Characteristics of heat and mass transfer in vapor absorption of falling film flow on a horizontal tube

The absorber is an important component in absorption machines and its characteristics have significant effects on the overall efficiency of absorption machines. This article reports on the results of numerical studies on the characteristics of falling film LiBr–H₂O solution on a completely wetted horizontal tube and the associated vapor absorption in the Reynolds number range of 5 < Re < 100. The boundary layer assumptions are used for the transport of mass, momentum and energy equations and the finite difference method is employed to solve the governing equations in the film flow. The heat and mass transfer coefficients are expressed in the forms of Reynolds number, Prandtl number and Schmidt number, at the usual absorber pressure and inlet solution concentration. The results can be used as a reference in designing actual absorption chiller.     

Comparison of numerical and experimental performance criteria of an ammonia–water bubble absorber using plate heat exchangers

A mathematical model for ammonia–water bubble absorbers was developed and compared with experimental data using a plate heat exchanger. The analysis was performed carrying out a sensitive study of selected operation parameters on the absorber thermal load and mass absorption flux. Regarding the experimental data, the values obtained for the solution heat transfer were in the range 0.51–1.21 kW m^?2 K^?1 and those of the mass absorption flux in the range 2.5–5.0 × 10^?3 kg m^?2 s^?1. The comparison between experimental and simulation results was acceptable being the maximum difference of 11.1% and 28.4% for the absorber thermal load and the mass absorption flux, respectively.     

Experimental study of an ammonia–water bubble absorber using a plate heat exchanger for absorption refrigeration machines

The development of absorption chillers activated by renewable heat sources has increased due mainly to the increase in primary energy consumption that causes problems such as greenhouse gases and air pollution among others. These machines, which could be a good substitute for compression systems, could be used in the residential and food sectors which require a great variety of refrigeration conditions. Nevertheless, the low efficiency of these machines makes it necessary to enhance heat and mass transfer processes in the critical components, mainly the absorber, in order to reduce their large size.This study used ammonia–water as the working fluid to look at how absorption takes place in a plate heat exchanger operating under typical conditions of absorption chillers, driven by low temperature heat sources. Experiments were carried out using a corrugated plate heat exchanger model NB51, with three channels, where ammonia vapor was injected in bubble mode into the solution in the central channel. The results achieved for the absorption flux were in the range of 0.0025–0.0063 kg m^?2 s^?1, the solution heat transfer coefficient varied between 2.7 and 5.4 kW m^?2 K^?1, the absorber thermal load from 0.5 to 1.3 kW. In addition, the effect of the absorber operating conditions on the most significant efficiency parameters was analyzed. The increase in pressure, solution and cooling flow rates positively affect the absorber performance, on the other hand an increase in the concentration, cooling, and solution temperature negatively affects the absorber performance.     

Absorption of organic fluid mixtures in plate heat exchangers

It is well known that the absorber is the key component in energy conversion systems that are based on absorption cycles. This paper describes an experimental investigation into the absorption process of organic fluid mixtures in an absorption system which has a spray and a plate heat exchanger. The absorber consists of an adiabatic mixing chamber with a spray, where the solution that is weak in refrigerant is sprayed into the refrigerant vapour. A two-phase mixture is formed and enters a plate heat exchanger, where the solution is cooled to complete the absorption process.We carried out experiments with different types of spray nozzles using the organic fluid mixtures methanol–tetraethyleneglycol dimethylether (TEGDME) and trifluoroethanol (TFE)–TEGDME. We analyse how the solution mass flow rate, absorber pressure and cooling water temperature affected the absorber performance and we discuss the results in terms of the absorber load, absorbed mass flux, degree of subcooling of the solution at the absorber outlet, solution film heat and mass transfer coefficients.The results indicate that the absorption system proposed is suitable for relatively low pressures. For water temperatures of 30 °C and absorber pressures between 2 and 6 kPa, the absorption rates for TFE–TEGDME were 1 to 2.5 g·s⁻¹·m⁻². The corresponding values for methanol–TEGDME with absorber pressures between 10 and 15 kPa were 0.4 to 1.2 g·s⁻¹·m⁻².     

