A Hybrid Numerical Methodology Based on CFD and Porous Medium for Thermal Performance Evaluation of Gas to Gas Micro Heat Exchanger

In micro heat exchangers, due to the presence of distributing and collecting manifolds as well as hundreds of parallel microchannels, a complete conjugate heat transfer analysis requires a large amount of computational power. Therefore in this study, a novel methodology is developed to model the microchannels as a porous medium where a compressible gas is used as a working fluid. With the help of such a reduced model, a detailed flow analysis through individual microchannels can be avoided by studying the device as a whole at a considerably less computational cost. A micro heat exchanger with 133 parallel microchannels (average hydraulic diameter of 200 m) in both cocurrent and counterflow configurations is investigated in the current study. Hot and cold streams are separated by a stainless-steel partition foil having a thickness of 100 μm. Microchannels have a rectangular cross section of 200 μm x 200 μm with a wall thickness of 100 μm in between. As a first step, a numerical study for conjugate heat transfer analysis of microchannels only, without distributing and collecting manifolds is performed. Mass flow inside hot and cold fluid domains is increased such that inlet Reynolds number for both domains remains within the laminar regime. Inertial and viscous coefficients extracted from this study are then utilized to model pressure and temperature trends within the porous medium model. To cater for the density dependence of inertial and viscous coefficients due to the compressible nature of gas flow in microchannels, a modified formulation of Darcy–Forschheimer law is adopted. A complete model of a double layer micro heat exchanger with collecting and distributing manifolds where microchannels are modeled as the porous medium is finally developed and used to estimate the overall heat exchanger effectiveness of the investigated micro heat exchanger. A comparison of computational results using proposed hybrid methodology with previously published experimental results of the same micro heat exchanger showed that adopted methodology can predict the heat exchanger effectiveness within the experimental uncertainty for both cocurrent and counterflow configurations

Enzymatic creatinine assays allowestimation of glomerular filtration rate in stages 1 and 2 chronic kidney disease using CKD-EPI equation

The National Kidney Disease Education Program group demonstrated that MDRD equation is sensitive to creatinine measurement error, particularly at higher glomerular filtration rates. Thus, MDRD-based eGFR above 60 mL/min/1.73 m2 should not be reported numerically. However, little is known about the impact of analytical error on CKD-EPI-based estimates. This study aimed at assessing the impact of analytical characteristics (bias and imprecision) of 12 enzymatic and 4 compensated Jaffe previously characterized creatinine assays on MDRD and CKD-EPI eGFR. In a simulation study, the impact of analytical error was assessed on a hospital population of 24 084 patients. Ability using each assay to correctly classify patients according to chronic kidney disease (CKD) stages was evaluated. For eGFR between 60 and 90 mL/min/1.73 m2, both equations were sensitive to analytical error. Compensated Jaffe assays displayed high bias in this range and led to poorer sensitivity/specificity for classification according to CKD stages than enzymatic assays. As compared to MDRD equation, CKD-EPI equation decreases impact of analytical error in creatinine measurement above 90 mL/min/1.73 m2. Compensated Jaffe creatinine assays lead to important errors in eGFR and should be avoided. Accurate enzymatic assays allow estimation of eGFR until 90 mL/min/1.73 m2 with MDRD and 120 mL/min/1.73 m2 with CKD-EPI equation.Peer reviewe

Numerical and Experimental Study of Microchannel Performance on Flow Maldistribution

Miniaturized heat exchangers are well known for their superior heat transfer capabilities in comparison to macro-scale devices. While in standard microchannel systems the improved performance is provided by miniaturized distances and very small hydraulic diameters, another approach can also be followed, namely, the generation of local turbulences. Localized turbulence enhances the heat exchanger performance in any channel or tube, but also includes an increased pressure loss. Shifting the critical Reynolds number to a lower value by introducing perturbators controls pressure losses and improves thermal efficiency to a considerable extent. The objective of this paper is to investigate in detail collector performance based on reduced-order modelling and validate the numerical model based on experimental observations of flow maldistribution and pressure losses. Two different types of perturbators, Wire-net and S-shape, were analyzed. For the former, a metallic wire mesh was inserted in the flow passages (hot and cold gas flow) to ensure stiffness and enhance microchannel efficiency. The wire-net perturbators were replaced using an S-shaped perturbator model for a comparative study in the second case mentioned above. An optimum mass flow rate could be found when the thermal efficiency reaches a maximum. Investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. From experiments, it could be observed that microchannels with S-shaped perturbators shifted the onset of turbulent transition to lower Reynolds number values. Experimental studies on pressure losses showed that the pressure losses obtained from numerical studies were in good agreement with the experiments (<4%)

L'Estimation de la filtration glomérulaire en 2014: Intérêts et limites des tests et formules

The accurate estimation of the glomerular filtration rate (GFR) is a goal of multiple interests regarding clinical, research and public health aspects. The strong relationship between progressive loss of renal function and mortality underlines the need for early diagnosis and close follow-up of renal diseases. Creatinine is the commonest biomarker of GFR in use. By reason of non-renal determinants of GFR, it is required to integrate creatinine values within equations that take in account its most important determinants (i.e. age, sex). The CKD-EPI 2009 equation is now recommended as the first line equation to estimate GFR within the general population. In this indication, it should replace MDRD that tends to overestimate the prevalence of stage 3 chronic kidney disease with GFR around 60 ml/min. However, many questions remain about the accuracy of GFR equations in specific situations such as extremes of age or body weight. The identification of new biomarkers, less determined by non-renal determinants, is of importance. Among these biomarkers, cystatin-C is more accurate to estimate GFR when it is combined to creatinine (i.e. equation CKD-EPI 2012). However the indications for using cystatin-C instead of creatinine alone are still unclear and its use remains limited in routine practice. In conclusion, neither biomarker nor equation gives an accurate estimation for the whole range of GFR and for all patient populations. Limits of prediction are relying on both biomarker's properties and the range of GFR that is concerned, but also rely on the measurement methods. Therefore, it is crucial to interpret the estimated GFR according to the strengths and weaknesses of the equation in use.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Power co-generation apparatus including a gas microturbine supplied by a multi-vortex combustion chamber with heat exchangers

info:eu-repo/semantics/publishe

Blade Analysis and Design Using an Implicit Flow Solver

peer reviewe

An optimization-based approach for the development of a combustion chamber for residential micro gas-turbine applications

In the field of micro gas turbine, attention must be paid on the design of the combustion chamber to reduce NOx formation without compromising combustion efficiency. To this goal flameless combustion represents an appealing solution. The present work aims at the design and optimization of a combustion chamber for a micro gas-turbine, operating in flameless combustion regime. The feasibility of such system is analysed with numerical simulations, using CFD-tools. Among the several configurations under investigations, it is possible to identify one that guarantees the maximum combustion efficiency, the minimum pressure losses as well as the minimum overall dimension.SCOPUS: ar.kinfo:eu-repo/semantics/publishe

Development of a Two-Stage Radial Inflow Turbine for a Mini-ORC

editorial reviewed-Ce travail est centré sur la construction d'un modèle 0-D de conception de roue turbine radial pour des fluides non idéaux. Ce modèle est appliqué au cas d'un cycle ORC de 10kW e à deux étages d'expansion. Le point de fonctionnement du système ainsi que le fluide de travail sont choisi sur base des contraintes propres aux turbomachines. Enfin, une vérification CFD sur base des roues concues est réalisée afin de valider le modèle, et de vérifier certaines caractéristiques de l'écoulement prédites sur base du modèle 0-D