Heat Transfer to Flat Strips Immersed in a Fluidized Bed

2011 ◽  
Vol 133 (7) ◽  
Author(s):  
Christopher Penny ◽  
Dennis Rosero ◽  
David Naylor ◽  
Jacob Friedman
Keyword(s):  
Heat Transfer ◽  
Power Generation ◽  
Nusselt Number ◽  
Fluidized Bed ◽  
Manufacturing Industry ◽  
Particle Diameter ◽  
Orientation Angle ◽  
Heat Treating ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Heat transfer to objects immersed in a fluidized bed has been studied extensively across a relatively large range of geometries, though most work has looked at cylinders, a geometry important in power generation fluidized bed applications. As the power generation industry has been the primary stimulant to fluidized bed heat transfer research, very little information is available regarding geometries significant in heat treating applications. In this work, heat transfer to thin flat strips immersed in a fluidized bed is examined. This geometry is important in the steel strap manufacturing industry where many manufacturers use environmentally damaging molten lead baths to heat-treat their product. In order to determine the feasibility of a fluidized bed heat treating system as an alternative to the more hazardous lead-based process, an experimental investigation has been conducted in which Nusselt number data for flat strips with widths in the range of 6.35–25.4 mm are obtained using a laboratory-scale fluidized bed (310 mm diameter). Aluminum oxide sand particles in the range of dp=145–330 μm (50–90 grit) are used as the fluidized media within the fluidized operating range from 0.15Gmf to approximately 10Gmf. The strip orientation angle θo was also varied to establish the position from which maximum heat transfer is obtained. It was found that a decrease in particle diameter, an increase in fluidizing rate, and an increase in sample diameter resulted in an increase in Nusselt number. It was also observed that for the smaller samples tested, a maximum Nusselt number plateau was reached, at approximately G/Gmf=2.5. Finally, it was shown that an increase in θo (from 0 deg to 90 deg) resulted in an increase in Nusselt number. A correlation for the maximum Nusselt number was developed, providing excellent agreement within ±15%.

Heat Transfer to Small Horizontal Cylinders Immersed in a Fluidized Bed

2006 ◽  
Vol 128 (10) ◽  
pp. 984-989 ◽  
Author(s):  
J. Friedman ◽  
P. Koundakjian ◽  
D. Naylor ◽  
D. Rosero
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Fluidized Bed ◽  
Fluidized Beds ◽  
Manufacturing Industry ◽  
Steel Wire ◽  
Small Diameter ◽  
Heat Treating ◽  
Packed Bed ◽  
Horizontal Cylinders

Heat transfer to horizontal cylinders immersed in fluidized beds has been extensively studied, but mainly in the context of heat transfer to boiler tubes in coal-fired beds. As a result, most correlations in the literature have been derived for cylinders of 25-50mm diameter in vigorously fluidizing beds. In recent years, fluidized bed heat treating furnaces fired by natural gas have become increasingly popular, particularly in the steel wire manufacturing industry. These fluidized beds typically operate at relatively low fluidizing rates (G∕Gmf<5) and with small diameter wires (1-6mm). Nusselt number correlations developed based on boiler tube studies do not extrapolate down to these small size ranges and low fluidizing rates. In order to obtain reliable Nusselt number data for these size ranges, an experimental investigation has been undertaken using two heat treating fluidized beds; one a pilot-scale industrial unit and the other a lab-scale (300mm diameter) unit. Heat transfer measurements were obtained using resistively heated cylindrical samples ranging from 1.3 to 9.5mm in diameter at fluidizing rates ranging from approximately 0.5×Gmf (packed bed condition) to over 10×Gmf using aluminum oxide sand particles ranging from dp=145-330μm (50–90 grit). It has been found that for all cylinder sizes tested, the Nusselt number reaches a maximum near 2×Gmf, then remains relatively steady (±5-10%) to the maximum fluidizing rate tested, typically 8-12×Gmf. A correlation for maximum Nusselt number is developed.

