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In this study, natural convection cooling phenomena in storage vaults for spent nuclear fuel were examined. The results revealed a relationship between the geometric length and heat flux of spent fuel cylinders. Such a correlation would be useful in dimensionally scaled storage vaults for experimental investigations of thermal and fluid flow properties. The relationship was applied to five scaled storage vaults, and the resulting thermal and fluid flow characteristics were investigated and compared with those of a full-scale storage vault. The thermal characteristics of the original and scaled storage vaults were in good agreement. The dimensionless temperature, dimensionless resident time, Euler number, and Richardson number were used to compare the behavior of structures with different dimensions, and the obtained data were in agreement within 5.1%. A 1/4-scale storage vault was constructed using the scaling methodology, and the temperature distribution was experimentally measured. The temperature distributions of the measured and simulated structures were found to be in good agreement, demonstrating that the proposed approach is effective for designing scaled-down storage vaults for experimental analyses.

In this study, a horizontal annular inlet was applied to a round-nozzle impactor in an effort to reduce the cut-off diameter. The collection efficiencies of round-nozzle impactors with either a vertical inlet, which has conventionally been used, or a horizontal annular inlet were investigated both numerically and experimentally. For the comparison, the nozzle width, nozzle-to-plate distance, impaction plate diameter, and flow rate of the aerosol drawn into the impactor were set the same for both the vertical inlet impactor and horizontal annular inlet impactor. A parametric study was performed to analyze the effects of the width of the horizontal annular inlet and nozzle throat length on the collection efficiency of round-nozzle impactors with nozzle Reynolds numbers ranging from 500 to 2600. By replacing the vertical inlet with the horizontal annular inlet, the square root of the Stokes number corresponding to the cut-off diameter could be reduced by approximately 30%. In other words, the value of (Stk 50 ) 1/2 was decreased from 0.49 to 0.32 or 0.33 when considering the effect of gravity.

Extreme ultraviolet lithography (EUVL) masks are vulnerable to particulate contamination due to the unavailability of pellicles. Particle deposition velocity is used to assess the level of particulate contamination. The particle deposition velocity onto a circular disk or a square flat plate situated perpendicular to the airflow was investigated. The numerical and experimental methods employed in this paper were validated by comparing the numerically simulated mean Sherwood numbers or the experimentally determined particle deposition velocities with the theoretically predicted values for the circular disk, representing a wafer, exposed to vertical airflow. Then, an equation for predicting the particle deposition velocity onto a square flat plate, simulating a EUVL mask, in vertical airflow was suggested by numerically obtaining the mean Sherwood number correlation, and validated through experiments.

We carried out a parametric study and optimization procedure to improve the performance of a corrugated louvered fin. Based on the results of the parametric study, we selected the fin pitch, louver pitch, and louver angle as the three most important parameters in performance. A full-factorial design was applied to the three chosen parameters for our design of experimental technique. We used the Kriging method and a micro-genetic algorithm to design an optimal corrugated louvered fin. The JF factor of the resulting optimum model was increased by 14–32% compared to that of the base model for Reynolds numbers in the range 0 ≤ ReLp ≤ 500. In addition, we propose a modified Suga–Aoki equation that can be used to estimate the optimum shape of a louvered fin with a smaller error than that produced by the original Suga–Aoki equation.

Deposition velocity of charged particles onto a horizontal flat plate in parallel airflow in the presence of an electric field was numerically investigated by employing a Lagrangian particle tracking approach. The plate length was set as 450 mm, i.e., the characteristic length of the next generation wafer. Either the face-up or the face-down critical surface was considered. The electric field strength and the particle density were varied. The particle deposition velocity was greatly influenced by the electric field strength for particles smaller than approximately 0.1 μm. The influence of the particle density was significant for particles larger than about 0.1 μm. In case of the face-down critical surface, the deposition velocity was greatly reduced for micrometer-sized particles, whereas it was estimated to be relatively high for sub-micrometer sized particles due to attractive electrophoresis and Brownian diffusion.

The thermal performance and uniformity of frost growth between the front- and rear-sides of microchannel heat exchangers was investigated as a function of the depth of the heat exchanger, the pitch of the fins, and the pitch of the channels. The microchannel heat exchangers exhibited frost formation that was concentrated on the front-side, resulting in significant frost growth non-uniformity between the front- and rear-sides, which led to rapid performance degradation. During the defrosting cycle, compared with natural convection, forced convection defrosting resulted in the unit retaining less water between the fins, leading to improved thermal performance. However, after a number of frosting and defrosting cycles, the relative improvement of forced convection became smaller. Thinner heat exchangers (i.e., with a smaller coil depth) exhibited more uniform frost growth between the front- and rear-sides of heat exchangers, and exhibited approximately 5% increase in the heat transfer rate per unit volume. A larger fin pitch showed less airflow blockage due to larger air pathway. In addition, heat exchangers with a larger fin pitch showed more uniform frost growth, as well as approximately 6% increase in the average heat transfer rate. Heat exchangers with a larger channel pitch exhibited a larger heat transfer rate during the early stages of the experiment; however, the frost growth was less uniform between the front- and rear-sides, leading to a rapid reduction in the heat transfer rate mid-way through the experiment.

