An experimental study on thermal mixing in a square body inserted inclined narrow channels

International Communications in Heat and Mass Transfer 39 (2012) 1245–1252

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An experimental study on thermal mixing in a square body inserted inclined narrow channels☆ Yasin Varol a,⁎, Besir Kok b, Hakan F. Oztop c, Ismail Turkbay d a

Department of Automotive Engineering, Technology Faculty, Firat University, 23119 Elazig, Turkey Technical Vocational School, Firat University, 23119 Elazig, Turkey Department of Mechanical Engineering, Technology Faculty, Firat University, 23119 Elazig, Turkey d Department of Mechanical Education, Firat University, 23119 Elazig, Turkey b c

a r t i c l e

i n f o

Available online 16 July 2012 Keywords: Thermal mixing Parallel jets Heat transfer

a b s t r a c t An experimental study has been performed on thermal mixing phenomena in a narrow channel by twin-jets at different temperatures. Water was used as working fluid and it is supplied by hot and cold taps. The channel has a circular exit hole to supply continuity of mass. An adiabatic square shaped object, which in the thickness of the channel, is inserted into the channel to control thermal mixing as a passive technique. Other parameters in experiments are ratio of flow rate of inlet fluid, inclination angle of the channel, jet diameter and jet velocities. Finally, a thermal mixing index was calculated from measured values of temperatures for different parameters. Temperature distribution is obtained for whole channel and isotherms are plotted. The obtained results indicated that higher thermal mixing efficiency is observed for ϕ = 60 o and inserted body can be a control parameter for thermal mixing for the same geometrical parameters. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Mixing processes are very crucial subjects in chemistry, mechanical engineering and environmental science. It can be classified in two separate groups such as flow mixing and thermal mixing. In flow mixing phenomena, the same fluid/fluids are mixed using mechanical devices such as propeller or jet mixing can be used [1]. In this context, jet mixed tanks are very popular due to low energy consuming, low investment and there is no complex mechanical part they have. This idea also can be used for thermal mixing process. Humprey et al. [2] numerically studied the time-dependent motion of a constant property, Newtonian fluid in a counter current shearing flow configuration. The three-dimensional flow and mixing characteristics of multiple and multi-set three dimensional confined turbulent round opposing jets in a novel in-liner mixer are examined numerically using the standard k-ε turbulence model by Wang and Mujumdar [3]. They indicated that multiple opposing jets achieve better mixing than single opposing jets in the study. Wang et al. [4] numerically studied the laminar flow in an in-line mixer based on opposing jet impingement. They found that unequal inlet momenta of

☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. Tel.: +90 424 237 0000x4219; fax: +90 424 236 7064. E-mail address: [email protected] (Y. Varol). 0735-1933/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2012.07.004

opposing jets obtained using both equal and unequal slot widths and the addition of baffles in the exit channel yield better mixing over shorter distances after impact. In their another study, they tested the effects of type of fluid as using air and water on flow and mixing effectiveness for various temperature differences between the confined opposing jets of different geometries. Beuf et al. [5] studied the influence of the geometry of the cell on mixing efficiency using three different geometries as circle, square and rectangle. They indicated that the flows in Hele–Shaw cells are generally laminar and it can be in a first approximation considered as quasi-two dimensional. They also showed that the rectangular geometry leads to a better mixing, but also that the aspect ratio of the rectangle play unexpectedly no important role on mixing. Walker et al. [6] made both numerical and experimental study to carry out mixing of coolant streams of different temperature in pipe junctions. In this way thermal fatigue may prevent in the pipe wall. They presented a distribution of time averaged mixing scalar for different velocity ratios. Wang et al. [7] studied jet mixing problem inside a slot experimentally. They tested the jet array effect on cooling performance. They also tested effect of orientation angle and H/D ratio. It is found that the acceptable uniform flow is observed for shallow as H/D = 1. Chang et al. [8] made a numerical analyzes to investigate the thermal mixing efficiency in Y-shaped channel. They solved two dimensional incompressible, steady state equations using Lattice Boltzmann method. They inserted different types of passive element to improve thermal mixing efficiency. It is demonstrated that the enhanced mixing efficiency is result of an increased intersection angle between the velocity vector and the temperature gradient within the channel.

