Thomas Bartzanas

Temperature, comfort and pollution levels during heat waves and the role of sea breeze

During the summer of 2007 several Greek regions suffered periods of extreme heat, with midday temperatures of over 40°C on several consecutive days. High temperatures were also recorded on the east coast of central Greece, where a complex sea breeze circulation system frequently develops. The more intense events occurred at the end of June and July. The highest temperatures were observed on 26 June and 25 July, while the sea breeze developed only on 25 July. Meteorological data collected at two sites—a coastal urban location and an inland suburban site that is not reached by the sea breeze flow—as well as pollution data collected at the urban site, were analysed in order to investigate the relationship between sea breeze development and the prevailing environmental conditions during these two heat wave events. The analysis revealed that sea breeze development affects temperature and pollution levels at the shoreline significantly, causing a decrease of ~4°C from the maximum temperature value and an increase of ~30% in peak PM10 levels. Additionally, several stress indices were calculated in order to assess heat comfort conditions at the two sites. It was found that nocturnal comfort levels are determined mainly by the urban heat island effect, the intensity of which reaches up to 8°C, while the applied indices do not demonstrate any significant daytime thermal stress relief due to sea breeze development.

INFLUENCE OF AN INSECT SCREEN ON GREENHOUSE VENTILATION

The influence of an insect screen on ventilation rate was experimentally investigated in a multispan glass–covered greenhouse equipped with a continuous roof vent, located at the University of Thessaly near Volos in the coastal area of eastern Greece. Microclimate variables as well as the ventilation rate were measured during summer. Two measuring techniques were used for the determination of ventilation rate: (1) the decay rate tracer gas technique, using N2O as tracer gas, and (2) the water vapor balance technique. The influence of the insect screen on ventilation rate was studied using a wind–related coefficient identified by fitting a simple linear model to the experimental values. The two measuring techniques gave similar results, but the water vapor balance technique provided a better fit to the experimental data. The wind–related coefficient significantly decreased when an insect screen covered the vent. Finally, the influence of the insect screen on the discharge coefficient was investigated. The discharge coefficient was correlated to the aerodynamic properties of the screen using porous media flow analysis.

Numerical prediction of fugitive dust dispersion on reclaimed land in Korea.

The Saemangeum reclaimed land located on the western coast of Korea is one of the worlds largest reclamation projects, developing 40,100 ha of land area. After construction of sea dikes in 2006, the exposed land area has been increasing in relation to the water level height. Dust from the exposed land, containing high amounts of salt, disperses to nearby areas. The dust is harmful to human and animal health as well as to plant growth. Therefore, an estimation of fugitive dust dispersion is necessary for the development of plans to manage the problem. Field experiments are ideal for understanding aerodynamic phenomena. However, finding a correlation between weather conditions and dust dispersion is a difficult task due to limited measuring points. Moreover, weather conditions are unstable, unpredictable, and cannot be artificially controlled. Field experiments also involve high labor and time expenses. To overcome these limitations, a computational fluid dynamics (CFD) model has been developed to analyze dust dispersion phenomena, both quantitatively and qualitatively, according to the topography of the area, under various weather conditions. Effort was devoted to improving the accuracy of the CFD model by taking into account topographical design, mesh structure, turbulence models, particle generation, and other factors influencing the final solution. Computed results of the 3-D developed CFD model were compared against experimental data. Results showed an average error of -6.8%, which is within the acceptable range. CFD-computed vertical log-profiles of dust dispersion were similar to the vertical profiles presented by an earlier study. CFD results showed that dispersion of fugitive dust was mainly affected by particle size, wind speed, wind direction, and topography of the area. The estimated dispersion distance, measured at a height of 3 m for the 10 µm particles and a wind speed of 1.7 m s-1, was 3100 m. A dispersion distance of 6300 m was obtained when wind speed was 3.9 m s-1 for the 10 µm particles. This study showed that a CFD model can be effectively used to supplement field experiments when analyzing dispersion of fugitive dust.

