Moisture diffusivity and transfer modelling in dry biscuit

In this investigation, the mechanism of mass (moisture and fat) transfer during deep-fat frying of robo (fried melon cake) was carried out. The plots of dimensionless concentration ratios against time were used to determine moisture transfer parameters including moisture diffusivity, moisture transfer coefficient, moisture transfer Fourier number, moisture transfer Biot number and activation energy and fat transfer kinetic parameters such rate constants and activation energy at different frying temperatures of 140, 155 and 170 0C. All moisture parameters increased linearly with frying oil temperatures except moisture transfer Biot number which deceased linearly with increase in temperature with correlation coefficient (R2) ≥ 0.99. There was also positive linear relationship between fat transfer rate constant and frying temperatures (R2 ≥ 0.99). An Arrhenius type of relationship was found between temperature and the effective moisture diffusivity and fat transfer rate constant (R2≥0.99) with activation energies values calculated as 25.66 and 42.18 kJ/mol, respectively. Both moisture loss and fat desorption after initial adsorption of 60 seconds were adequately modeled by the first-order kinetics adopted with high correlation coefficient (R2) value and were time and temperature dependent as revealed by Arrhenius analysis performed on the experimental data.

• Heat and moisture transfer model was developed for fixed-bed column walnut drying. • Temperature and moisture distributions in the column and single walnuts were simulated. • Mechanisms of heat and moisture transfer during the drying process were discussed. • Good predictability achieved for drying under step-down temperature conditions. This research was conducted to better understand the mechanisms of heat and moisture transfer of walnuts during drying in a fixed-bed column dryer with heated air, and to develop a mathematical model to simulate this process. Drying experiments were performed under constant temperature and step-down temperature conditions (43, 55, 65 and 75 °C) with fixed air velocity of 0.7 m/s. The nonequilibrium transfers of liquid moisture, water vapor and heat within the single walnuts (both shell and kernel), as well as between the walnuts and drying air in the column were simulated and studied using a finite element method. The numerical results showed that the distribution of temperature and moisture content in the single walnuts played an important role in the heat and moisture transfer in the deep-bed column. The drying characteristics of walnuts were significantly affected by the non-uniformity of drying conditions surrounding the walnuts (particularly for air temperature and humidity) along the column heights. The predicted values of walnut moisture contents, temperatures, and air relative humidity at different column heights agreed well with the experimental values (R adj 2 > 0.972) under different drying conditions. This study established the theoretical basis for improving and optimizing the walnut drying process in deep-bed dryers. The developed mathematical model can be used to describe the drying characteristics and understand the mechanisms of walnuts drying in deep bed dryers.

This paper reports the development of a mathematical model of moisture and heat transfer coupled with phase change materials (PCM) in porous textiles. On the basis of a finite volume difference scheme, the thermal buffering effect of PCM is simulated. The distributions of temperature and moisture concentration in porous textiles with PCM are numerically computed by this technique. For comparison, the distributions of temperature and moisture concentration in porous textiles without PCM are also numerically computed. A series of experiments measuring a fabric's surface temperature with PCM is conducted to validate the model, and there is reasonable agreement between the predictions and the measurements. The results indicate that this model is satisfactory.

This paper deals with the numerical and analytical modelling of two-dimensional heat and moisture transfer during drying of a spherical object. Drying is considered to be a process of simultaneous heat and moisture transfer whereby moisture is vapourized by means of a drying fluid (e.g. air), as it passes over a moist object. Numerical modelling of two-dimensional heat and moisture transfer during drying of a spherical object is carried out using an explicit finite-difference approach. Temperature and moisture distributions inside the object are determined by using the developed computer code. Moreover, the results predicted from the present model are compared with the experimental data available in the literature and a considerably high agreement is found. Copyright © 2003 John Wiley & Sons, Ltd.

Experimental study on the kinetics of water vapor sorption on selective water sorbents, silica gel and alumina under typical operating conditions of sorption heat pumps

Comparative study on the moisture transfer properties of shotcrete and casting concrete

A mathematical model, numerical algorithm, solution, and results of the computational experiment on a computer are developed to predict the process of heat and moisture transfer in porous media, taking into account such factors as the internal heat and moisture release of a porous natural product on the example of raw cotton and its products of hulling, seeds of various crops. It also considers the effect of temperature and moisture content changes in the environment on the storage and drying of porous materials. In this study, the developed mathematical support of the object under research makes it possible to predict the change in temperature and moisture content at arbitrary points of the porous body and serves to prevent the loss of quality and spontaneous combustion of materials under solar radiation and to analyze and make managerial decisions. Based on the method of coordinate-wise splitting, a numerical algorithm for calculating three-dimensional heat and moisture transfer problems in areas of the parallelepiped type is presented. An implicit second-order difference scheme for calculating the required functions is presented. Based on the numerical calculations performed, it was established that moisture and heat transfer and their exchange with the environment occurs in the upper layers of the cotton pile; in the inner layers, there is an increase in temperature and moisture due to the respiration of raw cotton and its seeds, which depends on the degree of moisture of the raw cotton and their products of hulling.

