Experimental and numerical investigation of an air-to-water heat pipe-based heat exchanger

An experimental and analytical investigation was conducted on an air-to-water heat exchanger equipped with six wickless heat pipes (thermosyphons) charged with water as the working fluid. The flow pattern consisted of a double pass on the evaporator and condenser sections. The six thermosyphons were all made from carbon steel, measured 2m in length and were installed in a staggered arrangement. The objectives of the reported experimental investigation were to analyse the effect of multiple air passes at different air inlet temperatures (100 to 250°C) and air mass flow rates (0.05 to 0.14kg/s) on the thermal performance of the heat exchanger unit including the heat pipes. The results were compared with a CFD model that assumed the heat pipes were solid rods with a constant conductivity. The conductivity of the pipes was extracted from modifications of correlations available in the literature based around the theory of Thermal Resistance. The results proved to be very accurate within 10% of the experimental values

Numerical prediction of air flow within street canyons based on different two-equation k-ε models

Numerical simulations on airflow within street canyons were performed to investigate the effect of the street aspect ratio and wind speed on velocity profiles inside a street canyon. Three-dimensional Standard, Renormalization Group (RNG) and Realizable k-ε turbulence model are employed using the commercial CFD code FLUENT to solve the Reynolds-averaged Navier-Stokes (RANS) equations. A comparison of the results from the presently adopted models with those previously published demonstrated that the k-e model is most reliable when simulating wind flow. The model is then employed to predict the flow structures in a street canyon for a range of aspect ratios (building height to street width ratio) between 0.5-2 at Reynolds number of 9000, 19200 and 30700 corresponding to the ambient wind speeds of 0.68m/s, 1.46m/s and 2.32m/s respectively. It is observed that the flow structure in the street canyon is influenced by the buildings aspect ratios and prevailing wind speeds. As the street aspect ratio increases, the air ventilation within the canyon reduces.</p

Writing and Mathematical Problem-Solving: Effects of Writing Activities on Problem-Solving Skills of Elementary Students

This action research study summarizes the use of writing activities with a group of 20 urban elementary students. The study investigates the effect of integrating supplemental writing activities with traditional mathematics instruction on student performance in problem solving, understanding of mathematical concepts, and willingness to participate in problem solving activities. Students met and participated in problem solving activities three times per week over an eight-week period. Students were asked to complete problem solving prompts and write about their problem-solving solutions. Performance data was collected using a problem solving writing rubric for evaluating student journal responses, anecdotal records, and classroom teacher response surveys

Interactions of shock tube exhaust flows with laminar and turbulent flames

The interactions of flow features emitting from open-ended shock tubes with free-standing propane flames have been investigated using high-speed schlieren imaging and high-frequency pressure measurements, with additional data from validated numerical modeling. Both compressed air-driven interactions with non-pre-mixed laminar diffusion flames (small-scale) and explosively-driven interactions with turbulent non-pre-mixed turbulent flames (large-scale) were tested for various flame locations and shock tube stagnation pressures (and therefore Mach numbers). In the small-scale tests it was observed that the flames were not significantly influenced by the passage of either the initial shock if placed close to the tube exit, or the weaker pressure waves downstream if the flame was placed further away. Four types of interaction were classified, three of which led to permanent extinguishment of the flames. The most effective mechanism of extinguishment for a flame in-line with the exhaust was the axial exhaust jet of expanding air, which served to push the flame off the fuel source either at close range (Type I) or more slowly at a distance (Type II), after which rapid cessation of combustion occurred. With the flame positioned to one side of the path of the jet, strong loop vortices achieved a similar overall outcome of extinguishment, albeit with very different flame behavior in reaction to the strong turbulence and vorticity induced by the passing flow (Type III). In all cases bar one, the disruption to the fire triangle caused by these flow effects was sufficient to extinguish – rapidly and permanently – the flame. However, at a sufficient lateral offset of the flame from the shock tube exit, the strength of rotating flow being entrained into the diffusing vortex ring was not sufficient to remove and disperse the heat from the extinguished flame (Type IV), such that re-ignition could occur. By contrast, in the large-scale tests with a significantly different shock pressure profile and a flame approximately 1 order of magnitude greater, extinguishment in all cases for all shock strengths and locations was achieved by the shock itself (accelerating combustion) and the following “blast wind” impulsively moving the flame off the fuel source, with the vortices having negligible effect at the given testing locations (Type V)

