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This work presents a methodology for the numerical simulation of reed valves of a small-size reciprocating compressor (approximately 200 mm in length, 60 mm in diameter), oil free and operating at high frequency. A fluidstructure interaction methodology is employed, where the fluid domain is modeled by the tridimensional transient Navier-Stokes equations, while the solid domain is governed by a beam model considering geometrical contacts between valves and stoppers (valve plate, piston, etc.) Results are shown for mass flow, pump power consumption and valve losses for evaporating temperatures ranging from -15 degrees Celsius to +40 degrees Celsius, and condensing temperatures from +45 degrees Celsius to +85 degrees Celsius. Finally, numerical results were compared to experimental data.

In this paper, an algebraic formulation which expresses the dimensionless rate of entropy generation as a function of the heat exchanger geometry (number of transfer units), the thermal-hydraulic characteristics (friction factor and Colburn j-factor), and the operating conditions (heat transfer duty, core velocity, coil surface temperature, and fluid properties) is derived. It is shown that for heat exchanger with constant wall temperature (i.e., condensers and evaporators), there does exist a particular number of transfer units which minimizes the dimensionless rate of entropy generation. An algebraic expression for the optimum heat exchanger effectiveness, based on the working conditions, heat exchanger geometry and fluid properties, is also presented. The theoretical analysis led to the conclusion that a high effectiveness heat exchanger does not necessarily provide the best thermal-hydraulic design.

Piston dynamics plays a fundamental role in two critical processes related to fluid flow in reciprocating compressors. The first is the refrigerant leakage through the radial clearance, which may cause considerable loss in the pumping efficiency of the compressor. The second process is the viscous friction associated with the lubricant film in the radial clearance; certainly a significant factor in the compressor energy consumption. In the present contribution a numerical simulation of the piston movement inside the cylinder of a reciprocating compressor is performed. The compressor considered here is a small hermetic compressor employed in domestic refrigerators. For the problem formulation both the axial and the radial piston motion is considered. In operation, the piston moves up and down along the axis of the cylinder, but the radial oscillatory motion in the cylinder bore, despite being usually small, plays a very important role on the compressor performance and reliability. The compromise between sealing of the gas leakage through the piston-cylinder clearance and the friction losses requires a detailed analysis of the oscillatory motion for a good design. The forces acting on the piston are the hydrodynamic force due to the pressure build up in the oil film (lubrication effects), the force due to the connecting rod, the viscous force associated with the relative motion between the piston and oil, and the force exerted by the gas on the top of the piston. All corresponding moments are also included in the problem formulation of the piston dynamics, in order to determine the piston trajectory, velocity and acceleration at each time step. The hydrodynamic force is obtained from the integration of the pressure distribution on the piston skirt, which, in turn, is determined from a finite volume solution of the time dependent equation that governs the oil flow. A Newton-Raphson procedure was employed in solving the equations of the piston dynamics. The results explored the effects of some design parameters and operating conditions on the stability of the piston, its sealing performance and friction losses. Emphasis was placed on investigating the influence of the pin location, radial clearance and oil viscosity on the piston dynamics. The complexity of the piston movement in reciprocating compressors was demonstrated and the detailed model presented can be employed as an useful tool for engineering design.

Heat and mass transfer processes are of paramount importance when designing some important compressor components, specially the ones that perform flow control, noise suppression and the compression process itself. In general, pressure losses, flow pulsation and heat exchanges must be minimized to achieve high efficiencies. On the other hand, the optimization of those parameters are subject to a compromise among noise generation, component manufacturing costs and reliability, hence the bigger is the amount of (correct) information available, the higher is the probability of a successful project. In this scenario, the use of Computational Fluid Dynamics (CFD) plays an important role in the development phase, since it allows the analysis of more options, faster and at lower expenses comparing to the “prototype-and-test” approach. It also spreads possibilities of alternatives prone to be considered in a technical trade-off and anticipates problems and design constrains prior to prototyping. This paper focus on case studies and designing problems solved by the authors using different CFD based approaches. The problems are described in terms of the approach used in the solution, boundary conditions, models adopted and results obtained.

Dissertação (mestrado) - Universidade Federal de Santa Catarina. Centro Tecnologico O propósito deste trabalho é estudar os efeitos da condução de calor bi-dimensional em tubos de calor de condutância constante e as conseqüências da modificação das condições impostas pelo meio exterior sobre o desempenho destes, em regime permanente. Para tanto, uma solução do problema utilizando um método analítico inverso é apresentada. Em decorrência da constatação da preponderância do processo de condução de calor, a equação de Laplace é resolvida simultaneamente no invólucro e na estrutura capilar. Com a imposição das hipóteses simplificativas cabíveis, resolve-se um problema de Neumann conjugado a um problema misto. As equações da temperatura são obtidas com a utilização da segunda identidade de Green. Faz-se uso de funções de Green modificadas para o problema de Neumann, expandidas parcialmente e auto-funções. São apresentados os métodos de construção de tais funções e os resultados para três condições de contorno diferentes, impostas na parede externa do tubo. A boa concordância verificada na comparação dos resultados com dados experimentais obtidos de trabalhos anteriores valida a metodologia adotada. Com base nestes resultados é levada a efeito uma análise da influência dos parâmetros que regulam à transferência de calor em tubos de calor sobre o comportamento térmico. A discrepância observada nas comparações com modelos existentes é explicada em função da relevância dos efeitos da condução de calor bi-dimensional em tubos de calor.

O projeto ótimo de compressores requer análises em diversos campos de comnhecimento, além de ser restrito por várias condições de contorno não necessariamente relacionadas ao ótimo de componentes isolados. O uso de programas de simulação tem mostrado ser uma ferramenta de projeto extremamente útil, tanto no teste de hipóteses prévio à construção de protótipos, a qual demanda muito tempo e tem alto custo. Este artigo descreve programas de simulação e abordagens teórico-experimentais usadas na Embraco para o projeto de compressores herméticos alternativos de refrigeração. Mais especificamente, o escopo se restringe à problemas de escoamento e transferência de calor, bem como alguns tópicos correlatos, em especial questões ligadas a ruído, projeto de motores e ao sistema de refrigeração em si.