectivity in far-infrared frequencies. The effect of various configuration parameters is quantified, with prediction of the net sustainability advantage. MTM window glass technology can be realized as a modification to commercial low-emissivity windows through the application of nanomanufactured films, creating the opportunity for both new and after-market sustainable construction.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051011-051011-10. doi:10.1115/1.4039925.

Lattice Boltzmann method (LBM) is performed to study numerically combined natural convection and surface radiation inside an inclined two-dimensional open square cavity. The cavity is heated by a constant temperature at the wall facing the opening. The walls normal to the heated surface are assumed to be adiabatic, diffuse, gray, and opaque while the open boundary is assumed to be black at ambient temperature. A Bathnagar, Gross and Krook (BGK) collision model with double distribution function (D2Q9-D2Q4) is adopted. Effects of surface radiation, inclination angle, and Rayleigh number on the heat transfer are analyzed and discussed. Results are presented in terms of isotherms, streamlines, and Nusselt number. It was found that the presence of surface radiation enhances the heat transfer. The convective Nusselt number decreases with increasing surface emissivity as well as with Rayleigh number, while the total Nusselt number increases with increasing surface emissivity and Rayleigh number. The inclination angle has also a significant effect on flow and heat transfer inside the cavity. However, the magnitude of total heat transfer decreases considerably when open cavity is tilted downward.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051012-051012-11. doi:10.1115/1.4039924.

In this study, forced convective heat transfer inside a circular tube automobile radiator is experimentally and numerically investigated. The investigation is carried out using Al2O3 and CuO nanofluids with water as their base fluid. A single radiator circular tube with the same dimensions is numerically modeled. Numerical model is validated using the experimental study results. In the experimental study, Al2O3 and CuO nanofluids of 0.05% volume concentrations (ϕ) were recirculated through the radiator for the Reynolds number (Re) between 260 and 1560. The numerical investigation is conducted for the nanoparticle volume concentration from 0% to 6.0% and 260 < Re < 1560. The investigation shows an enhancement of convective heat transfer coefficient (h) with the increase in nanoparticle volume concentration and with the Reynolds number. A maximum enhancement of 38% and 33% were found for Al2O3 and CuO nanofluids of ϕ = 1% and Re = 1560. For the same cooling load of the radiator, the pumping power can be reduced by 8% and 10%, when Al2O3 and CuO nanofluids (ϕ = 0.8%) were used. Enhancement in convective heat transfer can be utilized to reduce the radiator surface area required. However, the addition of nanofluid results in an enhancement of density (ρ) and viscosity (μ) along with a reduction in specific heat capacity (Cp). Hence, the selection of nanoparticle volume concentration should consider its effect on the thermophysical properties mentioned earlier. It is found that the preferred concentration is between 0.4% and 0.8% for both Al2O3 and CuO nanofluids. In our investigations, it is observed that the convective heat transfer performance of Al2O3 nanofluid is better than the CuO nanofluid.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051013-051013-15. doi:10.1115/1.4039966.

In this paper, a Ranque–Hilsch vortex tube (RHVT) has been optimized utilizing convergent (φ), straight, and divergent (θ) axial angles for hot-tube. Effects of divergent (θ) and convergent (φ) angles on the flow behavior have been investigated by computational fluid dynamic (CFD) techniques. By using a renormalization group (RNG) k–ε turbulence model based on finite volume method, all the computations have been carried out. The isentropic efficiency (ηis) and coefficient of performance (COP) of machine was studied under five different divergent angles (θ), namely 1 deg, 2 deg, 3 deg, 4 deg, and 6 deg, two different convergent (φ) angles (φ) namely 1 deg and 2 deg adjusted to the hot-tube. Furthermore, some geometrical and operational parameters including cold outlet diameter, hot-tube length, and different inlet pressures and mass flow rates have been analyzed in detail (spanwisely) in order to optimize the cooling efficiency of vortex tube (straight). The results show that utilizing the divergent hot-tubes increases the isentropic efficiency (ηis) and COP of device for most values of inlet pressures, and helps to become more efficient than the other shape of vortex tubes (straight and convergent). Finally, some results of the CFD models have been validated by the available experimental and numerical data, which show reasonable agreement, and others are compared qualitatively.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051014-051014-13. doi:10.1115/1.4040034.

