Laila Guessous

Effect of Compression on the Water Management of a Proton Exchange Membrane Fuel Cell With Different Gas Diffusion Layers

The gas diffusion layer (GDL) plays an important role in maintaining suitable water management in a proton exchange membrane fuel cell. The properties of the gas diffusion layer, such as its porosity, permeability, wettability, and thickness, are affected by the shoulders of the bipolar plates due to the compression applied in the assembly process. Compression therefore influences the water management inside fuel cells. A two-phase fuel cell model was used to study the water management problem in a proton exchange membrane fuel cell with interdigitated flow channels. The effect of the compression on the fuel cell performance was numerically investigated for a variety of GDL parameters. This paper focuses on studying the water management of fuel cells under compression for various types of gas diffusion layers. First, the deformation of a gas diffusion layer due to compression applied from the shoulders of the bipolar plates was modeled as a plain-strain problem and was determined using finite element analysis (FEA). The porosity and the permeability of the gas diffusion layer were then recalculated based on the deformation results. Next, the deformed domain from the FEA model was coupled with a fuel cell model, and the effects of the compression during the assembly process on the water management and fuel cell performance were studied for gas diffusion layers with different thicknesses, porosities, and compressive moduli. It was found that the deformation of the GDL results in a low oxygen concentration at the reaction site. The saturation level of liquid water increases along the flow direction, and is higher when the compression effect is considered in the simulation.

Fundamental correlations for laminar and turbulent free convection from a uniformly heated vertical plate

Abstract A dimensionless parameter ΠQ is introduced for the correlation of the heat transfer in natural convection induced by a constant wall heat flux. This dimensionless parameter depends on the Prandtl number, Pr, and the modified Rayleigh number, Ra ∗ , as Π Q ∼Ra ∗ /(1+Pr −1 ) . The development of ΠQ is based on physical arguments and allows the use of a single heat transfer correlation over the entire Prandtl number range. Dimensional arguments in terms of the laminar boundary layer and turbulent microscale concepts lead to a Nu∼ΠQ1/5 and Nu∼ΠQ1/4 dependence for the laminar and turbulent heat transfer, respectively. Comparison of these correlations with existing published data shows a good agreement.

A PSEUDO-SPECTRAL NUMERICAL SCHEME FOR THE SIMULATION OF STEADY AND OSCILLATING WALL-BOUNDED FLOWS

The objective of the present work is to present and validate a pseudo-spectral numerical scheme based on a variational formulation for the solution of three-dimensional, time-dependent wall-bounded forced and natural convective flows. One of the novel aspects of this numerical scheme is the use of rescaled Legendre-Lagrangian interpolants to represent the velocity and temperature in one direction. These interpolants were obtained by dividing the Legendre Lagrangian interpolants of same order by the square root of the corresponding weight used for Gauss-Lobatto quadrature. Results from two specific problems are presented as part of the validation process: Rayleigh-Bénard convection and steady and unsteady channel flow driven by an external oscillating streamwise pressure gradient.

Engine Lubrication System Analysis by Considering Aeration and Cavitation within the Rotating Oil Supply Passage

The connecting rod big-end bearing is one of the most heavily loaded components of the lubrication circuit system of modern combustion engines. The bearings oil supply has to be designed accordingly in order to ensure its sustainable operating reliability. As can be seen, the oil supply has to pass the main bearing and the rotating crankshaft before entering the connecting rod bearing. It is common knowledge that the centrifugal forces due to the crankshaft rotation affect the oil flow through the rotating supply passage into the rod bearing. The oil pump has to maintain a certain pressure level in the main oil gallery to overcome these centrifugal forces. In the early eighties, the second oil pressure limitation to the rod bearing operation was identified, which is lower than the centrifugal pressure limit. As a result of this, the dissolved air will be released in the rotating oil passage due to the pressure decrease caused by the centrifugal forces. Aeration or cavitation within the rotating oil supply passage occurs at much lower oil pressure. The detrimental effects of aeration or cavitation are loss of lubrication, overheating, metal erosion, mechanical shocking, and even system component failures. In this paper, a novel approach of identifying the second pressure limit is introduced by applying the CFD method. The three-dimensional, transient, multiphase flow in the transfer passage has been solved by considering its dynamic attributes. The characteristics of the aeration and cavitation flows and their effects on rod bearing oil supply have been investigated. The mechanisms underlying the interrupted oil supply to the rod bearing have been discussed in detail by studying the dynamics of aeration and cavitation. Finally, a map of the operational range has been established as an exemplary guideline for the design of engine lubrication systems and the key operating conditions including rotational speed and main bearing inlet pressure. Review led by Yuan-Ren Jeng

A Numerical Investigation of Bolt Underhead Temperature Evolution Under Various Fastening Conditions

The temperature rise that occurs due to frictional heating under a turning fastener head during the tightening process of bolted joints can have a significant effect on surface and thread wear and galling. In the present study, the spatial and temporal temperature rise in a bolt during the tightening process is numerically investigated for a variety of sliding and loading conditions. The effect of tightening speed, angle of turn, and frictional energy input is numerically investigated. Tightening speeds were varied between 1 rpm and 3000 rpm and the angle of turn was varied between 15 and 720 degrees past free spinning. Significant temperature rises of the bolt underhead were observed computationally for higher tightening speeds and angles of turn and the potential for localized melting or near-melting temperatures was shown. In the case of lower tightening speeds, the temperature rise was not as dramatic, but temperature increases are then observed along the length of the shank, showing the possibility of contributing to thermally induced galling between the threads. Due to the temperature variations observed in most cases in the underhead and along the bolt shank, this study indicates that such thermal effects should be considered when modeling the wear of bolted joints, particularly in cases involving larger tightening speeds or softer (lower stiffness) joints.

