임계열유속에 대한 임계전류밀도의 상사특성

박해균(Hae-kyun Park) 경희대학교 학술단체협의회 2022 高凰論集 Vol.70 No.-

임계열유속은 원자력발전소의 설계 및 안전과 관련된 매우 중요한 인자이다. 복잡한 수력학적 조건에서의 임계열유속 측정실험은 극한의 실험조건에 의한 실험설비의 파손 시 발생하는 경제적, 시간적 소요에 의해 연구자들로 하여금 많은 어려움을 겪게 한다. 본 연구는 이러한 임계열유속 측정의 난점을 극복하기 위해 열과 물질전달 간의 상사성을 이용한 대체가능 실험방법론을 제시한다. 임계전류밀도는 작업전극에 수소막이 덮이면서 나타나는 전기분해에서의 작동한계이다. 본 연구에서는 임계열유속과 임계전류밀도의 수력학적 유사성을 기반으로, 임계열유속에 대한 임계전류밀도의 상사특성을 탐구한다. 비등계에서의 기화현상을 모사하기 위해 묽은황산전기분해계를 사용하여 전기화학적으로 수소를 발생시키는 실험을 수행하였다. 예비실험으로써, 수소환원계의 핵생성특성을 측정하고 이를 비등계와 비교하였다. 비등계에서와 유사하게, 전류밀도가 증가하면 수소환원계의 활성핵생성밀도(Active nucleation site density)가 증가하였다. 반면, 비등계에서와는 반대로 수소기포의 이탈직경은 전류밀도에 따라 감소하였다. 결과적으로, 수소기포의 주기는 전류밀도에 따라 증가하였다. 임계전류밀도의 메커니즘을 밝히기 위해 기존 임계열유속 모형을 차용하고 이를 수소환원계에 활용가능한 형태로 수정하였다. 기존 비등계의 Dry spot 모형과 유사한 개념으로 ‘Insulation spot’ 모형을 개발하여 임계전류밀도에서의 수소기포배열을 평가하였다. 임계기포수가 6개인 경우 Insulated spot 모형에 의한 임계전류밀도는 측정된 임계전류밀도보다 7.4% 높은 값을 산출해내며 잘 예측한다. 본 연구에서 최초로 수소환원계에서의 Macrolayer가 확인되었다. 수소기포덩어리의 Hovering time과 Macrolayer의 두께를 활용한 기존 Macrolayer dryout 모형은 임계전류밀도 또한 4.37% 이내로 잘 예측한다. 이러한 결과는 임계열유속에서와 마찬가지로 임계전류밀도 또한 수력학적 거동이 지배적으로 작용함을 의미한다. 그러므로, 수소환원계의 작은 수소기포이탈직경을 고려하면 임계전류밀도는 임계열유속의 축소모형이라고 간주할 수 있겠다. 이러한 상사관계를 여러 기하학적 및 수력학적 조건으로 확장할 수 있다. 질량유량, 표면기울기, 도관의 크기 등에 대한 임계전류밀도의 영향은 임계열유속에서의 영향과 유사하게 나타남을 확인하였다. 본 연구에서 밝혀진 상사특성을 통해, 열과 물질전달 간의 상사성을 이상유동 현상의 범위로 확장함으로써 더욱 진보된 상사성을 확립할 수 있다. The critical heat flux (CHF) is one of the important design and safety parameters in the nuclear power plant. The measurements of the CHF for complex hydrodynamic conditions are difficult due to the extreme experimental conditions, which cause the failure of the test facility, resulting in financial and time demanding problems. The present study proposes a possible alternative experimental method to overcome the difficulty of the CHF experiment by extending the analogy relation between heat and mass transfers. The critical current density (CCD) is an operation limit in the water electrolysis when hydrogen film is formed on the working electrode. Based on the hydrodynamic similarities between the CHF and the CCD, analogous characteristics of the CCD with the CHF are explored. Dilute sulfuric acid electrolysis was employed to produce hydrogen gas, which simulates vaporization in the boiling system. As the preliminary investigation, the nucleation characteristics of the hydrogen evolving system are measured and compared with the boiling system. The active nucleation site density in the hydrogen evolving system increased with current density increased, similar to the boiling system. The hydrogen departure diameter decreased with the increased current density as opposed to the boiling system. Accordingly, the hydrogen bubble frequency increased as the current density increased. The existing CHF models are borrowed and customized to envisage the CCD mechanism. The hydrogen bubble configuration at the CCD is estimated by the ‘insulated spot model’, which is developed in a similar manner to the dry spot model in the boiling system. The CCD is predicted well when the critical number of bubbles is six, which yields 7.4% higher value than the measured one. The presence of the macrolayer in the hydrogen evolving system is firstly revealed in this study. The macrolayer dryout model also predicts the CCD within 4.37% by utilizing the thickness of macrolayer and hovering time of hydrogen mass. The results imply that the CCD is dominated by the hydrodynamic behaviors just like the CHF. Hence, the CCD can be regarded as a miniature CHF considering small bubble diameter of the hydrogen evolving system. The analogous relations are extended by applying to the various geometric and hydrodynamic conditions. It is confirmed that the influences of mass flux, surface inclination, channel gap size, etc on the CCD are similar to those on the CHF. The analogous characteristics proven in this study shed light on the improved analogy between heat and mass transfers by extending the scope to include the two-phase flow phenomenon.

