Dimitrios Tzempelikos

In the present case study, a moisture diffusion model is developed to simulate the drying kinetics of Lavandula x allardii leaves for non-stationary convective drying regimes. Increasing temperature profiles are applied over the drying duration and the influence of temperature advancing rates on the moisture removal and the drying rate is investigated. The model assumes a one-dimensional moisture transfer under transient conditions, which occurs from the leaf center to the surface by liquid diffusion due to the concentration gradient developed by the surface water evaporation caused by the difference of water vapor partial pressure between the drying medium and the leaf surface. A numerical solution of Fick’s 2nd law is obtained by an in-house code using the finite volume method, including shrinkage and a variable temperature-dependent effective moisture diffusion coefficient. The numerical results have been validated against experimental data for selected cases using statistical in...

Abstract. The improvement and optimization of air-distribution systems in drying kilns contributes to the pres-ervation of the quality, safety and shelf life of perishable products. The present study reports on the numerical solution of airflow within a two dimensional drying kiln enclosure loaded with wooden pallets. The performance of air flow field is examined with and without supply of wooden pallets. Different arrangements of the supplied wooden pallets are investigated as well as the use of a screen in order to improve the airflow filed. The steady state RANS equations that formulate the flow problem are solved along with a turbulence model that is the stan-dard k-εor the rng k-ε. The effect of the turbulence modeling is distinguished through direct comparisons of the derived airflow patterns. The results obtained show the presence of stagnant zones inside the pallets columns and above of them and the presence of recirculation regions in different zones of the drying kiln with...

Batch dryers are some of the most widespread equipment used for fruit dehydration. Nevertheless, the optimization of the air distribution inside the drying chamber of a batch dryer remains a very important point, due to its strong effect on drying efficiency as well as the uniformity of the moisture content of the drying products. A new scale laboratory batch-type tray air (BTA) dryer was designed, constructed and evaluated for the drying of several horticultural and agricultural products. The airflow field inside the dryer was studied through a commercial computational fluid dynamics (CFD) package. A three-dimensional model for a laboratory BTA dryer was created and the steady-state incompressible, Reynolds-Averaged Navier-Stokes equations that formulate the flow problem were solved, incorporating standard and RNG k-ε turbulence models. In the simulation, the tray, used inside the BTA drying chamber, was modeled as a thin porous media of finite thickness. The simulations for testin...

This paper presents the results of the airflow 2D numerical simulation inside an industrial unit designed for drying of wooden pallets. The airflow field profile was examined in different operation conditions by plotting velocity vector distribution, path lines and pressure contours for both loaded with the wooden pallets and unloaded drying room. The analysis of the obtained results shows the presence of stagnation zones between and above the pallet columns and recirculation regions in different zones of the drying room. Further work will focus on validation of the predicted CFD results by means of local measurements that will be taken on a full scale dryer.

This paper presents the results of the airflow 2D numerical simulation inside an industrial unit designed for drying of wooden pallets. The airflow field profile was examined in different operation conditions by plotting velocity vector distribution, path lines and pressure contours for both loaded with the wooden pallets and unloaded drying room. The analysis of the obtained results shows the presence of stagnation zones between and above the pallet columns and recirculation regions in different zones of the drying room. Further work will focus on validation of the predicted CFD results by means of local measurements that will be taken on a full scale dryer.

In order to overcome the lack of experimental data in the open literature and the necessity to validate numerical models, as well as increase the efficiency of the drying process, a new laboratory convective (LC) dryer has been designed, constructed and equipped with an integrated measurement and automated control instrumentation. The main sections of the LC dryer, which can be arranged for operation in a closed or open circuit mode through manually controlled dumpers, are the vertical flow drying chamber, the tube heat exchanger, the thermal boiler and finally the fan -motor with a smooth speed control unit. The experimental facility tested and monitored the moisture content removal of horticultural and agricultural products. The current paper outlines the methodology applied for the design and optimization of the LC dryer, which has been achieved through the analysis of the flow field by means of computational fluid dynamics (CFD). The prediction of the 3d flow problem was accompl...

Abstract In the present study, moisture content evolution of cylindrical quince slices during convective drying was modelled by using artificial neural networks (ANN). Quince slices with an average initial moisture content of 81% in wet basis (w.b.) or 4.27 kgwater/kgdry matter in dry basis (d.b.), were dried in a laboratory thermal convective dryer and experimental data of moisture content versus drying time was obtained for nine measurement groups of 40, 50 and 60 °C drying air temperature and 1, 2 and 3 m/s airflow velocity respectively. Different topologies of multilayer perceptron (MLP) ANN models containing a single or two hidden layers with a different number of hidden neurons and different types of transfer functions, have been investigated for predicting the moisture content evolution during drying. A group k-fold cross validation iteration procedure was performed for each developed ANN structure, in order to assess each model’s ability to estimate the moisture content of quinces on unseen data of air-drying temperature and airflow velocity combinations held out of the training process. For the cross validation of the developed ANN models, appropriate statistical evaluation indices were applied. The best performed ANN model based on the cross validation score metrics, contained two hidden layers with the sigmoid, softplus transfer functions and was composed by 90 artificial hidden neurons in each of the two hidden layers. A satisfying agreement of predictions with the experimental data was noticed, achieving coefficients of determination (R2) greater than 99% and root mean square error (RMSE) values less than 0.08 kgwater/kgdry matter.

