Moisture Transfer Mechanisms Research Articles

ОБҐРУНТУВАННЯ РОЗШИРЕНОГО СКЛАДУ СПОСТЕРЕЖЕНЬ НА ВОДНО-БАЛАНСОВИХ СТАНЦІЯХ ТА ДОСЛІДНИЦЬКИХ ГІДРОГЕОФІЗИЧНИХ ПОЛІГОНАХ

The current level of understanding of the subordination and multifactorial dependence of the determining processes in the atmosphere, lithosphere and hydrosphere of the Earth requires a corresponding reorganization of the basic system of environmental monitoring, improvement and expansion of research on water these stations, which can become the supporting 'nodes' of the balance of this system. The appearance of fundamentally new theoretical developments, modern devices and equipment, a large number of software tools, etc., prompts a significant reorganization and strengthening of the environmental monitoring system. The article substantiates an additional set of studies, which should be included in the regulation of observations at already existing water balance stations, with their mandatory modernization. Spheres are subject to control - the atmosphere, surface and underground (subsurface) hydrospheres, which change over time at different rates and pedosphere. A certain inertia of hydrogeological processes implies the possibility of using meteorological indicators, which can be used to predict changes in the moisture regime in the aeration zone and shallow groundwater in the near future; based on the reliably predictable changes of the latter - to forecast changes in interlayer groundwater resources, etc. It is proposed to include in the monitoring regulations the following indicators of the state of the environment, which will allow to identify and analyze the causes of changes in the water situation, balance and resources, to determine the mechanisms of moisture transfer and accumulation, as well as to build models and perform predictive assessments. The results of comprehensive research at the «Lutiz» landfill demonstrate significant variability of hydrogeophysical indicators and changes in the intensity of natural signals depending on the landscape timing, the latest tectonics, the geological basis and the composition of the overlying sediments. Original devices are presented, which are used to determine indicators of electric and thermal fields, the ratio of positive and negative air ions, etc.

Features of underground flow formation to the Golovesnya river (Desna basin) during the dry period 2007-2021

Purpose. To analyze the peculiarities of the dynamics of underground flow on the catchment of the Golovesnya River, a right-bank tributary of the Desna River, during the dry period of 2007-2021 in comparison with the period of 1960-1985 - before the beginning of noticeable climatic changes. Methods. The hydrodynamic finite-difference method of calculating the specific consumption of groundwater based on the data of routine observations of the levels of groundwater and surface water, the hydrodynamic method of calculating the components of the groundwater balance. Results. The specific flow of groundwater to the Golovesnya River was calculated based on the data of observations for 2007-2021; the share of groundwater flow in the total flow of the river was determined, changes in the dynamics of infiltration nutrition and the inflow of groundwater to the river were revealed, which can be associated with the sign of the charge of the static electric field of the surface layer of the atmosphere, an increase in air temperature, and an increase in the volume of moisture retention in the aeration zone after reducing GWT. Underground flow to the river largely compensates for the losses and stabilizes the river flow in low-water years and during prolonged hydrological drought. Important factors in the regulation of the groundwater regime in low-water periods (in addition to precipitation) are the level and flow of water in the river, as well as the loss of moisture due to the saturation of the aeration zone, and therefore, the lithological and granulometric composition of this zone. Losses in the aeration zone are also an important regulatory factor of balance and river flow in the scale of the Desna River catchment. The largest volumes of underground runoff to the Golovesnya River for 2007-2021 were set for the multi-water year 2016, the smallest - for 2021. By 2016, the chronological graphs show clearer seasonal patterns: repeatability of underground flow fluctuations in the spring and autumn seasons; opposite flow fluctuations in winter and spring and a relatively stable summer flow; typical for autumn are maximum, and for summer always minimum values ​​of reserves and infiltration recharge of groundwater. In the abundant water year of 2016, infiltration recharge and groundwater reserves during autumn and winter sharply decrease and acquire negative values ​​due to a significant decrease in GWT in the previous period, an unfavorable course of temperature and precipitation distribution, and increased outflow to the river. In 2017-2021, the seasonal differences in the recharge of the river with groundwater are almost equalized: the role of recharge increases during the summer period, and decreases during the spring and autumn seasons. The role of the condensation mechanism of moisture transfer in the aeration zone is increasing. During the summer, the lateral inflow of groundwater to the river was the highest in 2016-2018. Conclusions. Calculations and analysis of the dynamics of infiltration feeding and groundwater flow to the Golovesnya River have been performed. Changes in groundwater supply and discharge, especially in the period after 2015, have been identified, which can be interpreted as the consequences of adverse weather and climate conditions in 2014-2015. In 2018-2021, there was a decrease in groundwater and surface water resources, which corresponds to signs of hydrological drought.