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Y. Varol et al. / International Communications in Heat and Mass Transfer 39 (2012) 1245–1252

Nomenclature D ṁc ṁh MI n PE St t Tavg Tc Th WMI ΔT ϕ

Diameter (mm) Cold water mass flow (kg/s) Hot water mass flow (kg/s) Mixing index Number of jet Passive element Standard deviation of fluid temperature Time (s) Average temperature (°C) Cold jet temperature (°C) Hot jet temperature (°C) Uncertainty of mixing index Temperature difference between hot and cold water (°C) Inclination angle of the channel

Thermal mixing is also a very important application for T-junctions as given by Naik-Nimbalkar et al. [9]. Hu and Kazimi [10] made a numerical simulation on thermal striping for three-dimensional, unsteady turbulent model. They used two types of mixing tee configurations

and they modeled using commercial CFD code FLUENT. A similar work has been done by Kamide et al. [11] using water as a numerical work with finite difference method. They indicated that mixing behavior in the tee was characterized by the relatively large vortex structures defined by the diameters and the velocities in the pipes. Using of jets to enhance thermal mixing efficiency is very useful way and many of authors are worked on this subject as Shi et al. [12], Lou et al. [13], Chua et al. [14] and Devahastin and Mujumdar [15]. These authors mostly studied the jet mixing phenomena using numerical techniques. They observed that thermal mixing is mostly a function of temperature of jets. Sometimes, different shaped passive elements are used to control flow field, heat transfer and thermal mixing. Shan and Zhang [16] made a numerical calculation to investigate the different mixer configuration for an exhaust system of turbo-fan engine. They observed that the lobed forced mixer can increase the mixing efficiency by 65%, decrease the thrust coefficient by 3% only. Turki [17] used square cylinder to make a control mechanics for flow mixing numerically. Patil and Tiwari [18] made a numerical work to control laminar flow in a channel behind two inclined square cylinders. The main objective of this study is to understand the phenomena of flow and thermal mixing in a narrow channel with square object inserted. Twin jets are used at different temperature to supply water

Fig. 1. Diagram of experimental set-up.

Y. Varol et al. / International Communications in Heat and Mass Transfer 39 (2012) 1245–1252

into channel. This is good application for some sanitary system and mixture tanks. Based on above literature, there is no experimental work on thermal mixing in a narrow channel with passive element inserted. 2. Experimental set-up A schematic view of the experimental set-up is presented in Fig. 1 with equipment. In this experiment, two circular inlet jets are obtained. The water was used as working fluid. Each jet has different flow temperature. The temperatures were measured by T-type thermocouples located 100 mm distance in vertical and 133 mm distance in horizontal directions as given in Fig. 2 (b). The measuring points are named as T1, T2, T3,…..T20. We called station for each four thermocouples in a column. Thus, there are 5 stations in the experiment from left to right. This figure (Fig. 2 (a)) also shows the

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model experiment for channel with passive element. The passive element that inserted into the channel is 100 × 100 × 30 mm square adiabatic object. The channel can be an inclined position with inclination angle of ϕ. The channel can be seen from the side view in Fig. 2 (c). The length of the pipe is chosen to allow the flow to be fully developed at both inlet and exit. Hot water is supplied into the system using an electrical heater. Flowrates of hot and cold waters are measured by two different water rotameters. The inlet and outlet temperatures of water were also measured at the inlet and outlet pipe. The channel was insulated using rock wool at 10 cm thickness [19]. 3. Definition of mixing index The definition of the mixing index is the key results for this study. It is defined by Eq. (1) as used by many others earlier [3]. Here as value of

Fig. 2. a) Model of experimental set-up, b) dimensions of experimental set-up and location of thermocouples, c) side view.

