EFFECT OF BAFFLES GEOMETRY ON HEAT TRANSFER ENHANCEMENT INSIDE CORRUGATED DUCT

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 03, March 2019, pp Article ID: IJMET_10_03_057 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EFFECT OF BAFFLES GEOMETRY ON HEAT TRANSFER ENHANCEMENT INSIDE CORRUGATED DUCT Department of Mechanical Engineering, Faculty of Engineering, Almustansiriyah University, Iraq. ABSTRACT The turbulent heat transfer and friction inside a corrugated square duct inserted with various baffles geometries have been studied experimentally. Five types of baffles (flat, rectangular, semicircular, triangular and trapezoidal) are attached on top and bottom walls of the duct. The effects of duct wavy surface, baffle geometry, baffle position and flow Reynolds number are examined. Air is used as the working fluid with Reynolds number ranged from to under constant wall heat flux. Experimental results show obtained for average Nusselt numbers and friction factor. The results indicate that the trapezoidal baffled geometry provides a higher thermal performance than the other type baffled one. The present work showed that the highest thermal performance factor under the same pumping power obtained from the experiments, is about 2.26 times more than that of plain duct. Also, it is found that the thermal performance of the baffles attach on the bottom wall of the duct is higher than the other baffles attach on the top. Key words: Square duct, Nusselt number, Friction factor, inclined baffles. Cite this Article, Effect of Baffles Geometry on Heat Transfer Enhancement inside Corrugated Duct, International Journal of Mechanical Engineering and Technology, 10(3), 2019, pp INTRODUCTION One of the passive heat transfer augmentation techniques is using of baffle used in many industrial applications such as heat exchangers, internal cooling for turbines-blade, solarcollectors and used in electronic-devices cooling system. Baffles making a role in enhance heat-transfer enhancement by enlarge the area of the effective-surface and residence of timeperiod for the fluid heat transfer. Some parameters influence on the transferring heat and pressure-drop involves pitch ratio, orientation, blockage-ratio, baffles geometry and Reynolds number editor@iaeme.com

2 Heat transfer enhancement by baffles have been studied by various authors. Researcher [1] studied experimentally heat transfer and frictional losses in channel of rectangular section with two inclined solid and perforated baffles of the similar overall size on. The upstream baffle is mounted on the top surface (heated), while the orientation, shape and location of the other baffles was varied to attain the optimum geometry for augment transferring heat. The results of experimentation proved that the distribution of Nusselt-number depending on the orientation, location and second plate baffles geometry. Researcher [2] numerical study was conducted for laminar flow (periodic) of transferring heat and losing pressure in a channel that fitted with a staggered-baffles of diamond shaped. They found that the baffle-diamond with half-apex angle of 5 10 provided thermal performance slightly better than the flat baffle. Researcher [3] experimentally studied the effect of 60 -V-type turbulator-baffle on transferring heat and pressure-drop. Three types of various baffles-shape had e/h value of 0.1, 0.2 and 0.3 and PR values of 1, 2 and 3 are used a range of Reynold's number about , researcher [4].The results show that the maximum improvement thermal factor of 1.87 was achieved when used V-type baffle and PR=1.0 and e/h= 0.10 at lower Reynolds number. Researcher [5] studied experimentally transferring heat in a-channel with Z-ribs-shaped for various pitches at Re range of Zigzag-shape baffles are placed in a series-aligned on the top-wall fluxed isothermally. The Z-type baffles are 45 -inclined relative to the main flow direction are characterized at three baffle pitch ratios (P/H=1.5, 2 and 3) and the ratio of baffle- to channel-height (e/h=0.1, 0.2 and 0.3).The results show a significant effect of the presence on the transferring heat rate for Z-type baffle and friction loss over the plain channel with no baffle, Researchers [6-7]. This work aims to realize the effects of baffle's geometry and position of baffle on fluid characteristic, such as pressure drops, friction factor and heat transfer in corrugated square duct.five types of baffles (flat, rectangular, semicircular, triangular and trapezoidal) inserts in the duct which are manufactured locally and experimentally realized. Finally, a performance differentiation of all inserts shall be made to obtain the optimum geometry. 2. EXPERIMENTAL SETUP The experimental apparatus illustrates in Figure 1 that used in this study. It consists of a blower, inlet section, a test-section, exit section and instrumentations to measure temperatures, air velocity, pressure drop and electrical power input. The details of the apparatus are depicted as follows: (a) Air blower: An air blower of centrifugal kind, operates by an electrically-motor of 0.8 kw capacity and 2840 rpm, was used to supply the system with air at the required flow rate. Flow rate of an air can be controlled by manual control valve which is connected with blower. (b) Test section: The test section is a horizontal corrugated square duct has an internal size of 60*60 mm, the duct is manufactured from copper, 0.5 mm in thickness. The wavy surface of duct are manufactured locally by bending. The test section has 600 mm length, and an additional 600 mm (10 d h ), Researcher [8] hydrodynamic entry length so that flow would be fully developed as it enters the baffles section. The plate-type heater with rating (1000W, resistivity 4.9 Ohm/meter) is applied for heating the walls. To attain a constant heat flux boundary condition along the tested duct the heater is connected to a variac voltage transformer that supply an electric AC power to regulate the input voltage across it. To minimize the heat losses the test section is insulated with 50 mm-thick glass wool. Figures 2 & 3 show, a surface profiles of the duct wall, and the tested duct respectively. (c) Baffles: Flat, rectangular, semicircular, triangular and trapezoidal baffles are used in the present study as shown in Figure 4. The copper baffles are attached on the upper and then editor@iaeme.com

