Kyu Hyung Do

Analytical and experimental investigation on the operational characteristics and the thermal optimization of a miniature heat pipe with a grooved wick structure

Abstract A mathematical model for heat and mass transfer in a miniature heat pipe with a grooved wick structure is developed and solved analytically to yield the maximum heat transport rate and the overall thermal resistance under steady-state conditions. The effects of the liquid–vapor interfacial shear stress, the contact angle, and the amount of initial liquid charge have been considered in the proposed model. In particular, a novel method called a modified Shah method is suggested and validated; this method is an essential feature of the proposed model and accounts for the effect of the liquid–vapor interfacial shear stress. In order to verify the model, experiments for measuring the maximum heat transport rate and the overall thermal resistance are conducted. The analytical results for the maximum heat transport rate and the total thermal resistance based on the proposed model are shown to be in close agreement with the experimental results. From the proposed model, numerical optimization is performed to enhance the thermal performance of the miniature heat pipe. It is estimated that the maximum heat transport rate of outer diameter 3 and 4 mm heat pipes can be enhanced up to 48% and 73%, respectively, when the groove wick structure is optimized from the existing configurations. Similarly, the total thermal resistance of these heat pipes can be reduced by 7% and 11%, respectively, as a result of optimization.

Thermal Optimization of an Internally Finned Tube Using Analytical Solutions Based on a Porous Medium Approach

The present work deals with thermal optimization of an internally finned tube having axial straight fins with axially uniform heat flux and peripherally uniform temperature at the wall. The physical domain was divided into two regions: One is the central cylindrical region of the fluid extending to the tips of the fins and the other constituted the remainder of the tube area. The latter region including the fins was modeled as a fluid-saturated porous medium. The Brinkman-extended Darcy equation for fluid flow and two-equation model for heat transfer were used in the porous region, while the classical Navier-Stokes and energy equations were used in the central cylindrical region. The analytical solutions for the velocity and temperature profiles were in close agreement with the corresponding numerical solution as well as with existing theoretical and experimental data. Finally, optimum conditions, where the thermal performance of the internally finned tube is maximized, were determined using the developed analytical solutions.

Modeling and Thermal Optimization of a Micro Heat Pipe With Curved Triangular Grooves

Heat transfer and fluid flow characteristics in a micro heat pipe with curved triangular grooves are investigated using numerical and experimental methods. In the numerical part, a one-dimensional mathematical model for micro heat pipe with curved triangular grooves is developed and solved to obtain the maximum heat transport rate, the capillary radius distribution, the liquid and the vapor pressure distributions along the axial direction of the micro heat pipe under the steady-state condition. In particular, the modified Shah method is applied to calculate the pressure drop induced by the liquid-vapor interfacial shear stress. Experiments are conducted to validate the numerical model. In the experiments, the micro heat pipe with 0.56 mm in hydraulic diameter and 50 mm in length is tested. The experimental results for the maximum heat transport rate agree well with those of the numerical investigations. Finally, thermal optimization of the micro heat pipe with curved triangular grooves is performed using the numerical model.Copyright

Experimental Investigation on Thermal Characteristics of Heat Pipes Using Water-based MWCNT Nanofluids

In this paper, thermal characteristics of cylindrical grooved wick heat pipes with water-based MWCNT nanofluids as working medium are experimentally investigated. Volume fractions of nanoparticles are varied with 0.1% to 0.5%. Transient hot wire method developed in house is used to measure the thermal conductivity of nanofluids. It is enhanced by up to 29% compared to that of DI water. The thermal resistances and temperature distributions at the surface of the heat pipes are measured at the same evaporation temperature. The experimental results show that the thermal resistance of the heat pipes with water-based MWCNT nanofluids as working fluid is reduced up to 35.2% compared with that of heat pipe using DI water. The reduction rate of thermal resistance is greater than the enhancement rate of thermal conductivity. Finally, based on the experimental results, we present the reduction of the thermal resistances of the heat pipes compared with conventional heat pipes cannot be explained by only the thermal conductivity of water-based MWCNT nanofluids.

Effect of Initial Temperature of a Cylindrical Steel Block on Heat Transfer Characteristics of Staggered Array Jets During Water Jet Quenching

The effects of initial temperature of a cylindrical stainless steel block on heat transfer characteristics are investigated for a staggered-array circular jet during water jet quenching. The initial temperature of a block is set to change from 500°C to 1,000°C by an induction heating method. Transient temperature measurements are made at discrete locations near the surface along radial direction. The surface temperature and heat flux distributions can be numerically estimated by solving a two-dimensional axisymmetric inverse heat conduction problem. The heat transfer characteristics of staggered-array jets in the impingement jet zone are compared with those in the wall jet flow zone. The effect of initial temperature of the test block in the nucleate boiling regime is also examined to solve the difficulties in most water jet quenching of hot steel. The area-averaged heat flux curves are used to evaluate representative cooling performance of staggered-array jets. The maximum area-averaged heat flux linearly increases as the initial temperature of the test block increases. Regardless of initial temperature of the test block, the nucleate boiling regimes are remarkably similar to each other. The area-averaged heat flux is also provided for fully developed nucleate boiling.

The Study on the Lifetime Estimation using Fault Tree Analysis in Design Process of LNG Compressor

Fault Tree Analysis to predict the lifetime in the design process of LNG compressor is considered. Fault Trees for P & ID of the compressor are created. Individual components that comprise the compressor are configured with the basic event. The failure rates in the PDS and OREDA are applied. As results, the system failure rate and the reliability over time are obtained. Further, the power transmission and the shaft seal system is confirmed to confidentially importantly contribute to the overall lifetime of the system. These techniques will help to improve the reliability of design of large scale machinery such as a plant.

