Mass Transfer Coefficient For Evaporation Research Articles

Wetting and evaporative performance analysis of wet channels in indirect evaporative cooler with hydrophilic nano-coating

The wetting condition is an important factor that affects the cooling performance of an indirect evaporative cooler (IEC). Applying hydrophilic coatings to the wet channels is an effective way to enlarge the wetting area and enhance evaporative cooling. However, long-term hydrophilic durability tests for the coated surface are rarely discussed in existing studies. Moreover, the evaluation of performance improvement of a hydrophilic-coated IEC is usually based on testing the prototype as a whole, without giving attention to the internal wetting behavior inside the core. Therefore, a TiO2/SiO2 nano-coated polypropylene (PP) IEC is manufactured for studying the wetting and evaporative performance by considering the hydrophilic durability and internal wetting behavior with a specially-designed visualized experiment, aiming at promoting its application and guiding the design. Firstly, the microstructure, thermal conductivity, drop-surface characteristics and hydrophilic durability of the coated samples were studied. Secondly, the internal water distribution was studied under the influence of spray time (5 s ∼ 180 s), water flow rate (2 L/min ∼ 6 L/min), air velocity (2 m/s ∼ 3.5 m/s) and nozzle height (50 mm ∼ 250 mm) by combining fluorescence tracer and digital image processing methods. Lastly, the evaporative performance of the nano-coated IEC was tested under two spray strategies. Results show the hydrophilicity of polished nano-coated PP is durable in a dark and water-scouring environment with a steady contact angle of 30.9°. The coated core shows super hydrophilicity by forming a thin water film and being fully wetted within 60 s. Compared to the continuous spray, the intermittent spray can provide a larger evaporative mass transfer coefficient by 23.5% and a higher coefficient of performance by 114%.

Simulation of Mass and Heat Transfer in an Evaporatively Cooled PEM Fuel Cell

Evaporative cooling is a promising concept to improve proton exchange membrane fuel cells. While the particular concept based on gas diffusion layers (GDLs) modified with hydrophilic lines (HPILs) has recently been demonstrated, there is a lack in the understanding of the mass and heat transport processes. We have developed a 3-D, non-isothermal, macro-homogeneous numerical model focusing on one interface between a HPIL and an anode gas flow channel (AGFC). In the base case model, water evaporates within a thin film adjacent to the interfaces of the HPIL with the AGFC and with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell via the AGFC. The transport to the cathode side is shown to be partly limited by the ab-/desorption into/from the membrane. The cooling due to the latent heat has a strong effect on the local evaporation rate. An increase of the mass transfer coefficient for evaporation leads to a transport limited regime inside the MEA while the transport via the AGFC is limited by evaporation kinetics.

Comparative Performance evaluation of an active/passive solar Distillation system

No water, No life, No blue, No green. It is key for people prosperity. Human body uses water for all biological processes like digestion circulation transporting nutrients etc. The average consumption of water by human is 2.5 to 3.5 liter per day. In this methodology attempts has made to find out the effect of water depth on evaporative mass transfer coefficient of a passive single slope distillation system summers.The experiments has been conducted on south facing, single slope, solar still of 30° inclination of condensing cover, in summer climate for 24 hrs. on different five days for different five water depth from 0.04 to 0.18m.Objective is to study the variation in internal heat transfer coefficients depend to water depth in the still.

PRODUCTION ANALYSIS OF A TUBULAR SOLAR STILL

In this study, attempts were made to provide a group of complete heat and mass transfer correlations, and to propose a new heat and mass transfer model for a Tubular Solar Still (TSS) by taking account of thermal properties of the humid air inside the still. We developed a new experimental technique for directly measuring the evaporation rate from the saline water surface in the TSS and evaluated the evaporative mass transfer coefficient. Indoor experiments were conducted to investigate the production performance and the heat and mass transfer coefficients of the TSS. It was found that i) the heat and mass transfer coefficients, hcha, hcw, hcdha, hew can be expressed as functions of the temperature difference between the saline water and the cover, and ii) the conventional formula available for a basin type still overestimates the convective heat transfer coefficient, hcw for the TSS. Furthermore, in this study, the long-wave radiation interaction between the water surface and the tubular cover was shown theoretically.We also proposed a new heat and mass transfer model of TSS. The mass balance of water vapor inside the still was formulized for the first time. The proposed model was successful in the calculation of humid air properties (temperature, vapor density and relative humidity) and condensation flux on the tubular cover inner surface besides the saline water temperature, tubular cover temperature, trough temperature and evaporation flux. The validity of the model was evaluated from the comparison with field experiments in Fukui, Japan and in Hamuraniyah, UAE. It was seen that the calculated results also had a good agreement with the field data.

