Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.     

Mathematical modeling of drying characteristics of tropical fruits

In this study, a heat pump dryer was designed and produced in which drying air temperature was controlled with PID. PID controlled heat pump dryer was experimentally tested in drying tropical fruits such as kiwi, avocado and banana. Drying air temperature was kept at 40 °C with the accuracy of ±0.2 °C. Drying air velocity changed between 0.03 m/s and 0.39 m/s. Initial moisture contents of the kiwi, avocado and banana were 4.31, 1.51 and 4.71 g water/g dry matter, which were dried to 0.75, 0.35 and 0.5 g water/g dry matter moisture contents in 6 h. Afterwards the moisture ratios were analyzed with “STATGRAPHIC” computer program by using semi-theoretical models and empirical values. Correlation and standard error of estimation (SEE) and R² values were achieved.     

Structure and electrochemical hydrogen storage properties of A2B-type Ti–Zr–Ni alloys

Elemental substitution of part Ti by Zr has been carried out for Ti₂Ni alloy to form Ti_2−xZr_xNi (x = 0, 0.2, 0.4) alloys. Mechanical milling and subsequent heat treatment have been used to prepare non-equilibrium Ti–Zr–Ni alloys. The effects of Zr addition on the structure and discharge properties of Ti₂Ni alloy were investigated. The addition of Zr could enhance the discharge capacity of the non-equilibrium Ti₂Ni alloy at electrolyte temperatures of 313 and 333 K. For instance, the non-equlibrium Ti_1.6Zr_0.4Ni alloy had a stable discharge capacity of about 210 mAh/g at 313 K. However, the protective surface layer formed during heat treatment was destroyed at a high electrolyte temperature of 333 K, and thus a severe capacity loss during cycling.     

Evaluation of performance and energy consumption of an impinging stream dryer for paddy

Impinging stream dryer has proven to be an excellent alternative means for removing surface moisture of particulate materials. In this study, a coaxial two-impinging stream dryer prototype for paddy, whose surface moisture needs to be removed prior to subsequent processing, was developed and tested. The effects of various operating and geometric parameters, i.e., inlet air temperature, impinging distance, particle flow rate and particle feeding characteristics (single-point feeding vs. double-point feeding), on the overall performance (in terms of the volumetric water evaporation rate and volumetric heat transfer coefficient) and energy consumption of the dryer were then studied. It was found that the developed impinging stream dryer could reduce the moisture content of paddy by 3.4–7.7% (d.b.) within a very short period of time. The maximum value of the volumetric water evaporation rate was found to be about 198 kg_water/m³ h, while the maximum value of the volumetric heat transfer coefficient was about 7013 W/m³ K. The mean residence time of the particles (paddy) in the system was in the range of 1.81–2.42 s, leading to average drying rate in the range of 1.52–3.83 (% d.b.) s^?1, which is about 250 and 40 times higher than spouted-bed and fluidized-bed dryers, respectively. The lowest total specific energy consumption of the process was 5.1 MJ/kg_water when using double-point particle feeding at an inlet air temperature of 110 °C, an impinging distance of 5 cm and particle flow rate of 150 kg_{dry solid}/h.     

Modeling of a hazelnut dryer assisted heat pump by using artificial neural networks

The artificial neural network (ANN) approach is generic technique for mapping non-linear relationships between inputs and outputs without knowing the details of these relationships. In this paper, an application of the ANN has been presented for a PID controlled heat pump dryer. In PID controlled heat pump dryer, air velocity changed according to the temperature value which is set in process control device. Heat pump dryer was tested drying of hazelnut at 40 °C, 45 °C and 50 °C drying air temperatures. By training the experiment results with ANN, drying air velocities, moisture content of hazelnuts and total drying time were predicted for 42 °C, 44 °C, 46 °C and 48 °C drying air temperatures.     

Performances and improvement potential of solar drying system for palm oil fronds

In this study, an indirect forced convection solar drying system was tested for drying of palm oil fronds. The drying of 100 kg of palm oil fronds via solar drying system reduced the moisture content from 60% (w.b) to 10% (w.b) in 22 h (3 d of drying). During the drying process, the daily mean values of the drying chamber inlet temperature, drying chamber outlet temperature, drying chamber air temperature, and solar radiation ranged from 26 °C to 75 °C, 25 °C–65 °C, 26 °C–67 °C, and 96 W/m² to 1042 W/m² respectively, with corresponding average values of 53 °C, 46 °C, 48 °C, and 580 W/m². At average solar radiation of about 600 W/m² and air flow rate 0.13 kg/s, the collector, drying system and pick-up efficiencies were found about 31%, 19% and 67% respectively. The specific moisture extraction rate (SMER) was 0.29 kg/kWh. The exergy efficiency varied between 10% and 73%, with an average of 47%. In addition, the improvement potential of solar drying system for palm oil fronds ranged from 8 W to 455 W, with an average of 172 W.     

