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The objectives of this study are to investigate the effects of the power transient on critical heat flux (CHF), and to develop CHF and maximum heat flux (MHF) correlations in transient boiling systems with a countercurrent flow between the liquid and vapor flow. The test section consists of narrow, vertical, rectangular channels between parallel plates. Rate of change of wall temperature at the CHF point, (dTw/dt)CHF, and a nondimensional transient parameter, kappao =(L2/alphafTsat) (dTw/dt) CHF are introduced to evaluate the effects of power transients on CHF and MHF. Experimental ranges were 738.0-1,968.0 kg/m2s for mass flux, 7.0-17.5 C for inlet subcooling, and 3.0-8.0 mm for channel gap distance. The system pressure was kept constant at 1.0 atm. The experimental results show that CHF and MHF values increase with an increasing rate of change of wall temperature, and the increasing rate of CHF is higher in a wider channel gap distance. CHF and MHF increase linearly with increasing mass flux at the top ...

The objectives of this paper are to develop an advanced generator absorber heat exchanger cycle (WGAX) to reduce the generator exit temperature as low as possible using waste heat sources, and to compare it with the standard GAX cycle (SGAX). This paper performed parametric analysis to study the effects of the waste heat source temperature ( T w ) and the outlet temperature of a gas fired desorber (GFD), T g , on the cycle performance. Three different WGAX cycles (type A, type B and type C) were compared from the viewpoint of performance improvement. It was found that the effect of the waste heat source temperature ( T w ) on coefficient of performance (COP)s was negligible for a given GFD outlet temperature in the WGAX cycles. The GFD outlet temperature could be reduced down to 172°C with a higher COP β of WGAX cycle than the COP of the SGAX cycle. Therefore, the corrosion problem in the SGAX cycle at a higher T g than 200°C will be solved by adopting the WGAX cycles with a comparable COP . Type A had a merit from the viewpoint of the GAX effect while Type B had a merit from the viewpoint of exergy loss effect. In the WGAX cycles developed in this study, the GAX effect was dominant for a lower temperature than 181°C while the effect of exergy loss was dominant for a higher temperature than 181°C. It is found that the solution heated desorber should be placed below the GAXD to improve the cycle performance in the WGAX cycles. It is strongly recommended that there be a subcooling effect in the weak solution for cycle performance enhancement in the WGAX systems.

The objectives of this paper are to develop advanced energy management systems for an energy efficient and environmentally friendly society and to provide an assessment model for zero emission (of materials) and energy cascade systems. This paper introduces two types of advanced energy system models: energy cascade with zero emission by material circulation and a super environmentally benign industrial district from the viewpoint of complete energy utilization and environment conservation. An assessment model is developed to quantify the energy saving effect from the advanced energy cascade systems which consist of four major industry groups and 12 subgroups in Japan. The results show that an energy saving effect of 25% could be realized from the zero emission and energy cascade systems. It is, moreover, found that the minimum fuel and energy consumption rate could be obtained from an optimum area composition of major groups. This paper develops an energy cascade balance table which indicates the material and heat flow directions from a higher potential level to a lower potential level. The table proposes current energy consumption with respect to temperature level. The proposed energy models demonstrate how to maximize the heat utilization among various industries.

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re 1 , vapor Reynolds number Re v , inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.

The objectives of this article are to propose critical heat flux (CHF) and minimum heat flux (MHF) correlations in transient forced-convective boiling systems and to investigate the parametric effects on CHF and MHF. Experiments were conducted to obtain the CHF and MHF values in forced-convective boiling systems with vertical, parallel, rectangular channels between parallel plates. A nondimensional transient parameter,T=(Z2/αfTsat)(dtw/dt)CHFwas adopted to quantify the transient effects on CHF and MHF values. Experimental ranges are 718.0-2162.0 kg / m2 s for mass flux, 7.5-20.3°C for inlet subcooling, and 3.0-8.0 mm for channel gap distance. The system pressure is l.Oatm. The results show that CHF and MHF increased with increasing inlet subcooling. It was also found that CHF and MHF increased somewhat linearly with increasing mass flux at the top of the test section, while they were weakly dependent on the mass flux at the bottom of the test section. New CHF and MHF correlations were developed f...

