Pongjet Promvonge

Influence of Double-sided Delta-wing Tape Insert with Alternate-axes on Flow and Heat Transfer Characteristics in a Heat Exchanger Tube

Abstract The convective heat transfer and friction behaviors of turbulent tube flow through a straight tape with double-sided delta wings (T-W) have been studied experimentally. In the current work, the T-W formed on the tape was used as vortex generators for enhancing the heat transfer coefficient by breakdown of thermal boundary layer and by mixing of fluid flow in tubes. The T-W characteristics are (1) T-W with forward/backward-wing arrangement, (2) T-W with alternate axis (T-WA), (3) three wing-width ratios and (4) wing-pitch ratios. The experimental result reveals that for using the T-W, the increases in the mean Nusselt number ( Nu ) and friction factor are, respectively, up to 165% and 14.8 times of the plain tube and the maximum thermal performance factor is 1.19. It is also obvious that the T-W with forward-wing gives higher heat transfer rate than one with backward-wing around 7%. The present investigation also shows that the heat transfer rate and friction factor obtained from the T-WA is higher than that from the T-W. In addition, the flow pattern and temperature fields in the T-W tube with both backward and forward wings were also examined numerically.

Experimental Investigation on Turbulent Convection in Solar Air Heater Channel Fitted with Delta Winglet Vortex Generator

Abstract The paper presents an experimental study on the heat transfer and flow friction characteristics in a solar air heater channel fitted with delta-winglet type vortex generators (DWs). The experiments are conducted by varying the airflow rate for Reynolds number in the range of 5000 to 24000 in the test section with a uniform heat-flux applied on the upper channel wall. Firstly, the DW pairs are mounted only at the entrance of the lower wall of the test channel (called DW-E) to create multiple vortex flows at the entry. The effect of two transverse pitches ( R P = P t / H = 1 and 2) at three attack angles ( α = 30°, 45° and 60°) of the DW-E with its relative height, b/H = 0.5 (half height of channel) is examined. Secondly, the 30° DWs with three different relative heights ( b/H = 0.3, 0.4 and 0.5) are placed on the upper wall only (absorber plate, called DW-A) of the test channel. The experimental result reveals that in the first case, the 60° DW-E at R P = 1 provides the highest heat transfer and friction factor while the 30° DW-E at R P = 1 performs overall better than the others. In the second case, the 30° DW-A at b/H = 0.5 yields the highest heat transfer and friction factor but the best thermal performance is found at b/H = 0.4.

Thermal enhancement in a solar air heater channel using rectangular winglet vortex generators

The forced convection heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel solar air heater with rectangular winglet vortex generator (WVG) are experimentally investigated in this work. The rectangular winglet pairs are considered with two different arrangements by pointing upstream (PU) and pointing downstream (PD) of the flow. Ten pairs of the WVGs with various attack angles (α) of 60°, 45° and 30° are mounted on the test duct entrance wall to create longitudinal vortex flows over the tested channel. Measurements are carried out for the rectangular channel air heater of aspect ratio, AR = 10 and height, H = 30 mm with the WVG height, b/H = 0.4 and a transverse pitch ratio, P/H = 1. The flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the channel ranging from 5000 to 23,000. The experimental results show that the solar air heater channel with rectangular WVG provides significantly higher heat transfer rate and friction loss than the smooth wall channel. The use of larger attack angle value leads to higher heat transfer rate and friction loss than that of lower one. The PD-WVGs performs higher heat transfer rate and friction loss than the PU one for similar operating conditions. In comparison, the largest attack angle (α = 60°) of the PD-WVGs yields the highest increase in Nusselt number and friction factor while the lowest attack angle (α = 30°) of the PU-WVGs shows the best thermal performance.

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.

INFLUENCE OF NONUNIFORM TWISTED TAPE ON HEAT TRANSFER ENHANCEMENT CHARACTERISTICS

An experimental study of heat transfer and friction factor characteristics in a circular tube fitted by twisted tapes with nonuniform twist ratios is reported. The twisted tapes are used as swirl generators playing roles as heat transfer enhancers. The nonuniform twisted tapes examined in the present work have (1) sequentially increasing twist ratios (SL), (2) sequentially decreasing twist ratios (LS), (3) repeatedly increasing-decreasing twist ratios (SL/SL), (4) repeatedly decreasing-increasing twist ratios (LS/LS), (5) intermittently increasing-decreasing twist ratios (SL→LS), and (6) intermittently decreasing-increasing twist ratios (LS→SL). In addition, three different typical twisted tapes with twist ratios of 3.0, 4.0, and 5.0 (TT) were also evaluated. Apparently, LS yields monotonically increasing swirling intensity and SL decreasing swirling intensity, resulting in lower heat transfer rate and friction factor than the other four nonuniform twisted tapes, which possess extra fluid fluctuation. However, among the tapes tested, the SL/SL offers the maximum thermal performance factor of around 1.03, which corresponds to Nusselt number of around 36% and friction factor of 3.57 times over those of the plain tube.