Effects of Inlet Flow Boundary Conditions on the Thermal Performance of a Plate-Fin Heat Sink With an Impinging Flow

This paper discusses the effect of inlet flow boundary conditions on the performance of a plate-fin heat sink with an impinging flow. The inlet flow boundary conditions of the plate-fin heat sink are decided by duct configurations and heat sink geometries. First, velocity distributions according to the inlet flow boundary conditions of the plate-fin heat sink are obtained using numerical simulations. These results clearly show that the inlet flow boundary condition is divided into two branches: one is uniformly impinging flow and the other is non-uniformly impinging flow. Also, the fluid characteristics of the plate-fin heat sink with an impinging flow according to the inlet flow boundary conditions are experimentally examined by measuring the stagnation pressure distributions on the bottom of the plate-fin heat sink. Based on Do et al., correlations for pressure drop and thermal resistance of the plate-fin heat sink with uniformly impinging flow are proposed. Also, pressure drop and thermal resistance correlations of the plate-fin heat sink with uniformly impinging flow are compared with those of the plate-fin heat sink with non-uniformly impinging flow using correlations suggested by Kim et al. Finally, it is shown that the plate-fin heat sink with uniformly impinging flow has a lower thermal resistance than the plate-fin heat sink the non-uniformly impinging flow when the dimensionless length of the plate-fin heat sink is small and the dimensionless pumping power is large.Copyright

Experimental Investigation of Water Hammer Phenomena in a HelicalCoil Tube for Examination of Water Hammer Models

Extended Abstract Since all flows will eventually be changed, either suddenly or gradually, all pipeline systems inevitably experience transient effects [1]. These effects can normally produce sudden increase in pressure in the pipeline systems. These phenomena are called water hammer. Water hammer phenomena in the pipeline systems can result in not only damage to equipment, but also possible injury to plant personnel. Due to these safe problems, it is important to predict water hammer in the pipeline systems. In this study, water hammer phenomena in a helical coil tube are experimentally investigated. First of all, a friction factor is very important to estimate pressure propagation behaviour in water hammer prediction. Several models for the friction factors in a helical coil tube have been developed. However, there exists discrepancies between the models and there is no model for a friction factor of a helical coil tube in transition regime. Therefore, friction factors in a helical coil tube are experimentally obtained for laminar, transition, and turbulent regimes and compared with the previous models. Based on the comparison, the appropriate models are used for estimation of water hammer phenomena. Then, experiments are performed for water hammer phenomena. Because pressure propagation in water hammer phenomena is very fast, a helical coil tube should be long in order to have time enough to measure the pressure propagation. So, the length of a helical coil tube is determined as 100 m. For the implementation of sudden change of flows, a shutdown valve is located at the end of a helical coil tube. After the valve is suddenly closed, water hammer phenomena are occurred and the increased pressure wave travels along the tube. In order to measure the pressure propagation along the tube, pressure transducers with high frequency are installed every 10 meters along the tube. Experiments are performed as varying flow rate. As a result, the amplitude of pressure wave increases as increasing flow rate but the period of pressure wave is constant regardless to flow rate. Sometimes, the pressure in a pipeline drops to the vapour pressure at high flow rate. Then, cavitation is occurred in the tube. In this state, the amplitude of pressure wave is very large, it is not cyclic, and its period is not constant. Finally, experimental results for water hammer in a helical coil tube are compared with the models developed by previous researchers. In this study, the water hammer models are considered because they have high accuracy compared to the other models [1]. The water hammer models and the method how to solve them are well explained in [2, 3]. By comparison between experimental results and the models, it is shown that the unsteady friction coefficient model developed by Reddy et al. [4] corresponds well experimental results in this study. The results based on the other models have some deviations from the experimental results. In addition, when the cavitation is occurred during the water hammer phenomena, the pressure wave propagations are totally different from the water hammer models considered in this study. For this case, it is necessary to develop new models to predict pressure wave propagations under the cavitation.

Study on the Thermal Performance of a Solar Assisted Heat Pump System with a Hybrid Collector

>> In the present work, a solar assisted heat pump (SAHP) system with a hybrid collector was analyzed. For this, a simplified thermodynamic model was developed. Based on the proposed model, the heat transfer rate, COP, and the annual operating hour of the SAHP system were estimated. The effect of the variation of system design parameters on the performance of the system was also examined. From the results, the performance was improved with increasing the effectiveness of heat exchangers and decreasing the difference between the evaporation temperature and the outlet brine temperature of the hybrid collector loop. Finally, the performance of SAHP system with a hybrid collector was compared with that of conventional serial and parallel SAHP systems. The SAHP system with a hybrid collector was substantially better than a series system and slightly worse than a parallel system for both the yearly averaged heat transfer rate and COP. However, the annual operating hour of the SAHP system with a hybrid collector was much better than that of a parallel system.

Experimentally Evaluation of a Liquid Pool Spreading Model with Continuous Release

>> In this study, an experimental investigation is performed for evaluation of a liquid pool spreading model with continuous release. The model considered in this study was developed based on a concept which means that the liquid pool spreading is governed by a balance between an inertia force from gravity and a frictional force from friction with the ground under the whole base of the liquid pool. For evaluation of the model, experimental study is performed. Experimental apparatus is setup for measuring release rate, spreading velocity, and evaporation rate from a liquid pool. The experimental results are compared with results from the model. By applying release and evaporation rates obtained from experiments to solving the model, liquid pool radius variation according to time can be obtained. For evaluation of an effect of friction force in the spreading model, results obtained from the models with and without the friction force are compared with those obtained from the experiments. As a result, it is shown that there exists a large deviation between the results obtained from the model without the friction force and the experimental results. On the other hand, the tendency of liquid pool radius variation according to time is similar between the results obtained from the model without the friction force and the experimental results.