Hygrothermoelastic large deflection behaviour in a thin circular plate with non-Fourier and non-Fick law

The coupled fractional dual phase-lag hygrothermoelasticity theory, developed using fractional calculus principles, extends classical Fourier’s and Fick’s laws to a time-fractional differential equation. The concept is applied to a thin circular plate that is exposed to hygrothermal loadings. The finite integral transform method and decoupled technique are utilized to create closed-form expressions for various factors such as temperature, moisture, large deflection, and stresses. The study compares the results of the dual phase-lag model with those of classical and hyperbolic models. The phase-lags parameters play a crucial role in regulating the heat and moisture transfer mechanism

Study on the evolution of concrete pore structure and moisture transfer mechanism after high temperature based on different sequence NMR technology

This study investigates the influence of elevated temperatures on concrete's pore structure and moisture transfer mechanisms. Concrete samples exposed to temperatures of 200℃, 400℃, 600℃, and 800℃ were subjected to capillary water absorption (CWA) tests, and their properties were analyzed using Scanning Electron Microscopy (SEM) and Nuclear Magnetic Resonance (NMR) techniques employing different sequences (CPMG and SE-SPI). The results reveal after high temperature four distinct categories of pores within the concrete: micropores, mesopores, capillary pores, and macropores. Mesopores are in the highest abundance, followed by capillary pores, macropores, and micropores. As temperature increases, hydration reactions lead to an upsurge in micropores and a decline in the proportion of other pores. With further temperature escalation, the cement slurry's volume alteration causes pore expansion, resulting in a rise in capillary pores and macropores and a decrease in micropores. Furthermore, NMR technology, employing CPMG sequences, identifies three stages in the concrete's CWA process: rapid moisture transfer, moisture transfer rate descent, and moisture transition stage. SE-SPI sequence-based NMR results show that, following high-temperature exposure, moisture transfer within concrete progresses as follows: initially, water rapidly permeates mesopores and capillary pores from the exterior. As CWA time increases, these pores tend to saturate, and water gradually infiltrates micropores and macropores. High temperatures significantly impact the CWA process, enlarging pore sizes and altering the type of pore filling during capillary water absorption in concrete.

Evolution and Modelling of the Moisture Diffusion in Walnuts during the Combination of Hot Air and Microwave–Vacuum Drying

To understand the moisture transfer mechanism of walnuts during the combination of hot air (HA) and microwave–vacuum (MV) drying (HA-MVD) process, the drying characteristics and moisture diffusion characteristics of walnut during HA-MVD were investigated. The results indicated that the HA-MVD of walnuts occurred mainly in the falling-rate stage. The value of effective moisture diffusivity (Deff) dropped continuously with the decrease in moisture content (MC) during the HA drying, while switching to MV drying could truncate the decrease in Deff and still maintain a high value until the end of drying. The HA temperature, MC of the transition point, microwave power, and MV thermostatic temperature have significant effects on the moisture diffusion characteristics of walnuts. The values of Deff for walnuts ranged from 2.33 × 10−9 m2/s to 6.89 × 10−8 m2/s. The third-order polynomial prediction model of Deff related to the sample MC and drying conditions was established to describe the dynamic change in the Deff of walnuts during the HA-MVD process. The application of MVD in the final stage of drying could rapidly increase the internal vapor pressure of the walnuts, accelerate the diffusion speed of the internal moisture, and re-enhance the drying rate. The findings have practical value for the development of efficient and energy-saving drying methods in the walnut industry.

Drying Kinetics and Mass Transfer Characteristics of Walnut under Hot Air Drying

This study was conducted to investigate the drying kinetics and internal and external mass transfer characteristics of walnuts for an understanding of the drying mechanism. The drying characteristics, mass transfer characteristics, and color of walnut during hot air drying (HAD) were investigated under different initial moisture content (IMC) (0.35, 0.39, and 0.43 g water/g wet mass) and drying temperatures (50, 60, 70, and 80 °C). The results indicated that the IMC and drying temperature both have significant effects on the drying process of walnut, showing the higher the IMC, the longer the preheating time, the smaller the effective moisture diffusivity (Deff) and mass transfer coefficient (hm), and the longer the drying time, but reverse results for drying temperature. The values of Deff and hm for walnut ranged from 4.94 × 10−10 to 1.44 × 10−9 m2/s and 1.24 × 10−7 to 3.90 × 10−7 m/s, respectively. The values of activation energy for moisture diffusion and mass transfer ranged from 21.56 to 23.35 kJ/mol and 28.92 to 33.43 kJ/mol, respectively. Multivariate linear prediction models were also established for estimating the Deff and hm as a function of the HAD process parameters. The drying temperature has a greater effect on the walnut kernel lightness than the IMC. The Verma et al model could be used to describe the HAD process of the walnut. The findings contribute to the understanding of moisture transfer mechanisms in walnuts and have practical value for the evaluation and improvement of drying systems.