다양한 표면에서 유동 속도에 따른 풀 비등 열전달에 관한 연구

강동규(Dong-gyu Kang),이요한(Yohan Lee),서훈(Hoon Seo),정동수(Dongsoo Jung) 대한기계학회 2013 大韓機械學會論文集B Vol.37 No.4

본 연구에서는 열전달 표면의 형상과 그 위에서의 유동 속도의 변화에 따른 풀 비등 열전달계수의 변화를 살펴보기 위해 평판, 낮은 핀, Thermoexcel-E, Turbo-B 표면을 사용하여 유동 속도를 변화시켜가며 임계 열유속까지 열전달계수를 측정하였다. 작동 유체로는 증류수를 사용하였고 사각 평면 히터(9.53 x 9.53mm)를 이용하여 네 가지 표면에서 임계 열유속까지의 데이터를 얻을 수 있도록 장치를 제작하였고 60 에서 유동 속도를 0, 0.1, 0.15, 0.2m/s로 변화시켜가며 데이터를 취했다. 실험 데이터를 보면 모든 표면에서 유동이 있을 때의 임계 열유속은 유동이 없을 때에 비해 높은 것으로 나타났다. 또한 표면적의 증가와 기포 이탈에 충분한 핀 간격 등으로 인해 낮은 핀 표면의 임계 열유속은 평판이나 Turbo-B, Thermoexcel-E 표면보다 훨씬 놓았고 평판에 비해서는 무려 5배 정도의 향상을 보였다. 한편 대형 냉동기의 증발기용으로 개발된 Turbo-B와 Thermoexcel-E 표면은 물에서 기포의 이탈 지름이 크므로 열전달계수와 임계 열유속 모두 예상보다 큰 효과를 나타내지 않았다. 50kW/m2이하의 저열유속에서는 모든 표면에 대해 유동 속도 증가에 따라 열전달계수가 증가하였다. 결론적으로 핵발전소의 증기발생기에 적용하기에는 낮은 핀 형상의 표면이 가장 좋은 것으로 나타났다. In this study, a smooth flat surface, low fin, Turbo-B, and Thermoexcel-E surfaces are used to examine the effect of the flow velocity on the pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHFs). HTCs and CHFs are measured on a smooth square heater of 9.53 × 9.53 mm2 at 60°C in a pool of pure water at various fluid velocities of 0, 0.1, 0.15, and 0.2 m/s. Test results show that for all surfaces, CHFs obtained with flow are higher than those obtained without flow. CHFs of the low fin surface are higher than those of the Turbo-B and Thermoexcel-E surfaces due largely to the increase in surface area and sufficient fin spaces for the easy removal of bubbles. CHFs of the low fin surface show even 5 times higher CHFs as compared to the plain surface. On the other hand, both Turbo-B and Thermoexcel-E surfaces do not show satisfactory results because their pore sizes are too small and water bubbles easily cover them. At low heat fluxes of less than 50 kW/m2, HTCs increase as the flow velocity increases for all surfaces. In conclusion, a low fin geometry is good for application to steam generators in nuclear power plants.

열전달 촉진관에서 임계열유속까지의 풀 비등 열전달 계수

임병덕(Lim Byeongdeok),이요한(Yohan Lee),정동수(Dongsoo Jung) 대한설비공학회 2010 대한설비공학회 학술발표대회논문집 Vol.2010 No.11

In this work, nucleate pool boiling heat transfer coefficients(HTCs) of pure water on horizontal 26 fpi low fin, Turbo-B and Thermoexcel-E square surface of 9.53 mm length. Tested enhanced tubes HTCs are taken from 10 ㎾/㎡ to critical heat flux. Critical heat fluxes(CHFs) of all enhanced surfaces were greatly improved as compared to that of a plain surface in tested water. CHF of water on the 26fpi low fin surface, Thermoexcel-E surface, and Turbo-B was increased up to 320%, 275%, and 150% as compared to that of the plain surface. CHF of the Turbo-B enhanced surface were lower than that of the 26fpi low fin surface. The heat transfer enhancement ratios of the low fin surface and Turbo-B enhanced surface were 1.01~4.60, 0.84~4.22 respectively in the range of heat fluxes tested.

