Constant Vapor Research Articles

Experimental investigation on the flow boiling of R134a in a multi-microchannel heat sink

The present study reports on an experimental evaluation of flow boiling of R134a inside a multi-microchannel heat sink. The copper test section consisted of 27 parallel rectangular channels with 0.470mm depth, 0.382mm width, 0.416mm fin thickness and 40mm length. The experiments were performed at saturation temperatures of 18, 23 and 28°C, mass fluxes of 800, 1000 and 1200kgm−2s−1 and constant vapor quality at the heat sink inlet at 0.05. The wall heat flux was increased from 50kWm−2 until reaching critical heat flux, and maximum value of 460kWm−2 was reached. With the collected data from the experiments, the effects of mass flux, heat flux, saturation temperature and steam quality on the heat transfer coefficient are emphasized and the reasons are discussed. At low vapor quality, heat flux plays a major role in increasing the heat transfer coefficient, while the effect of mass flux is negligible. With rising heat flux, convective boiling begins to control the heat transfer mechanism, and the heat transfer coefficient increases with rising vapor quality and mass flux, until dry-out point. In all conditions, high heat transfer coefficients are obtained for high saturation temperatures. In open literature, the correlation generated by Mortada et al. for heat transfer coefficient is the one that makes the most accurate prediction for current study. Based on the experimental results, a correlation is proposed to calculate the heat transfer coefficients for R134a flow boiling inside the multi-microchannel at high mass and heat flux predicting the measured ones better than those in the literature.

Organic Flash Cycles: Off-design behavior and control strategies of two different cycle architectures for Waste Heat Recovery applications

Off-design characterization of energy systems has become interesting, especially for waste heat recovery application, where the heat source temperature and mass flow rate can vary over time. Low-grade heat is generally converted into power through ORC modules: the problem of the constant temperature evaporation lead to the definition of alternative architectures, among which organic flash cycles.In this work, the off-design behavior of two different architectures of single-stage Organic Flash Cycles has been analyzed in steady-state condition, for small scale waste heat recovery (WHR) purposes. The main difference between the two architecture is the regeneration: in the first architecture (Single-Stage Organic Flash Cycle SS-OFC), the liquid of the flash evaporator, after lamination is mixed with the vapor from the expander and then sent to the condenser; in the second architecture Single-Stage Organic Flash Regenerative Cycle, SS-OFRC, the liquid from the flash evaporator is mixed with the liquid from the condenser, to regenerate the cycle. The most appropriate fluid for the two cycles was selected from a list of sixteen fluids with the objective of minimizing volume flow rates and maximizing the system efficiency and i-Pentane was chosen. For the off-design behavior, a rotary volumetric expander derived from a Wankel engine was considered, taking into account the performance variation of the device at various rotating speed and pressure ratios. Three different control strategies were considered and compared in off-design analysis for both the cycle architectures: sliding-pressure, in which the expander speed was constant and flash pressure varied with the load; sliding-velocity, in which the load was controlled by the speed variation of the expander and flash pressure was retained constant; combined strategy in which the expander speed was varied to drive the flash pressure according to a function which maximized the system efficiency. Results showed that the efficiency of the two cycles was similar in all the operating field whatever was the control strategy considered: SS-OFRC demonstrated a better behavior at low temperatures of the heat source (<170 °C), while SS-OFC had a better efficiency at higher temperature. The maximum absolute efficiency difference in off-design conditions between the two cycles was lower than 0.3%. SS-OFRC however had a wider field of operation than SS-OFC, due to the better flexibility of this type of cycle. As for the control strategy, with both the architectures, combined strategy maximized the system efficiency and flexibility for every temperature and mass flow rate of the heat source considered.

Separation of Small Particles by Diffusio‐ and Thermophoresis in a Gas‐Liquid Cross‐Flow Array

AbstractA wet scrubber technique was studied for the separation of particles &lt; 2.5 μm from exhaust gas by a gas‐liquid cross‐flow array. The cleaning water of the separator flows vertically down along many regularly configured wires and the generated water film acts as an independent sink for particle collection. Due to the surface‐orientated horizontal particle movement caused by diffusio‐ (DP) and thermophoresis (TP), the particles can be captured by the water films. Since wastewater from the same production process can be used, the necessary cleaning water recycling costs can be saved due to the already available water treatment device. Experiments on a laboratory‐scale test rig show the particle grade removal efficiency (PGRE) for different inlet gas‐water temperature gradients and for a constant vapor injection.

