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In this work, flow and heat transfer in a channel having oval trench dimples were investigated numerically. 3-D channel flow with a cross-section of 300-mm width and 32-mm height were created using Computational Fluid Dynamic (CFD) with ANSYS, Fluent (V.15.0). Five oval trench dimples with 3-mm depth, 10.0-mm width and 45-mm length arranging with a single row and in-line configuration were located on the bottom surface of the channel. Reynolds number based on hydraulic diameter of the channel were fixed at Re=20,000 whereas a dimple inclined angle defined as the angle of dimple centreline to the mainstream was varied at 0, 15, 30 and 45 degrees. SST turbulent model was used to solve governing Equations. The result show that longitudinal vortex flow occurred at ?=15° to 45° which would be enhance heat transfer on the surface. When inclined angle became larger, the areas of Nusselt number and high total pressure coefficient took place at dimple edge in +Z direction. The peak of average spanwise Nusselt numbers took place for the case of ?=45°. Moreover, the area of high spanwise average Nusselt numbers (>100) for the case ?=30° was the largest.

The objective of this research is to study flame structure and heat transfer characteristics for the premixed flame jet from the swirling chamber. In this study, LPG and air was utilized as gas fuel and oxidizer for a premixed flame. The equivalence ratios () of LPG and air were considered at 0.8, 1.0, and 1.2 under a Reynolds number Re = 4,000. The swirl flame was generated by double tangential inlets in cylindrical chamber. The diameter of chamber was fixed at D = 20 mm and the hydraulic diameter of the inlet was Dh = 5 mm. In this study, the effect of chamber geometry on flame structure was investigated by varying the chamber from H = 2.2Dh to 7.0Dh. The structures and temperature of the free flame jet was recorded with camera and measured with a thermocouple. The heat transfer rate of impinging flame jet was also measured at distance from chamber outlet to flame impingement surface varying from L = 4Dh to 10Dh. The results show that the maximum of flame temperature occurs at =1.2. Impinging flame jet for case of chamber height at H = 4.6Dh and impingement distance at L = 4Dh give the highest heat transfer for all equivalence ratios due to the reaction zone of combustion reached to approach near the heat transfer surface.

This research aims to study the effect of pulsating frequency on flame structure and heat transfer characteristics of premixed flame from a pipe nozzle. The LPG and air were used as gas fuel and oxidizer. The equivalence ratios ( ) were evaluated at 0.8, 1.0, and 1.2 under a constant Reynolds number Re = 500. The effect of nozzle-to-impingement surface distance ratio was investigated at H = 2D to 10D, here D is the nozzle diameter at 12 mm. The frequency of pulsating (f) was varied from f = 0 to 10 Hz using a solenoid valve. The flame structures of free flame jet and the impinging flame jet were recorded with a digital camera. The average heat flux on impingement surface was measured with water cooling plate and evaluated from the heat balance of the cooling water. The results show that the pulsating of flame jet become having gap on flame and the mushroom appear at the end of flame. The size of mushroom structure becomes larger when increasing the frequency. While the non-pulsating jet did not appear in this structure. Pulsating flame jet can increase the overall average heat flux on the impingement surface up to about 12% for case of = 1.2 and H = 2D and f = 10 Hz. when compared to case of f = 0 Hz.

Jet impingement has been applied for internal cooling in gas turbine blades. In this study, heat transfer characteristics of impinging jets from a row of circular orifices were investigated inside a flow channel with rotations. The Reynolds number (Re) based on the jet mean velocity was fixed at 6,700. Whereas, the rotation number (Ro) of a channel was varied from 0 to 0.0099. The jet-to-impingement distance ratio (L/Dj) and jet pitch ratio (P/Dj) were respective 2 and 4, Dj is a jet diameter of 5 mm. The thermochromic liquid crystals (TLCs) technique was used to measure the heat transfer coefficient distributions on an impingement surface. The results show that heat transfer enhancement on a jet impingement surface depended on the effects of crossflow and Coriolis force. The local Nusselt number at X/Dj?20 on the leading side (LS) was higher than on the trailing side (TS) while heat transfer on the LS at 20?X/Dj?40 gained the lowest, compared to on the TS. The average Nusselt number ratios ( ) on the TS at Ro = 0.0049 gave higher than on the LS of around 2.17%. On the other hand, the on the TS at Ro = 0.0099 was less than the LS of about 0.08%.

