Experimental investigation of Marangoni condensation of ethanol–water mixture vapors on vertical tube

The heat transfer characteristics of the condensation of ethanol–water binary vapor on vertical tubes with the pipe diameter of 10 mm were investigated experimentally. The results showed that, with the change of the vapor-to-surface temperature difference, the condensation heat transfer coefficients revealed nonlinear characteristics with peak values under a wide variety of operating conditions. With the increasing pressure or velocity of the vapor, the heat transfer coefficients increased subsequently. The effect of vapor pressure or velocity on heat transfer coefficients reduced with the increasing ethanol mass fraction. It was noteworthy that, under low ethanol mass fractions (0.5–2%), the heat transfer coefficients augmented significantly, were about 5–8 times greater than that of pure steam. The comparison for different test blocks indicated that the condensation heat transfer coefficients for different pipe diameters were about the same value under the same operating condition. Significant heat transfer enhancement by Marangoni condensation could be achieved for full range of pipe diameter used in industrial condensers.     

Experimental investigation on heat transfer of forced convection condensation of ethanol–water vapor mixtures on a vertical mini-tube

In this paper, condensation heat transfer characteristics of ethanol–water vapor mixtures on a vertical mini-vertical tube with 1.221 mm outside diameter were investigated experimentally. The experiments were performed at different velocities and pressures over a wide range of ethanol mass fractions in vapor. The test results indicated that, with respect to the change of the vapor-to-surface temperature difference, the condensation curves of the heat transfer coefficients revealed nonlinear characteristics, and had peak values. At 2 % ethanol mass fraction in vapor, the condensation heat transfer coefficient value of the ethanol–water vapor mixture was found to have a maximum heat transfer coefficient of 50 kW m^?2 K^?1, which was 3–4 times than that of pure steam. The condensation heat transfer coefficients decreased with increased ethanol mass fraction in vapor. The vapor pressure and vapor velocity had a positive effect on the condensation heat transfer coefficients of ethanol–water vapor mixtures.     

Heat and mass transfer characteristics of simulated high moisture flue gases

The heat transfer process occurring in a condensing heat exchanger where noncondensible gases are dominant in volume is different from the condensation heat transfer of the water vapor containing small amount of noncondensible gases. In the process the mass transfer due to the vapor condensation contributes an important part to the total heat transfer. In this paper, the Colburn-Hougen method is introduced to analyze the heat and mass transfer process when the water vapor entrained in a gas stream condenses into water on the tube wall. The major influential factors of the convective-condensation heat transfer coefficient are found as follows: the partial pressure of the vapor p _v , the temperature of the outer tube wall T _w , the mixture temperature T _g , Re and Pr. A new dimensionless number Ch, which is defined as condensation factor, has been proposed by dimensional analysis. In order to determine the relevant constants and investigate the convection-condensation heat and mass transfer characteristics of the condensing heat exchanger of a gas fired condensing boiler, a single row plain tube heat exchanger is designed, and experiments have been conducted with vapor-air mixture used to simulate flue gases. The experimental results show that the convection-condensation heat transfer coefficient is 1.52 times higher than that of the forced convection without condensation. Based on the experimental data, the normalized formula for convention-condensation heat transfer coefficient is obtained. A heat transfer area m² - Ch condensation factor - c _p specific heat at constant pressure, J/(kg·K) - G mass flux Kg/(m²·s) - h heat transfer coefficient W/(m²·K) - J J-factor - Nu Nusselt number - pa pressure - Pr Prandtl number - Q heat transfer rate - q heat flux W/m² - r latent heat, kJ/kg - Re Reynolds number - Sc Schmidt number - T temperature, C or K - heat conductivity m W/(m·K) - density, kg·m³ - g gas - h moistened hot air - i interface - v vapor - w water     

Laminar film condensation from downward flowing superheated vapors onto a non-isothermal sphere

 A model is developed for the study of mixed convection film condensation from downward flowing superheated vapors onto a sphere with variable wall temperature. The model combined natural convection dominated and forced convection dominated film condensation, including effects of superheated vapor, pressure gradient and wall temperature variation can be solved numerically by the fourth-order Runge–Kutta technique. By the present numerical approach, the mean heat transfer is evaluated up to the critical angle of the condensate layer, φ_c. In general, the result of mean heat transfer shows that, as A, the wall-temperature amplitude, increases, the value of with inclusion of P, the pressure gradient effect, goes down slightly, however, the value of with the pressure gradient effect ignored will remain almost uniform. Further, for P=2.0, the mean heat transfer coefficient increases significantly, by 8.6–23.9%, depending on A, as the superheat parameter, S p, increases within a practical range. Received on 7 September 2000     

