Review on Thermal Performance of cross Flow Heat Exchanger Using Non-Circular Shape of Tubes

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International Journal on Theoretical and Applied Research in Mechanical Engineering (IJTARME)

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Review on Thermal Performance of cross Flow Heat Exchanger Using Non-Circular Shape of Tubes

1Snehal A. Powar, ²Ashish R. Wankhade, ³Neelam Gohel

Email: ¹[email protected], ²[email protected], ³[email protected] Abstract: The shape of tubes affects the thermal and

hydraulic performance of heat exchanger. Circular tubes are widely used in heat exchanger, circular shape causes severe separation of boundary layer and large wake region formation due to which produce high pressure drop observed in the shell side fluid. This study provides a literature review on different shape of tubes such as oval, cam and wing shape tubes used in cross flow heat exchanger. The friction factor for oval shape tubes are less compare to circular tubes, which helps to decrease the wake region, pressure drop and hence required less pumping power. Cam, wing shape tubes study shows increase in heat transfer rate as the angle of attack increases.

Keywords:Cam shape of tube, drag coefficient, friction factor,Nusselt number,Oval shape of tube, pressure drop, Wing shape of tube

I. INTRODUCTION

Heat exchangers have wide range of applications in the area of thermal power plant, automobile, waste heat recovery system, air conditioning and refrigeration system. The design of heat exchanger which should give high thermal performance and economical is preferable.

The design of cross flow heat exchanger using non circular shape tube provides high heat transfer rate, compact design, require less pumping power hence reduces the cost.

To manufacture the cross flow heat exchanger different shape of tubes are used. Circular tubes are widely used due to ease of manufacturing process, it causes severe separation flow and large wake region produces with high pressure drop.

To overcome the problem of flow separation and wake region one of the method is changing the shape of tube.

In recent years, lot of study is going on the change in shape of tubes. Study is carried out on different shapes such as oval, flat, elliptical, cam, wing, hexagonal.

A. Nouri-Borujerdi [1, 2], studied the heat transfer characteristics and pressure distribution of an isothermal cam shaped tube in cross flow. The angle of attack is varied in the range of 0°< α <180° with Reynolds number 1.5×10⁴<Reeq< 2.7×10⁴.The result show that the heat transfer from a cam shaped tube is maximum at α = 90° and minimum at α =30°. Cam shape tube give larger

value of St/Cd except at α = 90° and 120° relative to the circular tube.

Ala Hasan[3] studied the thermal-hydraulic performance of oval tubes in a cross-flow of air. It is seen from result that Nusselt number NuD for oval tubes are close to that for circular tube for Re_D< 4000 and for higher Reynolds number, NuD is lower than that of circular tube and it decreases with increase in the axis ratio R.

The drag coefficient Cd is also measured; it shows better combined thermal-hydraulic performance of oval tubes compare to circular tube.

AlaHasan [3] performs the investigation on circular and oval tube in an evaporative cooled heat exchanger under similar operating condition. Result found that average mass transfer Colburn factor (jm) for oval tube is 89%

that of circular tube and the average friction factor (f) for oval tube is 46% that of circular tube. This shows that oval tube have better combined thermal-hydraulic performance than circular tube.

Mesbah G. Khan [5],carried out the experiment for characterization of cross-flow cooling of air via an in- line elliptical tube array. For experimental analysis, range of Reynolds number for air side is 1×10⁴< Rea<

3.3×10⁴ and for water side 1×10³< 3.7 ×10³ is considered. Result shows that Nusselt number and hence the heat transfer rate increases with increase in Reynolds number in a power law fashion. Nu-Re correlation is found as Nua = 0.26Rea0.66

Sayed Ahmed E[8] study the cross flow air-cooling process via water-cooled wing-shaped tubes in staggered arrangement at different angles of attack. In this study both experimental and CFD analysis is carried out for different angle of attack. For water Rew = 500 and for air Rea=1800 to 9700.Results were obtained that wing shape tube bundle in heat exchanger gives best result for heat transfer coefficient, effectiveness, efficiency at zero angle of attack and as the value of Rea increases,Nua

increases whereasSta decreases. The highest value and lowest value of NuaandSta occurred at θ= 45ô, 135ô, 225ô, 315ô and at θ= 0˚, 180˚ respectively. From experimental results they develop new correlation for Nua and Sta number in terms of Rea , Pr and angle of attack and compare with previous work which shows thatNua increased by 24% compare to circular tube

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whereas by 76% compare to elliptic tube at zero angle of attack.