Mass transfer enhancement by the use of nanoparticles suspended in a liquid phase (known as nanofluids) has been studied in recent years with positive results. Different theories have been proposed to explain the improvement in mass transfer, however it has not been possible to elucidate a definite answer. While the theory is still uncertain the experimental work continues in areas that will benefit much such as non-reactive gas absorption. In this study carbon dioxide (CO2) absorption experiments are performed in a Taylor-Couette absorber at different rotational speeds. The base fluid for the experiments is methanol. Al2O3 and SiO2 nanoparticles are combined with methanol to produce nanofluids with the purpose of enhancing the absorption of the CO2 gas into the methanol. The system is equipped with a mass flow controller at the inlet and a mass flow meter at the outlet to obtain the absorption rate. The Taylor-Couette absorber performance is compared to a modified version in which trays were added to enhance the absorption rate. Experiments in co-current and counter-current flow modes are carried out. The results of continuous absorption are presented. In addition, the two-phase flow pattern of the CO2 gas bubbles and the liquid methanol in the Taylor-Couette absorber and the modified version is analyzed with pictures obtained by a high speed camera.Copyright © 2013 by ASME

In this paper, condensation and evaporation characteristics of low global warming potential (GWP) refrigerants such as R-1234yf and R-1234ze series are reviewed. This review focuses on heat transfer and pressure drop in plate heat exchangers. Mass flux is considered as an important factor while saturation temperature is not for condensation and evaporation process in plate heat exchangers. The dryout phenomenon occurs occasionally and gives greatly harmful impact on evaporation heat transfer. It is found that R-1234yf and R-1234ze(E) give slightly lower heat transfer performance than R-134a for both condensation and evaporation processes. Generally, low GWP refrigerants presented in this review give lower heat transfer coefficient and higher frictional pressure drop than the conventional refrigerants. Nevertheless, R-1234ze(Z) gives superior heat transfer performance than other refrigerants in condensation. R-32 gives remarkable performance in evaporation, but it gives relatively high GWP compared to other low GWP refrigerants.

The mass transfer enhancement (MTE) by a single emulsion droplet subjected to a Poiseuille flow is studied by using h-adaptive finite element approximation. The Navier–Stokes equations with the surface tension force at a droplet–fluid phase for the fluid flows are considered. The mass transfer is modeled by convection–diffusion equation. The interface capturing method with a conservative level set approach is employed to track the interface of a droplet. This is also incorporated with a mesh adaptivity for increasing the resolution in vicinity of the interface of the droplet. We numerically investigate the effect of hydrodynamic interactions of the droplet with the fluid on the mass transfer in varying physical parameters of the fluid flows and the suspended droplet in order to find the optimum conditions to invoke the enhanced MTE. It is found that Pe is an important factor to enhance the mass transfer transported by the convective current with diffusion. It is also found that Ca is an important factor for MTE considering disturbances and deformability of the droplet during the mass transfer process. It is found that the MTE decreases with increasing Ca in a high concentration region ([Formula: see text]) while MTE does not depend much on Ca in a low concentration region ([Formula: see text]).

In this article the absorption process in a smooth-tube heat exchanger (GAX absorber) with a porous medium is analyzed and a countercurrent model for GAX absorber that includes an absorption process within the porous medium is proposed. The absorption process is separated into three regions, a vapor region without a porous medium (region 1), a vapor region within a porous medium (region 2), and a liquid region within a porous medium (region 3). The effects of thermal conductivities and porosities of the porous medium on the absorption rate and the required tube length are investigated, and the Nusselt number within each region is also predicted. The results show that the absorption rate increases with an increase of the thermal conductivity and with a decrease in porosity of the porous medium. The optimum value of the ratio of thermal conductivities (k{sub p} / k{sub 1}) was found to be 5.0, and the optimum value of the porosity was found to be larger than 0.2.

