Condensation Of Pure Vapor Research Articles

Convective condensation in a stator–rotor–stator spinning disc reactor

Centrifugal intensification of condensation heat transfer in the rotor–stator cavities of a stator–rotor–stator spinning disc reactor (srs‐SDR) is studied, as a function of rotational velocity ω, volumetric throughflow rate , and average temperature driving force . For the current range of ω, heat transfer from the vapor bubbles to the condensate liquid is limiting, due to a relatively low gas–liquid interfacial area aGL. For rad s−1, a strong increase of aGL, results in increasing the reactor‐average condensation heat transfer coefficient hc from 1600 to 5600 W m−2 K−1, for condensation of pure dichloromethane vapor. Condensation heat transfer in the srs‐SDR is enhanced by rotation, independent of the vapor velocity. The intensified condensation comes at the cost of relatively high energy dissipation rates, indicating condensation in the srs‐SDR is more suited as a means to supply heat (e.g. in an intensified reactor‐heat exchanger), rather than for bulk cooling purposes. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3784–3796, 2016

Numerical study of nanofluids condensation heat transfer in a square microchannel

ABSTRACTThis paper presents a numerical study of nanofluids condensation heat transfer inside a single horizontal smooth square tube. The numerical results are compared with the previous experimental predictions. The numerical results show that the heat transfer coefficient could be improved within 20% by increasing the volume fraction of Cu nanoparticle by 5% or by increasing the mass flux from 80 to 110 kg/m2 s. Reducing the hydraulic diameter of the microchannel from 200 to 160 µm leads to an increase in the condensation average heat transfer coefficient by 10%. A new correlation estimating the Nusselt number for the condensation of nanofluids or pure vapor is proposed. It predicts average condensation heat transfer with a good agreement with those computed.

Effect of non-condensable gas on laminar film condensation of steam in horizontal minichannels with different cross-sectional shapes

In the present study, a 3-D numerical simulation of laminar film condensation of steam in the presence of non-condensable gas is performed in horizontal minichannels with six cross-sectional shapes based on the volume of fluid (VOF) method. Mixture of steam and oxygen enters the channel with uniform temperature, while the inlet volume fraction of oxygen increases from 0% to 3%. It is shown that the existence of non-condensable gas results in significant reduction in the mass transfer rate from vapor to liquid along the interface in the axial direction. And then the heat transfer coefficient of condensation with oxygen is demonstrated to decline sharply compared with that of the pure vapor condensation.

Forced Convective Condensation of R134A Vapor Inside a Vertical Microfin Tube

In this study, condensation of pure refrigerant R134a vapor inside a vertical 18° helical microfin tube was experimentally investigated. Tests were performed at saturation pressure of 5.7–5.9 bar with mass fluxes of 20–100 kg/m2s and heat fluxes of 1.7–5.3 kW/m2. The effects of mass flux and the temperature difference between the refrigerant and tube wall (ΔT) on the heat transfer performance were analyzed throughout experimental data. For experiments in which ΔT is more than 2.5°C, the average condensation Nusselt number showed a tendency to be independent from ΔT. Heat transfer enhancement ratio was found to be 1.59–1.71, which is always higher than the heat transfer area enhancement factor (1.55). Fins always act as a turbulence promoter in the given experimental data range. Finally, the most widely used heat transfer coefficient correlations for condensation inside microfin tubes were analyzed through the experimental data. Best fit was obtained with Yu and Koyama's correlation with an absolute mean deviation of 17% and Kedzierski and Goncalves's correlation with an absolute mean deviation of 19%.

Transient condensation of flowing vapor on a flat-plate: A scaling analysis

We perform an analysis of condensation of pure vapor flowing over a cooled flat plate. We estimate the time taken to achieve steady-state condensation by a transient analysis where we use results from previous studies that show that the time-dependent behavior is governed by the propagation of a kinematic wave along the condensate film. The steady-state time at any streamwise location along the plate depends on the steady-state film thickness at that location. Classical theories of laminar steady-state condensation are reviewed, and a scaling analysis is performed to capture the scalings for relevant quantities such as the liquid film thickness, liquid velocity, and heat transfer coefficient. The results from the scaling analysis are entirely consistent with the classical theories. Finally, the scaling analysis is extended to take into account effects of turbulence in the liquid film.

