Influence of variability in hygrothermal properties on analytical results of simultaneous heat and moisture transfer in porous materials

Depending on the data source used, the material hygrothermal properties that are used in the numerical analysis of simultaneous heat and moisture transfer will not be consistent. Differences in measurement methods and the individuality of specimens account for this. It is necessary to choose values from these different physical property sets to conduct a numerical calculation, which can cause the calculated results to differ. The subsequent range of variation in the calculated results should be quantitatively evaluated. In this study, the physical properties of several types of porous building materials were first gathered from four databases. The data were then categorized based on the kind of material and compared in terms of each physical property (density, porosity, specific heat, moisture capacity, thermal conductivity, and vapor permeability). The density, porosity, and specific heat varied by 10% on average, and the moisture capacity, thermal conductivity, and vapor permeability varied by 20% or more for all types of materials. In particular, the vapor permeability of plywood and moisture capacity of gypsum board differed by 50%. The influence that these physical property value variations had on hygrothermal calculation results was then quantitatively demonstrated for moisture and heat flow rate under a step change in the relative humidity or temperature of indoor air for a single layer wall. The moisture and heat flow rate into a single layer wall fluctuated by approximately 10%–40% due to differences in the vapor permeability and moisture capacity of the materials. For all types of materials, moisture was transferred more slowly than heat. Therefore, differences in moisture property values, such as vapor permeability and moisture capacity, influenced the results more significantly. Moreover, the moisture flow was accompanied by a phase change. The differences in moisture property values thus affected the heat flow.

Digital Modeling of Heat Transfer during the Baking Process

Numerical modeling and analysis of the baking process are challenging biochemical processes occurring in bread. These changes result from mass engineering tasks, usually characterized by the complex chain of chemical, physical, and heat transfer processes impacting the baking at the same time primarily caused by a variation of two dominating factors: (i) the heat and (ii) the internal moisture content at different temperatures and during the time’s process. This study presents an analysis of the 1-D computational fluid dynamics model for simultaneous heat transfer within a cylindrical bread sample. The numerical simulations were performed using the finite difference model (FDM) and the finite element model (FEM). In the first case, the proposed numerical model considered radiation and convection during sample heating and described the sample’s simultaneous heat, water, and vapor diffusion mechanisms. The calculations indicated that the FDM was susceptible to the time step; consequently, the range of 10 s and 100 s yielded the only relevant results. In the second case, the FEM was used to describe the phenomena of transportation during baking. Results obtained by the FEM showed a large temperature gradient near the surface. The study showed the presence of some critical cases that are considered the most influential on the stages of bread production. The first critical value is the time when the baking temperature reaches 100° C. The second critical value is the time when the liquid water content in the baking medium reaches its peak. The boundary conditions were examined and illustrated by figures in the center and the surface of the bread.

Burnout Investigation of Small Diameter Tubes Immersed in Nanofluids

This paper deals with research into pool boiling critical heat flux (CHF) of water–Al2O3, water–TiO2 and water–Cu nanofluids on horizontal stainless steel tubes. The experiments were conducted under atmospheric pressure. Nanoparticles were tested at concentrations of 0.001%, 0.01%, 0.1% and 1% by weight. Ultrasonic vibration was used in order to stabilize the dispersion of the nanoparticles. Although dispersants were not used to stabilize the suspension, the solutions tested showed satisfactory stability. Experimental measurements were performed with stainless steel tubes of three outside diameters: 1.6, 3 and 5 mm. Enhancement of CHF was observed to be independent of the concentration and material of the nanoparticles and tube diameter, with simultaneous heat transfer degradation. Built up during the boiling process, nanolayers improve substantially the heating surface wettability. A correlation is suggested for the CHF prediction during pool boiling of nanofluids.

ON PROBLEM OF CALCULATION OF CONDENSATION MOISTURE IN THE CAVES

Microclimate of karst caves are formed by the simultaneous process of heat and moisture (mass) exchange in the double-component thermodynamic system of “cave air - environment rock massif”. Cave formations in this case are belonged to the rock massif. The simultaneous heat-mass-transferring process can be considered as a simple process of transferring heat and mass, or isolation of heat transfer itself and the occurrences of evaporation and condensation for the purpose of simplifying the evaluation of the rests. Accordingly, relatively reliable must be the observations results on occurrences of evaporation and condensation in case when it is possible to isolate indicated simple processes from each other.