수분회수 냉각탑에 적용되는 중공사막 모듈의 유동특성에 관한 수치해석적 연구

박상철(Sang Cheol Park),박현설(Hyun Seol Park),이형근(Hyung Keun Lee),신원규(Weon Gyu Shin) 대한기계학회 2017 大韓機械學會論文集B Vol.41 No.8

본 연구의 목적은 수분회수 냉각탑에 설치된 엇갈림(staggered) 형태로 배치된 중공사막 모듈을 다공성 매질(porous medium)로 모델링 시 유동특성을 수치 해석적으로 검토하는 것이다. 1단으로 설치된 중공사막 모듈의 차압 데이터를 이용하여 다공성 매질의 모델링을 위한 압력-속도 2차 식을 도출하였다. 중공사막 모듈을 다공성 매질로 모델링한 경우 (“다공성 매질”)와 중공사막 모듈의 형상을 그대로 고려한 경우 (“멤브레인 모듈”)에 대해 유동 특성을 비교하였다. “다공성 매질”의 경우 “멤브레인 모듈”에 비해 유동에 의한 압력 변화는 0.6 % 미만의 적은 차이를 나타냈으나, 최대 유속은 약 2.5배, 평균 난류 운동에너지는 95배로 크게 나타났다. 이를 통해 다공성 매질로의 모델링은 압력강하는 잘 구현하나, 유속 및 난류 운동에너지는 잘 모사하지 못함을 알 수 있었다. The purpose of this study is to analyze the flow characteristics when a staggered hollow fiber membrane module is modeled as a porous medium. The pressure-velocity equation was used for modeling the porous medium, using pressure drop data. In terms of flow characteristics, we compared the case of the "porous medium" when the membrane module was modeled as a porous medium with the case of the "membrane module" when considering the original shape of the membrane module. The difference in pressure drop between the "porous medium" and "membrane module" was less than 0.6%. However, the maximum flow velocity and mean turbulent kinetic energy of the "porous medium" were 2.5 and 95 times larger than those of the "membrane module," respectively. Our results indicate that modeling the hollow fiber module as a porous medium is useful for predicting pressure drop, but not sufficient for predicting the maximum flow velocity and mean turbulent kinetic energy.

Modeling film boiling within chimney-structured porous media and heat pipes

Yeo, D.Y.,NO, H.C. Elsevier 2018 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.124 No.-

<P><B>Abstract</B></P> <P>A porous medium with separated paths for liquid and vapor flows does not fail even after part of the porous medium is dried out. Instead, a vapor film resides within the porous medium, and it grows very slowly. This heat transfer regime was named as “confined film-boiling regime” in this study, and a heat transfer model for this regime was suggested in this paper. Especially, this paper mainly focuses on heat transfer of a CRUD (Chalk River Unidentified Deposit), which is a naturally occurring porous medium found in nuclear reactors. In the present model, the balance between capillary pressure and pressure drops of liquid and vapor flows determined thickness of the vapor film. In addition, we assumed that capillary pressure was changed with applied heat flux. This assumption was validated with a planar heat pipe case: the root-mean-square-error (RMSE) was 16% for the model with the heat flux dependent capillary pressure, while one for a model with the constant capillary pressure was 790%. Furthermore, this approach also turned out to be valid for the case of the CRUD: the model predicted the wall superheat during the film boiling of the CRUD, and its relative error was only within 20% when it was compared with the measured wall superheats. Finally, sensitivity analysis for CRUD parameters found that the heat transfer performance of the CRUD is largely sensitive to chimney density and pore size.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Applied heat can increase capillary pressure during the film-boiling regime of the porous media. </LI> <LI> Film boiling of the chimney-structured porous media and heat pipe is analogous to the capillary limit. </LI> <LI> There is the optimum density of chimneys that maximize heat transfer through the porous medium. </LI> <LI> The chimney-structured porous medium with small pores is worse in heat removal than one with large pores. </LI> </UL> </P>

An Evaluation of Performance of Numerical models for the Flow Characteristics of Automotive Aftertreatment Device

정수진(Soo-Jin Jeong),김우승(Woo-Seung Kim),강우(Woo Kang),이제형(Je-Hyung Lee) 한국자동차공학회 2006 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-

