Makatar Wae-Hayee

Flow and Heat Transfer Characteristics of in-Line Impinging Jets With Cross-Flow At Short Jet-To-Plate Distance

The aim of this research is to numerically and experimentally study the flow and heat transfer characteristics of in-line impinging jets in cross-flow. The jets from a row of round orifices are perpendicularly impinged on the inner surface of a rectangular wind tunnel at a short distance between the orifice plate and impinged surface (H) of 2D, where D is a diameter of the orifice. The jet velocity was fixed corresponding to Re = 13,400 for all experiments, and the cross-flow velocity was varied at three different velocity ratios (velocity ratio, jet velocity/cross-flow velocity) of 3, 5, and 7. The heat transfer characteristic was visualized using a thermochromic liquid crystal sheet, and the Nusselt number distribution was evaluated by an image processing technique. The flow pattern on the impinged surface was also visualized by an oil film technique. The numerical simulation was used to explore a flow interaction between the impinging jets and cross-flow. The results indicated that Nusselt number peak increased by the increasing cross-flow velocity for short jet-to-plate distance. For the range determined, the maximum local Nusselt number peak was obtained at VR = 3 as the consequence of high velocity and high turbulence kinetic energy of jet impingement.

The effect of conical dimple spacing on flow structure and heat transfer characteristics of internal flow using CFD

In the present study, heat transfer and flow characteristics simulations over the surface of conical dimple were investigated. Single dimple row with inline arrangement was formed on the internal surface of the 3-D rectangular wind tunnel model. The air flow was perpendicular to the centre line of every dimple and the printed diameter of dimples on the surface was D=26.4mm. The depth of dimple on the surface of wind tunnel was H/D=2. The space between dimple-to-dimple was varied for S/D=1.125, 1.25,1.5, and 2. The Reynold number based on the hydraulic diameter of internal air flow was 20,000 depending on the wind tunnel hydraulic diameter. The numerical computation was applied with a Shear Stress Transport (SST) k-ω turbulence model. The average Nusselt number for the S/D=1.125 case is the highest. When the spacing becomes increase, the value of average Nusselt number tends to decrease.

The study of flow and heat transfer characteristics of impinging jet array mounting air-induced duct

Impinging jet is widely employed in thermal industrial applications due to having high heat transfer coefficient in impingement region. One method to increase heat transfer on an impingement surface is to increase turbulence intensity in jet flow. The mounting of an air-induced duct at nozzle outlet is a passive method to increase entrainment air resulting on increasing turbulence intensity. The aim of this research is to study flow and heat transfer characteristics of array of impinging jets mounting air-induced ducts. The investigation model was jets discharging from pipe nozzle having an inner diameter of d=17.2 mm and a length of 200 mm. Nozzle arrangement were inline configuration having 5 rows x 5 columns. A jet-to-jet distance (S) was S=6d, 8d and a jet-to-plate distance (H) was H=6d. The inner diameter (D) and the length (L) of the air-induced ducts were D=4d and L=4d, respectively. The Reynolds number was fixed at Re=20,000. In addition, the impinging jets without mounting the air-induced ducts were also investigated for benchmarking with the case of mounting the air-induced ducts. In the study, a thin foil technique was used to measure heat transfer on the impingement surface, and a computational fluid dynamic (CFD) using ANSYS, Fluent (V.15.0) was also adopted. The results showed that the effect of mounting air-induced duct can enhance entrainment air into the jet flow resulting on increasing of heat transfer of impinging jets on target surface, and the effects of mounting air-induced duct on increasing heat transfer in case of larger jet-to-jet distance (S/d=8) was more effective than that of smaller jet-to-jet distance (S/d=6).

