Water Vapor Dilution Research Articles

Effects of water sprays on hydrogen autoignition in heated air

The effects of water mist with different diameters and mass loadings on hydrogen auto-ignition in air with elevated temperatures and pressures are studied. It is shown that smaller droplets with larger mass loading are the most effective in inhibiting the auto-ignition process. Under elevated initial pressures and temperatures, the role of water mist in impacting auto-ignition is minimal, primarily because the ignition delay is substantially shorter than the droplet evaporation duration. As the initial temperature decreases, there exists a temperature value below which water mist markedly extends the ignition delay time, even escalating it by up to two orders of magnitude or causing a complete suppression of ignition. The rate of production for important radicals was analyzed. The data indicates that the evaporation of the water mist lowers the initial temperature, leading to a reduction in the rate of the temperature-sensitive fast chain branching reaction. In contrast, the third body reaction that generates HO2 is not sensitive to temperature. Meanwhile, the introduction of H2O results in an earlier activation of the third body reactions. Consequently, the production of HO2 is enhanced, emphasizing the role of subsequent slow chain propagation reactions that generate OH, thereby extending the ignition delay. By deactivating the mass transfer between two phases and introducing artificial species, it is evident that water vapor dilution and thermal effects predominantly influence the ignition delay, while its role as a direct reactant is minimal.

Investigation of the impact of nitrogen and water vapor dilution in oxidizer stream on flame structure, soot production and extinction of a laminar ethylene-fueled co-flow diffusion flame

This paper discusses the effect of nitrogen (N2) and water vapor (H2O) dilution in oxidizer stream on the flame structure, and soot production and extinction of laminar ethylene (C2H4)-fueled co-flow diffusion flame (CFDF). The total oxidizer stream flow rate is kept constant while varying the flow rates of air, N2, and H2O to achieve the desired oxygen concentration called oxygen index (OI). As OI reduces, the flame starts to lift off from the burner surface and shows upward and downward oscillations before extinction through blow-off. The OI at which the flame blows off is called the limited oxygen index (LOI), and it depends on burner configuration, fuel, and oxidizer flow conditions. LOI value for C2H4-fueled laminar CFDF were found to be 12.4% and 14% when the diluent was N2 and H2O, respectively. The experiment and simulation results are presented from 21% O2 concentration in oxidizer to very low oxygen concentration in oxidizer close to extinction. Through quantitative measurements of soot volume fraction (SVF), the result shows a notable reduction in the peak concentrations of SVF as the OI decreases. This decrease is more pronounced with H2O than N2 dilution. One-dimensional (1D) opposed flow diffusion flame simulations are performed to understand various effects of the addition of diluents like H2O and N2. It is observed that in addition to inert, thermal and diffusion effect of H2O in reduction of flame temperature and temperature-dependent production rate of key species, H2O also affects flame chemistry through OH + H2 = H + H2O, 2OH = O + H2O. As a result there is a further drop in the concentration of active radicals (like H, O) which leads to quicker soot inhibition and flame extinction.

Effects of Water Vapor Dilution on the Laminar Burning Velocity and Markstein Length of Ammonia/Water Vapor/Air Premixed Laminar Flames

Effects of Water Vapor Dilution on the Laminar Burning Velocity and Markstein Length of Ammonia/Water Vapor/Air Premixed Laminar Flames

On the evolutions of induction zone structure in wedge-stabilized oblique detonation with water mist flows

Two-dimensional wedge-stabilized oblique detonations in stoichiometric and fuel-lean H2/O2/Ar mixtures with water mists are studied using Eulerian-Lagrangian method. The effects of water droplet mass flow rate on flow and chemical structures in the induction zone, as well as physical / chemical roles of water vapor, are investigated. The results show that the oblique detonation wave (ODW) can stand in a range of water mass flow rates for both stoichiometric and fuel-lean mixtures. With increased droplet mass flow rate, the deflagration front in the induction zone is distorted and becomes zigzagged, but the transition mode from oblique shock wave (OSW) to ODW does not change. Moreover, the initiation and transition locations monotonically increase, and the OSW and ODW angles decrease, due to droplet evaporation and water vapor dilution in the induction region. For fuel-lean mixtures, the sensitivity of characteristic locations to the droplet loading variations is mild, which signifies better intrinsic stability and resilience to the oncoming water droplets. The chemical explosiveness of the gaseous mixture between the lead shock and reaction front is studied with the chemical explosive mode analysis. The smooth transition is caused by the highly enhanced reactivity of the gas immediately behind the curved shock, intensified by the compression waves. Nonetheless, the abrupt transition results from the intersection between the beforehand generated detonation wave in the induction zone and OSW. Beside, the degree to which the gas chemical reactivity in the induction zone for fuel-lean mixtures is reduced by evaporating droplets is generally lower than that for stoichiometric gas. Also, physical and chemical effects of water vapor from liquid droplets result in significant differences in ODW initiation and morphology. When abrupt transition occurs, both physical and chemical effects of water vapor influence transition location and ODW angle, but only physical one is important for smooth transition.

