Thermal conductivity moisture effects

As the cell walls are squeezed into polyhedra, a wall-thinning effect takes place, and liquid is drained from cell-wall faces into the lines of cell intersections to form ribs or struts, which are typically triangular in cross-section. This cell wall membrane thinning can continue to the point where the cell walls collapse and the cells open. This becomes a very important characteristic of most plastic foams, and affects properties such as thermal conductivity, moisture absorption, breathability, and load bearing. 

Effective thermal conductivity and effective moisture diffusivity are related to internal heat and mass transfer, respectively, while air boundary heat and mass transfer coefficients are related to external heat and mass transfer, respectively. The above transport properties are usually coefficients in the corresponding flow rate and driving force relationship. The equilibrium material moisture content, on the other hand, is usually related to the mass transfer driving force. 

L. I. Knab, D. R. Jenkins, and R. G. Mathey, "The Effect of Moisture on the Thermal Conductance of Roofing Systems," NBS Building Science Series 123, National Bureau of Standards, U.S. Dept, of Commerce, Washington, D.C., Apr. 1980. 

Thermal conductivity increases with increasing apparent density, volatile matter, ash, and mineral matter content. Due to the high porosity of coal, thermal conductivity is also strongly dependent on the nature of gas, vapor, or fluid in the pores, even for monolithic samples (van Krevelen, 1961). Moisture has a similar effect and increases the thermal conductivity of coal since its thermal conductivity value is approximately three times higher than that of dry coal (Speight, 1994, and references cited therein). However, the thermal diffusivity of coal is practically unaffected by moisture since the /Cp value is not essentially changed by moisture. 

C How does the condensation or freezing of water vapor in the wall affect the effeclivene.ss of the insulalion in the wall How does the moisture content affect the effective thermal conductivity of soil ... 

The cause of this difference could be a possible combined effect of the thermal conductance and the gas permeability. The thermal conductance is the lowest for blocks 16 poplar (see table I) and the permeability to gases is the lowest for poplar. These two factors combined could contribute to delay and slow down the production of volatile matter for blocks 16 of poplar. However, we must stress the fact that the difference between these blocks 16 poplar and the other samples is not that big, although significant in the case of factor a, the difference is S to 6 min for anhydrous wood samples, 20 to 30 min for wet wood samples, when moisture content generates differences between anhydrous and wet samples of the order of 60 min. 

However, for wet wood samples (H37), we observe a significant effect of the shape and of the species but without significant interaction between both factors. Energy flows are delayed for blocks 16 compared with cubes 4 and for poplar compared with beech, The effect of the lengthening of the wood blocks in the direction of the fibres as well as the effect of the species, act on the physical properties of wood samples in carbonization (decrease of the thermal conductance and of the permeability to gases). These effects could be enhanced by the moisture content of wood. 

The thermal conductivity of wood also increases greatly with its moisture content, as well as with other factors such as temperature and specific gravity. A number of proposed empirical equations indicate that the thermal conductivity of wood increases with increasing moisture content. These equations have been summarized by Siau (45) who proposed that the primary effect of water was to swell the cell wall and, thus, to provide a larger heat conduction area for a given wood. 

One way of expressing the insulating performance of a textile is to quote "effective thermal conductivity". Here the term "effective" refers to the fact that conductivity is calculated from the rate of heat flow per unit area of the fabric divided by the temperature gradient between opposite faces. It is not true condition, because heat transfer takes place by a combination of conduction through fibers and air and infrared radiation. If moisture is present, other mechanisms may be also involved. Research on the thermal resistance of apparel textiles [42-47], has established that the thermal resistance of a dry fabric or... 

However, it is not only wind that plays an important role in protection the effect of moisture is equally important. Water and moisture vapour both lead to a reduction in thermal insulation due to water having a higher thermal conduction... 

Effective moisture diffusivity and effective thermal conductivity are in general functions of material moisture content and tanperature, as well as of the material structure. Air boundary coeffiamts are functions of the conditions of the drying air, that is humidity, tonperature, and velocity, as well as system geometry. Equilibrium moisture content of a given material is a function of air humidity and temperature. The drying constant is a function of material moisture content, temperature, and thickness, as weU as air humidity, tonperature, and velocity. 

An eqnation has been derived relating the effective diffusivity of porous foodstuffs to various physical properties such as molecular weight, bulk density, vapor space permeability, water activity as a function of material moisture content, water vapor pressure, thermal conductivity, heat of sorption, and tanperature [80]. A predictive model has been proposed to obtain effective diffusivities in cellular foods. The method requires data for composition, binary molecular diffusivities, densities, membrane and cell wall permeabilities, molecular weights, and water viscosity and molar volume [81]. The effect of moisture upon the effective diffusivity is taken into account via the binding energy of sorption in an equation suggested in Ref. [77]. 

The effective thermal conductivity can be determined using the methods presented in Table 4.5, which includes the relevant references. Measurement techniques for thermal conductivity can be grouped into steady-state and transient-state methods. Transient methods are more popular because they can be run for as short as 10 s, during which time the moisture migration and other property changes are kept minimal. 

Despite the limited data of effective moisture diffusivity, a lot of data are reported in the literature for thermal conductivity. Data for mainly homogeneous materials are available in handbooks such as the Handbook of Chemistry... 

Effective moisture diffusivity Air boundary mass transfer coefficient Effective thermal conductivity Air boundary heat transfer coefficient... 

The transport properties of foods received much attention in the literature [184-188]. The main results presented by Saravacos and Maroulis [188] are summarized in this section. The results refer to moisture diffusivity and thermal condnc-tivity. Recently published values of moisture diffusivity and thermal conductivity in various foods were retrieved from the literature and were classified and analyzed statistically to reveal the influence of material moisture content and tempera-tnre. Empirical models relating moisture diffusivity and thermal conductivity to material moisture content and temperature were fitted to all examined data for each material. The data were screened carefully using residual analysis techniques. A promising model was proposed based on an Arrhenius-type effect of temperature, which uses a parallel structural model to take into account the effect of material moisture content. 

The thermal conductivity increases, in general, with increasing moisture content. Temperature has a positive effect, which depends strongly on the food material. The energy of activation for heat conduction E is, in general, higher in dry food materials. 

The effect of moisture in the cells is to change the foam dimensions and weight and, as will be discussed later, the thermal conductivity. ... 

Despite the limited data of effective moisture diffusivity, a lot of data are reported in the literature for thermal conductivity. Data for mainly homogeneous materials are available in handbooks such as the Handbook of Chemistry and Physics [91], the Chemical Engineers Handbook [92], ASHRAE Handbook of Fundamentals [93], Rohsenow and Choi [94], and many others. For foods and agricultural products, data are available in Refs. [83,88,95-97]. For selected pharmaceutical materials, data are presented by Pakowski and Mujumdar [98]. 

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