The subject of this study is numerical modelling of the fully coupled heat and moisture transfer in the building material, i.e., in a brick. The new mathematical model of equilibrium transport of heat and moisture in the form of continuous liquid medium (funicular region), discontinuous liquid medium (pendular region) and gaseous phase with dry air is proposed. As independent variables, the total mass moisture content and temperature are chosen. Both energy and moisture balance equations are mutually coupled. It is due to dependence of capillary and vapor pressures on the moisture content and temperature. Additionally, vaporization and condensation phenomena are also accounted for in the energy balance equation. Both transport equations are supplemented with boundary conditions expressing the coupled heat and moisture transfer at the dried wall or heat transfer at impermeable walls. Balance equations are discretized on the rectangular control volume grid in the framework of the in-house computational code. A grid-based method of generation of the control volume mesh is proposed, with ‘half’ control volumes related to boundary grid nodes created at domain walls. This approach enables deeper insight into implemented code as well as simplifies analysis of influence of selected terms in balance equations and implementation of complex hygro-thermal boundary conditions.

Moisture transfer in porous construction materials car Nomenclature c Specific heat capacity [J.kg−¹.K−¹] g Moisture flow [kg.m−².s−¹] ries many causes for their degradation: mould devel opment and freeze-thaw damage are favoured by the gl,r Wind-driven rain [kg.m−².s−¹] −2 accumulation of water, and chemicals such as chloride ions and carbon dioxide may accelerate the fracturing of cementitious composites. Over time, microscopic and macroscopic cracks progressively develop under the effects of mechanical loading and sorption/desorption cycles: their influence is to be accounted for in long-term hygrothermal performance assessments of the building envelope. Current simulation codes for heat and moisture transfer modelling in building facades do not allow accounting for the presence of cracks and defects in the material layers. The present work aims at integrat I Solar irradiance [W.m ] Kl Liquid permeability [s] kf ,l Fracture liquid permeability [s] Lv Latent heat of vaporization [J.kg−¹] pc Capillary pressure [Pa] pv Vapour pressure [Pa] q Heat flow [W.m−²] RH Relative humidity Rh Horizontal rainfall [mm.m−²] T Temperature [K] U Wind speed [m.s−¹] u Crack aperture [m] −3 ing such effects of damage in simulations at the scale of building facades. Experimental measurements of crack patterns were integrated into a newly developed w Moisture content [kg.m ] Greek symbols α Heat transfer coefficient [W.m −2.K−1] numerical model predicting coupled heat and moisture transfer in multi-layered components. The consequences of fractures on the hygrothermal performance of these components were then investigated by comparing damaged and undamaged materials in a series of simulation cases. Cracks were found to accentuate the amplitude of daily sorption/desorption cycles and moisture accumulation in the walls, particularly in case of impacting rain. Their impact on the overall thermal performance is small, although not negligible in case of water infiltration towards an insulation layer.

The ambient environment of architectural heritage is an important factor affecting its conservation. Two adjacent rows of Buddha statues in Grottoes No. 3 (semi-open) of Maijishan Grotto in Gansu, China, show apparent differences in the degree of deterioration. This study made a monitoring scheme of grottoes microenvironments such as air temperature, relative humidity, radiation, and surface temperature to explore the cause of the difference. A two-dimensional heat and moisture (HAM) transfer model was established and verified to simulate the temperature and humidity on the surface and inside of the Buddha statues. Then, temperature and water content fluctuation and the risks of thermal stress destruction on the surface and near the surface of the Buddha statues were evaluated. The results show that the radiation difference causes thermal stress and water content differences both in heights and in depths. This impact brought by the direct sunlight may contribute to the different deterioration on the two rows of Buddha statues. The eaves shaded the upper row of the Buddha statues much longer than the lower ones. Less severe fluctuation and differences in temperature and water content occur at the middle and upper points. This study evaluates the degradation of Grottoes No. 3 and has guiding significance for its preservation methods.

