• 설비공학논문집
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• 제16권1호
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• pp.16-25
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• 2004

The flow and heat/mass transfer in the falling-film of a heat exchanger can be influenced by the motion of the surrounding refrigerant vapor. In this study, the effect of the vapor flow direction on the absorption heat transfer has been investigated for a falling-film helical coil which is frequently used as the absorber of ammonia/water absorption refrigerators. The experiments were carried out for different solution concentration. The heat and mass transfer performance was measured for both parallel and counter-current flow. The effect of vapor flow on the heat and mass transfer is found to be increased with decreasing solution concentration. In the experiments with low solution concentration, whose vapor specific volume is great, the counter-current flow of vapor resulted in uneven distribution of falling-film and reduced the heat transfer performance of the absorber. The direction of the vapor flow hardly affected the thermal performance as the solution concentration became stronger since the specific volume of the ammonia/water vapor was much smaller than that of the water vapor.

• 설비공학논문집
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• 제16권12호
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• pp.1134-1139
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• 2004

The evaporative heat transfer coefficient of $CO_2$ (R-744) in a horizontal tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator (test section). The test section consists of a smooth, horizontal stainless steel tube of inner diameter of 7.75 mm. The experiments were conducted at mass flux of 200 to 500 kg/m$^2$s, saturation temperature of -5 to 5$^{\circ}C$, and heat flux of 10 to 40kW/m$^2$. The test results showed the heat transfer of $CO_2$ has a greater effect on nucleate boiling more than convective boiling. Mass flux of $CO_2$ does not affect nucleate boiling too much, and the effect of mass flux on evaporative heat transfer of $CO_2$ is much smaller than that of refrigerant R-22 and R-134a. In comparison with test results and existing correlations, correlations failed to predict the evaporative heat transfer coefficient of $CO_2$, therefore, it is necessary to develope reliable and accurate predictions determining the evaporative heat transfer coefficient of $CO_2$ in a horizontal tube.

• 설비공학논문집
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• 제25권8호
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• pp.427-431
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• 2013

Ground heat pump systems utilize the annually stable underground temperature to supply heat for space heating and cooling. The underground temperature affects not only the underground ecosystem, but also the performance of these systems. However, in spite of the widespread use of these systems, there have been few researches on the effect of the systems on underground temperature. In this research, case studies with numerical simulation have been conducted, in order to estimate the effect of ground heat pump systems on underground temperature. The simulation was coupled with the ground water-ground heat transfer model and the ground surface heat transfer model. In the result, it was found that the underground change depends on the heat transfer from the ground surface, the heat exchange rate, and the heat conductivity of soil.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1069-1074
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• 2004

This study investigated the effect of pressure variations on augmentation of heat transfer when the ultrasonic waves were applied. The augmentation ratio of heat transfer was experimentally investigated and was compared with the profiles of pressure distributions calculated applying a coupled finite element-boundary element method (coupled FE-BEM). As the ultrasonic intensities increase from 70W to 340W, the velocity of the liquid paraffin is found to increase as well as kinetic energy, This physical behavior known as quasi-Eckart streaming results from acoustic pressure variations in the liquid. Especially, the higher acoustic pressure distribution near two ultrasonic transducers develops more intensive flow (quasi-Eckart streaming), destroying the flow instability. Also, the profile of acoustic pressure variation is consistent with that of augmentation of heat transfer. This mechanism is believed to increase the ratio of hear transfer coefficient.

• 대한설비공학회지:설비저널
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• 제7권3호
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• pp.151-159
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• 1978

A heat transfer model for the case of simultaneous vibration of both the heated surface and its surrounding medium is constructed and the dimensional analysis is applied to this model in order to and the governing dimensionless Parameters in which the vibration effects the heat transfer. In the second Part of this study, an experimental investigation of the effect of vibration on natural convective heal transfer from spheres has been performed for the case of the external oscillatory motion being imposed on the heated surface which is immersed in an otherwise undisturbed air, The ranges of the experimental variables were: temperature difference 10 to $120^{\circ}C$; vibration frequency 10 to 120Hz; displacement amplitude 1.3 to 12.5mm. Three different diameter aluminum were used as the experimental models. Improvements in heat transfer due to vibration were observed, with the maximum increase being 330 Percent. A dimensionless correlation describing the measured heat transfer data is given.

