Effect Of Partial Wetting Research Articles

Simplified expressions of the transfer coefficients on a partially wet absorber tube

A simplified analytical solution of the governing differential equation for falling film absorption around a smooth horizontal tube is developed and compared to numerical results and experimental data from literature. Besides the assumption of a linear temperature profile, linear interfacial equilibrium together with the hypotheses of small mass-penetration are applied at the film interface to reach closed analytical expressions of the temperature and mass fraction fields, which can be used to estimate the local and average heat and mass transfer coefficients within falling film absorbers and generators. The noteworthy agreement with numerical results supports the simplifying assumptions introduced, and the inclusion of partial-wetting effects brings the results closer to the experimental data from previous literature, widening the applicability of this formulation.

Dynamics of non-wetting drops confined in a Hele-Shaw cell

We experimentally investigate the sedimentation of a non-wetting drop confined between two parallel walls. The whole system is immersed in a bath of liquid of low viscosity and a lubricating film may be dynamically formed between the drop and the walls of the cell. Depending on the thickness of the film and on the viscosity ratio between the droplet and the surrounding liquid, viscous dissipation localizes either in the lubrication layer or in the bulk of the drop. The velocity of the droplet is non-trivial as the thickness of the lubricating layer may depend on the interplay between interfacial tension and viscous dissipation. Interestingly, thin films whose nanometric thickness is set by long range intermolecular interactions may lubricate efficiently the motion of highly viscous droplets. We derive asymptotic models that successfully capture the settling velocity of the drop in the different regimes observed experimentally. The effect of partial wetting is finally illustrated by a sharp increase of the velocity of the drops that we attribute to a wetting transition.

Development and validation of an analytical formulation of the Nusselt and Sherwood numbers on a partially wetted absorber tube

With reference to a simplified analytical solution of the governing differential equation for falling film absorption, the corresponding formulation of the average Nusselt and Sherwood numbers obtained around a smooth horizontal tube is validated through a dedicated experimental investigation and data from previous literature. The assumption of a linear temperature profile together with the average interfacial mass fraction gradient and the hypotheses of small penetration for physical absorption are applied at the film interface to reach closed analytical expressions of average Nusselt and Sherwood numbers within falling film absorbers and generators. The inclusion of partial-wetting effects brings the results closer to the measurements and widens the applicability of these expressions.

Analytical solution of film mass-transfer on a partially wetted absorber tube

This work presents a two-dimensional analytical solution of the governing differential equation for falling film vapour-absorption around a plain horizontal tube. The solution of the species transport equation gives the LiBr mass fraction distribution within the liquid absorptive film flowing along the tube surface and can be used to characterize the mass transfer performance of falling film absorbers or generators. By means of the inclusion of partial wetting effects at reduced solution mass flowrates, this study obtains an analytical expression of the mass transfer coefficient of these devices applicable over an extended range of operative conditions. The hypotheses of small penetration for physical absorption and constant heat flux condition are applied at the film interface to reach a closed-form solution. Fourier method is used to solve the problem and the eigenvalues obtained from the characteristic equation depend on Lewis number, Biot number and the dimensionless heat of absorption. Given the boundary condition at the wall, the two-dimensional mass fraction field of the laminar film can be expressed analytically as a function of Schmidt, Reynolds numbers, the tube dimensionless diameter and the ratio of the wetted area to the total exchange surface. Finally, mass transfer coefficient and absorbed mass flux are locally and globally investigated as functions of the influent dimensionless groups to clarify their effects on the physical process and screen the potentiality of the model. Results show notable qualitative and quantitative agreement with previous numerical solutions and experimental results from previous literature. This model constitutes a widely applicable and time-saving tool for actual system simulations, design and control.

Is the productive performance of olive trees under localized irrigation affected by leaving some roots in drying soil?

Localized irrigation, which wets a part of the root zone only, is widely used in olive orchards. The effect of partial wetting of the root zone on plant performance is unknown. The aim of this work was to compare fruit and oil yields in olive trees under localized irrigation (LI treatment) to those from trees with the whole root zone under non-limiting soil water conditions (Pond treatment). The LI trees received enough water to replace the crop water needs. The study was made for three consecutive years in mature ‘Manzanilla de Sevilla’ olive trees growing under field conditions. Main variables were also measured in young trees of the same cultivar growing in 50L pots. Samples from rain-fed trees were also analysed. The LI trees showed similar water status than the Pond trees, but reduced stomatal conductance. As expected, the effect of the irrigation treatments on net photosynthesis was less evident. We observed a decrease in fruit yield in the LI trees, as compared to the Pond trees, likely because of the reduced gas exchange. No differences between the LI and Pond treatments were found for fruit weight, pulp:stone ratio, volume and the longitudinal and equatorial diameters. The LI treatment did not cause detrimental effect either on total oil content or virgin olive oil yield. The phenols content was greater in the LI treatment, but the (E)-hex-2-enal, one of the major volatile compounds and most related to the positive attributes of olive oil, was lower. No significant differences with the Pond treatment were found, however, in the sensory description of the oil flavours.