Numerical Study of Nanofluid Condensation Heat Transfer in a Square Microchannel

This study presents a numerical study of nanofluid condensation heat transfer inside a single horizontal smooth square tube. The numerical results are compared to previous experimental predictions, and show that the heat transfer coefficient can be improved 20% by increasing the volume fraction of Cu nanoparticles by 5% or increasing the mass flux from 80 to 110 kg/m2 s. Reducing the hydraulic diameter of the microchannel from 200 to 160 µm led to an increase in average condensation heat transfer coefficient of 10%. A new correlation estimating Nusselt number for condensation of nanofluids or pure vapor is proposed. It predicts average condensation heat transfer, with good agreement with the computed values.

Numerical Simulation of Laminar Film Condensation in a Horizontal Minitube with and Without Non-Condensable Gas by the VOF Method

Based on the volume of fluid (VOF) method, a steady three-dimensional numerical simulation of laminar film condensation of water vapor in a horizontal minitube, with and without non-condensable gas, has been conducted. A user-defined function defining the phase change is interpreted and the interface temperature is correspondingly assumed to be the saturation temperature. An annular flow pattern is to be expected according to a generally accepted flow regime map. The heat-transfer coefficient increases with higher saturation temperature and a smaller temperature difference between the saturation and wall temperatures, but varies little with different mass flux and degree of superheat. The existence of a non-condensable gas will lead to the generation of a gas layer between vapor and liquid, resulting in a lower mass-transfer rate near the interface and higher vapor quality at the outlet. In consequence, the heat-transfer coefficient of condensation with a non-condensable gas drops sharply compared with that of pure vapor condensation. Meanwhile, the non-condensable gas with a smaller thermal conductivity would cause a stronger negative effect on heat flux as a result of a higher thermal resistance of heat conduction in the non-condensable gas layer.

Measurement of enhanced heat transfer coefficient with perforated twisted tape inserts during condensation of R-245fa

The experimental conductive heat transfer results for flow through inserted perforated twisted tapes in a horizontal tube during condensation of pure R-245fa vapor. The test section consisting of two separate coaxial double pipes assembled in series, acted like a counter flow heat exchanger, where the refrigerant condensed inside the inner tube by rejecting heat to the cooling water flowing inside the outer tube in reversed direction. Data for three perforated twisted tapes having constant twist ratio of 7.1 mm and pitch of perforation as 12.5, 25.0 and 37.5 mm, inserted one by one in full length of test condenser by varying refrigerant mass flux from 100 to 200 kg/m2 s in steps of 50 kg/m2 s for the range of vapor quality from 0.1 to 0.9, were collected together with flow and without insert (plain tube). It has been found that the perforated twisted tape insert having pitch of perforation equal to in order of 12.5 mm gives the highest value of average heat transfer coefficient and is of the order of 37.5 % more than that of the plain one and the correlation predicts the experimental data within an error band of ±15 %.

Experimental investigation of condensation heat transfer on hybrid wettability finned tube with large amount of noncondensable gas

Condensation heat transfer outside horizontal plain and finned tubes with different surface wettability is experimentally studied. The methods of self-assembled monolayer coatings of n-octadecyl mercaptan with oxidation and etching treatments are employed to create the hydrophobic surfaces or superhydrophobic surfaces with nanostructures. The hydrophilic–hydrophobic (HH) hybrid and the hydrophilic–superhydrophobic (HS) hybrid surfaces based on the finned tubes are fabricated as well. The experimental results show that the HS hybrid finned tube achieves the highest condensation heat transfer performance in the presence of a large amount of noncondensable gas while it does not exhibit superior performance for the pure vapor condensation. The fraction of water vapor in the nitrogen–vapor mixture has significant influence on the condensation heat transfer coefficient. The dropwise condensation transforms gradually to the film condensation on both the superhydrophobic and the HS hybrid finned tubes with the increase of water vapor fraction.

Laminar film condensation of saturated vapor on an isothermal vertical cylinder

An analytical model for laminar film condensation of pure vapor on the outer surface of an isothermal vertical tube is presented. Based on the assumptions employed in the Nusselt’s analysis on a flat plate, the explicit expressions for the condensate film thickness as well as local and average Nusselt numbers are derived. The heat transfer from the liquid–vapor interface is dealt with for two contrary situations: with and without the condensate subcooling. The deviation of the Nusselt number of a cylinder from that of a flat plate is presented in terms of the ratio of the liquid film thickness to the radius of a tube and the Jakob number. The variations of the Nusselt number with the tube radii are calculated from the derived models to investigate the curvature effects of a condensing surface. It is demonstrated that the effect of the curvature on the condensation heat transfer becomes important when the radius of a tube is of the similar order of magnitude as the liquid film thickness.