Convective mass transfer coefficient for a hydrodynamically developed airflow in a short rectangular duct

In this paper, experiments are performed to determine the convective mass transfer coefficient for evaporation in a horizontal rectangular duct with an aspect ratio of 14.5:1. In the test facility, a short pan of water forms the lower panel of a long duct where a hydrodynamically fully developed laminar or turbulent airflow passes over the surface of the water. The measured convective mass transfer coefficients have uncertainties that are typically less than ±10% and are presented for Reynolds numbers ( Re) between 570 and 8100, Rayleigh numbers ( Ra) between 6300 and 83,000, inverse Graetz numbers ( Gz) between 0.003 and 0.04, and operating conditions factors ( H ∗) between −3.6 and −1.4. The measured convective mass transfer coefficients are found to increase as Re, Ra, Gz and H ∗ increase and these effects are included in the Sherwood number correlations presented in this paper, which summarize the experimental data.

Effect of water depths on heat and mass transfer in a passive solar still: in summer climatic condition

In this communication, an attempt has been made to find out the effect of water depth on evaporative mass transfer coefficient for a passive single-slope distillation system in summer climatic condition. The experiments have been conducted on a south facing, single slope, solar still of 30° inclination of condensing cover, in summer climatic condition for 24 h on different five days for different five water depths from 0.04 m to 0.18 m. The objective of the present paper is to study the behavioral variation in internal heat transfer coefficients with respect to the water depth in the still. It is understood that the heat transfer coefficients depends significantly on water depths. It is also observed that the nocturnal distillation is significant in the case of higher water depths because of reduced ambient and stored energy within it.

Kinetics of finite dose absorption through skin 2: Volatile compounds

A diffusion model to account for the disposition of an arbitrary dose of a (potentially) volatile compound applied to skin from a volatile vehicle is presented. In its most general form, the model allows for variable diffusivity of the permeant in the stratum corneum (SC) and must be solved numerically. However, for permeants having a constant diffusivity, absorption, and evaporation is characterized in terms of four dimensionless parameters—a reduced time τ, a fractional deposition depth in the SC f, a ratio of membrane capacity for the permeant to the applied dose β, and a ratio of evaporative mass transfer coefficient to diffusive permeability χ. An important combination of these parameters arises as the reduced dose Mr=(fβ)−1. Two cases are distinguished. In Case 1, corresponding to Mr≤1, the dose is less than that required to saturate the upper layers of the SC, and the shape of the absorption and evaporation profiles is independent of the dose. Analytical solutions to Case 1 may be derived for arbitrary initial distributions of the permeant; the solution for a square wave is presented. In Case 2, corresponding to Mr>1, absorption and evaporation approach steady-state values as the dose is increased. Numerical evaluations of this behavior are shown. Limiting behavior for the case of a highly volatile solvent applied to skin is discussed. A companion paper discusses the application of the model to the absorption and evaporation of benzyl alcohol from human skin in vitro. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association

Absorption and evaporation of N, N-diethyl- m-toluamide from human skin in vitro

The penetration of DEET through split-thickness cadaver skin was measured in non-occluded Franz cells placed either in a fume hood or on a laboratory workbench. DEET, dissolved in a small volume of ethanol and spiked with 14C radiolabel was applied to skin at doses from 0.02 to 11 000 μg/cm 2. DEET penetration was greater for cells placed on the workbench, and the percentage of radioactivity penetrated after 72 h increased gradually with dose, for doses up to 680 μg/cm 2. At higher doses, it declined. Percent penetration ranged from 11.5 ± 3.2% for a dose of 0.021 μg/cm 2 in the fume hood to 71.9 ± 5.5% for a dose of 260 μg/cm 2 on the workbench. Results were interpreted in terms of a diffusion/evaporation model having three parameters—a solubility value for the chemical in the upper stratum corneum, M sat; a mass transfer coefficient for evaporation, k evap; and a characteristic time for diffusion, h 2/ D. The parameters obtained from fitting the model to the data (normalized to the fume hood environment) were M sat = 18 μg/cm 2 and k evap = 2.6 × 10 −5 cm/h. The value of h 2/ D decreased from 16 h at a DEET dose of 25 μg/cm 2 to 10 h at 1480 μg/cm 2, consistent with an increase in skin permeability of about 1.5-fold over this dose range. This effect was confirmed by means of an additional study in which skin samples pretreated with increasing amounts of unlabeled DEET were washed and redosed with 14C-benzyl alcohol. A small (1.7-fold), but significant, increase in benzyl alcohol penetration with increasing amount of DEET was obtained. Thus, DEET enhanced its own skin permeation rate as well as that of another compound, but the effect was modest and not likely to be a major concern for compounds coadministered with DEET.