This paper reports the local multifaceted and area-averaged convective heat transfer coefficients (CHTCs) of longitudinal and transverse bricks arranged in lattice brick setting in tunnel kilns, using a three-dimensional (3D) computational fluid dynamics (CFD) model. A mesh sensitivity analysis was performed and the model was validated against reported experimental data in tunnel kilns. Three turbulence models were tested: the standard k–ε, re-normalization group (RNG) k–ε, and k–ω. The k–ω model provided the closest results to the experimental data. The CHTCs from the front, back, left, and right faces of the longitudinal and transverse bricks were calculated under various conditions. Area-averaged CHTCs for bricks were determined from the multifaceted CHTCs. Effects of rows, layers, and walls on faces and area-averaged CHTCs were investigated. A sensitivity analysis was performed to explore the effect of flow channels on the CHTCs. The numerical results showed that the CHTCs are enhanced by 17% for the longitudinal bricks and 27% for the transverse bricks when a uniform flow is reached in the tunnel kilns. Also, similar area-averaged CHTCs for the longitudinal and transverse bricks were obtained as a result of the uniform flow. Therefore, the specific energy consumption, quality, and quantity of brick production could be enhanced.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051015-051015-8. doi:10.1115/1.4040032.

This research concentrates on melting heat transfer in magnetohydrodynamics (MHD) flow of Sisko fluid bounded by a sheet with nonlinear stretching velocity. Modeling and analysis have been carried out in the presence of heat generation/absorption and magnetic field. Transformation procedure is implemented in obtaining nonlinear differential system. Convergence series solutions are developed. The solution for different influential parameters is analyzed. Skin friction coefficient and heat transfer rate are analyzed. It is observed that the qualitative results of magnetic field and melting heat transfer on velocity are similar.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051016-051016-8. doi:10.1115/1.4040033.

Vortex generators have been widely used to enhance heat transfer in various heat exchangers. Out of the two types of vortex generators, transverse vortex generators and longitudinal vortex generators (LVGs), LVGs have been found to show better heat transfer performance. Past studies have shown that the implementation of these LVGs can be used to improve heat transfer in thermoelectric generator systems. Here, a built in module in COMSOL Multiphysics® was used to study the influence of the location of LVGs in the channel on the comprehensive performance of an integrated thermoelectric device (TED). The physical model under consideration consists of a copper interconnector sandwiched between p-type and n-type semiconductors and a flow channel for hot fluid in the center of the interconnector. Four pairs of LVGs are mounted symmetrically on the top and bottom surfaces of the flow channel. Thus, using numerical methods, the thermo-electric-hydraulic performance of the integrated TED with a single module is examined. By fixing the material size D, the fluid inlet temperature $Tin$, and attack angle β, the effects of the location of LVGs and Reynolds number were investigated on the heat transfer performance, power output, pressure drop, and thermal conversion efficiency. The location of LVGs did not have significant effect on the performance of TEGs in the given model. However, the performance parameters show a considerable change with Reynold's number and best performance is obtained at Reynold number of Re = 500.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051017-051017-13. doi:10.1115/1.4040363.

This paper aims at providing further understanding on the fluid flow and heat transfer processes in unsteady rotating systems with mass transpiration. Such systems can be found in chemical separators, hydraulic systems, and printing devices. To this end, an unsteady viscous flow in the vicinity of an unaxisymmetric stagnation-point on a rotating cylinder is examined. The nonuniform transpiration and a transverse magnetic field are further considered. The angular speed of the cylinder and the thermal boundary conditions are expressed by time-dependent functions. A reduction of the Navier–Stokes and energy equations is obtained through using appropriate similarity transformations. The semisimilar solution of the Navier–Stokes equations and energy equation are developed numerically using an implicit finite difference scheme. Pertinent parameters including the Reynolds number and magnetic parameter and transpiration function are subsequently varied systematically. It is shown that the transpiration function can significantly affect the thermal and hydrodynamic behaviors of the system. In keeping with the findings in other areas of magnetohydrodynamics (MHD), the results show that the applied magnetic field has modest effects on the Nusselt number. However, it is demonstrated that the magnetic effects can significantly increase the imposed shear stress on the surface of the rotating cylinder.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051018-051018-9. doi:10.1115/1.4040282.

