Paulo Couto

TRANSIENT ANALYSIS OF SLIP FLOW AND HEAT TRANSFER IN MICROCHANNELS

Hybrid analytical-numerical solutions for transient flow and transient convective heat transfer within microchannels are presented. Analytical solutions for flow transients in microchannels are obtained by making use of the integral transform approach. The proposed model involves the transient fully developed flow equation for laminar regime and incompressible flow with slip at the walls in simple channel geometries. The solution is constructed so as to account for any general functional form of the time variation of the pressure gradient along the duct. Then, the transient-state convection heat transfer is solved for laminar slip flow inside microchannels formed by parallel-plates, making use of the generalized integral transform technique (GITT) and the exact analytical solution of the corresponding eigenvalue problem in terms of confluent hypergeometric functions so as to eliminate the transversal coordinate. The resulting system of transformed partial differential equations in the longitudinal coordinate is numerically solved by the Method of Lines as implemented in the routine NDSolve of the Mathematica system. Mixed symbolic-numerical algorithms are developed under the Mathematica platform.

Experimental Analysis of Supercritical Startup of Nitrogen/Stainless Steel Cryogenic Heat Pipe

An experimental investigation of the supercritical startup of a nitrogen/stainless steel cryogenic heat pipe is presented. A detailed description of the experimental setup used for this investigation is presented, and several data sets for the transient axial temperature distribution of the heat pipe are shown. A transient, one-dimensional model developed for microgravity environment available in the literature is used to obtain the internal pressure and working fluid axial distribution. The results showed that cryogenic heat pipes are very sensitive to parasitic heat loads (e.g., heat convection), even in rarefied atmospheres, and the parasitic heat loads can significantly change the operational temperature of the cryogenic heat pipe. The effects of parasitic heat loads must be accurately considered during the design stages. Also, the fluid charge plays an important role in the determination of the initial thermodynamic state of the cryogenic heat pipe. An excess of fluid charge may prevent a successful startup due to an increased vapor pressure, whereas a deficiency may prevent the startup due to lack of working fluid to sufficiently prime the heat pipe.

Analysis of supercritical startup of cryogenic heat pipes with parasitic heat loads

A mathematical model to predict the transient temperature profile of a cryogenic heat pipe during startup is presented. The improved model accounts for a known time-variable temperature boundary condition at the condenser region, while the boundary condition in the remaining length of the heat pipe is a radiative parasitic heat flux. The vapor pressure is modeled to determine the mass distribution of the saturated liquid inside the heat pipe. An axially grooved aluminum/oxygen cryogenic heat pipe is considered. Theoretical results for the axial temperature profile of the heat pipe for the startup period are compared to experimental μ-g flight data to validate the new model. Following the validation, the effects of some operational parameters over the axial temperature profile for the startup period are investigated to determine under what conditions the cryogenic heat pipe will start properly.

Measurement of thermophysical properties of ceramics by the flash method

The flash method, proposed by Parker, Butler, Jenkins and Abbott from the U.S. Navy Radiological Defense Laboratory in 1961, is the most popular method for measuring the thermal diffusivity of solids. In this method, the front surface of a small sample is subjected to a very short burst of radiant thermal energy. The resulting temperature rise on the opposite surface of the sample is measured and the thermal diffusivity is computed from the temperature rise versus time data. Also, the specific heat can be computed from the measured data, thus allowing for the calculation of the thermal conductivity. Several theoretical models are available for the flash method, which include adiabatic boundary conditions, heat losses, surface coating effects, among other aspects. In this paper, tests were made for the identification of thermo-physical properties of a Ceramic block. The Netzsch Nanoflash LFA 447/1 of LTTC/COPPE/UFRJ was used for the measurements.

