Byung Ha Kang

Flow and heat transfer correlations for porous fin in a plate-fin heat exchanger

The present experimental study investigates the impact of porous fins on the pressure drop and heat transfer characteristics in plate-fin heat exchangers. Systematic experiments have been carried out in a simplified model of a plate-porous fin heat exchanger at a controlled test environment. The porous fins are made of 6101 aluminum-alloy foam materials with different permeabilities and porosities. Comparison of performance between the porous fins and the conventional louvered fins has been made. The experimental results indicate that friction and heat transfer rate are significantly affected by permeability as well as porosity of the porous fin. The porous fins used in the present study show similar thermal performance to the conventional louvered fin. However, the louvered fin shows a little better performance in terms of pressure drop. For compactness of the heat exchanger, the porous fins with high pore density and low porosity are preferable. Useful correlations for the friction factor and the modified j-factor are also given for the design of a plate-porous fin heat exchanger.

Heat transfer in the thermally developing region of a pulsating channel flow

Abstract A study is made of the heat transfer characteristics of a fully-developed pulsating flow in a channel. The fluid at the channel inlet is of temperature T0, and the channel walls are at uniform temperaturer Tw Concern is directed to the thermally developing region. The unsteady Navier-Stokes equations are solved numerically to simulate a relatively slow throughflow at Re = 50, Pr = 0.7. Comprehensive time-dependent flow data are obtained for wide ranges of two key parameters, i.e. the pulsation amplitude 0 ⩽ A ⩽ 0.75, and the nondimensional pulsation frequency M up to 10.0. When M is low, the velocity profiles resemble much of the quasi-steady solutions. When M is large, the effects of oscillation are confined to a narrow zone adjacent to the walls. The changes in the Nusselt number Nu due to pulsation are pronounced in the entrance region, say X (Re · Pr ) , and the impact of pulsation on Nu is minor at far downstream locations. The effects of M on Nu are noticeable when M is small and moderate. At high pulsation frequencies, heat transfer is little affected by the addition of pulsation. Detailed analyses on local behavior of heat transfer are made by using Fourier-series representations of the numerical results. These exercises indicate that, due to pulsation, both heat transfer enhancement and reduction can be expected in various axial locations of the channel. Based on these numerical results, physically plausible explanations are offered to interpret the axial behavior of heat transfer.

Dynamic simulation of an absorption heat pump for recovering low grade waste heat

Abstract Numerical simulation was carried out to predict the transient operating characteristics and performance of an absorption heat pump recovering waste heat of 30°C–40°C. The effects of the temperature and the mass flow rate of the heat transfer medium, the heat transfer area of the system components, and the solution circulation rate on the system performance are investigated in detail. The results obtained indicate that a higher heating capacity is obtained with an increase in driving steam temperature, waste heat temperature, and mass flow rate of hot water and waste water. It is also found that the heating capacity becomes higher with an increase in heat transfer area of the system components except solution heat exchanger. An increase in solution flow rate leads to higher heating capacity but to lower COP. A practical absorption heat pump, installed at a chemical plant, was also simulated numerically. The simulation results were compared with the operation data of the practical system.

Forced convection heat transfer from two heated blocks in pulsating channel flow

Abstract The pulsating flow and attendant heat transfer characteristics from two heated blocks in a channel have been numerically investigated. At the channel inlet, a pulsating flow Ui, i.e. Ui = Uo(1 + A sin ωτ), is imposed with uniform temperature Tc. The block surfaces in the channel are at constant temperature TH. The channel walls are assumed to be adiabatic. Comprehensive time-dependent flow and temperature data are obtained and averaged over a cycle of pulsation in a periodic steady state. The effects of the important governing parameters, such as Reynolds number, Re, Strouhal number, St, pulsation amplitude, A, and the spacing between two blocks, w/H, on the heat transfer rate from the heated blocks and the flow behavior in the vicinity of the blocks are also investigated in detail. The results obtained indicate that the recirculation flows behind the downstream block as well as inside the inter-block region are substantially affected by Strouhal number St and inter-block spacing w/H. This, in turn, has a strong influence on the thermal transport from the heated blocks to the pulsating flow. The present results are also compared with those obtained for a steady non-pulsating flow, and the effect of pulsation on the transport process is scrutinized.

Thermal performance of aluminum-foam heat sinks by forced air cooling

Experiments have been carried out to investigate the heat transfer characteristics of an aluminum-foam heat sink placed on a heat source in a channel. The thermal performance of aluminum-foam heat sinks is evaluated in terms of the Nusselt number and thermal resistance of the heat sinks. The pore density of the aluminum-foam heat sinks and the Reynolds number are varied in the range of parameters: 10, 20, 40 pores per inch (PPI) and 710/spl les/Re/spl les/2900, respectively. It is found that thermal resistance is substantially reduced by employing an aluminum-foam heat sink with low pore density due to the relatively intense airflow through the heat sink. The aluminum-foam heat sink may provide 28% or higher thermal performance than a conventional parallel-plate heat sink of the same size. Further, the aluminum-foam heat sinks can dramatically reduce the overall mass of electronics-cooling devices owing to high porosity.

