A Network-of-Voids Model to Assess Wall Flow Patterns and Heat Transfer for Low Aspect Ratio Packed-Bed Reactors

2008 ◽  
Vol 6 (1) ◽  
Author(s):  
Peter I Chigada ◽  
Reginald Mann
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Fluid Flows ◽  
Packed Bed ◽  
Wall Heat Transfer ◽  
Transfer Coefficients ◽  
Catalytic Reactors ◽  
Heat Transfer Coefficients ◽  
Low Aspect Ratio ◽  
Random Placement

Exothermic packed bed catalytic reactors are usually characterised by a low diameter-aspect ratio to facilitate heat transfer. In operation, these reactors often exhibit localized regions with much higher temperatures referred to as hot spots. A new model based on a 2-D network-of voids (NoV) has been devised to explore wall heat transfer behaviour for such low aspect ratio packed tubes. Random placement of (packing) particles is used to provide a simple NoV framework for implicitly creating the tortuous fluid flows amongst the resulting randomized inter-connecting voids. This is a computationally tractable strategy for exploring the haphazard appearance of individual tube pin-hole burn-outs amongst the typically thousands, or tens of thousands, of tubes within high temperature industrial multi-tubular configurations. Although presently limited to 2-D, the model captures many natural features of the flow and heat transfer of randomly packed tubes, especially huge variations in wall and cross flows and consequently massive variations in local wall heat transfer coefficients along the length of individual tubes. The model is potentially superior to those based upon averaged properties, which do not properly distinguish the solid and fluid phases. The network-of-voids concept is readily extended to 3-D, in order to achieve geometric congruence of the model and assemblies of individual particles.

Heat Transfer with Chemical Reaction in Wall Heated Packed Bed Reactor

2014 ◽  
Vol 625 ◽  
pp. 722-725
Author(s):  
Duvvuri Subbarao ◽  
Reem Hassan Abd Elghafoor Hassan ◽  
Marappagounder Ramasamy
Keyword(s):  
Heat Transfer ◽  
Model Equation ◽  
Packed Bed ◽  
Packed Bed Reactor ◽  
Wall Heat Transfer ◽  
Transfer Coefficients ◽  
Exothermic Reactions ◽  
Packed Bed Reactors ◽  
Heat Transfer Coefficients ◽  
Endothermic Reactions

Information on wall heat transfer to packed bed reactors operating with exothermic or endothermic reactions is scarce. Overall wall heat transfer coefficients in a packed bed reactor in presence of an endothermic reaction are measured and observed to be smaller than the expected in the absence of reaction. This observation is in contrast with the reported observations with exothermic reactions in packed beds. A model equation based on energy balance is presented to explain the observations.

scholarly journals Experimental Heat Transfer Investigation of Stationary and Orthogonally Rotating Asymmetric and Symmetric Heated Smooth and Turbulated Channels

1992 ◽  
Author(s):  
H. A. El-Husayni ◽  
M. E. Taslim ◽  
D. M. Kercher
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Rotation Number ◽  
Symmetric Case ◽  
Wall Heat Flux ◽  
Heated Wall ◽  
Wall Heat Transfer ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Smooth Channel

An experimental investigation was conducted to determine the effects of variations in wall thermal boundary conditions on local heat transfer coefficients in stationary and orthogonally rotating smooth wall and two opposite-wall turbulated square channels. Results were obtained for three distributions of uniform wall heat flux: asymmetric, applied to the primary wall only; symmetric, applied to two opposite walls only; and fully-symmetric, applied to all four channel walls. Measured stationary and rotating smooth channel average heat transfer coefficients at channel location L/Dh = 9.53 were not significantly sensitive to wall heat flux distributions. Trailing side heat transfer generally increased with Rotation number whereas the leading wall results showed a decreasing trend at low Rotation numbers to a minimum and then an increasing trend with further increase in Rotation number. The stationary turbulated wall heat transfer coefficients did not vary markedly with the variations in wall heat flux distributions. Rotating leading wall heat transfer decreased with Rotation number and showed little sensitivity to heat flux distributions except for the fully-symmetric heated wall case at the highest Reynolds number tested. Trailing wall heat transfer coefficients were sensitive to the thermal wall distributions generally at all Reynolds numbers tested and particularly with increasing Rotation number. While the asymmetric case showed a slight deficit in trailing wall heat transfer coefficients due to rotation, the symmetric case indicated little change whereas the fully-symmetric case exhibited an enhancement.

scholarly journals Heat transfer in fluidized and fixed beds of adsorption chillers

