van der Ajj Ton Zanden

An approximate analytical solution of the hydrodynamic problem associated with an advancing liquid-gas contact line

Abstract An alternative to perturbation techniques or full numerical solution is developed for the free-surface problem associated with an advancing liquid-gas contact line. The method, which makes use of a local-wedge approximation to obtain the free-surface pressure variation, leads to a second-order ordinary differential equation for the meniscus shape. Both analytical considerations and comparisons with available full numerical solutions for capillary tubes confirm the validity of the meniscus equation up to values of the capillary number (Ca) of order 10−1. At higher Ca the equation retains its validity in the wall region, for which an approximate analytical solution is derived. Matching of this solution with the central circular meniscus profile leads to an analytical approximation for the advancing contact angle, which compares excellently with available data (Ca up to an order 1). In contrast with preceding analyses, the classical approximations—in particular, that of no slip—are assumed to retain their validity up to the order of a molecular dimension from the wall, at which point the true contact angle is reached. While this angle is again supposed to be equal to the static value, this assumption is not critical to the dynamic angle predicted.

Isothermal vapour and liquid transport inside clay during drying

ABSTRACT A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally oblained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.

The measurement of the diffusion coefficient of water in paints and polymers from their swelling by using an interferometric technique

An interferometric technique has been used to measure the swelling of a paint film caused by its absorption of water vapour. The diffusion coefficient of water in paint is obtained from the time evolution of the thickness of the paint film, when the paint film is subjected to an almost stepwise change in the relative humidity of the surrounding air. The measurements were possible in the range of the relative humidity of the air between 50 and 90%.

The measurement of the diffusion coefficient and the sorption isotherm of water in paint films

An experimental technique together with a numerical model is proposed with which the diffusion coefficient and the sorption isotherm of water in paint can be measured. Inside a closed vessel, paint films are on stainless-steel plates. Water is present as water vapour in the air and in the paint. After blowing dry or wet air through the vessel for some time, the situation moves to a new equilibrium. The relative humidity of the air inside the vessel is measured as a function of time. From fitting the theoretical/numerical model against the experimental values, follow the diffusion coefficient and the sorption isotherm of water in the paint. The results show large scattering. When the independently measured sorption isotherm is used as an input parameter in the model, the fitting procedure gives much smaller scattering for the diffusion coefficient.

Modelling diffusion-limited drying behaviour in a batch fluidized bed dryer

ABSTRACT Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.

The measurement of the sorption isotherm of water in paint films

A hygrometric method is presented with which the sorption isotherm of water in paint films can be measured. Only standard laboratory equipment is used. The principle of the technique is that a known amount of water is added to a system of paint and air. The relative humidity in the air is measured. The amount of water in the paint follows from a mass balance. A small leakage is incorporated in the model. The method is used on three alkyd paints and three latex paints.

A Procedure to Measure the Diffusion Coefficient of Water in Brick as a Function of the Water Concentration

A procedure to measure the diffusion coefficient of water in brick as a function of the water concentration has been proposed, based on a method of measuring the mass of a drying brick at both outer ends. In this method, the brick is subjected to one-dimensional drying. The procedure was tested using a simulation with realistic parameter values and experimental inaccuracies. An experiment consisted of 120 s of drying and measuring the masses every 10 s with an accuracy of 10−6 kg. The sample was first dried to a lower water content and the drying was stopped for some time to allow the concentration profile to approximate a known equilibrium concentration profile, after which another measurement was performed at that lower water content. Minimizing the differences between the masses, as measured and as it follows from the model, gave the value of the diffusion coefficient. The method failed for low water concentrations.

A Gravitational Procedure to Measure the Diffusion Coefficient of Water in Porous Materials: A Case Study on Concrete

A new procedure is presented with which the diffusion coefficient of water in partially saturated porous materials can be measured. The first step in the procedure is the creation of a non-equilibrium situation inside a sample by placing it into a centrifuge. In the second step, the mass of the sample is measured by hanging it from two cables, each connected to a balance. The comparison of the time evolutions of the measured masses and the masses as predicted using Ficks second law gives the diffusion coefficient.