Low-temperature Vapor Research Articles

Investigation of the Liquid–Vapor Separator Efficiency on the Performance of the Ejector Used as an Expansion Device in the Vapor-Compression Refrigeration Cycle

Abstract Ejector expansion refrigeration cycle with reference to the constant pressure mixing theory is investigated to display the effects of the liquid–vapor separator efficiency on the performance, entrainment ratio, and area ratio at various operation conditions. Reversible ejector assumption is used for the highest theoretical performance limit, whereas efficiency of the liquid–vapor separator and all ejector components is added to the model to calculate more realistic performance improvement potentials. R1234yf and R1234ze(E) having low global warming potential values are used in the analyses. Zero-dimensional thermodynamic models are constructed applying the conservation equations between the inlets and outlets of the refrigeration cycle and ejector components. Percentage performance decrease is higher when the mixing section and the separator efficiency is added to the model at higher condenser temperatures compared with the lower evaporator temperatures according to the investigated operation ranges. Vapor and liquid separation efficiency affects not only the performance but also the design of the ejector although it is an external component since it has influence on the area ratio and entrainment ratio. Finally, the difference between the percentage performance improvement of the reversible ejector cycle and the realistic ejector cycle including the separator and ejector components efficiencies is as high as 35% at the highest investigated condenser temperature for R1234yf.

Exergoeconomic investigation of a multi-generation system with CO2 as the working fluid using waste heat

A combined cycle coupling a supercritical power cycle with a transcritical refrigeration cycle with carbon dioxide as working fluid is evaluated and optimized from the exergoeconomic viewpoint. The system is designed to produce power, refrigeration, and heating simultaneously. If no net power is generated, the system can be described as a cogeneration. First, the cogeneration system working with various evaporation temperatures is optimized for having the lowest average cost of the products, which indicates that the system is preferred to operate at the lower evaporation temperature. Then for tri-generation, by increasing the power output and the pressure merging two sub-systems, the cost of the products (average and individual) reduces and the efficiency of the overall system increases. In addition, the cost of the power is the lowest while the cost of the refrigeration is the highest, and the cost of the heating is sensitive to the operation conditions of the overall system.

High temperature synthesis and characterization of ultrathin tellurium nanostructures

Thin tellurium (Te) has been predicted as a potential two dimensional system exhibiting superior thermoelectric and electrical properties. Here, we report the synthesis of high quality ultrathin Te nanostructures and the study of their electrical properties at room temperature. High quality ultrathin Te nanostructures are obtained by high temperature vapor phase deposition on c-plane sapphire substrates. The obtained nanostructures are as thin as 3 nm and exhibit α-Te phase with trigonal crystal structure. Room temperature electrical measurements show significantly higher electrical conductivity compared to prior reports of Te in bulk form or in nanostructure form synthesized by low temperature vapor deposition or wet chemical methods. Additionally, these nanostructures exhibit high field effect hole mobility comparable to black-phosphorous measured previously under similar conditions.

Experimental investigation on NH3–H2O generator-absorber heat exchange (GAX) absorption heat pump

Compared with conventional fuel-direct-burning heating systems, heat-driven absorption heat pump has high efficiency. To further improve the heating performance, generator-absorber heat exchange (GAX) absorption heat pump was developed, but its experimental research was not enough. The parameters tested were usually just COP and heating capacity in the existing literature, the variation of some internal parameters were rarely measured and analyzed. Besides, the latent heat exchange between the generator and the absorber could not continue normally under high thermal lift (TCON-TEVA) and low thermal thrust (TGEN-TABS), instead, more sensible heat is transferred, the temperature overlap decreases or even disappears, and the circulation process of GAX cycle approaches to that of single-effect cycle. This phenomenon is named “cycle degradation”, and is crucial to understand the working principle of GAX for optimal design and control. The cycle degradation was only simulated under low evaporation temperature, but has not been analyzed with experimental data, the influence rules of other external operational parameters on the phenomenon are also not clear. Therefore, a prototype of GAX absorption heat pump is built up, many internal and external parameters are measured and used to investigate the cycle degradation and overall performance under different testing conditions. During the experiment, the COP is in the range of 1.185–1.506, the cycle degradation occurs when the “lifting factor (Eq [18])” is lower than 2.3. Moreover, the performance improves with the increase of driving temperature or low-grade heat source temperature, and deteriorates due to the rise of supply water temperature.

