DEP46 - Artículos de revista

Hydrogen’s role in mitigating emissions variability: A chemical kinetic-thermodynamic digital framework for cleaner combustion technologies

2026-04-09T19:00:56Z

The combustion process in spark-ignition (SI) engines inherently presents cycle-to-cycle variations (CCV), leading to engine instability and variability in emissions formation. This work develops a digital framework that integrates chemical kinetics and a two-zone thermodynamic diagnostic model to understand the role of hydrogen in mitigating CCV and its impact on emissions formation. The framework predicts the crank angle degree resolved evolution of CCV of CO, H2, NO, and N2O in the engine combustion chamber’s burned gas zone. The framework is calibrated with experimental results from an SI engine working with gasoline-hydrogen fuel mixtures under stoichiometric and lean combustion conditions. This investigation has revealed that the formation of nitrogen-based emissions, particularly NO, exhibits higher variability than CO and exhaust unburnt H2, with coefficients of variation ranging from 7% to 35%. The high NO variability is attributed to the rapid decrease in NO destruction rates (i.e., kinetic “freezing”) at different in-cylinder pressure and temperature conditions within each thermodynamic cycle. It is elucidated that N2O formation occurs predominantly during the expansion and exhaust strokes. New knowledge has been created to understand how the thermochemical properties of hydrogen reduce NO cycle-to-cycle variability. A synergistic effect is unveiled, hydrogen enrichment leads to an engine operational shift towards a more dilute state (i.e., increased residual gases), where hydrogen’s combustion-enhancing properties (e.g., high flame speed, low ignition energy) are crucial for stabilising combustion and thus reducing NO formation variability. Furthermore, the work proposes a new predictive statistical model capable of describing NO dispersion using only the resi- dual–gas fraction and the mean NO level, offering a practical tool for engine calibration and emissions control. Research findings can guide the development of emissions abatement technologies for combustion-based pow- ertrains operating with hydrogen under lean combustion conditions, where conventional catalysts are less effective and understanding gains are highly significant. The proposed digital framework offers an emissions variability predictive tool facilitating the stable operation of clean powertrain for future energy systems.

Viscosity of [C4mim][(CF3SO2)2N], [C4mim][N(CN)2], [C2mim][C2H5SO4] and [Aliquat][N(CN)2] in a wide temperature range. Measurement, correlation, and interpretation

2026-02-16T20:05:41Z

The viscosity of four in-house-made ionic liquids, 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonylimide), ([C4mim][(CF3SO2)2N]), CAS RN 174899–83-3, 1-butyl-3-methyl-imidazolium dicyanamide, ([C4mim][N(CN)2]), CAS RN 448245–52-1, 1-ethyl-3-methyl-imidazolium ethylsulphate, ([C2mim][C2H5SO4]), CAS RN 342573–75-5 and methyltrialkyl(C8,C8,C10)ammonium dicyanamide [Aliquat][N(CN)2], CAS RN 63393–96-4, was measured in the temperature range 283.15 K − 373.15 K, at P = 0.1 MPa. Data obtained with an absolute relative uncertainty of Ur (η) = 0.02 was compared with available literature data for all ionic liquids, except for [Aliquat][N(CN)2], data herein presented for the first time. Both the Vogel-Tammann-Fulcher and IUPAC/IAPWS equations were applied to correlate the measured viscosity data as a function of temperature, the latter being more accurate. Values of η (298.15 K) are recommended for each ionic liquid studied. The viscosities of the different ionic liquids are discussed, considering their chemical structure differences.

Thermal Conductivity of Ionic Liquids and IoNanofluids. Can Molecular Theory Help?

2026-02-16T20:05:41Z

Ionic liquids have been suggested as new engineering fluids, specifically in the area of heat transfer, and as alternatives to current biphenyl and diphenyl oxide, alkylated aromatics and dimethyl polysiloxane oils, which degrade above 200 °C, posing some environmental problems. Addition of nanoparticles to produce stable dispersions/gels of ionic liquids has proved to increase the thermal conductivity of the base ionic liquid, potentially contributing to better efficiency of heat transfer fluids. It is the purpose of this paper to analyze the prediction and estimation of the thermal conductivity of ionic liquids and IoNanofluids as a function of temperature, using the molecular theory of Bridgman and estimation methods previously developed for the base fluid. In addition, we consider methods that emphasize the importance of the interfacial area IL-NM in modelling the thermal conductivity enhancement. Results obtained show that it is not currently possible to predict or estimate the thermal conductivity of ionic liquids with an uncertainty commensurate with the best experimental values. The models of Maxwell and Hamilton are not capable of estimating the thermal conductivity enhancement of IoNanofluids, and it is clear that the Murshed, Leong and Yang model is not practical, if no additional information, either using imaging techniques at nanoscale or molecular dynamics simulations, is available.

