Sensitivity of the Non-Equilibrium Approach for Mixture Condensation to Heat and Mass Transfer Correlations and Thermophysical Properties

The prediction of mixture condensation is still complex due to coupled heat and mass transfer and insufficient data of thermophysical mixture properties. This article analyzes the impact of various heat and mass transfer correlations on the non-equilibrium approach for mixture condensation in a vertical plain tube. Furthermore, the influence of thermophysical properties from different databases is investigated. The results are shown for ethanol-water, but allow conclusions to other fluid mixtures. They indicate that the liquid heat transfer coefficient in the non-equilibrium approach dominates the qualitative behavior of the condensation process, but the vapor mass transfer coefficient can only decrease or increase the quantitative level of the effective heat transfer with minor impact. More importantly, the logarithm in the vapor mass transfer term is central for the prediction of the condensation heat transfer. As this logarithm contains VLE data, it proves that there is a strong connection between VLE and overall prediction of mixture condensation. A demonstration of available data for thermophysical mixture properties of ethanol-water shows significant deviations, which affect the calculations as well. Besides, data from our own experiments are presented for mixture viscosity of ethanol-water. It is recommended to focus not only on improved heat and mass transfer correlations, but also on thermophysical properties and VLE data for a precise prediction of mixture condensation.

Inert Gas and Refrigerating Vapor Mass Flow Rates Ratio: A Much Promising Parameter for Diffusion-Absorption-Refrigeration Systems Performances Evaluation

This paper is consecrated to the thermodynamic study and analysis of diffusion-absorption-refrigeration (DAR) plants. The mass and energy balances analysis at the evaporator has allowed to highlight a new and original parameter, which can be used to analyze DAR system performances. It is the ratio of inert gas to refrigerant vapor mass flow rates at the evaporator inlets. This coefficient, which expression has been for the first time deduced mathematically, informs about the quality of the cycle and its performance, which are deeply affected by the growth of the inert gas flow energy expended to drive the refrigerant through the evaporator. The study shows that the coefficient of performance is decreasing with the increase of the mass flow rates ratio. The latter can be also used to find the optimal operating mode for the DAR machine with a specified working fluid.

Filmwise Condensation From Humid Air On a Vertical Superhydrophilic Surface: Explicit Roles of the Humidity Ratio Difference and the Degree of Subcooling

The process involving heat and mass transfer during filmwise condensation (FWC) in presence of non-condensable gases (NCG) has great significance in a large variety of engineering applications. The vapor mass flux leading to condensation and the resulting condensation heat transfer coefficient (CHTC) are dependent on the gradients of temperature and vapor mass fraction established near the condenser plate. The effects of the two most influencing thermodynamic parameters, i.e., the degree of subcooling and the difference of humidity ratio (between the free stream environment and on the condenser plate), have been characterized in this work both experimentally and through a mechanistic model. The vapor mass flux during condensation on a subcooled vertical superhydrophilic surface under a free convection regime is experimentally measured in a controlled environment (temperature and humidity) chamber. A mechanistic model, based on the similarity of energy and species transports, is formulated for the thermogravitational boundary layer over the condenser plate and tuned against the experimental results. Further, the model is used to obtain comprehensive data of the condensate mass flux and CHTC as functions of the salient thermal operating conditions over a wide parametric range. Results indicate that humidity ratio difference has a more pronounced influence on the condensation mass transfer rather than the degree of subcooling. The mechanistic model lends to the development of empirical correlations of condensate mass flux and CHTC as explicit functions of these two parameters for easy use in practical FWC configurations.

Influence of Concentration Distribution on Cross Diffusion Level in Building Room

The relationship of contaminant gas concentration distribution influence on cross diffusion character and level among temperature, humidity and contaminant gas concentration was obtained according to the non-equilibrium thermodynamic theory. The cross diffusion character and level under different contaminant gas concentration distribution were discussed, combining real temperature and humidity in building room. The results show that temperature grads and vapor mass grads are less than zero when contaminant gas mass grads and additional diffusion coefficient are both positive or negative, otherwise the two grads are more than zero. And the higher the initial temperature and humidity levels, the greater the absolute values of temperature grads and vapor mass grads, with the same contaminant gas mass grads and additional diffusion coefficient. While the influence of initial temperature level is finite, and that of initial humidity level is remarkable.

Growth and Collapse Dynamics of a Vapor Bubble near or at a Wall

This study investigated the dynamics of vapor bubble growth and collapse for a laser-induced bubble. The smoothed particle hydrodynamics (SPH) method was utilized, considering the liquid and vapor phases as the van der Waals (VDW) fluid and the solid wall as a boundary. We compared our numerical results with analytical solutions of bubble density distribution and radius curve slope near a wall and the experimental bubble shape at a wall, which all obtained a fairly good agreement. After validation, nine cases with varying heating distances (L2 to L4) or liquid heights (h2 to h10) were simulated to reproduce bubbles near or at a wall. Average bubble radius, density, vapor mass, velocity, pressure, and temperature during growth and collapse were tracked. A new recognition method based on bubble density was recommended to distinguish the three substages of bubble growth: (a) inertia-controlled, (b) transition, and (c) thermally controlled. A new precollapse substage (Stage (d)) was revealed between the three growth stages and collapse stage (Stage (e)). These five stages were explained from the out-sync between the bubble radius change rate and vapor mass change rate. Further discussions focused on the occurrence of secondary bubbles, shockwave impact on the wall, system entropy change, and energy conversion. The main differences between bubbles near and at the wall were finally concluded.