Thermal Performance Assessment of Turbulent Tube Flow Through Wire Coil Turbulators

This paper presents characteristics of turbulent convective heat transfer in a tube fitted with wire coil turbulators. Two different wire coils are introduced: (1) with typical/uniform coil pitch ratio (CR) and (2) with periodically varying three-coil pitch ratio. Various uniform coil pitch ratios (CR = 4, 6, and 8) and two periodically varying coil pitch ratios, the D-coil (decreasing three-coil pitch ratio arrangement) and DI-coil (decreasing/increasing three-coil pitch ratio arrangement), are experimentally investigated in a uniform heat flux tube. The experiments are performed for turbulent flows with Reynolds numbers ranging between 4500 and 20,000. All of the experimental results are compared with those obtained from using the plain tube, while the thermal performance factor is evaluated under an equal pumping power constraint. The experimental results show that the use of the tube fitted with all wire coils leads to an advantage on the basis of heat transfer enhancement over the plain tube with no insert. It is also observed that the uniform-pitch wire coil with higher coil pitch ratio (CR = 8) gives a higher thermal performance factor compared to ones with lower coil pitch ratios (CR = 4 and 6). In addition, for two periodically varying coil pitch ratios, the DI-coil performs with better heat transfer rate than the uniform-pitch ratio (CR = 6) and the D-coil for all Reynolds number ranges studied. The empirical correlations developed in terms of coil pitch ratios (CR), varying coil pitch ratios (D-coil and DI-coil), and Reynolds number are fitting the experimental data within plus or minus 3% and 5% for Nusselt number (Nu) and friction factor (f), respectively. The results of the thermal performance factor for various CR, D-coil, and DI-coil values are also determined.

Augmented heat transfer in rectangular duct with angled Z-shaped ribs

An experiment is focused on the design of the suitable ribs used for enhancing heat transfer in a rectangular duct heat exchanger by using wall heat transfer (Nusselt number), friction loss (friction factor) and thermal performance (thermal enhancement factor) data. The Z-shaped ribs were set on the rectangular duct at 30°, 45° and 60° and flat rib was set at 90° relative to the air flow direction. Reynolds numbers studied ranging from 5000 to 25,000 in the test section. The rectangular duct has aspect ratio, AR = 10 and height, H = 30 mm with the Z-rib height (e), e/H = 0.2 and the rib pitch (P), P/H = 3. The ribs were fitted in Z-shape (Z-rib) aligned in series on the whole area of the upper plate. The results of the Z-ribs show the significant increase in heat transfer rate and friction loss over the smooth channel. The 45° Z-rib provides the highest increase in the heat transfer rate and the best thermal performance

Thermal performance enhancement in solar air heater channel with periodically V-shaped baffles

The research paper presents a study on thermal performance enhancement in a rectangular channel heat exchanger mounted with periodically V-shaped baffles. The channel has an aspect ratio (width to height ratio), AR=10 and height, H=30 mm while baffle characteristics are the baffle to channel height ratio, e/H=0.2, 0.3 and 0.4; the baffle pitch to channel height ratio, PR=P/H=2 and 2.67; the attack angle (α) of 30° relative to the flow direction. The experiment has been conducted by varying air flow velocity in order to adjust Reynolds number range from 5000 to 25,000. The upper wall of the channel is uniformly heated as a constant heat flux while the rests are covered with thermal insulations to reduce heat loss to surroundings. The effects of the baffles on Nusselt number and friction factor have been examined. The overall performance of tested baffled channel is evaluated to obtain the degree of heat transfer enhancement and friction factor induced by baffles with respect to the smooth channel under similar flow conditions.

Drying kinetic of peppercorns in a rectangular fluidized-bed with wavy surfaces

The paper presents the comparative study of drying peppercorns in a rectangular fluidized-bed and a rectangular fluidized-bed with wavy surfaces. This experimental work has been carried out in two different drying bed configurations: rectangular bed and rectangular bed with wavy surface (called “wavy surface bed”). The wavy surface bed is used to produce a stronger turbulence flow inside. For each run of both beds, inlet hot air at 80°C is used for drying peppercorns with initial moisture content at about 82.5% w.b. The inlet air velocity is adjusted to be 1.0Umf, 1.1Umf and 1.3Umf (Umf= minimum fluidization velocity of peppercorns) as desired. Drying time and weight of peppercorns are measured in 5 minute intervals for 120 minutes for each run. The experimental results show that in the rectangular bed, the peppercorns show low oscillating and it becomes dense bed. This causes low heat transfer rate between peppercorns and hot air leading to slower mass transfer of moisture in the peppercorns. In the wavy surface bed, the peppercorns has high fluctuations due to unsymmetrical flow in the bed creating a sustainable oscillating flow with stronger turbulence of hot air and it causes dilute bed, resulting in higher heat and mass transfer in peppercorns. Closer examination reveals that for a similar operating condition, the rectangular wavy surface bed performs better than the rectangular bed.

Numerical heat transfer analysis in turbulent channel flow over a side-by-side triangular prism pair

Heat transfer and flow behavior in a channel fitted with a transverse triangular prism pair is numerically investigated in the turbulent flow regime for the Reynolds number ranging from 10000 to 50000. The aspect ratio of channel height to the prism base is fixed at 4.0 throughout the study. The Navier-Stokes equation, along with the energy equation, is solved using a finite volume method with the SIMPLE technique and the QUICK numerical scheme for coupling the discretized equations while the standard k-ɛ turbulence model is used for closure of the problem. The numerical result reveals that heat transfer augmentation in the channel with the built-in prism pair can be obtained. It is observed that as compared to a channel, the heat transfer is enhanced by about 17% for a single triangular prism and by some 85% for a triangular prism pair mounted on the channel wall. Effects of the clearance between the prisms on the heat transfer augmentation are presented. The heat transfer enhancement is due to the vortex formation or recirculation zone downstream of the prism elements. However, the presence of the prisms also leads to higher values of friction loss over the channel.