THE IMPACT OF INSECT SCREENS AND VENTILATION OPENINGS ON THE GREENHOUSE MICROCLIMATE

The objective of this work was to experimentally investigate the influence of vent type (side, roof, or both) and of anti-aphid insect screens on airflow, air temperature, and air vapor pressure deficit distribution in a round arch, mono-span greenhouse with vertical side walls. The greenhouse was equipped with two side roll-up vents and a flap roof vent. A tomato crop planted in double rows was cultivated inside the greenhouse. The three components of air velocity were measured by a 3-D sonic anemometer, and the air temperature and relative humidity were simultaneously recorded at several positions inside the greenhouse. Concerning the effect of insect screens, it was found that the mean value of the normalized air velocity was 58% lower in the greenhouse with insect screens on the side vent openings than in the case of a greenhouse without screens. Furthermore, the spatial heterogeneity of the microclimate variables was reduced with screens in the vent openings. When the ventilation was provided by side openings only, the air velocity inside the greenhouse was characterized by a strong air current near the greenhouse ground and low air velocity near the roof; when the ventilation was provided by roof vents, a circulating current prevailing at the center of the greenhouse was observed. The combined use of roof and side openings increased air velocity and decreased air temperature inside the greenhouse but also increased the spatial heterogeneity of the greenhouse microclimate compared to the cases with side or roof vents only. The most homogeneous climate conditions were achieved with the use of roof openings only. The results of this study provide a better understanding of the plant environment behavior under different vent configurations and a high-resolution database for validating on-going efforts with computer simulations.

Influence of heating system on greenhouse microclimate distribution.

Convective and radiative heat transfer from heating systems significantly determines greenhouse microclimate during the cold period of the year. These mechanisms are complicated because they combine free and forced convection modes, most often turbulent with different characteristics. The aim of the present study is to analyze the internal convective flows in a closed greenhouse caused by buoyancy forces from different configurations of heating systems. Numerical results obtained by the use of a commercial computational fluid dynamics code (ANSYS CFX) are compared to experimental measurements carried out in a full-scale experimental greenhouse with a tomato crop. The greenhouse was heated with a network of heating pipes and/or with an air heater. The standard k-e turbulence model was adopted to describe the turbulent nature of the flow and transported properties. The resistance of the crop to airflow and the heat and mass exchanges of the crop with the surrounding air were simulated using the equivalent porous medium approach. In general, good agreement was found, since the mean error between measured and simulated values for air velocity, air temperature, and air absolute humidity distribution was 16%. The combined use of heating pipes and air heater enhanced plant activity and reduced the condensation rate. This heating method led to an increase in energy consumption of up to 19%, but it also created a more heterogeneous climate distribution compared to the case in which only heating pipes were used. It was shown that the greenhouse air volume is split into two regions: one occupied by the crop where natural convection dominated, and one above the crop where the hot air from the air heater resulted in a different microclimate from the lower part of the greenhouse (crop level), and the convection mode changed to mixed or forced depending on the distance from the air heater.

A diagnostic system for improving biomass quality based on a sensor network.

Losses during storage of biomass are the main parameter that defines the profitability of using preserved biomass as feed for animal husbandry. In order to minimize storage losses, potential changes in specific physicochemical properties must be identified to subsequently act as indicators of silage decomposition and form the basis for preventive measures. This study presents a framework for a diagnostic system capable of detecting potential changes in specific physicochemical properties, i.e., temperature and the oxygen content, during the biomass storage process. The diagnostic system comprises a monitoring tool based on a wireless sensors network and a prediction tool based on a validated computation fluid dynamics model. It is shown that the system can provide the manager (end-user) with continuously updated information about specific biomass quality parameters. The system encompasses graphical visualization of the information to the end-user as a first step and, as a second step, the system identifies alerts depicting real differences between actual and predicted values of the monitored properties. The perspective is that this diagnostic system will provide managers with a solid basis for necessary preventive measures.

Numerical study of mechanically ventilated broiler house equipped with evaporative pads

Abstract One of the important factors to improve the broiler production is the provision of an optimum indoor environment (air quality, temperature, humidity, air velocity, gases and PMs concentration) with lower possible cost. The internal microclimate can be controlled either passively by selecting appropriate construction geometry and materials or actively by the ventilation systems and the electromechanical (E/M) equipment. In the case of broiler chamber the conditions that constitute optimum internal microclimate vary with respect the birds’ age. In the present work, the ventilation, inside a modern and fully automated broiler chamber equipped with fans and evaporative pads located in Central Greece, is simulated using Computational Fluid Dynamics (CFD) techniques. The transport phenomena inside the broiler house are described with Reynolds Averaged Navier Stokes (RANS) equations solved with the Finite Volume Method (FVM). The flow is assumed 3D, steady state and turbulent. The fans of the broiler chamber abduct air from the interior, forming inside negative pressure distribution, and are modeled as exhaust fans. The air enters the broiler house through evaporative pads which are simulated as porous media and as heat sinks, concurrently. The heat sink term is yielded analytically according to the external climatic conditions and the evaporative pads specifications. The litter and the animals are considered also as porous materials and sources of heat. The birds’ thermal properties and their heat emissions are computed according to their age, the measured birds’ volume, and height and meat composition. The developed CFD model is validated against measurements of temperature (16 points) and air velocity (6 points). According to the simulation results, it is drawn that the vertical temperature gradient should be taken into account when the operational sensors for the cooling devices are positioned inside the chamber since there is a deviation higher than 2 °C between the air content above and among the birds. Also various combinations of the available five fans, operating in two possible modes of the examined poultry chamber are studied in order to assess their effect to the internal microclimate. The operation of two or three central fans are proven to be the optimum choice in terms of temperature, ventilation and air velocity. The operation of only one fan fails to preserve the required temperature, while the operation of more than three fans does not improve the ventilation rates.