Study of turbulent flame characteristics of water vapor diluted hydrogen-air micro-mixing combustion

Hydrogen has been regarded as an important alternative fuel, especially in gas turbine applications. However, the premixed combustion of pure hydrogen has problems such as easily flashback and high NOx emissions. A more reasonable method of combustion is to use micro-mixing technology combined with water vapor dilution. This work experimentally investigated the influences of dilution rate (D) and equivalence ratio (φ) on the flame characteristics of hydrogen-air micro-mixing combustion diluted with water vapor. Results show that increasing D reduces the flame area by reducing the downstream OH radical signal, which is contrary to the effect of CO2 dilution. The flame local curvature radius shows that the micro-mixing combustion is a high turbulence intensity combustion, and the convex structure is more frequent. Moreover, increasing φ or decreasing D leads to a decrease in the instantaneous average curvature radius (Rave), indicating more small-scale wrinkle structures are generated. D and φ have a similar influence on the fluctuations of Rave and flame area in the time domain, while the fluctuation of the flame area is more evident near the lean-burn limit. When D = 25%, the frequency characteristics of the flame structure and chemical reactions are synchronized.

Autoignition and detonation development induced by temperature gradient in n-C7H16/air/H2O mixtures

The effects of water vapor dilution on autoignition and detonation development induced by an ignition spot with thermal non-uniformity in an n-C7H16/air mixture are numerically investigated. Zero-dimensional homogeneous ignition under constant-volume conditions is studied first. It is found that excitation time increases, whereas total heat release decreases with a H2O vapor mole fraction. Moreover, the role of H2O vapor diluents as a third body considerably influences the critical temperature gradient. One-dimensional autoignition and detonation development caused by temperature gradients in ignition spots is then studied. Three different autoignition modes are identified: (I) supersonic deflagrative wave, (II) detonative wave, and (III) subsonic deflagrative wave. It is found that H2O dilution has a slightly better performance on detonation suppression than CO2 dilution. The chemistry–acoustics interactions during autoignition development are weakened when the H2O mole fraction is increased. Besides, H2O vapor dilution can delay the detonation initiation and reduce detonation intensity. Furthermore, typical autoignition processes induced by a hotspot and the chemical effects of water vapor diluent are discussed. It is seen that the chemical effects of H2O dilution do not affect the lower limits of detonation development curves. Besides, the third body effect from the H2O vapor diluent is important in suppressing the detonation development for the investigated ignition spot size. Finally, the effects of equivalence ratios and ignition spot sizes on the autoignition modes of n-C7H16/air/H2O mixtures are studied. It is observed that the water vapor diluted mixtures with the fuel-lean condition are advantageous in inhibiting detonation from localized thermal non-uniformity.

Absolute and relative emissions analysis in practical combustion systems—effect of water vapor condensation

Accurate knowledge of the absolute combustion gas composition is necessary in the automotive, aircraft, processing, heating and air conditioning industries where emissions reduction is a major concern. Those industries use a variety of sensor technologies. Many of these sensors are used to analyze the gas by pumping a sample through a system of tubes to reach a remote sensor location. An inherent characteristic with this type of sampling strategy is that the mixture state changes as the sample is drawn towards the sensor. Specifically, temperature and humidity changes can be significant, resulting in a very different gas mixture at the sensor interface compared with the in situ location (water vapor dilution effect). Consequently, the gas concentrations obtained from remotely sampled gas analyzers can be significantly different than in situ values. In this study, inherent errors associated with sampled combustion gas concentration measurements are explored, and a correction methodology is presented to determine the absolute gas composition from remotely measured gas species concentrations. For in situ (wet) measurements a heated zirconium dioxide (ZrO2) oxygen sensor (Bosch LSU 4.9) is used to measure the absolute oxygen concentration. This is used to correct the remotely sampled (dry) measurements taken with an electrochemical sensor within the remote analyzer (Testo 330-2LL). In this study, such a correction is experimentally validated for a specified concentration of carbon monoxide (5020 ppmv).