Rubberwood (Hevea brasiliensis) is a major raw material used in furniture factories in southern Thailand. Proper kiln drying of the lumber plays a significant role on the efficient utilization of the final product. Overall quality-reducing defects in such a materials are inevitable, unless a correct and proper drying process is used. Therefore, the objective of the present study was to develop a three-dimensional numerical model of heat and moisture transfer during the drying process and to conduct physical experiments in order to compare the results between the two approaches. For this purpose, an innovative kiln dryer was designed in the laboratory which will also contribute to the prevention of defects in the wood. The results show that the samples dried at a temperature of 120 °C experienced faster decreases in their moisture content compared to those dried at temperatures of 100 °C and 80 °C. The drying temperature and moisture content of the samples were successfully predicted by the simulation model created in this investigation and the two approaches demonstrated good agreement for temperature and moisture with root mean square errors of 0.50 and 0.19, standard errors of 0.38 and 0.16, and mean absolute errors of 0.94 and 2.18, respectively. These results will help rubberwood drying facilities prevent wood deformation while reducing the drying time required.

The purpose of this study was to investigate experimentally the effects of impinging jets installed in a conventional rotary dryer for heat transfer enhancement. The metal drum had as insert a jet ...

LPG is increasingly used in households as it is a clean and environmentally friendly energy source. In this work, we experimentally tested the type of LPG burner used in Thailand. This work investigates the effect of varying the burner-to-plate distance on heat transfer from the flame produced by LPG combustion. The burner model KB-5 was tested to evaluate the heat transfer rate (Q) via a measuring plate heated directly from the burner. Flowing water was used to cool the measuring plate. Moreover, the inlet and outlet temperatures were measured. The burner-to-plate distance (H/d) was studied, with varying the flow rate of LPG. The results showed that the highest heat transfer rate was found at higher burner-to-plate distances when the LPG flow rate increased. In addition, the highest heat transfer rate occurred when the flame impinged on the heated surface. In this work, the total flame length should impinge at a surface approximately 70% of its length to get optimal results. The results of this work can be applied to optimize other types of domestic stoves. Furthermore, this promising result helps households and small businesses save costs on LPG stove fuel.

We experimentally and numerically investigated the effect of expansion pipe length on heat transfer enhancement and flow characteristics of impinging jet array with mounting expansion pipe. The inner diameter (d) and the length of each main pipe nozzle was d = 17.2 mm and 200 mm, respectively. Nozzle arrangement was distributed in 5 rows × 5 columns with an in-line configuration. The jet-to-wall distance (H) was fixed at H/d = 8, while the jet-to-jet spacing (S) was varied at S/d = 6 and 8. The expansion pipe length (L) was varied at L/d = 4, 6 and 8, and the inner diameter of the expansion pipe (D) was fixed at D/d = 4. Reynolds number of the jets was kept constant at 30000. The conventional impinging jets were also performed to compare with the case of jets with mounting expansion pipe. The 3-D numerical simulation with v2-f turbulence model was applied to simulate the flow characteristics. Results showed that the surrounding air was induced into the expansion pipe, and an entrainment of induced air became greater when expansion pipe was longer. This influenced more markedly on enhancement of Nusselt number at stagnation point for narrow jet-to-jet spacing (S/d = 6).