Light-trapping enhancement based on Ga2O3 nano-islands coated glass substrate

Nano-size textured topography is important to gain high absorption ratio of the incoming light for thin film solar cells. In this paper, a Ga₂O₃ nano-islands coated glass substrate is introduced by annealing Ga nano-islands in air. The aspect ratio of the islands can be easily controlled by tuning the average thickness of Ga film. Based on the proposed substrate, with average horizontal size of 500 nm and root-mean-square roughness of 80 nm of the nano-islands, the average reflectivity under AM 1.5 illumination spectrum can be limited at 6.6% when Si absorber layer is only 480 nm thick.     

Electrophoretic deposition of binary energetic composites

This work utilizes electrophoretic deposition (EPD) as a facile and effective method to deposit binary energetic composites. In particular, micron-scale aluminum and nano-scale copper oxide were co-deposited as a thin film onto a conductive substrate without the use of surfactants. For comparative purposes, films of this energetic mixture were also prepared by drop-casting (DC) the premixed suspension directly onto the substrate, then allowing the liquid to dry. The structure and microscopic features of the two types of films were compared using optical and electron microscopies. The films prepared using EPD had an appreciable density of 2.6 g/cm³, or 51% the theoretical maximum density, which was achieved without any further processing. According to the electron microscopy analysis, the EPD films exhibited much more uniformity in composition and film thickness than those produced by DC. Upon ignition, the EPD films resulted in a smoother and faster combustion event compared to the DC films. The dispersion stability was improved by adding water and decreasing the particle concentration, resulting in dispersions stable for >30 min, an ample amount of time for EPD. Patterned electrodes with fine feature sizes (20 × 0.25 mm) were then combined with EPD to deposit thin films of thermite for flame propagation velocity studies. The fastest velocity (1.7 m/s) was observed for an equivalence ratio of 1.6 ± 0.2 (Al fuel rich composition). This peak value was used to investigate the effect of film mass/thickness on propagation velocity. The deposition mass was varied from 20 to 213 μg/mm², corresponding to a calculated range of film thicknesses from 9.8 to 104 μm. At lower masses, a flame did not propagate, indicating a critical mass (20 μg/mm²) or thickness (9.8 μm). Over the range of thicknesses, in which self-propagating combustion was observed, the flame velocity was found to be independent of sample thickness. The lack of a thickness dependence suggests that under these particular conditions heat losses are negligible, and thus the velocity is predominantly governed by the intrinsic reactivity and heat transfer through the material.     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

Drop formation of swirl-jet nozzles with high viscous solution in vacuum-new absorbent in spray absorption refrigeration

In this paper, the drop formation properties of a lithium bromide salt solution Trane [Trane, Private communication, 1997, [1]], which is utilized in a new concept of spray absorber, is investigated. In the spray absorber of the absorption refrigeration cycles, the feasibility of forming droplets with an optimum diameter of 300 μm, calculated by the drop absorption model, were studied. To achieve above, a single nozzle spray chamber able to attain a low-pressure of 1.23 kPa (0.178 psia) pressure was built. The nozzles experimentally tested were swirl-jet nozzles. The differential pressure across the nozzles was varied from 50 to 200 kPa (7.25–29 psia). The flow rate in the experiment was varied between 0.018 and 0.043 kg/s (2.376–5.676 lb/min). The flow number that define the effective flow of the selected nozzles were 7.6 × 10^?7, 1.5 × 10^?6 and 2.3 × 10^?6 and the viscosity ratio of this disperse/continuous phase flow was 1300. The nozzles tested were able to produce drop sizes having a mean volumetric diameter (MVD) between 375 μm and 425 μm. Comparison of drop absorption model results to conventional absorber results shows a significant improvement in absorption.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

A thin film silicon anode for Li-ion batteries having a very large specific capacity and long cycle life

A thin film of Si was vacuum-deposited onto a 30 μm thick Ni foil from a source of n-type of Si, the film thickness examined being 200–1500 Å. Li insertion/extraction evaluation was performed mainly with cyclic voltammetry (CV) and constant current charge/discharge cycling in propylene carbonate (PC) containing 1 M LiClO₄ at ambient temperature. The cycleability and the Li accommodation capacity were found to depend on the film thickness. Thinner films gave larger accommodation capacity. A 500 Å thick Si film gave a charge capacity over 3500 mAh g⁻¹ being maintained during 200 cycles under 2 C charge/discharge rate, while a 1500 Å film revealed around 2200 mAh g⁻¹ during 200 cycles under 1 C rate. The initial charge loss could not be ignored but it could be reduced by controlling the deposition conditions.     