The objective of this paper is to present a review on the work done on different shape of tubes in cross flow heat exchanger. Tubes used for studies are oval, cam and wing shape tubes.

II. CROSS FLOW ACROSS THE CIRCULAR CYLINDER

Analysis of any heat exchanger both internal and external flow should be considered, the internal flow through the tubes and external flow over the tubes.When a fluid flow over a stationary solid body, body experiences two types of forces, drag and lift.The drag force is the net force exerted by a fluid on a body in the direction of flow due to combined effects of wall shear and pressure forces. The components of the pressure and wall shear forces in the normal direction to flow tend to move the body in that direction is called lift force.

The drag force FD depends on the density 𝞺 of the fluid, the upstream velocity V and size, shape and orientation of the body. The drag characteristics of a body is represented by the dimensionless drag coefficient CD, defined as

CD = FD 1

2ρAV² (1) Where A is the frontal area, the area projected on a plane normal to the direction of flow. The drag coefficient is primarily a function of the shape of tube, Reynolds number and surface roughness. The total drag coefficient is given by

CD = CD,friction + CD,pressure (2)

The wall shear stress is produces due to friction between fluid and tube wall, this is also called as skin friction drag, CD, friction. The friction drag is proportional to the surface area. Therefore larger surface area experiences a larger friction drag.

The pressure drag is also called as form drag because it is strongly depend on the form or shape of the body, CD. The pressure drag is proportional to the frontal area and the difference between the pressure acting on the front and back of the immersed body.

When a flow is separated from the body, it creates a separator region due to low pressure behind the body, where recirculation and backflow of fluid occurs. As the separated region increases, pressure drag is increases.

The separated region ends when the two separated flow streams reattach. The region of flow trailing the body where the effects of the body on velocity are felt is called wake as shown in fig (1). The wake region keeps on growing behind the body until the fluid in the wake region regain its velocity.

Figure1. Boundary Layer Separation and Wake Region [6]

Figure 2. Variation of Nuθalong the circumference of a circular cylinder [6]

The phenomena, affect the drag force also affect the heat transfer and this effect comes into Nusselt number. The variation in local Nusselt number Nuθ around the periphery of a cylinder is shown in fig.(2) . For all values of Nuθ is high at θ = 0˚ and it decreases as the value of θ increase, it is minimum at θ = 80˚, which is separation point in laminar flow. At θ = 90˚ sudden increase in value of Nuθ because of transition from laminar to turbulent flow. Because of thickening of the boundary layer it reaches to second minimum at θ

=140˚, which is separation point in turbulent flow. [6]

As the formation of wake region is large and also boundary layer separates in circular tube, so to reduce the formation of wake region and separation of flow, changing the shape of tube is one method. As we changes the shape of tube from circular to oval, elliptical, cam or wing shape, it help to reduce formation of wake region and separation of boundary layer. It helps to decrease the drag force on the tube.

III. OVAL SHAPE TUBES IN CROSS FLOW HEAT EXCHANGER

Study the thermal-hydraulic performance of oval shape tube compare to circular tube, experimentation is carried

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out in two evaporative cooled heat exchangers, in which one is having ovalshape of tube and other is having circular shape of tube under similar operating conditions.The heat exchanger cross-sectional area is88 × 130 × 250 mm³. The oval shaped tubes are manufacture from circular copper tube. The circular tube of 18 mm OD is heated then press and formed oval shape of tube having major axis of 25.3mm and minor axis of 8.2mm.

In an evaporative cooled heat exchanger, a thin layer of water is formed on the surface of tube by spraying water. The heat is transferred from hot water flowing inside the tube, to a thin layer of water on the surface of tube then to cold airpass over the tubes. Heat transfer from a thin layer of water to cooled air contains both latent and sensible heat. Latent heat is formed due to evaporation of sprayed water whereas sensible heat is formed due to temperature difference between sprayed water and cooled air. [3]

Figure 3. Distribution of Circular and Oval Tubes In Test Section [3]

Thermal Performance of Circular and Oval Shape Tubes

To analyzed the thermal performance of tubes heat flux is selected as thermal performance parameter θ which is defined as heat transfer per surface area given as follows:

Figure 4. Thermal Performance Parameter θ vs. Air Velocity[3]

θ= th1− th2

(th1− twb 1)D (3)

It is concluded from the graphFig (4), that θ is lower for oval tubes, on average it is 79% of that of circular tube, because circular tubes have large frontal area and the higher turbulence is induced on its backside which helps to increase the rate of heat and mass transfer.