Investigation on spray cyclic variations under idle operation of engine using optical diagnostics and statistical methods

Under idle operations of a spark-ignition direct-injection engine, issues such as misfire, unstable combustion, and power imbalance between individual cylinders are often encountered, which worsen the fuel economy and tailpipe emissions. These undesired phenomena have close relations with cyclic variations of the fuel sprays in the cylinder. In this article, the spray cyclic variations under idle operations have been investigated at a constant volume chamber using ultraviolet/visible laser absorption/scattering imaging technique and Mie scattering optical diagnostics combined with different statistical methods such as probability presence image, intersection over union, and edge fluctuation length. The variations in spray morphology of liquid/vapor phases and vapor mass distributions have been characterized. It was found that the cyclic spray variation after the end of injection is too large to ignore, implying that this cyclic variation should be taken into consideration when matching the spray to a combustion chamber or numerical modeling. The effects of injection pressure and fuel temperature on spray cyclic variations have been quantitatively examined. The results show that the higher injection pressure or the higher fuel temperature is, the larger variation in spray morphology and vapor mass distributions was observed, indicating that adopting an appropriately lower injection pressure or lower fuel temperature is helpful to a stable ignition and combustion under idle conditions for a non-homogeneous spark-ignition direct-injection engine.

Numerical study of the greenhouse solar drying of olive mill wastewater under different conditions

The aim of this work is to develop thermal modeling of the olive mill wastewater drying process in a greenhouse solar dryer. A configuration was thus proposed and simulated using the commercial software COMSOL Multiphysics in order to solve the conservation equations governing our problem. The resulting simulations are used to evaluate the temperature, velocity, and vapor mass fraction distributions after hours of sunshine and to provide a quantification of the drying process. The influence of the greenhouse effect on the drying kinetics is highlighted by comparing to open sun-drying results. The effect of some greenhouse geometric characteristics and external meteorological conditions are studied.

Variational Assimilation of Radar Data and GLM Lightning-Derived Water Vapor for the Short-Term Forecasts of High-Impact Convective Events

The assimilation of water vapor mass mixing ratio derived from total lightning data from the Geostationary Lightning Mapper (GLM) within a three-dimensional variational (3DVAR) system is evaluated for the analysis and short-term forecast (≤6 h) of a high-impact convective event over the northern Great Plains in the United States. Building on recent work, the lightning data assimilation (LDA) method adjusts water vapor mass mixing ratio within a fixed layer depth above the lifted condensation level by assuming nearly water-saturated conditions at observed lightning locations. In this algorithm, the total water vapor mass added by the LDA is balanced by an equal removal outside observed lightning locations. Additional refinements were also devised to partially alleviate the seasonal and geographical dependence of the original scheme. To gauge the added value of lightning, radar data (radial velocity and reflectivity) were also assimilated with or without lightning. Although the method was evaluated in quasi–real time for several high-impact weather events throughout 2018, this work will focus on one specific, illustrative severe weather case wherein the control simulation—which did not assimilate any data—was eventually able to initiate and forecast the majority of the observed storms. Given a relatively reasonable forecast in the control experiment, the GLM and radar assimilation experiments were still able to improve the short-term forecast of accumulated rainfall and composite radar reflectivity further, as measured by neighborhood-based metrics. These results held whether the simulations made use of one single 3DVAR analysis or high-frequency (10 min) successive cycling over a 1-h period.

Effect of Moisture Content on Mixing Air With Water-Liquid Flowing Through Inverted U-Tube for Power Plant Condenser Applications

Computational Fluid Dynamics (CFD) for air/water-vapor and water-liquid two-phase flow mixing with condensation in a vertical inverted U-tube is presented in this paper. This study is to investigate the flow behaviors and underlying some physical mechanisms encountered in air/water-vapor and water-liquid mixing flow when condensation is considered. Water-liquid flows upward-downward through the inverted U-tube while the air/water-vapor mixture is extracted from a side-tube just after the flow oriented downward. The CFD simulation is carried out for a side air/water-vapor mixture volume fraction (αm) of 0.2–0.7, water-vapor mass fraction (Xv) of 0.1–0.5 in the side air/water-vapor mixture and water-liquid mass flowrate (mw) of 2,4,6, and 8 kg/s. The present results reveal that, at lower air mass flow rate, no significant effect of Xv on the generated static pressure at the inverted U-tube higher part. However, by increasing the air mass flow rates, ma ≥ 0.001 at mw = 2 kg/s, and ma ≥ 0.00125 at mw = 4 kg/s, we can infer that the lowest static pressure can be attained at Xv = 0.1. This may be attributed to the increased vapor and air mass flow rates from the side tube which results in shifting the condensation from the tube highest part due to air accumulation. This leads to increasing the flow pressure and decelerating the water-liquid flow. Raising mw from 2 to 4 kg/s at the same vapor mass ratio results in a lower static pressure due to more condensation of water vapor. The turbulent intensity and kinetic energy starts to drop approximately at ma = 0.002 kg/s, and αm = 0.55–0.76 at mw = 2 kg/s for all Xv values but no noticeable change at mw = 4 kg/s occurs. These findings estimate the operational values of air and water mass flow rates for stable air entrainment from the side-tube. Increasing the air and vapor mass ratio over these values may block the evacuation process and fails the system continuance. Likewise more air entrainment from the side-tube will decelerate the water flow through the inverted U-tube and hence the flow velocity will decrease thereafter. Moreover, this study reveals that the inverted U-tube is able to generate a vacuum pressure down to 55.104 kPa for the present model when vapor condensation is considered. This generated low-pressure helps to vent an engineering system from the non-condensable gases and water vapor that fail its function if these are accumulated with time. Moreover, the water-liquid mass flow rate in the inverted U-tube can be used to sustain the required operating pressure for this system and extract the non-condensable gases with a less energy consuming system. The present CFD model provides a good physical understanding of the flow behavior for air/water-vapor and water-liquid flow for possible future application in the steam power plant.