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Fig. 3. a) Variation of mixing index with X coordinate, b) variation temperature of fluid _ h ¼ 0:05 kg=s; m _ c ¼ 0:033 kg=s and ϕ = 0°. with Y coordinate for m

Fig. 5. a) Variation of mixing index with X coordinate, b) variation temperature of fluid with _ h ¼ 0:05 kg=s; m _ c ¼ 0:05 kg=s, ΔT=15 °C and D=10 mm jet inlet. Y coordinate for m

a

b

c

d

24 .95 25 .85 26 .75 27 .65 28 .55 29 .45 30 .35

o

C

24 .4 25.3 26 .2 27.1 28 28.9 29.8 30.7 31.6

o

C

_ h ¼ 0:05 kg=s; m _ c ¼ 0:033 kg=s and ϕ = 0°, a) ΔT = 15 °C, D = 5 mm, b) ΔT = 20 °C, D = 5 mm, c) ΔT = 15 °C, D = 10 mm, d) ΔT= 20 °C, D = 10 mm. Fig. 4. Temperature field for m

Y. Varol et al. / International Communications in Heat and Mass Transfer 39 (2012) 1245–1252

a 24 .95 25 .85 26 .75 27 .65 28.55 29 .45 30 .35

b o

C

c 25 .84 26.36 26 .88 27 .4 27 .92 28.44 28 .96

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o

25 .78 26 .3 26 .82 27 .34 27.86 28 .38 28 .9

C

d o

C

26 .38 26.9 27.42 27 .94 28 .46 28 .98 29 .5

o

C

_ h ¼ 0:05 kg=s; m _ c ¼ 0:05 kg=s, ΔT = 15 °C and D = 10 mm jet inlet, a) ϕ = 0°, b) ϕ =30°, c) ϕ= 60°, d) ϕ = 90°. Fig. 6. Temperature field for m

thermal mixing decrease from 100% to zero, the thermal mixing efficiency of hot and cold water get better. MI ¼

St  100 ΔT

ð1Þ

In this equation, St shows the standard deviation between hot and cold water temperatures. Thus, standard deviation is given by

St ¼

s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi  2 ∑ni¼1 Th −Tavg =ðn−1Þ

ð2Þ

where ΔT is temperature difference between hot and cold water and Tavg average temperature. 4. Uncertainties The independent parameters measured in the experiments: temperatures in the channel. To carry out these experiments, accuracy of data logger is 0.001. The Uncertainty in the result having with odds is calculated by " WR ¼

∂R W ∂x1 1

2

 þ

∂R W ∂x2 2

2

 þ

∂R W ∂x3 3

2

 þ ::::::::::::::

∂R W ∂xn n

2 #1 =2

ð3Þ Following Eq. (3), St uncertainty for ΔT can be written as Tavg ¼

Fig. 7. a) Variation of mixing index with X coordinate, b) variation temperature of fluid _ h ¼ 0:05 kg=s, ΔT = 20 °C, ϕ = 60° and D = 5 mm jet inlet. with Y coordinate, for m

WTavg Tavg

Thot −Tcold 2

¼

"  WThot 2 Thot

ð4Þ

þ

  #1 = WTcold 2 2 Tcold

ð5Þ

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Fig. 9. a) Variation of mixing index with X coordinate, b) variation temperature of fluid with _ h ¼ 0:05 kg=s; m _ c ¼ 0:042 kg=s ΔT=15 °C, ϕ=30° and D=5 mm jet Y coordinate, for m inlet. _ h ¼ 0:05 kg=s, ΔT= 20 °C, ϕ = 60 and D = 5 mm jet inlet, Fig. 8. Temperature field for m _ c ¼ 0:033 kg=s, b) m _ c ¼ 0:042 kg=s, c) m _ c ¼ 0:05 kg=s. a) m

