CONVECTIVE HEAT TRANSFER ENHANCEMENT IN … · Thus the solution is of the form Fig. 1 (a) ... Convective Heat Transfer Enhancement In Low Reynolds Number of A Circular Pipe With

International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 2, Issue-9, Sept.-2014

Convective Heat Transfer Enhancement In Low Reynolds Number of A Circular Pipe With Full Length Twisted Tape Insert 6

CONVECTIVE HEAT TRANSFER ENHANCEMENT IN LOW REYNOLDS NUMBER OF A CIRCULAR PIPE WITH FULL LENGTH

TWISTED TAPE INSERT

1SUVANJAN BHATTACHARYYA, 2TAMAL ROY, 3NILADRI GHOSH, 4SATYAKI BANDYOPADHYAY, 5SUDIPTA SARKAR, 6PARTHA PANJA

1Assistant Professor, Mechanical Engineering Department, MCKV Institute of Engineering, Liluah, Howrah. West Bengal.

India. 2,3,4,5,6Undergraduate Students, Mechanical Engineering Department, Seacom Engineering College, Sankrail, Howrah. West

Bengal. India. E-mail: [email protected]

Abstract- 3-D numerical simulation is performed for heat transfer and fluid flow characteristics of the tubes fitted with twisted-tape inserts. The principle of heat transfer enhancement in the core flow of tube has been proposed to improve the temperature uniformity and reduce flow resistance. Twisted tape widths are 13mm have been investigate at low Reynolds number for different mass flow rates. The simulation results shows that the average overall heat transfer coefficient in circular plain tubes are enhanced with twisted tape of widths by as much as 90% at a constant tube-side temperature and the friction factor are enhanced. The twisted tape induced swirl flow heat transfer due to exponentially increasing heat inputs with various exponential periods and twisted tape induced pressure drop were systematically measured. Keywords- Twisted-tape, Laminar flow, Heat transfer enhancement, Low Reynolds number. I. INTRODUCTION Laminar flow is encountered in many industrial applications. In case of laminar flow, there is major thermal resistance in the bulk flow in addition to the dominant thermal resistance in the thin boundary layer adjacent to the flow. Twisted-tape inserts are, therefore, used to mix the gross flow effectively in laminar flow to reduce the thermal resistance in the core flow through the channel. Resistance to heat transfer from or to a fluid is mainly due to the slowly moving layers of fluid near the hot or cold wall, so as to enhance the heat transfer these layers should be disturbed or disturbed. In passive enhancement, to create this disturbance we provide some irregularities to the solid surface or wall. Transverse rib is made as integral surface to the duct wall. Farrel et al. Tested one fully-ribbed and two broken ribbed flat radiator tube. Olsson and Sunden tested two ribbed radiator tubes with airflow. The enhanced tubes showed higher friction factors than the smooth tube in both laminar and turbulent regions. Olsson and Sunden investigated the effect of rib configurations for the multiple V-ribbed channel. Initially, only full-length twisted-tape extending all over the length of the duct had been used. Subsequently, many variations of twisted tapes have been used to improve the thermohydraulic performance. Also, twisted tapes in combination with other enhancement techniques have been used. Saha

and Dutta have observed that, for regularly spaced twisted-tape elements, thermohydraulic performance of twisted tapes with multiple twists in the tape module is not much different from that with single twist in the tape module. Twisted tapes with gradually decreasing pitch perform worse than their uniform-pitch counterparts. Patil has worked with varying width twisted-tape inserts for which both friction factor and Nusselt number are lower than those with full-width twisted tapes. Saha et al. and Date and Saha have introduced regularly spaced twisted-tape elements which are better than full-length twisted tapes under certain circumstances. Li et al. Have designed an optimal multi-layer spacer with optimal non-woven nets in the outer layers and twisted tapes in the middle layer. Helical screw-tape inserts behave the same way as the twisted tapes. Twin and triple twisted tapes are also effective enhancement devices. Dewan et al. Have reviewed the studies on twisted tapes. Hong et al. Have employed evenly spaced twisted tapes in a convergent-divergent tube. Jagged twisted tapes perform better than the classical twisted tapes.



Fig. 1 (a) Full length twisted tape and (b) regularly spaced

twisted tubes. II. PHYSICAL MODEL The geometrical configurations of the twisted-tape inserts discussed in the present work are shown in Fig. 2. The length of the tube is 2000 mm and the inner diameter of the tube is 100 mm, which are fixed in this research. The twisted-tape inserts are made by uniformly 1mm thickness and 90 mm width. The twist ratio Y is defined as the ratio of length of one full twist pitch to diameter of the twist, which is fixed in the present study. In this section, tubes fitted with twisted-tape inserts are mainly investigated, and the results are compared with that of the smooth tube with no twisted-tape.

