Simulation-Based Design and Optimization of Frost-Free Refrigerators: A Thermoeconomic Approach

A steady-state simulation model was devised and its predictions for energy consumption were compared with experimental data obtained elsewhere, with differences not exceeding a 9% error band. The model was used to find out the evaporator air flow rate, the number of evaporator and condenser fins, the compressor size and efficiency, and the insulation thicknesses of both compartments which yield the minimum energy consumption when the total cost is constrained. The optimization led to a refrigeration system with energy consumption 18% lower than that observed for the baseline system without any cost penalty. The system performance for four different compressors including a high efficiency, a large capacity and a variable-speed one was also analyzed. According to the cost function adopted, the variable-speed compressor shall not cost 26% more than a single-speed one with the same piston displacement

Experiments on turbulent flows of elasto-viscoplastic fluids

In this thesis, we present an experimental investigation of turbulent flows of elasto-viscoplastic fluids. The motivation comes from the oil and gas industry, where turbulent flows of non-Newtonian fluids are frequently encountered. We characterize the turbulent flow of viscoplastic fluids, both under a fully turbulent flow and also when it is displaced by another fluid under turbulence in an eccentric annular geometry. Further, we investigate the fully turbulent flow of a drag reducing, elasto-viscoplastic wormlike micellar (or surfactant) solution, and compare those results to better known polymer solutions under turbulent flows. Our experiments in fully turbulent flows of viscoplastic Carbopol solutions show an enhancement of streamwise velocity fluctuations and a decrease in wall normal velocity fluctuations in comparison to water. As we increase the Reynolds numbers, the turbulence statistics approach Newtonian values. With regards to turbulent displacements of 0.15% Carbopol solutions in an eccentric annulus, we observe that the displacement is successful without the obstruction regardless of the displacing fluid. The obstruction at eccentricity e ≈ 0.5 is mostly detrimental to removal of the yield stress fluid stuck downstream of it. At high eccentricity values of e ≈ 0.7, the effect of the obstruction on the displacement of Carbopol is seen to be negligible. When wormlike micellar gels are submitted to a turbulent flow, the micellar structure near the wall appears to be mostly broken down during turbulent flow. Turbulent flows at low concentrations of surfactant show a Newtonian-like flow field throughout most of the duct, where the energy spectra shows a -5/3 power law scale with wavenumber. Conversely, energy spectra of the micellar solutions at large percentages of drag reduction show approximately a -3 power law decay. Moreover, a direct comparison of turbulent flows with flexible and rigid polymer solutions also shows similar turbulence statistics at approximately the same percentages of drag reduction. A -3 power law decay in the energy spectra is also observed with both flexible and rigid polymer solutions, and we hypothesize it may be a consequence of elasto-inertial turbulence.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat

Experimental evaluation of the pressure transmission in pipelines filled with yield stress fluids

Drilling fluids exhibit a viscoplastic behavior, which is defined by a yield stress. If the stress imposed to the fluid does not surpass the yield stress, the material behaves like an elastic solid; if the yield stress is exceeded, it flows like a viscous liquid. In well drilling operations, some valves installed on the drillpipe near the bottom of the hole are actuated by pressure differences in the drilling fluid. However, it has been argued that the pressure applied at the surface is not fully transmitted to the valve’s position, preventing its actuation. Therefore, the pressure transmission in viscoplastic fluids demands further investigation. In order to compensate for the lack of experimental studies about the problem, an experimental analysis of the pressure transmission in yield stress fluids has been performed in this work. The experimental rig consists of a long thermally-controlled helical pipe, on which are installed relative pressure transducers. Fluid is displaced by a helical pump at a controlled flow rate. During the experiments, the fluid is pressurized inside the closed pipeline while the pressures are measured and recorded. The results showed that, in agreement with literature, the pressure at one end of a closed pipeline filled with a yield stress fluid is not fully transmitted to the other end. Moreover, it was observed that the pressure gradient in the pressurized fluid is proportional to its yield stress, which indicates a relation between pressure transmission and the presence and magnitude of the yield stress. The experiments were compared to simulation work developed at the Research Center for Rheology and Non-Newtonian Fluids with good agreement.CAPES; PetrobrasFluidos de perfuração apresentam comportamento viscoplástico, que é definido por uma tensão limite de escoamento. Se a tensão imposta não supera a tensão limite de escoamento, o material se comporta como um sólido elástico, e quando esta tensão é excedida, o fluido escoa como um líquido viscoso. Durante o processo de perfuração de poços de petróleo, válvulas posicionadas na extremidade da coluna de perfuração são atuadas por diferenças de pressão no fluido. Argumenta-se, entretanto, que pressões aplicadas na superfície não são totalmente transmitidas até a válvula, impedindo sua operação. Logo, a transmissão de pressão em fluidos viscoplásticos demanda mais estudo. Para suprir a falta de estudos experimentais sobre o assunto, uma avaliação experimental da transmissão de pressão em um fluido viscoplástico é realizada neste trabalho. O aparato experimental construído para este propósito consiste de uma longa tubulação em formato helicoidal (serpentina) mantida sob temperatura controlada, ao longo da qual estão instalados transdutores de pressão relativa. O fluido é bombeado a uma vazão controlada por uma bomba do tipo helicoidal. Durante os testes, o fluido de trabalho é pressurizado na serpentina até um certo patamar ao mesmo tempo em que as pressões são medidas. Os resultados dos testes com fluidos viscoplásticos corroboram a literatura, que afirma que um fluido com tensão limite de escoamento confinado em uma tubulação fechada não transmite totalmente a pressão imposta na entrada ao longo do restante da tubulação. Além disso, foi observado que a diferença de pressão entre dois pontos da tubulação quando o fluido está em repouso é proporcional à tensão limite de escoamento deste fluido. Os resultados experimentais foram comparados com resultados de simulações numéricas de dois modelos matemáticos desenvolvidos no Centro de Pesquisas em Reologia e Fluidos Não newtonianos (CERNN), com boa concordância