Effects of water vapor dilution on the minimum ignition energy of methane, n-butane and n-decane at normal and reduced pressures

Water vapor dilution has great impact on fundamental combustion processes such as ignition, flame propagation and extinction. In the literature, there are many studies on how water vapor addition affects flame propagation and extinction limit. However, the influence of water vapor addition on ignition receives little attention. In this study, numerical simulations considering detailed chemical mechanisms are conducted for the ignition of methane, n-butane and n-decane/air/water vapor mixtures. The emphasis is spent on examining the effects of water vapor dilution on the ignition of these fuels at normal and reduced pressures. The minimum ignition energies (MIE) at different dilution ratios and initial pressures are obtained. It is found that at normal and reduced pressures, the MIE is proportional to the inverse of pressure and it increases exponentially with water vapor dilution ratio. A general correlation among the MIE, pressure and dilution ratio is proposed for each fuel. Furthermore, for stoichiometric methane/air/water vapor mixtures, the chemical and radiation effects of water vapor dilution are isolated and quantified. It is found that the three-body recombination reaction greatly increases the MIE and reduces the dilution limit.

Effects of water vapor dilution on trace gas flux, and practical correction methods

The effect of water vapor dilution on CH4 flux was examined here using dynamic chamber measurements at the forest floor of a black spruce forest in interior Alaska. CH4 and CO2 concentrations were differently diluted by increased water vapor in each chamber. After correction for water vapor dilution was applied, source (positive) CH4 flux was enhanced, whereas sink (negative) flux was reduced, and magnitude changed depending on the condition of water vapor. Several methods were examined to practically correct water vapor dilution for fluxes previously measured. Numerical correction for all data sampled at high frequency intervals (DAE-method) is highly recommended. Water vapor dilution correction for ensemble-averaged data (SA-method) is also applicable, though not recommended in case of long-term average data for accurate flux determination, especially for data obtained at irregular source strength plots. When evapotranspiration is measured in parallel to CH4 flux, the removal of the water vapor increment due to evapotranspiration in the chamber is another option for correction (HCE-method). This method resulted in similar accuracy to the DAE-method, as long as continuously sampled data were applicable. The application of the correction for water vapor dilution to previous tower-based flux data (measured over a wet sedge tundra in Barrow throughout the month of August, 2000) also resulted in an increase in source (positive) flux of 6.6% on average. This study concludes that either accurate flux must be determined using mixing ratio, or a water vapor dilution correction must be applied using simultaneously measured water vapor and trace gases.

The effects of water dilution on hydrogen, syngas, and ethylene flames at elevated pressure

This work investigates experimentally and numerically the kinetic effects of water vapor addition on the burning rates of H2, H2/CO mixtures, and C2H4 from 1atm to 10atm at flame temperatures between 1600K and 1800K. Burning rates were measured using outwardly propagating spherical flames in a nearly constant pressure chamber. Results show good agreement with newly updated kinetic models for H2 flames. However, there is considerable disagreement between simulations and measurements for H2/CO and C2H4 flames at high pressure and high water vapor dilution. Both experiments and simulations show that water vapor addition causes a monotonic decrease in mass burning rate and the inhibitory effect increases with pressure. For hydrogen flames, water vapor addition reduces the critical pressure above which a negative pressure dependence of the burning rate is observed. However, for C2H4 flames, the burning rate always increases with pressure. The results also show that water vapor addition has the same effect as a pressure increase for H2 and H2/CO flames, shifting the reaction zone into a narrower window at higher temperatures. For all fuels, water vapor addition increases OH formation via H2O+O while reducing the overall active radical pool for hydrogen flames. For C2H4, the additional HO2 production pathway through HCO results in a dramatic difference in pressure dependence of the burning rate from that observed for hydrogen. The present work provides important additions to the experimental database for syngas and C0–C2 high pressure kinetic model validations.

