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Title: Numerical simulation of supercritical heat transfer under severe axial density gradient in a narrow vertical tube

Abstract

A number of computational works have been performed so far for the simulation of heat transfer in a supercritical fluid. The simulations, however, faced a lot of difficulties when heat transfer deteriorates due either to buoyancy or by acceleration. When the bulk temperature approaches the pseudo-critical temperature the fluid experiences a severe axial density gradient on top of a severe radial one. Earlier numerical calculations showed, without exception, unrealistic over-predictions, as soon as the bulk temperature exceeded the pseudo-critical temperature. The over-predictions might have been resulted from an inapplicability of widely-used turbulence models. One of the major causes for the difficulties may probably be an assumption of a constant turbulent Prandtl number. Recent research, both numerical and experimental, indicates that the turbulent Prandtl number is never a constant when the gradient of physical properties is significant. This paper describes the applicability of a variable turbulent Prandtl number to the numerical simulation of heat transfer in supercritical fluids flowing in narrow vertical tubes. (authors)

Authors:
; ;  [1]
  1. Korea Atomic Energy Research Inst., 1045 Daedeokdaero, Daejeon (Korea, Republic of)
Publication Date:
Research Org.:
American Nuclear Society, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
OSTI Identifier:
22105945
Resource Type:
Conference
Resource Relation:
Conference: ICAPP '12: 2012 International Congress on Advances in Nuclear Power Plants, Chicago, IL (United States), 24-28 Jun 2012; Other Information: Country of input: France; 37 refs.; Related Information: In: Proceedings of the 2012 International Congress on Advances in Nuclear Power Plants - ICAPP '12| 2799 p.
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING; 22 GENERAL STUDIES OF NUCLEAR REACTORS; COMPUTERIZED SIMULATION; CRITICAL TEMPERATURE; DENSITY; FLUIDS; HEAT TRANSFER; PRANDTL NUMBER; TUBES; TURBULENT FLOW

Citation Formats

Bae, Y. Y., Hong, S. D., and Kim, Y. W.. Numerical simulation of supercritical heat transfer under severe axial density gradient in a narrow vertical tube. United States: N. p., 2012. Web.
Bae, Y. Y., Hong, S. D., & Kim, Y. W.. Numerical simulation of supercritical heat transfer under severe axial density gradient in a narrow vertical tube. United States.
Bae, Y. Y., Hong, S. D., and Kim, Y. W.. 2012. "Numerical simulation of supercritical heat transfer under severe axial density gradient in a narrow vertical tube". United States. doi:.
@article{osti_22105945,
title = {Numerical simulation of supercritical heat transfer under severe axial density gradient in a narrow vertical tube},
author = {Bae, Y. Y. and Hong, S. D. and Kim, Y. W.},
abstractNote = {A number of computational works have been performed so far for the simulation of heat transfer in a supercritical fluid. The simulations, however, faced a lot of difficulties when heat transfer deteriorates due either to buoyancy or by acceleration. When the bulk temperature approaches the pseudo-critical temperature the fluid experiences a severe axial density gradient on top of a severe radial one. Earlier numerical calculations showed, without exception, unrealistic over-predictions, as soon as the bulk temperature exceeded the pseudo-critical temperature. The over-predictions might have been resulted from an inapplicability of widely-used turbulence models. One of the major causes for the difficulties may probably be an assumption of a constant turbulent Prandtl number. Recent research, both numerical and experimental, indicates that the turbulent Prandtl number is never a constant when the gradient of physical properties is significant. This paper describes the applicability of a variable turbulent Prandtl number to the numerical simulation of heat transfer in supercritical fluids flowing in narrow vertical tubes. (authors)},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2012,
month = 7
}