Modelling of wood drying is analyzed. Wood drying involves moisture transfer from the interior of the wood to the surface, then from the wood surface to the surrounding air. These processes can be characterized by the internal and surface moisture transfer coefficients. A model of the two-dimensional moisture transfer is suggested to determine these coefficients in contrast to the one-dimensional model which was proposed in [12]. The model is based on a diffusion equation with a variable diffusion coefficient. The insufficiency of the one-dimensional model is considered. The influence of the geometry of a lumber on determination of the surface emission and diffusion coefficients and on the dynamics of drying is investigated.

The purpose of this paper is to present a numerical model of coupled heat, moisture transfer and their effects on the porosity changes of soil during the freezing process. Coupling among heat, moisture, and mechanical deformations is solved based on finite element method. The equilibrium equations of freezing process are investigated based on segregation theory. The numerical model is applied to a 2D problem of a laboratory frost heave test. Numerical simulation reveals the porosity characters during the freezing process. The frost depths and deformations also the porosity in consideration of the orthotropic properties of frozen soil is discussed.

A simplified model of heat and moisture transfer for tailrace tunnel ventilating in hydropower station

Estimation of groundwater recharge from rainfall is a key factor for determining groundwater resources in water development and management for supporting sustainable socio-economic development, especially for arid areas. The paper presents finite element modeling in the simulation of moisture transfer in unsaturated soils through the relationship between soil moisture, soil suction, unsaturated permeability and soil-moisture dispersivity. Those parameters required for soil moisture transfer are derived from the soil-water characteristic curve functions. Element sizes and time steps used in the modelling have been selected based on a detailed analysis of numerical simulation errors. The methodology had been applied to arid coastal plain area of Ninh Thuan province, Vietnam. Five subsurface soil types in the study area have been collected and analyzed, for saturated permeability, porosity, saturated soil water content, field moisture content, etc. Hourly rainfall data of the years 2014–2018 have been analyzed and grouped into different-duration rainfall events (1-hour, 2-hour, 3-hour and so on). The different rainfall durations and depths of rainfall events and temporal infiltration determined by the moisture transfer modelling have allowed determining the groundwater recharge from the rainfall data. The results show that during the rainy months from May to December 2014–2018, the groundwater recharge from the rainfall is very varying through the modeled soil profiles, from 0.280 m (silty clay) to 0.470 m (sandy silt), which is equivalent to 33.3%—55.2% of the rainfall depth during May—December. Lower infiltration in silty clay is due to low permeability and in the sand is due to low suction, and higher infiltration in silt and sandy silt is thanks to their higher moisture dispersivity. On average, in terms of annual rainfall and soil properties, the average infiltration during May—December is 0.380 m which is equivalent to 44.9% of the rainfall depth, which is about 289 106 m3 of rainwater infiltrated into the Quaternary aquifer over 760 km2 of the coastal plain of Ninh Thuan province. The results would be very useful for effective water resources development and management in a given specific hydrogeological condition for such a severe drought area where water is extremely essential.

The diffusion of moisture in paper is a complex phenomenon with pore diffusion dominating at low moisture contents and diffusion through fibres dominating at high moisture contents. Vapor diffusion through the pore space depends on the topology of the pore connections. Recently available three dimensional digital reconstructions of the pore space using X Ray Micro computed tomography (XμCT) enable us to determine the impact of the pore and fibre tortuosities and connectivity to moisture diffusion in an explicit manner. In this study, moisture diffusion was simulated through XμCT reconstructions of paper structures using a hybrid random walk algorithm that was developed to allow simultaneous diffusion in both the pore and fibre spaces with differential ‘intrinsic diffusivities’. The algorithm is specifically applied to simulate simultaneous diffusion under low and high relative humidity conditions where diffusion occurs predominantly through one medium i.e. pore space and high humidity conditions where both media (i.e. fibre and pore spaces) are highly conductive. The ‘intrinsic diffusivity’ of moisture through fibres was determined by using numerical simulation and experimental results. This intrinsic diffusivity a fundamental fibre characteristics is found to be independent of refining level but depends only the fibre moisture content under the conditions studied. The algorithm also allowed the determination of the anisotropy in diffusivity. One interesting result is that the anisotropy in diffusion is most significant at low moisture contents when diffusion through the pore space dominates. At high relative humidities (i.e. at high moisture contents), fibre conduction provides an alternative diffusion path, homogenizing diffusion to a large extent. As a result, diffusion becomes more isotropic with increased moisture contents in paper.