Computational Fluid Dynamics(CFD) has been widely used in the automotive industry to simulate the flow within the converter. Most of CFD models for predicting flow through converter assume a monolith resistance based on one-dimensional laminar flow and treat a monolith as porous medium. However, auto-catalyst is getting closer to engine to have fast warm-up and wide-angled diffusers are often employed to connect the upstream exhaust pipe to the monolith. Therefore, the question arises as to the accuracy of porous medium approach based on one-dimensional laminar flow and model validation studies are essential if it is to become a useful design tool. From these reasons, this paper evaluates the prediction performance of porous medium and three-dimensional multichannel model by comparing with experimental results for understanding the limitation of porous medium approach and suggesting guidelines for the development of more accurate flow model. For evaluating each models, this work calculates the flow mal distribution and pressure drop across the monolith of an axisymmetric catalyst assembly under steady state condition. The results shows that porous medium model under-predicts flow maldistributions in the monolith by 6.4% at maximum and three-dimensional multi-channel model also under-predicts by 10.3% at maximum. From the results of this work, it can be concluded that modification of porous medium approach is strongly required to account of additional entrance effect and detailed modeling of monolith channels can not be alternative for more accurate flow simulation of auto-catalyst due to the inaccuracy Averaged Navier Stokes k-ε type turbulence model under abrupt change of length scale condition.

Double resonant porous structure backed by air cavity for low frequency sound absorption improvement

Kim, Bo-Seung,Park, Junhong Elsevier 2018 COMPOSITE STRUCTURES -BARKING THEN OXFORD- Vol.183 No.-

<P><B>Abstract</B></P> <P>In this study, multi band sound absorbing structure with double resonant porous structure employing a slit porous medium was proposed. Sound absorbing materials are widely used for noise reduction in various industrial fields. When the thickness of the porous medium increases, better absorption efficiency occurs at low frequency bands. Since the thickness should be small, the noise reduction performance at low frequencies is unacceptable for practical applications. The proposed structure was composed of a slit absorber in helical shape attached to a perforated membrane backed by an air cavity. Its acoustic characteristics were controlled by adjusting the design parameters that are essential for specific noise control applications. To confirm the acoustic characteristics, three samples were made and comparison analysis was performed. Sound absorption coefficients were measured using two-microphone impedance tube to verify the acoustic performance. The proposed structure exhibited better low-frequency sound absorption compared with the results of a conventional porous medium. The measurements showed significant sound absorption improvement at low frequency bands without sacrificing performance in the mid- and high-frequency bands. Acoustic impedance of the DRPS was predicted and compared with the measured results. The sound absorption mechanism was composed of a Helmholtz resonator, a perforated membrane and the effect of the slit porous medium. A parametric study was conducted for achieving optimal sound absorption efficiency.</P>

Waste heat recovery of diesel engine using porous medium-assisted thermoelectric generator equipped with customized thermoelectric modules

Choi, Young,Negash, Assmelash,Kim, Tae Young Elsevier 2019 Energy conversion and management Vol.197 No.-

<P><B>Abstract</B></P> <P>In this study, thirty of customized bismuth-telluride (Bi<SUB>2</SUB>Te<SUB>3</SUB>) thermoelectric modules (TEMs) were fabricated for waste heat recovery of a diesel engine using a thermoelectric generator (TEG). By installing a plate-type porous medium whose porosity ranges from 0.121 to 0.516 in the TEG, the effects of the porosity on energy harvesting performance were investigated. Experimental results show that at the highest engine rotation speed of 1400 rpm, a maximum power output of 98.3 W was obtained using the lowest porosity (0.121), and a maximum energy conversion efficiency of 2.83% was obtained using the optimal porosity (0.416). The most significant improvements in the power output and conversion efficiency compared with the base case without porous media were 44.5% and 10.1% with porosities of 0.121 and 0.416, respectively, at the lowest engine speed of 1000 rpm. We concluded that the conversion efficiency and power output of the present TEG can be maximized via application of porous media with porosities of 0.461 and 0.32, respectively. The use of a porous medium with a porosity of <0.32 in the present TEG configuration should be avoided, as the backpressure would exceed the allowable limit of ~3 kPa for a passenger vehicle.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Bi<SUB>2</SUB>Te<SUB>3</SUB> thermoelectric modules were newly constructed for improved open circuit voltages. </LI> <LI> The performance of a thermoelectric generator was investigated for a diesel engine. </LI> <LI> The effects of porosity of the media on the power output and conversion efficiency were determined. </LI> <LI> Pressure drop measurements guide the optimal use of the porous medium. </LI> </UL> </P>

