The effect of blade angle on the flow and pressure distributions in the vicinity of the diffuser blades of a room air conditioner

Many studies on air-conditioning systems are more focused on the individual thermal comfort rather than the thermal efficiency, due to an increase in health concerns. There are several factors influencing the thermal comfort, such as temperature, humidity, convection and air movement, etc. Numerical analyses were performed to investigate the effect of blade angle on the flow characteristics in the vicinity of diffuser blades of a room air conditioner (RAC), with three different blade discharge angles of 45.1°, 58.6° and 116°. We used the commercial code FLUENT to calculate the two-dimensional steady thermal flow fields with different impeller rotational velocities. The angular velocities were located within the range from 900 rpm to 1200 rpm. Turbulence closure was achieved using a standard k-ɛ model. A moving reference frame (MRF) approach was adopted to simulate the flow field generated by the impeller in an RAC. The results were graphically depicted with various geometrical configurations and operating conditions. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Youn-Jea Kim received his B.S. degree in Mechanical Engineering from Sungkyunkwan University, Korea, in 1982. He then received his M.S. and Ph.D. degrees from the State University of New York at Buffalo in 1987 and 1990, respectively. Dr. Kim is currently a Professor at the School of Mechanical Engineering at Sungkyunkwan University in Korea. Dr. Kim’s research interests include gas dynamics, MEMS, and fluid-machineries, etc.     

A study on the mixing characteristics in a hybrid type microchannel with various obstacle configurations

Micromixer can be classified into two categories: active and passive mixers. An active mixer uses an external force, typically an electric power input, to stir the samples in the microchannel. Passive mixers, on the other hand, have either small protrusion in the microchannel or rough surfaces on the microchannel wall to enhance mixing without requiring any external energy input. In this study, we numerically investigated the mixing phenomena in the hybrid type microchannel combined both active and passive function. The modeling of passive type microchannel with different shaped obstacles (rectangular, triangular and cylindrical configurations, respectively) is conducted, which were mounted on top and bottom walls with groove pitch to height ratio. Also, we have accorded active effects as applied to electric fields and compared mixing performance with both types of microchannel. The incompressible Navier–Stokes equation is solved in this model, and the convection–diffusion equation was used to describe the concentration of the dissolved substances in the fluid. The concentration, flow and electric fields in the channel were calculated and the results were graphically depicted for various flow and electric conditions.     

Investigation of flow characteristics and eddy viscosity ratio for air-oil intermittent flow in horizontal pipe

Journal of Mechanical Science and Technology - Experiments and three-dimensional numerical computations are performed to investigate the air-oil intermittent two-phase flow in a horizontal pipe for...     

Numerical study on the mixing performance of a ring-type electroosmotic micromixer with different obstacle configurations

A new type of electrokinetic micromixer with a ring-type channel is introduced for fast mixing. The proposed mixer takes two fluids from different inlets and combines them in a ring-type mixing chamber. The fluids enter two different inlets (inner radius: 25 microm and outer radius: 50 microm), respectively. The total channel length is 500 microm, and four microelectrodes are positioned on the outer wall of the mixing chamber. The electric potentials on the four microelectrodes are sinusoidal with time, having various maximum values of voltage, zeta potential and frequency. Also, in order to compare the mixing performance with different obstacle configurations, we performed a numerical analysis using a commercial code, COMSOL. The concentration of the dissolved substances in the working fluid and the flow and electric fields in the channel were investigated and the results were graphically depicted for various flow and electric conditions.     

Two-dimensional two-fluid model for air-oil wavy flow in horizontal tube

This study concerns the development of a two-dimensional two-fluid model for wavy flows in horizontal tubes. To deal with the curved walls of the liquid and gas phases and the gas-liquid interface simultaneously, the bipolar coordinate system was used. Experiments on air-oil mixture flow in horizontal tubes with diameters of 20 and 40 mm were conducted to observe wavy flow patterns accompanying the two-dimensional (2D) and Kelvin-Helmholtz (KH) waves and to measure the pressure gradient under different flow conditions. Two different previous correlations for the interfacial friction factor were employed in the model for predicting the wavy flows with 2D and KH waves. Predictions of the model of the liquid film height, the average values of wall shear stresses of each phase, and the average interfacial shear stress were compared for different diameters and different superficial gas and liquid Reynolds numbers. Also presented are detailed predictions of the model for four different flow conditions, including the local values of interfacial shear stress, wall shear stress of the liquid phase, interfacial friction factor, liquid film height, and two-dimensional velocity distribution in the liquid phase at the cross-section of the tube.