Preliminary findings on seasonal heat stress of sheep.

This paper studies the heat stress conditions imposed onto sheep housed in a naturally ventilated sheep barn during the four seasons of a year. The barn is located near the east coast of central Greece. Heat stress is assessed by means of the hourly temperature-humidity index (THI). The analysis shows that animals were exposed to heat stress in 11%, 80% and 17% of spring, summer and autumn hours, respectively, whereas no heat stress was identified during winter. As expected, animals are more prone to severe heat stress during summer, as the daily maximum hourly values of THI remained higher than the extreme severe heat stress threshold (i.e. THI = 25.6) in 90% of the summer days. Additionally, THI remained higher than 25.6 during the vast majority of the summer daytime hours. Measures have to be taken so as to improve the indoor climate conditions and to alleviate animal’s heat stress.

Moisture content evaluation of biomass using CFD approach

In grass conservation systems, drying in the field is an essential process upon which the quality and quantity of the material to be conserved is dependent on. In this study a Computational Fluid Dynamics (CFD) model, previously validated, was used to assess qualitatively and quantitatively the field drying process of cut grass under different weather conditions and structural specifications of the grass. The use of the CFD model depicts the climate heterogeneity in the grass area with a special focus on moisture distribution, influence of the weather conditions, in order to create the possibility of applying the model as a decision support tool for an enhanced treatment of the grass after cutting.

Yearly numerical evaluation of greenhouse cover materials

Abstract The greenhouse internal microclimate, and consequently the plants growth, is formed from the combination of available solar radiation and ventilation. Transpiration and photosynthesis are affected by the PAR levels, as well as by the temperature, the wind velocity, and the air humidity etc. factors that vary along a day and along a year. The appropriateness of cover materials in a greenhouse depends on the crop as well as on the season of the year. Cover materials with high transmissivity in Photosynthetically active radiation (PAR) spectrum are desired during the winter but require significant ventilation during summer. Conversely covers with low transmissivity reduce the cooling requirements during summer but may prevent appropriate PAR to reach the plants during the winter. Consequently the evaluation of cover materials performance should be done along the whole year. In the present work the performance of four cover materials namely: Three-layer co-extruded film (3L), Ethylene vinyl acetate film (EVA), Thermal polyethylene film (TPE) and Rose Polyvinylchloride-based fluorescent (VPVC) were examined in terms of available PAR, temperature and air velocity in the plants’ level, for a tunnel type tomato greenhouse with side openings sited in the Central Greece. In order to evaluate the importance of the above mentioned parameters, the Ansys Fluent Computational Fluid Dynamic (CFD) code was used for a 2D simulation of transport phenomena inside the greenhouse like, the aerodynamics and the heat and radiation transfer in four wave length bands (Ultra Violet (UV), PAR, Near Infrared Radiation (NIR) and Infrared Radiation (IR)), using the method of finite volumes. The solar radiation was modeled with the Discrete Ordinates (DO) model and the daily variation of external solar radiation was introduced via User Defined Functions (UDFs) written in source code form. Plants were approached as porous materials. Unsteady simulations concerning eight characteristic days of the year (equinoxes, solstices and four intermediate days) were performed for 4 examined materials using climatic parameters regarding the area of Larissa, located in Central Greece. During the winter days the side openings are considered to be only 20% open with respect to the summer opening of them. Cover materials were numerically evaluated in terms of: (a) the available PAR distribution at crop level, (b) air velocity fields, (c) flow patterns’ discrepancies, and (d) the developed temperature vertical gradient distribution, during day time operation. From the simulations was concluded that the best material for the specific crop was the EVA.