A simulation and experimental study on primary cooler for COG heat pump

In this paper, a new type of integrated primary cooler that uses the low-temperature residual heat of coke oven gas to drive a lithium bromide absorption heat pump to cool the gas is proposed, and it can replace an evaporative cooler. The effect of the key parameters of the absorption heat pump, including refrigerant evaporation pressure, refrigerant concentration, refrigerant circulation, and low-temperature water evaporation pressure, on the efficiency of the primary cooler were studied with Aspen. Based on the optimization of the operation parameters, a verification experiment was carried out. The total average error from all the data was 5.76%, which indicates that the experimental and simulation results are in good agreement, and the heat pump primary cooling apparatus has prospects for broad application.

The self-protection effect of reactant gas on the moisture stability of CuSAPO-34 catalyst for NH3-SCR

The surface of CuSAPO-34 catalysts after NH3-SCR reaction (T ≤ 400 °C) is revealed to have residual adspecies. Cycle hydration methods are designed to explore the influence of the residual species upon the moisture stability of CuSAPO-34. It is demonstrated that the residual species on the catalyst surface are NH3 adspecies, which could help inhibit the transformation of Si(OAl)4 species to Si islands and prevent the decay of Brönsted acid sites (BAS) during the hydration treatment; the better preserved acidity and the existence of NH3 facilitate the preservation of isolated Cu2+ ions and their reducibility, and thus the SCR activity of the catalysts. The protective effect of residual NH3 species against the low-temperature water vapor is prominent when T ≤ 300 °C, which declines gradually and becomes weak when T > 400 °C due to the desorption of NH3 adspecies. The present work implies that the moisture stability of CuSAPO-34 catalyst could be simply ameliorated by locating it in an appropriate zone of the tailpipe to avoid the excessive desorption of the NH3 adspecies.

Sustainable desalination process selection: Decision support framework under hybrid information

Desalination technology has been recognized as a promising solution for mitigating the pressure caused by water shortage. In order to help the decision-makers to select the most sustainable desalination process/technology, a multi-criteria decision analysis method under hybrid information was proposed for sustainability ranking of alternative desalination processes. The interval AHP (Analytic Hierarchy Process) method was employed to determine the weights of the evaluation, and the TOPSIS (Technique for Order Preference by Similarity to an Ideal Solution) under hybrid information was employed to determine the sustainability ranking of the alternative desalination processes. Comparing with the previous studies about desalination process selection, the proposed method can achieve desalination selection based on the decision-making matrix composed by multiple types of data (i.e. crisp numbers, interval numbers and fuzzy numbers). Accordingly, the challenges of uncertainties and the quantification on the soft criteria can be successfully solved. The results based on the developed model can facilitate the decision-makers to select the most sustainability desalination technology with the considerations of economic, environmental. Technological and social pillars simultaneously. Four alternative desalination processes including multi-stage flash system (MSF), low temperature multi-effect distillation (LT-MED), reverse osmosis (RO), and vapor compression distillation (VCD) were studied by the proposed method in this study, and the closeness degree (CD) of LT-MED which represents its integrated sustainability performance is 0.6984, followed by the CDs of MSF (0.5602), VCD (0.5381) and RO (0.4116), respectively. Thus, the sustainability ranking from the best to the worst is LT-MED, MSF, VCD and RO. The results were validated by the sum weighted method, and sensitivity analysis was also carried out. The results reveal that the selection of LT-MED as the most sustainable is robust.

Dehydration Pathways of Gypsum and the Rehydration Mechanism of Soluble Anhydrite γ-CaSO4.

The dehydration products of gypsum under different temperature and water vapor pressure were investigated by thermodynamic theory. Additionally, the rehydration mechanism of soluble anhydrite was also studied by Monte Carlo (MC) simulations. The thermodynamic calculation results reveal that the dehydration mechanism of gypsum significantly depended on ambient temperature and water vapor pressure. In the high-temperature and low water vapor pressure region, gypsum dehydrates to form γ-CaSO4 in a single-step process (CaSO4·2H2O → γ-CaSO4); with increasing water vapor pressure, gypsum undergoes the CaSO4·2H2O → γ-CaSO4 → β-CaSO4·0.5H2O reaction path and as water vapor pressure increases further, the occurrence of a two-step conversion path CaSO4·2H2O → β-CaSO4·0.5H2O → γ-CaSO4 was observed. It was also found that gypsum is stable in the low-temperature and high water vapor pressure region and does not dehydrate to form any calcium sulfate hemihydrate. Finally, the rehydration mechanism of soluble anhydrite was studied by MC simulations. The simulation results are in agreement with the experimental data and support the finding that γ-CaSO4 rehydration forms CaSO4·0.67H2O in high relative humidity. Another important result revealed by the MC simulation is that γ-CaSO4 has an extraordinary ability to capture water molecules from an extremely dry atmosphere, which is very useful in some fields, such as in drying processes and even for extracting liquid water from extremely dry atmosphere.