Conference:
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  • The vertical upward flow of water in a heated tube at supercritical pressure is numerically simulated by a commercially available computational fluid dynamics code. The IAPWS-95 formulation is used to obtain the water properties, which vary substantially at supercritical condition. To match the simulation with the experiment performed by Yamagata et al. (Int. J. Heat Mass Transfer, Vol. 15, 1972), the mass velocity is set to be 1260 kg/m{sup 2}xs and the wall heat fluxes 233, 465, 698, and 930 kW/m{sup 2}. To examine the reliability of the turbulence model at the supercritical flow, a series of simulations are performedmore » with turbulence models: several Low- Reynolds number k-{epsilon} models, k-{omega} model, SST k-{omega}model, standard k-{epsilon} model, RNG k-{epsilon} model, and realizable k-{epsilon} model. For the last three turbulence models, the standard wall function and the enhanced wall treatment are used as wall boundary conditions. It is found that the predicted wall temperature is sensitive to the height of the grid next to the wall when the bulk enthalpy is around the pseudo-critical point. When the standard wall function is used, results from the RNG k-{epsilon} model and the standard k-{epsilon} model are identical, and the wall temperature predictions are lower than the experiment. Conversely, the predicted wall temperature is higher in the simulations with Low- Reynolds number k-{epsilon} models. The temperature difference between the predictions and the experiment becomes larger as the wall heat flux increases. Low-Reynolds number k-{epsilon} models result in extremely higher wall temperature than the experiment at the highest wall heat flux. Low turbulence kinetic energy resulting from the Low-Reynolds k-{epsilon} models reduces the heat transfer and causes higher wall temperature than the experiment. The mean flow fields and turbulence properties from each turbulence model are examined. It seems that the acceleration, which is caused by the density reduction as the bulk temperature increases, and the buoyancy lead to the inadequate prediction. Modification of the turbulence transport equation may be required to overcome these effects on the prediction of the wall temperature. (authors)« less
  • No abstract prepared.
  • Convection heat transfer of CO{sub 2} at supercritical pressures in a 0.27 mm diameter vertical mini tube was investigated experimentally and numerically for upward and downward flows at relatively low inlet Reynolds numbers (2900 and 1900). The effects of inlet temperature, pressure, mass flow rate, heat flux, flow direction, buoyancy and flow acceleration on the convection heat transfer were investigated. For inlet Reynolds numbers less than 2.9 x 10{sup 3}, the local wall temperature varies non-linearly for both flow directions at high heat fluxes (113 kW/m{sup 2}). For the mini tube used in the current study, the buoyancy effect ismore » normally low even when the heating is relatively strong, while the flow acceleration due to heating can strongly influence the turbulence and reduce the heat transfer for high heat fluxes. For relatively low Reynolds numbers (Re{sub in} {<=} 2.9 x 10{sup 3}) and the low heat flux (30.0 kW/m{sup 2}) the predicted values using the LB low Reynolds number correspond well with the measured data. However, for the high heat flux (113 kW/m{sup 2}), the predicted values do not correspond well with the measured data due to the influence of the flow acceleration on the turbulence. (author)« less
  • The convection heat transfer characteristics of supercritical CO{sub 2} in a vertical circular tube of 2 mm inner diameter were investigated experimentally for pressures ranging from 78 to 95 bar, inlet temperatures from 25 to 40 C, and inlet Reynolds numbers from 3800 to 20,000. The effects of the heat flux, thermo-physical properties, buoyancy and thermal acceleration on the convection heat transfer were analyzed. The experimental results show that for high inlet Reynolds numbers (e.g. Re = 9000) and high heat fluxes, a significant local deterioration and recovery of the heat transfer was found for upward flows but not formore » downward flows. Comparison of the experimental data for inlet Reynolds numbers from 3800 to 20,000 with some well-known empirical correlations showed large differences especially when the heat transfer deteriorates and then recovers when the effect of buoyancy is significant. The experimental data was used to develop modified local turbulent Nusselt number correlations for supercritical CO{sub 2} flowing in vertical small circular tubes. (author)« less
  • Water wall design is a key issue for supercritical Circulating Fluidized Bed (CFB) boiler. On account of the good heat transfer performance, rifled tube is applied in the water wall design of a 600 MW supercritical CFB boiler in China. In order to investigate the heat transfer and frictional characteristics of the rifled tube with vertical upward flow, an in-depth experiment was conducted in the range of pressure from 12 to 30 MPa, mass flux from 230 to 1200 kg/(m{sup 2} s), and inner wall heat flux from 130 to 720 kW/m{sup 2}. The wall temperature distribution and pressure dropmore » in the rifled tube were obtained in the experiment. The normal, enhanced and deteriorated heat transfer characteristics were also captured. In this paper, the effects of pressure, inner wall heat flux and mass flux on heat transfer characteristics are analyzed, the heat transfer mechanism and the frictional resistance performance are discussed, and the corresponding empirical correlations are presented. The experimental results show that the rifled tube can effectively prevent the occurrence of departure from nucleate boiling (DNB) and keep the tube wall temperature in a permissible range under the operating condition of supercritical CFB boiler. (author)« less