Chayut Nuntadusit

Effect of Twin Delta-Winged Twisted-Tape on Thermal Performance of Heat Exchanger Tube

This work aims at studying the effect of twin delta-winged twisted-tape insertion on heat transfer, pressure drop, and thermal performance characteristics of a heat exchanger tube. All twisted tapes used in this work were made of aluminum sheets twisted at a single twist ratio of 3.0. The twin delta wings were formed by extrusion of the tape at the center area at every twist length interval. For comparison, three different arrangements of the twin delta wings were: (1) the wing tips pointing upstream of the flow (TTW-up, twin delta-winged twisted tape in counterflow arrangement), (2) the wing tips pointing downstream of the flow (TTW-down, twin delta-winged twisted tape in co-flow arrangement), and (3) the wing tips pointing opposite direction (TTW-o, opposite winged twisted tape). The wing declination was arranged at an angle of 15° with respect to the tape surface. Effects of three different wing-tip angles of 20°, 40°, and 60° for a constant wing base were examined. The experiments were conducted using water as the test fluid in a uniform-heat-flux tube for Reynolds number between 5000 and 15,000. The results demonstrate that the TTW-up consistently provides greater heat transfer rate, friction factor, and thermal performance factor than the TTW-down and the TTW-o, at a similar condition. In addition, the heat transfer rate increases as the wing-tip angle decreases. Over the range investigated, the TTW-up with wing-tip angle of 20° gives the highest thermal performance factor of 1.26 along with a Nusselt number and friction factor of 2.57 and 8.55 times those of the plain tube.

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.

Characteristics of Various Film Cooling Jets Injected in a Conduit

Abstract: In the present study, film cooling characteristics by the jets through various easy‐to‐make straight holes and slots have been investigated. In this experiment, seven kinds of injection geometries were used. They were circular, rectangular, elliptic and oval holes and slots, respectively.

Drying of Rubber Sheet Using Impingement of Multiple Hot Air Jets

In the process for producing ribbed smoked rubber sheet, the rubber sheet drying is the most time and energy consumption process. This research study the possibility in develop new drying system using array of hot air jets impinging directly on rubber sheet in order to reduce time for drying rubber in smoked room and increase productivity of rubber sheets. In the study, the array of jets from jet plate with drilled jet holes in staggered arrangement impinged on the both side of rubber sheet. The effects of jet velocity (Vj=10, 16, 23 m/s), jet temperature (Tj=50, 60, 70°C) and the distance from jet outlet to rubber surface (L=4D, 6D, 8D which D is diameter of jet hole) on drying rate were investigated by measuring weight of rubber sheet change with time. The heat transfer rate on impinged surface was also measured by attaching a heat flux sensor on impinged wall. The results showed that the convective heat transfer coefficient increased when the jet velocity was increased and when the distance from jet was decreased particularly in jet directly impinged region. It was found that the enhancement in heat transfer rate from jets cannot increase the rubber drying rate for all cases because the drying rate depend on rubber property. The process of rubber sheet drying can be divided in 2 periods; in the first drying period, the drying rate decreases with decreased moisture content. The drying rate depended on the initial moisture content and the condition of external effect such as jet velocity, jet temperature and distance from jet outlet to rubber surface. In second drying period, the moisture content is below 20% dry basis. In this period, the drying rate is almost constant near zero. It was also found that the drying for case of L=6D was higher than case of L=4D and 8D. The optimum condition for rubber sheet drying without defects on rubber property after drying was L=6D and Tj=70°C.

Effect of Combined Microwave Heating and Impinging Hot-Air on Rubberwood Drying

Applying microwave heating and impinging hot-air is one of the most interesting methods to increase the higher drying rates of rubberwood drying based on acceptable quality. A maximum microwave power level of 200W at a frequency of 2.45GHz with maximum working temperature of 70°C, only hotair (70°C) and combined microwave (200W) - hotair (70°C) were choosed to evaluate the effect of rubberwood drying by different width sizes (1, 2, 3 and 4 in.) by 46 in. length by 1 in. thick. In all cases, the drying time is reduced significantly from 168 h to less than 8-15 h in various wood widths and resulted in saving to an extent of about 91% of drying time from initial moisture content ranges of 73%-49% to 15% percent of moisture level. Drying stresses from prong test no found during drying and total color of rubberwood changed after high temperature drying is a natural surface when compared to fresh wood. The values of six strength compared to the reference values are concentrated in the ranges of 16.9-23.9 (11.0)MPa for shearing strength parallel to grain, 4291.1-6701.6 (4350)N for hardness, 73.3-110.2 (66.0)MPa for MOR, 7059.5-12856.7 (9240.0) MPa for MOE, 27.2-14.3 (5.0)MPa for compressive strength perpendicular to grain and 60.6-35.7 (32.0)MPa for compression strength parallel to grain. These results show that it is possible to develop a drying process for rubberwood using microwave-hot air in investigating further in this area.

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).

Heat Transfer Enhancement of Impinging Jet from Pulse Jet Combustor

The aims of this study are to investigate flow characteristics in pulse jet combustor and heat transfer characteristics of impinging jet from pulse jet combustor. The combustor is Helmholtz type which has single combustor chamber and single tailpipe. The inner diameter (D) of tailpipe was 47 mm and tailpipe length was about 16D. The effect of number of air inlets: single inlet, double inlets with 90o apart, double inlets with 180o apart and triple inlets with 90o apart on flow and heat transfer characteristics were studied. A water cooled heat flux sensor was applied to measure heat transfer rate on the surface at stagnation point. The jet-to-plate distance was varied at L=1D, 2D, 4D, 6D and 8D. Two of pressure transducers were mounted on the wall of combustion chamber and on the wall of tailpipe at 4D from tailpipe outlet to measure pressure simultaneously. It is found that the variation of pressure near the tailpipe outlet is strongly depended on air inlet configurations. The pressure variations in pulse jet combustor could be preliminary related to the temperature and velocity of jet from tailpipe and heat transfer rate on jet impingement surface.

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.