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In this paper, a thorough model for the porous diffusion layer of a polymer electrolyte fuel cell (PEFC) is presented that accounts for multicomponent species diffusion, transport and formation of liquid water, heat transfer, and electronic current transfer. The governing equations are written in nondimensional form to generalize the results. The set of partial differential equations is solved based on the finite volume method. The effect of downscaling of channel width, current collector rib width, and diffusion layer thickness on the performance of polymer electrolyte membrane (PEM) fuel cells is systematically investigated, and optimum geometric length ratios (i.e., optimum diffusion layer thicknesses, optimum channel, and rib widths) are identified at decreasing length scales. A performance number is introduced to quantify losses attributed to mass transfer, the presence of liquid water, charge transfer, and heat transfer. Based on this model it is found that microchannels (e.g., as part of a tree network channel system in a double-staircase PEM fuel cell) together with diffusion layers that are thinner than conventional layers can provide substantially improved current densities compared to conventional channels with diameters on the order of 1 mm, since the transport processes occur at reduced length scales. Possible performance improvements of 29, 53, and 96 % are reported.

In this study, pulsed laser based curing of a printed nanoink (nanoparticle ink) combined with moderate and controlled substrate heating was investigated to create microconductors at low enough temperatures appropriate for polymeric substrates. The present work relies on (1) the melting temperature depression of nanoparticles smaller than a critical size, (2) DOD (drop on demand) jettability of nanoparticle ink, and (3) control of the heat affected zone induced by pulsed laser heating. In the experiments, gold nanoparticles of 3–7 nmdiameter dissolved in toluene solvent were used as ink. This nanoink was printed on a polymeric substrate that was heated to evaporate the solvent during or after printing. The overall morphology of the gold microline was determined by the printing process and controlled by changing the substrate temperature during jetting. In addition, the printed line width of about 140 m at the room temperature decreased to 70– 80 m when the substrate is heated at 90° C. By employing a microsecond pulsed laser, the nanoparticles were melted and coalesced at low temperature to form a conductive microline which had just 3–4 times higher resistivity than the bulk value without damaging the temperature sensitive polymeric substrate. This gold film also survived after Scotch tape test. These are remarkable results, considering the fact that the melting temperature of bulk gold is 1064° C and the polymeric substrate can be thermally damaged at temperatures as low as 500° C. DOI: 10.1115/1.1924627

In this study, a novel concept is investigated, according to which the traditional ribbed flow delivery systems are replaced with permeable porous fluid distributors, which circumvent a number of known performance hindering drawbacks. A thorough single-phase model, including the conservation of mass, momentum, energy, species, and electric current, using Butler-Volmer kinetics, is numerically solved in three dimensions, to investigate the impact of different flow configurations on the performance of hydrogen fuel cells. It is found that cells with porous gas distributors generate substantially higher current densities and therefore are more advantageous with respect to mass transfer. Another advantage of porous flow distributors is the potential for higher power densities and reduced stack weight.

The work presented in this paper focuses on the effect of jet pulsation on the heat transfer and fluid dynamics characteristics of single and double jet impingement on a constant heat flux heated surface. Specifically, the influence of frequency, amplitude and in particular, of the phase difference of the two jets on the temperature distribution of the heated surface is examined. The simulations are conducted using a novel, remeshed Smooth Particle Hydrodynamics (SPH) methodology that is based on particle discretization of the governing compressible Navier-Stokes equations. It was found that the strong aerodynamic and thermal interaction that exists between the gaseous jets and the impingement surface leads to non-linear system responses; with serious heat transfer implications. Dynamical systems analysis leads to the identification of intermittent periodic/chaotic behavior above a threshold value of the Reynolds number. As a result, a reduction in the maximum plate temperature in a window of periodic behavior was discovered.