Evaluation of electrodeposited Mn‐Co protective coatings on Crofer 22 APU steel

AbstractInterconnects used in Solid Oxide Cells stacks require protective coatings to lower their parabolic rate constant and block chromium evaporation (on the air side). In this work four different protective coatings on steel are evaluated for their high temperature corrosion resistance and electrical conductivity. A commercial electroplating process was used for the preparation of coatings with different Mn/Co ratios on Crofer 22 APU steel. Oxidation of samples was performed in air at 800°C for 1000 hours. Postmortem analysis of the coated samples was performed by scanning electron microscopy and x‐ray diffractomettry. Based on the results, influence of the Co/Mn ratio on the resulting corrosion properties are discussed. Parabolic rate constant of the coated samples is the lowest for the MnCo sample, whereas electrical resistance is the lowest for the Co sample, which has a corrosion rate similar to the not‐coated alloys.

Non-isothermal evaporation in a sessile droplet of water-salt solution

Experimental data on nonisothermal evaporation of sessile droplets of water-salt solutions (LiBr + H2O; CaCl2+H2O) were obtained. Evaporation of droplets of volatile liquids occurs with almost constant evaporation rate, and the problem is solved in a stationary approximation. High-temperature evaporation of water-salt solutions leads to significant difficulties at modeling the heat and mass transfer. In this case, the evaporation rate substantially decreases with time. With the growth of salt concentration in the solution from 11% to 60%, the partial pressure of water vapor at the interface falls by an order of magnitude. In this work, we have performed simulation, considering diffusion in solutions, a non-isothermal character, and the Stefan flow, and proposed a simple method for calculating the mass flow. The resulting technique can qualitatively and quantitatively predict the solution behavior with a significant change in the external boundary conditions in time.

Stomatal conductance increases with rising temperature

ABSTRACTStomatal conductance directly modifies plant water relations and photosynthesis. Many environmental factors affecting the stomatal conductance have been intensively studied but temperature has been largely neglected, even though it is one of the fastest changing environmental variables and it is rising due to climate change. In this study, we describe how stomata open when the temperature increases. Stomatal conductance increased by ca 40% in a broadleaf and a coniferous species, poplar (Populus deltoides x nigra) and loblolly pine (Pinus taeda) when temperature was increased by 10 °C, from 30 °C to 40 °C at a constant vapor pressure deficit of 1 kPa. The mechanism of regulating stomatal conductance by temperature was, at least partly, independent of other known mechanisms linked to water status and carbon metabolism. Stomatal conductance increased with rising temperature despite the decrease in leaf water potential, increase in transpiration, increase in intercellular CO2 concentration and was decoupled from photosynthesis. Increase in xylem and mesophyll hydraulic conductance coming from lower water viscosity may to some degree explain temperature dependent opening of stomata. The direct stomatal response to temperature allows plants to benefit from increased evaporative cooling during the heat waves and from lower stomatal limitations to photosynthesis but they may be jeopardized by faster depletion of soil water.

Modeling of the lithium hydride hydrolysis under low relative humidity

A reactional mechanism describing the hydrolysis of lithium hydride (LiH) under moist atmosphere is proposed and modeled. It involves the formation of lithium oxide (Li2O) at low vapor pressure and both lithium oxide and lithium hydroxide (LiOH) at higher vapor pressures, with diffusion of water in these layers. A numerical model based on this mechanism is implemented with COMSOL Multiphysics to simulate the hydrolysis of LiH particles in open system (constant water vapor pressure). Kinetic parameters such as rate constant of reactions and diffusion coefficient of water in Li2O and LiOH are first fitted against experimental data. The best agreements are obtained when the diffusion coefficient of water is 10 times higher in LiOH than in Li2O. The resulting model accurately predicts the hydrolysis rates experimentally measured for a wide range of water vapor pressures (0.3–17 Pa). The hydrolysis mechanisms are compatible with a Li2O production limited by water diffusion and a LiOH production governed by the kinetics. Finally, simulation suggest that the thickness of the Li2O layer does not depends much on the water vapor pressure whereas that of LiOH increases drastically at high water vapor pressures.