Fig. 2. The full length twisted-tape

III. GOVERNING EQUATIONS AND

BOUNDARY CONDITIONS The present work based on the assumption that the fluid flow and heat transfer processes are laminar and in steady-state, and the heat loss to the environment is neglected. Laminar models are available, and we used that.

The solution for the laminar boundary layer on a flat pipe was obtained by Prandtl's student H. Blasius [15] in 1908. For two dimensional, steady, incompressible flow with zero pressure gradient, the governing equation of motion are;

With boundary conditions, At y=0, u=0, v=0

At y= , u=U,

Blasius reasoned that the velocity profile, u/U, should be similar for all values of x when plotted versus a non dimensional distance from the wall; the boundary-layer thickness, δ, was a natural choice for non dimensionalizing the distance from the wall. Thus the solution is of the form

Where,

Introducing the stream function, Ψ, where

And,

Satisfies the continuity equation above identically; substituting for u and v into the above equations reduces the equation to one in which Ψ is the single dependent variable. Defining a dimensionless stream function as

makes f(η) the dependent variable and η the independent variable. With Ψ defined by above equation and η defined also in the above equations. The velocity component s are given by

And,

By differentiating the velocity components , it also can be shown that



And,

IV. GRID GENERATION AND

NUMERICAL METHOD The grid generations and numerical simulations are performed using commercial Ansys software package Gambit 2.2.30 and Fluent 6.3.26, respectively. Fig. 3 presents the mesh generation of the computation domain using hybrid method of size function and boundary layer technique. Medium grid are employed to generate mesh in the interior of tube and boundary layers, respectively. In all cases, the grid is finer close to the wall and the vicinity of the twisted tapes where the temperature and velocity gradients are large.

Fig. 3. Grid generation of the duct and twisted-tape

V. NUMERICAL SOLUTION OF LAMINAR

HEAT TRANSFER Theoretical equations for laminar model in a circular tube of a100 mm in diameter and a 1000 mm long with the twisted-tape insert were numerically solved for heating of air The twisted-tape of width (w = 90 mm), thickness (T= 10 mm), total length (l = 2000 mm), pitch of 180◦rotation and twist ratio (y = 3) was The unsteady fundamental equations for laminar heat transfer are expressed in the three dimensional coordinate. The twisted-tape 3D geometry-data file

was created by AutoCAD Design Engineering Software Tools. The control volume discretization equations were derived from these fundamental equations by using the hybrid scheme (Patankar, 1980). The thermo-physical properties for each control volume are given as those at each volume-temperature. The procedure for the calculation of the flow field is the SIMPLEST algorithm which stands for semi-implicit method for pressure-linked equations (Patankar and Spalding, 1972). The three-dimensional grid system is established using the commercial code ANSYS ICEM CFD 14.0 (ANSYS, Inc.). The fluid domain is discretized with unstructured tetrahedral elements and in order to get more precise results, the inner regions especially close to the wall of the inserts are meshed into much finer cells due to the existence of narrow regions produced by the edges of the inserts. Mesh adaption is adopted to implement mesh refinement based on the gradients of pressure, velocity, temperature etc. in the whole domain, until the specified number of grid nodes is achieved. RESULTS AND DISCUSSION The variations of Nusselt number with duct axis for smooth duct and with twisted-tape are shown in Fig. 8. It is clear that the smooth tube without twisted tape inserts has the lower Nusselt number range than the twisted-tape with the duct. The residuals is also converge are shown in fig. 4. The streamline contours in the computational domain of the tubes at different Re are presented in Fig. 10. It is clearly seen that the twisted tape induces swirling flow; the streamline contour in the tubes they only have uniformly right twist inserts. However, there are significant changes in the streamline contour of the tube without twisted tape. The tube without twisted tape has the same streamline contour with the tube with the twisted tape before the upstream of the twist, since they have the same right twists, but the change occurs in the middle region of the tube due to the left twist.

Fig. 4. Converge residuals



Fig. 5. streamline contours of the Twisted –tape

Fig. 6. Temperature variation inside the duct

The variations of temperature with duct axis for along with twisted tape shown in Fig. 6and Fig. 7. It is clear that the temperature is going on increasing along with the duct axis and it is expected.