Combined Thermal-Hydraulic Performance

Pressure drop in air flow over the oval and circular tube bank is measured to calculate friction factor. The friction factor is calculated as

𝑓 =_0.5𝜌𝑣^∆𝑝

𝑥2 (4)

Where∆𝑝 is pressure drop across one tube row.The mass transfer Colburn factor 𝑗_𝑚 is calculated using equation (5)whereSh is Sherwood number, convective mass transfer coefficient given by 𝑆𝑕 = 𝑕_𝑚𝑙 𝐷 and Sc is Schmidt number, given by𝜗 𝐷.

𝑗𝑚 = 𝑆𝑕 𝑅𝑒𝑆𝑐^1/3 (5) Fig (5) show the graph of f vs. Re, it show that the friction factor (f) for oval shape tubes have lower value compare circular shape of tubes. Lower friction factor requires less energy for air movement across the tube to achieve required heat transfer rate and as the friction factor f, for oval tube is 46 % that of circular tube, the use of oval tubes save the energy.In similar fig (5) graph of 𝑗𝑚 vs Re and 𝑗𝑚/𝑓 vs Re as shown to analyzed the combined thermal-hydraulic performance of oval tubes compare to circular tube. The graph show that the value of mass transfer Colburn factor 𝑗𝑚 for oval tube is 89%

that of circular tube and the ratio of 𝑗𝑚/𝑓 for oval tube is 1.93-1.96 times that of circular tube.

Figure 5. Mass Transfer Colburn Factor jm and Friction Factor f For Circular and Oval Tubes[3]

IV. CAM SHAPE OF TUBE IN CROSS FLOW HEAT EXCHANGER

The experiments are conducted on cam shape of tube to study the pressure distribution and drag coefficient over the surface of tube. The tubes are made up of copper

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plate having thickness of 0.3 mm and a length of 120 mm. The three tubes have same diameters D = 22 mm and d = 12 mm with different distance between the centers, l = 11 mm, 29 mm, 66mm. To measure the pressure distribution over the tube surface, 20 holes are drilled with 18° of intervals at the centre. The angle of attack is measured between flow direction and the axis of tube in clockwise direction. [2]

Figure 6. Schematic of Cam Shaped Tube and Thermocouple Location In Cross Flow [2]

The pressure coefficient Cp is calculated as:

𝐶𝑝 =𝑃𝑖− 𝑃_∞

1

2𝜌𝑈_∞² (6) The pressure drag coefficient CD is defined as:

𝐶𝐷 = 𝐶𝑝,𝑖𝑐𝑜𝑠 𝜓𝑖∆𝑆𝑖/𝐷𝑒𝑞 20

1

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Where, P – Pressure,U_∞ - Velocity, 𝜓_𝑖- hole angle, 𝑆_𝑖- streamline coordinate

From experimental data graphs are plotted, Fig. (7) show the pressure distribution over the surface of cam shape tube at 𝛼 = 0°, U = 15m/s for different Reeq and corresponding l/Deq. Positive and negative values from X-axis indicates (S/Deq) the measured distance along upper and lower part of the cylinder respectively. For l/Deq = 0.4, 0.8, 1.1, flow separates at 𝑆/𝐷𝑒𝑞 =

±0.75, ±0.85, ±0.5and reattach at𝑆/𝐷𝑒𝑞 = 1.3, 0.8 . This pressure distribution decreases the drag produce around the surface of tube.

Figure 7. Pressure Coefficient For Three Different l/Deq

and Reynolds Number [2]

Figure 8.Pressure drag coefficient of a cam shaped and circular tubes vs. angle of attack [2]

As shown in Fig. (8) at different values of Reynolds number, curves are repeated after every 150 degree.

Maximum value of CD is at 𝛼 = 90° 𝑎𝑛𝑑 270° and minimum value of CD is appear at 𝛼 = 30°, 180°, 330°.

At𝛼 = 180°, the laminar boundary layer is formed over the most part of the tube surface and separation is delayed which help to reduce the extent of the wake region and the magnitude of the form drag.

Figure 9. The average Nusselt number ratio of the cam- shaped tube to an equivalent circular tube [2]

Fig. (9)show The maximum value of Nucam/Nucir>1 at α

= 90˚ and 270˚, corresponding values are 1.05 and 1.8 for various Reynolds number. The Minimum value of Nucam/Nucir≤ 1 at α = 180˚ is 0.87, this is because large wake region form below critical Reynolds number over a large region of cam shape tube.

V. WING SHAPE TUBES IN CROSS FLOW HEAT EXCHANGER

The experiments were conducted in open wind tunnel.