Heat Transfer Characteristics for Condensation of R134a in a Vertical Smooth Tube

In this study, condensation of pure refrigerant R134a vapor inside a smooth vertical tube was experimentally investigated. The test section was made of a copper tube with inside diameter of 7.52 mm and length of 1 m. Experimental tests were conducted for mass fluxes in the range of 20–175 kg/m2s with saturation pressure ranging between 5.8 and 7 bar. The effects of mass flux, saturation pressure, and temperature difference between the refrigerant and tube inner wall (ΔT) on the heat transfer performance were analyzed through experimental data. Obtained results showed that average condensation heat transfer coefficient decreases with increasing saturation pressure or temperature difference (ΔT). In addition, for the same temperature difference (ΔT), heat can be removed from the refrigerant at a higher rate at relatively low pressure values. Under the same operating conditions, it was shown that average condensation heat transfer coefficient increases as mass flux increases. Finally, the most widely used heat transfer coefficient correlations for condensation inside smooth tubes were analyzed through the experimental data. The best fit was obtained with Akers et al.'s (1959) correlation with an absolute mean deviation of 22.6%.

Dropwise condensation on superhydrophobic nanostructured surfaces: literature review and experimental analysis

It is well established that the dropwise condensation (DWC) mode can lead up to significant enhancement in heat transfer coefficients as compared to the filmwise mode (FWC). Typically, hydrophobic surfaces are expected to promote DWC, while hydrophilic ones induce FWC. To this end, superhydrophobic surfaces, where a combination of low surface energy and surface texturing is used to enhance the hydrophobicity, have recently been proposed as a promising approach to promote dropwise condensation. An attractive feature of using superhydrophobic surfaces is to facilitate easy roll-off of the droplets as they form during condensation, thus leading to a significant improvement in the heat transfer associated with the condensation process.High droplet mobility can be obtained acting on the surface chemistry, decreasing the surface energy, and on the surface structure, obtaining a micro- or nano- superficial roughness. The first part of this paper will present a literature review of the most relevant works about DWC on superhydrophobic nanotextured substrates, with particular attention on the fabrication processes. In the second part, experimental data about DWC on superhydrophobic nanotextured samples will be analyzed. Particular attention will be paid to the effect of vapour velocity on the heat transfer. Results clearly highlight the excellent potential of nanostructured surfaces for application in flow condensation applications. However, they highlight the need to perform flow condensation experiments at realistic high temperature and saturation conditions in order to evaluate the efficacy of superhydrophobic surfaces for practically relevant pure vapor condensation applications.

An analytical method for Wilson point in nozzle flow with homogeneous nucleating

The prediction of properties at the Wilson point is the key to both theoretical and numerical researches of the condensation of pure vapor or moist gas by homogeneous nucleation. An analytical solution for position and flow properties of Wilson point in nozzle flow at low pressure was presented. This method is adopted the classical nucleation rate and the droplet growth rate by Young and based on Lagrangian momentum, energy and Clausius–Clapeyron equations. Compared with the rapid change of the heat release rate, the changes of many properties and parameters are reasonably simplified or ignored. In addition, the expansion rate coefficient obtained by Rayleigh line relationships was considered. The position of Wilson point and the corresponding properties can be efficiently and effectively found by this method, only with the given values of the nozzle geometry, inlet stagnation pressure and temperature. The analytical solution agrees well with the numerical results and the experiment by Wyslouzil. Finally, some important results about the variation tendency of the parameters were obtained to illustrate the power and contribution of the present method, which would be helpful to research in further theory, simulation and practice.

Condensation on downward-facing surfaces subjected to upstream flow of air–vapor mixture

This paper reports experimental results for film condensation on vertical and downward inclined smooth and grooved surfaces subjected to ascending streams of a vapor–air mixture. An experimental facility was built in order to evaluate the heat fluxes for different surface inclinations, surface sub-cooling temperatures and air to vapor ratios in the mixture. Two experimental procedures were employed to evaluate the heat absorption rate during the condensation process. The employment of grooved surfaces resulted in a 10% enhancement of the condensation rate in the case of the pure vapor condensation and a negligible effect when noncondensable gases (NCGs) were present in the mixture.