     

Numerical analysis of the zeta potential regarding the characteristics of a ring-type electro-osmotic mixer

The micromixer that is applied for the electro-osmotic effect in this study, which is a passive type, takes two fluids from different inlets and combines them into a single channel. The fluids then enter the mixing chamber with different inner and outer radii. Four microelectrodes are positioned on the outer wall of the mixing chamber. The electric potentials on the four microelectrodes are sinusoidal for certain values of the time, voltage, zeta potential, and frequency. In order to check the validity of the two-dimensional numerical analysis that already has been performed, we performed a three-dimensional numerical analysis using a commercial code, COMSOL. The incompressible Navier-Stokes equation is solved in this model, with a slip boundary condition on the inner and outer walls of the mixing chamber. The results of two- and three-dimensional models are graphically depicted and compared from the viewpoint of streamlines and concentration.     

Validation and updating in a large automotive vibro-acoustic model using a P-box in the frequency domain

In this paper, a model validation framework is proposed and applied to a large vibro-acoustic finite element (FE) model of a passenger car. The framework introduces a p-box approach with an efficient quantification scheme of uncertainty sources and a new area metric which is relevant to the responses in the frequency domain. To prioritize the input uncertainties out of the enormous FE model, the experts’ knowledge is utilized to select candidate input parameters which have large potential influences on the response of interests (ROI) among several thousands of input parameters. Next, a variance-based sensitivity analysis with an orthogonal array is introduced in effort to quantify the influence of the selected input parameters on the ROIs. The employment of the eigenvector dimension reduction method and orthogonal combinations of interval-valued input parameters provides the p-box of the ROI even if the size of the FE model is very large. A color map and the u-pooling of the p-boxes over the frequency band as well as the p-box at different frequencies are introduced to assess the model error and quantitative contributions of the aleatory and the epistemic input uncertainties to the overall variability of the ROIs in the frequency domain. After assessing the model error, the FE model is updated. It was found that the sensitivity results and the experts’ knowledge about the associated components effectively determine the modifications of the component models and the input parameter values during the updating process.     

High-efficiency design optimization of a centrifugal pump

Design optimization of a backward-curved blades centrifugal pump with specific speed of 150 has been performed to improve hydraulic performance of the pump using surrogate modeling and three-dimensional steady Reynolds-averaged Navier-Stokes analysis. The shear stress transport model was used for the analysis of turbulence. Four geometric variables defining the blade hub inlet angle, hub contours, blade outlet angle, and blade angle profile of impeller were selected as design variables, and total efficiency of the pump at design flow rate was set as the objective function for the optimization. Thirty-six design points were chosen using the Latin hypercube sampling, and three different surrogate models were constructed using the objective function values calculated at these design points. The optimal point was searched from the constructed surrogate model by using sequential quadratic programming. The optimum designs of the centrifugal pump predicted by the surrogate models show considerable increases in efficiency compared to a reference design. Performance of the best optimum design was validated compared to experimental data for total efficiency and head.     

Comparative Study of Hydrogen Embrittlement of Three Heat-resistant Cr-Mo Steels Subjected to Electrochemical and Gaseous Hydrogen Charging

Hydrogen embrittlement (HE) behaviors of low carbon steel and three heat-resistant Cr-Mo steels having different Cr contents were compared through tensile testing, silver decoration, and blistering observation after electrochemical hydrogen charging and hardness testing after gaseous hydrogen charging. It was observed that higher Cr content caused a longer suppression in charging hydrogen into the steels. However, under hydrogen supersaturation conditions, the higher the strength of the heat-resistant steels, the poorer the HE resistance after electrochemical charging. In contrast, the higher the Cr content, the better the HE resistance under high-temperature gaseous charging conditions.