Heat and mass transfer boundary conditions at the surface of a heated sessile droplet

This work numerically investigates how the boundary conditions of a heated sessile water droplet should be defined in order to include effects of both ambient and internal flow. Significance of water vapor, Marangoni convection, separate simulations of the external and internal flow, and influence of contact angle throughout drying is studied. The quasi-steady simulations are carried out with Computational Fluid Dynamics and conduction, natural convection and Marangoni convection are accounted for inside the droplet. For the studied conditions, a noticeable effect of buoyancy due to evaporation is observed. Hence, the inclusion of moisture increases the maximum velocities in the external flow. Marangoni convection will, in its turn, increase the velocity within the droplet with up to three orders of magnitude. Results furthermore show that the internal and ambient flow can be simulated separately for the conditions studied, and the accuracy is improved if the internal temperature gradient is low, e.g. if Marangoni convection is present. Simultaneous simulations of the domains are however preferred at high plate temperatures if both internal and external flows are dominated by buoyancy and natural convection. The importance of a spatially resolved heat and mass transfer boundary condition is, in its turn, increased if the internal velocity is small or if there is a large variation of the transfer coefficients at the surface. Finally, the results indicate that when the internal convective heat transport is small, a rather constant evaporation rate may be obtained throughout the drying at certain conditions.

Batch distillation of binary mixtures: preliminary analysis of optimal control

The present work concerns the distillation of a binary non-ideal zeotropic mixture in an N-stage batch column under total condensation and constant vapour flow rate assumptions. The aim is to maximize the final product of desired purity using the reflux as a control parameter. We discuss methodological aspects of the numerical resolution, and apply the direct method based on a full discretization of the optimal control problem. We show that if the number of plates is just sufficient or is big enough to assure the desired purity of the product, the optimal solution is very close to the cyclic-like policy reported by other authors in a different context.

The origin of anomalous water diffusion in silica glasses at low temperatures

AbstractAnomalous water diffusion into SiO2 glass was observed in a low temperature range, below ~850°C, under a constant water vapor pressure of 355 Torr (47.3 kPa). Both the effective water diffusion coefficient and water solubility exhibited an anomalous time dependence. For example, water solubility in the low temperature range increased initially, achieving much higher values than expected based on extrapolation from higher temperature data, and then decreased with time toward an equilibrium value. This phenomenon was reported earlier, but a complete explanation was not possible; a new model is presented based upon glass surface compressive stress generation and subsequent surface stress relaxation. Water diffusion can promote stress generation and stress relaxation, both of which affect the reaction between diffused molecular water and the glass structure. By considering these stress effects, the anomalous water diffusion behavior in silica glass is explained. Furthermore, the same model can account for the reversal of external tensile and compressive stress effects on water solubility and diffusivity in silica glass observed after a few hours of heat treatment at 650°C in 355 Torr water vapor pressure.

Coffee-stain growth dynamics on dry and wet surfaces

The drying of a drop containing particles often results in the accumulation of the particles at the contact line. In this work, we investigate the drying of an aqueous colloidal drop surrounded by a hydrogel that is also evaporating. We combine theoretical and experimental studies to understand how the surrounding vapor concentration affects the particle deposit during the constant radius evaporation mode. In addition to the common case of evaporation on an otherwise dry surface, we show that in a configuration where liquid is evaporating from a flat surface around the drop, the singularity of the evaporative flux at the contact line is suppressed and the drop evaporation is homogeneous. For both conditions, we derive the velocity field and we establish the temporal evolution of the number of particles accumulated at the contact line. We predict the growth dynamics of the stain and the drying timescales. Thus, dry and wet conditions are compared with experimental results and we highlight that only the dynamics is modified by the evaporation conditions, not the final accumulation at the contact line.

Effect of capillary pressure and salinity on CO2 solubility in brine aquifers

CO2 storage is a process in which CO2 is injected into depleted oil and gas reservoirs or deep aquifers to store CO2 through several mechanisms. Since supercritical CO2 phase and saline brine are immiscible at a wide range of pressure and temperature in a permeable medium, capillary pressure exists between the CO2 and saline brine. Although there have been many efforts to study solubility of CO2 in brine aquifers, the most published models neglect the capillary pressure effect during phase behavior calculations to investigate the amount of dissolution trapping. This work focuses on effects of capillary pressure and salinity on solubility of CO2 in the region, where brine saturation increases from a dried zone and/or below residual saturation toward a 100% water saturated zone at the transition zones of CO2 saturations. For this purpose, we developed a new model accounting for inequalities of aqueous and gas phase pressures to calculate the solubility of CO2 in the brine phase. Peng-Robinson equation of state and criterion for local equilibrium are applied. The capillary pressure is a function of phase saturation. We verified the proposed method against the experimental measurement. Subsequently, we performed the case studies including the capillary pressure effect on the solubility of CO2, water vaporization and K-values. The results indicate that increasing capillary pressure leads to a reduction in CO2 solubility. The importance of this effect on CO2 solubility depends on the capillary pressure curve which represents petrophysical properties. In a case considered in this paper, the decrease of CO2 solubility is 23% when the capillary pressure increases from 0.48 to 9.45MPa. Additionally, effects on equilibrium constant (K-values) and water vaporization were also investigated in the presence of capillary pressure, which can be used to obtain accurate simulation results in commercial reservoir simulators. Furthermore, we analyzed the influence of salinity on CO2 solubility and compared the results with and without the capillary equilibrium. Comparisons between different salinities reveal that CO2 solubility decreases nearly 49% when the salinity increases from 0 to 4mol aqueous NaCl solutions. It is the first attempt to theoretically investigate the capillary pressure effect on the subject of CO2 geological storage.

