Air-steam Mixture Research Articles

Development of a general correlation for free convection vapor condensation over a horizontal tube in the presence of a noncondensable gas

In applications involving external condensation in the presence of noncondensable gases (NCG), there is a growing interest in horizontal tubes because they minimize condensate resistance, and hence reduce the adverse effects of NCGs. In the literature, experimental heat transfer rates are summarized in empirical correlations that fit the underlying data, but do not cover the spectrum of possible steam-NCG mixtures and thermal conditions. In this work, we aim at developing a general correlation that is applicable to a wide range of physical and geometrical conditions.We first present a CFD model for laminar free convection regimes, taking into account the resistance of the gas phase and the liquid condensate. The model is validated against recent condensation data on steam-N2 and steam-CO2 mixtures. The predictions agree well with the data, being mostly within the experimental uncertainty.Subsequently, close to 200 parametric CFD simulations are conducted for steam-air mixtures across a wide range of steam mass fraction (0.05–0.99), wall temperature subcooling (5-50 K), pressure (1–5 bar) and tube diameter (0.01–0.04 m). The resulting data are cast in a compact correlation that is compatible with the heat and mass transfer analogy. The proposed correlation is in good agreement with empirical correlations in the literature.

Investigation of the heat transfer coefficient during the condensation of small quantities of water vapour from a mixture with a high proportion of non-condensable gas in a horizontal smooth tube

During heat recovery of exhaust gas energy in a shell and tube heat exchanger, the exhaust gas is cooled down and the water vapour content may condense when the corresponding temperature is reached. In this case, a condensation heat exchanger allows recovering the latent heat in addition to the sensible heat and is therefore beneficial for a preferably high heat recovery. The database and algorithms for the design of this kind of apparatus are still nowadays quite week because only little experimental work is realized with condensation of water vapour from a mixture with a very high proportion of non-condensable gas in horizontal tubes. The actual research work improves this database with a considerable amount of experimental data and proposes a correlation for the prediction of the heat transfer coefficient in an exhaust gas heat exchanger in which parts of the gas humidity condense. All experiments were carried out under conditions, which are typical for this kind of heat exchangers, hence the presented results can be used for the design of condensation heat exchangers in industrial heat transfer systems. A gas inlet temperature of 120 °C, a range of the air-steam mixture Reynolds number 11,000 ≦ Re ≦ 31,000, three tube lengths between 1.87 m and 3.04 m, water vapour volume fractions of 7 % to 14.5 % as well as tube diameters of 22 mm and 28 mm were selected to derive the correlation. Furthermore, additional tube lengths (0.574 m, 0.973 m, 1.373 m) were investigated to determine the parameters influencing the condensation rate and comprehensive location profiles over the tube length. A correlation for a correction factor fC is derived, which enables to calculate the increase of the heat transfer coefficient due to condensation from the dry heat transfer coefficient without condensation. This dry heat transfer coefficient is calculated by the measured values in the inlet and outlet for a purely dry air-steam mixture cooling without condensation. The correction factor fC depend on the water vapour volume fraction, the Reynolds number and the geometry ratio between the inner diameter and the tube length.

Experimental study of steam and steam–air mixture condensation over vertical chrome-plated tube and polished tube exterior surface

In this article, a series of steam condensation experiments were performed on the outer surface of chrome-plated tube and polished tube under steam and steam–air mixed conditions to study their heat transfer characteristics and whether they could be used to optimize Passive Containment Cooling System in advanced nuclear reactors. The effects of total pressure, air mass fraction and subcooling on condensation heat transfer of the chrome-plated tube and polished tube were analyzed. Moreover, basing on the existing experiment, a smooth tube that has the same size as the polished tube and chrome-plated tube was tested under the same conditions. The experimental phenomena indicated that filmwise condensation and dropwise condensation co-exist on the tube exterior surface, and more droplets were observed on the exterior surface of the chrome-plated tube. Under the steam condition, with the fixed wall subcooling, condensation heat transfer coefficient of the polished tube and chrome-plated tube shows different change rules under different pressure: the condensation heat transfer coefficient of the polished tube increases with the increase of pressure, while the chrome-plated tube decreases with the increase of pressure. Compared with the smooth tube, the heat transfer performance can be improved by using the chrome-plated tube and polished tube. Under the steam–air condition, the existence of air deteriorates heat transfer performance seriously. The heat transfer performance of the chrome-plated tube and the polished tube are both improved compared with the smooth tube when the air mass fraction is below 60℃. When the air mass fraction is over 60℃, the heat transfer ability of these tubes are similar to the smooth tube. Besides, within the measurement parameters of our experiment, the increase in total pressure has a positive effect on steam condensation with air.