The objective of this research is to investigate the heat transfer of an impinging jet array under a fully developed flow. 5 × 5 jets discharged from pipe nozzles with a diameter of d = 17.2 mm and a length of 300 mm were evenly spaced in an in-line arrangement. The effects of the jet-to-jet spacing (S) and the jet-to-plate distance (H) were varied at S/d = 4, 6, 8 and H/d = 2, 4, 6, 8. The jet Reynolds numbers were varied at Re = 10,000, 20,000, 30,000 and 40,000. The temperature distributions on the impingement surface were measured using an infrared camera and were then calculated to evaluate the Nusselt numbers. The results showed that the highest average Nusselt number was found at H/d = 4 for all jet-to-jet spacings (S/d). A heat transfer correlation was performed based on the peak of the average Nusselt number occurring at H/d = 4. This was in contrast to the case of impinging jet array discharged from orifice nozzles, in which the average heat transfer decreased monotonically with jet-to-plate distance (H).

This paper presents the results of flow and heat transfer characteristics of vortex generating jet on a flat plate. A circular jet from the pipe nozzle is injected perpendicularly to the mainstream flow on flat plate Longitudinal vortex flow is induced for enhancing heat transfer on a constant heat flux surface. The effect of jet-mainstream flux ratio (J) on heat transfer was investigated by fix the mainstream flow at 10/s and varied the jet velocity corresponding to three jet-mainstream momentum flux ratios, J , of 0.06, 2.25, and 12.25. Temperature distribution on heat transfer surface with constant heat flux is captured by an infrared camera. The flow field is also investigated with numerical simulation. The results reveal that all heat transfer rates on the surface are higher than under the No-jet conditions due to generated counter-rotating vortex pair. Simulation results indicate that the low momentum flux vortex of J = 0.06 moves close to the surface and directly affects heat transfer enhancement, with a maximum increase of 40%. With a greater jet velocity the vortex pair moves away from the surface, the mainstream flows around the more stronger jet, and counter-flow secondary vortex is formed downstream of jet exit. Thus for J = 2.25, the disturbed mainstream exerts more influence on the surface heat transfer, up to a maximum of 50%. For J = 12.25, a maximum heat transfer, at 68% better than the No-jet scenario, is clearly achieved through the secondary vortex over a longer and larger area.

This study aimed to determine the equilibrium moisture content and to develop an indirect measurement technique for the moisture content (MC) by observing temperature and vapor pressure during hot ...

Flow and heat transfer characteristics of impinging jet from pipe nozzle with air-augmented duct were experimentally and numerically investigated. The effects of air-augmented duct geometry on heat transfer enhancement were concerned. The experimental parameters included a diameter (D) and a length (L) of air-augmented duct in the range of D = 2d, 3.3d, 6d, and L = 2d, 4d, 6d where d was the inner diameter of main pipe nozzle at 17.2 mm. The distance from air-augmented duct outlet to impingement surface (S) at S = 2d, 4d and 6d were considered. The conventional impinging jet was also studied to compare the results with the case of an air-augmented duct. The result comparison was based on constant jet mass flow rate by fixing the jet Reynolds number of conventional pipe at Re = 20,000. The temperature distributions on the impingement surface were measured by using a thermal infrared camera, and profiles of velocity and turbulence intensity of the jet were measured by using hot-wire anemometer. The 3-D numerical simulation with SST k-ω turbulence model was also applied to reveal the flow characteristics. The results show that the heat transfer rate on the impingement surface for the case of an air-augmented duct in conditions of 2d ≤ D ≤ 4d and L = 2d is noticeably higher than the case of conventional impinging jets due to increasing air entrainment. The heat transfer rate for the case of D = 6d, L = 2d at S = 2d, is the largest by getting 25.42% higher compared to a conventional impinging jet.