This research examined the orientation effect for a cylindrical heat sink used to cool an LED light bulb. Natural convection and radiation were taken into account in the simulation, and the numerical model was validated experimentally. Changes in the cooling performance of a cylindrical heat sink with increasing angle of inclination were examined. The orientation effect was analyzed by considering the flow characteristics and drag coefficient, and the influence of geometric parameters on the cooling performance of a cylindrical heat sink was evaluated. The orientation effect was intensified when the number of fins and the fin length increased, whereas the influence of fin height on the orientation effect was relatively insignificant. Finally, a correlation was proposed to predict the Nusselt number around an inclined cylindrical heat sink.

This study aims to achieve an in-depth understanding of frost layer growth behavior on cryogenic surfaces (below −100 °C) under forced convection conditions. Carrying out frosting experiments using liquid nitrogen led to observations of the characteristic phenomenon in the frost layer growth behavior. Particularly, frost layers were growing separately in the leading edge and downstream of the cooling surface under the unique frosting mechanism. Irregularly sized shrub-shaped frost crystals, which were not observed in the previous studies, appeared at the initial frosting stage. The effect of operating conditions on the average frost thickness was investigated. As the cooling surface temperature decreased, the average frost thickness decreased. The increase in air velocity yielded an increased frost layer thickness.

The present work tested the degradation of two polymeric materials under conditions routinely encountered in nuclear applications. Polymers in the form of plastic bags are commonly used along with metal containers to store and transport nuclear materials. The role of the bag, also known as bag-out bag, is to prevent contamination in case the metal containers are breached. Polymeric materials of interest are sPVC and aromatic polyether urethane (APU-ether), which in this study were exposed to alpha, gamma and thermal conditions. These conditions were specifically chosen to simulate the harsh environments normally encountered in the storage of nuclear materials. The physical, chemical and mechanical properties of these materials were investigated by a series of characterization methods before and after exposures. Our results highlight the superior stability of APU-ether under thermal aging and ionizing radiation and provide new insights into the durability of polymeric materials used in nuclear applications. These findings will serve as the foundation for a rational selection of new bag-out bag materials to be considered in the nuclear industry.

The heat transport and fluid flow with forced convection across a locally heated square cylinder near a wall were investigated using numerical simulations. A uniform heat flux was applied to half of the square cylinder to simulate local heating. The parameters that were varied in this study were the Reynolds number and the distance between the wall and the square cylinder. Critical points were identified at which the system exhibited a transition between oscillatory flow and steady flow. Each of these flow regimes was investigated, and the effects of the fluid flow on the heat transport were examined.

Light-emitting diode (LED) lighting offers greater energy efficiency than conventional lighting. However, if the heat from the LEDs is not properly dissipated, the lifespan and luminous efficiency are diminished. In the present study, a heat sink of LED lighting was optimized with respect to its fin-height profile to obtain reliable cooling performance for high-power LED lighting applications. Natural convection and radiation heat transfer were taken into consideration and an experiment was conducted to validate the numerical model. Fin-height profiles reflecting a three-dimensional chimney-flow pattern were proposed. The outermost fin height, the difference between fin heights, and the number of fin arrays were adopted as design variables via sensitivity analysis, and the heat sink configuration was optimized in three dimensions. Optimization was conducted to simultaneously minimize the thermal resistance and mass. The result was compared with the Pareto fronts of a plate-fin heat sink examined in a previous study. The cooling performance of the optimized design showed an improvement of more than 45% while preserving a mass similar to that of the plate-fin heat sink.

Disulfonated poly(arylene ether sulfone) (BPS) random copolymers, prepared from a sulfonated monomer, have been considered for use as membrane materials for various applications in water purification and power generation. These membranes can be melt-processed to avoid the use of hazardous solvent-based processes with the aid of a plasticizer, a low molecular weight poly(ethylene glycol) (PEG). PEG was used to modify the glass transition temperature and melt rheology of BPS to enable coextrusion with polypropylene (PP). Our previous paper discussed the miscibility of BPS with PEG and the influence of PEG on the glass transition of BPS. In this study, the rheological properties of disulfonated poly(arylene ether sulfone)s plasticized with poly(ethylene glycol) (PEG) are investigated to identify coextrusion processing conditions with candidate PPs. The effects of various factors including PEG molecular weight, PEG concentration, temperature and BPS molecular weight on blend viscosity were studied. The rheological data effectively lie on the same master curve developed by Bueche and Harding for non-associating polymers such as poly(methyl methacrylate) (PMMA) and polystyrene (PS). Although sulfonated polysulfone contains ionic groups, the form of its viscosity versus shear rate (or frequency) behavior appears to be dominated by the relaxation of polymer entanglements.

