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Turbulent periodic flow and heat transfer in a rectangular channel with detached V-baffles
(2017-10-01)
Kaewkohkiat, Y.
;
Promvonge, P.
;
Eiamsa-ard, S.
This paper presents a numerical analysis of turbulent periodic flow and heat transfer in a rectangular channel with detached V-baffles. The computations are based on the finite volume method with the SIMPLE algorithm for handling the pressure–velocity coupling and using the QUICK scheme for the convection terms. Air is used as the test fluid with the air flow rate in terms of Reynolds numbers ranging from 3000 to 20,000. The effects of different detached-clearance ratios (c/H, CR) of 0.0, 0.05, 0.1, 0.15, and 0.2, baffles-pitch to square channel-diameter ratio (pitch ratio (p/H), PR) is 1.0, baffles-height to square channel-diameter ratio (blockage ratio (b/H), BR) is 0.10, and attack angle (α) is 45◦ on heat transfer, friction factor and thermal enhancement factor are investigated numerically. It is found that a pair of counter-rotating vortices (P-vortex) caused by the baffles can induce impingement/attachment flows repeatedly on the rectangular channel walls leading to a greater increase in the heat transfer over the test channel. The maximum thermal performance and heat transfer are found to be about 1.5 and 3.3, respectively for CR = 0.05 and Re = 3000, while the highest pressure loss is about 21.5 in the case of CR = 0.2 and Re = 20,000.
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Numerical heat transfer study in a round tube with 60° V-shaped rings
(2017-06-01)
Kaewkohkiat, Y.
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Tamna, S.
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Pairoj, W.
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Promvonge, P.
Turbulent flow and heat transfer behaviors in a 3-dimensional isothermal tube mounted repeatedly with 60° V-shaped rings (V-rings) are numerically investigated. The computation based on the finite volume method was conducted for the airflow rate in terms of Reynolds number (Re) in the range of 4000 to 20,000. The ring parameters include three flow blockage ratios, (BR=e/D=0.1, 0.15 and 0.2) and a single ring pitch ratio, PR=1. The computed result shows that friction factor and Nusselt number (Nu) increase with the increment in BR and the maximum thermal enhancement factor (η) of about 1.73 with Nu/Nu<inf>0</inf> = 4.5 is found at BR=0.1 and Re=4000. For the need of maximum heat transfer enhancement (Nu/Nu<inf>0</inf>), the case of BR=0.2 V-ring is selected and modified since this case provides the highest heat transfer. The modification is made by cutting off some area of both ends of the BR=0.2 V-ring and the effect of cutting off (called end-cut ratio, ER=c/e) on thermal performance is also examined. The study reveals that for the end-cut V-ring, the friction factor is considerably decreased while the heat transfer rate reduces moderately with increasing ER. The end-cut V-ring yields the maximum η of 1.64 at ER=0.9, Re= 4000 with Nu/Nu<inf>0</inf> = 5.4. Thus, the Nu/Nu<inf>0</inf> of the end-cut one is about 20% higher than that of the BR=0.1 V-ring.
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Thermal characteristics in turbulent channel flows with V-baffle vortex generators
(2014-01-01)
Kaewkohkiat, Y.
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Jedsadaratanachai, W.
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Eiamsa-Ard, P.
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Promvonge, P.
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Eiamsa-Ard, S.
Experimental work has been performed to examine the effects of V-baffle vortex generators on the heat transfer and friction characteristics in a channel under a constant heat flux boundary condition. In the experiments, V-baffles at different pitch ratios (PR=P/e = 4.0, 6.0 and 8.0) were carried out. Measurements were carried out for a channel of one aspect ratio, AR = W/H = 4.0 and duct height, H= 40 mm with baffle height, e = 8 mm. Experiments were conducted for the Reynolds number range of 6000 to 22,000. The distributions of temperature and local Nusselt number on bottom channel wall were observed with thermochromic liquid crystal (TLC) sheet. Isothermal friction factors were also taken and presented. The obtained results demonstrate that heat transfer increases with the increase of Reynolds number, whereas friction factor (f) shows the opposite trend. Experimental results also show that the channels with V-baffle at the highest pitch ratio (PR = 8.0) provide highest heat transfer than others. © 2014 Asian Institute of Technology.
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Heat transfer enhancement in a channel with rib-groove turbulators
(2010-04-01)
Kaewkohkiat, Y.
;
Kongkaitpaiboon, V.
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Eiamsa-Ard, S.
;
Pimsarn, M.
This paper presents the effects of the rib-groove turbulators on the heat transfer and friction characteristics in a rectangular channel. The experiments encompass the Reynolds number range from 1800 to 10,000; pitch ratios (PR=P/e) 6.6-13.3 by using air as the working fluid. The obtained results demonstrate that heat transfer rate in term of Nusselt number (Nu) increases with the increase of Reynolds number, whereas friction factor (f) shows the opposite trend. Both Nusselt number and friction factor increase with decreasing pitch ratio. It is also observed that heat transfer rate and friction factor for the channels with rib-groove turbulators are higher than those for the smooth channel under similar test conditions. In addition, the correlations for heat transfer rate in term of Nusselt number (Nu) and friction factor (f) for channel with rib-groove turbulators are also presented. © 2010 American Institute of Physics.
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Combustion of rice husk in a fluidized bed combustor with wavy-surfaced chambers
(2009-01-01)
Kaewkohkiat, Y.
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Eiamsa-Ard, S.
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Wongcharee, K.
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Thungsotanon, D.
;
Promvonge, P.
The paper deals with an experimental investigation on effects of percent excess airs (EA = 15%, 30%, 45%, 60%, and 75%) on axial/radial temperature distributions, gas emissions and combustion efficiency in a fluidized bed combustor (FBC) with wavy surfaces using rice husk as the fuels. The combustion chamber shape is cylindrical with 200 mm in diameter (D) and 2400 mm in height (H). The top part of the chamber is designed and constructed to be wavy surface walls. In constructing the wavy surface bed at the top chamber, five modules of converging-diverging nozzles are inserted into the bed at which each module has a throat diameter of 100 mm and height of 200 mm. In the experiments, the axial/radial temperature distributions inside the combustor are measured for each percent excess air used while the feeding rate of rice husk is kept constant at 95 kg/hr. From experimental results, the emissions of flue gases are found to be CO = 308 ppm and NO<inf>x</inf> = 298 ppm, at EA = 75%. At all percent excess airs (EA) the axial/radial temperature distributions in the chamber are nearly uniform.
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Combustion behavior in a dual-staging vortex rice husk combustor with snail entry
(2008-11-01)
Eiamsa-ard, S.
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Kaewkohkiat, Y.
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Thianpong, C.
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Promvonge, P.
The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO<inf>2</inf>, O<inf>2</inf>, NO<inf>x</inf>), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO<inf>2</inf> = 8.1%, O<inf>2</inf> = 9.3%, CO = 352 ppm, NO<inf>x</inf> = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk. © 2008 Elsevier Ltd. All rights reserved.

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