We studied the flow and heat transfer characteristics in a rotating serpentine passage with ribbed walls. The channel length-tohydraulic diameter ratio of the rotating serpentine passage (L/Dh), the rib height-to-hydraulic diameter ratio (e/Dh), rib angle of attack (a), the rib pitch-to-height (p/e) ratio and aspect ratio (AR) were fixed at 11.33, 0.13, 90°, 10 and 1, respectively. Numerical simulations were performed at a constant Reynolds number, Re = 10000. The rotation number (Ro) levels were 0.0, 0.1, 0.2 and 0.3. The distribution of local heat transfer coefficients and the flow field in a rotating serpentine channel were studied by numerical simulations using the commercial software ANSYS ver.15.0 (Fluent). The results show that the local heat transfer at the first-pass trailing and the second-pass leading surfaces has significantly higher rates than at the first-pass leading and second-pass trailing surfaces, due to rotational effects with Coriolis forces that are high in the middle region between two ribs, and very low immediately before and after the ribs. In addition, the overall thermal hydraulic performance with rib-roughened walls was best at the highest rotation number tested (Ro = 0.3).

The heat transfer performance of swirling impinging jets (SIJs) was investigated and compared with that of the conventional jet (CIJ). The swirling jets were induced by helical rod inserts (HRs) fitted with pipe nozzles. The helical rod inserts with two different rod diameter to nozzle diameter ratios (d/D) of 0.46 and 0.64 were employed for comparison. Jet-to-plate distance to nozzle diameter ratio (L/D) was varied from 1 to 5 while Reynolds number was fixed at 20,000. The temperature and Nusselt number distributions on the impinged plate were measured and evaluated using thermochromic liquid crystal (TLC) sheet and image processing technique. The experimental results showed that the swirling jet (SJ) at d/D=0.64 gave higher average heat transfer rate than the SJ at d/D=0.46 up to 6.41 % and the conventional jet (CJ) up to 35.05 %. In addition, Nusselt number peak of swirling jets increased as jet-to-plate distance decreased.

A PV integrated greenhouse dryer has been fabricated with a unique solar collector and tested in no-load condition under forced convection mode. Various important thermal performance indicators such as heat utilisation factors, convective heat transfer coefficient, coefficient of performance and percentage of net heat gain have been evaluated to validate the effectiveness of modifications. Energy, electrical and exergy efficiencies have been also analysed for installed PV system for exhaust fan. Greenhouse dryer experiments have been conducted for with and without solar collector conditions. The enhancement in the highest convective heat transfer coefficient for the greenhouse dryer with solar collector condition is 150% in comparison to the absence of solar collector. Heat utilisation factor and coefficient of performance are found 10.1% and 7.9%, respectively, higher than greenhouse dryer without solar collector. The maximum energy and exergy efficiencies are found 16.8% and 21.4% for greenhouse dryer with and without solar collector, respectively. These results represent the effectiveness of solar collector placed inside the dryer and insulated north wall. The designed greenhouse dryer with solar collector is proposed as most suitable dryer for crop drying in the temperature range of 40–70 °C.

A modified greenhouse dryers with solar air heating collector were used for bitter gourd flakes drying under natural and forced convection modes till stagnation in their moisture evaporation. Experiments were performed simultaneously for open sun, natural and forced convection modes to validate the thermal modeling and compare drying kinetic results. The predicted temperatures of bitter gourd flakes, room air temperature and moisture evaporation rate show the fair agreement with the experimental observations within the root mean square of percentage deviation ranges from 6.10 to 10.41% and coefficient of correlation ranges from 0.96 to 0.99 under natural convection mode. Similarly, for the forced convection mode, coefficient of correlation ranges from 0.96 to 1 and root mean square of percentage deviation ranges from 3.74 to 7.55%. Logarithmic and Midilli-Kucuk models are selected as best curve fitting techniques for non-linear regression analysis for bitter gourd flakes drying under natural and forced convection modes respectively based on SSE, R-square, adjusted R-square and RMSE values. Proposed thermal models will be helpful in designing energy efficient greenhouse drying system for given crop mass and greenhouse dryer location.