The Gaussian diffusion sphere model (GDSM) is proposed to predict the average deposition velocity of particles onto a flat plate exposed to parallel airflow after considering the combined effects of electrophoresis and thermophoresis. This model can account for convection, Brownian diffusion, gravitational settling, thermophoresis, and electrophoresis, and it provides fast calculation times and accurate predictions. Using the GDSM, the effects of the deposition surface size on the deposition velocity are analyzed. When the gravitational effect is dominant for a face-up surface or the attractive electrophoresis effect is dominant, the deposition velocity is estimated to be independent of the deposition surface size. Deposition under the influence of thermophoresis depends on the deposition surface size due to the formation of a thermal boundary layer. Deposition velocities for a 450-mm-long surface are studied under a temperature difference of 40 K and for electric field strengths ranging from 0 to 1000 V/cm.

meta-Aminophenol was used during nucleophilic step copolymerization to end-cap partially disulfonated bisphenol A based random copolymers with controlled oligomeric molecular weights. The amine end groups were thermally reacted with a tetrafunctional epoxy reagent to produce networks. Very high gel fractions, up to 99%, and ductile film formation were achieved. The oligomer was further functionalized with acryloyl chloride, phenylethynyl phthalic anhydride, and maleic anhydride to obtain several novel free radically cross-linkable oligomers. The structure and molecular weights (Mn) were established with 1H NMR spectroscopy of the oligomers and end groups. TGA analysis demonstrated the high thermal stability of these oligomers; DSC investigations showed that the oligomers had curing exotherms from 140 to 330 °C and that residual casting solvent during cross-linking was necessary to avoid vitrification during membrane formation. In addition to the thermally cross-linkable systems, photocross-linking was dem...

We report a LDC (load duration curve) method to determine the optimum size of CHP (combined heat and power) units. The method gives the appropriate capacity graphically from the LDC of a building's heating demand. The method can be applied to the most common CHP units that are connected to the electrical grid, installed with thermal storage and auxiliary heat sources, and operated by a traditional heat-led strategy. The LDC method is simple and requires less information than existing sizing methods. Our method is in agreement with existing methods within 2.7–12.6% for internal combustion engine-driven CHP units, and 17.1–32.1% for Stirling engine-driven CHP units.

A field test of a 70 ㎾ heat pump system with flue gas heat recovery was performed by an experiment at theKorea Institute of Energy Research. The flue gas is exhausted from a 320 RT absorption chiller-heater in the heating season.Using this flue gas, source water of the heat pump is heated by a condensed-type heat exchanger in the chimney. Theoperating characteristics of the heat recovery heat pump system were analyzed. Based on the results of the experiments, operating maps were obtained, and an optimum operating range is suggested, in which the return and heat source watertemperature are 51 ℃ and 31 ℃ , respectively. Additionally, economic analysis of this system was conducted and about 50%energy cost savings can be expected in the heating season. Key words Condensing heat recovery(응축열회수), Flue gas(배가스), Heat recovery system(열회수 시스템), Latent heat(잠열), Condensation(응축)†Corresponding author, E-mail: yslee@kier.re.kr 기호설명COP H ： 난방성능계수 Q cond ： 응축(난방)열량 [ ㎾ ] Q evap ： 증발열량 [ ㎾ ]

A major, unprecedented improvement in the durability of polymer electrolyte membrane fuel cells is obtained by tuning the properties of the interface between the catalyst and the ionomer by choosing the appropriate dispersing medium. While a fuel cell cathode prepared from aqueous dispersion showed 90 mV loss at 0.8 A cm(-2) after 30,000 potential cycles (0.6-1.0 V), a fuel cell cathode prepared from glycerol dispersion exhibited only 20 mV loss after 70,000 cycles. This minimum performance loss occurs even though there was an over 80% reduction of electrochemical surface area of the Pt catalyst. These findings indicate that a proper understanding and control of the catalyst-water-ionomer (three-phase) interfaces is even more important for maintaining fuel cell durability in typical electrodes than catalyst agglomeration, and this opens up a novel path for tailoring the functional properties of electrified interfaces.

This paper presents a numerical optimization of louvered fins to enhance the JF factor in terms of the design parameters, including the fin pitch, the number of louvers, the louver angle, the fin thickness, and the re-direction louver length. We carried out a parametric study to select the three most important parameters affecting the JF factor, which were the fin pitch, number of louvers, and the louver angle. We optimally designed the louvered fin by using 3rd-order full factorial design, the kriging method, and a micro genetic algorithm. Consequently, the JF factor of the optimum model increased by 16% compared to that of the base model. Moreover, the optimum model reduced the pressure drop by 17% with a comparable heat transfer rate.

We describe a numerical model for predicting performance of fin-and-tube heat exchangers for air-source heat pumps that takes into account airflow reduction due to the accumulation of frost. The thermal resistances between the air, frost, and refrigerant are analyzed to evaluate the effects of frost growth. We compare the results of our model with experimental data for a fin-and-tube heat exchanger and with the results of existing models. We conduct experiments to obtain the correlations between the pressure drop and the Reynolds number, the temperatures of the air and the cold surface, and the humidity. The simulated heat-transfer rates and frost mass agreed with the experimental data to within 7% and 9%, respectively.

The finite element approach is applied to predict crack patterns in a single or composite material under loadings. Crack patterns are represented as variations of a field variable. These variations are determined from the solution of a coupled system of equations consisting of an Allen-Cahn or Ginzburg-Landau type field equation and elasticity equations based on phase field theory. This representation does not require tracking crack tips as in the conventional finite element approach for the modeling of crack propagation problems. For a numerical solution for the system, a finite element algorithm is proposed and implemented into the finite element program “FEAP”. Several numerical simulations are performed and analyzed to predict the crack patterns in 2D single or composite materials under the loadings.

