Convective Evaporation Research Articles

Boiling heat transfer of a ternary refrigerant mixture inside a horizontal smooth tube

Abstract A theoretical model for predicting the forced convective boiling heat transfer coefficients of ternary mixtures is presented. The effect of mass diffusion near the gas-liquid interface is considered in both nucleate boiling and forced convection vaporization contributions. The predicted heat transfer coefficients are compared with experimental data, which are obtained for an HFC32: 125: 134a (23: 25: 52 wt%) mixture inside a horizontal smooth tube 6.0 mm i.d. and 4.0 m long, and in the ranges of heat flux 10–20 kW m−2 and of mass flux 150–400 kg m−2 s−1. The present predictive method shows reasonable agreement with the experimental data within an accuracy of ±30%.

Wave and Turbulent Film Condensation on a Vertical Surface. Correlation for Average Heat-Transfer Coefficient.

The experimental study of body forced convection of pure vapor on a vertical surface (l=2980 mm) is presented. CFC11, CFC113 and HCFC123 are used as the working fluids. The flow patterns of condensate film are classified into four kinds of flow : L flow (laminar flow), L-S flow (laminar and sine wave flow), L-S-H folw (laminar, sine wave and harmonic wave flow), L-S-H-T flow (laminar, sine wave, harmonic wave and turbulent flow). The nondimensional correlations for predicting the average heat-transfer coefficients are obtained for four kinds of flow and compared with the experimental results, respectively. Assuming more simple four kinds of flow patterns, which are L flow, S flow (sine wave flow), H folw (harmonic wave flow) and T flow (turbulent flow), simple nondimensional correlations for predicting the average heat-transfer coefficients are also obtained for S flow, H flow and T flow. The nondimensional correlations for combined H and T flow are also obtained.

Diurnal variations of net radiation at a tropical station ? Osu; Nigeria

The diurnal variations of net radiation have been studied by analysing one year data measured at a tropical station, Osu (7.43° N, 4.58° E), in Nigeria. The maximum net daytime flux (which occurs around 14h local time) varies in the course of the year from 382.6±136.7Wm−2 in the wet season (April–October) to 480.3±61.8 Wm−2 in the dry season (November–March). The low values (and large fluctuations) of the hourly means recorded during the wet (monsoon) season are attributed to the important roles that the convective clouds and water vapour play in the atmospheric radiation budget, which is very pronounced in the tropical areas of West Africa.

Effect of lubricating oil contamination on evaporation in refrigerants R12 and R22

An investigation has been made into the effect of oil concentration on evaporation heat transfer coefficients in refrigerant-oil mixtures flowing in a horizontal tube. A new correlation is presented for heat transfer coefficients in convective evaporation of refrigerant-oil mixtures that predicts the results of the present study within approximately ±20%. The paper reports measurements of evaporation heat transfer coefficients in refrigerants R12 and R22, both oil-free and with two concentrations of Shell Clavus 32 oil. A 1·8 m long ⅜ in O/D copper tube (8·05 mm I/D) was used, at evaporation temperatures of −5°C, 0°C and +5°C. Heat flux and mixture mass velocity were kept constant at 2500 W m−2 and 155 kg m−2 s−1, respectively, and measured coefficients were in the range of 1400 to 3900 W m−2 K−1. The results showed that, for a complete evaporator, 2% oil may be expected to increase the heat transfer coefficient by 12%, but 10% oil returns the coefficient to oil-free values.

Effect of lubricating oil contamination on evaporation in refrigerants R12 and R22

An investigation has been made into the effect of oil concentration on evaporation heat transfer coefficients in refrigerant-oil mixtures flowing in a horizontal tube. A new correlation is presented for heat transfer coefficients in convective evaporation of refrigerant-oil mixtures that predicts the results of the present study within approximately ±20%. The paper reports measurements of evaporation heat transfer coefficients in refrigerants R12 and R22, both oil-free and with two concentrations of Shell Clavus 32 oil. A 1·8 m long ⅜ in O/D copper tube (8·05 mm I/D) was used, at evaporation temperatures of −5°C, 0°C and +5°C. Heat flux and mixture mass velocity were kept constant at 2500 W m−2 and 155 kg m−2 s−1, respectively, and measured coefficients were in the range of 1400 to 3900 W m−2 K−1. The results showed that, for a complete evaporator, 2% oil may be expected to increase the heat transfer coefficient by 12%, but 10% oil returns the coefficient to oil-free values.