High-temperature oxidation of Moss—Mo3Si alloys doped with Sc or Y in air and in a steam-air mixture

For continuation of investigations aimed at studying the effect of rare earth elements (RRE) on the behavior of metal-silicide molybdenum alloys in oxidizing environments, the results of experimental modeling of high-temperature oxidation of alloys based on Mo—15 (at.%) Si and doped with Sc or Y in the isothermal mode in the air and in a steam-air environment are presented. New data on the effect of REE on oxidation resistance of alloys of the Mo—Si system of hypoeutectic composition, the formation of scale on their surface, the phase composition and the structure of oxidation products was obtained.

Numerical prediction of the chemical indicator response used in steam sterilisers

Steam sterilisation is a commonly used method in the sterilisation of surgical instruments. To ensure the sterility of the sterilised goods an evaluation of the sterilisation process is required. This might be achieved either through physical measurements or indicators. Optimal sterilisation results are achieved by removing the air from the sterilisation chamber. In this paper a new computational fluid dynamics (CFD) based approach is presented, which allows to calculate the steam distribution within a sterilisation chamber with focusing on hollow loads. Additional measurements were performed using a self-developed measurement chamber to validate the CFD model. A modified process challenge device (PCD) with different tube lengths in combination with a chemical indicator (CI) was tested, to identify the volumetric influence of the lumen on the resulting air-steam mixture therein. A numerically efficient model was developed to determine a steam volume fraction threshold leading to a response of the CI. This study aims to predict the volumetric amount of steam which is necessary in order to pass a PCD test fitted with a CI. Both the CFD model and the measurements showed that often an insufficient steam penetration is indicated by PCDs which can lead to an insufficient sterilisation of hollow loads.

Condensation heat transfer for downward flows of superheated steam-air mixture in a circular pipe

We measured the radial and axial temperature distributions of a saturated steam-air mixture in a vertical circular pipe (diameter, 49.5 mm; cooling height, 610 mm) in our previous study. Those of a superheated steam-air mixture in the same experimental setup were measured in this study. The condensation heat flux qc and heat transfer coefficient hc for the superheated mixture were evaluated, and they were compared with those for the saturated mixture. There was no large difference in qc between the saturated and superheated mixtures. The heat transfer coefficient hc (which was evaluated for the saturation temperature) for the superheated mixture agreed with that for the saturated mixture. hc computed with the diffusion layer model for saturated temperature agreed well with the hc measured for the superheated mixture.

Influence of drying mode and feed per tooth rate on the fine dust creation in pine and beech sawing on a mini sash gang saw

The experimental results of the study focused on the effect of drying processes of modified air drying and warm air-steam mixture drying of pine and beech wood on the size of sawdust particles created in cutting using PRW-15M sash gang saw, are presented in the paper. Particle size analysis of dry sawdust was performed using two methods—sieving method and laser diffraction analysis. The results showed that the drying process did not affect the general particle size distribution of the sawdust, but the content of very fine dust particles in beech wood sawdust is many times higher when the beech wood was dried in the warm air-steam mixture drying process and sawn at a feed per tooth of 0.105 mm.

Моделирование окисления расплава активной зоны ядерного реактора при наличии оксидной корки на поверхности расплава

At a severe accident of nuclear power plants with light-water reactors, the most effective way to localize the forming melt (corium) is to keep it in the cooled reactor vessel, the integrity of which depends on the value of heat flux from the melt to the reactor vessel. In this case, one of the critical processes is the melt oxidation by a water steam or a steam-air mixture. It process can lead to a significant increase in the thermal load on the reactor vessel due to a heat of exothermic reactions of oxidation of reducing agents, which presents in the melt, a thickness decreasing of the metallic part of the molten pool, and a hydrogen release. All of these factors strictly depends on the rate of oxidation. When considering the conditions of melt oxidation, it taken into account that for the accepted scenarios of a severe accident, the most realistic situation is the presence of a solid-phase oxide layer (oxidic crust) on the melt surface. Under these conditions, a dependence for calculating the rate of core melt oxidation based on the diffusion model proposed and its validation by using the obtained experimental data performed.