The test section of size 305 × 305 × 780 𝑚𝑚³ is placed in the middle of the wind tunnel. The wing- shaped tube dimensions are shown in fig.(10). Tubes are fixed in such way that angle of attack can be adjusted by turning the tubes. The air was passed over the tubes bundle and water was flowing inside the tubes. Air was heated with the help of 4kW electric heater up totemperature of56.5 ± 1.5℃DBT and 26 ± 1.5℃

WBT. A 134A refrigeration system used to maintain inlet temperatureof water at 10.8 ± 1.5℃. T-type of

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thermocouples was used to measure the temperature and air side pressure drop measured by an electric micro- manometer. [8]

Figure 10. Dimensions of wing shape tube in mm [8]

Figure 11. Schematic Drawing of Experimental Setup [8]

Table 1. Drawing Legend Sr.

No.

Description Sr.

No.

Description

1 Electrical Heater 9 Water Control valves

2 Honey Comp 10 Water pump

3 Pitot-Static Tube 11 Electrical micro- manometer 4 Pressure Taps 12 Water tank 5 Test Section 13 Evaporator

6 Gate 14 1.5 hp Compressor

7 Fan 15 Expansion valve

8 Electrical motor

Experimental data was collected after 120-160 min, the steady state is reached. The water flow rate was measured at the beginning of test run. The mean air velocity was calculated by eq.(8). For analysis, Reynolds number of water (Rew =500) kept constant and air side Reynolds number is varied from Rea=1800 - 9700.

𝑉𝑎𝑖 = 2𝑔(𝜌_𝑤

𝜌𝑎 𝑓)∆𝑕𝑑𝑦𝑛 (8) Heat transfer on water side and air side are calculated by equation (9) and (10) respectively.

𝑄𝑤 = 𝑚𝑤× 𝐶𝑝𝑤× 𝑇𝑤𝑒− 𝑇𝑤𝑖 = 𝑚𝑤𝐶𝑝𝑤∆𝑇𝑤 (9) 𝑄𝑎 = 𝑚𝑎× 𝐶𝑎𝑓× 𝑇𝑎𝑖− 𝑇𝑎𝑒 = 𝑚_𝑎𝐶𝑎𝑓∆𝑇𝑎 (10) The average waterside and airside heat transfer Q was taken for analysis as,

𝑄 =𝑄_𝑎+ 𝑄_𝑤

2 (11) Heat transfer through condensation and radiation is neglected. Therefore, the overall heat transfer rate 𝑄 = 𝑄𝑐𝑜𝑛𝑣𝑒𝑐𝑡𝑖 o𝑛= 𝑕𝑎× 𝐴𝑠𝑜 × ∆𝑇𝑙𝑛 (12) Where, 𝐴𝑠𝑜 is the total outer surface area for the tubes and ∆𝑇𝑙𝑛 is the logarithmic mean temperature difference calculated as follows:

∆𝑇𝑙𝑛 = 𝑇_𝑎𝑖 − 𝑇_𝑎𝑒 𝑙𝑛^(𝑇_(𝑇^𝑎𝑖^−𝑇^𝑠⁾

𝑎𝑒−𝑇_𝑠)

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Air side average heat transfer coefficient is calculated as:

𝑕_𝑎 = 𝑄

𝐴_𝑠𝑜∆𝑇_𝑙𝑛 (14) The Reynolds Number 𝑅𝑒𝑎 is given by:

𝑅𝑒𝑎 =𝜌𝑎𝑓𝑉𝑎𝑖𝐷𝑒𝑞

𝜇𝑎𝑓 (15) The Nusselt Number 𝑁𝑢_𝑎 is given by:

𝑁𝑢𝑎 =𝑕_𝑎𝐷_𝑒𝑞

𝑘𝑎𝑓 (16)

Where,𝐷𝑒𝑞is outer equivalent diameter of the tube and 𝑘𝑎𝑓 is the air thermal conductivity, W/mK.

As shown in fig (12), Rea plotted on X-axis and Nua on Y-axis for different angle of attack. Graph show that Nusselt number increases as Reynolds number and angle of attack increases. Increase in Nusselt number helps increase in the turbulent intensity and also to enhance the convective heat transfer.