This paper scrutinizes the impact of thermal radiation and applied magnetic field on Jeffrey fluid with peristalsis. The effects of Joule heating and viscous dissipation are retained. Convective conditions are imposed for the heat and mass transfer analysis. Lubrication approach is considered for the analysis. Expressions for pressure gradient, stream function, temperature, concentration, and heat transfer coefficient are developed and physically interpreted through illustrations. It is revealed that temperature enhances for higher estimation of Brinkman and Hartmann numbers, while it decays for larger Biot number. Furthermore, the concentration decreases for varying Schmidt number. Heat transfer coefficient has an oscillatory behavior for larger estimation of radiation parameter.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051019-051019-5. doi:10.1115/1.4040279.

Wax deposition in oil pipelines brings a critical operational challenge in the oil development, and the indirect thermal washing is a most common and effective method of wax cleaning. The temperature field in thermal washing is the basis for making a reasonable plan to wash and remove wax well. In this paper, the wells of sucker rod pump in Da Qing oil field are selected as research objects, a new method which is based on heat-fluid coupling method is proposed for predicting temperature field during the thermal washing process. The temperature field of the annulus of tubing and casing and the temperature field of the annulus of rod and tubing are simulated with different thermal washing parameters. In the indirect thermal washing, the temperature in annulus of tubing and casing gradually decreases from wellhead to the bottom, while the temperature in the annulus of rod and tubing increases from bottom to the wellhead. With the increase of temperature and flow rate of thermal washing fluid, the temperature in annulus of tubing and casing and the temperature in annulus of rod and tubing are both increasing, but the rise rate is different at different depths. Compared to the measured results, the coincidences rate is in the range of 93.67%–99.31%. The research results can guide effectively the thermal washing operation.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051020-051020-9. doi:10.1115/1.4040280.

The novel adaptive thermal metamaterial developed in this paper provides a unique thermal management capability that can address the needs of future spacecraft. While advances in metamaterials have provided the ability to generate materials with a broad range of material properties, relatively little advancement has been made in the development of adaptive metamaterials. This metamaterial concept enables the development of materials with a highly nonlinear thermal conductivity as a function of temperature. Through enabling active or passive control of the metamaterials bulk effective thermal conductivity, this metamaterial that can improve the spacecraft's thermal management systems performance. This variable thermal conductivity is achieved through induced contact that results in changes in the F path length and the conductive path area. The contact can be generated internally using thermal strain from shape memory alloys, bimetal springs, and mismatches in coefficient of thermal expansion (CTE) or it can be generated externally using applied mechanical loading. The metamaterial can actively control the temperature of an interface by dynamically changing the bulk thermal conductivity controlling the instantaneous heat flux through the metamaterial. The design of thermal stability regions (regions of constant thermal conductivity versus temperature) into the nonlinear thermal conductivity as a function of temperature can provide passive thermal control. While this concept can be used in a wide range of applications, this paper focuses on the development of a metamaterial that achieves highly nonlinear thermal conductivity as a function of temperature to enable passive thermal control of spacecraft systems on orbit.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051021-051021-14. doi:10.1115/1.4040277.

This work investigates numerically by finite volume method using Low k–ω model, the forced turbulent convection through a channel roughened by seven heated ribs arranged transversely. These ribs of rectangular cross section have a blocking ratio H/h = 10 and pitch ratio λ/h = 3. The modeling the problem parameters are Reynolds number, ranging from 5480 to 68500, and the width of the first rib L1 ranging from 0.5h to 15h. The objective of this study is to look for the width of the first rib L1 which induces the best heat transfer. The flow configurations of identical ribs from the first one generate a large eddy spreading along the top of the two first ribs, blocking the flow of the first cavity. The widening of the first rib may solve this problem. This flow configuration is required in several engineering applications necessitating the flow periodicity starting from the first cavity. The streamlines show that the first rib acts as a forward facing step when L1 > 5h. The effect of the width of the first rib is highlighted by velocity, pressure, turbulent kinetic energy and temperature profiles. Nusselt number distributions confirm that the widening of the heated surface is not recommended for improving heat transfer in spite of flow periodicity in all cavities (roughness d-type). The best improvement in heat transfer of 18%, compared to a smooth wall is obtained for thinnest first rib of L1/h = 0.5. However, this configuration provides a minor heat exchange at the first pitch and the second rib, which might be a disadvantage for the material structure.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051022-051022-9. doi:10.1115/1.4040278.