Dynamics and useful heat of the discharge stage of adsorptive cycles for long term thermal storage

Interest towards adsorption heat storage, especially for long-term (seasonal) applications, is growing. The previous studies have always treated the heat storage cycle as a fully temperature-initiated process, similarly to common adsorption cooling and heating cycles. However, in long term storage applications, the discharge stage of the cycle is initiated by a jump of pressure rather than by a traditional drop of temperature. This requires specific investigations on the useful heat recoverable as well as on the adsorption dynamics.In the present paper, an appropriate experimental methodology was applied for studying the heat discharge stage. Three well known adsorbents (Mitsubishi AQSOA FAM-Z02, silica Siogel and composite LiCl/silica) were experimentally tested in a lab-scale heat storage unit, evaluating the effect of cycle operating parameters. The results, elaborated in terms of useful heat recoverable from the charged adsorbent, highlighted that the evaporation temperature and the flow rate of heat transfer fluid have a great influence on the adsorption dynamics and the useful heat. For the silica gel and FAM Z02, the maximum heat storage capacity 450 kJ/kg is reached at the evaporation temperature of 25 °C. The composite performs better at low evaporation temperatures, allowing heat upgrade even at 5 °C. The flow rate of the heat transfer fluid has a more significant effect on FAM Z02 than on the other adsorbents, for which an optimal flow rate of 1.2 kg/min was found.

Synthesis, growth, optical, photoconductivity and specific heat properties of potassium borodicitrate (KBDC) single crystal

Nonlinear optical high-quality single crystals of potassium borodicitrate (KBDC) were grown by low-temperature solution growth slow evaporation technique. The crystal structure and quality of the grown crystal were examined by single-crystal X-ray diffraction and high-resolution X-ray diffraction analysis, respectively. The transmittance spectrum measured from the as-grown crystal exhibits wide range of transparency from UV to visible region. The third-order nonlinear optical properties of the grown crystal were studied using Z-scan technique. The third-order nonlinear susceptibility (χ3) of KBDC single crystal is found to be 4.1130 × 10−9 esu. The laser-induced damage threshold for the grown crystal was measured using Nd:YAG laser. Furthermore, the photoconductivity and specific heat capacity of the crystal were measured, and the observed results are discussed in detail.

Optimization of pulsed current in resistance spot welding of Zn-coated hot-stamped boron steels

Metallic coatings have been widely used for hot-stamped boron steel in automotive applications. Surface coating in hot-stamped boron steels has improved corrosion resistance. However, a thick coating layer degrades resistance spot weldability. In the case of a Zn-coated hot-stamped boron steel, the high contact resistance and material stiffness with low vaporization temperature of the alloyed layers result in a narrow current passage at the faying surface. Expulsion occurs at low current due to rapid heat development, resulting in a narrow weld current range. The pulsed current can control the heat input by applying a fast cool time during welding to improve the mechanical properties of the joint and acceptable current range. In this study, Zn-coated hot-stamped steels were resistance spot welded using various pulse current procedures, and welding condition was optimized via three pulsed current steps. Step 1: Contact area was controlled without nugget formation. Step 2: Nugget growth was minimized and contact area was maximized. Step 3: The maximum acceptable current range and nugget diameter were achieved. These welding conditions improved the acceptable current range and mechanical properties of spot welds due to increased spot weld size.