2019 ◽  
Vol 128 ◽  
pp. 01003 ◽  
Author(s):  
Jaroslaw Krzywanski ◽  
Karolina Grabowska ◽  
Marcin Sosnowski ◽  
Anna Zylka ◽  
Anna Kulakowska ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Fluidized Bed ◽  
Fluidized Beds ◽  
Fixed Bed ◽  
Wall Heat Transfer ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Innovative Idea ◽  
Fixed Beds

An innovative idea, shown in the paper constitutes in the use of the fluidized bed of sorbent, instead of the conventional, fixed-bed, commonly used in the adsorption chillers. Bed–to–wall heat transfer coefficients for fixed and fluidized beds of adsorbent are determined. Sorbent particles diameters and velocities of fluidizing gas are discussed in the study. The calculations confirmed, that the bed–to–wall heat transfer coefficient in the fluidized bed of adsorbent is muchhigher than that in a conventional bed.

Low-Aspect-Ratio Rib Heat Transfer Coefficient Measurements in a Square Channel

1998 ◽  
Vol 120 (4) ◽  
pp. 831-838 ◽  
Author(s):  
M. E. Taslim ◽  
G. J. Korotky
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Aspect Ratio ◽  
Transfer Coefficient ◽  
Flow Direction ◽  
Turbine Blades ◽  
Transfer Coefficients ◽  
Square Channel ◽  
Low Aspect Ratio ◽  
The Heat Transfer Coefficient

Cooling channels, roughened with repeated ribs, are commonly employed as a means of cooling turbine blades. The increased level of mixing induced by these ribs enhances the convective heat transfer in the blade cooling cavities. Many previous investigations have focused on the heat transfer coefficient on the surfaces between these ribs and only a few studies report the heat transfer coefficient on the rib surfaces themselves. The present study investigated the heat transfer coefficient on the surfaces of round-corner, low-aspect-ratio (ARrib = 0.667) ribs. Twelve rib geometries, comprising three rib height-to-channel hydraulic diameters (blockage ratios) of 0.133, 0.167, and 0.25 as well as three rib spacings (pitch-to-height ratios) of 5, 8.5, and 10 were investigated for two distinct thermal boundary conditions of heated and unheated channel walls. A square channel, roughened with low-aspect-ratio ribs on two opposite walls in a staggered manner and perpendicular to the flow direction, was tested. An instrumented copper rib was positioned either in the middle of the rib arrangements or in the furthest upstream location. Both rib heat transfer coefficient and channel friction factor for these low-aspect-ratio ribs were also compared with those of square ribs, reported previously by the authors. Heat transfer coefficients of the furthest upstream rib and that of a typical rib located in the middle of the rib-roughened region of the passage wall were also compared.

Effect of Rib Configuration and Lateral Ejection on a High Aspect Ratio Trailing Edge Channel

2013 ◽  
Author(s):  
Rui Kan ◽  
Li Yang ◽  
Jing Ren ◽  
Hongde Jiang
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Local Heat Transfer ◽  
Large Aspect Ratio ◽  
Thermochromic Liquid Crystal ◽  
Transfer Coefficients ◽  
Trailing Edge ◽  
Average Heat ◽  
Edge Channel ◽  
Heat Transfer Coefficients

Thermal issues of gas turbine blade trailing edge are complex and challenging, due to limited space for effective cooling. The trailing edge cavities are usually large aspect ratio ducts with lateral ejection. The objective of this study is to investigate the effects of different rib configurations and lateral ejection on heat transfer characteristics inside a trailing edge channel. The investigations were conducted on a large aspect ratio wedge-shaped channel with square ribs of e/Dh = 0.05, under Reynolds number 15,000. Twelve different configurations were tested: 1)three rib types, including a symmetry V-shaped rib configuration and two non-symmetry V-shaped rib configurations, of which the rib angles are 60 degrees, 2) two rib pitches, P/e = 10 and P/e = 5, 3) two flow directions, with an open tip outlet or with lateral ejection. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal experimental method. The configurations were also investigated numerically for the detailed flow field and for the validation of CFD codes. Results show that with lateral ejection, the heat transfer coefficients decrease from inlet to outlet. The heat transfer near the ejection holes is enhanced while heat transfer coefficients near the wall opposite to the exit holes decrease. The curvature of the streamlines creates a large separation area near the end of the channel and thus results in low local heat transfer coefficients. The P/e = 10 configurations have higher average heat transfer compared with P/e = 5 configurations. Average heat transfer coefficient is the highest with the center of the V-shaped rib placed at the middle of the channel, and is the lowest when the V-shaped rib center is located near the narrow part of the channel.