Pressure and partial wetting effects on superhydrophobic friction reduction in microchannel flow

Friction reduction in microchannel flows can help alleviate the inherently taxing pumping power requirements associated with the dimensions involved. One possible way of achieving friction reduction is through the introduction of surface microtexturing that can lead to a superhydrophobic Cassie-Baxter state. The Cassie-Baxter state is characterized by the presence of air pockets within the surface microtexturing believed to act as an effective “shear free” (or at least shear reduced) layer, decreasing the overall friction characteristics of the surface. Most work in this area has concentrated on optimizing the surface microtexturing geometry to maximize the friction reduction effects and overall stability of the Cassie-Baxter state. However, less attention has been paid to the effects of partially wetted conditions induced by pressure and the correlation between the liquid-gas interface location within the surface microtexturing and the microchannel flow characteristics. This is mainly attributed to the difficulty in tracking the interface shape and location within the microtexturing in the typical top-down view arrangements used in most studies. In this paper, a rectangular microchannel with regular microtexturing on the sidewalls is used to visualize and track the location of the air-water interface within the roughness elements. While visually tracking the wetting conditions in the microtextures, pressure drops versus flow rates for each microchannel are measured and analyzed in terms of the non-dimensional friction coefficient. The frictional behavior of the Poiseuille flow suggests that (1) the air-water interface more closely resembles a no-slip boundary rather than a shear-free one, (2) the friction is rather insensitive to the degree of microtexturing wetting, and (3) the fully wetted (Wenzel state) microtexturing provides lower friction than the non-wetted one (Cassie state), in corroboration with observations (1) and (2).

1D and 2D simulations of partially wetted catalyst particles: A focus on heat transfer limitations

Hydrodynamics and transport phenomena inside Trickle Bed Reactors are strongly modified when superficial liquid velocity is not sufficient to insure a perfect wetting of catalyst particles. For this reason, the impact of partial wetting on catalyst effectiveness has been widely studied in past, in case of isothermal reactions. Heat transfer limitations, inside or outside catalyst particles, have also been investigated, but only in case of total wetting. In the present study, the effect of partial wetting is quantified numerically for various kinetics, mass and heat transfer limitations. When possible, an analogy is proposed between 1D and 2D models of particles. Robust rules are proposed to take into account for partial wetting in 1D-radial models of particles. The case of partial evaporation inside catalyst pores is also studied. Evaporation phenomena is approximated following a simple approach. Performed calculations show that the effect of partial wetting can affect very strongly catalyst effectiveness factors, especially in case of evaporation in the pores. The use of 1D approximations gives a good prediction of the global catalyst efficiency, as far as boundary conditions are symmetric between heat and mass transfer. Otherwise, 1D models are not relevant anymore.

Modelling of a hollow fibre ceramic contactor for SO 2 absorption

Process intensification allows significant improvements in chemical manufacturing and processing, leading to cheaper, safer and cleaner technologies. Recovery of sulfur dioxide from gas emissions using a membrane device instead of dispersive absorption (e.g. scrubbers) may intensify the process and it is in the spotlight of many investigations. This work develops the modelling of a ceramic hollow fibre membrane contactor used as membrane device in SO 2 absorption. The modelling of mass transfer through the membrane is performed in order to evaluate the technical possibilities of the process. The effect of partial wetting on the mass transfer and the influence that operation conditions have on the process efficiency are evaluated. Solvent selection is an important variable to be taken into account in the technical application of the process; this variable is studied through the Henry's law constant ( H).

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate.

Recent advances in the analysis and design of trickle-bed reactors

This paper summarizes some progress in our understanding of trickle-bed reactors. The particular topics discussed in this review are the basic hydrodynamics, flow regime transition, pressure drop and holdup calculations, use of tracers, modelling of partial wetting effects, reactor design of and catalyst testing in trickle beds. The rapid advances made in these areas are critically examined and some problems which need further investigation are outlined.

The effect of partial wetting of the root zone on yield and water use efficiency in a drip-and sprinkler-irrigated mature grapefruit grove

The effect of partial wetting of the root zone on yield and water use efficiency in a drip- and sprinkler-irrigated mature grapefruit grove was tested in a long-term experiment from 1976 to 1979. Three different percentages of the surface soil areas (∼ 30%, 40% and 70%) were wetted by the use of single and double drip laterals and sprinklers, respectively. Irrigation frequencies were 3 and 7 days for the drip treatments and 14 and 21 days for the sprinkler-irrigated plots.