Heat and mass transfer analogy for condensation of humid air in a vertical channel

This study examines energy transport associated with liquid film condensation in natural convection flows driven by differences in density due to temperature and concentration gradients. The condensation problem is based on the thin-film assumptions. The most common compositional gradient, which is encountered in humid air at ambient temperature is considered. A steady laminar Boussinesq flow of an ideal gas–vapor mixture is studied for the case of a vertical parallel plate channel. New correlations for the latent and sensible Nusselt numbers are established, and the heat and mass transfer analogy between the sensible Nusselt number and Sherwood number is demonstrated. Received on 15 November 1999     

Buoyancy effects on the mass transfer in absorption with a nonabsorbable gas

This paper describes the vapor side buoyancy effects on the mass transfer in absorption in the presence of a nonabsorbable gas. Experimental results on a diffusion-absorption refrigerator (DAR) indicate that the vapor side buoyancy effects on the mass transfer are significant when the density of the nonabsorbable gas is significantly less than that of the absorbable gas. A rectangular enclosure absorption problem is first solved to demonstrate the buoyancy effects without the presence of a forced flow. Then, mixed convection heat transfer in a circular pipe is simulated in such a way as to be analogous to the mixed convection mass-transfer problem in the DAR absorber. Finally, the vapor side mixed convection absorption between parallel plates is simulated including the effects of the absorbed mass on the mass balance. The Sherwood number dependence on the mass transfer Grashof number and Reynolds number as well as the effects of the suction boundary conditions are discussed. Each of these simulations had individual limitations, but, taken together, they illuminate the major aspects of the absorption physics.     

Effects of vapor pressure/velocity and concentration on condensation heat transfer for steam–ethanol vapor mixture

When a steam–ethanol vapor mixture condenses on a vertical flat plate, the form of the condensate film changes and many drops are created. This non-film condensation is called pseudo-dropwise or Marangoni condensation. This paper aims to study the main influencing factors on the Marangoni condensation of steam–ethanol vapor.The factors include the ethanol concentration, vapor pressure, vapor velocity and vapor-to-surface temperature difference. The experiments show that the heat transfer coefficient has a maximum value of approximately 42 kW/m² K when the ethanol concentration is 1%. At the low concentrations of 0.5, 1, 5.1 and 9.8%, the condensation heat transfer is greater than for pure steam. In addition, the heat transfer for all vapor mixtures increases with both the rise of vapor pressure and vapor velocity.     

R410A condensation inside a commercial brazed plate heat exchanger

This paper presents the heat transfer coefficients and the pressure drop measured during HFC-410A condensation inside a commercial brazed plate heat exchanger: the effects of saturation temperature, refrigerant mass flux and vapour super-heating are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature and great sensitivity to refrigerant mass flux and vapour super-heating. At low refrigerant mass flux (<20 kg/m² s) the saturated vapour condensation heat transfer coefficients are not dependent on mass flux and are well predicted by Nusselt [W. Nusselt, Die oberflachenkondensation des wasserdampfes, Energy 60 (1916) 541–546, 569–575] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (>20 kg/m²s) the saturated vapour condensation heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [W.W. Akers, H.A. Deans, O.K. Crosser, Condensing heat transfer within horizontal tubes, Chem. Eng. Prog. Symp. Series 55 (1959) 171–176] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by Webb [R.L. Webb, Convective condensation of superheated vapor, ASME J. Heat Transfer 120 (1998) 418–421] model. A simple linear equation based on the kinetic energy per unit volume of the refrigerant flow is proposed for the computation of the frictional pressure drop.     

Experimental study on the condensation of ethanol–water mixtures on vertical tube

The condensation heat transfer of the ethanol–water mixtures on the vertical tube over a wide range of ethanol concentrations was investigated. The condensation curves of the heat flux and the heat transfer coefficients revealed nonlinear characteristics and had peak values, with respect to the change of the vapor-to-surface temperature difference. This characteristic applies to all ethanol concentrations under all experimental conditions. With the decrease of the ethanol concentrations, the condensation heat transfer coefficient increased notably, especially when the ethanol concentration was very low. The maximum heat transfer coefficient of the vapor mixtures increased to 9 times as compared with that of pure steam at ethanol vapor mass concentration of 1%. With the increase of the ethanol concentrations, the condensation heat transfer coefficient decreased accordingly. When the ethanol concentration reached 50%, the heat transfer coefficient was smaller than that of the pure steam.     

