• Clean Technology
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• v.7 no.4
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• pp.273-280
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• 2001

An absorber is a major component in the absorption refrigeration systems and its performance greatly affects the overall system performance. In this study, experimental analyses on heat transfer characteristics for removal of absorption heat in ammonia-water bubble mode absorber were performed. Heat transfer coefficients were estimated as the variations of input gas flow rate, solution flow rate, temperature, concentration, absorber diameter and height, and input flow direction. The increase of gas and solution flow rate affects positively in heat transfer. However, the increase of solution temperature and concentration affects negatively. Moreover, under the same Reynolds Numbers, countercurrent flow is superior to cocurrent flow in heat transfer performance. In addition, from these experimental data, empirical correlations which can explain easily the characteristics of heat transfer are derived.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.10
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• pp.820-826
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• 2009

The anisotropic phonon conductions with varying Joule heating rate of the silicon film in Silicon-on-Insulator devices are examined using the electron-phonon interaction model. It is found that the phonon heat transfer rate at each boundary of Si-layer has a strong dependence on the heating power rate. And the phonon flow decreases when the temperature gradient has a sharp change within extremely short length scales such as phonon mean free path. Thus the heat generated in the hot spot region is removed primarily by heat conduction through Si-layer at the higher Joule heating level and the phonon nonlocality is mainly attributed to lower group velocity phonons as remarkably dissimilar to the case of electrons in laser heated plasmas. To validate these observations the modified phonon nonlocal model considering complete phonon dispersion relations is introduced as a correct form of the conventional theory. We also reveal that the relation between the phonon heat deposition time from the hot spot region and the relaxation time in Si-layer can be used to estimate the intrinsic thermal resistance in the parallel heat flow direction as Joule heating level varies.

• Transactions of the Korean hydrogen and new energy society
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• v.34 no.6
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• pp.741-747
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• 2023

In this study, the heat transfer characteristics were numerically analyzed to investigate the possibility of utilizing cooling water using liquid nitrogen. From the study, as the mass flow rate of the hot fluid increased, the heat transfer rate increased by 8.9-81.7%. And lowering the inlet temperature of the hot fluid resulted in increase in the heat transfer rate by 33.8-71.5%. As for the filling level of liquid nitrogen, as higher filling level led to a decrease in the outlet temperature and an increase in the overall heat transfer coefficient.

• Solar Energy
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• v.16 no.2
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• pp.65-77
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• 1996

The operating thermal chracteristics of direct contact liquid-ice heat exchanger was experimentally investigated. In this paper, The effects of Ice Packing Factor(IPF), the inlet temperature and the flow rate of Heat Transfer Fluid(HTF) were stuided in the liquid-ice heat exchanger. Thermal stratification in liquid-ice heat exchanger was established clearly and faster at the higher inlet temperature and flow rate of HTF. At the end of melting of the lower flow rate is cleared the thermal stratification in liquid-ice heat exchanger. The temperature stratification is long with higher value of IPF of liquid-ice heat exchanger. The mean temperature of liquid-ice storage was changed rapidly with increasing flow rate and inlet temperature of HTF. The gradiant of ratio of total energy to latent energy was found higher with increasing inlet temperature and flow rate.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.5 s.236
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• pp.633-642
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• 2005

Nucleate pool boiling experiments for R11 under a constant wall temperature condition were carried out. A microscale heater array was used for the heating and the measurement of high temporal and spatial resolution by the Wheatstone bridge circuit. Very sensitive heat flow rate data were obtained by the control for the surface condition with high time resolution. The measured heat flow rate shows a discernable peak at the initial growth stage and reaches an almost constant value. In the thermal growth region, bubble shows a growth proportional to $t^{\frac{1}{5}}$. The bubble growth behavior is analyzed with a dimensionless parameter to compare with the previous results in the same scale. As the wall superheat increases, the departure diameter and the departure time increase, and the waiting time decreases. But the asymptotic growth rate is not affected by the wall superheat change. The effect of the wall superheat is resolved into the suggested growth equation. Dimensionless parameters of time and bubble radius characterize the thermal growth behavior well, irrespective of wall condition. The comparison between the result of this study and the previous results shows a good agreement at the thermal growth region. The quantitative analysis for the heat transfer mechanism is conducted with the measured heat flow rate behavior and the bubble growth behavior. The required heat flow rate for the volume change of the observed bubble is about twice as much as the instantaneous heat flow rate supplied from the wall.