A comparison study between the realistic random modeling and simplified porous medium for gamma-gamma well-logging

Rasouli Fatemeh S. 한국원자력학회 2024 Nuclear Engineering and Technology Vol.56 No.5

The accurate determination of formation density and the physical properties of rocks is the most critical logging tasks which can be obtained using gamma-ray transport and detection tools. Though the simulation works published so far have considerably improved the knowledge of the parameters that govern the responses of the detectors in these tools, recent studies have found considerable differences between the results of using a conventional model of a homogeneous mixture of formation and fluid and an inhomogeneous fractured medium. It has increased concerns about the importance of the complexity of the model used for the medium in simulation works. In the present study, we have suggested two various models for the flow of the fluid in porous media and fractured rock to be used for logging purposes. For a typical gamma-gamma logging tool containing a137Cs source and two NaI detectors, simulated by using the MCNPX code, a simplified porous (SP) model in which the formation is filled with elongated rectangular cubes loaded with either mineral material or oil was investigated. In this model, the oil directly reaches the top of the medium and the connection between the pores is not guaranteed. In the other model, the medium is a large 3-D matrix of 1 cm3 randomly filled cubes. The designed algorithm to fill the matrix sites is so that this realistic random (RR) model provides the continuum growth of oil flow in various disordered directions and, therefore, fulfills the concerns about modeling the rock textures consist of extremely complex pore structures. For an arbitrary set of oil concentrations and various formation materials, the response of the detectors in the logging tool has been considered as a criterion to assess the effect of modeling for the distribution of pores in the formation on simulation studies. The results show that defining a RR model for describing heterogeneities of a porous medium does not effectively improve the prediction of the responses of logging tools. Taking into account the computational cost of the particle transport in the complex geometries in the Monte Carlo method, the SP model can be satisfactory for gamma-gamma logging purposes.

Experimental study on combustion instability and attenuation characteristics in the lab-scale gas turbine combustor with a sponge-like porous medium

김영재,이대근,김용모 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.4

The present study has experimentally investigated the combustion instability and its attenuation characteristics in the lab-scale swirlstabilized premixed combustor with a sponge-like porous medium. Unlike the conventional premixed burners, this model combustor has the unique features including a porous dump plane and an acoustic cavity, which was devised for attenuating the combustion instability. When replacing the dump plane with a non-porous medium, the burner becomes the conventional design. In order to evaluate the damping effects of the porous medium on the unstable flame dynamics, various acoustic and photonic measurements and flame visualization were made. Special emphasis is given to the effects of the acoustic cavity length on the stabilization characteristics. Results showed that the model combustor with the porous dump plane and the acoustic cavity exhibited dramatic attenuation of the pressure oscillation intensity by up to about 40 %. The attenuation was increased with increasing the acoustic cavity length. It was also found that the attenuation is effective not only for the fundamental resonance but also for its higher harmonics. Based on the experimental results, detailed discussions are made for the combustion instability and its attenuation characteristics in the model gas turbine combustor with porous and nonporous media.

Numerical simulation of droplet merging and chemical reaction in a porous medium

Yu, Hanjun,Son, Gihun,Shim, Woosup Elsevier 2017 International Communications in Heat and Mass Tran Vol.89 No.-

<P><B>Abstract</B></P> <P>The chemical reaction between droplets merging in a porous medium is numerically investigated by solving the conservation equations of mass, momentum and chemical concentration in the internal and external regions of the porous medium. A level-set formulation for two-phase flows is extended to include the effects of porosity and mass transfer with chemical reaction. The numerical result for one-dimensional mass transfer with chemical reaction shows good agreement with the exact solution. The numerical method is applied to investigate the effects of droplet size, impact velocity and porosity on the droplet merging and the associated chemical reaction in a porous medium.</P>