Thermodynamic analysis and comparison of four insulation schemes for liquid hydrogen storage tank

Hydrogen has more energy per unit mass (141.8 MJ/kg) than any other fuel but also has the lowest gaseous density (0.084 kg/m3), and liquid hydrogen (LH2) storage is a solution with high energy density. However, LH2 storage has the characteristics of low temperature (20 K) and easy evaporation, putting forward higher requirements for insulation system. At present, the improvement and optimization of insulation system consisting of spray on foam insulation (SOFI), multilayer insulation (MLI) and variable density MLI (VDMLI) is a hot topic. Considering the considerable sensible heat of hydrogen, this paper introduces self-evaporation vapor cooled shield (VCS) into the above-mentioned insulation systems to fully recover its sensible heat to improve insulation performance. A thermodynamic model has been established to study the insulation properties of composite insulation system for LH2 tank, and the results have good agreement with test data. Coupling effects among them are analyzed and the optimization strategies of MLI, VDMLI and VCS are quantitatively explained to obtain better insulation performance. The heat flux and temperature profile of four insulation structures (MLI, VDMLI, MLI + VCS and VDMLI + VCS) have also been quantitatively analyzed, which can provide guidance for insulation system optimization. The insulation performance of composite insulation system under different vacuum conditions have also been compared.

Mechanical Properties of Films of Graphene Oxide Doped with Chitosan

Films of graphene oxide containing 3 wt % chitosan are obtained via the low-temperature evaporation of a stable mixture of aqueous suspensions. The obtained films are characterized via elemental analysis, IR spectroscopy, and X-ray photoelectron spectroscopy. It is also found that introducing the additive substantially increases the mechanical strength and plasticity of the film. The ultimate tensile strength and elongation upon breaking are thus 85.8 MPa and 2.53% for the composite films, respectively. These values are higher than the ones for a pure film of graphene oxide (55.1 MPa and 0.83%, respectively).

The effects of Na on the growth of Cu2ZnSnSe4 thin films using low-temperature evaporation process

Cu2ZnSnSe4 (CZTSe) absorbers were deposited on borosilicate glass substrate using the low-temperature process, and different Na incorporation methods were applied to investigate the effects of Na on the CZTSe growth. Na was diffused into some of the absorbers after growth, which led to strongly improved device performance compared with Na-free cells. With the post-deposition treatment, the effect of Na on CZTSe growth was excluded, and most of Na was expected to reside at grain boundaries. The conversion efficiency of the completed device was improved due to the enhancement of open circuit voltage and fill factor. The efficiency of 2.85% was achieved at substrate temperature as low as 420 °C.

Geyser boiling in a thermosyphon with nanofluids and surfactant solution

Experiments with a two-phase closed thermosyphon are carried out with the aim of investigating the influence of different working fluids on geysering. The length of the analysed device is 1.800 mm and its inner diameter is 20 mm. It is filled with various working fluids: from pure deionised-water and deionised-water plus sodium dodecyl sulphate (surfactant) as reference fluids to two different gold nanofluids, a silica nanofluid, and a nanofluid with nanohorn nanoparticles. Thermal performance of the thermosyphon is investigated with respect to time averaged parameters, such as amount of heat transferred, thermal resistance, and temperature distribution in the working fluid sump, but also with respect to geysering frequency and time dependent internal pressure. The inlet temperature of heating water is controlled between 35 °C and 85 °C and of the cooling medium between 15 °C and 25 °C.The most important conclusion is that geysering is strongly affected not only by nanoparticles, but also by chemical stabilisers like sodium dodecyl sulphate or polyvinylpyrrolidone. It is shown experimentally that nanofluids consisting of base fluid, chemical stabiliser, and nanoparticles in some cases diminish or even completely suppress geysering. Effects related to the nanoparticles alter the solid-vapour interface (a nanoporous layer on the evaporator wall) and the vapour-liquid interface (bubble surface). The influence of nanoparticles is restricted to the evaporator section. Replacing water with one of the other studied solutions/suspensions shifts the working point of the thermosyphon, changing the amount of transferred heat (increase even up to 80% for low evaporator temperature) and operating temperature levels.

A nanopump for low-temperature and efficient solar water evaporation

Zr(Ti)O2 nanotube membranes with superhydrophilic vertical channels and strong light absorption function as a nanopump for solar water evaporation.

The Effect of Filler Content on the IR Spectra of Poly(p-xylylene)–Sulfide Nanocomposites