열전달 촉진 표면에서 임계 열유속까지의 물의 풀 비등 열전달계수

이요한(Yohan Lee),강동규(Kang Dong Gyu),정동수(Dongsoo Jung) 대한설비공학회 2011 설비공학 논문집 Vol.23 No.3

In this work, nucleate pool boiling heat transfer coefficients(HTCs) of pure water are measured on horizontal 26 fpi low fin, Turbo-B and Thermoexcel-E square surfaces of 9.53 ㎜ length. HTCs are taken from 10 ㎾/㎡ to critical heat flux for all surfaces. Test results show that critical heat fluxes(CHFs) of all enhanced surfaces are greatly improved as compared to that of a plain surface. CHFs of water on the 26 fpi low fin surface, Thermoexcel-E surface, and Turbo-B are increased up to 320%, 275%, and 150% as compared to that of the plain surface, respectively. CHF of the Turbo-B enhanced surface is lower than that of the 26 fpi low fin surface due to the surface geometry. The heat transfer enhancement ratios of the Thermoexcel-E surface, low fin surface and Turbo-B enhanced surface are 1.6~2.9, 1.6~2.1, 1.4~1.7 respectively in the range of heat fluxes tested. Judging from these results, it can be said that these types of enhanced surfaces can be used in heat transfer applications at high heat fluxes.

임계압력 근처에서의 환형관 채널에 대한 열전달 특성 연구

홍성덕(Sungdeok Hong),천세영(Seyoung Chun),김세윤(Seyun Kim),백원필(Wonpil Baek) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4

An experimental study on heat transfer characteristics near the critical pressure has been performed with an internally-heated vertical annular channel cooled by R-134a fluid. Two series of tests have been completed: (a) steady-state critical heat flux (CHF) and (b) heat transfer tests for pressure reduction transients through the critical pressure. In the present experimental range, the steady-state CHF decreases with the increase of the system pressure For a fixed inlet mass flux and subcooling, the CHF falls sharply at about 3.8 MPa and shows a trend toward converging to zero as the pressure approaches the critical point of 4.059 MPa. The CHF phenomenon near the critical pressure does not lead to an abrupt temperature rise of the heated wall because the CHF occurred at remarkably low power levels. In the pressure reduction transient experiments, as soon as the pressure passed through the critical pressure, the wall temperatures rise rapidly up to a very high value due to the occurrence of the departure from nucleate boiling. The wall temperature reaches a maximum at the saturation point of the outlet temperature, then tends to decrease gradually.

얇은 히터 thermal activity의 올바른 결정을 위한 가열 방법에 따른 실질 가열 두께 연구

정우현(Woo Hyun Jung),조항진(HangJin Jo) 대한기계학회 2020 대한기계학회 춘추학술대회 Vol.2020 No.12

In various industrial field such as nuclear engineering (fuel cladding) and electronics (chip cooling), a critical heat flux of thin heater are getting a lot of interests due to its unusual characteristic, lower CHF value than the thick heater. In case of the thin heater, generally the critical heat flux has a tendency to decrease as the thickness of heater decreased. This phenomenon is interpreted as the thermal property effect of heater, represented by the thermal activity parameter that is composed of heater thickness, density, thermal capacity, and thermal conductivity. In this paper, it is claimed that the heater thickness parameter could be changed according to the heating method; Joule heating method (direct heating) or conduction heating methods (direct heating). By solving the conduction problem of heater numerically with the dry patch mechanism concept for the CHF, giving thinner thickness for the Joule heating method as an effective heating thickness showed consistency on the numerical results.

