Drying moisture conduction coefficient

In drying, the heat transfer coefficient h and the mass transfer coefficient K between the drying gas and the wet material, the heat diffusivity /Cj, and moisture (mass) diffu-sivity coefficients, and the thermal conductivity and moisture conductivity coefficients are the parameters of interest. Although several methods are employed for the determination of these parameters, here only the techniques and equipments that allow their determination via a drying experiment will be described. 

The right side of Equation 2.25 contains known data when the weight of the wet material is measured as a function of time and when we have already determined, according to the previons section, the mass transfer coefficient K from data obtained in the constant rate drying period. The first eigen-valne can be determined numerically (by trial aud error or by iteratiou) from the transcendental equation. (The value of the V ranges between 0 and m. Ref. [2].) The mass diffnsivity coefficient can be calculated directly from Equation 2.22 if t-i is known. When the moisture capacity coefficient is determined separately [2,19], then the moisture conductivity coefficient can also be determined from Equation 2.23. 

Here, /r is the moisture conduction coefficient with the dimension of a diffusion coefficient. However, unlike the latter, the moisture conduction coefficient strongly depends on the moisture content of the drying good. Often, with decreasing moisture content, k also decreases. 

In the initial constant rate drying period, a knowledge of the heat diffusivity and heat conductivity coefficients of the wet material is necessary because they are the controlling factors for heat transport within the material. In this section of drying it is presumed that the pores of the wet material are saturated by moisture. Consequently, the above-mentioned characteristics of a heterogeneous material consisting of a solid skeleton and, within this, a capillary system (filled... 

The transport properties discussed above (moisture diffu-sivity, thermal conductivity, interface heat, and mass transfer coefficients) describe completely the drying kinetics. However, in the literature sometimes (mainly in foods, especially in cereals) instead of the above transport properties, the drying constant K is used. The drying constant is a combination of these transport properties. 

An information flow diagram for a drying model appropriate for this method is shown in Figure 4.16. This model can calculate the material moisture content and temperature as a function of position and time whenever the air humidity, temperature, and velocity are known as a function of time, together with the model parameters. If the model takes into account the controlling mechanisms of heat and mass transfer, then the transport properties (moisture dif-fusivity, thermal conductivity, boundary heat and mass transfer coefficients) are included in the model as parameters. If the dependence of drying conditions (material moisture content, temperature, and thickness, as well as air humidity, temperature, and velocity) on transport properties is also considered, then the constants of the relative empirical equations are considered as model parameters. In Figure 4.16 the part of the model that contains equations... 

While the puU-olf test is now used extensively for purposes of quality control, only a limited number of studies have been conducted using the approach to assess effects of concrete quality, surface conditions, environment during application of the FRP, and durability. Sen et al. used the test to assess the durability of the bond of two different carbon fabrics and five epoxy systems under four different exposure conditions and concluded that exposure to wet-dry cycles resulted in the greatest deterioration due to moisture absorption in the epoxy. Malvar et al. tested a bond of CFRP to concrete under combinations of temperature and humidity level for a period of 7 days and reported that the maximum number of failures was seen after exposure to an environment consisting of 95% relative humidity (RH) and a temperature of 38° C (Malvar et al. 2003). While these sets of experiments provide valuable data, they do not allow for an assessment of systems with respect to crucial characteristics such as diffusion coefficients of moisture uptake and the actual deterioration of the resin system used to bond the FRP to the concrete as a result of exposure (Karbhari and Ghosh 2009 Zilch and Miihlbauer 2007 RizkaUa et al. 2008 Keller and Zhou 2006 El Damatty et al. 2005 Shayan et al. 2003). 

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