Binary nanoemulsions, nano-sized oil-droplet suspensions in binary solution (H2 O/LiBr), are developed to enhance the heat and mass transfer performance of absorption refrigeration systems. In this study, a novel four-step method is proposed to prepare the stable oil-in-binary solution (O/S) emulsion. To stabilize the nanoemulsions in a strong electrolyte, a polymeric stabilizer (Gum Arabic) is used as a steric stabilizer. The droplet size and the thermal conductivity of binary nanoemulsions are measured by the dynamic light scattering method and the transient hot-wire method, respectively. It is concluded that the ratio of 2:1 (oil:surfactant) is the best condition for distribution stability. It is also found that the measured thermal conductivity of the oil-in-water nanoemulsion enhances up to 6.4% at 0.1 vol% of oil, and the binary nanoemulsion enhances up to 3.6% at 1.0 vol% of oil in 30 wt% H2 O/LiBr compared with the estimated one from the Maxwell’s model. This result is compared with electric conductivity of LiBr solution and it is found both conductivities have similar trend. It is finally proposed that the thermal conductivity of the binary nanoemulsion could be enhanced by adding nano-sized droplets of n-decane oil, which has a lower thermal conductivity than that of the base fluid.Copyright © 2010 by ASME

The objectives of this study are to investigate the combined heat and mass transfer enhancement using binary nanofluids as a working fluid in a H2O/LiBr absorber. The result of heat and mass transfer experiment with the additives (arabicgum, 2E1H) showed that the heat and mass transfer performance of binary nanofluid with 2E1H enhanced significantly. In the case of 0.01 wt% Al2O3 binary nanofluids with 2E1H, the vapor absorption rate increased up to 77% in comparison with that without the additives. The heat transfer rate of 0.01 wt% Al2O3 binary nanofluids with 2E1H increased up to 19%. Based on the experimental results, it is recommended that the Al2O3 binary nanofluid be good with 2E1H to improve the heat and mass transfer performance.

Binary nanoemulsions, oil-droplet suspensions in binary solution (H2 O/LiBr), are developed to enhance the heat and mass transfer performance of absorption refrigeration cycles. This paper studies the formation and stability of n-decane in H2 O/LiBr nanoemulsions produced by using polyoxyethylene lauryl ether and polyoxyethylene sorbitan monolaurate as surfactant. To stabilize the nanoemulsions in a strong electrolyte, polymeric stabilizer (gum Arabic) is used as a steric stabilizer. The droplet size and the thermal conductivity of binary nanoemulsions are measured by the dynamic light scattering method and the transient hot-wire method, respectively. It is found that the effective thermal conductivity of binary nanoemulsions (1.0 vol % of n-decane in 30 wt % H2 O/LiBr) enhances up to 3.59% with the average droplet size of 44.3 nm. The stability has more significant effect on the thermal conductivity enhancement than the initial drop size.Copyright © 2009 by ASME

This paper develops a flooding correlation from experiments in a fluted tube with a twisted insert for application to absorption heat pump systems. Fluted tubes have been used to enhance both heat and mass transfer during absorption and desorption processes. In the present tests, a plastic twisted tape was inserted to increase the absorption rate in counter current absorption. The effects of the twisted insert and the angle of inclination on the flooding were examined. Water and ethyl alcohol solution flows downward in the fluted tube while air flows counter current in an upward direction. The flooding mechanism in a fluted tube with a twisted insert was analyzed by visual observation and experimental correlations for the flooding for both vertical and nearly horizontal positions were developed. The experimental data from this paper were compared with those for smooth tubes from the literature. The results show that the flooding initiated from the bottom for the vertical tube and from the top of the tube for the nearly horizontal tube. The fluctuation of the pressure drop for the vertical tube was much stronger than that for the horizontal tube. The effect of the position of the insert was not significant for the nearly horizontal tube while it was somewhat significant for the vertical tube. The superficial flooding vapor velocity for the nearly horizontal position is lower than that for the vertical position for a given liquid mass flow rate.

The objectives of this paper are to develop a new method of wettability measurement and to study the effect of surface roughness on the wettability in a LiBr/H2O falling film absorber. Three absorber tubes with micro-scale roughness are tested in a falling film absorber installed in a test rig. Inlet solution temperature (30–50°C), concentration (55–62 weight % of LiBr) and mass flow rate (0.74–2.71 kg/min) are considered as key parameters. A new method is proposed to estimate the wettability of a tube by measuring a minimum mass flow rate to wet the tube completely. Reynolds number is ranged from 30 to 120 by controlling the inlet mass flow rate. The wettability for the structured surfaces was higher than that for the bare tube. The wettability decreased linearly along the vertical location as low as 65% for the bare tube and 75% for the micro-scale scratched tubes. The wettability increased with increasing the solution temperature and the solution mass flow rate. The effect of film Reynolds number on the wettability in the bare tubes was more significant man that in the micro-scale scratched tubes. The experimental results of wettability can be used in the design of absorbers with micro-scale roughness.

