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For the successful design of heat exchangers, it is very important to understand local heat transfer phenomena. Because complex flow phenomena affect heat transfer in the cross flow heat exchanger, it is difficult to measure local heat transfer coefficients by conventional methods of heat transfer measurements. In the present study, a naphthalene sublimation technique is employed to measure the local heat transfer coefficients for the cross flow heat exchanger and the experiments are performed for the staggered array of circular tubes. Local and average Nusselt numbers for various air velocities and distances between tubes are investigated, and empirical correlations are obtained using these data.
In the present study, the 2-fluid nozzle and 3-fluid nozzle to atomize the diesel and water with air for the fuel reformer of SOFC system were experimentally examined. In the 2-fluid nozzle, the diesel and water were alternately atomized due to bislug flow pattern, and it implies that the mixing of both liquids strongly affects the atomization pattern. On the other hand, in the 3-fluid nozzle, the diesel and water were atomized simultaneously due to the separated injection channels without mixing problem. Therefore, compared to the 2-fluid nozzle, the 3-fluid nozzle is suitable for the stable operation of the fuel reformer. In case of the 3-fluid nozzle, Type A where the air was supplied through the central channel was the most efficient.
In the present study, the atomization characteristics of a twin-fluid nozzle, which will be the component of a fuel reformer in 1 ㎾ SOFC system, were preliminarily investigated. Diesel and air were used as the test fluids. With two air cap sizes(1.6 and 4.5 ㎜) of the nozzle, the atomization of diesel fuel was visualized, and the approximate spray angle, the diesel flow rate and the average droplet size(SMD) of diesel were examined with the air flow rate and the difference of height(between nozzle and diesel level in diesel container). As the air flow rate and the difference of height were increased, the diesel flow rate sprayed from the nozzle was increased. For the air cap sizes of 1.6 and 4.5 ㎜, the average droplet sizes ranged from 20 to 80㎛, and from 50 to 250㎛, respectively. From this study, it was confirmed that the twin-fluid nozzle could be applied in the 1 ㎾ fuel reformer.
Performance of the water/air direct contact air conditioning system, in which heat and mass are directly transferred between air and water droplet, is simulated using semi-empirical method. Direct contact system improves transport efficiency compared to conventional indirect contact system. In this study, correlations for which represent the capacity of direct contact system are derived as a function of air and water flowrate from the experimental data. Cooling and heating performance of the water/air direct contact air conditioning system are evaluated using these correlations.
Fouling is very serious problem in heat exchanger because it rapidly deteriorates the performance of<br/> heat exchanger. Cross-flow heat exchanger with vortex generators is developed, which enhance heat<br/> transfer and reduce fouling. In the present heat exchanger, shell and baffle are removed from the<br/> conventional shell-and-tube heat exchanger. The naphthalene sublimation technique is employed to<br/> measure the local heat transfer coefficients. The experiments are performed for single circular tube,<br/> staggered array tube bank and in-line array tube bank with and without vortex generators. Local and<br/> average Nusselt numbers of single tube and tube bank with vortex generator are investigated and<br/> compared to those of without vortex generator.