Experimental investigation of heat transfer performance of double pipe u-bend heat exchanger using full length twisted tape.

Yadav, Anil Singh

Introduction

Heat transfer process occurs in a wide variety of engineering situation. But the heat transfer coefficients are generally low. Hence in some cases a heat transfer augmentation scheme in necessary. A detailed survey of various techniques to augment convective heat transfer is given by Bergles [11]. Twisted tape techniques have been used to augment heat transfer in double pipe heat exchanger. As per the studies of Hong and Bergles [3], Saha and Chakrabarty [5], Saha Dutta [6] etc. Bhatia, Kumar and Sud [1] carried out study of heat transfer to heated air in a pipe heated by the condensing steam outside it. They found that in case of Twisted tape the maximum increase in heat transfer was approximately 70% at a pitch to diameter ratio of 1.0 and in case of coiled wire the maximum increase in heat transfer was approximately 30% at a pitch to diameter ratio of 1.0.

Kumar and Sud [2] found that coiled type of swirl generator causes the increase of 72% in heat transfer but frictional power also increase 90%. For water flowing through a vertical stainless steel tube, pitch to diameter ratio of 1.0 to 5.5 was used. Hong and Bergles [3] found that in the case of high Prandtl Number fluid in laminar flow the heat transfer rate increases considerably for a moderate increase in pressure drop. Saha and Date [4] have considered twisted tape element connected by thin circular rods. They observed that the pressure drop associated with the full-length twisted tape could be reduced without impairing the heat transfer augmentation rates in certain situation. Saha and Chakraborty [5] investigated the use of turbulences promoters with water. Instead of single twist in the tape module multiple twist tape considered. They observed that there is drastic reduction in pressure drop, which is in excess of the reduction in heat transfer.

Saha and Dutta [6] investigated the use of turbulences promoters with short length Twist tape and regularly spaced Twist tape element. They achieved better thermodynamics performance with short length Twist tape and regularly spaced Twist tape element instead of full-length Twist tape while working with Twisted tape. Saha and Langille [7] have considered different types of strips. The strips are of longitudinal rectangular, square and crossed cross-section and full length and short length as well as regularly spaced types. They observed that the short length strips perform better than the full-length strips. Friction factor reduces by 8-58% and Nusselt no reduces by 2-40% for short length strips. For regularly spaced strips elements, Friction factor increase by 1-35% and Nusselt no increase by 15-75%.

Smith and Meyer [8] compare three different heat transfer enhancement method namely: Microfins, Twist tape and High fins to that of smooth tube. They observed that the heat transfer coefficient are increased by approximately 46%, 87% and 113% on average compared to those of smooth tube, using respectively Twist tape, High fins and Micro fins. They also observed that on average the pressure drop of Micro fins tube is 38% higher than those of smooth tube. High fins tube increase the pressure drop by 81% in comparison to smooth tube and Twist tape increase the pressure drop by 148%.

Heat transfer and pressure drop characteristics of laminar water flow through a circular tube with longitudinal inserts were experimentally studied by Hsieh and Huang [9]. Testing was performed on tubes with square and rectangular as well as crossed longitudinal strip inserts with Aspect ratio AR=1 and 4. Solanki, Prakash and Gupta [10] conducted experimental and theoretical studies of laminar forced convection in tubes with polygon inner cores.

[FIGURE 1 OMITTED]

It is well known that the twisted tape increase the Pressure drop considerably and hence the pumping power as compared with flow in smooth tube. Hence it become necessary to take up this study on half length tape would involve supposedly lesser pressure drop and pumping power as compared with full length twisted tape. A twisted tape in a tube is shown in Fig. 1 & 2.

[FIGURE 2 OMITTED]

Hence the following aims were sets:

To obtain carefully controlled experimental data on heat transfer and pressure drop for flow of hot/cold fluid in a

a. Plain heat exchanger.

b. Heat exchanger with full length twisted tape.

To carry out performance evaluation of the above heat exchanger on the basis of equal mass flow rates and unit pressure drop.

Experimental setup

A schematic layout of the test loop is shown in Fig. 3. The set-up consisted of:

1. An oil tank with heater of 0.64[m.sup.3] capacity placed on floor. 2. An overhead water tank 0.5[m.sup.3] capacity located at an elevation of 2.75 meters. 3. Double pipe u bend heat exchanger. 4. Measuring devices like Rotameter, temperature indicator, and pressure gauge. 5. Twisted tape. 6. Gear pump.

[FIGURE 3 OMITTED]

The oil tank is placed on the floor and is provided with heating coil of variable input. The tank dimensions are 0.8m x 0.8m x 1m. The tank is provided with PVC tube of 1.85m long and 5cm diameter, which is connected to 1.5HP motor. The motor outlet is connected to the inlet of heat exchanger through pipe to circulate hot oil in ckt. This pipe is connected to inner tube of heat exchanger through flange coupling. This pipe is provided with different measuring devices like rotameter, temperature indicator and pressure gauge.

An overhead tank is a Sintex tank of 0.5[m.sup.3] located at a height of 2.75m from the floor. The flow rate of water is kept constant at the rate of 15 Lit/min. Test section is double pipe heat exchanger of u bend type as shown in Figure 3. The Heat Exchanger consists of 4m lengths in each arm and 0.465m length of u-bend section. The heat exchanger is made-up of stainless steel tubes. The inner diameter of inner tube is 2.11cm and outer diameter of inner tube is 2.5cm. Inner diameter of annulus pipe is 5cm. The two straight legs of inner tube are connected to U-bend section with the help of flange coupling. The test section was heavily insulated by asbestos rope insulation.