The greenhouse dryers consist opaque and insulated north wall with solar air heating collector at the bottom. It has been used for bitter gourd flakes drying under natural and forced modes. Thermal models were developed for the both modes of drying. Experiments were performed concurrently for open sun, natural and forced modes to compare their thermal modeling and drying kinetics results. Energy analysis includes; embodied energy, CO2 emission, CO2 mitigation and economic evaluation were done for both modes of drying. The predicted values of bitter gourd flakes and room air temperature, and moisture evaporation rate showed the fair agreement with the experimental observations within the root mean square of percentage deviation ranged from 3.25% to 5.82% and 2.83% to 5.26%, and the coefficient of correlation ranged from 0.98 to 0.99 and 0.99 to 1 under natural and forced modes, respectively. The final moisture ratio was achieved 0.12 after 15 h, 0.14 after 16 h and 0.25 after 21 h under forced, natural and open sun modes respectively. Crop drying under forced mode is found significantly faster than other modes of drying. The Logarithmic model was selected as best curve fitting technique for non-linear regression analysis for forced mode. The energy payback time of developed dryers under natural and forced modes is 1.68 and 2.35 years, respectively. The net CO2 mitigation of bitter gourd flakes drying is 33.04 and 36.34 t under natural and forced modes, respectively.

Journal of Current Science and Technology, 9, 1, 49-58

This study describes heat transfer and flow characteristics for various angles of inclined-pin staggered arrays in a rectangular channel. The pin array consists of 10 staggered rows of cylindrical pins. Influence of the pin inclined angle is studied at θ = 30o, 45o, 60o, 90o, 120o, 135o, and 150o. A thermochromic liquid crystal sheet is used for temperature measurement. Flow field and heat transfer characteristics are predicted by numerical simulation. From the study, certain pin inclined angles have shown a favorable effect on the heat transfer enhancement with an array of pins. Only pin arrays with inclined angles at θ = 120o and 135o could enhance heat transfer on the surface when compared with conventional right-angle pin. Jet-like flows behind the pin move the counter-rotating vortices with high velocity (CRH) that induce airflow attachment to the heat transfer surface down to the lower surface. So, CRH has a significant effect on heat transfer promotion. Pin inclination angles at θ = 120o, 135o and 150o could greatly reduce the pressure drop and enhance the thermal performance factor. The greater number of rows in the test section has been proven crucial for some inclination angles in promoting heat transfer performance.

The purpose of this research was to investigate experimentally and numerically the effects of an impinging jet array with entrained air ducts on heat transfer. Straight pipes with 17.2 mm inner diameter (d) and 300 mm length (l) were in a 5 × 5 nozzle array with in-line configuration. The length of air duct (L) was varied among L/d = 2, 4, or 6, and the diameter of the duct (D) satisfied D/d = 2, 4, or 6. The jet-to-surface distance (H) was tested at H/d = 4, 6, or 8; and the jet-to-jet spacing (S) was among S/d = 4, 6, or 8. Reynolds number (Re) in the jets was from 10,000 to 40,000. Conventional impinging jets were also examined for benchmarking. Flow characteristics obtained numerically from v2f turbulence model were also presented to explain the heat transfer mechanisms. The results indicated that heat transfer rates on the target surface became higher when the duct was elongated, and was more widely spread when the duct diameter was expanded. These effects were more marked in cases with narrow jet-to-jet spacing (S/d = 4) and with large jet-to-surface distance (H/d = 8). Moreover, a heat transfer correlation was constructed from the experimental results to predict heat transfer rates within the studied ranges of parameters.