The methods of optimization and regulation of the convective drying process of materials in drying installations

In this work, based on fundamental principles well-established in the field of drying technology, optimization for the process of material drying involves controlling the mechanism of moisture transfer by influencing diffusion and thermo-diffusion processes. Based on the Kirpichov criterion, a quantitative measure of moisture transfer dynamics is ensured, while Nusselt numbers help control temperature gradient and efficient moisture removal. The article proposes the use of empirical relationships between Nusselt numbers and problem parameters such as moisture content, temperature, and airflow velocity. Optimizing drying parameters based on the proposed equations can contribute to improving drying quality, reducing process time, and lowering energy consumption. The proposed methods of moisture and temperature gradient control within the material are sustainable and allow us to achieve uniform drying without causing excessive stresses or deformation.

Массопроводность дисперсных и листовых материалов в условиях термодинамического равновесия

The intensification of drying of wet capillary-porous bodies and their structural and deformation transformations is determined by the internal mechanism of moisture transfer in the material being dried. The scientific papers of A.V. Lykova, S.P. Rudobashty, B.S. Sazhina, E.N. Ochneva, N.V. Churaeva are devoted to the study of mass conductivity of various bodies. They confirm the importance to determine the characteristics of internal moisture transfer in the form of liquid and vapor to calculate the intensity of moisture exchange between the surface of a wet material and the coolant and to establish the ratio of moisture and heat flows not only during the drying process, but also during storage of materials. For dispersed and sheet capillary-porous materials of different composition and porous structure, determining the intensity of internal moisture transfer during their interaction with the environment is a relevant area of the research. It determines the drying method, thermal treatment conditions and energy efficiency. Dispersed and sheet capillary-porous materials of various shapes, sizes, structures, and humidity have been used as the objects of the study. To determine the mass conductivity parameters of a wet body during its interaction with a coolant, a thermodynamic method is used. It is based on the use of experimental desorption isotherms and calculated parameters characterizing the porous structure of the material. The authors have applied the method to calculate the mass conductivity coefficients of dispersed and sheet materials based on experimental data obtained under conditions of thermodynamic equilibrium of a wet body and gas. Calculated curves of the dependence of effective mass conductivity coefficients and the moisture content of the dried material are obtained considering environmental parameters. The data obtained on the patterns and the changes in mass conductivity coefficients makes it possible to establish the types of relationships between moisture and the material with the mechanisms of its transfer for a number of dispersed and sheet bodies. The data obtained can be used for the kinetic calculation of the drying process and to determine the conditions of their storage, as well as to identify the conditions for increasing the energy efficiency of dryers with convective supply.

The Behaviour of Wood Fibre Insulation Systems for Solid Wall Buildings: Lessons from a Long-term Monitoring Study

The majority of the traditional building stock in the UK is made of solid brick walls. Internal wall insulation is one of the possible interventions to preserve the external appearance of a building while improving the energy efficiency of its walls. However, it can lead to moisture-related risks, such as excess moisture accumulation and mould growth. This paper presents an overview of the lessons learnt from the long-term monitoring of two case-study solid wall buildings, insulated with wood fibre-based systems and monitored for a year. The aim of this monitoring campaign was to provide evidence on the main moisture transfer and storage mechanisms occurring in solid walls insulated with wood fibre-based systems under actual environmental conditions. Results showed that the dynamic interaction between the insulated wall and moisture sources could be controlled by means of an appropriate specification of the insulation system, considering the influence of orientation, material properties and indoor moisture loads. It was also found that construction moisture influenced the initial conditions of the interface, but mitigation strategies can be considered to reduce mould growth risk during installation and before the drying of construction moisture.

