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Title: Mixed convection through vertical porous annuli locally heated from the inner cylinder

Abstract

The purpose of the present study is to examine the influence of both aiding and opposing external flows on the buoyancy-induced natural convection in vertical porous annuli. The effects of radius ratio are taken into account in a numerical study. Measurements of heat transfer coefficients in aiding and opposing flows cover the free to forced convective heat transfer regimes. Mixed convection in a vertical annulus filled with a saturated porous medium is numerically and experimentally investigated. Calculations are carried out under the traditional Darcy assumptions and cover the ranges 10 {le} Ra {le} 200 and 0.01 {le} Pe {le} 200. Both numerical and experimental results show that the Nusselt number increases with either Ra or Pe when the imposed flow is in the same direction as the buoyancy-induced flow. When the imposed flow opposes buoyancy-induced flow, the Nusselt number first decreases with an increase of the Peclet number and reaches a minimum before increasing again. Under certain circumstances, the Nusselt number for a lower Rayleigh number may exceed that for larger value. Nusselt numbers are correlated by the parameter groups Nu/Pe{sup 1/2} and Ra/Pe{sup 3/2}. Good agreement exists between measured and predicted Nusselt numbers, and the occurrence of a minimummore » Nusselt number in mean flow that opposes buoyancy is verified experimentally.« less

Authors:
 [1];  [2]
  1. (Univ. of Arizona, Tucson (United States))
  2. (Colorado State Univ., Fort Collins (United States))
Publication Date:
OSTI Identifier:
5020244
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Heat Transfer (Transactions of the ASME (American Society of Mechanical Engineers), Series C); (United States); Journal Volume: 114:1
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING; ANNULAR SPACE; CONVECTION; POROUS MATERIALS; BENCH-SCALE EXPERIMENTS; CALCULATION METHODS; CYLINDERS; DARCY LAW; NUMERICAL ANALYSIS; NUSSELT NUMBER; CONFIGURATION; ENERGY TRANSFER; HEAT TRANSFER; MASS TRANSFER; MATERIALS; MATHEMATICS; SPACE; 420400* - Engineering- Heat Transfer & Fluid Flow

Citation Formats

Choi, C.Y., and Kulacki, F.A. Mixed convection through vertical porous annuli locally heated from the inner cylinder. United States: N. p., 1992. Web. doi:10.1115/1.2911239.
Choi, C.Y., & Kulacki, F.A. Mixed convection through vertical porous annuli locally heated from the inner cylinder. United States. doi:10.1115/1.2911239.
Choi, C.Y., and Kulacki, F.A. Sat . "Mixed convection through vertical porous annuli locally heated from the inner cylinder". United States. doi:10.1115/1.2911239.
@article{osti_5020244,
title = {Mixed convection through vertical porous annuli locally heated from the inner cylinder},
author = {Choi, C.Y. and Kulacki, F.A.},
abstractNote = {The purpose of the present study is to examine the influence of both aiding and opposing external flows on the buoyancy-induced natural convection in vertical porous annuli. The effects of radius ratio are taken into account in a numerical study. Measurements of heat transfer coefficients in aiding and opposing flows cover the free to forced convective heat transfer regimes. Mixed convection in a vertical annulus filled with a saturated porous medium is numerically and experimentally investigated. Calculations are carried out under the traditional Darcy assumptions and cover the ranges 10 {le} Ra {le} 200 and 0.01 {le} Pe {le} 200. Both numerical and experimental results show that the Nusselt number increases with either Ra or Pe when the imposed flow is in the same direction as the buoyancy-induced flow. When the imposed flow opposes buoyancy-induced flow, the Nusselt number first decreases with an increase of the Peclet number and reaches a minimum before increasing again. Under certain circumstances, the Nusselt number for a lower Rayleigh number may exceed that for larger value. Nusselt numbers are correlated by the parameter groups Nu/Pe{sup 1/2} and Ra/Pe{sup 3/2}. Good agreement exists between measured and predicted Nusselt numbers, and the occurrence of a minimum Nusselt number in mean flow that opposes buoyancy is verified experimentally.},
doi = {10.1115/1.2911239},
journal = {Journal of Heat Transfer (Transactions of the ASME (American Society of Mechanical Engineers), Series C); (United States)},
number = ,
volume = 114:1,
place = {United States},
year = {Sat Feb 01 00:00:00 EST 1992},
month = {Sat Feb 01 00:00:00 EST 1992}
}
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  • Low Rayleigh number (R < 500) thermal convection is examined experimentally and analytically for a vertical circular cylinder filled with saturated porous materials. The cylinder is heated from below and cooled from above. The upper boundary is permeable to flow. Results are reported for the critical Rayleigh number at the onset of convection (Rc) and the Nusselt (Nu) vs. Rayleigh (R) number correlation. The convective structure is deduced from temperature measurements. The lateral walls have a stabilizing effect and tend to increase Rc relative to the case of a laterally unbounded porous layer. The preferred convective patterns in the cylindermore » tend to be nonaxisymmetric. The experimental results are in good agreement with analytic predictions. A linear stability analysis is used to calculate Rc and the structure of the convective modes for a range of aspect ratios (radius/height). An integral technique is applied to obtain heat transfer rates for 1 < R/Rc < 5.5. 20 references.« less
  • MHD mixed convection flow about a vertical cylinder embedded in a porous medium is considered using non-Darcian model. Variable heat transfer boundary condition is incorporated. A transformation that enables solving for the entire mixed convection regime is introduced. Results are obtained using a finite difference scheme. The effect of the applied magnetic field on the heat transfer coefficient and on the wall shear stress is presented for the entire mixed convection regime including pure forced and pure natural convection limits. The magnetic field is found to have different behavior in the forced convection dominated regime other than that in naturalmore » convection dominated regime.« less
  • A numerical analysis is made of the unsteady flow and heat transfer characteristics of mixed convection in a vertical block-heated channel with and without installing an inclined plate above an upstream block. Parameter studies including the inclined plate angle, Reynolds number (ranging from 260 to 530), and Grashof number (in the range of 0--3,200,000) on heat transfer performance have been explored in detail. The results show that the installation of an inclined plate in the vertical block-heated channel can effectively augment the blocks` heat transfer performance in the channel. This can be applied to heat exchangers.
  • A numerical investigation has been conducted to evaluate the effects of diameter ratio and aspect ratio in natural convection of gases within vertical annuli. The inner cylinder is maintained at uniform heat flux and the outer cylinder at constant temperature. The horizontal top and bottom walls are insulated. Detailed result of {le} 15, 1 {le} A {le} 10, and 100 < Ra{sub L}* < 10{sup 7}. The inner wall temperature is a function of diameter ratio and aspect ratio. The heat transfer results have been compared with those for isothermal heating, and have been found to be higher. The innermore » wall temperature is a function of diameter ratio and aspect ratio. The heat transfer results have been compared with those for isothermal heating, and have been found to be higher. The inner diameter is seen to be the appropriate length scale for high Rayleigh number flows and/or high radius ratios, and the radius ratio effect on heat transfer is seen to be insignificant for radius ratios greater than 10. The heat transfer results based on the inner diameter are in very good agreement with published experimental results, although these experiments were conducted for very high aspect ratio. Heat transfer correlations are provided.« less