Salient features of heat transfer during vigorous surface evaporation and vaporization of sedimentation products in a rotor film-type evaporator

UDC 535.52:66.045 Processing of solutions and suspensions that yields a dry end product is a promising application of rotor film-type evaporators (RFEs), which make it possible to replace periodic multistage processes with continuous combined processes. In view of this, it is a very urgent matter to study the heat exchange during deep concentration of solutions and suspensions. As shown by many studies, heat exchange in RFEs is complex and is determined by both the hydrodynamic conditions in the evaporator and the properties of the product being processed, the design features and rotation frequency of the rotor, and the heat flux density, Despite the complexity of the process, numerous attempts have been made to obtain an equation to describe it. In studies [1, 2] on heat exchange in an articulated-rotor RFE during evaporation of toluene, water, ethyl alcohol, and sugar syrup Dieter concluded that the heat-transfer coefficient does not depend on the spraying density and the heat flux density and increases with the rotation frequency of the rotor. The data reported by different authors for articulated-rotor evaporators are very contradictory. In [3] Marchenko obtained a criterial dependence by generalizing experimental data on vaporization of water, but it takes no account of the influence of the rotation frequency of the rotor on the heat-transfer coefficient. In [4] Kibrik established a relation between the heat-transfer coefficient and the design features of the rotor. In a study of convective heating and evaporation of distilled water, glycerin solutions, and solutions of S-150 additive in benzine in an articulated-rotor RFE [5] Golova obtained criterial equations, which in our opinion take insufficient account of the influence of the design features of the rotor, since the only the influence of the number of rotors was considered. Shevchuk [6] investigated how the heat transfer is affected by a large number of factors: the flow rate of the liquid, the physicochemieal properties of the products studied (distilled water, aqueous solutions of glycerin and esters of unsaturated higher fatty acids), the rotation frequency of the rotor, the direction of the specific heat flux (during convective heat exchange), the instantaneous length of the evaporator, and the design features of the rotor. The studies were carried out in glass and metal evaporators. After analyzing the hydrodynamic and heat-exchange laws by using a semi-empirical theory of turbulent transfer, Nikitin and Fedorov [7] obtained computational relations for determining the heat-transfer coefficient in the RFE heating zone during processing of pseudoplastic solutions. In our opinion, they did not take sufficient account of the influence that the rotor design features and the variation of the non-Newtonian liquids have in the process of solvent removal. The first attempt to study heat exchange during concentration of NaCI and NaNO 3 solutions to obtain a dry product in a RFE with rigidly fastened blades was made by Lysenko [8]. The average heat-transfer coefficient was found to increase with the heat flux. It is difficult to agree with that conclusion, when the physics of the process and the relation obtained are considered. The heat exchange in a RFE during concentration of azo dyes (they were obtained in dry form) was studied by Nagieva [9]. Arbitrarily dividing the RFE with a combined rotor (with hinged blades at the top and rigid blades at the bottom) into three zones (a zone of heating and onset of evaporation of the initial product, a zone of intensive evaporation, and a drying zone), she obtained criterial relations for each zone and for each product studied. It should be noted that the rotor design features and the non-Newtonian properties of the suspension in the second zone are not taken into account in those relations.

Droplet and vapor transport in a turbulent jet

An experimental study of the role of large-scale structures in droplet and vapor transport in dropletladen jets has been conducted. Flow visualization was used to document the effect of droplet injection location on dispersion and to characterize the convective transport of droplet vapor. Experiments were conducted in an acoustically forced turbulent jet with point-source injection of a monodisperse-droplet stream. A reactive Mie scattering technique was employed to visualize vortex structures, droplet distribution. and vapor transport simulataneously in the near-field region of the jet. Two droplet sizes, 40 and 100 μm, wre investigated, resulting in Stokes numbers of 0.9 and 5.6, respectively. The injection location was varied from the jet centerline to the jet edge. The results show that injection location has a substantial impact on both droplet and vapor convection. In general, for a given injection location the 40 μm droplets experienced greater dispersion than the 100 μm droplets. For a given injection location the 40 μm droplets experienced greater dispersion than the 100 μm droplets. For a given droplet size (Stokes number), dispersion increased as the injection location moved radially ontward from the centerline. For both Stokes mumbers investigated, droplets injected into the core region of the jet (R0<0.2) experienced limited lateral transport. Droplet injected closer to the edger of the jet (R0=0.4), however, underwent substantial radial transport. Vapor transport followed similar trends. The slip velocity between the droplet and jet flow has a significant impact on vapor transport as well.

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel (Change in heat transfer along flow)

The objective of this paper is to make clear how heat transfer coefficient changes along the flow with the passing bubbles through a narrow vertical rectangular channel (20 mm wide, 2 mm deep and 450 mm long). The experiments were done using subcooled water of 80, 60, and 40 K at atmospheric pressure in which the air bubbles were injected into the channel at a designated period from 0.125 to 1.0 s and their length was controlled to be equal to 0.03, 0.02, and 0.01 m. The experiment shows that the heat transfer coefficients decrease along the flow and then reach a constant value beyond a certain distance from the leading edge of the heated surface where the flow becomes fully developed in both the velocity and the thermal conditions. Under the fully developed conditions, the heat transfer coefficients are predicted well by the existing theoretical analysis in which both the convective term and evaporation on the interface are ignored.