Evaluation of a portable chamber system for soil CO2 efflux measurement and the potential errors caused by internal compensation and water vapor dilution

A portable, easy-to-install chamber system for rapid measurement of soil CO2 efflux is needed for the immediate analysis of CO2 flushes after events such as tillage and grassland renovation. We conducted field measurements and simple simulations to validate the effectiveness of a portable chamber system with an infrared CO2 gas analyzer (IRGA) and a temperature/humidity sensor (the SIR system). We confirmed that a measurement period of 60-180 s after the closure of the chamber was sufficient for the calculation of soil CO2 efflux. We compared the SIR system with a closed static chamber system and an automated open/closed chamber system. The efflux data obtained with the SIR system were linearly related to the data obtained with the other systems, and the relationships between the soil temperature and efflux were similar in the three systems. These results show that the SIR system provided soil CO2 efflux values with a reasonable accuracy relative to the other systems. We also examined how the calculated efflux values were affected by the internal compensation of the IRGA for environmental parameters and water vapor dilution in the chamber. The potential error caused by the default internal compensation of the IRGA was within 3.1% under general environmental conditions. The effect of water vapor dilution was large (>20%) for small CO2 effluxes. The combined effects of the default internal compensation and water vapor dilution were larger for small CO2 effluxes than for large CO2 effluxes, and the effects became larger under low air pressure conditions. We recommend that accurate environmental compensation and water vapor correction be applied for analysis of small CO2 effluxes when there is a rapid increase in water vapor, especially under low air pressure conditions.

Flow-through respirometry applied to chamber systems: Pros and cons, hints and tips

Flow-through respirometry is a powerful, accurate methodology for metabolic measurement that is applicable to organisms spanning a body mass range of many orders of magnitude. Concentrating on flow-through respirometry that utilizes a chamber to contain the experimental animals, we describe the most common flow measurement and control methodologies (push, pull and stop-flow) and their associated advantages and disadvantages. Objective methods for calculating air flow rates through the chamber, based on the body mass and taxon of the experimental organism, are presented. Techniques for removing the effect of water vapor dilution, including the direct measurement of water vapor pressure and mathematical compensation for its presence, are described and evaluated, as are issues surrounding the analysis of one or both of the respiratory gases (oxygen and carbon dioxide), and issues related to the mathematical correction of wash-out phenomena (response correction). Two important biomedical applications of flow-through respirometry (metabolic phenotyping and room calorimetry) are discussed in detail, and we conclude with a list of suggestions aimed primarily at investigators starting out in applying flow-through respirometry.

A new approach for flow-through respirometry measurements in humans

Indirect whole room calorimetry is commonly used in studies of human metabolism. These calorimeters can be configured as either push or pull systems. A major obstacle to accurately calculating gas exchange rates in a pull system is that the excurrent flow rate is increased above the incurrent flow rate, because the organism produces water vapor, which also dilutes the concentrations of respiratory gases in the excurrent sample. A common approach to this problem is to dry the excurrent gases prior to measurement, but if drying is incomplete, large errors in the calculated oxygen consumption will result. The other major potential source of error is fluctuations in the concentration of O(2) and CO(2) in the incurrent airstream. We describe a novel approach to measuring gas exchange using a pull-type whole room indirect calorimeter. Relative humidity and temperature of the incurrent and excurrent airstreams are measured continuously using high-precision, relative humidity and temperature sensors, permitting accurate measurement of water vapor pressure. The excurrent flow rates are then adjusted to eliminate the flow contribution from water vapor, and respiratory gas concentrations are adjusted to eliminate the effect of water vapor dilution. In addition, a novel switching approach is used that permits constant, uninterrupted measurement of the excurrent airstream while allowing frequent measurements of the incurrent airstream. To demonstrate the accuracy of this approach, we present the results of validation trials compared with our existing system and metabolic carts, as well as the results of standard propane combustion tests.

Dilution effects of superheated water vapor on turbulent premixed flames at high pressure and high temperature

Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H 2O to those of air and H 2O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO 2, reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, S T, normalized by laminar burning velocity, S L, was much weaker than that of CO 2 dilution. The mean volume of the turbulent flame region defined as the region between 〈 c〉 = 0.1 and 〈 c〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO 2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO 2. The emission indices of CO and NO x were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.