St ¼ Thot −Tavg WSt Tt

¼

"  WThot 2 Thot

ð6Þ

þ

WTavg

!2 #1 =

Tavg

ΔT ¼ T1 −T2 WΔT ¼ ΔT

WMI ¼ MI

"  WT1 2 T1

2

ð7Þ ð8Þ

þ

  #1 WT2 2 =2 T2

"   #1  WΔT 2 =2 WSt 2 þ St ΔT

ð9Þ

ð10Þ

Calculations show that the total uncertainty in calculating for MI; WMI = 0.46%. 5. Results and discussion An experimental study has been performed in this work to see the effects of parameters on thermal and flow mixing under twin-jet flow. The studied parameters are inclination angle of the channel, diameters of the inlet jet nozzle, temperature difference between two inlet jets and flow rate of the jets. Finally, mixing index and temperature distribution will

be presented in the next parts of the study. Also temperature distribution is plotted via isotherms for each case. Fig. 3 (a) and (b) illustrates variation of mixing index (MI) with X coordinate and variation of fluid temperature with Y coordinate at the station 5, respectively. The governing parameters for this figure are _ h ¼ 0:05 kg=s; m _ c ¼ 0:033 kg=s and ϕ = 0°. Variation of mixing m index with X coordinates for the indicated parameters is given in Fig. 3 (a). This figure is plotted using definition of mixing index which is calculated from Eq. (1). It is started from a huge value and decreases along the channel. An increasing is formed at the location of square object. MI fluctuating along the channel at the D =10 mm, ΔT = 20 °C and D = 5 mm, ΔT = 20 °C. This mean thermal mixing is better along the channel at lower temperature difference for these parameters. Also as it is seen from the figure thermal mixing is better at D = 5 mm nozzle diameter. Fig. 3 (b) gives variation temperature of fluid with Y coordinate with same parameters as Fig. 3 (a). It compares the effect of both temperature difference and jet inlet diameters on temperature variation. As seen from Fig. 3 (b), for D = 5 mm, ΔT = 20 °C, temperatures values are almost constant with Y direction. D = 10 mm, ΔT =15 °C and D = 10 mm, ΔT = 20 °C exhibit similar trend and a low temperature is formed at Y = 0.16 m due to bigger jet diameter. The lowest value is formed at this point for D = 10 mm, ΔT = 15 °C. Fig. 4 indicates temperature distribution for different studied parameters as Fig. 3. It is noted that these temperatures distributions are shown for t = 450 s which is approximately steady-state regime. Inlet jets and outlet hole are symbolized with different colored arrows. Here, blue, red and black arrows are shown cold fluid, hot fluid and output hole, respectively. The left and right columns are presented the different ΔT values. Thus, effects of ΔT on temperature variations can be seen at different jet velocities. As indicated in the

Y. Varol et al. / International Communications in Heat and Mass Transfer 39 (2012) 1245–1252

Fig. 4 (a) and (b), hot fluid mostly sits at the right top corner due to hot inlet jet. The fluid impinges onto top corner of passive element (PE) and come back and makes circulation at this point. Thus, hot fluid captured in that area. Due to high jet velocity, cold fluid directly moves under the PE. For lower jet velocity, above the PE more fluid is heated. In Fig. 4 (c) and (d), the hot fluid deviated from its original path after impinging onto PE and in (c) second impingement is occurred onto channel boundary. It is an interesting result that higher temperature has disappeared near the exit for ΔT = 20 °C and D = 10 mm. Fig. 5 (a) and (b) illustrates the mixing index and temperature _ h ¼ 0:05 kg=s; m _ c ¼ 0:05 kg=s, ΔT = 15 °C values with location for m and D = 10 mm jet inlet diameter, respectively. In this case, there is huge difference between ϕ = 0° and other inclination angles as seen from Fig. 5 (a). A stable variation is formed for ϕ= 30°, 60° and 90°. In other words, values of mixing index goes to zero after x = 0.32 m. It means that well thermal mixing is achieved at those inclination angles. In Fig. 5 (b), higher temperature is measured around Y = 0.24 m for ϕ = 0°. At Y = 0.32 m, the minimum temperature value is obtained for ϕ = 30°. The figure also indicated that temperatures at the middle points are higher than that of edge nodes except ϕ= 30°. In the horizontal position of the channel (ϕ= 0°), due to oscillation of flow, temperature values present zig-zag shaped distribution. For other values