R1234yf condensation inside a 3.4 mm ID horizontal microfin tube

This paper shows experimental results about R1234yf condensation inside a microfin tube with an inner diameter at the fin tip of 3.4 mm. R1234yf is a new environmentally friendly refrigerant, with a Global Warming Potential lower than 1, therefore it matches the new environmental laws. Experimental tests are carried out for mass velocities from 100 to 1000 kg m−2 s−1, vapor qualities from 0.95 to 0.2, at saturation temperature of 30 °C and 40 °C. The experimental results show that heat transfer coefficient increases when both mass velocity and vapor quality increase. Frictional pressure gradient increases with mass velocity at constant vapor quality, whereas at constant mass velocity it increases with vapor quality up to a maximum, after which it slightly decreases. The experimental heat transfer coefficient and pressure drop are also compared against the values predicted by empirical correlations available in the open literature.

Carbon-coated SiO/ZrO2 composites as anode materials for lithium-ion batteries

A combination of high-energy ball milling and constant pressure chemical vapor deposition was used to prepare carbon-coated SiO/ZrO2 composites. It was found that the as-prepared composites were composed of amorphous carbon, amorphous SiO, and paracryslalline ZrO2. The electrochemical analysis results revealed excellent electrochemical performances for the composites, including a high initial discharge capacity (1737mAhg−1), a remarkable cyclic stability (reversible capacity of 721mAhg−1 at 800mAg−1, after 100 cycles), and a good rate capability (870mAhg−1 at 800mAg−1). These features demonstrate that these composites are promising alternative candidates for high-efficiency electrode materials of Li-ion batteries.

Isotopic Fractionation in Wintertime Orographic Clouds

AbstractThe Isotopic Fractionation in Snow (IFRACS) study was conducted at Storm Peak Laboratory (SPL) in northwestern Colorado during the winter of 2014 to elucidate snow growth processes in mixed-phase clouds. The isotopic composition (δ18O and δD) of water vapor, cloud water, and snow in mixed-phase orographic clouds were measured simultaneously for the first time. The depletion of heavy isotopes [18O and deuterium (D)] was greatest for vapor, followed by snow, then cloud. The vapor, cloud, and snow compositions were highly correlated, suggesting similar cloud processes throughout the experiment. The isotopic composition of the water vapor was directly related to its concentration. Isotopic fractionation during condensation of vapor to cloud drops was accurately reproduced assuming equilibrium fractionation. This was not the case for snow, which grows by riming and vapor deposition. This implies stratification of vapor with altitude. The relationship between temperature at SPL and δ18O was used to show that the snow gained most of its mass within 922 m above SPL. Relatively invariant deuterium excess (d) in vapor, cloud water, and snow from day to day suggests a constant vapor source and Rayleigh fractionation during transport. The diurnal variation of vapor d reflected the differences between surface and free-tropospheric air during the afternoon and early morning hours, respectively. These observations will be used to validate simulations of snow growth using an isotope-enabled mesoscale model with explicit microphysics.

Shell-Side Flow Condensation of R410A on Horizontal Tubes at Low-Mass Fluxes

An investigation of refrigerant R410A condensation on a shell and tube heat exchanger simulation is conducted. Tests are on the outside of a horizontal smooth tube, a herringbone tube, and a newly developed three-dimensional-enhanced tube, called the enhanced tube (EHT) tube, all of the same outer diameter. Experiments were conducted at a constant saturation temperature of 45 °C, a constant inlet vapor quality of 0.8, a constant outlet vapor quality of 0.1, and mass fluxes ranging from 5 kg/(m2 s) to 50 kg/(m2 s). At low-mass velocities, the smooth tube shows superior performance over the herringbone tube and the EHT tube. The cause might lie in surface tension effects that result in liquid inundation at the lower portion of the tube, thickening the film on the tube and deteriorating the heat transfer performance. Analyses were conducted to find a suitable correlation of the experimental data.

