June Kee Min

Analysis of the absorption process on a horizontal tube using Navier–Stokes equations with surface-tension effects

Abstract A Navier–Stokes procedure has been developed to investigate the absorption phenomena about the free-falling film flow on a horizontal tube. The fully elliptic equations of momentum, temperature and concentration are solved by using the SIMPLER algorithm, which incorporates the QUICK scheme and the incomplete Cholesky conjugate gradient method for accuracy and efficiency. Taking account of the surface-tension effects, the free-surface location is carefully traced by the MAC method in a time-accurate manner. The details of the flow and heat/mass transfer phenomena, such as the free-surface location, the streamlines, the formation of recirculating region and the rate of absorption, are seen to be well captured. The results are presented for various flow rates; the surface tension is found to play an important role in dictating the flow field especially when the flow rate is low.

Analysis of Two-Way Fluid-Structure Interaction and Local Material Properties of Brazed Joints for Estimation of Mechanical Integrity

Recent years have witnessed a strong need for eco-friendly and energy-efficient systems owing to global environmental problems. A heat exchanger is a well-known mechanical rig that has long been used in many energy systems. The use of a heat exchanger in an airplane engine has been attempted. In this case, the heat exchanger should be redesigned to be compact, lightweight, and highly reliable, and the issue of mechanical integrity gains importance. Therefore, in this study, we proposed a method for evaluating the mechanical integrity of a tube-type heat exchanger. A U-shaped single tube was used as an example, and its behavior and stress distribution were studied using fluid-structure interaction (FSI) analysis.

Measurement of pressure distribution inside a cross-corrugated heat exchanger using microchannel pressure tappings

This paper suggests a novel method to measure pressure distribution inside a cross-corrugated heat exchanger using microchannel pressure tappings. The microchannel-embedded corrugated sheet is fabricated. In particular, the hydraulic diameter of the non-circular microchannel is measured. The specially designed pressure chamber is made to control the reference pressure. The pressure response is examined theoretically and experimentally in terms of delay time and peak pressure. The delay time shows linear decreases in the length of the microchannel and the peak pressure also decreases with increasing frequency of the reference pressure. The microchannel pressure tapping is applicable to steady pressure measurement but not to transient pressure measurement. Finally, 20 corrugated sheets and 1 microchannel-embedded corrugated sheet are stacked to form the cross-corrugated heat exchanger. The static and stagnation pressures inside the heat exchanger are measured. The measured friction factor is compared with that in previous reports.

Numerical Study on Surface Air-Oil Heat Exchanger for Aero Gas-Turbine Engine Using One-Dimensional Flow and Thermal Network Model

: 오일의 대류열전달 계수 k : 열전도 계수 Key Words: Computational Fluid Dynamics(전산유체역학), Flow and Thermal Network(열유동 네트워크), Heat Exchanger(열교환기), Surface Air-Oil Cooler(SAOHE), 초록 : 항공기용 가스터빈 엔진에 있어서, 기어 어셈블리 및 전자장비에 사용되는 오일의 냉각을 위하여 열교환기가 사용되며 이를 Surface air-oil heat exchanger (SAOHE) 라고 한다. 이 열교환기는 엔진 팬 케이싱 내부에 설치되며 기어박스 시스템 및 전자장비로부터 바이패스 덕트 후류 쪽으로 열을 소산시킨다. 본 연구의 목적은 SAOHE의 설계를 위한 효율적인 수치해석방법을 개발하는 것이다. SAOHE 설치에 따른 핀에서의 열공력학적 성능을 평가하기 위하여 다공성 모델을 활용한 2 차원 수치해석을 수행하였고, 열교환기 성능평가에 대해 시간 및 비용적으로 효과적인 1 차원 열유동 네트워크 프로그램을 개발하였다. 이 프로그램을 이용하여 열교환기의 압력강하 및 열전달 성능을 예측하였고, 1차원 열유동 네트워크 프로그램을 검증하기 위해 2차원 전산해석 결과 및 실험 결과와 비교하였다. Abstract: In an aero gas-turbine engine, a surface air-oil heat exchanger (SAOHE) is used to cool the oil system for the gearboxes and electric generators. The SAOHE is installed inside the fan casing of the engine in order to dissipate the heat from the oil system into the bypass duct stream. The purpose of this study was to develop an effective numerical method for designing an SAOHE for an aero gas-turbine engine. A two-dimensional model using a porous medium was developed to evaluate the aero-thermal performance of the fins of the heat exchanger, and a one-dimensional flow and thermal network program was developed to save time and cost in the evaluation of the heat exchanger performance. Using this network program, the pressure drop and heat transfer performance of the heat exchanger were predicted, and the results were compared with two-dimensional computational fluid dynamics results and experiment data for validation.