Heat transfer from pulsating flow in a channel filled with porous media

Abstract A numerical study is made of heat transfer characteristics from forced pulsating flow in a channel filled with fluid-saturated porous media. The channel walls are assumed to be at uniform temperature. The Brinkman-Forchheimer-extended Darcy model is employed. The time-dependent, two-dimensional governing equations are solved by using finite-volume techniques. Numerical solutions are obtained for quasi-steady periodic states. Flow and temperature fields are examined over ranges of the principal parameters, i.e. the amplitude of flow pulsation A, the pulsation frequency parameter M [≡H( ω 2ν ) 1 2 ] , the Darcy number Da (≡ K H 2 ) , the thermal conductivity ratio R κ (≡ κ eff κ ) , and the heat capacity ratio R c {≡ (ϱC p ) eff [e(ϱC p )] } . The impact of pulsation is discernible in the cycle-averaged temperature distribution. In comparison with the case of non-pulsating flow, the presence of flow pulsation brings forth a reduction in heat transfer in the entrance region and an enhancement of heat transfer at moderate downstream regions. Farther downstream, the influence of pulsation is meager. The magnitudes of changes in heat transfer depend upon A, M, Da, Rκ, and Rc. The effect of pulsation on heat transfer between the channel wall and the fluid is more pronounced for small M and large A. Explicit influences of Da, Rκ, and Rc on the flow and heat transport characteristics are also scrutinized.

THERMAL INTERACTION BETWEEN ISOLATED HEATED ELECTRONIC COMPONENTS IN PULSATING CHANNEL FLOW

The characteristics of a pulsating flow and the associated thermal transport from two heated blocks, representing energy dissipating electronic components with different heights, in a channel have been numerically investigated. At the channel inlet a pulsating sinusoidal flow is imposed at a uniform temperature. The surfaces of the blocks are taken at a constant higher temperature. The channel walls are assumed to be adiabatic. Results on the time-dependent flow and temperature field are obtained and averaged over a cycle of pulsation. The effect of the important governing parameters, such as the Strouhal number and the dimensionless heights of the blocks, on the flow and the heat transfer is investigated in detail. The results indicate that the recirculating flow behind the second block, as well as that in the interblock region, are substantially affected by the pulsation frequency and the heights of the blocks. These, in turn, have a strong influence on the thermal transport from the heated elements to ...

The lowest oscillation mode of a pendant drop

The lowest oscillation mode of a pendant drop has long been conceived to be the longitudinal vibration, i.e., periodic elongation and contraction along the longitudinal direction. However, here we experimentally show that the rotation of the drop about the longitudinal axis is the oscillation mode of the lowest resonance frequency. This rotational mode can be invoked by periodic acoustic forcing and is analogous to the pendulum rotation, having the frequency independent of the drop density and surface tension but inversely proportional to the square root of the drop size.

Effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of horizontal tubes

An experimental study has been conducted to investigate the effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of various kinds of copper tubes. Plain, spiral, corrugated, and low-finned tubes were selected as test tubes. In this work, to increase the wettability of distilled water on copper tubes, a novel hydrophilic surface treatment method using plasma was employed. The experiments show that every kind of hydrophilic surface treated tube tested in the work exhibits superior evaporation heat transfer performance as compared with that of the same kind of untreated tube. It is found out that during the evaporation process, the high wettability of the surface obtained through hydrophilic treatment induces film flow on the tubes while sessile drops are formed on untreated tubes. The film has a smaller thickness as well as a greater heat transfer area than the sessile drops, and this yields higher heat transfer rate for hydrophilic surface treated tubes than that for untreated tubes.

Performance analysis of a metal-hydride heat transformer for waste heat recovery

Metal-hydride systems are applicable as energy-conversion devices, such as cooling machines, heat pumps and heat transformers. Of particular interest are metal-hydride heat transformers, since they have more advantages over absorption or adsorption heat transformers for high-temperature boosting. Thus, growing attention has been given to metal-hydride heat transformers for the effective use of industrial waste heat. Thermal analysis on a module-type of metal-hydride heat transformer has been carried out to predict the performance of the system. The hydride reactor is considered as a coupled cylindrical tube module in the modelling. The effects of various operating conditions, as well as the design parameters, on the system performance have been extensively investigated. The optimal time of cycle period could be chosen from the numerical results, considering the performance. It is also found that the COP and heating output are increased as the waste-heat temperature is increased. The system performance could also be enhanced at the design phase by increasing the heat-transfer coefficient and the efficiency for internal heat exchange and by reducing the connection tube volume between reactors. The present results are also compared with the previous experimental results of metal-hydride heat transformers, as well as those of absorption heat transformers with LiBr-water.