Fig. 7. Temperature profile along duct axis

The variations of pressure with duct axis for along with twisted tape shown in Fig. 9. It is clear that the pressure is going on decrease along with the duct axis and it is expected. The more the pressure drop, more the mixing on fluid will occur.

Fig. 8. Variation of Nusselt Number along duct axis

Fig. 9. Pressure drop along duct axis

CONCLUSION In this investigation, a numerical prediction has been conducted to study the heat transfer and flow characteristics of varying geometric parameters in laminar flow at low Reynolds number. Heat transfer enhancement characteristics of a tube fitted with twisted- is numerically studied and analyzed minimal entropy generation principle in the present paper. As the Reynolds number increases, the swirling disturbance generated by twisted-tape intensifies, thus causing higher temperature and velocity gradient near the tube wall. Therefore both the heat transfer coefficient and the friction factor increase. The contour plots of predicted velocity intensity and streamlines are presented for further understanding of the mechanism of compound heat transfer enhancement. It reveals that the mainly promote laminar intensity and heat transfer in the near wall zone. And it is also expected that the pressure drop will decrease along the duct axis. REFERENCES

[1] Ashis K. Mazumder, Sujoy K. Saha., Enhancement of

thermo-hydraulic performance of turbulent flow in rectangular and square ribbed ducts with twisted-tape inserts., ASME Journal of Heat Transfer., Vol.130, 081702, 2008.

[2] Jung-Yang San, Wen-Chieh Huang., Heat transfer enhancement of transverse ribs in circular tubes with consideration of entrance effect., International Journal of Heat and Mass Transfer., Vol.49, 2965–2971, 2006.


Convective Heat Transfer Enhancement In Low Reynolds Number of A Circular Pipe With Full Length Twisted Tape Insert

10

[3] Robert Kiml, Adrian Magda, Sadanari Mochizuki, Akira Murata., Rib-induced secondary flow effects on local circumferential heat transfer distribution inside a circular rib-roughened tube., International Journal of Heat and Mass Transfer., Vol.47, 1403–1412, 2004.

[4] S.K. Saha, A. Dutta, S.K. Dhal., Friction and heat transfer characteristics of laminar swirl flow through a circular tube fitted with regularly spaced twisted tape elements., International Journal of Heat and Mass Transfer., Vol.44, 4211-4223, 2001.

[5] S. Eiamsa-ard, K. Wongcharee , S. Sripattanapipat., 3-D Numerical simulation of swirling flow and convective heat transfer in a circular tube induced by means of loose-fit twisted tapes., International Communications in Heat and Mass Transfer., Vol.36, 947–955, 2009.

[6] S. Eiamsa-ard, C. Thianpong, P. Eiamsa-ard., Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes., Experimental Thermal and Fluid Science., Vol.34, 53–62, 2010.

[7] S. Eiamsa-ard, C. Thianpong, P. Eiamsa-ard, P. Promvonge., Convective heat transfer in a circular tube with short-length twisted tape insert., International Communications in Heat and Mass Transfer., Vol.36, 365–371, 2009.

[8] S. Eiamsa-ard, C. Thianpong, P. Promvonge., Experimental investigation of heat transfer and flow friction in a circular tube

fitted with regularly spaced twisted tape elements., International Communications in Heat and Mass Transfer., Vol.33, 1225–1233, 2006.

[9] S. Eiamsa-ard, P. Seemawute, K. Wongcharee., Influences of peripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in laminar and turbulent tube flows., Experimental Thermal and Fluid Science., 2010.

[10] S. Eiamsa-ard, P. Promvonge., Performance assessment in a heat exchanger tube with alternate clockwise and counter-clockwise twisted-tape inserts., International Journal of Heat and Mass Transfer., Vol.53, 1364–1372, 2010.

[11] S. Eiamsa-ard, K. Wongcharee, P. Eiamsa-ard, C. Thianpong., Heat transfer enhancement in a tube using delta-winglet twisted tape inserts., Applied Thermal Engineering, Vol.30, 310–318, 2010.

[12] P. Promvonge, S. Eiamsa-ard., Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert., International Communications in Heat and Mass Transfer., Vol.34, 849–859, 2007.

[13] Shyy Woei Chang, Yih Jena Jan, Jin Shuen Liou., Turbulent heat transfer and pressure drop in tube fitted with serrated twisted tube., International Journal of Thermal Sciences., Vol.46, 506-518, 2007.

Documents

CONVECTIVE HEAT TRANSFER ENHANCEMENT IN … · Thus the solution is of the form Fig. 1 (a) ... Convective Heat Transfer Enhancement In Low Reynolds Number of A Circular Pipe With