Possible New Heat Transfer Fluid: The IoNanofluid of 1-Ethyl-3-methylimidazolium Dicyanamide + Nano-Titanium Oxide─Studying Its Thermal Conductivity and Viscosity

2026-02-16T20:05:39Z

Ionic liquids with the dicyanamide anion, namely, with 1-alkyl-imidazolium cations, have been receiving attention recently due to their potential applications. The utilization of these liquids as heat transfer fluids, specifically in small heat exchangers and microchannels for microprocessor cooling, is presently deemed highly feasible, as it can be both more efficient and environmentally acceptable. The design of a heat transfer equipment that makes use of fluids requires knowledge of their thermophysical properties. In this regard, dispersions of nanoparticles have been extensively studied in recent years to improve thermal conductivity or obtain desirable optical properties. IoNanofluids is what we have taken to name the result of such dispersions in ionic liquids. In this paper, we report measurements of the thermal conductivity and viscosity of the IoNanofluid of 1-ethyl-3-methylimidazolium dicyanamide, [C2mim][N(CN)2], with 0.5% mass fraction of TiO2 nanoparticles (diameter 20 nm) in the temperature range (293 < T/K < 343), at P = 0.1 MPa. Reasonable enhancements were found for thermal conductivity and viscosity, which were temperature-dependent. The IoNanofluid was found to behave as a non-Newtonian fluid in most of the temperature range studied. A discussion about the possible use of this IoNanofluid as a heat transfer fluid shows that it has very promising properties to be used in heat transfer applications.

Ionic Liquids—A Review of Their Toxicity to Living Organisms

2026-02-11T20:01:04Z

Ionic liquids (ILs) were initially hailed as a green alternative to traditional solvents because of their almost non-existent vapor pressure as ecological replacement of most common volatile solvents in industrial processes for their damaging effects on the environment. It is common knowledge that they are not as green as desired, and more thought must be put into the biological consequences of their industrial use. Still, compared to the amount of research studying their physicochemical properties and potential applications in different areas, there is a scarcity of scientific papers regarding how these substances interact with different organisms. The intent of this review was to compile the information published in this area since 2015 to allow the reader to better understand how, for example, bacteria, plants, fish, etc., react to the presence of this family of liquids. In general, lipophilicity is one of the main drivers of toxicity and thus the type of cation. The anion tends to play a minor (but not negligible) role, but more research is needed since, owing to the very nature of ILs, except for the most common ones (imidazolium and ammonium-based), many of them are subject to only one or two articles.

Thermophysical Properties of 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C6mim][(CF3SO2)2N]—New Data, Reference Data, and Reference Correlations

2026-02-11T20:01:03Z

Published data on the thermophysical properties of ionic liquids are normally in disagreement if results from different laboratories, using different samples and different measurement protocols, are compared. This fact was recognized years ago at the level of the International Union of Pure and Applied Chemistry (IUPAC), which established IUPAC Project 2002-005-1-100 (Thermodynamics of ionic liquids, ionic liquid mixtures, and the development of standardized systems), with the main objective of recommending a reference ionic liquid, making reference-quality measurements on selected thermophysical properties of both the pure ionic liquid and its mixtures, establishing recommended values for the properties measured, and providing recommendations on measurement methods. The ionic liquid chosen was 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C6mim][(CF3SO2)2N], because of its stability, low viscosity compared with that of most common ionic liquids, low water solubility, ease of preparation and purification, and commercial availability. Due to its hydrophobicity, it is capable of being obtained very pure, with water amounts as small as 20 ppm. This paper reports new results obtained with the sample of [C6mim][(CF3SO2)2N] synthesized in the IUPAC project, namely on density, speed of sound, surface tension, and refractive index, as well as thermal conductivity of a commercial sample at P = 0.1 MPa, as a function of temperature, and proposes reference data and reference data correlations for the density, speed of sound, heat capacity, surface tension, viscosity, electrical conductivity, thermal conductivity, refractive index, ion self-diffusion coefficient, and melting temperature of this ionic liquid at 0.1 MPa, as a function of temperature, using these and other data reported up to July 2020. Rheological measurements demonstrated that this ionic liquid is Newtonian.