Ordered morphologies in disulfonated poly(arylene sulfide sulfone nitrile) (SPSN) copolymers were generated via thermal annealing followed by multiblock copolymer synthesis. While SPSN random copolymers (R-SPSN) showed featureless morphologies, the SPSN multiblock copolymers (B-SPSN) exhibited cocontinuous lamellar morphologies with a center-to-center interdomain size of up to 40 nm. In spite of the well-ordered, interconnected hydrophilic domains, the water self-diffusion coefficient (e.g., D = (0.7–2.0) × 10–10 m2 s–1) and proton conductivity (e.g., σ = 0.16–0.20 S cm–1 in deionized water at 30 °C) through B-SPSN were lower than those of the corresponding R-SPSN (e.g., D = (3.5–3.9) × 10–10 m2 s–1 and σ = 0.21 S cm–1) due to the relatively lower water uptake of the B-SPSN after thermal annealing. The reduced water uptake of B-SPSN was beneficial to reduction of peroxide degradation rate. Thermal annealing produced significant gains in morphological ordering and finer control over desired membrane proper...

An analysis is presented to evaluate the temperature response in fins with internal heat generation due to heat conduction described by Fourier and non-Fourier laws. For the influence of non-Fourier heat conduction, the analysis is formulated by considering different boundary conditions in order to understand practical fin design problems. The isothermal base or convection-heated base with a convection tip condition for longitudinal and pin fins with uniform thickness are adopted for the analysis. A method for determining the instantaneous fin efficiency for non-Fourier heat conduction is elaborated. The deviation of the temperature response of the non-Fourier model from the Fourier system is systematically studied with several possible design variables, e.g., Fourier, Vernotte numbers, the effect of boundary conditions, and heat generation parameters. The non-Fourier fin efficiency is always lower than the Fourier value for identical design parameters. This decremented value is dependent upon the Vernotte and Fourier numbers. The heat generation in the fin amplifies both the Fourier and non-Fourier fin efficiency.

In this study, the friction factor and Nusselt number in a plate heat exchanger with dimples and protrusions were investigated according to geometric and operating conditions. The numerical results of a steady-state laminar model for the laminar region and a steady-state SST (shear stress transport) k–ω turbulence model for the turbulent region were in good agreement with an unsteady analysis. For efficient analysis, a correlation for the critical Reynolds number was derived as a function of the geometric parameters. Based on these results, Nusselt number and friction factor correlations applicable to various geometric parameters and operating conditions (500 ⩽ ReH ⩽ 15,000) were obtained. Multi-objective optimization was carried out, where the objective function was a function of the normalized Nusselt number and normalized friction factor. To investigate the variation of the optimal points according to weighting factors, a Pareto front was generated using a micro-genetic algorithm. When JF factor was used as a single-objective function, the performance of the optimal point was enhanced by 28% compared with that of the reference model. The optimal point was independent of the Reynolds number.

Heat pumps have been promoted as energy-efficient heating and cooling systems. However, in the heating mode, heat pump performance is often significantly degraded by the frost layer that forms on the fin surface of the heat exchanger due to the low temperature of the outdoor unit. In this research, we experimentally investigated the thermal performance and frosting behavior at various locations on a plate-fin and tube heat exchanger with respect to refrigerant flow direction (counter-flow and parallel-flow) and fin surface treatment (bare, hydrophilic, hydrophobic, and hybrid). On the bare heat exchanger, the frost growth was more uniform with a counter-flow than with a parallel-flow, and thus the overall heat transfer rate was higher under counter-flow condition. The performance difference between the counter-flow and parallel-flow conditions became more significant when the heat exchanger was operated at lower refrigerant temperature and air velocity. Although the initial heat transfer rate of the hydrophilic unit was the highest among the surface-treated heat exchangers, this unit also suffered a dramatic reduction in the heat transfer rate because of the fast frost growth and non-uniform frosting behavior on its rear side. On the other hand, although the hydrophobic heat exchanger had a slightly lower initial heat transfer rate than the hydrophilic unit, it exhibited a consistent heat transfer rate throughout the experiment, owing to the frost retardation effect and the relatively uniform frosting behavior on its rear side. Each of the hybrid hydrophilic–hydrophobic surface-treated heat exchangers showed a lesser reduction of the heat transfer rate than the hydrophilic unit, and a higher initial heat transfer rate than the hydrophobic unit.

The analysis of fins using arc length to determine the minimum volume for a given heat transfer rate is demonstrated by a unified formulation for fully and partially wet surfaces of longitudinal and pin fins. The shape of the optimum profile, a circular arc, was identical for both fin geometries under the same surface conditions. The optimum shape for partially wet surfaces had two different circular arcs for dry and wet surfaces. Unlike the previous study, the base thickness of circular profiles was less than that of parabolic profiles, regardless of the surface conditions. The minimum fin volume for the circular profile was always less than the corresponding parabolic profile. The difference in volume between these profiles reached a maximum for fully wet surfaces, and was also dependent upon the psychrometric properties and heat rate constraint. The optimum wet circular arc resulted in a maximum 156% savings in fin material, compared with the parabolic arc for pin fins; this percentage was up to 135% for longitudinal fins. Nevertheless, the savings for longitudinal and pin fins were only 25.9% and 22.0%, respectively, for dry surfaces; these results differed from published values.