To support an effort to setup an industrial scale production facility to produce metal substrates coated with porous boiling surface (PBS) coating to enhance boiling heat transfer performance of these metal substrates, an axisymmetric transient heat transfer model with boundary conditions varying both in time and length dimensions has been proposed and solved to obtain the temperature evolution along the inner surface of a long finned tube heating and cooling in a multizone furnace. Experiments for finned tube heating and cooling were conducted using a single-zone batch furnace, and the experimental data obtained were compared with the simulation results to establish reasonable confidence in the proposed model and boundary conditions. A parametric study on several important operating parameters was conducted to gain better insights that can be used in making design and operating decisions. If required, the model can conveniently be extended to other types of substrates and furnace dimensions.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051023-051023-7. doi:10.1115/1.4040284.

The liquid refrigerant defrosting (LRD) is a defrosting method which leads the liquid refrigerant in the high-pressure reservoir to the frosting evaporator. The refrigeration process is continuous during the defrosting period, which increases the defrosting frequency. Compared with the traditional defrosting method, no large fin spacing should be left to reduce the defrosting frequency. The system can recover all the defrosting cooling capacity to improve the subcooling, so that the indoor air temperature fluctuations are avoided. In order to explore the effect and the rule of the LRD, the defrosting experiments were carried out in different frosting mass under the condition of the cold storage temperature of −20 °C. The defrosting time, temperature rise value, cooling capacity, and compressor power consumption value were calculated at the different frosting mass. Interpolation and applying the curve fitting equation helps to obtain remaining values. The relative humidity was calculated by the frosting mathematical model. Finally, the relationship between the coefficient of performance (COP) and the defrosting cycle (the sum of the defrosting time and the frosting time) was obtained. The experiments and theoretical research showed that the fluctuating value of cold storage temperature was about 5 °C and the defrosting time was about 30 min during the defrosting process. In the case of the relative humidity of 70%, 80%, 90%, the optimum defrosting cycle of the experiment was 16.4, 10.9, 7.5 h and the frosting mass was 2.66, 2.90, 3.22 kg, and the maximum COP was 1.51, 1.48, 1.45.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Thermal Sci. Eng. Appl. 2018;10(5):054501-054501-4. doi:10.1115/1.4039783.

Condenser performance benefits afforded by dropwise condensation have long been unattainable in steam cycle power plant condensers due to the unavailability of durable and long-lasting hydrophobic surface treatments. However, recent work in superhydrophobic coating technology shows promise that durable coatings, appropriate for use on condenser tubes in steam cycle power generation systems, may soon become a reality. This work presents a nanoscale, vapor phase deposited superhydrophobic coating with improved durability comprised of several layers of rough alumina nanoparticles and catalyzed silica with a finishing layer of perfluorinated silane. This coating was applied to solid, hemicylindrical test surfaces fabricated from several common condenser tube materials used in power generation system condensers: Titanium, Admiralty brass, Cupronickel, and Sea Cure stainless steel as well as 304 stainless steel stock. The development evolution of the coating and its effect on condensation behavior on the above materials are presented. Results show that the performance enhancement, measured in rate of heat transfer spikes corresponding to condensate roll-off events, was best for the titanium surface, which produced 64% more events than the next most active material when coated using the most durable surface treatment tested in this work.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):054502-054502-5. doi:10.1115/1.4040276.

The gradient porous materials (GPMs)-filled pipe structure has been proved to be effective in improving the heat transfer ability and reducing pressure drop of fluid. A GPMs-filled pipe structure in which radial pore-size gradient increased nonlinearly has been proposed. The field synergy theory and tradeoff analysis on the efficiency of integrated heat transfer has been accomplished based on performance evaluation criteria (PEC). It was found that the ability of heat transfer was enhanced considerably, based on the pipe structure, in which the pore-size of porous materials increased as a parabolic opening up. The flow resistance was the lowest and the integrated heat transfer performance was the highest when radial pore-size gradient increasing as a parabolic opening down.

Commentary by Dr. Valentin Fuster