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Diverse cross-corrugated surface geometries were considered to estimate the sensitivity of four variants of k-e turbulence models (Low Reynolds, standard, RNG and realizable models). The cross-corrugated surfaces considered in this study are a conventional sinusoidal shape and two different asymmetric shapes. The numerical simulations using the steady incompressible Reynolds-averaged Navier- Stokes (RANS) equations were carried out to obtain the steady solutions of the flow and thermal fields in the unitary cell of the heat exchanger matrix. In addition, the experimental test for the measurement of local convective heat transfer coefficients on the heat transfer surfaces was performed by means of the Transient liquid crystal (TLC) technique in order to compare the numerical results with the measured data. The features on detailed flow structure and corresponding heat transfer in the unitary cell of the matrix type heat exchanger are compared and analyzed against four different turbulence models considered in this study.

In general, thermal residual stress is generated in casting products as a result of non-uniform cooling, phase transformation from liquid to solid states, and thermo-mechanical constraints. Thus, understanding accurate stress distributions is important in designing a casting part to predict its real structural stability. However, the residual stress that occurs in the casting process is often not considered in the mechanical design process to reduce engineering efforts and time. We conducted a structural analysis that compares two cases with and without considering casting residual stress in large-scale sand castings and obtained significantly different results. We introduced an interfacing algorithm (casting-structural interface) to map the casting residual stress with structural stress and combined the results of finite different analysis for casting and finite element analysis for structural design. This work represents our preliminary studies on improving the design accuracy of a large-scale casting part and shows the importance of considering the casting residual stress. A difference of about 41% in maximal principle stress is obtained with and without considering the casting residual stress.

In industrial fields of machine and aerospace, cooling systems consisting of channels are widely used to increase energy efficiency and prevent system overheat. In cooling channels, a reduced pressure drop, an enhanced heat transfer, and a short channel length are considered key design requirements for optimizing the total volume and weight of a system. In this work, we improved heat transfer efficiency by using milli-scale wavy structures inside the channel. By optimizing the inner structures through computational fluid dynamics analysis and Taguchi method, the Nusselt number increased by approximately 11.7% with a similar pressure drop compared with that of a normal channel for a Reynolds number of 1000.

Erratum to “Transient performance prediction of an axial compressor considering VIGV operation speeds” [JMST 28 (10) (2014) 4099~4107] Donghyun Kim, Sangjo Kim, Changmin Son, Kuisoon Kim, Myungho Kim and Seongki Min Department of Aerospace Engineering, Pusan National University, Busan, 609-735, Korea School of Mechanical Engineering, Pusan National University, Busan, 609-735, Korea The 4th R&D Institute, Agency for Defense Development, Daejeon, Korea

In the present study, transient performance and flow characteristics of a 1.5-stage compressor with VIGV are investigated using CFD. The level of accuracy of CFD approach is evaluated by comparing the predicted performance against experimental data of a 3.5-stage compressor. The main objective of the present research is to quantitatively estimate the performance of transonic axial-flow compressor under transient operation of VIGV compared to steady-state performances on static IGV condition. The present study is performed to investigate the effect of VIGV movement on the performance characteristics when the VIGV setting angle varies with time. The VIGV is set to rotate in the clockwise and counterclockwise directions. The results were compared with steady-state predictions. Reasonable differences are observed in mass flow rate predictions (2%∼6%) between the transient and the steady-state calculations as VIGV operation speed varies. As VIGV operation speed increases, the difference in mass flow rate also increases. Therefore, a change in work input of the compressor is confirmed due to changes in mass flow rate. This is mainly caused by the changes in deviation and incidence angles depending on the operation speed and operation direction (clockwise and counterclockwise). The changes in deviation and incidence angles reflected on the entropy distribution. Even in mid-span, there is variation in angles depending on the operation speed and direction.

We numerically investigated the behavior of the water meniscus formed between a flat surface and a tip surface, which is flat or circular in shape, using the lattice Boltzmann method (LBM). The shape of the water meniscus formed between the flat bottom surface and the tip surface depends on the tip shape and the interaction between the water meniscus and the bottom or tip surface. The interaction is determined by the contact angles of the bottom and tip surfaces, resulting in different contact lengths between the water meniscus and the bottom or tip surface. The difference in these contact lengths depends on the effects of both the tip curvature and the interaction between the water meniscus and the bottom or tip surface. We classified the shapes of the water meniscus into seven different patterns as a function of the contact angles of the flat bottom and tip surfaces: concave, semi-concave, inverse semi-concave, column, convex, semi-convex, and inverse semi-convex.

A tube type heat exchanger is often the only solution when minimum pressure loss is a requirement. In addition, small diameter tubes are preferable because of an increased heat transfer area within an acceptable pressure loss limit. The present work reports on both an analytic model and experimental results with regards to the pressure drop and heat transfer characteristics of compact straight, C-curved, and U-curved tubes. The inner diameter of the tube (D) for our selected heat exchanger type was 1.26 mm with a thickness of 0.12 mm and a total length of 150.8 D. For the experiment, pressurized nitrogen gas bottles were used rather than an air compressor system in order to simplify the test facility. Hence the pressure conditions were easily set at 10, 30, and 50 bar corresponding to a range of Reynolds numbers from 10000 to 50000. To elevate the air temperature outside the tube (from 100°C to 400°C), an electric furnace was installed around the “test tube”. An analytic model to determine the pressure loss through curved tubes-referred to as the modified friction factor- is proposed. Good agreement was found between the modified friction factor and existing correlations, thus confirming the suitability of this model for determining pressure losses for different shape of tubes. The average measured Nusselt numbers were within 10- 15% of the Dittus-Boelter and Gnielinski correlations.

The analysis of the time-dependent and two-dimensional fluid flow and heat transfer around a single row of tubes in a channel is performed numerically. Due to its fundamental significance and practical importance, aerodynamic and heat transfer characteristics of tube bundle have been paid great attention by many researchers. In the present study, the immersed boundary method is applied by using a Cartesian grid system. Numerical solution for the governing equations of mass, momentum and energy conservation is obtained with the finite volume method. To validate the numerical approach with the immersed boundary method, the results have been compared with published data. The generation and evolution of vortical structures, wake interactions and their effects on the drag, lift and heat transfer are analyzed at different Reynolds numbers. The effect of hydraulic boundary layer development on the fluid flow and heat transfer is also investigated.