Potential well formation over a locally heated water surface

A model of the air-steam flow rising above a locally heated water surface is developed. The model is based on the system of free convection equations in the Boussinesq approximation and is implemented numerically. The adequacy of the numerical model is supported by the order of magnitude agreement between numerical predictions and experimental data, and with the analytical calculation of the slip velocity of water droplets based on Stokes drag. The temperature and velocity fields in the air-steam mixture are calculated. Based on the velocity field, a relation is proposed for calculating the potential field (potential) caused by the region of lowered dynamic pressure in the centre of the heated spot. The calculated potentials are compared with the potentials in other works using empirical approaches based on the processing of experimental data.

Condensation Heat Transfer for Downward Flows of Steam-Air Mixture in a Circular Pipe

Condensation Heat Transfer for Downward Flows of Steam-Air Mixture in a Circular Pipe

The Specifics of Design and Prediction of Thermohydraulic Characteristics of Thermosiphons

Recommendations are given on designing thermosiphons (TS) prepared on the basis of the experience that the authors gained in designing more than 4000 TSs for heat-recovery steam generators and the results of long-term investigations and tests carried out to study thermohydraulic and corrosion processes in vertical and inclined TSs filled with steam-water mixture. The results of similar studies performed by other authors are analyzed. References are given to the regulations on the design of heat transfer equipment for thermal and nuclear power stations. The requirements for TS construction materials, the degree of TS filling with water, and the TS vacuum level are outlined. Whether the evacuation of TS may be abandoned was assessed. Measures are proposed to prevent depressurization on water freezing in TSs. Recommendations are given on the calculation of thermohydraulic characteristics and steam-and-gas distribution in vertical or inclined TSs. The conditions of displacement by a steam flow of noncondensable gases, including air, to the top of the condensation zone in a thermosiphon are considered. Various regimes of axial heat transport and heat transfer are described for pure steam condensation (including film condensation, bubble condensation, distribution of different regimes of steam condensation, and condensate cooling along the height of the condensation zone during “flooding”). Heat transfer in the steam condensation from an air-steam mixture (the diffusion component of thermal resistance), maximum power, and the conditions for poorer cooling of the TS heating zone (“flooding,” steam separation at the upper generatrix of the heating zone in an inclined thermosiphon) are examined. Thermosiphons feature a countercurrent flow of steam and its condensate in a single flow with the same mass flowrates. Hence, a special case of “flooding” affecting the maximum power of the thermosiphon, the effect of the incoming steam flow on the heat transfer rate during film condensation, steam-and-gas distribution in a top-plugged channel, corrosion processes in thermosiphons, and hydrogen diffusion through the thermosiphon wall should also be studied.

Steam Condensation Heat Transfer During Initial Blow-Down Period of a Severe Nuclear Accident

Abstract Heat transfer coefficient (HTC) relations developed using steady-state experimental data are used for capturing the complete heat transport characteristic in a severe nuclear accident. It is important to verify the applicability of these correlation(s) at an early stage of the accident where heat transfer is transient in nature. In this paper, an experimental study is executed for this purpose. High-pressure steam (at 0.26 MPa (2.6 bar) and 0.41 MPa (4.1 bar) absolute pressure) is leaked into the closed containment initially filled with atmospheric air, and filmwise condensation is studied on an isothermally maintained vertical stainless steel test plate. During the experiment, temperature variation across the test plate at specified locations and inside the containment are recorded using the microthermocouples. The steam–air mixture composition is also examined using an online mass-spectrometry system. An inverse heat conduction (IHC) technique, validated using air-jet impingement heat transfer data, is used to estimate the time-varying condensation heat flux. It is found that the existing correlations based on the steady-state experimental data predict the transient condensation flux quite well, except in very early transient situation with a time scale of ∼20 s.

Influence of Cooling Medium Flow Character on the Operation of Heat Exchangers with Steam Condensation Inside Tubes

The influence of the character of cooling medium motion in a heat exchanging apparatus on the process of steam condensation in the latter is considered in the cases of cross flow past a tube bank in the apparatus, longitudinal flow past tubes in the bank in the regimes of forward flow and countercurrent flow, superheated steam condensation, and of nonuniform distribution of the cooling medium in different zones of the apparatus. It has been established that in the case of cross flow past the first rows of the tube bank, cooling medium zones with reduced heat transfer efficiency appear in the apparatus. The dependence of the pressure losses by the steam, which condenses in a heat exchanger in regimes of countercurrent flow, forward flow, and cross flow, is investigated. Experimental data on the superheated steam temperature have been obtained and generalized. The positive influence of the throttle on the line of removal of the steam–air mixture to a steam–jet ejector in nonuniform cooling of the parallel channels of the condenser has been established experimentally, and the presence of the optimal size of the throttle on the line of removal of noncondensing gases has been confirmed.

