• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.28 no.10
• /
• pp.1139-1146
• /
• 2004

The effect of triangular tabs attached at the perimeter of jet nozzle on heat transfer enhancement was investigated experimentally. The modified flow structure was visualized using a smoke-wire method. Four different types of jet nozzle having 0, 4, 6 and 8 tabs were tested at jet Reynolds number Re=15,000 to investigate the effect of tabs on the variation of heat transfer rate. The local and average Nusselt numbers are increased with increasing the number of tabs. At nozzle-to-plate distance of L/D=4, the average Nusselt number was increased about 9.9% at Re=15,000 in the impingement region for the case of 8 tabs attachment. As the nozzle-to-plate distance increases, however, the heat transfer enhancement effect of triangular tabs is reduced. For the case of 4 tabs, the heat transfer enhancement is not so distinctive at L/D=8. As the protrusion depth of tabs into the jet flow increases, the heat transfer rate is also enhanced when the nozzle-to-plate distance is smaller than L/D=6.

• Proceedings of the SAREK Conference
• /
• 2009.06a
• /
• pp.1503-1506
• /
• 2009

Bubble jet loop heat pipe is a newly devised variation of heat pipe in which heat is effectively transported by the latent heat of evaporation and condensation as well as the heat capacity of circulating working fluid. The circulatory and oscillating motion of the working fluid becomes possible by the motion of bubble jet which is generated at a narrow circular gap. These bubbles are condensed at the condensing section. Bubble jet loop heat pipe makes it possible to carry heat long distances upward and horizontal directions. Because Its structure is a very simple and a low cost, it is available for the floor heating, vinyl house heating, the defrosting of heat pump system and home refrigerator.

• Journal of computational fluids engineering
• /
• v.14 no.1
• /
• pp.70-77
• /
• 2009

In this study, time-dependent numerical analysis was carried out to investigate the plasma jet impingement on a flat plate, and a compressible form of two-dimensional inviscid gas dynamics equations were solved using the flux corrected transport algorithm. The mathematical modeling of Joule heating in the polycarbonate capillary bore and the mass ablation from the bore wall was incorporated in the numerical analysis and the series of computation was performed for three cases depending on the distance of the opposing plate from the capillary exit. The computational results reveal that the presence of the opposing plate does not affect the flow conditions inside the capillary when compared to the case of open-air plasma discharge. In the exterior region, the flow structure shows the typical supersonic underexpanded jet which consists of the strong Mach disk in front of the opposing plate and the barrel shock at the side of the jet. It is found that the shock evolution becomes more quasi-steady when the plate distance decreases. Also, the effects of the distance between the capillary bore exit and the opposing plate on the flow conditions along the opposing plate are investigated and the pressure variation on the plate shows more complicated interaction between the plasma discharge and the opposing plate when the location of plate becomes closer to the capillary exit.

• 한국연소학회:학술대회논문집
• /
• 2006.04a
• /
• pp.105-110
• /
• 2006

Many previous works have been performed to provide correlations of flame length, theoretically and experimentally. Most of these results studied were conducted in vertical turbulent flame with no coaxial air condition. The present study analyzes the flame length scaling with coaxial air. In turbulent hydrogen non-premixed jet flames with coaxial air, flame length scaling theoretically proposed so far has been related with the concept of a far-field equivalent source. At high coaxial air to fuel velocity ratio, $U_A/U_F$, however, this scaling theory has some difference with experimental flame length data. This difference is understood to be due to the fact that the theory is based on far-field notion, while the effect of coaxial air on jet flame occurs in the region near the nozzle exit. Therefore, we define effective jet density $P_{eff}$ involving the concept of near-field so that effective jet diameter can be extended to the near-field region. In this condition, we modify the correlation and compare with experimental data.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.6 no.2
• /
• pp.45-52
• /
• 2002

An experimental investigation on the structure and dynamic behavior of two dimensional over-expanded air jets exiting into water was carried out. The hish speed digital video imaging and static pressure distribution measurement were made to characterize the structure and time-dependant behavior of the jets. Mach number at the jet exit was 2.0 and was slightly less than the value predicted by the ideal nozzle calculation. Variance of jet spreading angle at different stagnation condition was measured as a function of mass flow rate. Periodic nature of the air jet distortion in water was observed and the frequency of the repetition was approximately 5-6 Hz for all cases tested. Three characteristic length scales were defined to characterize jet structure. $L_1$, maximum width of the plume when the periodic instability occurs, $L_2$, width of the jet where secondary reverse flow entrained jet flow and $L_3$, distance from the jet exit to the location where entrainment of the secondary reverse flow occurs. The ratio of $L_1$ and $L_2$ decreased with increasing stagnation pressure, i.e. mass flow rate. $L_3$ increased with increasing stagnation pressure. The temporal behavior of static pressure measurements also showed peak around frequency of 5, which corresponds the frequency obtained by visual measurements