Numerical investigation of the three-dimensional pressure distribution in Taylor Couette flow

Copyright © 2017 by ASME. An investigation is conducted on the flow in a moderately wide gap between an inner rotating shaft and an outer coaxial fixed tube, with stationary end-walls, by threedimensional Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD), using the realizable k - ϵ model. This approach provides three-dimensional spatial distributions of static and dynamic pressures that are not directly measurable in experiment by conventional nonintrusive optics-based techniques. The nonuniform pressure main features on the axial and meridional planes appear to be driven by the radial momentum equilibrium of the flow, which is characterized by axisymmetric Taylor vortices over the Taylor number range 2.35 × 106 ≤ Ta ≤ 6.47 × 106. Regularly spaced static and dynamic pressure maxima on the stationary cylinder wall follow the axial stacking of the Taylor vortices and line up with the vortex-induced radial outflow documented in previous work. This new detailed understanding has potential for application to the design of a vertical turbine pump head. Aligning the location where the gauge static pressure (GSP) maximum occurs with the central axis of the delivery pipe could improve the head delivery, the pump mechanical efficiency, the system operation, and control costs.Published versio

Convective heat transfer in airflow through a duct with wall thermal radiation

This paper presents a numerical investigation on airflow through a heated horizontal rectangular duct wherein the model considers the combined modes of natural and forced convection heat transfer and the thermal radiation from duct walls. The duct periphery is differentially heated with known temperature profiles imposed on the two opposite vertical sidewalls while the other two walls are treated as adiabatic. The air enters into the duct hydrodynamically fully developed and flows steadily under laminar conditions undergoing thermal development within the duct. Considering several temperature profiles on the two vertical sidewalls, the numerical simulation generates the heat transfer rates and associated fluid flow patterns in the duct for a range of airflow rates, duct aspect ratios and surface emissivity. The variation of local Nusselt number at duct walls and the fluid flow patterns are critically examined to identify thermal instabilities and the significance of wall thermal radiation effects on the overall heat transfer rates

Conceptual and Numerical Analysis of Active Wingtip Vortex Cancellation in Propeller-Driven Electric Aircraft

As battery and electric motor technology continues to advance rapidly, propeller-driven electric aircraft are likely to become a significant part of the aviation market in the near future. One proposed design configuration for electric aircraft involves using large, wingtip- mounted propellers to actively cancel wingtip vortices, a method called active wingtip vortex cancellation (AWVC). By reclaiming part of the kinetic energy that would otherwise be lost to tip vortex formation, drag is decreased. In addition, the induced spanwise flow and upwash from the propeller causes the spanwise lift distribution to remain more uniform at the wingtips, increasing lift. Previous wind tunnel testing of this configuration characterized a significant increase in lift and decrease in drag, particularly in low-aspect-ratio configurations. This paper builds on that research by examining several test cases with a 3D, transient, viscous, sliding mesh CFD analysis in an effort to validate numerical methods for future conceptual design studies. In addition, many practical considerations regarding the implementation of this design are analyzed. Geometry from the aforementioned wind tunnel literature was reconstructed and analyzed. CFD indicated an 18.1% increase in lift and 5.1% increase in net thrust was possible solely through the phenomenon of AWVC. Furthermore, this CFD analysis matched wind tunnel data to within approximately 1%, validating the CFD approach for the analysis of more exotic configurations involving active wingtip vortex cancellation