Comparison of Numerical Results for Laminar Wavy Liquid Film Flows down a Vertical Plate for Various Time-Differencing Schemes for the Volume Fraction Equation

Liquid film flows are classified into waveless laminar, wavy laminar, and turbulent flows depending on the Reynolds number or the flow stability. Since the wavy motions of the film flows are so intricate and nonlinear, studies on them have largely been experimental. Most numerical approaches have been limited to the waveless flow regime. The various free surface-tracking schemes adopted for this problem were used to more accurately estimate the average film thickness, rather than to capture the unsteady wavy motion. In this study, the wavy motions in laminar wavy liquid film flows with Reynolds numbers of 200-1000 were simulated with various numerical schemes based on the volume of fluid (VOF) method for interface tracking. The results from each numerical scheme were compared with the experimental results in terms of the average film thickness, the wave velocity, and the wave amplitude.

NUMERICAL STUDY OF THE HIGH-SPEED BYPASS EFFECT ON THE AERO-THERMAL PERFORMANCE OF A PLATE-FIN TYPE HEAT EXCHANGER

The high-speed bypass effect on the heat exchanger performance has been investigated numerically. The plate-fin type heat exchanger was modeled using two-dimensional porous approximation for the fin region. Governing equations of mass, momentum, and energy equations for compressible turbulent flow were solved using ideal-gas assumption for the air flow. Various bypass-channel height were considered for Mach numbers ranging 0.25-0.65. Due to the existence of the fin in the bypass channel, the main flow tends to turn into the core region of the channel, which results in the distorted velocity profile downstream of the fin region. The boundary layer thickness, displacement thickness, and the momentum thickness showed the variation of mass flow through the fin region. The mass flow variation along the fin region was also shown for various bypass heights and Mach numbers. The volumetric entropy generation was used to assess the loss mechanism inside the bypass duct and the fin region. Finally, the correlations of the friction factor and the Colburn j-factor are summarized.

A Numerical Study on Plate-Type Heat Exchanger Using One-Dimensional Flow Network Model and Porous-Media Model

Abstract A typical heat exchanger, found in many industrial sites, is made up of a large number of unitary cells, whichcauses difficulties when carrying out full-scale three-dimensional numerical simulations of the heat exchanger to analyzethe aero-thermal performance. In the present study, a three-dimensional numerical study using a porous media model was carried out to evaluate the performance of the heat exchanger modelled in two different ways ： full-scale and simplified. The pressure drop in the air side and gas side along with the overall heat transfer rate were calculated using a porousmedia model and the results were then compared to results obtained with a one-dimensional flow network model. The comparison between the results for two different geometries obtained using a porous media model and a one-dimensional flow network model shows good agreement between the simplified geometry and the one-dimensional flow network model.The full-scale geometry shows reasonable differences caused by the geometry such as sudden expansion and contraction.

Heat Exchanger Ranking Program Using Genetic Algorithm and ε-NTU Method for Optimal Design

