Cristiana Brasil Maia

Analysis of the Airflow in a Prototype of a Solar Chimney Dryer

The development of alternative energy sources is increasingly becoming a necessity. In this context, solar energy stands out because it is a renewable and low pollutant source of energy. The solar chimney uses solar energy to generate hot airflow that can be used to dry agricultural products or generate electric power. This paper presents an experimental study of the airflow inside a solar chimney. The tests were performed in a prototype built on the campus of Universidade Federal de Minas Gerais, with appropriate dimensions to dry agricultural products. The values of temperature, velocity, solar radiation, and humidity inside and outside the device were monitored over a nine month period, allowing the characterization of the airflow and the determination of the optical properties of the materials used.

Fundamental Studies of Adhesion of Dust to PV Module Surfaces: Chemical and Physical Relationships at the Microscale

Photovoltaic (PV) module soiling is a growing area of concern for performance and reliability. This paper provides evaluations of the fundamental interactions of dust/soiling particles with several PV module surfaces. The purpose is to investigate the basic mechanisms involving the chemistry, morphology, and resulting particle adhesion to the first photon-incident surface. The evaluation and mapping of the chemistry and composition of single dust particles collected from operating PV module surfaces are presented. The first correlated direct measurements of the adhesive force of individual grains from field-operating collectors on identical PV module glass are reported, including correlations with specific compositions. Special microscale atomic force microscopy techniques are adapted to determine the force between the particle and the module glass surface. Results are presented for samples under dry and moisture-exposed conditions, confirming the effects of cementation for surfaces having soluble mineral and/or organic concentrations. Additionally, the effects of hydrocarbon fuels on the enhanced bonding of soiling particles to surfaces are determined for samples from urban and highly trafficked regions. Comparisons between glass and dust-mitigating superhydrophobic and superhydrophilic coatings are presented. Potential limitations of this proximal probe technique are discussed in terms of results and initial proof-of-concept experiments.

Fundamental studies of the adhesion of dust to PV module chemical and physical relationships at the microscale

PV module soiling is a growing area of concern for performance and reliability. This paper provides evaluations of the fundamental interactions of dust/soiling particles with a several PV module surfaces. The purpose is to investigate the basic mechanisms involving the chemistry, morphology and resulting particle adhesion to that first photon-incident surface The first-time evaluation and mapping of the chemistry of single dust particles from operating PV module surfaces is presented. The first direct measurements of the adhesive force of individual grains are reported, including correlations to the specific surface chemistry. Special nanoscale techniques using atomic force microscopy (AFM) are adapted to determine the force between the particle and the surface. Results are presented for samples under dry and moisture-exposed conditions confirming the effects of cementation for surfaces having organic/soluble mineral concentrations. Additionally, the effects of hydrocarbon fuels on the enhanced bonding and adhesive force of soiling particles to surfaces are determined for samples from urban and highly-trafficked regions. Comparisons between glass and superhydrophobic and superhydrophilic coatings are presented, showing the effectiveness of the lower-surface energy conditions on the particle adhesion. The potential, limitations of this novel proximal probe technique are discussed in terms of the results and initial, proof-of-concept experiments.

Soiling particle interactions on PV modules: Surface and inter-particle adhesion and chemistry effects

The understanding of the fundamental physics and chemistry of dust and the interaction of these soiling fragments with the PV module surface and each other is potentially important to developing viable mitigation approaches. This paper builds on our previous reports and observations investigating individual soiling particle adhesion on PV module glass using microscale proximal probe techniques. Specifically, in this presentation we report on the refinement of those adhesive force measurements by gaining and including information on the contact area of those particles with the surface, the specific chemistry of interactive surfaces, and the quantification of the force values using materials standard. We also investigate the adhesive forces holding the soiling particles together and the effects of the critical parameters of surface compositional properties, moisture (humidity), and hydrocarbons. This allows for the comparisons of the inter-particle adhesion to the adhesive force holding the particle to the glass module surface. These evaluations are performed on soiling particles collected from operating modules in differing climate zones in Brazil and Middle-East Gulf regions.

Assessment of the Fluid Dynamics Aspects of a Vehicle Ventilation System

Abstract Vehicular air conditioning systems are directly affected by the ducts and fans of the ventilation system. This paper presents an experimental study of the influence of the geometry of the air ducts and fans on the performance of an automotive air conditioning system. The flow rate, pressure drops and power requirement demand were evaluated for different geometrical and operational conditions. A test bench was used that constituted the air-box of an automotive air conditioning system, composed of the air ducts, the centrifugal blower, and an instrumental panel. Two different geometrical fans and ducts were tested. The experimental results showed a great potential for performance improvement. Significant gains were achieved for the pressure drop, flow rate and air distribution at the branch ducts with the modifications implemented. The experimental results also showed that the power consumption slightly increased with the considered modifications.

