Two-phase flow of R-134a refrigerant during flow boiling through a horizontal circular mini-channel

This research focuses on heat transfer to R-134a during flow boiling in a 1.75 mm internal diameter tube. Flow visualisation and heat transfer experiments are conducted to obtain heat transfer coefficients for different flow patterns. The measured data in each flow regime are compared with predictions from a three-zone flow boiling model. The calculations are in fair agreement with the experimental results which correspond in particular to slug flow, throat-annular flow and churn flow regimes under conditions of low heat flux.     

Unveiling the Molecular Origin of Vapor-Liquid Phase Transition of Bulk and Confined Fluids

At temperatures below the critical temperature, discontinuities in the isotherms are one critical issue in the design and construction of separation units, affecting the level of confidence for a prediction of vapor–liquid equilibriums and phase transitions. In this work, we study the molecular mechanisms of fluids that involve the vapor–liquid phase transition in bulk and confinement, utilizing grand canonical (GCE) and meso-canonical (MCE) ensembles of the Monte Carlo simulation. Different geometries of the mesopores, including slit, cylindrical, and spherical, were studied. During phase transitions, condensation/evaporation hysteretic isotherms can be detected by GCE simulation, whereas employing MCE simulation allows us to investigate van der Waals (vdW) loop with a vapor spinodal point, intermediate states, and a liquid spinodal point in the isotherms. Depending on the system, the size of the simulation box, and the MCE method, we are able to identify three distinct groups of vdW-type isotherms for the first time: (1) a smooth S-shaped loop, (2) a stepwise S-shaped loop, and (3) a stepwise S-shaped loop with just a vertical segment. The first isotherm type is noticed in the bulk and pores having small box sizes, in which vapor and liquid phases are close and not clearly identified. The second and the third types occurred in the bulk, cylindrical, and slit mesopores with sufficiently large spaces, where vapor and liquid phases are distinctly separated. Results from our studies provide an insight analysis into vapor–liquid phase transitions, elucidating the effect of the confinement of fluid behaviors in a visual manner.     

A miniature chemiresistor sensor for carbon dioxide

A carpet-like nanostructure of polyaniline (PANI) nanothin film functionalized with poly(ethyleneimine), PEI, was used as a miniature chemiresistor sensor for detection of CO₂ at room temperature. Good sensing performance was observed upon exposing the PEI–PANI device to 50–5000 ppm CO₂ in presence of humidity with negligible interference from ammonia, carbon monoxide, methane and nitrogen dioxide. The sensing mechanism relied on acid–base reaction, CO₂ dissolution and amine-catalyzed hydration that yielded carbamates and carbonic acid for a subsequent pH detection. The sensing device showed reliable results in detecting an unknown concentration of CO₂ in air.     

Hybrid flow analyzer for automatic hollow-fiber-assisted ionic liquid-based liquid-phase microextraction with in-line membrane regeneration

The proof-of-concept of a new methodology for in-line hollow-fiber (HF)-assisted three-phase liquid-phase microextraction (LPME) allowing for handling of the feed and acceptor aqueous solutions and of minute volumes of the organic extracting phase in a programmable flow mode is reported in this paper. The flow analyzer fosters in-line anchoring of ionic-liquid-laden extracting solution (10 % (v/v) methyltrioctyl ammonium chloride in kerosene) in the pores of a single-strand microporous polypropylene HF, and regeneration of the liquid-phase membrane itself for each individual analysis cycle in a fully automated mode. Using hexavalent chromium as a model analyte and 1,5-diphenylcarbazide as a chromogenic probe in the acceptor solution, the flow-based HF-LPME hyphenated system was harnessed to the clean-up of troublesome samples (viz., domestic wastewater and soil leachates) with concomitant enrichment of target species. Distinct extraction modes and chemistries were assessed for enhanced Cr(VI) permeability. A single sample plug was subjected to a twofold backward–forward flow extraction so as to decrease the thickness of the boundary layer at the HF shell side for improved extraction efficiency. Under the optimized physicochemical variables, a limit of detection of 4.6 μg?L^?1 Cr(VI), a dynamic linear range of up to 500 μg?L^?1 and intermediate precision better than 10 % were obtained for a sample volume of 2.8 mL buffered at pH 4 and a volume of organic extractant of 120 μL, with an enrichment factor of ca. 11 for a sample residence time in the donor compartment of merely 4.5 min. Analyte recoveries in domestic wastewaters were ≥83 % using external calibration with relative standard deviations better than 14 %, thereby demonstrating the expedient clean-up of samples with elevated content of dissolved organic carbon. The automatic HF-LPME method was validated in terms of bias against the SRM 2701 (NIST soil) preceded by the EPA alkaline digestion method 3060A. No significant differences between Cr(VI) concentration as obtained with the automatic HF-LPME system (546?±?57 mg?kg^?1) and the certified value (551.2?±?17.2 mg?kg^?1, expressed as mean ± combined uncertainty) were encountered at the 0.05 significance level.     

Single strand hollow fiber membrane (SSHFM): an on-line sample preparation for the flow based colorimetric determination of free iron in fruit juices

A single strand hollow fiber membrane (SSHFM) was developed for the on-line sample preparation for the flow based colorimetric determination of free iron levels in fruit juices. The SSHFM, as used, could separate Fe²⁺ from some spectrophotometric interfering agents in the fruit juice, such as pigments, solid suspensions and polysaccharides. The screening process was likely to have been primarily based on dialysis, wherein only ions or molecules that are smaller than the pores of the membrane can diffuse through while relatively larger molecules or particles could not. Two flow modes, a continuous and stopped flow, were studied. Factors that influenced the sensitivity (%dialysis) of the method, such as the flow rate, sample volume, flow direction and stopped flow time, were optimized. The stopped flow mode was found to be relatively more sensitive than the continuous flow mode and displayed a linear range of 1-30 mg L⁻¹ Fe²⁺, a limit of detection of 0.5 mg L⁻¹, and a % relative standard deviation of less than 2% (n = 8) for the analysis of 10 mg L⁻¹ Fe²⁺ spiked grape juice samples. A sample throughput of 24 samples h⁻¹, was attained without any further sample treatment.     

Nanothin Polyaniline Film for Highly Sensitive Chemiresistive Gas Sensing

This study developed a facile technique for site‐specific synthesis of nanometer‐thick polyaniline (PANI) film for fabrication of field‐effect transistor/chemiresistor sensors. The nanothin film had a thickness of 9–20 nm and was of carpet‐like morphology offering a highly accessible surface geometry that enhanced gas adsorption and promoted surface reaction/interaction. When applied for sensing of ammonia (NH₃) and nitrogen dioxide (NO₂), the performance of the nanothin film device was comparable to that of a 1‐dimensional (1‐D) nanostructure, with a great advantage in ease of processing. Sensing mechanism study indicated electrostatic gating as the dominating mode of sensing.