The problem of improving the tractive and dynamic performance of locomotives by means of using axle boxes with inclined radius links was investigated. The kinematics of axle boxes with different variants (different angles) and designs of inclined radius links, based on the vector contour method for link mechanisms, was studied. The results of a bogie design with vertically inclined radius links were used for the improvement of vertical load distribution for locomotives. The behavior of a locomotive running in a curve is more complicated because the lateral forces appear between a flange of the wheel and the head of the rail in the contact zone, and the values of wheel slip are higher than for straight sections of track. These two factors are the main reasons for wheel wear. Theoretical and experimental investigations to reduce wear and to improve vehicle dynamics were made, and the results obtained show that an effective solution can be reached by the use of inclined links to the longitudinal axis of a bogie in the horizontal plane. Questions about the development of a bogie with a radial installation and active steering are discussed.

Frosting experiments were conducted on surface-treated (hydrophilic, hydrophobic, and dual) louvered-fin heat exchangers with varying fin pitches, and the frosting behavior of the rows was investigated in terms of the experimental conditions. In the early stage of experiments, three of these surface-treated heat exchangers exhibited the highest heat transfer rate at 16 FPI (fins per inch), in particular, the hydrophobic unit had the smallest reduction in the heat transfer rate during the latter stage due to the frost-retardation effect. Hence, the hydrophobic unit had the largest overall heat transfer rate. For the hydrophilic and dual-treated heat exchangers, the first row (front side) showed more active frost growth than the second row (rear side), and thus the leading-edge effect was demonstrated in these two heat exchangers. By contrast, with the hydrophobic heat exchanger, the leading-edge effect was not observed when the refrigerant temperature was higher than −10 °C, or the airflow rate was higher. The ratio of remained water after defrosting was the highest for the hydrophobic unit, but this unit still showed the highest overall heat transfer rate during the repeated frosting and defrosting experiments cycle, due to frost retardation.

We report a facile method for fabricating polymer hierarchical structures, which are the engineered, ratchet-like microscale structures with nanoscale dimples, for the directional movement of droplets. The fabricated polymer hierarchical structures with no surface modifier show hydrophobic, superhydrophobic, or omniphobic characteristics depending on their intrinsic polymer properties. Further treatment with a surface modifier endows the polymer surfaces with superomniphobicity. The fabricated polymer substrates with no surface modifier enable the movement of the water droplet along the designed track at almost no inclination of the substrate.

A double-differential transform method has been used to analyze a cascaded rectangular–triangular fin losing heat by simultaneous convection and radiation to its surroundings. The base of the fin is heated by convection by a hot fluid at a uniform temperature. The thermal conductivity of the fin material assumed to be a linear function of temperature. An exact analytical solution is developed for a pure convection cascaded rectangular–triangular fin of constant thermal conductivity. Results are presented for the temperature distribution for a range of dimensionless parameters governing the problem. Both single- and two-material cascaded rectangular–triangular fins are analyzed. An optimization study has been performed to establish data for optimum heat transfer rate, cold fluid Biot number, fin efficiency, and fin effectiveness. If properly designed, a cascaded rectangular–triangular fin can be more effective than a rectangular or a triangular fin.

Fuel cells are promising devices for clean power generation in a variety of economically and environmentally significant applications. Low-temperature proton exchange membrane (PEM) fuel cells utilizing Nafion require a high level of hydration, which limits the operating temperature to less than 100 ∘C. In contrast, high-temperature PEM fuel cells utilizing phosphoric acid-doped polybenzimidazole can operate effectively up to 180 ∘C; however, these devices degrade when exposed to water below 140 ∘C. Here we present a different class of PEM fuel cells based on quaternary ammonium-biphosphate ion pairs that can operate under conditions unattainable with existing fuel cell technologies. These fuel cells exhibit stable performance at 80–160 ∘C with a conductivity decay rate more than three orders of magnitude lower than that of a commercial high-temperature PEM fuel cell. By increasing the operational flexibility, this class of fuel cell can simplify the requirements for heat and water management, and potentially reduce the costs associated with the existing fully functional fuel cell systems. There is intensive research underway into the development of fuel cells. Here, the authors present a proton exchange membrane fuel cell based on quaternary ammonium-biphosphate ion pairs, offering promising performance under a wide range of conditions that are unattainable with conventional technologies.

This study presents an analytical work on the performance and optimum design analysis of porous fin of various profiles operating in convection environment. Straight fins of four different profiles, namely, rectangular, convex parabolic and two exponential types are considered for the present investigation. An analytical technique based on the Adomian decomposition method is proposed for the solution methodology as the governing energy equations of porous fins for all the profiles are non-linear. A comparative study has been carried out among the results obtained from the porous and solid fins, and an appreciable difference has been noticed for a range of design conditions. Finally, the result shows that the heat transfer rate in an exponential profile with negative power factor is much higher than the rectangular profile but slightly higher than the convex profile. On the other hand, the fin performance is observed to be better for exponential profiles with positive power factor than other three profiles. A significant increase in heat transfer through porous fins occurs for any geometric fin compared to that of solid fins for a low porosity and high flow parameter.