Thermal-hydraulic performance assessment of a micro shell-and-tube heat exchanger operating under part-load conditions in the NET Power cycle recuperator

2026-02-10T20:01:36Z

This paper numerically investigates the thermal-hydraulic performance of a micro shell-and-tube heat exchanger (MSTHE) for application in the thermal recuperator of the innovative oxy-combustion-based NET Power cycle, operating under cycle-relevant part-load conditions. The aim is to support the technological transition from the established printed circuit heat exchangers (PCHE) to MSTHE, which offer a lower inertia, cost-effective, and maintenance-friendly high-performance alternative. To this end, a thermal-hydraulic computational model of the MSTHE was developed, capable of capturing the rapid variation of the supercritical CO2 (scCO2) properties and the partial filmwise condensation of the turbine exhaust gases. Results show that the MSTHE must contain at least 60,000 tubes so that the pressure drop on the tube-side is lower than 1 bar at nominal conditions. The MSTHE effectiveness decreases from 89.2% to 65.1% as the cycle load is reduced from 100% to 20%. The overall heat transfer coefficient decreases gradually between 100% and 40% cycle load, drops sharply between 40% and 30%, and then stabilizes between 30% and 20% cycle load. This stabilization is attributed to the abrupt local increase of the heat capacity on the scCO2-side during the pseudo-critical phase transition, which also enhances local condensation heat release and thickens the condensate film on the shell-side. However, it was found that this phenomenon induces strong axial temperature gradients that may induce thermal stresses, representing a trade-off to the proposed compact design. While the floating microtube bundle of MSTHEs can accommodate these thermal stresses, the rigid compact block structure of PCHE is more prone to damage, revealing an addi- tional key advantage of MSTHEs.

Analysis of compliant tube properties and operating conditions in a Liebau pump

2026-02-03T20:01:06Z

This study examines asymmetric pumping in Liebau pumps, a type of valveless pump that generates unidirec- tional flow through the periodic compression of a flexible (compliant) tube. This pumping has applications in biomedical devices, microfluidics, and organ support. We investigate how the properties of the compliant tube and the operating conditions affect the pump performance, using dimensionless parameters. Experiments were performed with different configurations, varying the tube material (latex and rubber) and the fluid (water and water–glycerine mixture). The results indicate that the flow rate and resonant period depend on the stiffness of the tube and the viscous effects. It was observed that for small values of the Womersley number (𝑊𝑜2), viscous effects significantly reduce the flow rate. In contrast, for large Womersley values, the semiempirical models previously proposed adequately predict the experimental behaviour. Effects such as the compliant tube depression, which were not accounted for in previous models, were also found to influence performance. This work extends the analysis of this type of pump to unexplored conditions, with the aim of expanding knowledge and enabling the use of Liebau pumps in real-world applications.

Thermodynamic characterization of the (CO2 + O2) binary system for the development of models for CCS processes: Accurate experimental (p, ρ, T) data and virial coefficients

2026-03-23T09:11:50Z

Continuing our study on (CO2 + O2) mixtures, this work reports new experimental (p, ρ, T) data for two oxygen-rich mixtures with mole fractions x(O2) = (0.50 and 0.75) mol·mol−1, in the temperature range T = (250–375) K and pressure range p = (0.5–20) MPa, using a single-sinker densimeter. Experimental density data were compared to two well-established equation-of-state models: EOS-CG and GERG-2008. In the p, T-range investigated, the EOS-CG gave a better reproduction for the equimolar mixture (x(O2) = 0.5), whereas the GERG-2008 performed significantly better for the oxygen-rich mixture (x(O2) = 0.75). The EOS-CG generally overestimates the density, while the GERG-2008 underestimates it. This complete set of new experimental data, together with previous measurements, is used to calculate the virial coefficients B(T, x) and C(T, x), as well as the second interaction virial coefficient B12(T) for the (CO2 + O2) system.

