• International Journal of Aeronautical and Space Sciences
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• v.11 no.1
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• pp.19-25
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• 2010

A computational study for the optimal design of heat exchangers (HX) used in a high temperature and high pressure system is presented. Two types of air to air HX are considered in this study. One is a single-pass cross-flow type with straight plain tubes and the other is a two-pass cross-counter flow type with plain U-tubes. These two types of HX have the staggered arrangement of tubes. The design models are formulated using the number of transfer units ($\varepsilon$-NTU method) and optimized using a genetic algorithm. In order to design compact light weight HX with the minimum pressure loss and the maximum heat exchange rate, the weight of HX core is chosen as the object function. Dimensions and tube pitch ratio of a HX are used as design variables. Demanded performance such as the pressure loss (${\Delta}P$) and the temperature drop (${\Delta}T$) are used as constraints. The performance of HX is discussed and their optimal designs are presented with an investigation of the effect of design variables and constraints.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.10
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• pp.1033-1040
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• 2011

The premixed burner is a very strong candidate for using household boiler burner system because it has high efficiency, low emission and can be used in compact boiler system. Usually, household boiler burner systems use a Bunsen burner, which consists of an inner rich premixed flame and fuel burned completely by a secondary air supply. It has a relatively long flame length and operates in a high excess of air, so it is difficult to fit such a burner into a high efficiency compact boiler. In this paper, the characteristics of a premixed combustion burner for surface media such as metal fiber, ceramic, and SUS fin were evaluated. In particular, the flow velocity over the burner surface for the cold flow characteristics of the surface material were measured and adjusted. The combustion tests were carried out by taking pictures of the flame and measuring the flame temperature. The amounts of CO and NO were measured and the characteristics of the surface burner materials, combustion chamber, and heat exchangers were evaluated for various excess air ratios and heating values.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.2
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• pp.216-224
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• 2001

Experiments have been performed to investigate evaporative heat transfer characteristics of R-134a flowing in a small diameter tube. Test section was made of stainless steel tube with an inner diameter of 2.2mm and was uniformly heated by electric current which was applied to the tube wall. The local saturation temperature of refrigerant flowing in a tube is calculated from the measured local saturation pressure by using an equation of state. Inner wall temperature was calculated from measured outer wall temperature, accounting for heat generation in the tube and one dimensional heat conduction through the tube wall. Mass quality of refrigerant flowing in a tube was calculated by considering energy balance in the pre-heater and the test section. Heat flux was varied from 19 to 64kW/$m^2$, and mass flux was chanted from 380 to 570kg/$m^2$s for each heat flux condition. From this study, heat transfer in a small diameter tube is affected by heat flux as well as mass flux for a wide range of mass quality. Heat transfer coefficient in a small diameter tube is much greater than that in medium sized tubes. Test results in this study are compared with Gungor and Winterton correlation, which gives an absolute average deviation of 27%.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.3
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• pp.690-700
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a circular cylinder were examined in a wind tunnel. In order to produce strong thermal stratifications, a compact heat exchanger type variable electric heater is employed. Linear temperature gradient of up to $250^{\circ}C/m$ can be well sustained. The velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured and the smoke wire flow visualization method was used to investigate the wake characteristics. It is found that the temperature field effects as an active contaminant, so that the mean velocity and temperature profiles can not sustain their symmetricity about the wake centerline when such a strong thermal gradient is superimposed. It is evident that the turbulent mixing in the upper half section is stronger than that of the lower half of the wake in a stably stratified flow.

• 한국신재생에너지학회:학술대회논문집
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• 2007.11a
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• pp.19-22
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• 2007

In this study, we investigated operating characteristics of natural gas fuel processor for polymer electrolyte membrane fuel cells (PEMFCs). The fuel processor consists of a natural gas reformer, a water-gas shift reactor, a heat-exchanger and a burner, in which the overall integrated volume is exactly(exceptionally) small, namely, about 10L except outer insulation. The producted hydrogen is $1Nm^3/hr$ and the maximum thermal efficiency is ${\sim}76%$(low heating value) at full operating load. A compact and highly efficient $1Nm^3/hr$ class natural gas fuel processor was developed at UNISON is an advantage for application in residential PEMFCs co-generation systems.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.8
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• pp.1195-1202
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• 2004

In this paper an experimental study was investigated for two-phase flow distribution in compact heat exchanger header. A test section was consisted of the horizontal bottom dividing header($\phi$: 5 mm, L: 80 mm) and 10 upward circular mini channels ($\phi$: 1.5 mm, L: 850 mm) using an acrylic tube. Three different types of tube intrusion depth were tested for the mass flux and inlet mass quality ranges of 50 - 200 kg/$m^2$s and 0.1 - 0.3, respectively. Air and water were used as the test fluids. The distribution of vapor and liquid is obtained by measurement of the total mass flow rate and the calculation of the quality. Two-phase flow pattern was observed, and pressure drop of each channel was measured. By adjusting the intrusion depth of each channel an uniform liquid flow distribution through the each channel was able to solve the mal-distribution problem.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.1
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• pp.30-36
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• 2010

The heat flowrate and pressure drop of $CO_2$ in a multi-tube-in-tube helical coil type gas cooler were investigated experimentally. The mass flowrate of $CO_2$ and coolant were varied from 0.06 to 0.075 [kg/s], respectively and the cooling pressure of gas cooler were from 8 to 10 [MPa]. The heat flowrate of $CO_2$ in the test section is increased with the increase in mass flowrate of coolant, the cooling pressure and mass flowrate of $CO_2$. The pressure drop of $CO_2$ is decreased with the decrease in mass flowrate of coolant and $CO_2$, but decreased with increase in cooling pressure of $CO_2$. The heat flowrate of $CO_2$ in the multi-tube-in-tube helical coil type gas cooler is greatly higher than that of $CO_2$ in the double pipe type gas cooler, while the pressure drop of $CO_2$ in the multi-tube-in-tube helical coil type gas cooler is greatly lower than that of $CO_2$ in the double pipe type gas cooler. Therefore, in case of the application of $CO_2$ at the multi-tube-in-tube helical coil type gas cooler, it is expected to carry out the high-efficiency, high-performance and compactness of gas cooler.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.323-327
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• 2009

To extract hydrogen for stack, fuels such as LPG and LNG were reformed in the fuel processor, which is comprised of desulfurizer, reformer, shift converter, CO remover and steam generator. All elements of fuel processor are integrated in a single package. Highly active catalysts (desulfurizing adsorbent, reforming catalyst, CO shift catalyst, CO removal catalyst) and the various burners were developed and evaluated in this study. The performance of the developed catalysts and the commercial ones was similar. 1 kW, 5 kW class fuel processor systems using the developed catalyst and burner showed efficiency of 75 %(LHV, for LNG). The start-up time of the 1 kW class fuel processor was less than 50 minutes and its volume including insulation was about 30 l. The start-up time of 3 kW and 5 kW class fuel processors with the volume of 90 l and 150 l, respectively, was about 60 minutes. In the case of LPG fuel, efficiency, volume and start-up time of 1kW class fuel processor showed 73 %(LHV), < 60 l and < 60 min, respectively. Advanced fuel processor showed more highly efficiency and shorter start-up time due to the improvement of heat exchanger and operating method. 1 kW and 3 kW class fuel processors have been evaluated for reliability and durability including with on/off test of developed catalysts and burner.