Jinho Lee

Bio: Jinho Lee is an academic researcher from Yonsei University. The author has contributed to research in topics: Convection & Porous medium. The author has an hindex of 17, co-authored 65 publications receiving 1271 citations.

Heat transfer enhancement of copper-water nanofluids in a lid-driven enclosure

Abstract: A numerical study is conducted to investigate the transport mechanism of mixed convection in a lid-driven enclosure filled with nanofluids. The two vertical walls of the enclosure are insulated while the horizontal walls are kept at constant temperatures with the top surface moving at a constant speed. The numerical approach is based on the finite volume technique with a staggered grid arrangement. The SIMPLE algorithm is used for handling the pressure velocity coupling. Numerical solutions are obtained for a wide range of parameters and copper-water nanofluid is used with Pr = 6.2 . The streamlines, isotherm plots and the variation of the average Nusselt number at the hot wall are presented and discussed. It is found that both the aspect ratio and solid volume fraction affect the fluid flow and heat transfer in the enclosure. Also, the variation of the average Nusselt number is linear with solid volume fraction.

Frost retardation of an air-source heat pump by the hot gas bypass method

Abstract: This study is concerned with a hot gas (refrigerant) bypass method to retard the formation and propagation of frost in an air-source heat pump. The feasibility of the hot gas bypass method was investigated experimentally and the method's performance is compared with that of a normal, 1.12 kW capacity air-source heat pump system with no defrost equipment such as an electric resistance heater. Results indicate that the hot gas bypass method is useful for retarding the formation and growth of frost at the outdoor coil. The best performance is shown under a bypass refrigerant flow rate of 0.2 kg/min (20% of the whole system refrigerant flow rate). During 210 min of heat pump operation, the hot gas bypass method improved COP and heating capacity at an average of 8.5% and 5.7%, respectively, relative to the normal system.

The application of photo-coupler for frost detecting in an air-source heat pump

Abstract: This experimental study is carried out to investigate reliability and effectiveness of a new method of using photo-coupler for detecting frost formation in an air source heat pump, and further to determine the most efficient initiation point of the defrost cycle. This new method of using photo-coupler as a frost sensing device is evaluated by comparing its performance with conventional time control defrost system in which defrost cycle is set to start at predetermined interval, e.g. about at every 1–1.5 h. Results indicate that overall heating capacity of photo-coupler detection method (case IV) is 5.5% higher than that of time control method. It is also shown that for maximum efficiency the defrost cycle must be initiated before the frost build-up area exceeds 45% of total front surface of the outdoor coil.

Numerical investigation of the effect of magnetic field on the onset of nanofluid convection

Abstract: The present analysis aims at investigating the effect of a uniform vertical magnetic field on the onset of convection in an electrically conducting nanofluid layer with a new set of physical boundary condition. It is assumed that the value of the temperature can be imposed on the boundaries, but the nanoparticle fraction adjusts together with effects of Brownian and thermophoresis so that the nanoparticle flux is zero on the boundaries. Using the Galerkin method, the critical Rayleigh number on the onset of convection and the corresponding wave number are obtained in terms of various parameters numerically. The numerical computations are presented for water-based nanofluids with Al2O3 and Cu nanoparticles. It is found that the volumetric fraction of nanoparticle, the Lewis number, the modified diffusivity and the density ratios have a destabilizing effect, while the magnetic field has stabilizing effect on the system. The zero flux nanoparticle boundary condition has more destabilizing effect than the constant nanoparticle boundary conditions for Al2O3–water nanofluid, while reverse for Cu–water nanofluid.

Thermal instability in a rotating porous layer saturated by a non-Newtonian nanofluid with thermal conductivity and viscosity variation

Abstract: The stability of a non-Newtonian nanofluid saturated horizontal rotating porous layer subjected to thermal conductivity and viscosity variation is investigated using linear and nonlinear stability analyses. The model used for the non-Newtonian nanofluid includes the effects of Brownian motion and thermophoresis. The Darcy law for the non-Newtonian nanofluid of the Oldroyd type is used to model the momentum equation. The linear theory based on the normal mode method, and the criteria for both stationary and oscillatory modes are derived analytically. A weak nonlinear analysis based on the minimal representation of truncated Fourier series method containing only two terms is used to compute the concentration and thermal Nusselt numbers. The results obtained during the analysis are presented graphically.

Stagnation-point flow of a nanofluid towards a stretching sheet

Abstract: This communication reports the flow of a nanofluid near a stagnation-point towards a stretching surface. The effects of Brownian motion and thermophoresis are further taken into account. The analytic solutions are developed by homotopy analysis method (HAM). Special emphasis has been given to the parameters of physical interest which include stretching ratio a / c , Prandtl number Pr , Lewis number Le , Brownian motion number Nb and thermophoresis number Nt . It is observed that reduced Nusselt number is an increasing function of ratio a / c . The comparison of the present results with the existing numerical solutions in a liming sense is also shown and this comparison is very good.

Refrigeration And Air Conditioning

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Mixed convection flow in a lid-driven inclined square enclosure filled with a nanofluid

Abstract: This work is focused on the numerical modeling of steady laminar mixed convection flow in a lid-driven inclined square enclosure filled with water–Al2O3 nanofluid. The left and right walls of the enclosure are kept insulated while the bottom and top walls are maintained at constant temperatures with the top surface being the hot wall and moving at a constant speed. The developed equations are given in terms of the stream function–vorticity formulation and are non-dimensionalized and then solved numerically subject to appropriate boundary conditions by a second-order accurate finite-volume method. Comparisons with previously published work are performed and found to be in good agreement. A parametric study is conducted and a set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles and enclosure inclination angle on the flow and heat transfer characteristics. It is found that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by inclination of the enclosure at moderate and large Richardson numbers.

Nanofluid flow and heat transfer in a rotating system in the presence of a magnetic field

Abstract: In this paper the magnetohydrodynamic (MHD) nanofluid flow and heat transfer between two horizontal plates in a rotating system is analyzed. The lower plate is a stretching sheet and the upper one is a solid permeable plate. The basic partial differential equations are reduced to ordinary differential equations which are solved numerically using the fourth-order Runge–Kutta method. Different types of nanoparticles such as copper, silver, alumina and titanium oxide with water as their base fluid have been considered. Velocity and temperature profiles as well as the skin friction coefficient and the Nusselt number are determined numerically. The influence of pertinent parameters such as nanofluid volume fraction, magnetic parameter, wall injection/suction parameter, viscosity parameter and rotation parameter on the flow and heat transfer characteristics is discussed. The results indicate that, for both suction and injection the Nusselt number has a direct relationship with the nanoparticle volume fraction. The type of nanofluid is a key factor for heat transfer enhancement. The highest values are obtained when titanium oxide is used as a nanoparticle. Also it can be found that the Nusselt number decreases with the increase of the magnetic parameter due to the presence of Lorentz forces.