An accurate thermodynamic model to characterise dissociating N2O4 at vapour–liquid equilibrium states

2026-04-07T07:46:38Z

A new thermodynamic model is presented, capable of accurately representing the vapour–liquid equilibrium pressures and densities, and liquid phase densities and enthalpies of dissociating dinitrogen tetroxide (N2O4 ⇄ 2NO2). The model is based on the Peng-Robinson equation of state coupled with advanced mixing rules. The -required but non-measurable- critical coordinates of the pure components forming the reactive mixtures are optimized, within a variability range defined in a previous study, to fit experimental vapour–liquid equilibrium data. The optimized parameters are then validated by comparing calculated thermodynamic properties with available experimental data in the subcritical region. The negligible impact of the higher temperature reaction 2NO2 ⇄ 2NO + O2, within the vapour–liquid equilibrium region where the optimisation is performed, is also proven. The resulting model is finally compared with the currently most accurate available equation of state, showing comparable results when considered both the scatter in available experimental data and the relative simplicity of the proposed equation of state. In particular, the proposed model demonstrates the satisfactory capability of a cubic equation of state to accurately reproduce both saturation pressures and saturation densities without requiring volume translation.

Zeta potential of CO2-rich aqueous solutions in contact with intact sandstone sample at temperatures of 23 °C and 40 °C and pressures up to 10.0 MPa

2026-03-19T13:13:05Z

Despite the broad range of interest and applications, controls on the electric surface charge and the zeta potential of silica in contact with aqueous solutions saturated with dissolved CO2 at conditions relevant to natural systems, remains unreported. There have been no published zeta potential measurements conducted in such systems at equilibrium, hence the effect of composition, pH, temperature and pressure remains unknown. We describe a novel methodology developed for the streaming potential measurements under these conditions, and report zeta potential values for the first time obtained with Fontainebleau sandstone core sample saturated with carbonated NaCl, Na2SO4, CaCl2 and MgCl2 solutions under equilibrium conditions at pressures up to 10 MPa and temperatures up to 40 °C. The results demonstrate that pH of solutions is the only control on the zeta potential, while temperature, CO2 pressure and salt type affect pH values. We report three empirical relationships that describe the pH dependence of the zeta potential for: i) dead (partial CO2 pressure of 10-3.44 atm) NaCl/Na2SO4, ii) dead CaCl2/MgCl2 solutions, and iii) for all live (fully saturated with dissolved CO2) solutions. The proposed new relationships provide essential insights into interfacial electrochemical properties of silica-water systems at conditions relevant to CO2 geological storage.

Dew points for hydrogen-rich (hydrogen + propane) and (hydrogen + n-butane) mixtures determined with a microwave re-entrant cavity resonator

2026-03-20T11:58:18Z

Dew-point measurements were performed for hydrogen-rich binary mixtures with hydrocarbon mole fractions of 0.09995 C3H8, 0.1499 C3H8, 0.0513 n-C4H10, and 0.09888 n-C4H10 using a modified microwave re-entrant cavity apparatus. Isochoric measurements were conducted in the temperature range of (256 to 286) K with pressures up to 6.8 MPa and 2.0 MPa for mixtures containing propane and n-butane, respectively. The combined expanded uncertainties (k = 2) in dew-point temperature and pressure were estimated to be between (0.35 and 2.0) K and (0.0011 and 0.030) MPa. The agreement with predictions of the GERG-2008 equation of state is within 5.5%. However, better agreement with predicted values was achieved using a cubic equation recently developed in-house. Moreover, experiments with short- and long-term exposure of pressure sensors to hydrogen have shown that pressure measurement, and thus the uncertainty, can be affected; further investigation on this matter is therefore inevitable to improve hydrogen thermophysical property data.

Hubs and clusters approach to unlock the development of carbon capture and storage – Case study in Spain