3 Effect of Baffles Geometry on Heat Transfer Enhancement inside Corrugated Duct to the lower wall of the duct with height of 175 mm, width of each baffles is about 2 mm less than the width of the duct.the distance between two successive baffles (pitch,bp) is (50 mm). All details of baffles are demonstrated in Figure 5. d) Instrumentation: The temperatures of duct surface (Ts) on the upper, lower and side walls are measured by 12 thermocouples (K-,Type, range: 0 C,to 800 C) of 1 mm diameter located along the test section as shown in Figure 6. And another two thermocouples are positioned upstream and downstream of the test duct to measure the inlet and outlet air temperatures. These thermocouples are connected to a Bench type 12 channels temperature recorder SD card time data logger model BTM-4208SD digital recorder. To measure the axial pressure drops across the test section two static pressure taps are located at the top of the duct wall, which used to evaluate average friction factor. The pressure drop is measured by digital manometer (±2000 mbar, range differential, input Model PM- 9100). Digital vane-type anemometer is used to measure the air velocities. Reynolds number of range of is adopted. The data of electric power input, the surfaces temperature, inlet and outlet air flow temperatures, and the manometers readings are recorded when the steady state is attained. Figure 1 Schematic diagram of apparatus editor@iaeme.com

4 Figure 2 Surface profiles of the duct wall Figure 3 The tested duct Figure 4 The tested baffles editor@iaeme.com

5 Effect of Baffles Geometry on Heat Transfer Enhancement inside Corrugated Duct Figure 5 Schematic diagram of baffles Figure 6 Positions of the thermocouples on the test sectio 3. THEORETICAL ANALYSIS The aims of current work is to inquiry the Nusselt number, friction factor and thermal enhancement factor. Reynold's number is an independent parameter. The Reynolds number based on the hydraulic diameter of the square duct which is given by Researcher [9]: Re= Where dh: is the hydraulic diameter of square duct,(m) dh=4a c /p (2) u : is the average velocity, (m/s). Heat carried by air is given by Researcher [9]: Qhc= c p ( ) (3) Where = mass flow rate, (kg/s) is given by = (4) Assumption: steady-state conditions of transferring heat transfer equals to losing heat loss in test section is given by: (1) editor@iaeme.com