• 대한기계학회논문집
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• 제19권9호
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• pp.2386-2398
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• 1995

The paper describes the results of an experimental investigation of the heat transfer rate on the endwall surface within the plane turbine cascade passage and includes the effect of the heat transfer for the two different boundary layer thicknesses and Reynolds numbers. The limiting streamlines on the endwall surface have been visualized by the oil film method in order to compare with the endwall heat transfer. The hue-capturing method using the termochromatic liquid crystals with great spatial resolution has been used to provide the local distribution of the endwall heat transfer coefficients. Because the detailed contours of the local heat transfer coefficients over the entire endwall can be obtained from the hue-capturing method, it has been possible to obtain information on the endwall heat transfer within the plane turbine cascade passage from these heat transfer contours.

• 설비공학논문집
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• 제15권4호
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• pp.274-286
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• 2003

Numerical simulations are performed to investigate the turbulent convective heat transfer of the supercritical carbon dioxide flows in vertical and horizontal square ducts. The gas cooling process at the supercritical state experiences a sudden change in thermodynamic and transport properties. This results in the extraordinary variations of the heat transfer coefficients in the supercritical state, which are much different from those of single or two phase flows. Algebraic second moment closure which can include the effects of large thermophysical property variations of carbon dioxide and of buoyancy is employed to model the Reynolds stresses and turbulent heat fluxes in the governing equations. The previous correlations for the turbulent heat transfer coefficient for the supercritical carbon dioxide flows couldn't reflect the buoyancy effect. The present results are used to establish a new heat transfer coefficient correlation including the effects of large thermophysical property variation and buoyancy on in-duct cooling process of supercritical carbon dioxide.

• 대한기계학회논문집B
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• 제33권1호
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• pp.17-27
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• 2009

Water jet impingement cooling is used to remove heat from high-temperature surfaces such as hot steel plates in the steel manufacturing process (thermo-mechanical cooling process; TMCP). In those processes, uniform cooling is the most critical factor to ensure high strength steel and good quality. In this study, experiments are performed to measure the heat transfer coefficient together with the inverse heat conduction problem (IHCP) analysis for a plate cooled by planar water jet. In the inverse heat transfer analysis, spatial and temporal variations of heat transfer coefficient, with no information regarding its functional form, are determined by employing the conjugate gradient method with an adjoint problem. To estimate the two dimensional distribution of heat transfer coefficient and heat flux for planar waterjet cooling, eight thermo-couple are installed inside the plate. The results show that heat transfer coefficient is approximately uniform in the span-wise direction in the early stage of cooling. In the later stage where the forced-convection effect is important, the heat transfer coefficient becomes larger in the edge region. The surface temperature vs. heat flux characteristics are also investigated for the entire boiling regimes. In addition, the heat transfer rate for the two different plate geometries are compared at the same Reynolds number.

• Advances in nano research
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• 제15권3호
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• pp.191-201
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• 2023

In order to develop better method to enhance and control the flow and heat transfer inside the radiator of electronic device, the synergistic effect of MHD nanofluids and porous medium on the flow and heat transfer in rectangular opened channel is simulated using Lattice Boltzmann method. Three nanofluids of CuO-water, Al₂O₃-water and Fe₃O₄-water are studied to analyze the influence of the type of nanofluid on the synergistic effect. The simulation results show that the porous medium can increase the flow velocity in fluid zone adjacent to the porous medium and enhance the heat transfer on the surface of the channel. Under no magnetic field, when the porosity of porous medium is 0.8, the Nusselt number is 4.46% higher than when the porosity is 0.9. Al₂O₃-water has the best heat transfer effect among the three nanofluids. At Ф=0.06, Ha=100, θ=90°, ε=0.9, Nu of Al₂O₃-water is 6.51% larger than that of CuO-water and 5.05% larger than that of Fe₃O₄-water. Magnetic field enhances seepage in porous medium and inhibits heat transfer in the bottom wall. When Ha=30 and 60, the inhibiting effect is the most significant as the magnetic field angle is 90°. And when Ha=100, the inhibiting effect is the most significant as the magnetic field angle is 120°.

• 설비공학논문집
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• 제18권10호
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• pp.842-849
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• 2006

In this study, the effect of carbon nanotubes (CNTs) on boiling heat transfer is investigated. Three refrigerants of R22, R123, R134a, and water are used as base working fluids and 1% of CNTs by volume is added to the base fluids to study the effect of CNTs. All data are obtained at the pool temperature of $7^{\circ}C$ for all refrigerants and $100^{\circ}C$ for water in the heat flux range of $10{\sim}80\;kW/m^2$. Test results show that CNTs increase the boiling heat transfer coefficients for all fluids. Especially, large enhancement was observed at low heat flutes. With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generations. Fouling was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion should be examined for their application in pool boiling heat transfer.