An analytical method for determining the temperature dependent moisture diffusivities of pumpkin seeds during drying process

This paper presents an analytical method, which determines the moisture diffusion coefficients for the natural and forced convection hot air drying of pumpkin seeds and their temperature dependence. In order to obtain scientific data, the pumpkin seed drying process was investigated under both natural and forced hot air convection regimes. This paper presents the experimental results in which the drying air was heated by solar energy.     

Thermal modeling of a natural convection greenhouse drying system for jaggery: An experimental validation

The aim of this work is to develop a thermal model so as to predict the jaggery temperature, the greenhouse air temperature and the moisture evaporated (jaggery mass during drying), during the drying of jaggery under natural convection conditions. The experiment was conducted separately for 0.75 kg and 2.0 kg of jaggery pieces having dimensions of 0.03 × 0.03 × 0.01 m³ for complete drying. The jaggery was dried in a roof-type even span greenhouse with floor area of 1.20 × 0.78 m². Experiment was carried out during February 5–8, 2004 at IIT Delhi (28°35′N 72°12′E) from 10 am to 5 pm. A computer program was developed in MATLAB software so as to calculate the jaggery temperature, the greenhouse air temperature and the moisture evaporated and was also used to predict the thermal performance of the greenhouse on the basis of solar intensity and ambient temperature. The software developed was experimentally validated. It was shown that the analytical and experimental results for jaggery drying are in good agreement.     

Grape drying by using hybrid photovoltaic-thermal (PV/T) greenhouse dryer: An experimental study

A hybrid photovoltaic-thermal (PV/T) greenhouse dryer of 100 kg capacity has been designed and constructed at Solar Energy Park, Indian Institute of Technology, New Delhi (28°35′N, 77°12′E, 216 m above MSL), India. The developed dryer has been used to dry the Thompson seedless grapes (Mutant: Sonaka) when DC fan was in operation for forced mode convection. The drying of grapes was also performed in open as well as shade for comparison. Experiments were conducted for drying of grapes in the month of April, 2007. Various hourly experimental data namely moisture evaporated, grape surface temperatures, ambient air temperature and humidity, greenhouse air temperature and humidity, etc. were recorded to evaluate heat and mass transfer for the proposed system. It has been found that the value of the convective heat transfer coefficient for grapes (GR-I) lies between 0.26 and 0.31 W/m² K for greenhouse and 0.34–0.40 W/m² K for open conditions, respectively and that for grapes (GR-II) lies between 0.45–1.21 W/m² K for greenhouse and 0.46–0.97 W/m² K for open conditions, respectively.     

Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer

In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 °C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 × 12.5 × 25 mm) and cylindrical shaped pumpkin slices (35 × 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m² C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature.     

Drying of sweet basil with solar air collectors

In this study, sweet basil was dried and its drying parameters were investigated experimentally and theoretically by using newly developed solar air collectors. Proper temperatures were chosen to dry sweet basil and experiments were carried out at different flow rates. At the end of drying experiments, it was determined that total mass of sweet basil decreased from 0.250 kg to 0.029 kg. In drying sweet basil, dimensionless moisture ratios were decreased rapidly to 300 min for 0.012 kg/s, 360 min for 0.026 kg/s, and 450 min for 0.033 kg/s. It was observed that the efficiency of collector was increased at the same rate with air flow changed between 29 and 63%. Among the models in the literature, Page Model was found to suit best for drying sweet basil. Furthermore, a novel mathematical model rendering more valid results for sweet basil and leafy products was developed.     

Experimental Studies for Convective Drying of Potato

An experimental facility was built at the Indian Institute of Technology Delhi in order to examine the characteristics of convective drying of a moist object. The test facility consists of an inlet section, a divergent and convergent section, a settling chamber, a test section, and an outlet section. Initial moisture content and time-dependent moisture content of a rectangular shaped moist object (4 cm × 2 cm × 2 cm) are measured by this test facility. The potato slice was selected as a sample moist object. Moisture content was measured at different air temperatures of 40, 50, 60, and 70°C with an air velocity of 2 m/sec. The density of potato slice was determined for various drying temperatures. The volume shrinkage during drying decreased almost linearly with moisture content. The percentage air pores and porosity increased gradually with decreasing moisture content and increasing drying air temperature. Volumes of water, air, and solid content of potato were determined at different drying air temperatures. The results are validated with theoretical data.     