6 Qh c =Q convection. (5) Transferring heat by convection in test section is given by Researcher [9]: Q conv. =ha s (T wm T avg. ) (6) where As = is the surface area of heat transfer,(m 2 ), in this study the baffles surface area is neglected, Researcher [10] which is given by: As=(4a.Lt) (7) Where a : is the side length of square duct. L t : is the test section length. Average mean wall temperature (T wm ) and bulk temperature (T avg. ) are illustrated in (8 and 9-respetivelly). = /12 (8) T avg. = (9) Average value of both coefficient of transferring heat (h) and Nusselt number Nu are shown in eq. (10 and 11) : h Nu=hd h /k (11) Friction-factor was computed along test section length Researcher [9]: f=2δpd h /ρu 2 L t (12) Where Δp = the pressure drop over the test section The thermal improvement factor definitions as the ratio of the coefficient of transferring heat of enhanced surface to that of a plain surface at same pumping power, Researcher [11]: TEF=(Nu/Nu s )/(f/f s ) 1/3 (13) The friction-factor and heat transfer verification of plain square duct of non-baffle is performed by rapprochement with previous values under similar condition as shown in Figure 7 and 8, respectively. Results of the present square- plain duct are found to be good agreement with those obtained from the foundation correlations of researcher [8] found in the open literature, less than (±0.5% deviations). The equations of the friction-factor and Nusselt-number and for turbulent flow in the square duct with fixed heat flux are as follows: Correlation of researcher [8]: Nu o =0.023Re 0.8 Pr 0.4 (14) Correlation of researcher [8]: f o =(0.79lnRe 1.64) -2 (15) 4. RESULTS AND DISCUSSIONS The average Nusselt number for plain and baffled duct through a range of flow Reynold'snumber of is illustrated in Figure 9.Higher is reported average Nusselt number for baffled duct than plain duct since baffles interrupt the wall boundary layer causing (10) editor@iaeme.com

7 Effect of Baffles Geometry on Heat Transfer Enhancement inside Corrugated Duct reattachment, flow separation, and also generate intense, highly secondary flows and vortex pairs, which increase the turbulent transport in this duct. In general this enhance the heat transfer. Variation of the average Nusselt number enhancement ratio (Nu/Nup) for different geometry and position of baffle is plotted in Figure 10. It's clear from this figure that the enhancement ratio is decreased with the Reynolds number incresed for all of studied cases. The maximum value is obtained for trapezoidal baffles attached on the lower duct wall which is found to be about time over the corrugated plain duct and the minimum value is found to be about time over the plain duct for flat baffles which attached on the upper duct wall. Figure 11 shows the effect of using wavy surface and baffles-turbulators on the pressure drop across the tested duct as shown in terms of friction. It is noted from this figure when using baffles-turbulators leads to a substantial increase in friction factor over the plain duct. Figure 12 presents the differentiation of friction-factor ratio, f/f p with Reynold's-number for different geometries and positions baffles. It is observed that the f/f p increases when Reynold's number increased. The use of rough surface of duct and inserts the inline baffle produce to an extremely increasing in friction-factor in differentiation with plain duct without baffle. Maximum friction factor ratio for the flat baffle which attached on upper wall duct is found to be about times over the plain duct and the minimum friction factor ratio for the flat baffle which attached on the lower wall duct is observed to be 3.5-4times over the plain duct depend on Reynold's number values. Figure 13 shows the thermal improvement factor for the baffled duct.the performance factor tends to decrease with the Reynolds number is rise. The maximum thermal enhancement factor is found about of 2.26 for the trapezoidal baffle attach on the lower wall duct. Figure 7 Nusselt number as function of Reynolds number for square duct editor@iaeme.com

8 Figure 8 Friction factor as function of Reynolds number for square duct Figure 9 Variation of Nusselt number with Reynolds number editor@iaeme.com