The effect of the speed of rotation on heat transfer characteristics inside a ribbed serpentine channel was studied experimentally. In this study, ribs with square cross sections were located on the leading side (LS) and trailing side (TS) of the serpentine channel wall. The serpentine channel had an aspect ratio (AR) of 1.0 and a hydraulic diameter (Dh) of 50 mm. The rib pitch-to-height ratio (p/e), the rib height-to-hydraulic diameter ratio (e/Dh), the channel length-to-hydraulic diameter ratio (L/Dh), and radius ratio of the channel (r/Dh) were fixed at 10, 0.1, 8, and 5, respectively. There were four different types of geometries for the present study, viz. a smooth wall, a 90° ribbed wall, a 60° V-ribbed wall, and a 60° V-broken ribbed wall. The Reynolds number (Re) based on the hydraulic diameter of the channel was constant at 10,000, and the rotation numbers (Ro) were varied in the range from 0.0 to 0.30. The heat transfer coefficients in a serpentine channel were measured experimentally using the steady thermochromic liquid crystals (TLC) technique. In addition, the friction factor and the thermal-hydraulic performance were measured for each case. The results showed that the ribbed wall attained better regional heat transfer performances than the smooth wall for both stationary and rotating conditions at almost all locations. The heat transfer in the first pass was the highest on the TS wall, followed by the LS wall. A similar trend also occurred in the turn region. However, in the second pass, the heat transfer was the highest for the LS wall, followed by the TS wall. Due to the rotation, the Coriolis force acted on the TS wall in the first pass and on the LS wall in the second pass, while the direction of the centrifugal force was aligned with the radius of the rotation. As a result, the average heat transfer in the first pass was larger than it was in the second pass. The maximum average heat transfer and thermal performance inside a serpentine channel occurred in the case when there was a broken V-60° rib, i.e., it was increased up to about 2.1–2.4 in the range of Ro = 0.0–0.15. The thermal performance factor for the broken V-60° rib at Ro = 0.2–0.3 gained about 2.4–2.6, which was near the case of the V-60° ribs.

The aim of this research was to study flow and heat transfer characteristics with a row of inclined pins in a rectangular channel. The cylindrical pins of diameter D = 10 mm were mounted on the heat transfer surface in spanwise direction of the channel. The pin-to-pin distance was fixed at S = 2D. Two of pin heights H were studied, namely short pins or long pins with H = 2D or H = 3.2D, respectively. The effects of pin inclination angle were investigated for θ=30o, 45o, 60o, 90o, 120o, 135o, and 150o in the range of Reynolds number from 14,000 to 32,860. The local temperature on endwall surface was measured using a thermochromic liquid crystal sheet coupled with image processing. Numerical simulations were used to obtain three-dimensional flow fields and heat transfer rates on pin surfaces and endwall surface. Results for the case H/D = 2 show that pins inclined by θ=30o, 45o and 60o can enhance heat transfer downstream of the pin row, from the baseline case of pins with θ=90o. The heat transfer was improved by a pair of counter-rotating vortices near the heat transfer surface. These vortices induced the main flow to attach on the heat transfer surface. The pin inclinations θ=120o and 135o somewhat improved heat transfer behind the pins. However, the inclination θ=150o gave the lowest heat transfer rate on the endwall surface. The heat transfer on endwall surface with H/D = 3.2 was poorer than with H/D = 2 because the counter-rotating vortices did not reach the heat transfer surface. The inclination θ=30o with H/D = 2 gave the best thermal performance. The local Nusselt number on the rear side of pin surfaces with H/D = 3.2 and inclinations θ=30o, 120o, 135o and 150o tended to increase from the case H/D = 2, due to a jet-like flow attachment on the pin surfaces. However, the local Nusselt number on pin surfaces with H/D = 3.2 and inclinations θ=45o and 60o tended to be lesser than with H/D = 2, due flow separation downstream far from the pins.