Energy and exergy analysis of timber dryer assisted heat pump

In this research, poplar and pine timbers have been dried from the moisture contents of 1.28 kg water/kg dry matter and 0.60 kg water/kg dry matter to 0.15 kg water/kg dry matter in heat pump dryer functioning on the basis of 24 h operation. The change in weight in all of the timbers was followed in the drying chamber and drying stopped when the desired weight was achieved. At 40 °C dry bulb temperature, 0.8 m/s air velocity, and initial moisture content of the poplar timbers 1.28 kg water/kg dry matter, the moisture content was reduced to 0.15 kg water/kg dry matter moisture content in 70 h, and the moisture content of the pine timbers which was 0.60 kg water/kg dry matter was reduced to the same amount in 50 h. All data collected while drying were saved on computer and analysed afterwards. For this system, energy analysis was made to determine the energy utilization. Exergy analysis was accomplished to determine of exergy losses during the drying process.     

Investigation of the intermediate oxidation regime of Diesel fuel

A very high temperature fuel-air mixture is necessary for the thermal partial oxidation process of hydrocarbon fuels in order to have a high reaction temperature which accelerate the reaction kinetics. For Diesel fuel and due to the ignition delay time behavior, different oxidation behavior can be realized at different preheating temperatures. In this work, the intermediate oxidation region of Diesel fuel is investigated. By making use of the ignition delay time behavior, an vaporizer like tube reactor is constructed in order to enable a very high preheating temperature without the risk of self-ignition in a time-independent experiment. The oxidation behavior of Diesel fuel in air is investigated numerically and experimentally. In the numerical part, the ignition delay time was estimated using CHEMIKIN tools for different air-fuel mixtures at different temperatures. The evaporation behavior of the Diesel fuel-air mixtures are investigated at relatively high air preheating temperatures ranging from 500 °C up to 680 °C. The amount of the process air was varied from an air ratio λ = 0.35 to λ = 0.6. The experiments are also performed with N₂ as an evaporation media and compared with those performed with air to detect any temperature increase in the case of Diesel-air mixtures. The amount of heat release in the low chemistry region as well as in the intermediate region is calculated for the case of Diesel/air mixtures.The experiments show that four different oxidation region of Diesel fuel can be distinguished depending on air inlet temperatures and on the air ratio. At a temperature lower than 723 K (450 °C), no chemical reaction takes place. The cool flame reactions start at temperatures above 723 K (450 °C). However, no stable cool flame can be achieved unless the air preheating temperature reached about 753 K (480 °C). The cool flame region is extended up to about 873 K (600 °C), at which the intermediate regime started. This regime stabilized to a short range up to about 923 K (650 °C) after which ignition takes place.     

超声波预处理对污泥干燥速率影响研究

在超声波预处理污泥干燥实验基础上,分析了超声波预处理技术对污泥干燥速率的影响,并对污泥干燥过程进行了指数拟合,结果表明.超声波预处理能有效提高污泥干燥效率,其改善的程度取决于不同的实验条件;超声波预处理能够加速表面自由水分蒸发和快速结束污泥恒速干燥阶段;增大超声波功率可以更好地改善污泥的干燥特性,但改善的程度随着十燥时间的延长而放缓;超声波处理时间过长可能会增大污泥的黏度,反而不利于污泥脱水干燥.     

Comparison of different methods for the determination of moisture content in biomass

The purpose of this study was to compare different methods for the determination of moisture in biomass. Twenty different biomass materials from various places in Europe were investigated for total moisture using oven drying in air at three different temperatures (80, 105 and 130 °C), distillation with xylene, and freeze drying. In addition, the materials were used for the comparison of different methods for the determination of moisture in the analysis sample. In all cases CEN TC335 Solid Biofuels—Methods for determination of moisture content—Oven dry method was used as the reference method. Significant differences were obtained between the oven-drying methods in air at various temperatures. Similar results were found for some materials when comparing the reference method with xylene distillation and freeze-drying methods. In some cases the discrepancies were explained by the loss of volatile organic compounds during the oven-drying step at elevated temperatures. Small differences were found in the comparison of methods for the determination of moisture in the analysis sample.     