Heat and mass transfer during ethanol evaporation on the walls of a flat channel at forced convection of humid air

• The process of joint heat and mass transfer during the flow of moist air in a flat channel at ethanol evaporation on the walls was studied. A similarity in distributions of velocity, temperature, and component concentration profiles has been found. • The contribution of the heat flux components to the total energy exchange on an adiabatic wall has been determined. It was found that with an increase in the moisture content, the contribution of the convective component decreases. • Condensation of steam intensifies the process of ethanol evaporation, as a result of which the mass flows of water and ethanol vapors increase with increasing inlet air humidity. • The use of ethanol as an evaporative agent allows obtaining of the high values of the air cooling parameter, including those at high humidityφ in → 100%. The results of numerical investigation of heat and mass transfer in the laminar flow of humid air in a plane-parallel channel with adiabatic walls from the surface of which ethanol is evaporated are presented. The main attention is paid to studying the influence of air humidity at the inlet in the entire range of its change from absolutely dry to fully saturated state ( φ = 0 ÷ 100 % ) on the heat and mass transfer intensity and cooling efficiency. It has been established that ethanol evaporation and steam condensation occur simultaneously on the channel walls. The relationship between the components of the heat and diffusion flows on the wall is determined depending on the initial air humidity. An increase in the humidity of steam at the channel inlet is accompanied by an increase in the gas temperature at the outlet of the evaporation cell and, accordingly, deterioration in the cooling efficiency. Nevertheless, even when a steam-air mixture in the saturation state ( φ = 100 % ) with ethanol as an evaporation agent is fed to the cell inlet, the efficiency of such a cell remains high and approximately equal to the theoretical limit of water evaporation into dry air.

Effect of Surface Inclination on Filmwise Condensation Heat Transfer During Flow of Steam–Air Mixtures

Abstract Empirical/semi-empirical correlations are available in the literature to quantify the effect of several major parameters, like bulk pressure, non-condensable gas mass fraction, and wall subcooling, on condensation heat transfer coefficient (HTC). However, despite numerous applications of condensation on inclined flat plates, there is a lack of understanding of the effect of surface inclination on condensation heat transfer. Accordingly, a dedicated experimental program was undertaken to investigate the effect of surface inclination angle on filmwise steam condensation. Experiments were performed at different bulk pressures (1.7–4.2 bar absolute) and steam-air mass fractions (ranging from pure steam, i.e., 0% to 40% w/w air), with the steam-air mixture flowing over a flat test plate (Re range, 4200–4800). In each run, the inclination angle of the test surface was varied from −90 deg (condensation underneath the horizontal surface, facing downward) to +90 deg (condensation over the horizontal surface, facing upward) in increments of 15–20 deg (inclination angle θ measured from vertical). The results reveal an intriguing trend: for pure steam condensation, the HTCs decrease as the plate is inclined in either direction from the vertical, and the variation is nearly symmetric for both upward- and downward-facing configurations. On the other hand, for steam condensation in the presence of air, the HTCs decrease monotonically for upward-facing configurations, while they increase slightly (10–20%), and decrease subsequently (for θ < −70 deg) for downward-facing cases. Finally, the HTCs for inclined orientations are compared with the HTC in the standard vertical configuration to quantify the effect of inclination angle.

Investigation on steam direct contact condensation injected vertically at low mass flux, part II: Steam–air mixture experiment

An experimental study is conducted on the condensation behavior, onset of chugging, fluid oscillation, and pressure oscillation when a steam–air mixture is injected vertically into subcooled water at a low mass flux. Air is present in the form of dispersive small bubbles after condensation at a low water temperature and in the shape of a toroid at a high water temperature. Bubble resonance is first observed experimentally; a large bubble is almost stationary below the pipe outlet and air is sustainably released by small bubbles from the upper surface. The upper air mass fraction of chugging is obtained and compared with a transient conduction–diffusion model and momentum balance model. The dependence of different parameters on the amplitude, frequency, and probability of fluid oscillation is analyzed. A new frequency correlation modified with probability is developed and shows good agreement with the experimental data. During bubble resonance, the wave shape of pressure oscillation is harmonic with the frequency at about 35 Hz, and the intensity is larger than that under pure steam conditions. When bubble resonance does not occur, the presence of air can reduce the pressure oscillation intensity.