9 Effect of Baffles Geometry on Heat Transfer Enhancement inside Corrugated Duct Figure 10 Variation of Nusselt number ratio, Nu/Nu p with Reynolds number Figure 11 Variation of Friction factor with Reynolds number editor@iaeme.com

10 Figure12 Variation of Friction factor ratio, f/f p with Reynolds number Figure 13 Variation of thermal enhancement factor with Reynolds number editor@iaeme.com

11 Effect of Baffles Geometry on Heat Transfer Enhancement inside Corrugated Duct 5. CONCLUSIONS An experimental study is presented to investigate of the heat transfer characteristics and friction loss in a corrugated square duct with various baffle geometries aligned along the upper and lower duct walls. Conclusions emerging from the results of the present experimental study include the following: 1- Generally, the average Nusselt-number for baffled duct is higher than the smooth duct. 2- The Nusselt-number enhancement ratio (Nu/Nu p ) slightly decrease with increasing Reynolds number for all of studied cases. 3- The maximum Nusselt-number enhancement ratio are recorded at a range time over the corrugated plain duct for trapezoidal baffles attached on the lower duct wall and the minimum Nusselt-number enhancement ratio are recorded at a range time over the corrugated plain duct for flat baffles which attached on the upper duct wall. 4- Using turbulators- baffles leads to a substantial increase in friction-factor over a plain duct. 5- Maximum ratio of friction-factor of flat baffle which is attached on upper wall duct is found to be about times over the plain duct and the minimum friction factor ratio for the flat baffle which attached on the lower wall duct is recorded about 3.5-4times over the plain duct depend on Reynold's-number. 6- The maximum thermal enhancement factor is found about of 2.26 for the trapezoidal baffle is that attach on the lower wall duct. REFERENCES [1] Dutta, P., and Hossain, A., Internal cooling augmentation in rectangular channel using two inclined baffles, International Journal of Heat and Fluid Flow, 26 (2), 2005, pp [2] Somchai Sripattanapipat and Pongjet Promvonge, Numerical analysis of laminar heat transfer in a channel with diamond-shaped baffles, International Communications in Heat and Mass Transfer, Vol.36, 2009, PP [3] Promvonge, P., Heat transfer and pressure drop in a channel with multiple 60 V-baffles, International Communications in Heat and Mass Transfer, 37(7), 2010, pp [4] Murata A., Mochizuki S., Comparison between laminar and turbulent heat transfer in a stationary square duct with transverse or angled rib turbulators, Int. Commun. Heat and Mass Transfer, Vol.44, 2001 pp [5] Parkpoom Sriromreun, Chinaruk Thianpong and Pongjet Promvonge, Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles, International Communications in Heat and Mass Transfer, Vol. 39, 2012, PP [6] Ahn S.W., The effects of roughness types on friction factors and heat transfer in roughened rectangular duct, Int. Commun. Heat and Mass Transfer, Vol.28 (7), 2001,pp [7] Promvonge P., Thianpong C., Thermal performance assessment of turbulent channel flow over different shape ribs, Int. Commun. Heat Mass Transfer, Vol , pp [8] Adrian Bejan and Allan D. Kraus, Handbook of, Heat Transfer, 2003, Canada editor@iaeme.com

12 [9] Incropera, F., and Dewitt, P. D., Introduction to Heat Transfer, John Wiley & Sons Inc., Fifth edition, [10] Rajendra Karwa,Maheshwari,B.K.,Nitin Karwa, Experimental study of heat transfer enhancement in an asymmetrically heated rectangular ducts perforated baffles, International Communications in Heat and Mass Transfer,Vol.32, 2005, pp [11] Sombat Tamna, Warakom Nerdnoi, Chinaruk Thianpong and Pongjet Promvonge, Numerical.Heat Transfer Study in a Square Channel with Zigzag-Angled Baffles, The Second TSME International Conference on Mechanical Engineering,19-21 October, 2011, Krabi editor@iaeme.com