Kui Soon Kim

Review of Heat Exchanger Studies for High-Efficiency Gas Turbines

Air transportation has been being expanded remarkably, and its growth is expected to continue in the coming decades. Environmental issues and airlines require gas turbine manufacturers to produce environmentally friendly gas-turbine engines with lower emissions and improved specific fuel consumption. These requirements can be met by incorporating heat exchangers into gas turbines for intercooling and recuperation. Relevant research in such areas as the design of a heat exchanger matrix, materials selection, and manufacturing technology and optimization has been carried out by a variety of researchers. These works are reviewed in this paper. The recent advance in technologies appears to herald the development of intercoolers and recuperators for civil aeroplane gas turbines. Based on results reported in previous studies, potential heat exchanger designs for an aero gas turbine recuperator, intercooler, and cooling-air cooler are suggested.Copyright

CFD and Thermo-Mechanical Analysis for Heat Exchanger Used in Aero Engine

The multi-physics analysis using both the CFD and thermo-mechanical analysis is carried out to estimate the life of the heat exchanger which is operated under the conditions of high temperature and high pressure. First CFD analysis is carried out to obtain the distribution of flow, pressure and temperature around heat exchanger. The distribution of pressure, temperature and heat transfer coefficient obtained from the CFD analysis is transferred to the thermo-mechanical analysis using finite element analysis technique and is used as data to calculate the mechanical and thermal stress distribution in the heat exchanger. For the CFD analysis, it is considered a segment of heat exchangers using the symmetric and periodic conditions. For the thermo-mechanical analysis, the present finite element model considered both a segment and a half of full geometry by using the symmetric and periodic conditions. Alloy 625 is used for the present heat exchanger design due to its high strength at the elevated temperatures. The temperature-dependent physical properties of Alloy 625 for the thermo-mechanical analysis are used in a temperature ranges of 300∼1100K. Fatigue analysis is performed using a Goodman-diagram to assess the life of the present heat exchanger.Copyright

The effects of heat transfer evaluation methods on Nusselt number for mini-channel tube bundles

Heat transfer performance of tube bundles, which are widely used in practical applications, has been extensively investigated. Most previous experimental and numerical works for tube bundles have been performed with tube diameter in a range of 10 to 51mm and a Reynolds number of 8,000≤Re≤30,000. Recently, tube bundles with small diameter have garnered interest, since a mini-channel tube provides greater compactness. The present work investigates the applicability of previous correlations available in the open literature to tube bundles with small diameter of 1.5mm and a Reynolds number of 3,000≤Re≤7,000. Experiment results and a commercial CFD package were used to analyze the thermal-hydraulic performance of the tube bundles. The average convective heat transfer coefficients of tube bundles in most previous works were evaluated based on the difference between the bulk fluid temperature and the average surface temperature of the tubes. However, the tubular heat exchanger design process makes use of the concepts of overall thermal resistance and the LMTD, which is defined from the temperature difference between two working fluids. This paper examines the variation of the shell-side convective heat transfer coefficients of tube bundles caused by the two different analysis methods. The comparison showed that the average Nusselt number evaluated based on the LMTD method is 22.6% smaller than that based on the surface average temperature. This shows that it is necessary to consider the discrepancy in the heat transfer coefficient definition for proper design of heat exchangers. Also, the Zukauskas correlation developed for larger diameter tubes and higher Reynolds number are still in good agreement with experimental data for the present small diameter tube bundle within a discrepancy of 4.7%.

Numerical Analysis of Phase Change and Natural Convection Phenomena During Pipe Freezing Process

A numerical analysis is performed in this study to see the characteristics of the ice plug formation and the heat transfer during pipe freezing process. The unsteady three dimensional computer code which solves incompressible Navier-Stokes equations including energy equation has been developed in this study. The computed results for verification including phase interface and flow patterns showed good agreements with others. And additional pipe freezing experiments for verification showed that maximum flow rate for the complete plug formation is highly dependent on the pipe diameter and length of jacket containing refrigerant (LN2 ). The results of pipe freezing experiments could be also used as a guideline and to the proper computation. It was found that the effects of pipe diameter and freezing jacket length on the ice plug formation are significant. It was also found that this bi-cellular vortex oscillates in an axial section (r – z plane) as moves to downstream. Finally, the numerical results showed that the characteristics of the flow in the mushy region influence the shape and the freezing time of ice plug formation.Copyright