The effect of the nature of a filler and its concentration (C ~ 0–100 vol %) on the IR spectra (500–4000 cm–1) of thin nanocomposite films based on poly(p-xylylene) and sulfides (PPX–PbS, PPX–CdS, and PPX–S) was studied. Films with a thickness d ~ 0.5 μm are produced via low-temperature vapor deposition polymerization on silicon substrates. The common and specific variations in the IR-spectra of nanocomposite films are identified. The low-frequency shifts of C–H out-of-plane deformation vibrations of aromatic rings and the changes in characteristic band intensities of PPX are observed with increasing filler concentration. Additional IR-bands arise at 1000–1800 cm–1 for PPX–PbS and PPX–CdS films and at 3100–3600 cm–1 for PPX–CdS ones. However, the bands attributed to C–C and C–H stretch vibrations of the ring and С–Н deformation vibrations of СН2-groups are found to lose much of their intensity in PPX–S films. The occurrence of complementary IR bands in nanocomposites is due to the formation of (1) complex sulfoxide phases with PbS nanoparticles, (2) C–O and C=O groups because of the oxidation of PPX polymer chains in the PPX–PbS films, and (3) complex compounds with sulfo (SO4), hydroxyl (OH), and carboxylate (СОО–) groups in the PPX–CdS films. Furthermore, some IR bands in PPX–PbS and PPX–CdS films are structural modifications of the substituted aromatic ring due to loss of symmetry.

Synthesis of few-layer 2H-MoSe2 thin films with wafer-level homogeneity for high-performance photodetector

Abstract The unique structural and physical properties of two-dimensional (2D) atomic layer semiconductors render them promising candidates for electronic or optoelectronic devices. However, the lack of efficient and stable approaches to synthesize large-area thin films with excellent uniformity hinders their realistic applications. In this work, we reported a method involving atomic layer deposition and a chemical vapor deposition chamber to produce few-layer 2H-MoSe2 thin films with wafer-level uniformity. The reduction of MoO3 was found indispensable for the successful synthesis of MoSe2 films due to the low vaporization temperature. Moreover, a metal-semiconductor-metal photodetector (PD) was fabricated and investigated systematically. We extracted an ultrahigh photoresponsivity approaching 101 A/W with concomitantly high external quantum efficiency up to 19,668% due to the produced gain arising from the holes trapped at the metal/MoSe2 interface, the band tail state contribution, and the photogating effect. A fast response time of 22 ms was observed and attributed to effective nonequilibrium carrier recombination. Additionally, the ultrahigh photoresponsivity and low dark current that originated from Schottky barrier resulted in a record-high specific detectivity of up to 2×1013 Jones for 2D MoSe2/MoS2 PDs. Our findings revealed a pathway for the development of high-performance PDs based on 2D MoSe2 that are inexpensive, large area, and suitable for mass production and contribute to a deep understanding of the photoconductivity mechanisms in atomically thin MoSe2. We anticipate that these results are generalizable to other layer semiconductors as well.

Proposal and thermodynamic analysis of a combined open-cycle absorption heat pump and thermal desalination system driven by high-humidity exhaust gas

A combined system based on an open-cycle absorption heat pump (OAHP) and a low-temperature multi-effect evaporation (LT-MEE) water desalination process is configured, modeled, and analyzed. The system is applicable to be run by high-humidity gases, and the flue gas from the combustion of natural gas is used here. Compared with conventional absorption heat pumps, the OAHP is characterized by a gas-solution direct-contact absorber, where not only the sensible heat but also the latent heat and water vapor in flue gas are partially recovered. The OAHP-MEE system has good internal synergy, as demonstrated by a 36%–74% water production gain over the simple system when driven by flue gas with temperature of 120 °C–250 °C. With the increase of the feed-gas temperature, the combined system becomes increasingly favorable over the simple system in energy utilization and water production. For a specified feed gas condition, an optimal water recovery rate of OAHP and a maximum number of effects of MEE exist, mutually leading to a maximum water output. Using a cooler to cool the strong solution entering the absorber not only has a favorable effect on water production, but also widens the parameters' ranges, thus offering good flexibility for design and operation.

Flammability parameters of liquified natural gas

The use of liquefied natural gas (LNG) is constantly growing. However, safety issues regarding cryogenic storage and transportation systems are still to be fully resolved. In particular, the evaluation of the efficiency of inerting systems for low-temperature LNG vapour is essential. In this work, the variation of the flammability range (in terms of lower and upper flammability limits and minimum oxygen concentration) obtained by adding nitrogen to pure air for some representative LNG mixtures has been evaluated at ambient temperature and at temperatures below 0 °C by using a detailed kinetic model entitled KIBO, which has been proved to be reliable for the description of C0C4 reactions in oxidative conditions, and by the limiting burning velocity theory.Strong differences are reported among pure methane and natural gas mixtures for all the investigated temperatures. The effect of composition is therefore relevant. Furthermore, the obtained results suggest that the lower flammability limit is determined by thermal aspects at high temperature only, whereas at low temperature, kinetic limitations are more relevant.