A greenhouse dryer under forced convection mode is designed and fabricated with the integration of solar collector and variable speed exhaust fan. The developed system is used for bitter gourd flakes drying under three different air mass flow rates (0.0275, 0.0551, and 0826 kg/s). Moisture content of bitter gourd flakes was decreased effectively from 96.8% to 12.2% in 17 h with optimum air mass flow rate 0.0551 kg/s, whereas open sun drying has taken 26 h to reach 20.7% moisture content. The average greenhouse dryer efficiency was found to be 19.7% at 0.0551 kg/s air mass flow rate. Shrinkage (in terms of percentage) of dried bitter gourd flakes was found to be higher as 74% at 0.0275 kg/s air mass flow rate because of higher greenhouse room air temperature. Hardness of dried bitter gourd flakes was found to be highest as 365 g at 0.0275 kg/s air mass flow rate due to less air exchange rate and high inside room temperature. On the basis of statistical analysis, Prakash and Kumar model and Logarithmic model were selected as best drying models for greenhouse and open sun drying, respectively. The dehydration of higher moisture content crops inside developed greenhouse dryer was found to be more consistent. The designed greenhouse system is recommended for small farmers.

JOURNAL OF CURRENT SCIENCE AND TECHNOLOGY, 8, 2, 67-74

JOURNAL OF CURRENT SCIENCE AND TECHNOLOGY, 8, 2, 57-65

The heat transfer characteristics were experimentally investigated in a wind channel with different types of cut baffles for heat transfer augmentation. The aim of using zigzag-cut baffles is to create 3D flow structure behind the baffles instead of transverse vortex flow leading to enhance heat transfer. In this study, 4 types of baffles were examined; conventional baffle (Rectangular cross section with no cut), baffle with rectangular zigzag-cut, baffle with triangle zigzag-cut at 45 degree and at 90 degree. All of the baffles have the same height at H = 15 mm and flow blocking area. In the experiment, the row of seven baffles was attached on the inner surface of wind channel. The effects of pitch spacing length were also investigated at baffle pitch distance P/H = 4, 6 and 8 (H: Height of baffle). The experiments were performed at constant Reynolds number (Re) of 20000. The heat transfer patterns via Thermochromic liquid crystal sheet were visualized and recorded with a digital camera. The recorded images were then analyzed with image processing technique to obtain the distribution of Nusselt number. The flow characteristics pass through the baffles were also numerically studied with CFD simulation for understanding the heat transfer characteristics. The friction losses were measured to evaluate the thermal performance for each baffle. It was found that the baffle with rectangular zigzag-cut gives the best thermal performance due to heat transfer augmentation in upstream and downstream side of baffle.

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 ...

An effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet in the case of low jet-to-plate distance at H = 2D was experimentally and numerically investigated. In the experiments, the air jet from orifice impingement on the wall of wind tunnel while a cross-flow was simultaneously induced normal to the jet flow. The jet velocity was fixed while the cross-flow velocity was varied corresponding to velocity ratios (jet velocity/cross-flow velocity) VR = 3, 5 and 7. The temperature distribution on an impinged surface was visualized by using thermochromic liquid crystal sheet (TLCs), and Nusselt number distribution was evaluated by using image processing method. The flow pattern on impingement surface was visualized by using oil film technique. The numerical simulation was carried out for a better understanding of the jet flow in the cross-flow. The results show that Nusselt number peak shifts downstream and the Nusselt number peak increases with increasing cross-flow velocity.

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 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.

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.

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.

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...

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.

Heat transfer enhancement in a tube using twisted tapes with alternate axes at different alternate lengths (in term of alternate length to twist length ratio, l/y = 0.5, 1.0, 1.5 and 2.0) has been investigated in the present work. The twisted tapes with both uniform and non-uniform alternate lengths (TAs and N-TAs) as well as a typical twisted tape (TT) were comparatively tested for Reynolds number between 5000 and 21,500. The experimental results revealed that all the TAs and N-TAs yielded higher Nusselt number and friction factor than the TT and the Nusselt number and friction factor obtained were considerably increased with decreasing the alternate length (l). Heat transfer and friction factor by the N-TAs were found to be directly dependent of alternate length rather than the variation of the length. In the present study, the optimum tradeoff between the enhanced heat transfer and increased friction was found by using the TA at l/y = 0.5, giving the highest heat transfer rate as well as the maximum thermal performance factor. In addition, empirical correlations for Nusselt number, friction factor and performance evaluation criteria to assess the real benefits in using the TAs/N-TAs of the enhanced tube were also determined.