The batch-type dryer is one of the most popular equipment for fruit drying. However, the optimization of the air distribution inside the drying chamber remains a very important issue, due to its strong impact on efficient drying and uniformity of moisture content of products. A new scale laboratory batch-type, tray air dryer has been designed, constructed and evaluated for drying several horticultural and agricultural products. The flow field inside the dryer is studied through computational fluid dynamics (CFD). A three-dimensional model for laboratory dryer has been created and the steady state incompressible, Reynolds-Averaged Navier-Stokes (RANS) equations that formulate the flow problem are solved, incorporating standard and RNG k-ε turbulence models. The simulations are conducted for testing chamber average velocity of 2,9 m/s at ambient temperature. The CFD models are evaluated by comparing airflow patterns and velocity distributions to measured data. Numerical simulations an...

The current paper demonstrates how to introduce a combination of CFD (Computational Fluid Dynamics) and virtual instrumentation to undergraduates in a research setting in the framework of a fluid mechanics course. A laboratory research project has been developed to enable students to compare and analyze pressure and velocity measurements obtained experimentally in a "Venturi tube" setup to those generated using a commercial CFD software package. An old but proven laboratory setup has been retrofitted with computer data acquisition system and custom written software has been developed to suit the experimental module. Pressure measurements received along the tube for five Reynolds numbers are directly compared with corresponding predictions using CFD code and all are contrasted with the 1-d inviscid flow solution. It is shown that, although undergraduate students may have limited knowledge in the underlying theories and equations used by CFD codes to solve flow problems, the...

The improvement and optimization of air-distribution systems in drying kilns contributes to the preservation of the quality, safety and shelf life of perishable products. The present study reports on the numerical solution of airflow within a two dimensional drying kiln enclosure loaded with wooden pallets. The performance of air flow field is examined with and without supply of wooden pallets. Different arrangements of the supplied wooden pallets are investigated as well as the use of a screen in order to improve the airflow filed. The steady state RANS equations that formulate the flow problem are solved along with a turbulence model that is the standard k-� or the rng k-�. The effect of the turbulence modeling is distinguished through direct comparisons of the derived airflow patterns. The results obtained show the presence of stagnant zones inside the pallets columns and above of them and the presence of recirculation regions in different zones of the drying kiln with the two tu...

A laboratory scale convective dryer equipped with advanced instrumentation was developed in order to study the drying kinetics of quince slices. Experiments were carried out for air temperatures 40 o C, 50 o C and 60 o C and 2 ms -1 bulk velocity. The whole process occurred within the falling rate period. Results showed that a temperature increase from 40 to 60 o C produced a decrease of the total time of drying of about 54%. Eight thin-layer drying models were incorporated to fit the time evolution of the moisture data using non-linear regression analysis. Among the various models, the Weibull formula was best fitted to measurements. For the range of the drying temperatures examined, Fick's law of diffusion was used to determine the effective moisture diffusivity, which varied between 3.23 × 10 -10 m 2 s -1 and 7.82 × 10 -10 m 2 s -1 . Assuming an Arrhenius type model for the drying process, a value of 38.291 kJmol -1 was estimated for the activation energy.

Aiming to overcome the lack of experimental data in the open literature, a laboratory scale convective dryer (LSCD) was designed, constructed and equipped with an integrated measurement and automated control instrumentation. The LSCD is under operation for testing and monitoring the drying characteristics of various horticultural and agricultural products. The main sections of the LSCD, which can be arranged for operation either in a closed or open circuit mode through manually controlled dumpers, are the vertical flow drying chamber, the tube heat exchanger, the thermal boiler and finally the fan -motor with a smooth speed control unit. The current paper outlines the design aspects of the facility along with the measurement and control instrumentation and reviews some experimental studies of drying behaviour of quince slices under different conditions concerning the air velocity and temperature. The experimental results have been used for the validation of the Page thin layer dryin...

The objective of the present work is to study the drying kinetics of quince slices during convective drying. The details of the laboratory-scaled convective dryer are presented. Experiments were carried out at air temperatures 40 °C, 50 °C and 60 °C and 2 m s−1 bulk velocity. The whole process occurred within the falling rate period. Results showed that the temperature of the air stream has a significant effect on the drying curves. In particular, a temperature increase from 40 to 60 °C produced a decrease of the total time of drying of about 54%. Eight thin-layer drying models were used to fit the temporal distributions of the moisture data using non-linear regression analysis. Among the various models, the Weibull formula was best fitted to measurements. For the range of the drying temperatures examined, Fick's law of diffusion was used to determine the effective moisture diffusivity, which varied between 3.23 × 10−10 m2 s−1 and 7.82 × 10−10 m2 s−1. Assuming an Arrhenius type model for the drying process, a value of 38.291 kJ mol−1 was estimated for the activation energy.