Enio Pedone Bandarra Filho

Entropy Generation in Thermal Radiative Loading of Structures with Distinct Heaters

Thermal loading by radiant heaters is used in building heating and hot structure design applications. In this research, characteristics of the thermal radiative heating of an enclosure by a distinct heater are investigated from the second law of thermodynamics point of view. The governing equations of conservation of mass, momentum, and energy (fluid and solid) are solved by the finite volume method and the semi-implicit method for pressure linked equations (SIMPLE) algorithm. Radiant heaters are modeled by constant heat flux elements, and the lower wall is held at a constant temperature while the other boundaries are adiabatic. The thermal conductivity and viscosity of the fluid are temperature-dependent, which leads to complex partial differential equations with nonlinear coefficients. The parameter study is done based on the amount of thermal load (presented by heating number) as well as geometrical configuration parameters, such as the aspect ratio of the enclosure and the radiant heater number. The results present the effect of thermal and geometrical parameters on entropy generation and the distribution field. Furthermore, the effect of thermal radiative heating on both of the components of entropy generation (viscous dissipation and heat dissipation) is investigated.

Application of adaptive control in a refrigeration system to improve performance

This paper reports an experimental investigation using adaptive control in a refrigeration system. This system is composed basically of a semi-hermetic compressor, concentric tubes heat exchangers, the condenser and evaporator, and thermostatic expansion valve (TXV). The refrigerant used in the refrigeration system was the HCFC-22 and also an AC frequency inverter was used to control the speed of the compressor. The temperatures were measured by PT-100 sensors and the pressures using piezoresistive pressure transducers. Data acquisition was implemented using the Labview software. An electronic card with analogical signal output was also used. These analogical signals were converted into digital through a Programmable Logic Controller (PLC) and then sent to the computer through a PC serial port. Tests were performed with a variable speed compressor in the range from 30 to 70 Hz and the experimental results showed that the highest performance (COP) was attained by working in the range of 50 Hz, using adaptive fuzzy control.

Nucleate Boiling in Large Arrays of Impinging Water Sprays

The nucleate boiling of subcooled water, under 100 cm2 square arrays of impinging sprays, was experimentally investigated. Three types of commercially available full cone pressure nozzles, of distinct flow capacities, allowed for runs where the average impinging coolant mass flux spanned the 0.3–7.2 kg/m2-s range. Array geometry was varied adjusting nozzle-to-nozzle and nozzle-to-impingement surface distances. Experimental construction allowed for good drainage of spent coolant and unrestricted air entrainment to spray cones. The average heat flux through the heated, upward-facing, copper impingement surface was found to be equal to the sum of single-phase and nucleate boiling heat flux components. The phase-change component was experimentally observed to depend upon wall excess temperature only. The proposed heat transfer correlation reproduces original experimental data with a mean absolute error of 10.6%. Non-critical-heat-flux (non-CHF) cooling capacity and efficiencies of up to, respectively, 2000 kJ/kg and 83% were observed.

Heat Transfer Coefficient Correlation for Convective Boiling Inside Plain and Microfin Tubes Using Genetic Algorithms

Two-phase flow heat transfer has been exhaustively studied in recent years. However, in this field, several questions remain unanswered. Heat transfer coefficient prediction related to nucleate and convective boiling has been studied using different approaches—numerical, analytical, and experimental. In this study, an experimental analysis, data representation, and heat transfer coefficient prediction of two-phase heat transfer in nucleate and convective boiling are presented. An empirical correlation is obtained, based on a genetic algorithms search engine, of a dimensional analysis of the two-phase flow heat transfer problem.

Analysis of the Drop-in Operation of a Refrigeration System by the Response Surface Methodology

The objective of this study is to evaluate the gains in a drop-in process of refrigerants in a basic refrigeration system using the Response Surface Methodology to optimization. A central composite design and analysis of variance are also used. In addition, the model will be analyzed and compared to experimental results, consolidating various parameters to get the best regions in a broad universe of options, with a reduced number of experiments. The response surface methodology is a set of statistical tool, which describes the entire region of interest through experimental specific points, by appropriate polynomial equation. The experimental results were extracted from a cooling test bench compound for a reciprocating compressor, evaporator, condenser and expansion valve. R22, R290, R1270, R438A, R404A, R134a, R410A and R32 were tested at the same experimental apparatus, the lubrificating oil was replaced in some cases. The higher value of coefficient of performance and cooling capacity were investigated. The results obtained with the MSR technique showed good agreement with the experimental work, (R2 exceeding 0.8) which indicates, good representation of the output variables of the cooling system. The tested optimization tools were able to identify areas of maximum, allowing for better comparison of results obtained by different refrigerants.

PHOTOTHERMAL CONVERSION USING GRAPHENE/WATER NANOFLUIDS

The dispersion of particles in the nanometer range 1 to 100 nm in a liquid medium, known as nanofluids, has been shown by several authors as an optimization alternative for conventional solar absorption systems and heat transfer systems due to its unique properties. The present study experimentally investigated the behavior of Graphene/Water nanofluids under realistic conditions of photothermal conversion using direct absorption solar collector (DASC). Nanofluids at low volumetric concentrations – 1 to 300ppm were obtained by dilution and sonication process. An apparatus was built for the simultaneous exposure of different concentrations of nanofluids and the base fluid to solar radiation without any solar concentrating device. The results showed that nanoparticles have excellent photo-thermal conversion capacity, even at low concentrations, however, the Specific Absorption Rate (SAR) decreased with nanoparticle’s addition to the base fluid. After ten hours of testing, the samples showed agglomeration and precipitation problems, a challenge for future applications.