Heat Transfer to Small Cylinders Immersed in a Fluidized Bed

2005 ◽  
Author(s):  
Jacob Friedman ◽  
Polo Koundakjian ◽  
Dennis Rosero
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Fluidized Bed ◽  
Fluidized Beds ◽  
Manufacturing Industry ◽  
Steel Wire ◽  
Small Diameter ◽  
Heat Treating ◽  
Packed Bed ◽  
Pilot Scale

Heat transfer to horizontal cylinders immersed in fluidized beds has been extensively studied, but mainly in the context of heat transfer to boiler tubes in coal-fired beds. As a result, most correlations in the literature have been derived for cylinders of 25–50mm diameter in vigorously fluidizing beds. In recent years, fluidized bed heat treating furnaces fired by natural gas have become increasingly popular, particularly in the steel wire manufacturing industry. These fluidized beds typically operate at relatively low fluidizing rates (G/Gmf < 5) and with small diameter wires (1–6mm). Nusselt number correlations developed based on boiler tube studies do not extrapolate down to these small size ranges and low fluidizing rates. In order to obtain reliable Nusselt number data for these size ranges, an experimental investigation has been undertaken using two heat treating fluidized beds; one a pilot-scale industrial unit and the other a lab-scale (300mm diameter) unit. Heat transfer measurements were obtained using resistively heated cylindrical samples ranging from 1.3 mm to 9.5 mm in diameter at fluidizing rates ranging from approximately 0.5 × Gmf (packed bed condition) to over 10 × Gmf using aluminum oxide sand particles ranging from dp = 145–330 μm (50 to 90 grit). It has been found that for all cylinder sizes tested, the Nusselt number reaches a maximum near 2 × Gmf, then remains relatively steady (± 5–10%) to the maximum fluidizing rate tested, typically 8–12 × Gmf. A correlation for maximum Nusselt number is developed.

Effect of Periodicity of Non-Uniform Sinusoidal Side Heating on Natural Convection in an Anisotropic Porous-Enclosure

2011 ◽  
Vol 110-116 ◽  
pp. 1613-1618 ◽  
Author(s):  
S. Kapoor ◽  
P. Bera
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Stream Function ◽  
Numerical Study ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Porous Enclosure

A comprehensive numerical study on the natural convection in a hydrodynamically anisotropic as well as isotropic porous enclosure is presented, flow is induced by non uniform sinusoidal heating of the right wall of the enclosure. The principal directions of the permeability tensor has been taken oblique to the gravity vector. The spectral Element method has been adopted to solve numerically the governing differential equations by using the vorticity-stream-function approach. The results are presented in terms of stream function, temperature profile and Nusselt number. The result show that the maximum heat transfer takes place at y = 1.5 when N is odd.. Also, increasing media permeability, by changing K* = 1 to K* = 0.2, increases heat transfer rate at below and above right corner of the enclosure. Furthermore, for the all values of N, profiles of local Nusselt number (Nuy) in isotropic as well as anisotropic media are similar, but for even values of N differ slightly at N = 2.. In particular the present analysis shows that, different periodicity (N) of temperature boundary condition has the significant effect on the flow pattern and consequently on the local heat transfer phenomena.

Enhancement of Heat Transfer to an Air Jet Impinging on a Flat Plate

Volume 1 ◽  
2004 ◽  
Author(s):  
D. P. Mishra ◽  
D. Mishra
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Flat Plate ◽  
Average Nusselt Number ◽  
Cooling System ◽  
Reynolds Numbers ◽  
Heated Plate ◽  
Maximum Heat ◽  
Air Jet ◽  
Maximum Heat Transfer

An experimental investigation of the impinging jet cooling from a heated flat plate has been carried out for several Reynolds numbers (Re) and nozzle to plate distances. The present results indicate that the maximum heat transfer occurs from the heated plate at stagnation point and decreases with radial distances for all cases. The maximum value of the stagnation as well as average Nusselt number is found to occur at separation distance, H/D = 6.0 for Re = 55000. An attempt is also made to study effects of nozzle exit configuration on the heat transfer using a sharp edged orifice for same set of Reynolds numbers and nozzle to plate distance. The stagnation Nusselt numbers of sharp orifice jets are found to be enhanced by around 16–21.4% in comparison to that of square edged orifice. However, the enhancement in the average Nusselt number of sharp orifice is found to be in the range of 7–18.9% as compared to the square edged orifice. The maximum enhancement of 18.9% in average Nu is achieved for Re = 55 000 at H/D = 6. Two separate correlations in terms of Nuo, Re, H/D for both square and sharp edged orifice are obtained which will be useful for designing impinging cooling system.