A one-dimensional spot welding model

A one-dimensional model is proposed for the simulations of resistance spot welding, which is a common industrial method used to join metallic plates by electrical heating. The model consists of the Stefan problem, in enthalpy form, coupled with the equation of charge conservation for the electrical potential. The temperature dependence of the density, thermal conductivity, specific heat, and electrical conductivity are taken into account, since the process generally involves a large temperature range, on the order of 1000K. The model is general enough to allow for the welding of plates of different thicknesses or dissimilar materials and to account for variations in the Joule heating through the material thickness due to the dependence of electrical resistivity on the temperature. A novel feature in the model is the inclusion of the effects of interface resistance between the plates which is also assumed to be temperature dependent. In addition to constructing the model, a finite difference scheme for its numerical approximations is described, and representative computer simulations are depicted. These describe welding processes involving different interface resistances, different thicknesses, different materials, and differentvoltageforms.The differencesin the processdue toAC orDC currents are depicted as well.

Investigation of the Validity of the Carslaw and Jaeger Thermal Theory under Different Working Conditions

The temperature rise caused by frictional heat generation is of great importance in many tribological problems. However, due to a lack of a reliable and general experimental methods to measure the flash temperature on the contact surface, numerical simulations are widely used to evaluate the flash temperature under different operating conditions. The theoretical solutions developed by Carslaw and Jaegerare among the most commonly used solutions in tribology to calculate surface temperatures. Carslaw and Jaeger utilized mathematical methods to develop an analytical temperature solution under the assumption that the calculation domain is insulated and semi-infinite in size; however, real working conditions may not satisfy these assumptions. Therefore, in this study, a transient three-dimensional numerical thermal model using second-order finite difference methods to solve the transient heat conduction equation was utilized to evaluate the effect of domain size, heat source position, and material properties on the accuracy of the temperature prediction of the Carslaw and Jaeger solution for low Peclet numbers. Results indicate that for very short sliding times, the Carslaw and Jaeger solution is close to the numerical result; the domain size and heat source position have little influence. However, as time increases, the deviation between the Carslaw and Jaeger solution and the numerical results becomes increasingly larger, indicating that the mathematical solution may no longer be utilized without introducing significant errors. The length of time during which the Carslaw and Jaeger solution performs adequately is a function of the domain size, heat source position, and material properties.

Industrial Mentors: An Often Untapped Resource in Undergraduate Research Programs

Students taking part in a 10-week summer research experience for undergraduates (REU) program in the department of mechanical engineering at Oakland University receive three levels of mentorship: from faculty, graduate students and researchers/engineers from industry. Industrial mentors, all of whom volunteer to take part in the experience, play a variety of roles as part of the program and are viewed by the authors as an often untapped resource in undergraduate research programs. This paper focuses on the experience gained from involving industrial mentors in the REU program and on the lessons learned: what worked, what didn’t work and what improvements can be made in the following years.Copyright

Improving Radiator Design Through Shape Optimization

The current emphasis by auto manufacturers on improving the quality and fuel economy of their vehicles requires enhancements in the efficiency and operation of all engine components, including those in the engine cooling system. Improvements in the pressure drop and flow homogeneity in the water tank of a radiator are needed to reduce the power demands on the vehicle water pump and increase the lifetime of the radiator. The latter criterion is particularly important in the reduction of premature fouling and failure of heat exchangers. Rather than relying on ad hoc geometry changes with the goal of improving the performance of radiators, the coupling of CFD flow simulations with numerical shape optimization methods could assist in the design and testing of automotive heating and cooling components. This paper describes ongoing efforts to develop a fully automated suite of virtual tools to aid in the design and testing of automotive heating and cooling components. The development and validation of optimization criteria for pressure drop and mass flow rate distribution in a water-to-air automotive heat exchanger are discussed, as well as the methodology used to automate the mesh generation, CFD simulation, and shape optimization procedures using the GAMBIT® and FLUENT® software combined with an in-house code.Copyright

A Numerical Investigation of the Effects of Flow Pulsations on the Drag Force Over a Cylinder

Flow over a cylinder is a fundamental fluid mechanics problem that involves a simple geometry, yet increasingly complex flow patterns as the Reynolds number is increased, most notably the development of a Karman vortex with a natural vortex shedding frequency fs when the Reynolds number exceeds a value of about 40. The goal of this ongoing study is to numerically investigate the effect of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the drag force over and vorticity dynamics behind a circular cylinder. This paper reports on initial results involving unsteady, laminar and incompressible flows over a circular cylinder. Sinusoidal free-stream pulsations with amplitudes Av varying between 25% and 75% of the mean free-stream velocity and frequencies f varying between 0.25 and 5 times the natural shedding frequency were considered. Of particular interest to us is the interaction between the pulsating frequency and natural vortex shedding frequency and the resulting effects on drag. Interestingly, at frequencies close to the natural frequency, and to twice the natural frequency, a sudden drop in the mean value of the drag coefficient is observed. This drop in the drag coefficient is also accompanied by a change in the flow and vortex shedding patterns observed behind the cylinder.© 2011 ASME