Hydrophilic, hydrophobic, and dual (hydrophilic and hydrophobic) coatings were applied to louvered-fins typically used for heat pump heat exchangers, and the characteristics and performance of the heat exchangers under both frosting and wet conditions were compared according to surface treatment. The hydrophilic heat exchanger had the highest air-side pressure drop under frosting conditions. The hydrophobic unit had a lower air-side pressure drop than the others (hydrophilic and dual), due to frost retardation, and the reduction of the heat transfer rate was also smaller. The dual-fin heat exchanger exhibited frost retardation only in the early stage of the experiment, and the heat transfer rate was slightly greater than that of the hydrophilic unit. In wet-condition experiments for evaluating the evaporating performance of surface-treated heat exchangers, the hydrophilic unit had a lower air-side pressure drop than the others, due to the thin film of water condensation on the fins. The differences in the heat transfer rates of the heat exchangers were not significant.

This paper presents an analytical analysis of both Fourier and non-Fourier heat conduction in the absorber plates of a flat-plate solar collector. Separation of variables was employed to develop the model. For the analysis, a repetitive heat transfer module was used for the solution of parabolic and hyperbolic equations. From the practical point of view, two types of boundary conditions were separately chosen. A numerical technique based on the finite difference method was employed to determine the temperature for validation purposes. A comparative investigation was carried out to understand the requirements for use of the non-Fourier heat conduction model easily. A significant difference in the temperatures obtained from the Fourier and non-Fourier models was observed for lower values of the Fourier number and higher values of the Vernotte number. Finally, the effect of the boundary conditions on the Fourier and non-Fourier heat transfer was demonstrated.

A solely analytical method is investigated to determine the minimum fin shape of wet fins subject to variable heat transfer coefficients. The analysis is applicable to any surface condition, including dry, fully wet and partially wet. An optimization procedure based on the variational principle is derived to develop an analytical formulation. The effects of various design parameters, including base temperature, relative humidity, surrounding temperature and surrounding pressure, are studied to identify the dependent variables affecting the minimum envelope shape to transfer energy efficiently. The analysis also shows the influence of the variable heat transfer coefficient on the temperature distribution and profile shape with the intention of optimizing the envelope shape criterion. A convergent optimum profile shape is established for the constant heat transfer coefficient or the temperature-dependent heat transfer coefficient with a decreasing function along the length of the fin. On the other hand, divergent profiles under wet conditions may transfer heat optimally for an increasing heat transfer coefficient from the fin base to the fin tip. A simple method is highlighted to produce a new profile by choosing a constraint tip temperature to improve the shape near the tip in order to have an ease in manufacturing process.

This study presents a numerical model that is applicable to fin-and-tube condensers with complex refrigerant circuitry, and can be used to describe the exergy loss on the refrigerant side. The model was validated by comparing numerical results with experimental data for an R410A multi-pass condenser. The modeled and measured heat transfer rates were observed to agree with an error of less than 10%. Refrigerant-side performance was analyzed according to the design parameters using the numerical model, and appropriate performance evaluation criteria (PEC) were established for the refrigerant side of the condenser. A methodology was developed for refrigerant circuitry design based on entropy generation minimization (EGM). The resulting refrigerant circuit design enhanced heat transfer performance and lowered entropy generation in comparison to simple refrigerant circuitries.

Exact and approximate techniques to determine the performance of annular fins with a step rectangular profile are developed. A simple approximate method is proposed to analyze the prevalent heat transfer characteristics of the fin array. An algebraic expression based on the mean value approach is used as an approximation tool, and the temperature distribution in the fins is determined using an exponential function. A method based on a modified Bessel function formulation is employed for exact analysis. The analyses are extended to optimize fins based on the principle of maximizing heat transfer rate for a given volume. The results obtained from the exact and approximate analyses are presented in a one-to-one comparative manner to allow for a wide range of practical design variables. The error in the approximate analysis calculations is investigated, and it is found to be well within engineering accuracy requirements. It is expected that the approximate analytical tool designed here will be extremely useful for designers who want to easily determine design parameters. Because the approximate methodology is so simple, all calculations can easily be done.