The aim of this research is to enhance the heat transfer on an impinged surface under an impinging jet array by minimizing a cross-flow effect. Conventional round orifices (aspect ratio, AR = 1) are substituted by the elongated orifices with aspect ratio AR = 4 and 8 with the same jet exit area. Two types of orifice arrangements; in-line and staggered arrays are compared. The experimental investigation was carried out at constant distance from orifice plate to impinged surface H = 2DE (DE is equivalent diameter of orifice). The heat transfer characteristic was visualized using thermochromic liquid crystal sheet (TLCs) and the Nusselt number distribution was evaluated by an image processing technique. The flow characteristic on the impinged surface was also visualized by oil film technique. The results show that the cross-flow in a case of the jets issued from the orifices with AR = 4 is considerably less significant than that in cases of the ones delivered from the orifices with AR = 1 and 8. At Reynolds number of 13,400, the Nusselt numbers for the jet arrays issued from the elongated orifices with AR = 4 with in-line and staggered arrangements are respectively 6.04% and 12.52% higher than those for the case of AR = 1.

Forced convective heat transfer on the impinged plate associated with swirling impinging jets (SIJ) issuing from nozzles inserted by twisted tapes has been investigated. Swirling impingement jets with several swirl rates were generated by twisted tapes at different twist ratios ( y/W = 3, 4, 5, and 6). The experiments were performed by locating nozzle at 4 different jet-to-plate spacings of L/D = 2, 4, 6 and 8. A jet Reynolds number varied between 4000 and 16000. Attributing to a high momentum transfer rate, an efficient heat transfer was obtained by using the jet with a small jet-to-plate spacing and the twist tape with a large twist ratio at high Reynolds number. At small jet-to-plate spacings ( L/D = 2 and 4), swirling impinging jets gave higher heat transfer rate than conventional impinging jets while at large jet-to-plate spacings ( L/D = 6 and 8), the opposed result was obtained. Over the range examined, only SIJ induced by the twisted tape at a twist ratio of 6 consistently provided higher average Nusselt numbers than CIJ.

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.

Flow and heat transfer characteristics of swirling impinging jet (SIJ) were studied experimentally at constant nozzle-to-plate distance of L = 4D. The swirling jet is generated by inserting twisted tapes within a pipe nozzle. Effects of swirl on the impinged surface are investigated at twist ratios (y/W) of ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52. The flow patterns of the free swirling jet and the swirling impinging jet were visualized by mixing dye with the jet flow. Distributions of temperature and convective heat transfer coefficient on the impinged surface were measured with thermochromic liquid crystal (TLC) sheet and image processing technique. Additionally, an oil film technique was performed as a complementary technique for flow visualization on the impinged surface. The experimental results reveal that there appear to be two peaks of heat transfer in the jet impingement region. The heat transfer enhancements in jet impingement region can be achieved at a low twist ratio of 3.64 which corresponds to the swirl number of 0.4.

This paper presents the heat transfer and flow characteristics in a channel with different types of transverse perforated ribs. The effects of perforation/hole inclination angle (θ = 0°, 15° and 30°) and a location of hole on the rib (h = 0.2H, 0.5H and 0.8H), have been examined. The investigation was performed at constant Reynolds number ( Re ) of 60,000. The experimental heat transfer results via Thermochromic liquid crystal sheet are reported along with the numerical flow characteristics. The results reveal that due to jet-like flows impinging on the surface, the inclined perforated rib considerably improve the heat transfer immediately downstream from the ribs, compared to straight perforated and solid ones, resulting in superior overall heat transfer performance.