Parametric Analysis of Heat Transfer on Multistage Cryogenic Radiator

A theoretical and experimental parametric study of the heat-transfer phenomena on a multistage passive cryogenic radiator is presented. This investigation was performed in the frame of a cooperative effort between Clemson University and the Federal University of Santa Catarina. Passive cryogenic radiator technology is under development at the Satellite Thermal Control Laboratory at the Federal University of Santa Catarina, where two experimental prototypes have been built and ground tested. The mathematical model, developed to predict the temperature distribution on the radiator stages, was used to study the sensitivity coefficients with respect to the design parameters. The design parameters considered are the radiator stages surface emissivity, the multilayer insulation effective emissivity, the radiator supports global conductance, and the thermal load over the radiator stages. This sensitivity analysis showed that the thermal joint conductance between the stages and the support structure (aluminum-Teflon®) plays an important role in the temperature distribution of the radiator. An experimental study was conducted within the Mechanical Engineering Department of Clemson University to gather thermal conductance data for comparison with the theoretical results. The thermal conductance data were incorporated into an analytical model developed for the prediction of the transient temperature behavior of a multistage cryogenic radiator for spacecraft applications. The data were also compared with the recently developed model for the prediction of thermal conductance of polymer and metal joints. Ultimately, conclusions are presented about the importance of the thermal conductance between the polymer support structure and the passive cryogenic radiator stages in the temperature distribution of the radiator.

Development of a certified reference material: ethanol in water - a practical case

O presente trabalho descreve o desenvolvimento de um material de referencia certificado (MRC) referente a solucoes de etanol em agua em cinco concentracoes diferentes, abordando todas as etapas necessarias para o desenvolvimento de um MRC, conforme prescreve as ISO GUIDE 30, 31, 32, 33, 34 e 35. O desenvolvimento deste MRC originou-se nas determinacoes da Portaria Inmetro n° 006 de 2002, que assegura que todos os etilometros a serem utilizados no Brasil deverao ter os seus modelos aprovados, e as verificacoes realizadas a partir de ensaios com solucoes de etanol em agua. Apos um treinamento, em fevereiro de 2003, no The Federal Institute for Material Research and Testing - BAM, que e um instituto metrologico alemao designado, a Dquim iniciou o processo de desenvolvimento deste MRC, o qual foi concluido em agosto de 2004. No Inmetro desenvolveu-se inicialmente o procedimento de preparo individual, onde as solucoes do MRC sao preparadas em garrafas de 0,5 L. Atualmente esta se estudando, no Inmetro, o desenvolvimento do procedimento em batelada, o que corresponde ao preparo das solucoes do MRC em garrafas de 5,0 L, com sequente distribuicao em garrafas de 0,5 L. Pode-se considerar que o processo de desenvolvimento deste material de referencia certificado esta num periodo continuo, onde melhorias estao sendo viabilizadas, como a utilizacao da tecnica de DSC para a determinacao da pureza do etanol, e a finalizacao do desenvolvimento do procedimento em batelada, que sera um importante passo para a agilizacao no processo de preparo destas solucoes. Desta forma, o desenvolvimento deste MRC contribui de forma significativa, para confiabilidade nas medicoes realizadas com etilometros, tendo incerteza de medicao declarada. Outro ponto importante e que a partir deste desenvolvimento, possibilita-se a sociedade um MRC que pode ter aplicacoes variadas, nao se restringindo somente a ensaios com etilometros.

Transient temperature behavior of multistage cryogenic radiators : Model and experimental validation

The Satellite Thermal Control Laboratory of the Federal University of Santa Catarina Is one of the major groups engaged in the Uniespaco Program, financed by the Brazilian Space Agency. To provide the cryogenic thermal control requirements of the infrared radiation sensors of the small-sized Brazilian satellites, a multistage passive cryogenic radiator is under development. It is designed to operate at 145 K with a cooling capacity of 0.1 W in the primary stage. The secondary stage can dissipate 1 W at 250 K. A brief literature review is presented, showing the world state of the art in satellite radiators. An analytical model, developed to predict the temperature behavior of the radiator, is presented. The experimental setup constructed to generate data to be compared with the theoretical results is also presented. The comparison between theory and data is good, demonstrating that the technology of design and development of the passive cryogenic radiator is completely dominated by the Satellite Thermal Control Laboratory.