The influence of plate corrugations geometry scale factor on performance of plate heat exchanger as condenser of vapour from its mixture with noncondensing gas

The condensation of steam from a steam-air mixture is studied experimentally at three samples of Plate Heat Exchanger (PHE) channel. For three test samples corrugations are similar but differ in scale with the height 5, 7.5 and 10 mm. The results of tests are compared to calculations with one dimensional mathematical model of the process. The identification of correlations for local heat and mass transfer coefficients, as also friction factor in condensing two-phase flow, is made by comparison of computed results with tests data. The correlations for condensing two-phase flow are based on single-phase correlations for considered channels. For mass transfer coefficients, the best results are obtained using a stagnant film model for accounting transverse mass flux influence. For the prediction of pressure losses in two-phase condensing flow, the correlation with Lockhart-Martinelli parameters is used for zones of low liquid flow rates. For other channelzones are proposed its combination with an equation based on a dispersed annular model of flow structure. The presented equations can be used for calculation of heat and mass transfer intensity and pressure losses in two-phase condensing flow when designing PHE for condensation of steam from its mixture with air by local process parameters.

Predicting cutting power for band sawing process of pine and beech wood dried with the use of four different methods

Wood drying is an important stage in the woodworking process. After drying, wood is subject to a re-sawing process, for which a high quality surface, low material loss, and high efficiency are often required. In this paper, forecasted values were presented of cutting power for the re-sawing process of pine and beech wood that were dried with four different methods. Forecasting of cutting power for an industrial band saw machine that works daily in a Polish sawmill was determined. Values of cutting power were forecasted for a full range of feed speeds of the analyzed band saw machine. The achieved results allowed the observation of noticeable changes in the forecasted cutting power for a given sawing process as a function of the wood drying method applied. Significant changes were observed for pine wood, especially between air-dried pine wood and wood dried with warm air-steam mixture, and between pine wood dried in a conventional kiln and with warm air-steam mixture.

ГАЗООТВОДНАЯ СИСТЕМА НА НАЛИВНЫХ СУДАХ

Various gas discharge systems in tanks of oil tankers, their advantages and disadvantages are analyzed. the method of calculation of the gas discharge system to determine the volume flow of the steam-air mixture from a separate tank when loading the vessel and the resistance of the pipeline system in the first approximation is proposed.

Numerical Simulation of Steam Condensation in a Steam-Gas Mixture Flow in a Variable-Section Channel with a Bundle of Smooth Horizontal Tubes

In this paper, the results from calculations of heat and mass transfer in a variable-cross-section channel with a bundle of smooth horizontal tubes, on the surface of which steam from a moving steam-gas mixture (SGM) condenses, are presented. The decrease in the channel’s cross section and, accordingly, the number of tubes in the vertical rows along the SGM movement provides the mixture with approximately constant velocity as the steam condenses. The mathematical model used in this study is described in detail in our previous publications. The two-dimensional equations of single-phase hydrodynamics, energy, and diffusion are solved for the external SGM flow. The condensation process is modeled at the level of the boundary conditions on the tube surface, taking into account a moving laminar condensate film. The heat transfer through the tube wall from the film to the cooling water is described using a one-dimensional model of the wall. To account for the irrigation of the bundle’s lower tubes with condensate formed on the upper tubes (inundation effect), a simplified model is used. The data on the velocity fields and impurity concentration in the condenser and the heat transfer characteristics are presented. The calculation results of the heat transfer coefficients on the tubes of the first vertical row of the bundle and the heat transfer coefficients for individual sections of the simulated condenser containing several tube rows at a 0–8.5% volume fraction of air in the SGM at the inlet to the apparatus are compared with experimental data. A quite satisfactory agreement between the calculated and experimental data is obtained, which confirms the efficiency of the used model. The calculated data on the local velocity fields and the composition of the steam-air mixture indicate a significant heterogeneity of these characteristics. This complicates the development of relatively simple engineering methods for calculating the heat load of condensers at high air concentrations. The calculations were performed using in-house CFD-code ANES.