Air-Side Heat Transfer and Pressure Drop Characteristics of Peripheral Fin Heat Exchangers

We present an experimental evaluation of the peripheral finned-tube heat exchanger. In this novel compact evaporator geometry, the air-side is composed by an arrangement of open-pore cells formed by radial fins whose bases are attached to the tubes and whose tips are connected to peripheral fins. Each fin arrangement is made up of six radial fins and six peripheral fins forming a hexagon-like structure. The air-side fin configuration is composed of three levels of fin arrangement, each characterized by the length of radial fin and mounted with a 30° offset from its neighboring level. Experimental data on the air-side heat transfer and pressure drop were generated in an open-loop wind tunnel calorimeter. A one-dimensional theoretical model based on the theory of porous media has also been developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny-Carman model with the particle diameter definition due to Whitaker and the friction factor correlation due to Ergun. The model overpredicts the air-side thermal conductance by less than 15% for air flow rates higher than 14 L/s. The air-side pressure drop is underpredicted by the model, but still within the limits encountered in the literature. The analysis is complemented with an entropy generation minimization analysis in order to demonstrate the procedure for obtaining an optimized configuration of the heat exchanger.

ON LEVEL SETS WITH "NARROW THROATS" IN LINEAR DIFFERENTIAL GAMES

Examples with zero-sum linear differential games of fixed terminal time and a convex terminal payoff function depending on two components of the phase vector are considered. Such games can have an indifferent zone with constant value function. The level set of the value function associated with the indifferent zone is called the "critical" tube. In the selected examples, the critical tube and the neighboring level sets exhibit "narrow throats". Presence of such throats requires extremely precise computations for constructing the level sets. The paper presents different forms of critical tubes with narrow throats and indicates the combinations of problem parameters that can produce them.

A Comparison between Anterior Threaded Cages vs. Posterior Pedicle Screw Instrumentation in Terms of Foraminal Distraction and Lumbar Lordosis

A cadaveric study was performed to compare the effect of anterior versus posterior distraction instrumentation on lumbar foraminal height and lordosis. Pedicle screws and interbody instrumentation can be used to distract vertebral bodies, thereby increasing foraminal height. BAK interbody cages were inserted anteriorly at L4-L5 and L5-S1 in ten spines. Isola instrumentation was applied posteriorly in six spines at L4, L5, and S1. Both anterior instrumentation and posterior pedicle screw instrumentation provided significant increase in foraminal height with slight decrease in total lumbar lordosis. Foraminal height increased with larger cages at the level in which the cages were first applied, and the addition of cages at the neighboring level had a little effect. However, no statistically significant differences were found with different sizes of cages at the neighboring level. The maximum change in foraminal height occurred with 10 mm of posterior distraction, but a plateau effect was observed over six mm. Distraction of the lumbar spine is effective in increasing foraminal height with relatively small losses of lumbar lordosis. Similar increases were obtained with the anterior and posterior approaches. The surgeon should choose a particular method based on the patient's pathology.