The objective of this paper is to establish a model for inducement of Marangoni convection in aqueous LiBr solution with heat transfer additives. Basic mechanism of Marangoni convection was introduced from the viewpoints of the surface tension and the interfacial tension gradients. The Wilhelmy plate method was used to measure the surface tension and the interfacial tension of various heat transfer additives such as n-Octanol, 2-Octanol, 3-Octanol, 4-Octanol, n-Decanol, 2-Decanol, 3-Decanol and 2-ethly-1-hexanol. It was found that the salting-out model can not be a general criterion for inducement of Marangoni convection. The salting-out effect is a trigger for inducement of Marangoni convection before the additive solubility while the imbalance of the surface tension and the interfacial tension is a trigger for inducement of Marangoni convection after the solubility. The surface tension gradients by the solution temperature is not a trigger for inducement of Marangoni convection. The interfacial tension gradients by the LiBr concentration and the solution temperature can not be an initial cause for inducement of Marangoni convection. However, the magnitude of the interfacial tension plays an important role for inducement of Marangoni convection.

The colors of thin and thick layers of graphene and graphene oxide films on either SiO 2 or Si 3 N 4 grown silicon substrates were generated by a theoretical calculation procedure. The effects of the thicknesses of the material and the dielectric layers on the visibility of the graphene-based materials were investigated. The theoretical investigation was supplemented by measurements of the thicknesses of the material layers using either an atomic force microscope or a profilometer, depending on the thickness range. By combining the color calculation procedure with the measured thickness profiles, optical images of graphene-based materials on dielectric substrates can be reconstructed. The reconstructed image corresponds well to the real microscope image, which suggests that the image reconstruction procedure is a convenient way to investigate colors and determine the thickness of graphene-based materials.

It has been experimentally demonstrated that suspensions of carbon nanotubes (CNTs) and nanofibers (CNFs) significantly increase the thermal conductivity of nanofluids; however, a physically sound theory of the underlying phenomenon is still missing. In this study, the nonlinear nature of the effective thermal conductivity enhancement with the particle concentration of CNT and CNF nanofluids is explained physically using the excluded volume concept. Specifically, the number of contacting CNTs and CNFs could be calculated by using the excluded volume concept, where the distance for heat to travel in a cylinder between the contacting cylinders in the thermal network of percolating CNTs and CNFs increased with the excluded volume. In contrast to the effective thermal conductivity model of Sastry et al (2008 Nanotechnology 19 055704) the present revised model could reproduce the nonlinear increase of the thermal conductivity with particle concentration, as well as the dependence on the diameter and aspect ratio of the CNTs and CNFs. It was found that the alignment of CNTs and CNFs due to the long range repulsion force decreases the excluded volume, leading to both the convex and concave nonlinear as well as linear increase of the thermal conductivity with particle concentration. The difference between various carrier fluids of the suspensions could be explained as the result of the change in the excluded volume in different base fluids.

This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

The enhancement of bendability of flexible nanoelectronics is critically important to realize future portable and wearable nanoelectronics for personal and military purposes. Because there is an enormous variety of materials and structures that are used for flexible nanoelectronic devices, a governing design rule for optimizing the bendability of these nanodevices is required. In this article, we suggest a design rule to optimize the bendability of flexible nanoelectronics through neutral axis (NA) engineering. In flexible optical nanoelectronics, transparent electrodes such as indium tin oxide (ITO) are usually the most fragile under an external load because of their brittleness. Therefore, we representatively focus on the bendability of ITO which has been widely used as transparent electrodes, and the NA is controlled by employing a buffer layer on the ITO layer. First, we independently investigate the effect of the thickness and elastic modulus of a buffer layer on the bendability of an ITO film. Then, we develop a design rule for the bendability optimization of flexible optical nanoelectronics. Because NA is determined by considering both the thickness and elastic modulus of a buffer layer, the design rule is conceived to be applicable regardless of the material and thickness that are used for the buffer layer. Finally, our design rule is applied to optimize the bendability of an organic solar cell, which allows the bending radius to reach about 1 mm. Our design rule is thus expected to provide a great strategy to enhance the bending performance of a variety of flexible nanoelectronics.