An experimental study of _{N 2} dilution effects on _{CH 4} – _{O 2} flame stabilization characteristics in a two-section porous medium

Kim, Seung Gon,Lee, Dae Keun,Noh, Dong-Soon Elsevier 2016 Applied thermal engineering Vol.103 No.-

<P><B>Abstract</B></P> <P> <SUB> N 2 </SUB> dilution effects on <SUB> CH 4 </SUB> – <SUB> O 2 </SUB> flame stabilization in a two-section porous medium burner were experimentally investigated for its potential application to the destruction of highly <SUB> N 2 </SUB> -diluted <SUB> CF 4 </SUB> . The burner was made up of two axially-stacked silicon carbide foams having different pore sizes. Various flame behaviors, such as stable flame, flashback, blowout, extinction and transient flames, were observed and, for the stable flame, temperature distributions and exhaust gas emissions were measured. Results showed that the flame was only stable in a belt-like region on the domain of fuel flow rate and oxygen concentration. The region boundaries, termed as <SUB> O 2 </SUB> -enriched and <SUB> O 2 </SUB> -deficient limits, were determined for various equivalence ratios. It was found that our burner configuration was able to sustain the submerged combustion even under the <SUB> N 2 </SUB> dilution amounting to at least 10.4 times of <SUB> CH 4 </SUB> flow rate. In addition, temperature measurements showed that the stable flames could have their peak temperatures higher than 1600 ° C , which is known to be a thermal destruction temperature of <SUB> CF 4 </SUB> . These results may indicate that porous media burners can practically applied in destructing highly <SUB> N 2 </SUB> -diluted <SUB> CF 4 </SUB> with a reduced fuel consumption.</P> <P><B>Highlights</B></P> <P> <UL> <LI> N<SUB>2</SUB> dilution effects on various flame behaviors in a two-section porous medium. </LI> <LI> O<SUB>2</SUB>-enriched and O<SUB>2</SUB>-deficient limits of stable flames. </LI> <LI> High temperature combustion to destruct highly-diluted nondegradable gases. </LI> <LI> Estimated fuel consumption being less than those of commercial devices. </LI> </UL> </P>

Modeling heat transfer through chimney-structured porous deposit formed in pressurized water reactors

Yeo, D.Y.,NO, H.C. Elsevier 2017 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.108 No.1

<P><B>Abstract</B></P> <P>The model was developed to predict the heat transfer within the Chalk River Unidentified Deposit (CRUD), which is a kind of fouling found on the fuel rods of pressurized water reactors (PWR). The CRUD tends to develop steam chimneys that separate the liquid from the vapor phase. Therefore, the model describes the CRUD as a porous medium with steam chimneys. Unlike the previous approaches that assume that the evaporation takes place at the lateral surfaces of the chimneys, in the present study it is postulated that the vapor is generated by the bubble nucleation at the CRUD–clad interface, as observed via the visualization study for the chimney-structured porous medium. The generated bubble escapes through the steam chimney. The heat transfer in the CRUD can be described by three mechanisms of heat removal, which are nucleate boiling, liquid convection in the CRUD, and forced convective heat removal from the surface of the CRUD. The predicted CRUD–clad interface temperatures and overall heat transfer coefficients were compared to the experimental results, which were produced under the simulated PWR conditions (approximately 15MPa, 300°C). The prediction data presented better agreement with the experimental data; the normalized Root Mean Square Error (RMSE) of the present model is 18.6% in contrast with the 42.4% obtained with the Cohen model. After the validation with the experimental data, the prediction of temperature in the model was used to investigate how the heat transfer characteristics tended to change within the CRUD. Furthermore, the parametric study regarding the CRUD properties revealed that the effect of permeability on heat transfer is not significant in the nucleate boiling regime.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We propose a new heat transfer model and boiling model for the porous medium in PWRs. </LI> <LI> We validate the model against experimental data under the simulated PWR conditions. </LI> <LI> The liquid convection in the deposits contributes significantly to heat transfer. </LI> <LI> The effect of permeability on heat transfer is found to be insignificant. </LI> </UL> </P>