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.14 no.5
• /
• pp.1234-1243
• /
• 1990

Double coaxial are jets(annular and coaxial air jets) between which propane gas is fed was selected to study the structure of diffusion flames in turbulent shear flow. Schlieren and direct photographs are taken to visualize the flame structure. Mean and fluctuating temperatures and ion currents were measured to investigate the macroscopic and the instantaneous flame structure. The objective of this study is to understand the interaction between combustion and mixing process especially in the transition region of turbulent shear flow. The investigation reported in this paper focuses on the macroscopic and the instantaneous structures of three flames obtained. The increased mixing effect resulting from increase of Reynolds number of central air jet makes the flame bluish and short. When the velocity of surrounding air stream is higher than that of central air jet, the instantaneous flame structure is composed of coherent structure. It is considered that the flame structure of transitional region of mixing layer depends on the structure of mixing layer of non-reacting conditions.

• 한국가시화정보학회:학술대회논문집
• /
• 2005.12a
• /
• pp.36-39
• /
• 2005

Information on temporal evolution of whole velocity fields is essential for physical understanding of a complicated turbulent flow and was obtainable using dynamic PIV because of advances of high-speed imaging technique, laser and electronics. A dynamic PIV systme consists of a high-speed CMOS camera having $1K\times1K$ pixels resolution at 1 KHz and a high-repetition Nd:Yag pulse laser. In order to validate its performance, the dynamic PIV system was applied to a turbulent jet whose Reynolds number is about 3000. The particle images of $1024\times512$ pixels were captured at a sampling rate of 4 KHz. The dynamic PIV system measured successfully the temporal evolution of instantaneous velocity fields of the turbulent jet, from which spectral analysis of turbulent structure was also feasible.

• 한국연소학회:학술대회논문집
• /
• 2000.12a
• /
• pp.113-122
• /
• 2000

Dynamic structures of unsteady $CH_4$/Air jet diffusion flames with flame-vortex interaction were numerically investigated. A time-dependent, axisymmetric computational model was adopted for this calculation. Two step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including gravitational effect show that the large outer vortices and the small inner vortex street can be well simulated without any additional disturbances in the downstream of nozzle tip. It was found that the temperature and species concentrations had various values for the same mixture fraction in flame-vortex interaction region. This unsteady jet flame configuration accompanying flame-vortex interaction is expected to give good implications for the structure of turbulent flames.

• 한국전산유체공학회:학술대회논문집
• /
• 2003.10a
• /
• pp.141-142
• /
• 2003

When the under-expanded supersonic jet impinges on the perpendicular plate, it is well known that the self-induced flow oscillation occurs at the specific conditions. This phenomenon is related with the noise problems of aeronautical and other industrial engineering. But, the very complicated flow field is formed and it is difficult to clear the flow structure and the mechanism of oscillation. This paper aims to clear the characteristics of flow field and the wave pattern during the under-expanded supersonic jet impinges on the plate. The numerical calculation was carried out using the TVD numerical method. In this paper, the flow visualization, the pressure fluctuation on the surface of plate and the mechanism of oscillation are discussed.

• Proceedings of the KSME Conference
• /
• 2000.11b
• /
• pp.267-271
• /
• 2000

The local heat transfer rate of an axisymmetric submerged air jet impinging on normal to a heated flat plate was investigated experimentally with varying solidity of mesh screen. The mean velocity and turbulent Intensity profiles of streamwise velocity component were measured using a hot-wire anemometry. The temperature distribution on the heated flat surface was measured with thermocouples. The screen installed in front of the nozzle exit(behind of 35mm) modify the jet flow structure and local heat transfer characteristics. For higher solidity screen, turbulence intensity at core lesion is high and increases the local heat transfer rate at nozzle-to-plate spacings(L/D<6). For larger nozzle-to-plate spacings(L/D>6), however, the turbulent Intensities of all screens tested in this study approach to an asymptotic curve, but the small mean velocity at the core region reduces the local heat transfer rate for high solidity screens.