A simulation program was developed to evaluate the heat transfer performance of a multi-pass fin-tube evaporator, considering the pressure drop in the distributor and capillary tubes. The effect of capillary tube length for each pass was analyzed with various inlet air flow types and distributions. The appropriate capillary tube length distribution and correlation were deter-mined for various inlet air flow types and distributions. The correlated results agreed well with the simulation, with an average error of less than 7%. By applying an optimal capillary tube length distribution, the heat transfer rate was increased by 4～5% compared to cases with uniform tube- length distributions, for each of the inlet air flow types and distributions considered in this study.Keywords:Fin-and-tube evaporator(휜-튜브 증발기), Simulation(시뮬레이션), Capillary tube(모세관), Distributor(분배기), Air flow distribution(공기유속 분포) †Corresponding author Tel.: +82-2-2220-0426; fax: +82-2-2295-9021 E-mail address: ksleehy@hanyang.ar.kr

Like dry surface fins, development of the boundary layer over a wet fin causes the heat transfer coefficient to vary over the fin surface. The present study analytically determines the performance of different fins geometries by analyzing the temperature-dependent thermal conductivity of the fin material and the variable heat transfer coefficient under wet surface conditions. Rectangular, triangular, convex (parabolic), and exponential geometric longitudinal fins were analyzed by the differential transform method under dehumidifying surface conditions. The mass transfer process was calculated by adopting the humidity ratio as a polynomial function with fin surface temperature, which was determined from the psychrometric correlation using regression analysis. The effect of the wet surface, the variable conductivity, and the heat transfer coefficient of different profiles on the temperature and fin efficiencies were studied comparatively. A new expression based on the transformed method was formulated appropriately to determine the heat transfer rate as nonlinear terms associated with it. Optimum design analysis was also carried out, and from these results, fin performance and optimization parameters were identified for variations of the same design constants with different geometric fins.

The cross-linkable copolymers (SHQx-TFVys) with varying degrees of sulfonation (DS) from 70 to 95% were prepared from potassium-2,5-dihydroxybenzenesulfonate (SHQ), decafluorobiphenyl (DFBP), and 4-(trifluorovinyloxy)-biphenyl-2,5-diol (TFVOH) as a cross-linkable moiety. To develop a highly stable polymer electrolyte membrane (PEM) for application in polymer electrolyte fuel cells (PEFC)s, cross-linked membranes were prepared by chemical cross-linking. The cross-linked membranes were synthesized by varying the amount of TFVOH (5–30 mol %) in order to achieve desirable PEM properties. The structures of the cross-linkable monomer and polymers were investigated by 1H and 19F NMR and FT-IR spectra. The cross-linked membranes exhibited good glass transition temperature and thermal stability up to 239–271 °C and 290–312 °C, respectively. The crosslinked membranes (DS range 80–95%) exhibited higher proton conductivity(0.098–0.151 S/cm) than Nafion 212 (0.092 S/cm). Moreover, all membranes possessed lower methano...

This study presents an analysis for establishing a method to determine an optimum profile of a fin by minimizing its volume for a given heat transfer duty under wet environments with variable thermal conductivity and heat transfer coefficient. A polynomial relationship between the humidity ratio and temperature for mass transfer is employed for predicting its energy transfer accurately. Both fully and partially wet surfaces were investigated over a wide range of thermopsychrometric parameters according to the suitability of any application. The optimality criteria were derived as a function of temperature-dependent parameters, and optimization was carried out by minimizing the fin volume for a constrained heat transfer rate and design constants. From the result, it can be highlighted that the minimum envelope shape of wet fins is a strong function with the heat transfer coefficient. However, the thermal conductivity effect on the optimized fin shape shows marginally.

A general procedure for the optimal design of a micro-channel heat exchanger for heat pump systems is presented. For this design, a performance analysis program that can reflect the various geometric variables of the micro-channel heat exchanger was developed. The deviation between simulated and experiment results of previous research was within 4% for the heat transfer rate. To prove the feasibility of the optimal design process, the performance of the reference heat exchanger was compared to that of the optimized heat exchanger. The JF air and PEC V of the optimized heat exchanger were enhanced by 14% and 26%, respectively.Keywords:Condenser(응축기), Micro-channel(미세유로), Optimal design(최적설계), Approximate optimization method(근사최적화 기법) †Corresponding author Tel.: +82-2-2220-0426; fax: +82-2-2295-9021 E-mail address: ksleehy@hanyang.ac.kr 기 호 설 명 A : 전열 면적 [m 2 ]D core : 열교환기 폭 [mm] f : 마찰인자 F d : 핀 폭 [mm]F h : 핀 높이 [mm] 그리스 문자 F p : 핀 간격 [mm]F th : 핀 두께 [mm]h : 대류 열전달계수 [W/m

This study presents a simple approach to estimate the capacity of combined heat and power (CHP) units suitable for installation in high heat-to-power ratio (HPR) buildings when excess electricity cannot be sold back to the grid. The two different methods used in this approach, a graphical method and a simple equation, are capable of predicting the optimum capacity to within 7% and 15%, respectively, of the methods presented in previous studies. In particular, the simple equation has the advantage of selecting the optimum capacity using the base and peak of the demand power load. The two easy-to-use methods can be used with any building, region, and energy-price structure that satisfies the conditions considered in this study, and will be of great use to future CHP owners.