• 한국신재생에너지학회:학술대회논문집
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• 2007.11a
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• pp.542-545
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• 2007

A semi-closed loop ground heat exchanger is proposed and its performance is compared through the measuring the effective thermal conductivity of the ground. In-situ tests based on the line source model are carried out to evaluate the thermal characteristics of each ground heat exchanger which has different penetration water flow rate. The test results show the increasing effective thermal conductivity of ground as the penetration water flow rate(PWFR) is increased. Therefore, the higher thermal performance of the proposed semi-closed ground heat exchanger can be expected.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.2
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• pp.720-729
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• 1996

An analysis of convective boiling heat transfer for refrigerant mixtures is performed for an annular flow to investigate the degradation of the heat transfer rate. Annular flow is selected in this study because a great portion of the evaporator in the refrigeration and air conditioning system is known to be in the annular flow regime. Mass transfer effect due to composition difference between liquid and vapor is included in this analysis, which is considered to be one of driving forces for the mass transfer at the interface. Due to the concentration gradient at the interface the mass transfer is interfered, so is the evaporative heat transfer at the interface. The mass transfer resistance makes the interface temperature slightly higher and, as a result, the heat transfer coefficients decrease compared with those without mass transfer effects. The degradatioin of the heat transfer rate reaches its maximum at a certain composition. The composition difference between vapor core and vapor at the interface has a direct effect on the temperature difference between the vapor core and the interface and the degradation of the heat transfer rate. Correction factor $C_{F}$ for the mixture effects is added to the correlation for pure substances and the flow boiling heat transfer coefficients can be calculated using the modified equation.n.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.175-180
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• 2001

In this study, the minimum heat flux conditions are experimentally investigated for the spray cooling of hot plate. The hot plates are cooled down from the initial temperature of about $900^{\circ}C$, and the local heat flux and surface temperatures are calculated from the measured temperature-time history. The results show that the minimum heat flux point temperatures increase linearly resulting from the propagation of wetting front with the increase of the distance from the stagnation point of spray flow. However, in the wall region, the minimum heat flux point temperature becomes independent of the distance. Also, the experimental results show that the velocity of wetting front increases with the increase of the droplet flow rate.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.10
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• pp.974-981
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• 2001

In this study, the minimum heat flux conditions are experimentally investigated for the spray cooling of hot plate. The hot plates are cooled down from the initial temperature of about$ 900^{\circ}C$, and the local heat flux and surface temperatures are calculated from the measured temperature-time history. The results show that the minimum heat flux point temperatures increase linearly resulting from the propagation of wetting front with the increase of the distance from the stagnation point of spray flow. However, in the wall region, the minimum heat flux point temperature becomes independent of the distance. Also, the velocity of wetting front increases with the increase of the droplet flow rate.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.2
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• pp.15-20
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• 2018

The objective of this study is to numerically investigate the thermal characteristics of an automobile exhaust-based heat exchanger for automotive thermoelectric power generation with various exhaust gas mass flow rates and temperatures. The heat exchanger for automotive thermoelectric power generation has a square-type pin installed inside, so the maximum amount of heat can be transferred to the thermoelectric element from the heat energy coming from the automobile exhaust gas. The exhaust gas mass flow rate changed from 0.01, to 0.02, to 0.03 kg/s, and the exhaust gas temperature changed from 400, to 450, to 500, to 550, to $600^{\circ}C$, respectively. A numerical simulation was conducted by using the commercial program ANSYS CFX v17.0. Consequently, the exhaust gas pressure difference between the inlet and the outlet of the heat exchanger is determined according to the flow rate of the exhaust gas. When the mass flow rate of the exhaust gas increased, the pressure difference between the inlet and the outlet of the heat exchanger increased, but the exhaust gas pressure difference between the inlet of the heat exchanger and the outlet did not vary with the exhaust gas temperature. Therefore, in order to obtain the maximum surface temperature from the heat exchanger, the exhaust gas mass flow rate should be lower, and the exhaust gas temperature should be higher.