Effect of CuO/ZnO catalyst preparation condition on alcohol-assisted methanol synthesis from carbon dioxide and hydrogen

CuO/ZnO catalysts are synthesized using a co-precipitation method with different precipitation temperatures (298–353 K) and pH values (5–9). A conventional precipitation is compared to an ultrasonic-assisted precipitation at each precipitating temperature. Methanol is directly synthesized from CO₂ and H₂ (1:3 mol ratio) through an alcohol-assisted reaction (423 K, 5 MPa, 24 h) by using different alcohols (ethanol, propanol and butanol) as a medium. There are two parts for the challenge of this research, including the preparation of CuO/ZnO catalysts using an ultrasonic-assisted precipitation and, methanol synthesis through an alcohol-assisted method. It is found that the precipitation temperature and pH value significantly affect the catalyst properties and the reaction activity. An ultrasonic irradiation helps facilitate the crystalline phase formation and decrease precipitation temperature. The highest yield of methanol is obtained when CuO/ZnO is precipitated at 333 K from the conventional precipitation (31%) while it is at 313 K from the ultrasonic-assisted precipitation (32%). In addition, the different type of alcohol strongly affects methanol yield and CO₂ conversion. The use of larger alcohol molecules offers higher CO₂ conversion but lower methanol yield.     

A study on slow evaporation of liquids in a dual-porosity porous medium using square network model

Slow evaporation of a liquid is studied in a two-dimensional pore square network of aspect ratio 1 with three sides insulated and one side exposed to air for drying. In this study, the external transfer resistance and liquid-film effects are ignored while the capillary effects dominate viscous and gravity forces in the hydrophilic network. The square domain is divided into two layers with distinct porosities and particle sizes such that the two layers are exposed to drying alternately. A 100 × 100 network simulation of two cases of the exposed larger-pore layer shielding the smaller-pore layer, and the exposed smaller-pore layer shielding the larger-pore layer, lead to dramatically different responses in terms of the liquid evaporation plots and saturation distributions. The former case retains moisture in the inner smaller-pore layer till the entire outer larger-pore layer is dry, and is characterized by decaying liquid evaporation plots. The latter case leads to loss of moisture in both the exposed smaller-pore layer (due to evaporation) and the inner larger-pore layer (due to capillary pumping), and is characterized by bilinear evaporation plots (with an initial faster evaporation followed by a subsequent slower one). A case study that imposes uniform porosity in the two layers but keeps particle sizes different in the two layers indicate that though the pattern of saturation distribution during evaporation may remain similar to the earlier cases, but the evaporation plots are significantly different. An experimental validation of the simulation is undertaken with the help of a smaller 12 × 12 network where saturation patterns and evaporation plots are replicated well by the simulation. However the presence of surface liquid films created due to surface roughness as well as the capillary-suction driven liquid redistribution may be the cause of the large mismatch in the drying time of the network.     

Convective drying of an individual iron ore pellet – Analysis with CFD

The objective of this paper is to model convective drying of an individual iron ore pellet placed in a free stream of air with the aim to clarify the different stages of drying. A numerical model taking into account capillary flow of liquid moisture and internal vapor flow is developed and implemented in a commercial available software for Computational Fluid Dynamics where also the flow around the pellet is simulated, yielding heat- and mass transfer coefficients as a function of position. A real pellet is optically scanned for its geometry and simulations of the drying are compared to experiments with very good agreement. The result clearly shows four stages of drying; (i) evaporation of liquid moisture at the pellet surface, (ii) surface evaporation coexisting with internal drying as the surface is locally dry, (iii) internal evaporation with completely dry surface and (iv) internal evaporation at boiling temperatures. A moisture front moving towards the core of the pellet will start to develop at the second drying stage and the results show that the front will have a non-symmetrical form arising from the surrounding fluid flow.     

Model for predicting evaporation characteristics of vegetable oils droplets based on their fatty acid composition

In this work, a model for predicting evaporation characteristics (constant of evaporation and evaporation time) of cottonseed oil and diesel fuel has been developed and validated experimentally in the temperature range of 684–917 K under atmospheric pressure.The experimental study is based on the fibre-suspended droplet evaporation technique. The theoretical model for predicting evaporation characteristics is based on the determination of transport properties and thermodynamic properties of different phases of cottonseed oil using the properties of its predominant fatty acids (linoleic, oleic and palmitic). Results show that taking into account convection in the quasi-steady model by the correlation of Ranz and Marshall is enough to give a good prediction of the constant of evaporation of diesel fuel in the studied temperature range. For cottonseed oil, the quasi-steady model gives a good prediction for temperatures from 684 K to 773 K while for temperatures from 773 K to 917 K, it is necessary to take into account the convection and the influence of the heating period of the droplet for a good prediction of the constant of evaporation. For the duration of heating and evaporation time, the model gives a rather good prediction for cottonseed oil for the temperature range from 840 K to 917 K.