Heat Mitigation Benefits of Urban Trees: A Review of Mechanisms, Modeling, Validation and Simulation

Modeling, validating, and simulating are three essential parts in investigating the heat mitigation benefits of urban trees (BUT). Therefore, 81 relevant studies from the last ten years are reviewed, analyzed, and summarized in this study. Three main ways for urban trees to adjust the environment are summarized, including shade creation and radiation modification, cooling effects of transpiration, and airflow blocking and modification effects. Research works are analyzed with regard to four categories: (1) heat and moisture exchange mechanisms and their mathematical modeling; (2) verification of modeling predictions based on measurements; (3) thermal performance simulation and prediction; and (4) environmental assessment and human thermal comfort analyses. Future research opportunities are discussed: (1) conduct real-time and in-depth measurements to analyze the mechanisms of heat and moisture transfer of trees in different areas; (2) develop tree radiation attenuation, airflow resistance, and transpiration models to accurately describe heat and moisture transfer processes in the urban environment; and (3) establish a three-dimensional numerical simulation method that can accurately simulate the urban thermal environment with trees. This review provides researchers with an overview and potential research opportunities on the thermal effects of urban trees.

A Review: Study on the Enhancement Mechanism of Heat and Moisture Transfer in Deformable Porous Media

The heat and moisture transfer process in deformable porous media commonly exists in material drying, solid waste treatment, bioengineering, and so on. The transfer process is accompanied by deformation of the solid skeleton and pore interface structure, which limits the transfer rate and affects quality. Microwave and ultrasound are the main representatives of reinforcement technology. However, as the moisture decreases, the energy utilization efficiency of microwaves decreases significantly. Based on the experimental and theoretical methods, the enhancement mechanism of ultrasound on the process is studied, which provides guidance for the wide application of ultrasonic enhancement. With the increase in ultrasound power, the pore area and the moisture effective diffusion coefficient gradually increase. A macroscope mathematical model for ultrasonic-coupled thermal-hydro-mechanical modeling is developed, and the results show that ultrasound increases the temperature gradient within material, resulting in higher moisture transmission rates with an ordered direction, and the alternating expansion and compression process results in smaller macroscopic deformations. Subsequently, the drying kinetic characteristics of typical deformable porous media such as municipal sludge, porous fibers, and activated alumina particles are investigated. The process parameters of the ultrasonic assisted drying system are optimized using the response surface method and artificial neural network model.

Enhancing the Performance of Roller-Compacted Concrete Pavement by Synergetic Improvement of Packing Density, Lubrication, and Moisture State of Recycled Concrete Aggregate

Roller-compacted concrete pavement (RCCP) is considered superior to other pavement types with reference to cost, ease of construction, and performance. However, the aggregate demand is significantly higher in RCCP than in the conventional concrete pavement. It is predicted that natural minable limestone sources would be exhausted in India in the next 30 to 40 years. One way to reduce the natural aggregates (NA) requirements in RCCP is through the integration of recycled concrete aggregates (RCA). However, the physical property of RCA is significantly inferior to that of NA owing to the presence of adhered mortar (AM), which increases the water demand by 2.3 to 4.6 times and affects the compactness and hardened-state behavior of RCCP. This study has tried to enhance the compactness and improve the performance of RCCP containing RCA (coarse, fine, and total RCAs) through different synergetic approaches. This, in turn, enhances the interlocking capacity using the particle packing approach, followed by mitigating the negative effects of AM by altering the moisture states, then improving the compactability and lubricating the matrix with superplasticizers. The results depict that altering the moisture states alone could adversely affect the RCCP performance because of the moisture transfer mechanism from the hydraulic gradient. Moreover, the inclusion of superplasticizers in different moisture states could manifest better aggregate rearrangement and compactability. Further, it could improve the tensile behavior of the RCCP compared with the concrete containing NA. These findings favor the complete replacement of NA by RCA for low-volume rural road construction.

АНАЛІЗ ГЕОФІЗИЧНИХ, МЕТЕОРОЛОГІЧНИХ ТА ГІДРОГЕОЛОГІЧНИХ ДАНИХ ДЛЯ ПОЯСНЕННЯ НЕВІДПОВІДНОСТЕЙ МІЖ ІНФІЛЬТРАЦІЄЮ ТА АТМОСФЕРНИМИ ОПАДАМИ