This paper describes the main features and results of a numerical investigation of molten microdroplet impact and solidification on a colder flat substrate of the same material that melts due to the energy input from the impacting molten material. The numerical model is based on the axisymmetric Lagrangian Finite-Element formulation of the Navier ‐Stokes, energy and material transport equations. The model accounts for a host of complex thermofluidic phenomena, exemplified by surface tension effects and heat transfer with solidification in a severely deforming domain. The dependence of the molten volume on time is determined and discussed. The influence of the thermal and hydrodynamic initial conditions on the amount of substrate melting is discussed for a range of superheat, Biot number, and impact velocity. Multidimensional and convective heat transfer effects, as well as material mixing between the droplet and the substrate are found and quantified and the underlying physics is discussed. Good agreement in the main features of the maximum melting depth boundary between the present numerical results and published experiments of other investigators for larger (mm-size) droplets was obtained, and a complex mechanism was identified, showing the influence of the droplet fluid dynamics on the substrate melting and re-solidification. @DOI: 10.1115/1.1391274#

The basic problem of the impact and solidification of molten droplets on a substrate is of central importance to a host of processes. An important and novel such process in the area of micromanufacturing is solder jetting where microscopic solder droplets are dispensed for the attachment of microelectronic components. Despite the recent appearance of a few numerical studies focusing on the complex transient aspects of this process, no analogous experimental results have been reported to date to the best of our knowledge. Such a study is reported in this paper. Eutectic solder (63Sn37Pb) was melted to a preset superheat and used in a specially designed droplet generator to produce droplets with diameters in the range 50–100 μm. In a first series of experiments, the size, temperature, and impacting speed of the molten droplets were maintained constant. The primary variable was the temperature of the substrate that was controlled in the range from 48°C to 135°C. The dynamics of molten solder microdroplet impact and solidification on the substrate was investigated using a flash microscopy technique. The time for the completion of solidification from the moment of a solder droplet impact on the substrate varies between 150 μs and 350 μs. The dynamic interaction between the oscillation in the liquid region and the rapid advance of the solidification front was visualized, quantified, and presented in this paper. In a second series of experiments, the evolution of the wetting angle between the spreading drop and the substrate was recorded and analyzed. No quantitative agreement with Hoffman’s correlation for wetting was found. It was established that the wetting angle dynamics is strongly coupled with the evolution of the droplet free surface. Two successive regimes were distinguished during the spreading. The influence of the initial impact velocity and substrate temperature on the dynamics of the measured wetting angle was described in both regimes. To the best of our knowledge, this study presents the first published experimental results on the transient fluid dynamics and solidification of molten microdroplets impacting on a substrate at the above-mentioned time and length scales that are directly relevant to the novel solder jetting technology. [S0022-1481(00)01403-1]

This paper combines a theoretical model with experimental measurements to elucidate the role of key operating parameters affecting solder microdroplet deposition in the electronics manufacturing industry. The experimental investigation is used to evaluate the final deposit (bump) shapes and trends predicted by the model. The effects of substrate temperature, material composition, layer thickness, and thermal contact resistance (including surface oxidation) are delineated. Solder-deposit shape comparisons between experiments and modeling suggest that the value of thermal contact resistance may change with process parameters, and is probably dependent on the solder phase. It is established that inferences regarding the overall shape or solidification times of solder bumps using limited modeling trends should be made only after careful consideration of the substrate composition, accurate representation of the thermal contact resistance, and adequate resolution of the fluid dynamical oscillatory motion and its effects on solidification rates. It is shown that modeling tools can be used in conjunction with experiments to promote our fundamental understanding of the transport processes in the complex solder jetting technology.

Nanofluids have been proposed to improve the performance of microchannel heat sinks. In this paper, we present a systematic characterization of aqueous silica nanoparticle suspensions with concentrations up to 31 vol %. We determined the particle morphology by transmission electron microscope imaging and its dispersion status by dynamic light scattering measurements. The thermophysical properties of the fluids, namely, their specific heat, density, thermal conductivity, and dynamic viscosity were experimentally measured. We fabricated microchannel heat sinks with three different channel widths and characterized their thermal performance as a function of volumetric flow rate for silica nanofluids at concentrations by volume of 0%, 5%, 16%, and 31%. The Nusselt number was extracted from the experimental results and compared with the theoretical predictions considering the change of fluids bulk properties. We demonstrated a deviation of less than 10% between the experiments and the predictions. Hence, standard correlations can be used to estimate the convective heat transfer of nanofluids. In addition, we applied a one-dimensional model of the heat sink, validated by the experiments. We predicted the potential of nanofluids to increase the performance of microchannel heat sinks. To this end, we varied the individual thermophysical properties of the coolant and studied their impact on the heat sink performance. We demonstrated that the relative thermal conductivity enhancement must be larger than the relative viscosity increase in order to gain a sizeable performance benefit. Furthermore, we showed that it would be preferable to increase the volumetric heat capacity of the fluid instead of increasing its thermal conductivity.