Experimental evaluation of a low-power direct air-cooled double-effect LiBr–H2O absorption prototype

This paper describes a new small air-cooled double-effect LiBr–H₂O absorption prototype directly powered by fuel and discusses the experimental findings for some tests carried out in Madrid in 2007, with natural gas as energy source. The prototype, which has been designed to supply 7 kW of cooling power, was able to chill water up to 7–18 °C under extreme outdoor temperatures. A new flat-sheet adiabatic absorber was used allowing it to operate at outdoor temperatures about 45 °C without any sign of crystallization. A mean daily coefficient of performance (COP) of about 1.05 was obtained. Since this absorption machine does not need cooling tower, there is neither water consumption nor Legionella pollution. Moreover, it is a quite compact unit. The ratio of cooling power over volume is about 6.0 kW/m³, while for the only air-cooled absorption chiller, Rotartica 045v, in the marked until 2009 this ratio is 4 kW/m³. When comparing with electric chillers presently on the market, this prototype was found to have a cooling cost approximately 15.9% higher and an environmental impact 16.7% lower. The absorption prototype is a more environmentally friendly solution as it does not emit fluorinated refrigerants.     

Ferritic stainless steels as bipolar plate material for polymer electrolyte membrane fuel cells

Both interfacial contact resistance (ICR) measurements and electrochemical corrosion techniques were applied to ferritic stainless steels in a solution simulating the environment of a bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC). Stainless steel samples of AISI434, AISI436, AISI441, AISI444, and AISI446 were studied, and the results suggest that AISI446 could be considered as a candidate bipolar plate material. In both polymer electrolyte membrane fuel cell anode and cathode environments, AISI446 steel underwent passivation and the passive films were very stable. An increase in the ICR between the steel and the carbon backing material due to the passive film formation was noted. The thickness of the passive film on AISI446 was estimated to be 2.6 nm for the film formed at −0.1 V in the simulated PEMFC anode environment and 3.0 nm for the film formed at 0.6 V in the simulated PEMFC cathode environment. Further improvement in the ICR will require some modification of the passive film, which is dominated by chromium oxide.     

Effect of chitosan on stabilization of acetates-containing solution: A novel precursor for LiMn2O4 film deposition

Due to the adequate viscosity of the chitosan-added precursor solutions, the films deposited from the chitosan-added precursor solution showed a higher deposition rate than the ones from the PVP-added solution under the same coating parameters. Furthermore, the chitosan-added precursor solution remained stable without any precipitation for at least 10 months. On the other hand, without the addition of chitosan, the precursor solution showed apparent precipitation after being stirred for 12 h. The enhanced stability of the precursor solution by the addition of chitosan is attributed to the complexation between metal ions and the –NH₂ groups of chitosan. And the electrochemical behavior for the deposited films calcined at 700 °C for 1 h was also characterized by charge–discharge test. The result revealed that the film deposited from chitosan-containing precursor solution possesses an initial discharge capacity of 134 mAh g⁻¹ and about 9% capacity loss after 50 charge/discharge cycles, which is better than the one deposited from chitosan-free precursor solution with an initial discharge capacity of 108 mAh g⁻¹ and 24% capacity loss after 50 cycles.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Experimental investigation of thermal performance of a double-flow solar air heater having aluminium cans