: 질량 유량 NTU : 전달단위수 Nu : Nusselt수 Pr : Prandtl 수 p : 압력차 q : 열전달량 Key Words: CFD(전산유체역학), Genetic Algorithm(유전알고리즘), Heat Exchanger(열교환기), e-NTU(유용도-전달단위수), Sizing(사이징), Optimization(최적화) 초록: 오늘날 고성능컴퓨터로 인해 많은 산업분야에서 전산해석이 사용되고 있다. 하지만 정해진 컴퓨터자원과 시간에 의해 3차원 풀 스케일 해석에서는 많은 어려움 등이 있다. 본 연구에서 e-NTU식과 열교환기 성능의 데이터베이스를 이용해 열교환기의 성능예측프로그램을 개발하였다. 다양한 타입의 열교환기 형상정보와 성능데이터베이스를 구축하였고, 이를 바탕으로 정해진 작동 조건에서 열교환기의 성능을 계산하였다. 계산된 정보를 바탕으로 최적의 사이즈를 갖는 형상을 찾기 위해 유전알고리즘(Genetic Algorithm)을 이용하였다. 계산을 위해 상용 소프트웨어인 MATLAB과 REFPROP이 사용되었다. Abstract: Today, computational fluid dynamics (CFD) is widely used in industry because of the availability of high-performance computers. However, full-scale analysis poses problems owing to the limited resources and time. In this study, the performance and optimal size of a heat exchanger were calculated using the effectiveness-number of transfer units (e-NTU) method and a database of characteristics heat exchanger. Information about the geometry and performance of various heat exchangers is collected, and the performance of the heat exchanger is calculated under the given operating conditions. To determine the optimal size of the heat exchanger, a Genetic Algorithm (GA) is used, and MATLAB and REFPROP are used for the calculation.

Numerical Study of Wavy Film Flow on Vertical Plate Using Different Turbulent Models

* Rolls-Royce University Technology Center, Pusan Nat’l Univ.,** School of Mechanical Engineering, Kyungpook Nat’l Univ.(Received October 10, 2012 ; Revised November 18, 2013 ; Accepted November 18, 2013) Key Words: Vertical Plate(수직평판), Free Suface Flow(자유표면유동), Turbulent Model(난류모형), Computational Fluid Dynamics(전산유체역학), Wavy Film Flow(파동액막류)초록: 액막류는 다양한 산업분야에 적용되는 쉘-튜브 열교환기의 주요 열교환기구로 오랫동안 연구되어 왔다. 액막류의 한쪽 경계는 고정벽에 접하고 있지만 반대편에서는 기체 영역과 경계를 형성하므로 액막 레이놀즈 수가 증가함에 따라 쉽게 불안정해지는 특징을 가지고 있다. 따라서 레이놀즈 수가 증가함에 따라 자유표면 파동 현상이 나타나는데, 층류 영역에서는 큰 진폭의 고립파가, 난류 천이 이후에는 낮은 진폭의 물결파가 나타난다. 액막류의 열전달 성능은 액막의 두께에 의해 크게 지배받는데 액막류에 동반된 파동은 액막 두께의 시공간적 변화를 의미하는 것이므로 이에 대한 정보를 해석적으로 수집하는 것은 액막류 열전달 성능을 예측하는데 필수적이다. 본 연구에서는 낮은 진폭의 물결파를 동반한 난류 액막류에 대하여 여러 가지 난류 모형을 적용한 해석결과들을 실험결과와 비교함으로써 난류 모형들에 대한 평가를 실시하였다.Abstract: Film flows applied to shell-and-tube heat exchangers in various industrial fields have been studied for a long time. One boundary of the film flow interfaces with a fixed wall, and the other boundary interfaces with a gaseous region. Thus, the flows become so unstable that wavy behaviors are generated on free surfaces as the film Reynolds number increases. First, high-amplitude solitary waves are detected in a low Reynolds number laminar region; then, the waves transit to a low-amplitude, high frequency ripple in a turbulent region. Film thickness is the most significant factor governing heat transfer. Since the wave accompanied in the film flow results in temporal and spatial variations in film thickness, it can be of importance for numerically predicting the films wavy behavior. In this study, various turbulent models are applied for predicting low-amplitude ripple flows in turbulent regions. The results are compared with existing experimental results, and finally, the applied turbulent models are appraised in from the viewpoint of wavy behaviors.

Numerical Study of Surface Heat Transfer Effects of Multiple Fan-Shaped Small-Scale Fins

In this work, we study a heat transfer enhancement technology using fan-shaped small-scale fins. Fins having a thickness of 10 move up-down by a pulsating flow. Owing to these motions, the heat transfer on a surface increases dramatically. The two-way FSI (fluid-structure interaction) method was applied for the analysis, and the analysis model was evaluated using a single fin model by comparing the experimental results. In summary, a maximum 40% increase in heat transfer capacity using a single and multiple small-scale fins was obtained in comparison with the results obtained without using fins. From this work, we believe that the proposed method can be a promising method for heat transfer enhancement in real applications.