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of inclination angle temperatures are oscillated at low frequency. It means that thermal mixing becomes better. For this case isotherms are plotted in Fig. 6. Fig. 6 (a) shows that presentation of PE into the channel divides the channel to two parts as hot and cold of fluid. In the middle of the channel values of temperature equal to arithmetic mean of inlet temperatures. However for other values of inclination angles, hot fluid cumulates upstream region of the channel. At the exit of the channel flow temperatures are almost equal for other inclination angles. As seen from the figures at different inclination angles, heated fluid become lesser with increasing of inclination angle. As we indicated above that the homogeneity in temperature distribution is the most important aim of this work. In this study, effect of ratio of inlet flowrate is investigated to see the efficiency on thermal mixing. With this aim flow rate of hot jet _ h ¼ 0:033 kg=s to is taken as fixed while cold jet changes from m _ c ¼ 0:05 kg=s. For this case variation of mixing index with channel m length is presented in Fig. 7 (a) and temperature variation is plotted in Fig. 7 (b). As seen from Fig. 7 (a), MI is lower and more uniform _ h ¼ 0:05 kg=s while other fluctuating, especially around PE. Also at m when look thermal changes along station 5 at Fig (b), temperature _ h ¼ 0:05 kg=s. These mean is nearly constant along the column at m thermal mixing is better in the channel when flow rate between hot and cold jets is lower.

a 25 .98 26 .42 26 .86 27 .3 27 .74 28 .18 28 .62

o

C

b 26 .04 26 .4 26 .76 27 .12 27 .48 27 .84 28 .2

o

C

c 28 .74 29.32 29 .9 30.48 31 .06 31 .64 32 .22

o

C

_ h ¼ 0:05 kg=s; m _ c ¼ 0:042 kg=s ΔT = 15 °C, ϕ = 0° and D = 5 mm jet inlet b) m _ h ¼ 0:05 kg=s; m _ c ¼ 0:042 kg=s ΔT = 15 °C, ϕ = 30° and D = 5 mm jet Fig. 10. Temperature field, a) m _ h ¼ 0:05 kg=s; m _ c ¼ 0:042 kg=s ΔT = 20 °C, ϕ = 60° and D = 5 mm jet inlet. inlet, c) m

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Y. Varol et al. / International Communications in Heat and Mass Transfer 39 (2012) 1245–1252

Fig. 8 is plotted to show the affect of ratio of inlet flowrate on temperature distribution. As seen from the figures well thermal mixing is _ c ¼ 0:033 kg=s in Fig. 8 (a). For increasing of flow rate performed for m of cold jet hot fluid locates at the top-left corner due to domination of cold jet as seen from Fig. 8 (b) and (c). Finally Fig. 9 (a) and (b) compares the effect of passive element on mixing index and variation temperature of fluid with Y coordinate, respectively. The figures indicate that inserting of passive element (PE) into the channel disturbs the flow and mixing index increases around the PE. Near the exit of the channel same mixing index value is performed for both cases. Also, temperatures values are decreased due to presence of PE. It means that PE can be used as control element for flow and thermal mixing. Fig. 10 (a) to (c) also compares the temperature distributions with (right column) and without (left column) PE. As given in the figures presence of PE changes the temperature distribution inside the channel for all parameters. 6. Conclusions An experimental work has been performed in this study for different parameters such as jet temperatures, jet diameters, temperature difference, inclination angle of the channel and flow rates. The main findings can be listed as • Mixing index decreases from inlet of the fluid to exit part of the channel almost for all cases due to increasing of thermal mixing. • Thermal mixing is the main function of temperature of inlet jets. • Temperature distribution depends on mainly flowrate inside the channel. • Presence of passive element, affects the mixing index. It behaves like a curtain between hot and cold jet inlets. Thus, mixing index decreases around the body. It means that the insertion of the body can be used as control parameter for flow and thermal mixing. • Another effective parameter on thermal mixing is the inclination of the channel. It affected from this parameter, higher thermal mixing efficiency is observed for ϕ = 60°. Acknowledgments Authors thank the Firat University scientific and research fund for their valuable financial support with a project number 1747.

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