Mass transfer correlations for rotating drum bioreactors

Evaporative cooling is extremely important for large-scale operation of rotating drum bioreactors (RDBs). Outlet water vapour concentrations were measured for a RDB containing wet wheat bran with the aim of determining the mass transfer coefficient for evaporation from the bran bed to the headspace. Mass transfer was expressed as the mass transfer coefficient times the area for transfer per unit volume of void space in the drum. Values of ka′ were determined under combinations of aeration superficial velocities ranging from 0.006 to 0.017 ms −1 and rotation rates ranging from 0 to 9 rpm. Mass transfer coefficients were evaluated using a variety of residence time distributions (RTDs) for flow in the gas phase including plug flow and well-mixed and a Central Jet RTD based on RTD studies. If plug flow is assumed, the degree of holdup at low effective Peclet ( Pe eff) numbers gives an apparent under-estimate of ka′ compared with empirical correlations. Values of ka′ calculated using the Central Jet RTD agree well with values of ka′ from literature correlations. There was a linear relationship between ka′ and effective Peclet number: ka′=2.32×10 −3 Pe eff.

An experimental study on evaporative heat transfer coefficient and applications for passive cooling of AP600 steel containment

After TMI and Chernobyl accidents, many efforts have been made to enhance the nuclear safety with passive features. Among such passive features, the passive containment cooling system (PCCS) has been suggested by Westinghouse in the AP600 plant. The containment with PCCS is a dual containment, and consists of a stainless steel vessel and a concrete wall. In the gap between these structures, air and water can counter-currently pass and cool the steel surface. This paper experimentally investigates evaporative heat and mass transfer at the surface of a falling water film with counter-current air flow in a vertical duct with one-side heated plate. Experiments included various conditions of mass flow rate of film and air. Experimental results show the strong effects of water temperature and air mass flow rate, but little effect of the water flow rate. Also, simple analyses based on heat and mass transfer analogy were performed to evaluate the experimental results. With experimental data, a new correlation on evaporative mass transfer coefficient was developed, and with the correlation, the containment pressure and temperature was calculated for the design basis accident of AP600 by the use of CONTEMPT4/MOD5 code implementation.

Modelling of the rate of oil spill disappearance from seawater for Kuwaiti crude and its products

In this paper a semi-analytical model has been developed to estimate the amount of oil disappeared from an oil spill floating on seawater surface. The model considers evaporation, dissolution and sedimentation of oil components. Crude oil and wide boiling range fractions are divided into several pseudocomponents using distribution model developed by Riazi (Ind. Eng. Chem. Res., 36 (10) (1997) 4299–4307). Heavy components with densities above density of water, sink to the bottom of sea while light components vaporize or dissolve into water. In the model, oil spill thickness is considered variable versus time. The input data for the model are distillation data (or composition) for oil, API gravity of oil, initial volume of oil spill, initial area of oil spill, air temperature, wind speed and water surface velocity. The only adjustable parameter is a constant in the relation for the mass transfer coefficient for evaporation. The model estimates area, volume and composition of oil spill versus time. It also calculates the amount of oil vaporized, dissolved or sunk into water versus time. Three sets of experiments were conducted for a Kuwaiti export crude oil and four different Kuwaiti petroleum products also used for export to determine the rate of oil disappearance at ambient temperature. Data obtained in these experiments were used to determine the model parameter as well as evaluation of the proposed model.

Evaporation at a Liquid Surface Due to Jet Impingement

Experiments were performed to determine mass transfer coefficients for evaporation from a water surface on which an air jet impinged. During the course of the experiments, parametric variations were made of the jet velocity and diameter, the separation distance between the jet origin and the water surface, the diameter of the water surface, and the degree of insulation of the water containment pan. It was found that for all of the investigated operating conditions, the dimensionless mass transfer coefficient varied with the 0.8 power of the jet Reynolds number. Furthermore, the transfer coefficient decreased linearly as the separation distance between the jet origin and the water surface increased, with the most significant decreases occurring at relatively small values of the surface-to-jet diameter ratio. At larger diameter ratios, the transfer coefficient was relatively insensitive to the separation. In general, the larger the diameter of the water surface, the lower the transfer coefficient. Comparisons with the literature showed that the dimensionless mass transfer coefficients for impingement on a liquid surface are lower than those for impingement on a solid surface.