Parametric studies and correlations for combined conduction-mixed convection–radiation from a non-identically and discretely heated vertical plate

The present paper reports the parametric studies and correlations for the problem of combined conduction-mixed convection–radiation from a non-identically and discretely heated vertical plate. Three discrete heat sources of non-identical heights but with identical volumetric rate of heat generation are assumed to be flush-mounted in a thin vertical plate. The longest and the shortest heaters are provided at the leading and trailing edges of the plate, while the remaining heater is located centrally. The governing fluid flow and heat transfer equations are considered in their full strength without the boundary layer approximations and are solved using the finite volume method. A computer code is written to solve the problem and various parametric studies have been performed. The relative roles of free convection, forced convection and radiation in various fluid flow and heat transfer results have been elucidated. In conclusion, based on a large set of data generated from the code, correlations for maximum non-dimensional plate temperature, average non-dimensional plate temperature and mean friction coefficient have been evolved.     

Calculating mass transfer from vertical wet fabrics using a free convection heat transfer correlation

In this note, the evaporation rate from a vertical wet fabric sheet is calculated using a free convection heat transfer correlation. Chilton–Colburn analogy is used to derive a mass transfer correlation from a heat transfer correlation proposed by Churchill and Chu for free convection from a vertical isothermal plate. The mass transfer rate obtained from this expression has shown excellent agreement with experimental data.     

A Non-isothermal Pore Network Drying Model with Gravity Effect

The concept of immiscible displacement as an invasion percolation (IP) process driven by heat and mass transfer is used in a pore network model for convective drying of capillary porous media. The coupling between heat and mass transfer occurs at the liquid–gas interface through temperature-dependent equilibrium vapor pressure and surface tension as well as the phase change enthalpy (in evaporation and condensation). The interfacial effects due to capillary forces and gravity are combined in an invasion potential; viscous forces are neglected. Simulation results show stabilized invasion patterns and finite drying front width by the influence of gravity.     

Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO<Subscript>2</Subscript> Sequestration

Dissolution of CO₂ into brine causes the density of the mixture to increase. The density gradient induces natural convection in the liquid phase, which is a favorable process of practical interest for CO₂ storage. Correct estimation of the dissolution rate is important because the time scale for dissolution corresponds to the time scale over which free phase CO₂ has a chance to leak out. However, for this estimation, the challenging simulation on the basis of convection–diffusion equation must be done. In this study, pseudo-diffusion coefficient is introduced which accounts for the rate of mass transferring by both convection and diffusion mechanisms. Experimental tests in fluid continuum and porous media were performed to measure the real rate of dissolution of CO₂ into water during the time. The pseudo diffusion coefficient of CO₂ into water was evaluated by the theory of pressure decay and this coefficient is used as a key parameter to quantify the natural convection and its effect on mass transfer of CO₂. For each experiment, fraction of ultimate dissolution is calculated from measured pressure data and the results are compared with predicted values from analytical solution. Measured CO₂ mass transfer rate from experiments are in reasonable agreement with values calculated from diffusion equation performed on the basis of pseudo-diffusion coefficient. It is suggested that solving diffusion equation with pseudo diffusion coefficient herein could be used as a simple and rapid tool to calculate the rate of mass transfer of CO₂ in CCS projects.     

Experimental studies on mixed convection heat transfer in laminar flow through a plain square duct

This paper reports the findings of experimental studies on combined free and forced convection through a plain square duct in laminar region. The test fluid flows through an inner square duct, hot water at high flow rate circulated through a annular channel formed between square duct and circular tube, in counter current fashion to attain a nearly uniform wall temperature conditions. The importance of mixed convection is judged by the value of the Richardson number (Ri). It was observed that at low Reynolds number, heat transfer was mainly governed by mixed convection. However at higher values of Reynolds number, heat transfer was significantly dominated by forced convection. It was found that Reynolds number higher than 1050 for water and 480 for ethylene glycol resulted in laminar forced convention heat transfer. The empirical correlation developed for Nusselt number in terms of Grashoff number and Graez number, was found to fit with experimental Nusselt number within ±11 and ±12?% for water and ethylene glycol respectively.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in small diameter tubes

The condensation heat transfer of pure refrigerants, R-22, R-134a and a binary refrigerant R-410A flowing in small diameter tubes was investigated experimentally. The condenser is a countflow heat exchanger which refrigerant flows in the inner tube and cooling water flows in the annulus. The heat exchanger is smooth, horizontal copper tube of 1.77, 3.36 and 5.35 mm inner diameter, respectively. The length of heat exchanger is 1220, 2660 and 3620 mm, respectively. The experiments were conducted at mass flux of 200–400 kg/m² s and saturation temperature of 40°C. The main results were summarized as follows: in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski and Wu and Little correlation. The new single-phase correlation based on the experimental data was proposed in this study. In case of two-phase flow, the condensation heat transfer coefficient of R-410A for three tubes was slightly higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. The condensation heat transfer coefficient for R-22, R-134a and R-410A increased with increasing mass flux and decreasing tube diameter. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensation heat transfer. Therefore, the new condensation heat transfer correlation based on the experimental data was proposed in the present study.     

Heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a heat exchanger

This paper describes heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a fin-tube heat exchanger. The experiments in which the moist air was passed into the heat exchanger coated with sorption material were conducted under various conditions of air flow rate (0.5–1.0 m/s) and the temperature of brine (14–20°C) that was the heat transfer fluid to cool the air flow in the dehumidifying process. It is found that the sorption rate of vapor is affected by the air flow rate and the brine temperature. Meanwhile, the attempt of clarifying the sorption mechanism is also conducted. Finally the average mass transfer coefficient of the organic sorbent coated on heat transfer surface of a fin-tube heat exchanger is non-dimensionalzed as a function of Reynolds number and non-dimensional temperature, and it is found that the effect of non-dimensional temperature on them is larger than Reynolds number .     

Experimental study on the application of paraffin slurry to high density electronic package cooling

Experiments were performed by using water and paraffin slurry to investigate thermal characteristics from a test multichip module. The parameters were the mass fraction of paraffin slurry (0, 2.5, 5, 7.5%), heat flux (10, 20, 30, 40 W/cm²) and channel Reynolds numbers. The size of paraffin slurry particles was within 10–40 μm. The local heat transfer coefficients for the paraffin slurry were larger than those for water. Thermally fully developed conditions were observed after the third or fourth row. The paraffin slurry with a mass fraction of 5% showed the most efficient cooling performance when the heat transfer and the pressure drop in the test section were considered simultaneously. A new correlation for the water and the paraffin slurry with a mass fraction of 5% was obtained for a channel Reynolds number over 5300. Received on 25 January 1999     

Effect of salt on boiling heat transfer of ammonia-water mixture

Nucleate pool boiling heat transfer coefficients were determined experimentally for NH₃–H₂O, NH₃–H₂O–LiNO₃ and NH₃–H₂O–LiBr mixtures. Both the salts were effective in increasing the heat transfer coefficient of NH₃–H₂O mixture. A concentration of 10 mass% of the salts in water, produced the greatest enhancement in heat transfer coefficient at all the range of pressure, heat flux and ammonia concentration studied in this investigation. The experiments indicated that ammonia concentration also has the impact on the augmentation of heat transfer coefficient in NH₃–H₂O binary mixture by the addition of salts. For the solution of ammonia mass fraction 0.30, high concentration of LiBr gives the highest heat transfer coefficient, for ammonia mass fraction of 0.25, high concentration of LiNO₃ gives the maximum heat transfer coefficient, for ammonia mass fraction of 0.15, both the salts are equally effective in increasing the heat transfer coefficient.     

Effects of wetted walls on laminar natural convection between vertical parallel plates with asymmetric heating

A numerical analysis is carried out to investigate the effect of latent heat transfer, in association with the evaporation or condensation of the water vapor on the wetted channel walls which are under asymmetric heating, on the natural convection heat transfer. Major nondimensional groups identified are Gr _T , Gr _M , Pr and Sc. Results for Nusselt and Sherwood numbers are specifically presented for an air-water system under various heating conditions to illustrate latent heat transfer during the evaporation and condensation processes. The effects of the channel length and wetted wall temperatures on the momentum, heat and mass transfer are examined in great detail.     

Nucleate pool boiling of aqueous solution of citric acid on a smoothed horizontal cylinder

Nucleate pool boiling heat transfer coefficients were measured during pool boiling of the mixtures of Citric acid/water on a horizontal heated Cylinder. The experiment was done at atmospheric pressure and heat fluxes up to 113 kW m⁻² and mass fraction range 0.1496–0.613 over all ranges of mass fraction, the heat transfer coefficients of the mixtures are markedly less than those in single component substances and, in particular, are dramatically deteriorated in the vicinity of both single component substances. An applicability of existing correlations to the present experimental data is discussed. As a result, it is difficult for any existing correlation to predict the true values of pool boiling heat transfer coefficients over all ranges of mass fraction in mixtures of citric acid/water. Available correlation results were not exactly adapted to experimental data and for the best estimation, a new modified model based on Stephan-Kroner has been achieved with reasonable accuracy. Also the status of bubble generation showed that nucleation site density is strictly functioning of heat flux.