임계 열유속 근방까지의 풀 비등 열전달계수

박기정(Ki-Jung Park),정동수(Dongsoo Jung) 대한설비공학회 2008 설비공학 논문집 Vol.20 No.9

In this work, pool boiling heat transfer coefficients(HTCs) of 5 refrigerants of differing vapor pressure are measured on horizontal smooth square surface of 9.52 ㎜ length. Tested refrigerants are R123, R152a, R134a, R22, and R32 and HTCs are taken from 10 ㎾/㎡ to critical heat flux of each refrigerant. Wall and fluid temperatures are measured directly by thermocouples located underneath the test surface and by thermocouples in the liquid pool. Test results show that pool boiling HTCs of refrigerants increase as the heat flux and vapor pressure increase. This typical trend is maintained even at high heat fluxes above 200 ㎾/㎡. Zuber's prediction equation for critical heat flux is quite accurate showing a maximum deviation of 21% for all refrigerants tested. For all refrigerant data up to the critical heat flux, Stephan and Abdelsalam's well known correlation underpredicted the data with an average deviation of 21.3% while Cooper's correlation overpredicted the data with an average deviation of 14.2%. On the other hand, Gorenflo's and lung et al's correlation showed only 5.8% and 6.4% deviations respectively in the entire nucleate boiling range.

탄소나노튜브 및 그래핀 나노유체 사용시 장기운전에 따른 열전달성능 변화에 대한 연구

김영훈(Kim Young-Hun),김남진(Kim Nam-Jin) 한국태양에너지학회 2017 한국태양에너지학회 논문집 Vol.37 No.1

Critical heat flux refers to the sudden decrease in boiling heat transfer coefficient between a heated surface and fluid, which occurs when the phase of the fluid near the heated surface changes from liquid to vapor. For this reason, critical heat flux is an important factor for determining the maximum limit and safety of a boiling heat transfer. Recently, it is reported that the nanofluid is used as a working fluid for the critical heat flux enhancement. However, it could be occurred nano-flouling phenomena on the heat transfer surface due to nanoparticles deposition, when the nanofluid is applied in a heat transfer system. In this study, we experimentally carried out the effects of the nano-fouling phenomena in oxidized multi-wall carbon nanotube and oxidized graphene nanofluid systems. It was found that the boiling heat flux decreased by hourly 0.04 and 0.03 kW/㎡, also the boiling heat transfer coefficient decreased by hourly 11.56 and 10.72 W/㎡·K, respectively, in the thermal fluid system using oxidized multi-wall carbon nanotube or oxidized graphene nanofluid.

나노 구조가 형성된 열전달 표면에서의 임계 열유속 증진 메커니즘

김동억(Dong Eok Kim) 대한기계학회 2014 大韓機械學會論文集B Vol.38 No.7

나노 구조가 형성된 열전달 표면에서 유체의 비등 시 임계 열유속 값이 나노 구조가 없는 표면보다 현저히 증가한다는 것은 잘 알려진 사실이다. 다수의 물리적 메커니즘들이 이러한 나노 구조에서의 임계 열유속 증진 현상을 설명하기 위해 제안되어 왔다. 하지만 지금까지 대부분의 연구들은 정성적인 결과를 제시해 왔으며, 이러한 현상을 일반적으로 설명할 수 있는 이론은 아직 확립되지 않았다. 본 연구에서는 나노 구조가 형성된 표면에서의 임계 열유속 증진에 관한 정량적인 메커니즘을 증기 반동력 및 표면 접착력에 기초하여 제안하고자 한다. 특히, 본 연구에서는 임계 열유속 증진 현상을 표면에 형성된 나노 구조로 인한 액체, 증기, 고체의 삼중선 길이의 증가 및 나노 구조와 액체 사이의 접착력에 근거하여 설명하였다. The critical heat flux (CHF) on a heat transfer surface with nanostructures is known to be significantly better than that on flat surfaces. Several physical mechanisms have been proposed to explain this phenomenon. However, almost all studies conducted so far have been qualitative, and a generalized theory has not yet been established. In this study, we developed a quantitative mechanism for CHF enhancement on a surface with nanostructures, based on vapor recoil and surface adhesion forces. We focused on the increase in the length of the triple contact line owing to the formation of nanostructures and the adhesion force between them and the liquid.

분무냉각에 있어서 임계열유속 상관식에 관한 연구

김영찬 ( Yeung Chan Kim ) 한국액체미립화학회 2018 한국액체미립화학회지 Vol.23 No.3

The critical heat flux of spray cooling were measured on the test surface of 10 mm diameter made by stainless steel. The experiments were carried out for the droplet-flow-rate of 0.00002~0.003 ㎥/(㎡s) and liquid subcooling temperature of 40~75℃. Experimental results showed that the critical heat flux of spray cooling increased remarkably with the increase of droplet-flow-rate. Meanwhile, the effect of liquid subcooling on critical heat flux of spray cooling appeared weakly under the present experimental conditions. In additions, correlation between the dimensionless critical heat flux and Weber number based on droplet-floe-rate was developed for experimental results.