Thermochemical heat storage materials – Performance of mixed salt hydrates

Thermochemical heat storage is highly promising, in particularly with a view to long-term heat storage. For the implementation of heat storage in households, thermochemical reactions in the low temperature range below 120°C are important. Especially salt hydrates such as MgCl2, CaCl2 or MgSO4 were tested with micro gravimetric methods for their suitability. However, the cycle stability of consecutive charging (dehydration) and discharging (hydration) reactions of these materials was low and could be improved only by control of the water uptake (i.e. discharging time) to prevent overhydration. In contrast, mixtures of these salt hydrates showed significant improvements in cycle stability, mass and enthalpy balances. The experiments also showed that the cycleability of all investigated materials increased if hydration and dehydration reactions were performed under constant vapor pressure of 21mbar. Contrary to other materials, the mixture of CaCl2 and MgCl2 showed good cycleability under all tested conditions. In addition, the mixture showed superior kinetic properties. Additionally, there is evidence of tachyhydrite (CaMg2Cl6⋅12H2O) formation during cycling of the mixture by the use of XRD after the thermal analysis. Further investigations will be performed to identify further synergies, ideal mixing ratios and formed phases.

Determination of temperature and concentration of a vapor–gas mixture in a wake of water droplets moving through combustion products

Characteristic temperatures and concentrations of a vapor–gas mixture in a wake of water droplets moving through combustion products (initial temperature 1170 K) were determined using the Ansys Fluent mathematical modeling package. We investigated two variants of motion: motion of two droplets (with sizes from 1 mm to 3 mm), consecutive and parallel, and motion of five staggered droplets. The influence of the relative position of droplets and also of distances between them (varied from 0.01 mm to 5 mm) on temperatures and concentrations of water vapor was established. The distances determine the relation between the evaporation areas and the total volume occupied by a droplet aggregate in the gas medium. The results of modeling for conditions that take into account vaporization on the droplet surface at average constant values of evaporation rate and also with consideration of the change in the latter, depending on the droplet temperature field, are compared. We determined conditions under which the modeling results are comparable for the assumption of a constant vaporization rate and with regard to the dependence of the latter on temperature. The earlier hypothesis on formation of a buffer vapor layer (“thermal protection”) around a droplet, which decreases the thermal flow from the external gas medium, was validated.

Evaluation of plant waste used as mulch on soil moisture retention

Soil mulching is a technique commonly used in arid and semi-arid areas to prevent erosion and reduce evaporation of soil water. In this study, an experiment quantified the effect of three organic wastes (shredded date palm leaf (&lt;em&gt;Phoenix dactylifera&lt;/em&gt;), cereal straw, and pine bark) used as mulch on moisture retention in two soils with different textures, S1 (loam) and S2 (silty loam). Each soil was added to a height of 15 cm in square containers 31 cm wide and 20 cm high. The treatments were administered a day after soil saturation with water by covering each soil with 2 cm of mulch. Soils without mulching were used as control treatments. The experiment took place inside a greenhouse until a total loss of soil moisture was achieved. Temperature (T) and relative humidity (RH) values were measured throughout the experiment. During the first 16 days, high and practically constant evaporation was observed in the soils without mulching (1813 g H&lt;sub&gt;2&lt;/sub&gt;O/day per m&lt;sup&gt;2&lt;/sup&gt; in S1 and 1792 g H&lt;sub&gt;2&lt;/sub&gt;O/day per m&lt;sup&gt;2&lt;/sup&gt; in S2). Linear regressions were established to quantify the water loss over time in each treatment. The results show that all three organic byproducts were effective in reducing water loss in both soils, with significant differences observed between pine bark and the other two materials. The palm leaf was as effective as the cereal straw, so it is considered appropriate for use as a mulching material in areas like ours where its abundance and ease of use provide further environmental benefits.

Effects of vapour pressure on the motion of cavitation bubble

ABSTRACTTwo models of bubble dynamics with constant vapour pressure and vapour content were newly developed. The mass flux of water vapour at bubble wall was simplified to facilitate the analysis. Based on these two models and the approximation of the mass flux, more simplified equations corresponding to the models were deduced from dynamic equation of bubble corrected by Sochard et al. MATLAB and the Runge–Kutta algorithm were applied to calculate and analyse the dynamical behaviour of cavitation bubbles under an ultrasonic field. Numerical results reveal that bubbles under the constant vapour pressure model exhibit larger amplitudes and move more violently to generate acoustic cavitation effect than the constant vapour content model. Nevertheless, the kinetic characteristics of bubbles under the constant vapour content model are similar to those of bubbles without water vapour. Moreover, vapour content exerts certain influence on the maximum radius, minimum radius and collapse intensity of bubbles.