Convection-Evaporation Feedback in the Equatorial pacific

The coupling between convection and surface evaporation is investigated to assess the importance of evaporative cooling in regulating the tropical sea surface temperature. It is found that such a coupling is scale dependent. On timescales of several days, convective activity enhances surface evaporation, which together with the decrease of surface solar radiation, acts to cool the sea surface. However, on scales of climatic interest, convection acts to reduce surface evaporation. High sea surface temperature gives rise to more convective activity, which through interaction with the large-scale circulation, increases the low-level large-scale convergence and decreases the surface wind, leading to low evaporation in spite of the increased surface-air humidity difference. Therefore, although individual convective events can significantly enhance surface evaporation on short timescales, the long-term average effect of convection is to suppress surface evaporation at high SST due to its interaction with the large-scale circulation. One potential implication of this result is that evaporative cooling on climate timescales may not provide a negative feedback on the sea surface temperature of warm oceans with convectively disturbed tropospheres.

Multitechnique characterization of polysulfone gas permeation membranes

Integrally thin skinned asymmetric polysulfone membranes developed for gas separation applications were prepared by casting a dope solution on a glass plate. A dry/wet phase inversion process with a forced convective evaporation step is then used to form a skin. The membrane is immersed in water before undergoing a solvent exchange and drying process. The thin skin of these membranes allows gas to selectively diffuse through the membrane when the surface is without defects. The gas transport properties (selectivity and flux) are measured using standard techniques and are reported on Table II.Scanning electron microscopy is an important method of examining the cross-sections and surfaces of these membranes to determine the surface integrity, skin thickness, and porosity of the supporting substructure. The ideal integrally thin skin thickness is 1000 Å or less. We further characterized these membranes using Atomic Force Microscopy to image the membrane surface and X-ray Photoelectron Spectroscopy to obtain chemical information about the polysulfone surface.

95/01897 Correlation of convective vaporization on banks of plain tubes using refrigerants

95/01897 Correlation of convective vaporization on banks of plain tubes using refrigerants

Convective Vaporization Data for Pure Refrigerants in Tube Banks Part II: Enhanced Tubes

Convective vaporization data were obtained for two different enhanced tube geometries in a staggered tube bank made of 18.9 mm (0.75 inch) outside diameter tubes having an equilateral pitch-to-diameter ratio of 1.25. Data are presented for pure refrigerants R-11, R-123, and R-134a at saturation temperatures of 4.4°C (40°F) and 26.7°C (80°F). The data span vapor qualities of 0.0 to 0.9, mass velocities (based on minimum flow cross section area) of 10 to 30 kg/(m2· s) (7,400 to 22,000 lb/h·ft2), and heat fluxes of 15 to 45 kW/m2 (4,760 to 14,300 Btu/h·ft2). The apparatus allowed independent control of vapor quality, mass velocity, and heat flux. The data were correlated in the form of an asymptotic model with a root mean squared error of less than 15%. A modified superposition model with a suppression factor of unity correlated all the Tu-B (R-11 and R-123) data and approximately 90% of the W-SE (R-134a) data within ±20%. The experimental results for the two enhanced tube geometries tested show that the tub...

Shell-Side Boiling in Flooded Refrigerant Evaporators Part I: Integral Finned Tubes

Convective vaporization data were obtained on a staggered tube bank made of 18.9 mm (0.75 inch) outside diameter integral fin tubes (1024 fins/m; 26 fins/inch) with an equilateral pitch-to-diameter ratio of 1.25. Data are presented for pure Refrigerant-11 with saturation temperatures of 4.4°C (40°F) and 26.7°C (80°F). The data span vapor qualities of 0 to 0.9, mass velocities (based on minimum flow cross section area) of 7 to 18 kg/m2·s (5,200 to 13,300 lb/h·ft2), and heat fluxes of 15 to 45 kW/m2 (4,800 to 14,300 Btu/h·ft2). The data were taken in an apparatus that allows independent control of vapor quality, mass velocity, and heat flux. In contrast to the data presented on enhanced tubes in Part 2, the convective effects strongly influence the integral finned tube performance. The Chen superposition model (with or without using a suppression factor) largely under-predicted the data. The data were correlated in the form of an asymptotic model. The correlations fit 92% of the data within ±20%.

Pool and forced convective vaporization of saturated liquid mixtures

Vaporization of liquid mixtures is characterized by the decline of heat transfer, which depends on the mixture composition. As the total heat transfer is a function of the convective evaporation and the nucleate boiling mechanisms, the effect of liquid composition and the other variables of each mechanism have been analysed in this article. The analysis has been performed to identify the fundamental changes in nucleate flow boiling compared to nucleate pool boiling and boiling of the pure components. In the nucleate pool boiling region, the influence of mixture composition on the basic variables, i.e. bubble-departure diameter, frequency of departure and boiling site density, is presented. Models of the heat-transfer process which take mass transfer into account are compared with experimental data both of nucleate pool boiling and of nucleate flow boiling to predict the available data.