We report an experimental investigation of a novel, high performance ultrathin manifold microchannel heat sink. The heat sink consists of impinging liquid slot-jets on a structured surface fed with liquid coolant by an overlying two-dimensional manifold. We developed a fabrication and packaging procedure to manufacture prototypes by means of standard microprocessing. A closed fluid loop for precise hydrodynamic and thermal characterization of six different test vehicles was built. We studied the influence of the number of manifold systems, the width of the heat transfer microchannels, the volumetric flow rate, and the pumping power on the hydrodynamic and thermal performance of the heat sink. A design with 12.5 manifold systems and 25 μm wide microchannels as the heat transfer structure provided the optimum choice of design parameters. For a volumetric flow rate of 1.3 l/min we demonstrated a total thermal resistance between the maximum heater temperature and fluid inlet temperature of 0.09 cm2 K/W with a pressure drop of 0.22 bar on a 2×2 cm2 chip. This allows for cooling power densities of more than 700 W/cm2 for a maximum temperature difference between the chip and the fluid inlet of 65 K. The total height of the heat sink did not exceed 2 mm, and includes a 500 μm thick thermal test chip structured by 300 μm deep microchannels for heat transfer. Furthermore, we discuss the influence of elevated fluid inlet temperatures, allowing possible reuse of the thermal energy, and demonstrate an enhancement of the heat sink cooling efficiency of more than 40% for a temperature rise of 50 K.

A self-contained, small-volume liquid cooling system for thin form-factor electronic equipment (e.g., blade server modules) is demonstrated experimentally in this paper. A reciprocating water flow loop absorbs heat using mesh-type microchannel cold plates and spreads it periodically to a larger area. From there, the thermal energy is interchanged via large area, low pressure drop cold plates with a secondary heat transfer loop (air or liquid). Four phase-shifted piston pumps create either a linearly or radially oscillating fluid flow in the frequency range of 0.5–3 Hz. The tidal displacement of the pumps covers 42–120% of the fluid volume, and, therefore, an average flow rate range of 100–800 ml/min is tested. Three different absorber mesh designs are tested. Thermal and fluidic characteristics are presented in a time-resolved and a time-averaged manner. For a fluid pump power of 1 W, a waste heat flux of 180 W/cm2(ΔT=67 K) could be dissipated from a 3.5 cm2 chip. A linear oscillation flow pattern is advantageous over a radial one because of the more efficient heat removal from the chip and lower hydraulic losses. The optimum microchannel mesh density is determined as a combination of low pump losses and high heat transfer rates.

An experimental study on the transient outward solidification of a transparent binary alloy from an internally cooled horizontal pipe is reported in this paper. Two-wavelength holographic interferometry was used to visualize the density field in the liquid phase and to measure the local heat and mass transfer rates at the mush/liquid interface at characteristic times, Stefan numbers, and initial concentrations. The presence of double diffusive convection in the liquid phase greatly affected the local Nusselt and Sherwood numbers. The dependence of the pipe wall temperature, the solidified volume fraction, and the shape of the solid/mush and mush/liquid interfaces on the Stefan number, time, and initial concentration were also determined. The flow field in the liquid phase was visualized with the help of shadowgraphs. Interface photographs presented in this paper demonstrate the effect of the initial concentration on the structure of the mush/liquid interface, which varies from rather smooth to highly dendritic.