EFFECT OF COMBUSTION AIR DILUTION BY WATER VAPOR OR NITROGEN ON NOX EMISSION IN A PREMIXED TURBULENT NATURAL GAS FLAME: AN EXPERIMENTAL STUDY

ABSTRACT A study of the effects of combustion air dilution by water vapor or nitrogen on NOx emission in a premixed turbulent natural gas flame was performed for lean conditions. NOx concentrations were measured with dry air, water vapor dilution or nitrogen dilution of the combustion air at varying air factor at three different power/pressure settings. Results showed for all cases a significant reduction in NOx emission as an effect of combustion air dilution by water vapor and nitrogen for constant Tad. The results also showed that the reducing effect of water vapor dilution is larger than the reducing effect of nitrogen dilution. It is therefore concluded that water vapor dilution of the combustion air not only reduces NOx emission in premixed turbulent flames by lowering flame temperature and oxygen deficiency, but also by chemical action.

Puhimau Thermal Area: A Window into the Upper East Rift Zone of Volcano, Hawaii?

We report the results of two soil CO2 efflux surveys by the closed chamber circulation method at the Puhimau thermal area in the upper East Rift Zone (ERZ) of Open image in new window volcano, Hawaii. The surveys were undertaken in 1996 and 1998 to constrain how much CO2 might be reaching the ERZ after degassing beneath the summit caldera and whether the Puhimau thermal area might be a significant contributor to the overall CO2 budget of Open image in new window. The area was revisited in 2001 to determine the effects of surface disturbance on efflux values by the collar emplacement technique utilized in the earlier surveys. Utilizing a cutoff value of 50 g m−2 d−1 for the surrounding forest background efflux, the CO2 emission rates for the anomaly at Puhimau thermal area were 27 t d−1 in 1996 and 17 t d−1 in 1998. Water vapor was removed before analysis in all cases in order to obtain CO2 values on a dry air basis and mitigate the effect of water vapor dilution on the measurements. It is clear that Puhimau thermal area is not a significant contributor to Open image in new window CO2 output and that most of Open image in new window CO2 (8500 t d−1) is degassed at the summit, leaving only magma with its remaining stored volatiles, such as SO2, for injection down the ERZ. Because of the low CO2 emission rate and the presence of a shallow water table in the upper ERZ that effectively scrubs SO2 and other acid gases, Puhimau thermal area currently does not appear to be generally well suited for observing temporal changes in degassing at Open image in new window.

Packaging trade-offs for an LSI-oriented very high-speed computer, the HITAC M-200H: Tsuneyo Chiba, Akira Masaki, Kennichi Furumaya and Satoshi Hososaka. IEEE Trans. Components, Hybrids, Mfg Technol.Chmt-4, (2) 166 (June 1981)

Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H2O to those of air and H2O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO2, reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, ST, normalized by laminar burning velocity, SL, was much weaker than that of CO2 dilution. The mean volume of the turbulent flame region defined as the region between 〈c〉 = 0.1 and 〈c〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO2. The emission indices of CO and NOx were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.

Computer-aided design of VLSI circuits: Arthur Richard Newton. Proc. IEEE69 (10), 1189 (October 1981)

Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H2O to those of air and H2O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO2, reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, ST, normalized by laminar burning velocity, SL, was much weaker than that of CO2 dilution. The mean volume of the turbulent flame region defined as the region between 〈c〉 = 0.1 and 〈c〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO2. The emission indices of CO and NOx were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.

Measurement and analysis of gas exchange during exercise using a programmable calculator.

Although exercise testing is useful in the diagnosis and management of cardiovascular and pulmonary diseases, a rapid comprehensive method for measurement of ventilation and gas exchange has been limited to expensive complex computer-based systems. We devised a relatively inexpensive, technically simple, and clinically oriented exercise system built around a desktop calculator. This system automatically collects and analyzes data on a breath-by-breath basis. Our calculator system overcomes the potential inaccuracies of gas exchange measurement due to water vapor dilution and mismatching of expired flow and gas concentrations. We found no difference between the calculator-derived minute ventilation, CO2 production, O2 consumption, and respiratory exchange ratio and the values determined from simultaneous mixed expired gas collections in 30 constant-work-rate exercise studies. Both tabular and graphic displays of minute ventilation, CO2 production, O2 consumption, respiratory exchange ratio, heart rate, end-tidal O2 tension, end-tidal CO2 tension, and arterial blood gas value are included for aid in the interpretation of clinical exercise tests.