A Model to Estimate Ambient Conditions and Behavior of the Airflow Inside a Solar Chimney

This chapter presents models to estimate the environmental conditions and the behavior of the airflow within a prototype of a small-scale solar chimney located in Belo Horizonte, Brazil. A correlation from the literature for diffuse radiation, based on clearness index and global radiation, was evaluated. A model from the literature was used to estimate the ambient temperature of the device. The results of both parameters were compared with the experimental data. An energy balance was applied to find the heat interactions between the ground, airflow, coverage, and environment, based on the estimated incident solar radiation and ambient temperature. Literature correlations were used to estimate the convective heat transfer coefficients. Consolidated correlations were then applied to estimate the mass airflow rate and outlet temperature of the airflow inside the prototype. The results were compared with the experimental data for 4 days in autumn and good agreements were found. The model was then used to estimate the airflow parameters for 1 year. The analysis performed was transient, with results provided for each hour of the day, for 365 days. These results showed good accordance with the experimental data. The greater differences were found in the mass flow rate at night.

A Mathematical Model for the Exhaust Gas Temperature Profile of a Diesel Engine

This work presents a heat transfer model for the exhaust gas of a diesel power generator to determine the gas temperature profile in the exhaust pipe. The numerical methodology to solve the mathematical model was developed using a finite difference method approach for energy equation resolution and determination of temperature profiles considering turbulent fluid flow and variable fluid properties. The simulation was carried out for engine operation under loads from 0 kW to 40 kW. The model was compared with results obtained using the multidimensional Ansys CFX software, which was applied to solve the governor equations of turbulent fluid flow. The results for the temperature profiles in the exhaust pipe show a good proximity between the mathematical model developed and the multidimensional software.

Determination of the Diameter of a Cylinder to Insertion in a Low-Speed Wind Tunnel Based on EFD and CFD

The objective of this study was to determine the ideal diameter of a circular cylinder for insertion into the test section of a small wind tunnel, so that there was no flow blockage and influence of the developing wind tunnel boundary layer in the flow around the cylinder. The approaches used were the experimental fluid dynamics (EFD) by the use of Pitot tube and hot wire anemometer (HWA) and the computational fluid dynamics (CFD) simulations, performed with ANSYS CFX software. For this, it was first determined the boundary layer thickness along the test section without the cylinder, through computer simulations. Based on this information, several simulations were performed with cylinders of different diameters to quantify their influence on the rotational flow. From the four studied cylinder diameters, it was found that only the one with 30 mm did not cause bending of the streamlines at the intersection with the boundary layer developing in the tunnel. Moreover, this was the minimum diameter of the cylinder so that the tests with Pitot probe could be used without disturbances caused by its dimensions in the flow to be characterized. After analyzing these results, the cylinder of 30 mm was built and the experimental tests were performed, validating the numerical results.

Investigation of the Predictive Ability of Two Advection Schemes on the Formation of a Turbulent Separation Bubble in a Boundary Layer Wind Tunnel

This paper presents a study that correlates the capacity of two advection schemes in foreseeing flow separation inside a boundary layer wind tunnel (BLWT herein after). The geometry of the BLWT forces the generation of a turbulent separation bubble. Numerical simulations were carried out with the commercial Computational Fluid Dynamics software ANSYS-CFX®. The high-resolution advection scheme is shown to be more appropriate than the upwind scheme in predicting flows where properties are subject to strong gradients, such as pressure and velocity.

Analysis of the Impact of the New Emissions Limits on the Temperatures of the Vehicle Floor

The Kyoto Protocol established the reduction of pollutant emissions limits for all sectors of industrial economy in 8%, compared to 1990´s levels, to be adopted in the period between 2008 and 2012. Individual countries defined a progressive scale for the emission reduction applied to automotive vehicles. These new emission limits are reached altering the calibration of the Electronic Central Unit (ECU), altering the volume and the composition of the catalytic converters and also adding new components to the engine, such as EGR (exhaust gas recirculation) system and phasing sensor. This work evaluates the impact of these modifications in the exhaust system temperatures and in the peripherical devices. In order to meet the requirements of the new emissions limits, the volume of the catalytic converter is higher, increasing the heat rejected. It provokes a temperature raise on the exhaust system and under the vehicle pavement, which impact the functionality of some components and also the passengers thermal comfort. It is observed that the new emission standards in Brazil resulted in an increase of the vehicle temperatures, affecting the passengers’ thermal comfort, and eventually producing more emissions due to the use of an air conditioning system.