A simplified approach to transient convective droplet evaporation and burning

A simplified approach to transient convective droplet evaporation and burning

Comparative study of cooling curves with JIS silver specimens and alloy 600 specimens in relation to additive effectiveness

Quenching- oil cooling curves obtained using a silver specimen (Japanese Industrial Standard K 2242) in-dicate three stages of cooling: vapor blanket, boiling, and convection. The temperatures at which the va-por blanket stage transfers to the boiling stage and the boiling stage transfers to the convection stage are referred to as the characteristic temperature (CT) and the convection- stage starting temperature (CSST), respectively. As the amounts of CT- improving additives are increased in increments of 1,2,3,6, 8, 10, and 12%, the CTs become higher, as clearly shown in the cooling curves obtained using a silver specimen. Likewise, as the amounts of CSST- improving additives are increased from 1 to 2 to 3%, the CSSTs become lower. These tendencies are similarly observed when using an Alloy 600 specimen, and both additives effectively improve cooling performance. However, in the case of 6,8,10, and 12% CT- im-proving additions, the differences among the additive concentrations are more pronounced in cooling curves obtained using the silver specimen than in those obtained using the Alloy 600 specimen. This can be attributed to the greater temperature sensitivity of the silver specimen thermocouple. In field quench-ing operations, such phenomena as insufficient hardness, inverse quench hardening, and unstable distor-tion can be remedied by additive treating. However, one must know what type of additive and how much to use. To this end, cooling curve measurement using silver specimens is useful.

Correlation of Convective Vaporization on Banks of Plain Tubes Using Refrigerants

This article provides a correlation for convective vaporization on banks of plain tubes. New data are provided for R-113 and R‐123 vaporization on 16.8‐mm‐O.D. tubes over a wide range of vapor qualities (0.1 to 0.9) with low mass velocities (8 kg / m2 s to 40 kg / m2 s) for a wide range of heat flux. The correlation is based on three data sets: the present data, the R-113 data of Comwell and Scoones, and that of Jensen and Hsu. The asymptotic model (exponent of 3) provided the best correlation, having a mean square error of 14.6%. The asymptotictype correlation provided better results than the superposition model. A disturbing anomaly was noted in the lack of agreement between pool boiling on a single tube and pool boiling on one heated tube in a bundle for the Comwell and Scoones data. More work is required to understand this situation. The data were analyzed to interpret whether convection causes boiling suppression. Assuming that the pool boiling data are correct, it appears that the higher-mass-velocity data of Jensen and Hsu, and of Comwell and Scoones, may experience suppression.

Wave and Turbulent Film Condensation on a Vertical Surface. Correlation for Local Heat-Transfer Coefficient.

The experimental study of body forced convection of pure vapor on a vertical surface (l=2980 mm) is presented. CFC11, CFC113 and HCFC123 are used as the working fluids. The flow patterns of condensate film are classified into four kinds of flow : L flow (laminar flow), L-S flow (laminar and sine wave flow), L-S-H folw (laminar, sine wave and harmonic wave flow), L-S-H-T flow (laminar, sine wave, harmonic wave and turbulent flow). The nondimensional correlations for predicting the average heat-transfer coefficients are obtained for four kinds of flow and compared with the experimental results, respectively. Assuming more simple four kinds of flow patterns, which are L flow, S flow (sine wave flow), H folw (harmonic wave flow) and T flow (turbulent flow), simple nondimensional correlations for predicting the average heat-transfer coefficients are also obtained for S flow, H flow and T flow. The nondimensional correlations for combined H and T flow are also obtained.

STUDY OF HEAT AND MASS TRANSFER DURING IR DRYING OF PAPER

ABSTRACT A mathematical model has been developed to study the drying of paper using a gas-fired IR dryer. The model accounts for various phenomena : water and vapour mass transfer, conduction, convection and radiation heat transfer. The phenomenological equations are solved with a finite difference scheme, including a modified upwind differencing scheme to account for water migration within the paper sheet. The simulation results illustrate the basic underlying phenomena involved in IR paper drying and can be instrumental to the engineer to make the detailed analyses of such a drying process. A sensitivity analysis has shown that the drying rate is most sensitive to parameters governing the IR beat transfer process whereas the paper sheet temperature is most sensitive to parameters governing the mass transfer process with the surroundings.

Convective Vaporization of Pure Refrigerants in Enhanced and Integral-Fin Tube Banks

Convective Vaporization of Pure Refrigerants in Enhanced and Integral-Fin Tube Banks