Heteropolyacids (HPAs) are nano-size metal–oxygen clusters with anion acids stronger than the conventional solid acids. Insoluble and high surface area solid acids, i.e. Cs 2.5 H 0.5 PW 12 O 40 (CsPW) and H 3 PW 12 O 40 /SiO 2 (PWS) are dispersed throughout E nd-group cross-linkable, S ulfonated F luorinated B i- P henol (ESF-BP) copolymer to produce organic–inorganic nanocomposite proton exchange membranes (PEMs). The molecular structure of ESF-BP copolymer is identified by 1 H NMR spectra. The FT-IR of prepared CsPW and PWS inorganic powders show that the primary Keggin structure remains almost unaltered. The PEMs properties including density, water uptake, water diffusivity, swelling, ion exchange capacity (IEC), oxidative stability and proton conductivity are investigated to characterize the influence of added HPAs on the composite PEMs. The intermolecular interaction between HPAs and the sulfonic group of the copolymer affects the physico-chemical characteristics of membranes. The addition of HPAs to the polymer matrix increases oxidative stability, water uptake and conductivity but decreases the IEC, the water diffusivity and the density of membrane. The cells with the nanocomposite membranes have better performance than the cell with the plain polymer membrane. The prepared composite membrane using dispersion of PWS through the ESF-BP has superior characteristics and voltage–current responses in the PEMFC rather than using CsPW.

A methodology for determining the optimum pin fin profile is introduced to minimize the fin volume for a constrained heat transfer rate under dehumidifying surface conditions. In this methodology, the mass transfer is evaluated using the polynomial variation of humidity ratio with temperature. A scheme is developed for solving the optimum conditions derived as a function of unknown temperature-dependent parameter and tip temperature under both the fully and partially wet surface conditions. The effect of psychrometric properties of the surrounding air on the optimum wet fin profile has been examined. The analysis presented in this study is pertinent to the dry, fully wet, and partially wet surface conditions. In every case study of optimum wet fins, the excess temperature at the tip vanishes with respect to the surrounding temperature. A non-linear temperature distribution in the optimum wet fin has been identified.

A radial heat sink was investigated, considering both natural convection and radiation. Experiments were performed to validate numerical results and a close agreement was found. The effect of radiation on total heat transfer was examined by varying emissivity, and it was found that the maximum radiation contribution was 27%. The radial heat sink was optimized to maximize thermal performance. The thermal performance was enhanced by 12.3% while the mass increased by 20%. Radiation (emissivity) had a significant effect on the optimum long fin length when only thermal performance was considered. The average thermal resistance decreased by 8.7% for the optimized model with the same mass as the reference model, and the effect of radiation on total heat transfer was greater than the effect of natural convection.

Fundamental studies of a new thermal annealing strategy for maximizing proton conductivity and single cell performance of glassy sulfonated hydrocarbon fuel cell membrane materials under both fully and partially hydrated conditions are reported. Directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers (BisA-XX, XX is the mole percent of hydrophilic moieties, XX = 20 to 40) with 2,2′-isopropylidene diphenol swivel ((CH3)2–C) units in their polymer backbone were cast from N,N-dimethylacetamide (DMAc) and dried under two different protocols; one set was dried at 60 °C (BisA-XX_60 °C), and the other at 150 °C under vacuum immediately after the initial drying (BisA-XX_150 °C). Small amounts of DMAc solvent remained during the second drying step which depressed the glass transition temperatures (Tg) of the BisA copolymers lower than 150 °C. BisA-XX_150 °C samples were essentially thermally annealed when dried at 150 °C. This increased the density of BisA copolymer chains; their Tg values were increased. Moreover, T1 values of the protons in BisA-XX aromatic phenylene rings measured via solid-state NMR technique were lower as a result of the improved 1H–1H dipolar interaction. Interestingly, proton conductivity was improved after thermal annealing, possibly because the sulfonic acid density in the fully hydrated state (i.e., IECv(wet)) was enhanced. The synergistic effect of reduced water uptake and more developed hydrophilic–hydrophobic nanochannel formation may be important. The thermal annealing significantly influenced proton conductivity particularly at a low humidity (30 to 80% relative humidity (RH)). As a result, BisA-XX_150 °C samples derived from identical materials exhibited electrochemical polymer electrolyte membrane fuel cell (PEMFC) performances similar and superior to Nafion® 112 and BisA-XX_60 °C samples measured at 65% RH at 80 °C, respectively.

In this study, the cooling performance and mass of a pin-fin radial heat sink were optimized. A radial heat sink with pin fins was examined numerically to obtain a lighter heat sink while maintaining a similar cooling performance to that of a plate-fin heat sink investigated in a previous study. Both natural convection and radiation heat transfer were considered. Experiments were performed to validate the numerical model. The average temperature and mass of the heat sink for various types of fin arrays were compared to determine an appropriate reference configuration. The effects of various geometric parameters on the thermal resistance and mass of the heat sink were investigated; these indicated that the system was sensitive to the number of fin arrays, as well as the length of the long and middle fins. Multidisciplinary optimization was carried out using the three design variables to minimize the thermal resistance and mass simultaneously, and Pareto fronts were obtained with various weighting factors. A design for the optimum radial heat sink is proposed, which reduces the mass by more than 30% while maintaining a similar cooling performance to that of a plate-fin heat sink.

A semi-analytical method was described to determine the thermal performance of fin-and-tube heat exchangers with orthotropic fin materials and wet surfaces. The effect of the orthotropic thermal conductivity was investigated over a range of design parameters for both wet and dry fins, and design optimization was carried out using a generalized Newton–Raphson method. The optimized designs were analyzed to gain insight into the relative importance of the different parameters under varying conditions, and a set of curves was determined to provide design optimization guidelines.