Study of Heat Transfer Characteristics and Kerf Quality of Flame Jet Cutting

The aim of this research is to study heat transfer rate of impinging flame jet and cutting quality of steel plate using flame jet. The cutting torch was used for heating on the impingement surface, and it was used for cutting the steel plate samples. LPG at constant flow rate of 0.14 kg/s was mixed with pure oxygen at varied flow rate corresponding to equivalence ratio, =0.78, 0.93 and 1.16. The nozzle-to-plate distance was examined at h=3, 4, 5, 6, 7 and 8 mm. Heat transfer rate on the impingement surface was measured using water cooled heat flux sensor. In order to investigate cutting quality, steel plate with 6 mm in thickness was cut by this flame jet with cutting speed at 260 mm/min. The surface roughness, slag quantity and kerf characteristics were considered for cutting quality. The results show that the flame jet for condition of =0.78 at h=4 mm gives the highest heat transfer rate. The flame jet for condition of =0.93 at h=6 mm is optimal for using cutting steel plate in this study.

Heat Transfer Enhancement of Impinging Row Jets in Cross-Flow with Mounting Baffles on Surface

The aim of this research is to enhance heat transfer on a surface of row of impinging jets in cross-flow by mounting some baffles on the surface. A row of 4 jets with inline arrangement discharging from round orifices impinged normally on inner surface of wind tunnel with simulated cross-flow. The orifice diameter (D) was 13.2 mm. The jet-to-surface distance and jet-to-jet distance were fixed at H=2D and S=3D, respectively. Four couples of baffles with V-shaped arrangement at attack angle, θ=30o, were mounted on surface in upstream or downstream of impinging jets and the location of baffles attachment is L=1.5D apart from the jet impingement region. The velocity ratios (Jet velocity/cross-flow velocity) were varied from VR=3, 5 and 7 while the jet velocity was kept constant corresponding to Re=13,400. The experimental investigation was carried out for heat transfer characteristic by using Thermochromic Liquid Crystal sheet, and heat transfer coefficient distributions were evaluated using an image processing method. The results show that the impinging jets with mounting the baffles in the upstream region of jet impingement region can enhance the heat transfer rate throughout VR.

Flow and Heat Transfer Characteristics of Impinging Jet from Expansion Pipe Nozzle with Air Entrainment Holes

Flow and heat transfer characteristics of impinging jet from expansion pipe were experimentally and numerically investigated. The expansion pipe nozzle was drilled on expansion wall for increasing an entrainment of ambient air into a jet flow. The diameter of round pipe nozzle was d=17.2 mm and the diameter of expansion pipe was fixed at D=68.8 mm (=4d). The number of air entrainment holes was varied at 4, 6 and 8 holes, and the expansion pipe length was examined at L= 2d, 4d and 6d. In this study, the expansion pipe exit-to-plate distance was fixed at H=2d and the Reynolds number of jet was studied at Re=20,000. Temperature distribution on the impinged surface was acquired by using an infrared camera. The numerical simulation was carried out to reveal the flow field. The results show that the ambient air enters through the holes and subsequently blocked the entrainment of ambient air into the jet flow. It causes to enhance the heat transfer particularly at stagnation point higher than the case of conventional pipe: 4.68% for 4 holes at L=2d, 6.4% and 6.28% for 4 holes and without holes at L=4d and 5.48% for 8 holes at L=6d.

Investigation of Flow and Heat Transfer Characteristics of Annular Impinging Jet

Flow and heat transfer characteristics of impinging jet from annular pipe were experimentally and numerically investigated. To generate annular jet, the cylindrical rod with 12.7 mm in diameter was inserted at center of pipe nozzle which has inner diameter (D) of 28.6 mm. The jet-to-plate distance (H) was examined in the range of 2D, 4D, 6D and 8D. The jet Reynolds number was fixed for all experiments at Re=20,000 based on averaged velocity and pipe inner diameter. The conventional jet was also studied for comparison. The temperature distribution on the impingement surface was measured using an infrared camera. The numerical simulation was carried out to visualize the flow behavior. The results show that the heat transfer of annular jet is higher than that the case of conventional pipe jet at low jet-to-plate distance (H=2D); however, the ones of annular and conventional jet are comparable when jet-to-plate distance becomes higher than H=6D.