A traditional and non-standard approach to the analysis of estimated values of groundwater infiltration feeding using the charge sign of the electric field of the surface atmosphere is considered. A comparison of the results of hydrogeological calculations and observations from specific electrophysical and meteorological factors made it possible to explain the discrepancies between the values of infiltration nutrition and the amount of precipitation. The daily values of groundwater recharge were determined based on 41-year observations in wells in the city Khmelnyk. The average long-term value of infiltration recharge of groundwater from GWT 0.8–2.3 m was 145 mm, from GWT 2.7–4.5 m – 14.7 mm (fluctuations in the range from 129 mm to negative values). A significant correlation between the annual values of infiltration and precipitation at GWT0.8–2.3 m was observed only at the first stage of observations (1980–1988). However, there are also significant contradictions in the ratios of infiltration and precipitation, which could be explained only by involving the data of our own electrophysical observations. For the period from 2008 to 2017, the negative values of the "infiltration-temperature" correlation coefficients for the summer season changed to positive ones, which suggests an increase in the importance of moisture transfer mechanisms in the form of steam. Data on the charge sign of the static electric field of the surface layer of the atmosphere support this conclusion. At high values of the intensity of the static electric field (E) with a negative sign, the direction of moisture movement has an upward character, thanks to which evaporation from the aeration zone increases several times compared to what happens at zero values of E. And the intensity of the electric field with a positive sign forms a downward movement of moisture, which leads to an increase of GWT. From this, the cases when low values of infiltration nutrition were obtained with a significant amount of precipitation become more understandable. Conversely, the low amount of precipitation in 2014–2017 (average value 524 mm) was accompanied by high groundwater recharge (160 mm – 10 % above the norm), stable underground flow to the river in the range of 90–100 m3 /year/m and by the growth of GWT due to the positive values of the static electric field and the decrease in wind strength. This electrical factor has such a powerful influence that it is able to neutralize and effectively counteract the negative impact on groundwater recharge of temperature rise and air humidity deficiency. Since electrodynamic processes have a significant and sometimes decisive role not only in moisture transport in the aeration zone, but also in regional processes of groundwater feeding, the creation of artificial positively charged static fields above the soil surface can become the most effective safeguard against the depletion of groundwater reserves during hydrogeological drought.

A meshless method for modeling the microscopic drying shrinkage cracking processes of concrete and its applications

The dry shrinkage cracking of concrete structures has a great influence on their mechanical strengths. However, traditional numerical methods have some limitations in dealing with the shrinkage cracking processes of concrete. Based on this background, the traditional smoothing kernel function in Smoothed Particle Hydrodynamics (SPH) method has been improved, which can realize the simulations of progressive cracking processes of concrete; The SPH forms of humidity conduction equations have been derived, which can realize the simulations of humidity expansions in SPH; The relationships between the humidity gradient and concrete shrinkage stress is defined, which is embedded into SPH constitutive equations to realize the simulations of concrete humidity – stress- damage coupling problems; The random aggregate generating method and the “region searching” method have been proposed to realize the generations of concrete meso-structures in SPH. Firstly, the microscopic dry shrinkage cracking processes of a circular concrete model is simulated, and the rationality of the improved method is verified by the comparisons with previous experimental results. Then the influences of different factors (aggregate percentages, ambient humidity, shrinkage coefficient and moisture diffusivity) on the dry shrinkage cracking morphologies as well as the moisture transfer mechanisms of concrete are systematically discussed, results show that: The aggregate percentage affects the depths the “advantage cracks” penetrate into the concrete model; With the decrease of ambient humidity and moisture diffusivity, the density of shrinkage cracks increase sharply, and the crack length also increases; The penetration depth of the “advantage cracks” increases with the increase of the shrinkage coefficient. The research results can provide some references for the comprehensive understandings of the mechanisms of concrete shrinkage cracking and moisture migration. Meanwhile, it can also provide a certain understanding for the applications of SPH method into simulations of humidity – stress- damage coupling problems.

Determination and comparison of effective moisture diffusivity of carrot (core and cortex) during hot air drying

AbstractThe moisture content and shrinkage of a sample are often ignored when determining its effective moisture diffusivity (EMD), which will result in a larger error in simulating the dynamic moisture diffusion process during drying. This work aimed to determine the EMDs of cortex and core in axial and radial directions considering the combined influence of moisture content and shrinkage of material. The drying and shrinkage characteristics of cortex and core were investigated under different hot air drying conditions, and the EMDs of the cortex and core in different drying stages were determined by the slope method considering the moisture content and shrinkage. The results showed that the EMDs were dependent on the carrot component rather than its directions. The EMDs of the core and cortex were different. It decreased slowly at the early drying stage, and decreased sharply at the later drying stage, and increased with the hot air temperature increasing. A third order polynomial relationship was established to correlate the EMDs with the moisture content and hot air temperature (R2 > 0.9460), and was verified by experiments. The maximum weighted absolute percentage error of the moisture content from simulation and experiment was only 6.11%, more accurate than that based on a constant effective moisture diffusivity (CEMD) for characterizing the intrinsic moisture diffusion in carrot during drying.Practical applicationsThe EMD is an important parameter that characterizes intrinsic moisture diffusion in material and often used for modeling and calculation in food drying. The investigation methods developed in this study can be applied for determining the EMD of samples with shrinkage and variable moisture content. This study would be helpful for understanding the moisture transfer mechanism and optimizing operation conditions in carrot drying industry.