The problem of transient double diffusion in a composite layer is studied numerically. The composite layer consists of a fluid region extending over a fluid-saturated permeable substrate. Initially, the fluid in the system is motionless, isothermal, and stably stratified with a linear salt distribution. A constant uniform heat flux is then suddenly applied to the bottom wall of the system. The resulting coupled flow, temperature, and concentration fields as they evolve in time are obtained numerically. The flow in the fluid region was determined by solving the complete form of the two-dimensional laminar governing equations subjected to the usual Boussinesq approximations. The flow in the porous region was modeled using the general flow model, which includes both the effects of macroscopic shear (Brinkman effect) and flow inertia (Forchheimer effect). Interesting results were obtained and are presented in a systematic manner so as to document the effect of changing the important system parameters, which include the height of the permeable substrate, its permeability, and the ratio of the thermal to the solutal Rayleigh number. It was found that these parameters had a major impact on the system behavior and their effects are thoroughly discussed.

This paper reports a theoretical study of conjugate film condensation on one side of a vertical wall and boundary layer natural convection on the other side. Each phenomenon is treated separately and the solutions for each side are matched on the wall. The main heat transfer and flow characteristics in the two counterflowing layers, namely, the condensation film and the natural convection boundary layer, are documented for a wide range of the problem parameters. In addition, the wall heat flux and the wall temperature distribution resulting from the interaction of the two heat transfer modes (condensation and natural convection) are determined. Important engineering results regarding the overall heat flux from the condensation side to the natural convection side are summarized at the end of the study.

Natural convection of water has numerous thermal engineering applications ranging from heat exchangers and solidification or melting processes to water circulation in the sea, lakes, and estuaries. Unlike most of the fluids commonly used in engineering applications that possess monotonic, almost linear for small temperature ranges, density-temperature relationships, water possesses a density maximum at 4C at atmospheric pressure. The presence of this density maximum greatly affects water natural convection and yields flows markedly different from those of other fluids, or even water for temperature ranges away from its density maximum. The present experimental investigation focuses on the high Rayleigh number regime (Ra > 10{sup 9}). Unlike in previous investigations the flow in the system is turbulent. The experimental apparatus is a rectangular enclosure of height to length aspect ratio {approx}1/2 and height {approx}25 cm. The chief contribution of the present study, in addition to determining the temperature and flow fields in the enclosure, is that a general, easy to use correlation is proposed for the Nusselt number (representing the overall heat transfer through the layer). This correlation is accurate (when the density extremum exists in the system) regardless of the relative magnitude of the top wall temperature and the densitymore » extremum temperature.« less

Film boiling is the mode if boiling during which the hot surface is separated from the vaporizing liquid by a nearly continuous film vapor. Film boiling is usually considered a very undesirable boiling regime since it is a relatively quiet and inefficient mode of heat transfer, particularly as compared to nucleate boiling. It is customary to analyze the two-phase flow regime of laminar flow film boiling by assuming the two-phase flow regime of laminar flow film boiling by assuming an idealized vapor film flow characterized by a smooth liquid-vapor interface. However, during stable flow film boiling, the wavy nature of the liquid-vapor interface and its role in local heat and mass transport have been largely ignored. The vapor interface is rarely stationary. Interfacial waves may substantially augment the heat transfer rates throughout the layer. The present analysis treats stagnation point flow film boiling on a sphere immersed in a subcooled liquid. The effect of system parameters on the dynamic behavior of the liquid-vapor interface as well as the response to step changes in the temperature and velocity fields are investigated. The study corrects and extends the theoretical analysis previously developed by Sheppard and Bradfield (1972). The new theoretical model notmore » only allows for the influence of liquid velocity, vapor superheat, and liquid subcooling on the wavy nature of the liquid-vapor interface, but it also sheds some light on the response of the system of perturbations in heat flux and superheat and into the conditions leading toward instances of liquid-solid contact during stable flow film boiling.« less

This paper establishes a theoretical framework for the minimization of entropy generation (the waste of exergy, or useful energy) in extended surfaces (fins). The entropy generation rate formula for a general fin is derived first. Based on this general result, analytical methods and graphic results are developed for selecting the optimum dimensions of pin fins, rectangular plate fins, plate fins with trapezoidal cross section, and triangular plate fins with rectangular cross section.