2026-04-08T08:46:57Z

Many countries have assigned an indispensable role for carbon capture and storage (CCS) in their national climate change mitigation pathways. However, CCS deployment has stalled in most countries with only limited commercial projects realised mainly in hydrocarbon-rich countries for enhanced oil recovery. If the Paris Agreement is to be met, then this progress must be replicated widely, including hydrocarbon-limited countries. In this study, we present a novel source-to-sink assessment methodology based on a hubs and clusters approach to identify favourable regions for CCS deployment and attract renewed public and political interest in viable deployment pathways. Here, we apply this methodology to Spain, where fifteen emission hubs from both the power and the hard-to-abate industrial sectors are identified as potential CO2 sources. A priority storage structure and two reserves for each hub are selected based on screening and ranking processes using a multi-criteria decision-making method. The priority source-to-sink clusters are identified indicating four potential development regions, with the North-Western and North-Eastern Spain recognised as priority regions due to resilience provided by different types of CO2 sources and geological structures. Up to 68.7 Mt CO2 per year, comprising around 21% of Spanish emissions can be connected to clusters linked to feasible storage. CCS, especially in the hard-to-abate sector, and in combination with other low-carbon energies (e.g., blue hydrogen and bioenergy), remains a significant and unavoidable contributor to the Paris Agreement’s mid-century net-zero target. This study shows that the hubs and clusters approach can facilitate CCS deployment in Spain and other hydrocarbon- limited countries.

Vapor-liquid equilibria and excess enthalpies of the binary systems 1-pentanol or 2-pentanol and 1-hexene or 1,2,4-trimethylbenzene for the development of biofuels

2026-03-25T12:27:38Z

Accurate experimental data of vapor-liquid equilibria (VLE) and excess enthalpies are reported for four binary systems: (1-pentanol þ 1-hexene), (2-pentanol þ 1-hexene), (1-pentanol þ 1,2,4- trimethylbenzene) and (2-pentanol þ 1,2,4-trimethylbenzene). An isothermal total pressure cell was used for measuring VLE at T ¼ 313.15 K. The data were fitted using Margules, Wilson and NRTL equations. Excess enthalpies were measured at two different temperatures T ¼ (298.15 and 313.15) K using an isothermal flow calorimeter and were correlated by the Redlich-Kister equation. All systems present a positive deviation from the Raoult's Law. An azeotropic behavior with maximum pressure is observed for the mixtures 1-pentanol or 2-pentanol with 1,2,4-trimethylbenzene. In addition, an endothermic behavior, which increases with temperature, is obtained when the alcohols are mixed with these hydrocarbons.

Viscosities of binary mixtures containing 2-butanol + hydrocarbons (2,2,4-trimethylpentane or 1,2,4-trimethylbenzene) at high pressures for the implementation of second generation biofuels

2026-01-21T18:38:24Z

2-Butanol is an alcohol which can be from renewable origin, and it is clasiffied as second-generation biofuels. Before the implementation of some biofuels, thermophysical and thermodynamic properties are required. To fulfil this objective, dynamic viscosities of (2-butanol + 2,2,4-trimethylpentane) and (2-butanol + 1,2,4-trimethylbenzene) mixtures are reported at four temperatures (293.15, 313.15, 333.15, 353.15) K, pressures up to 140 MPa and mole fractions of 2-butanol x = (0.3, 0.5, 0.8, 1). The measurements were carried out using an accurate vibrating-wire viscometer with a relative expanded uncertainty of 1.5% for a coverage factor k = 2. Experimental data were fitted to a modified VFT model obtaining good results in agreement with the uncertainty.

Characterization of cellular structure appearance in ethanol expanding spherical flames

2026-04-08T06:20:06Z

The objective of this work is to characterize the combustion process of ethanol flames under cellular conditions. Ethanol is considered an alternative fuel and can be used to replace fossil fuels. To investigate the behavior of ethanol as a fuel, some of its combustion properties are measured and characterized, such as laminar burning velocity and flame front stability, which strongly depend on the appearance of cellularity on the flame. The study is developed in a cylindric constant volume combustion bomb instrumented with Schlieren technique to visualize ethanol flames and make an optical diagnosis of the combustion process. Some cellular parameters are proposed to characterize the cellular structure of the flame, which quantitatively define the appearance and cellularity development, such as cellular radius, the time for the cellular structure apparition on the flame and the influence of cellularity on the burning velocity. Other dimensionless parameters that can help to determine the influence of cellularity in the combustion process and compare between different flames. An I-Optimal design of experiments is proposed in this work to characterize the flame stability of ethanol, design the experimental testing and develop predictive models for the proposed cellular parameters. The proposed area of study to assure cellular flames is delimited by an initial temperature of 343K, initial pressures from 0.15 MPa to 0.30 MPa and equivalence ratios ranging from 0.8 to 1.4. Images confirm that models predict correctly the cellular radius and others cellular parameters, and the appearance of cellularity affects the burning velocity generating an auto-turbulence in the flame which enhance it. Predictions of cellular radius obtained with developed model are in accordance with the results obtained by other works.