In this paper extensive numerical investigation of the heat transfer characteristics and the pressure force of jet impingement from the single row and multiple rows on a fixed and moving flat surface are reported. The computations were carried out over a wide range of parameters: relative nozzle-to-surface distance (H/d) from 0.5 to 6, relative nozzle to nozzle distances (S/d) from 4 to 10, jet angle from 45° to 90°, relative velocity ratio (Vplate/Vj) i.e. ratio of surface velocity to jet velocity from 0 to 1. The jet Reynolds number (Re) of 2,500, 3,400, 10,000, 20,000, and 23,000 and the number of jet rows of 1, 2, 4, and 8 have been used. It was found that the numerical accuracy by SST k-ω model is reasonably high to allow for a discussion of the main flow and heat transfer characteristics. The jet impingement heat transfer performance is generally enhanced with the increase of jet Reynolds number and jet angle and with the decrease of surface distance (H/d), jet distance (S/d) and the relative velocity ratio (Vplate/Vj) within the range examined. The pressure force coefficients on the impingement surface are relatively insensitive to Re number and the velocity ratio within the range examined, while it has highly dependent on H/d, S/d and jet angle. For multiple rows of aligned jet holes, the flow pattern exhibited a different shape due to the different intensity of the interference between adjacent air jets. The effect of multiple rows with regards to the impact on average Nu and pressure force coefficient for different geometry variations such as Re, H/d, S/d, VR and ɵ is negligible compared to the single row by approximately 9 and 13% in average respectively. Based on the computed results, equations of dimensionless parameters are correlated.
Understanding the flow pattern of the gas jets in packed beds can have
considerable significance in improving reactor design and process
optimization. This study researches the fuel diffusion in the radial
direction and the flame length in a packed bed of a Parallel Flow
Regenerative (PFR) Shaft kiln. This kiln is characterized that the fuel is
injected vertically in the packed bed using a lot of lances in the
cross-section while the combustion air is distributed continuously. Such a
large, packed bed has to be approximated as a porous media. This assumption
is used to model the reactive flow in the kilns. Using a box with 700
spheres of 52 mm spheres in Body-Centered Cube (B.C.C.) arrangement the
local concentrations of injected nitrogen in airflow were measured. The
measured values match approximately with those calculated with the Porous
Media Model (PMM). The studied parameters are the number of burners and
burner arrangements. The radial mixing of fuel and air in a packed bed is
relatively bad. Therefore, a lot of burners are needed for better
temperature homogenization in the cross-section.
The effect of jet Reynolds number, jet exit angle, the nozzle to surface distance, jet to jet spacing on the heat transfer, and pressure force performance from multiple impinging round jets on a moving curved surface have been numerically evaluated. Two correlations are developed and validated for the average Nu number and the pressure force coefficient and the agreement between the CFD and correlations was reasonable. The surface motion effect becomes more pronounced on the Nu number distribution for low jet Re number, high jet to jet spacing, large jet to surface distance, and angled jets. The pressure force coefficient is highly dependent on the jet to surface distance and jet angle but relatively insensitive to jet Re number and jet to jet spacing.
Understanding the flow pattern of the gas jets in packed beds can have considerable significance in improving reactor design and process optimization. This study researches the fuel diffusion in the radial direction and the flame length in a packed bed of a Parallel Flow Regenerative (PFR) Shaft kiln. This kiln is characterized that the fuel being injected vertically into the packed bed using a lot of lances in the cross-section while the combustion air is distributed continuously. The packed bed approximated as a porous media and the measured values match approximately with those calculated with the Porous Media Model (PMM). The radial mixing of fuel and air in a packed bed is relatively bad. The flame length increases a little bit with the particle diameter. The fuel velocity has a negligible influence on the flame length for values larger than 20 m/s. The flame length decreases with increasing the excess air numbers, especially for value lower than 1.2 the flame length increases very strong.
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