A new approach for measuring the air gap between diaper and baby skin

The air gap between baby skin and top sheet of disposable diapers is the main place where baby’s metabolic heat, perspiration, and moisture produced by evaporation of urine stay and transfer. Heat and moisture transfer underneath a diaper are greatly affected by the shape and size of the air gap, therefore, it is of significant importance to explore the air gap between diaper and baby skin for comfort and skin health of babies. Currently, it is still a big problem to accurately measure and characterize the air gap of diaper for scholars. In this article, a new approach to measure the air gap, combining three-dimensional scanning, liquid silicone rubber filling, and image processing methods, was proposed to obtain the shape and size of the air gap between diaper and baby skin by region, for quantitative research on heat and moisture transfer mechanism of diapers. In addition, several indicators, including air volume, air gap thickness, and contact area, as well as clothing area factor of the diaper, were chosen to quantify the air gap size and distributions. After verification of accuracy and repeatability, this new approach can effectively measure the size of the air gap underneath the diaper, and provide a feasible way for exploration on quantitative relationship between thermal and moisture comfort of diapers and their structural design, fit, and diapering habits. Furthermore, diaper designers and baby caregivers can obtain guidelines of diaper structural design, wearing and change to improve the microclimate in the diaper for baby comfort and skin health.

Drying characteristics of concrete under surface low air pressure

The humidity change of C30, C50, and C80 concretes under surface low air pressure and atmospheric pressure were tested. Based on the mechanism of moisture transfer under low air pressure, a theoretical model of concrete humidity field considering the effect of pressure was established. The model was used to analyze the changes of concrete internal humidity field when the surface pressure changes. The results show that the relative humidity values of each concrete under low air pressure are lower than that of atmospheric pressure. And the effect is greater near the drying surface. The improved calculation model was used to calculate the humidity change of concrete under low air pressure. The calculated values were in good agreement with the experimental values. It can be used to study the internal humidity field of concrete under different pressure conditions.

Mechanism of moisture adsorption in plant fibers surface-modified with glycerol evaluated by LF-NMR relaxation technique

Surface modification by humectants is an important technology to improve product quality in textile field, healthcare, tobacco processing and paper-making industry. As a common humectant glycerol is applied to keep the moisture adsorbability of the plant fibers during manufacturing. The effects of glycerol on the moisture adsorption of the plant fibers were studied by analyzing the induced differences of bulk and surface physicochemical property with XRD, FTIR, SEM characterizations. The improvement of moisture adsorption capacity of the modified plant fibers was caused by the increased active adsorption sites, while the moisture diffusion resistance increased simultaneously with glycerol indicated by a declining Deff. LF-NMR relaxation spectra demonstrated the water state and distribution in the plant fibers were changed by loading glycerol. The moisture transfer mechanisms induced by glycerol were also investigated. Free water failed to materialize in the plant fibers treated with glycerol, immobile water existed preferentially during the adsorption, and bound water presented increasing after the immobile water was saturated. These findings are referenced values to improve the manufacturing processes of moisture-retaining properties of different functional plant fibers.Graphical abstract

DERIVATION OF THE EQUATION OF UNSTEADY-STATE MOISTURE BEHAVIOUR IN THE ENCLOSING STRUCTURES OF BUILDINGS USING A DISCRETE-CONTINUOUS APPROACH

Two differential equations of moisture transfer based on the theory of moisture potential have been considered. The first equation includes the record of moisture transfer mechanisms of vapor and liquid phases and their relationship. The second equation is a simplified form of the first equation which makes it possible to apply a discrete-continuous approach. The peculiar properties of the boundary conditions setting of the outside air for temperature and humidity fields have been presented. It is proved that the use of the discrete-continuous method provides high accuracy of calculations and can be used in engineering practice to assess the unsteady humidity regime of enclosing structures.

Heat and moisture transfer studies on walnuts during hot air drying in a fixed-bed column dryer

• 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.