Heat Transfer Coefficients for Simultaneously Developing Flow in Rectangular Tubes

2000 ◽  
Author(s):  
Srinivas Garimella ◽  
William J. Dowling ◽  
Mark Van derVeen ◽  
Jesse D. Killion
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Heat Exchangers ◽  
Operating Conditions ◽  
Tube Length ◽  
Turbulent Regime ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Developing Flow ◽  
Rectangular Tubes

Abstract A study of heat transfer in simultaneously developing flow through rectangular tubes is presented in this paper. Heat transfer coefficients were measured for three different tube sizes and shapes (Dh = 2.21 mm, α = 0.050; Dh = 3.02 mm, α = 0.108; and Dh = 1.74 mm, α = 0.029), which correspond to typical dimensions used in automotive heat exchangers. For each of these tubes, several different tube lengths were tested to measure the effect of developing flow on the Nusselt number. The study primarily focussed on the laminar and transition regimes, with some data in the turbulent regime, which is typical of the operating conditions for many automotive heat exchangers. The results demonstrate that developing flow enhances Nusselt numbers, especially for the short tubes used in heater cores, although for the geometry range studied, the effect of aspect ratio was not very significant. Heat transfer correlations were developed from the data, with excellent agreement between the data and the values predicted by these correlations. These correlations accounted for the effects of Reynolds number (118 < Re < 10671) Prandtl number (6.48 < Pr < 16.20), and bulk-to-wall property variations (0.243 < μb/μw < 0.630), and geometric features such as tube length, hydraulic diameter, and aspect ratio.

Studies on Condensation of Refrigerants in a High Aspect Ratio Microchannel and in a Novel Micro-Groove Surface Heat Exchanger: Development of Micro-Condensers in Compact Two Phase Cooling Systems

2007 ◽  
Author(s):  
Serguei Dessiatoun ◽  
Sourav Chowdhury ◽  
Ebrahim Al-Hajri ◽  
Edvin Cetegen ◽  
Michael Ohadi
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Temperature Drop ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Groove Surface ◽  
Heat Transfer Coefficients ◽  
Surface Condenser ◽  
Two Phase Cooling

Three different refrigerants, R134a, R245fa and HFE7100 were analyzed as working fluids for two-phase cooling of high heat flux electronics in a 0.7 mm hydraulic diameter 190 mm long high aspect ratio minichannel and in a newly developed micro-groove surface condenser. The latter comprised of a micro-groove surface with rectangular grooves of 84 μm in hydraulic diameter with an aspect ratio of 10.6 and headers that directed the refrigerant flow into the grooves. It was concluded that in the minichannel R245fa provides higher heat transfer coefficients compared to R134a with a significantly higher pressure drop. The saturation temperature drop in the same channel created a significant temperature drop for HFE7100 that make the application of such minichannels for cross-flow condensers with this fluid unpractical. The microgroove surface condenser provided significantly higher heat transfer coefficients compared to the minichannel condenser. The pressure drop in the micro-groove surface condenser was extremely low and imposed just 1C temperature drop on HFE7100 at it highest heat flux. The mass flux of refrigerant in the micro-groove surface condenser is significantly lower compared to conventional mini and microchannel condensers. In its current configuration, the microgroove surface condenser benefits from the possibility of an increase in mass flux resulting in a significant increase in heat transfer coefficient and just a moderate increase in pressure drop.

New Approach to Determine the Heat+transfer Coefficients for Cooling Tower Packing

1989 ◽  
Vol 203 (2) ◽  
pp. 75-84 ◽  
Author(s):  
M A Younis
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Cooling Tower ◽  
Empirical Relationship ◽  
Packed Bed ◽  
Transfer Coefficients ◽  
Water Contact ◽  
Heat Transfer Coefficients ◽  
Tower Packing

The heat input response technique was developed to determine the heat-transfer coefficients for a forced draught cooling tower packing. The method was applied to a counter current type air-water contact system in a packed bed. A temperature change was alternately imposed on inlet air at the tower bottom and on inlet water at the tower top. Outlet temperatures of air and water were measured with time. From zeroth moments of these temperature changes, water-film and air-film heat-transfer coefficients have been estimated. Finally, the effect of the water/air loading ratio and the packing material, such as wood, aluminium wire netting and plastic PVC on the values of the heat-transfer coefficients was experimentally tested. An empirical relationship between the gas-side heat-transfer coefficient, liquid-side heat-transfer coefficient and water/gas ratio has been established within the range of the tested data.

Local and average interphase heat transfer coefficients in a randomly packed bed of spheres

AIChE Journal ◽  
1968 ◽  
Vol 14 (3) ◽  
pp. 483-490 ◽  
Author(s):  
Bernard M. Gillespie ◽  
Edward D. Crandall ◽  
James J. Carberry
Keyword(s):  
Heat Transfer ◽  
Packed Bed ◽  
Transfer Coefficients ◽  
Interphase Heat Transfer ◽  
Heat Transfer Coefficients
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