Comprehensive Solution for Transient Flow in Heterogeneous Porous Media

Solutions of the hydraulic diffusivity equation are of utmost importance for many reservoir engineering problems. Despite all the efforts, there is still a need for the development of rigorous and comprehensive solutions for transient flow problems in heterogeneous oil reservoirs. This study demonstrates the use of an integral transform approach to obtain such a rigorous and comprehensive solution for the hydraulic diffusivity equation in heterogeneous porous domain. The reservoir heterogeneities can be approximated by any continuous differentiable function. The presented general solution and its derivation are valid for multi-dimensional problems in any orthogonal coordinate system. It has the advantage of rigorously solving the hydraulic diffusivity equation for transient, late-transient and steady-state (or pseudo-steady-state) flow regimes in a single formulation that allows the consideration of variable flowrates. In this work, applications of the general solution for one-dimensional problems in the Cartesian and radial coordinate systems are presented, showing comparisons of the results obtained with a finite difference numerical scheme. The solution presented can be used to analyze buildup, drawdown and interference test data, making it a useful tool for pressure transient analysis applied to reservoir engineering problems.

Transient Solution for the Energy Balance in Porous Media Considering Viscous Dissipation and Expansion/Compression Effects Using Integral Transforms

Monitoring of downhole flowing temperatures in oil wells is gaining attention in the recent years due to the possibility of exploring these data for reservoir characterization and determination of inflow profiles along the well completions, leading to an increased interest in the development of solutions for the equations governing the thermal behavior of a reservoir. In this work, it is proposed to use the generalized integral transform technique (GITT) to provide solutions for the energy balance equation, considering the thermal effects related to fluid flow. A formal and general solution for the energy balance in the porous media is presented and validated. It is presented the application of the proposed solution to one-dimensional and two-dimensional problems in the Cartesian coordinate system. The two-dimensional problem, which considers heat transfer to the surrounding impermeable formations, is tackled by a single domain formulation. The mathematical approach taken in these solutions is rigorous, valid for all flow regimes (transient, late-transient and pseudo-steady-state/steady-state) and for any orthogonal coordinate system, presenting the possibility of achieving differentiable and stable solutions with controlled accuracy. The solution comprises an important contribution to support the application of temperature data to reservoir engineering problems.

A Comprehensive Approach for Assessing the Impacts of Wettability on Oil Production in Carbonate Reservoirs

This paper discusses the impacts that rock wettability may have upon the production and recovery of oil with waterflooding in carbonate reservoirs and how it should be modeled. A broad review of the state of the art has been conducted surveying existing disagreements and knowledge gaps, basic definitions, as well as the correct understanding of the physical phenomena and identification of the characteristics of the various wettability scenarios.Case studies conducted with a black oil reservoir simulator evaluated the impact of different wettability scenarios on oil production and recovery. A comprehensive approach considering all the parameters involved in the wettability modeling was applied to the case studies, showing how the behavior of the reservoir varies as a function of their wettability.This paper shows how relative permeability and capillary pressure should be varied to correctly represent different wettability scenarios and consequently assess its impacts on oil production and recovery.The case studies show that the evaluation of the volume of oil in the reservoir is impacted by wettability through the irreducible water saturation and primary drainage capillary pressure and must be considered in the analyses. In long term analyses, mixed-wet scenarios have a higher oil production and recovery. In medium and short term, the water-wet scenarios have the higher recovery, but in relation to oil production, these scenarios are negatively influenced by the smaller volume of oil in place.The main contribution of this paper is the simultaneous analyses of all the parameters involved in the modeling of wettability showing how they impact the behavior of a reservoir. It shows how the parameters must be varied in a heterogeneous reservoir and how heterogeneity impacts the relevance of wettability in the studies.Copyright