THEORETICAL AND EXPERIMENTAL STUDY OF CONVECTIVE CONDENSATION INSIDE A CIRCULAR TUBE

This paper presents a theoretical modeling and numerical and experimental investigations of the laminar convective condensation inside a circular smooth tube. The developed model includes the interaction between the surface tension, gravity, and shear stresses at the vapor‐liquid interface and its cross influence on heat transfer. The influence of the gravity force, tube diameter, and temperature drop on the heat transfer at in-tube condensation of pure ethanol vapor is considered. The heat transfer coefficient as a function of the inclination angle of the condenser tube and the temperature drop between the vapor saturation and the wall temperatures is measured. A comparison of the experimental and numerical data is performed. The results of the numerical calculation are in good agreement with the experimental results. Numerical experiments have been carried out with the aim of predicting the time length of the transition to the steady-state regime after an abrupt change of the gravity level. It is shown that the transition time to the steady-state regime increases with an increase in the diameter of the condenser tube. The transition time to the steady-state regime under microgravity conditions is longer than that under normal gravity for a tube with a diameter larger than the value close to the capillary length of the working liquid.

Experimental Study of Laminar Convective Condensation of Pure Vapor Inside an Inclined Circular Tube

Convective condensation of pure ethanol vapor inside a smooth tube of inner diameter 4.8 mm and of length 200 mm is studied. The experiments have been carried out at temperature 58°C corresponding to the pressure of 440 mbar, the vapor mass velocity varying from 0.24 to 2.04 kg/(m2 s). The dependency of the Heat Transfer Coefficient (HTC) is investigated experimentally both subject to the temperature difference between the saturated vapor and the wall and subject to the condenser inclination. The results show that the HTC reduces with growth of the temperature difference. The dependency of the HTC on inclination has a maximum in the range 15°–35° due to the complex gravity drainage mechanism of the condensed liquid. The results could be useful for development of compact effective cooling systems for space and ground application.

Revisit of Laminar Film Condensation Boundary Layer Theory for Solution of Mixed Convection Condensation With or Without Noncondensables

This work presents a unique and unified formulation to solve the laminar film condensation two-phase boundary layer equations for the free, mixed, and forced convection regimes in the absence or presence of noncondensables. This solution explores the vast space of mixed convection across the four cornerstones of laminar film condensation boundary layer theory, two established by Koh for pure vapor condensation in the free or forced convection regimes and the other two established by Sparrow corresponding to condensation with noncondensables. This formulation solves the space of mixed convection completely with Koh and Sparrow’s solutions shown to be merely four specific cases of the current solution.

Exergy Efficiency of Two-Phase Flow in a Shell and Tube Condenser

This study deals with a comprehensive efficiency investigation of a TEMA “E” shell and tube condenser through exergy efficiency as a potential parameter for performance assessment. Exergy analysis of condensation of pure vapor in a mixture of non-condensing gas in a TEMA “E” shell and tube condenser is presented. This analysis is used to evaluate both local exergy efficiency of the system (along the condensation path) and for the entire condenser, i.e., overall exergy efficiency. The numerical results for an industrial condenser with a steam–air mixture and cooling water as working fluids indicate significant effects of temperature differences between the cooling water and the environment on exergy efficiency. Typical predicted cooling water and condensation temperature profiles are illustrated and compared with the corresponding local exergy efficiency profiles, which reveal a direct (inverse) influence of the coolant (condensation) temperature on the exergy efficiency. Further results provide verification of the newly developed exergy efficiency correlation with a set of experimental data.

Condensation from a vapor-gas mixture on a plane surface

An approach is developed, which was previously suggested for the investigation of intensive condensation of pure vapor in application to a one-dimensional steady-state problem of condensation in the presence of a noncondensable component. Results are obtained, which make possible the estimation of the parameters of the limiting modes of condensation from a vapor-gas binary mixture.

Condensation heat transfer of R-134a inside a microfin tube with different tube inclinations

Experimental heat transfer studies during condensation of pure R-134a vapor inside a single microfin tube have been carried out. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The data are acquired for seven different tube inclinations, α, in a range of −90 to +90° and three mass velocities of 54, 81, and 107 kg/m 2-s for each inclination angle during condensation of R-134a vapor. The experimental results indicate that the tube inclination angle of, α, affects the condensation heat transfer coefficient in a significant manner. The highest heat transfer coefficient is attained at inclination angle of α = +30°. The effect of inclination angle, α, on heat transfer coefficient, h, is more prominent at low vapor quality and mass velocity. A correlation has also been developed to predict the condensing side heat transfer coefficient for different vapor qualities and mass velocities.