Figure 12.Nua vs. Rea For Different Angle of Attack [8]

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Figure 13.Nua vs. angle of attack (θ1, 2, 3) at different Rea

[8]

Fig.13 show that, Nua versus angle of attack (θ1,2,3) at different Reynolds number are plotted. The highest values of Nua are occurred at θ1,2,3=45˚,135˚,225˚,315˚

whereas lowest values of Nua are occurred at θ1,2,3=0˚,180˚. Based on experimental results, correlation between Nua, Rea, Pr and at different angle of attack is obtained as follows:

𝑁𝑢𝑎= 𝑎. 𝑅𝑒𝑎𝑏 . 𝑃𝑟¹³. 1 + 𝜃

𝜃90 (17) Where a, b, c are constant, mention in table, this correlation is applicable for1800 ≤ 𝑅𝑒_𝑎≤ 9700.

Table.2. Constants for Proposed Correlation for Nusselt Number [8]

Angle A B C

0°≤ 𝜃_1,2,3 ≤ 45° 0.52 0.567 0.272 135°≤ 𝜃1,2,3≤ 180° 1.1323 0.5631 -0.6737 180°≤ 𝜃_1,2,3≤ 225° 0.2334 0.5732 0.6829 315°≤ 𝜃1,2,3≤ 360° 2.327 0.5741 -0.9691

Figure14. Comparison of Present Nuavs Rea Results with Different Tube Shapes at0° Angle of Attack [8]

The result of new correlation between Nua- Rea at different angle of attack, obtained from wing shaped tube is compared with other shape of tubes such as circular and elliptical at zero angle of attack fig.(14).

The study shows that Nua increases, by 24% compare to circular tubes and by 76% compared to elliptical tubes bundle.

SUMMARY

The thermal –hydraulic performance of oval shape tube shows better performance than circular tube. Friction factor for oval shape tube is decreases compare to circular tube which helps to reduce wake region and hence it reduces the drag force, pressure drop. Study of cam shape tube show the reduction in pressure drag coefficient and increase in heat transfer coefficient. In case of wing shape tube, Nusselt number for air side increases with increase in Reynolds number and angle of attack. The heat transfer enhancement and better thermal-hydraulic performance is achieved by changing the shape of tube. It also helps to reduces large wake region and separation of flow, which reduces the drag force. The heat exchanger with different shape of tube requires less pumping power, which helps to save energy. The size of heat exchanger is reduced which gives the compact and less costly heat exchanger.

NOMENCLATURE

Alphabet –Upper Case CD Pressure drag coefficient Cp Pressure coefficient

Deq Equivalent circular diameter, m Q Heat transfer rate, W

S Streamline coordinate U Velocity, m/s

Greek Letters

α angle of attack, thermal diffusivity θ thermal performance parameter 𝞺density

𝞵dynamicviscocity 𝞓difference

Dimensionless number f Friction factor,

jmmass transfer Colburn factor Nu Nusselt number

Re Reynolds number Sc Schmidt number Sh Sherwood number Subscripts

a Air eq equivalent i Inlet

k thermal conductivity m mass flow rate o Outlet

w water

∞freestream

REFERENCES

[1]. A.Nouri-Borujerdi ,A.M. Lavasani,

"Experimental study of forced convection heat transfer from a cam shaped tubes in cross

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flows”, International Journal of Heat and Mass Transfer, 50 (2007) 2605-2611

[2]. A.Nouri-Borujerdi, A.M. Lavasani, “Pressure loss and heat transfer characterization of a cam shaped cylinder at different orientation”, Journal of Heat Transfer, Vol. 130/124503-1 (Dec. 2008).

[3]. AlaHasan, Kai Siren, “Performance investigation of plain circular and oval tube evaporatively cooled heat exchanger”, Applied Thermal Engineering 24 (2004) 777-790 [4]. G. P. Merker, H. Hanke, “Heat transfer and

pressure drop on the shell-side of tubes-banks having oval-shaped tubes”, Int. J. Heat Transfer, Vol. 29, No. 12,pp 1903-1986 [5]. Mesbah G. Khan, Amir Fartaj, David S.-K.

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[6]. S. Tiwari, D. Maurya , G. Biswas, V. Eswaran,

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[7]. S. A. Nada, H. El-Batsh, M. Moawed, “Heat transfer and fluid flow around semi-circular tube in cross flow at different orientations”, Heat Mass Transfer (2007) 43:1157-1169 [8] Sayed Ahmed E. Sayed Ahmed, Emad Z.

Ibrahiem, Osama M. Mesalhy, Mohamed A.

Abdelatief, “Study of cross flow air-cooling process via water-cooled wing-shaped tubes in staggered arrangement at different angles of attack, Part 2: heat transfer characteristic and thermal performance criteria”, World academy of science , Engineering and Technology, Vol:7 2013-06-25

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