Khalid N. Alammar

Quantitative measurements of laminar hydrogen gas-jet diffusion flames in A 2.2 s drop tower

Drop tower experiments were performed to identify buoyancy effects in laminar, hydrogen gas jet diffusion flames. Quantitative rainbow schlieren deflectometry was utilized to optically measure distributions of refractive index and, thus, temperature and oxygen molar percentage assuming chemical equilibrium in the flame. Test conditions consisted of atmospheric pressure flames burning in quiescent air at jet exit Reynolds numbers of 40 and 70, burner diameters of 0.30 and 1.19 mm, and jet exit Froude numbers of 1150 to 219,900 in earth gravity. Results show that the stoichiometric flame surface reached steady microgravity conditions, although the surrounding flow was evolving at the end of the 2.2 s drop period. In microgravity, the axial diffusion was important throughout the flow field. The maximum temperature and flame length were independent of gravity. Gravity affected the flame width, although the effect diminished at higher jet exit Froude numbers. For a given jet exit Reynolds number, the shape of the microgravity flame was independent of the burner diameter.

Effects of buoyancy on transitional hydrogen gas-jet diffusion flames

ABSTRACT Experiments were conducted to isolate effects of buoyancy in gas-jet diffusion flames undergoing transition by hydrodynamic instability of the fuel jet. The jet Reynolds number was kept below the critical value for turbulent pipe flow to ensure an initially laminar fuel jet. Fast-reacting, nonsooting hydrogen fuel was used to characterize flame properties by jet Reynolds number and jet Froude number, which were varied independently. The jet Froude number was controlled by varying burner diameter and operating pressure in an Earth-gravity facility and by varying gravitational acceleration in the 2.2-s drop tower. Flame characteristics were obtained using rainbow schlieren deflectometry, a line-of-sight optical diagnostic technique. Results show significant effects of buoyancy on near-field structure and transition from laminar to turbulent flame. Data suggest that a buoyant transitional flame could become laminar in the absence of gravity.

Computational Analysis of Turbulent Flow Around a Dimpled and Smooth Sphere

Steady turbulent flow around a 43-mm diameter smooth sphere and one with 245 round dimples was simulated. The turbulent flow around the sphere was attained by placing a turbulator 9 mm’s upstream of the center point. For comparison, the turbulator was also placed around the dimpled sphere. The simulation revealed stable vortical flow structure inside the dimples. A stable vortex pair in the wake region was predicted in both cases. Predicted separation point over the smooth sphere was further downstream than in the case of dimpled sphere. The predicted drag coefficient for the smooth sphere was 40% lower than that of the dimpled sphere, which was 0.35. Drag predictions are compared with previously published measurements.Copyright

Simulation of Fully-Developed Average Turbulent MHD Pipe Flow With Heat Transfer

Using a zero-equation turbulence model, fully-developed average turbulent MHD pipe flow with wall heating was simulated. Uncertainty was approximated through grid-independence and model validation. Effect of Reynolds, Hartmann, and Prandtl numbers on heat transfer characteristics was investigated. With increasing Hartmann number, heat transfer was shown to increase towards the side layer. Increasing the Prandtl number was shown to enhance heat transfer. Increasing the Reynolds number decreased the effect of the Hartmann number.Copyright

Flow and Heat Transfer Characteristics Downstream of A Porous Sudden Expansion

Incompressible, axisymmetric laminar flow downstream of a porous expansion is simulated. Effect of the Darcy number and inertia coefficient on flow and heat transfer characteristics downstream of the expansion is investigated. The simulation revealed circulation downstream of the expansion. Decreasing the Darcy number is shown to decrease circulation. The Nusselt number, friction coefficient, and pressure drop are shown to increase, while reattachment and location of maximum heat transfer move upstream with decreasing Darcy number. Similar effects are observed with increasing inertia coefficient.

Flow and Heat Transfer Characteristics of Heterogeneous Nanofluids in Pipes

Heterogeneous nanofluid flow in pipes is simulated. Assuming incompressible, axisymmetric, and laminar flow, effect of nanoparticle distribution and Prandtl number on flow and heat transfer characteristics is investigated. With nanoparticle overall volume concentration of 0.05, up to 20% heat transfer enhancement was predicted for fully developed heterogeneous flow compared to homogeneous nanofluid. At the entrance region, the enhancement is shown to increase with increasing Prandtl number.Copyright

Laminar Flow and Heat Transfer Characteristics of Colloid Suspensions in Water: A Numerical Study

Numerical analysis is performed to examine axisymmetric laminar flow and heat transfer characteristics of colloidal dispersions of nanoparticles in water (nanofluids). Effect of volume fraction on flow and heat transfer characteristics is investigated. Four different materials, Alumina, Copper, Copper Oxide, and Graphite are considered. Heat transfer and property measurements were conducted previously for Alumina nanofluid. The measurements have shown that nanofluids can behave as homogeneous mixtures. It is found that oxide-based nanofluids offer the least heat transfer enhancement compared to elements-based nanofluids. When normalized by friction pressure drop, it is shown that graphite can have the highest effective heat transfer enhancement. For a given volume flow rate, all nanofluids exhibited linear increase in heat transfer enhancement with increasing colloids volume fraction, up to 0.05.Copyright