This study experimentally investigates a device for inserting an absorbing plate made of aluminium cans into the double-pass channel in a flat-plate solar air heater (SAH). This method substantially improves the collector efficiency by increasing the fluid velocity and enhancing the heat-transfer coefficient between the absorber plate and air. These types of collectors had been designed as a proposal to use aluminium materials to build absorber plates of SAHs at a suitable cost. The collector had been covered with a 4-mm single glass plate, in order to reduce convective loses to the atmosphere. Three different absorber plates had been designed and tested for experimental study. In the first type (Type I), cans had been staggered as zigzag on absorber plate, while in Type II they were arranged in order. Type III is a flat plate (without cans). Experiments had been performed for air mass flow rates of 0.03 kg/s and 0.05 kg/s. The highest efficiency had been obtained for Type I at 0.05 kg/s. Also, comparison between the thermal efficiency of the SAH tested in this study with the ones reported in the literature had been presented, and a good agreement had been found.     

Measurement and modeling of liquid film thickness evolution in stratified two-phase microchannel flows

Polymer electrolyte membrane (PEM) fuel cells incorporating microchannels (D < 500 μm) can benefit from improved fuel delivery and convective cooling. However, this requires a better understanding of two-phase microchannel transport phenomena, particularly liquid–gas interactions and liquid clogging in cathode air-delivery channels. This paper develops optical fluorescence imaging of water films in hydrophilic channels with varying air velocity and water injection rate. Micromachined silicon test structures with optical access and distributed water injection simulate the cathode channels of a PEM fuel cell. Film thickness data vary strongly with air velocity and are consistent with stratified flow modeling. This work facilitates the study of regime transitions in two-phase microchannel flows and the effects of flow regimes on heat and mass transfer and axial pressure gradients.     

Hydrodynamics of quenching with impinging free-surface jet

The hydrodynamics of jet impingement quenching of a stainless steel specimen has been studied experimentally. The specimen is heated to an initial temperature of about 900 °C and then quenched by a subcooled free-surface water jet. High-speed imaging shows that the free-surface of the water film in the wetted region is smooth. The water film outside the wetted region is deflected away from the surface and then breaks into droplets due to surface tension and shear forces. The splashed droplet velocity is found to be low up to a wetting front radius of 6 mm (r/d_J ≈ 2), beyond which it increases rapidly before reaching a constant value at a wetting front radius of about 8 to 10 mm (2.67 ? r/d_J ? 3.34). The water film velocity at the wetting front is calculated using the single-phase boundary layer model suggested by Watson [2]. At moderate subcooling, the splashed droplet velocity up to a wetting front radius of 10 mm (r/d_J ≈ 3.34) is found to be much lower than the estimated single-phase film velocity. The study shows that although the wetted region may appear devoid of any bubbles, strong two-phase flow occurs within this region.     

Electrochemical performance of amorphous-silicon thin films for lithium rechargeable batteries

The effect of deposition temperature and film thickness on the electrochemical performance of amorphous-Si thin films deposited on a copper foil is studied. The electrochemical properties show optimum conditions at 200 °C deposition, and thinner films exhibit superior electrochemical performance than thicker ones. A film of 200 nm Si deposited at 200 °C exhibits excellent cycleability with a specific capacity of ∼3000 mAh g⁻¹. This is probably due to optimization between the strong adhesion by Si/Cu interdiffusion and the film stress.     

Thermal and mechanical performance of sealed,gas-filled,flat plate solar collectors

The study includes calculations for both the thermal performance and the mechanical behaviour of a gas-filled, flat plate solar collector without external gas expansion, i.e., a collector with varying gas volume and gas pressure and movement in both cover glass and absorber. Classical theories for the thermal performance are combined with a finite-element method to investigate which factors have an impact from the mechanical stress point of view.This article describes major results for collectors with copper and aluminium absorbers combined with different inert gases. It is shown that a collector may be designed which uses less material than a standard collector but achieves at least the same thermal performance, by using a thinner collector and a thinner absorber and a suitable gas filling other than air. If copper is used in absorber and tubes, a 0.15 mm thick absorber together with a tube-to-tube distance of 103 mm results in the same performance as a 0.3 mm absorber with a 144 mm tube-to-tube distance, but the former will use 25% less material. The use of copper can be further reduced if the absorber is made of aluminium and the tubes are made of copper. The factor of safety for thick (>0.5 mm) aluminium absorbers is, however, not as large as it is for copper absorbers.