Heat Transfer Due to Natural Convection in a Periodically Heated Slot

2012 ◽  
Vol 135 (2) ◽  
Author(s):  
M. Z. Hossain ◽  
J. M. Floryan
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Temperature Field ◽  
Natural Convection ◽  
Average Nusselt Number ◽  
Horizontal Direction ◽  
Wave Number ◽  
Lower Wall ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Heat transfer resulting from the natural convection in a fluid layer contained in an infinite horizontal slot bounded by solid walls and subject to a spatially periodic heating at the lower wall has been investigated. The heating produces sinusoidal temperature variations along one horizontal direction characterized by the wave number α with the amplitude expressed in terms of a suitably defined Rayleigh number Rap. The maximum heat transfer takes place for the heating with the wave numbers α = 0(1) as this leads to the most intense convection. The intensity of convection decreases proportionally to α when α→0, resulting in the temperature field being dominated by periodic conduction with the average Nusselt number decreasing proportionally to α2. When α→∞, the convection is confined to a thin layer adjacent to the lower wall with its intensity decreasing proportionally to α−3. The temperature field above the convection layer looses dependence on the horizontal direction. The bulk of the fluid sees the thin convective layer as a “hot wall.” The heat transfer between the walls becomes dominated by conduction driven by a uniform vertical temperature gradient which decreases proportionally to the intensity of convection resulting in the average Nusselt number decreasing as α−3. It is shown that processes described above occur for Prandtl numbers 0.001 < Pr < 10 considered in this study.

Enhancement of Heat Transfer Behind Sliding Bubbles

2007 ◽  
Author(s):  
Marcelino Figueroa ◽  
D. Keith Hollingsworth ◽  
Larry C. Witte
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Temperature Response ◽  
Thin Foil ◽  
Lower Surface ◽  
Camera System ◽  
Maximum Heat ◽  
Maximum Heat Transfer

The time-dependent temperature distribution on an inclined, thin-foil uniform-heat-generation heater was used to infer the surface heat transfer enhancement caused by the passage of an FC-87 bubble sliding beneath the lower surface of the heater. A two-camera system was used: one camera recorded color images of a liquid crystal layer applied to the upper (dry) side of the heater while a second camera simultaneously recorded the position, size, and shape of the bubble from below. The temperature response of the heater could then be correlated directly to the bubble characteristics at any given time during its passage. Data along the line bisecting the bubble wake from 9 bubbles comprising 54 bubble images were analyzed. Heat transfer in the wake behind sliding cap-shaped bubbles is very effective compared to the natural convection that occurs before the passage of the bubble. Maximum values of heat transfer coefficient in the range of 2500 W/m2K were produced in very sharply peaked curves. The point of maximum cooling measured as a fraction of the local driving temperature difference before the bubble passage was identified and correlated with some success to the streamwise length of the bubble. The location of the maximum heat transfer coefficient was reasonably correlated to bubble width. The level of the maximum heat transfer coefficient when cast as a Nusselt number based on bubble width grew to a saturation value as the bubble moved across the plate. A constant value of Nusselt number requires that the heat transfer coefficient falls as the bubble grows past some critical bubble size. This behavior was observed for the larger bubbles.

A Correlation for Maximum Heat Transfer to Cylinders and Spheres in Gas-Fluidized Beds

2018 ◽  
Author(s):  
Mirza M. Shah
Keyword(s):  
Heat Transfer ◽  
Fluidized Beds ◽  
Particle Diameter ◽  
Absolute Deviation ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Surface Diameter ◽  
Data Points ◽  
Gas Fluidized Beds ◽  
Cylinders And Spheres

A general correlation is presented for predicting maximum heat transfer coefficient for surfaces submerged in gas-fluidized beds. It has been verified with data for horizontal and vertical cylinders and spheres in beds of a wide variety of particles and gases. The gases include air, cryogens, methane, CO2, ammonia, and R-12. The range of parameters includes: heat transfer surface diameter 0.05 to 220 mm, particle diameter 31 to 15000 μm, pressure 0.026 to 0.95 MPa, and temperature 13 to 1028 °C. The 363 data points from 53 sources are predicted with a mean absolute deviation of 16.2 %. Several other correlations were also compared to the same data but had much larger deviations.

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