Rotameter is used to measure the flow rate of oil in the inner tube. Rotameter is connected at inlet to inner pipe of heat exchanger. The range of rotameter is 0-50 Lit/minute. Two Burdon pressure gauges are used at inlet section and another at outlet section of hot oil. The range of Pressure gauge is 0-5 kg/[cm.sup.2]. The difference in reading of inlet and outlet pressure gauge gives the pressure drop in heat exchanger. Four Digital Thermometers are used at inlet and outlet section of each hot and cold fluid.

In all experiments twisted tapes were made out of 0.8mm thick stainless steel strip. The width of which was 1mm less than inside diameter of test section. The strip were at first pushed into a tube and then one end of the strip was tightened in a vice keeping tube in perpendicular position and other end was twisted by long. Full length twisted tape were manufactured in the Amsler torsion testing machine to the desired twist ratio and were later on inserted in the test section.

Experimental procedure and Data reduction

First the plain tube double pipe heat exchanger (i.e. without turbulator) was tested. At the beginning of series of tests, the hot oil was circulated through inner tube and cooling water through annulus tube in counter-flow configuration. The air was bled at various locations. The flow rate of water was fixed to 15 Lit. /min. The cooling water coming in heat exchanger is at room temperature. First the oil flow rate was fixed to 2 Lit/min. A prescribed heat input was given to the oil in oil tank and sufficient state. Usually 1/2 hour was required for the attainment of steady state for a run. Once the steady state was reached the flow rates of hot and cold fluid, temperature reading at inlet and outlet section of hot and cold fluid and burdon pressure gauge readings were taken. The flow rate of cold water was kept constant and above procedure was repeated for different flow rates of hot fluid.

After completing the test with plain heat exchanger (i.e. without turbulator), the u bend double pipe heat exchanger was removed from loop. Then full-length twisted tape was inserted into the both straight legs (4m each) of the u-tube. The tape was inserted from one side and pulled from other end by thread or thin wire. Then the heat exchanger was connected in loop and takes various readings. Transformer oil was circulated inside tube and cold water through annulus in counter flow arrangement.

Experiments were conducted over the following range of various parameters:

The flow rate of oil                       ([M.sub.H]) = 4, 8,
                                             12,18,24,30 (all Lit/min)
The flow rate of water                     ([M.sub.C]) = 15 Lit/min.
                                             (Constant)
ID of inner tube                           di = 0.0211 m
OD of inner tube                           do = 0.025 m
ID of outer tube                           Di = 0.05 m

Test length of heat exchanger:
For heat transfer                          = 8m
For pressure drop                          = 8.46m
Heat exchanger area                        [A.sub.0] = 0.628[m.sup.2]
The water temperature at inlet             = 25[degrees]C (ambient
                                             temperature)
Twist ratio for full length twisted tape   = half pitch/Tube inside
                                             diameter= 7
Thickness of Twisted tape                  = 0.8mm
Length of Twisted tape                     = 4m (2-piece)

Heat input was determined from the enthalpy rise of the fluid. A linear variation in the bulk temperature was assumed over the test length. The tube wall inside temperature was calculated by one dimensional conduction equation. The average wall temperature and the bulk mean temperature were combined with heat flux to give the Nusselt No. all the fluid properties were evaluated at the mean film temperature. Pressure drop data were obtained under isothermal condition and the fanning fraction factor was calculated.

Result and discussion

After having studied the heat transfer and pressure characteristic, it becomes necessary to combine these to evaluate the performance of full-length tapes. For this purpose, their performance was studied for each heat flux separately for equal mass flow rates, and unit pressure drop.

Equal mass flow rate basis

Fig.4 shows the performance evaluation for the full-length tapes on equal mass flow rates basis. This is a simple criterion for performance evaluation.

[FIGURE 4 OMITTED]

Fig.4 shows that the average heat transfer coefficient inside tube increases with increase in the flow rate of fluid in each case. On comparing the different curves it has been observed that heat transfer performance of full-length twisted tape is maximum followed by smooth tube. The heat transfer coefficient is increased by approximately 60% on average compared to those of smooth tubes using full-length twisted tape.

The increase in heat transfer coefficient from smooth tube to twisted tape can be well understood by boundary layer phenomenon. In smooth tubes the flow is stream lined flow.

Due to slip condition the fluid in contact with tube (wetted perimeters) flow at very slow speed than inner core of tube. Due to this boundary layer thickness is high and heat transfer is retarded. The boundary layer thickness may be reduced by fitting turbulators to heat transfer surfaces. These twisted tape tabulators interrupt the fluid flow so that a thick boundary layer cannot form.

Unit pressure drop basis

Unit pressure drop basis is an important criterion in heat exchange equipment design. An augmentative technique, which is effective from the heat transfer point of view, may fail in case it results in a pressure drop penalty greater than what the equipment can handle.

The increase in pressure drop is certainly a disadvantage resulting out of the use a turbulence promoter. The advantage gained in terms of increase of average heat transfer coefficient by using a turbulence promoter is partially offset by the increased pumping power requirements. In order to study the relative advantage of turbulence promoter vis-a-vis its disadvantage. The study of the parameter heat transfer coefficient per unit pressure drop appears to be appropriate. Fig. 5 shows the plots of hi/[DELTA]P against Flow rate.

Thermal performance ratio of the heat exchanger is ratio of heat transfer coefficient to pressure drop. Thermal performance ratio = hi/[DELTA]P ([mk.sup.-1][s.sup.-1]). On comparing the different curves of figure 5 it has been observed that heat transfer performance of smooth tube is maximum followed by full-length twisted tape. It has been observed that thermal performance of smooth tube are better than full length twisted tape by 1.7- 2.1. Thermal performance decreases with use of turbulators because of increase in pressure drop is more than increase in heat transfer coefficient.

[FIGURE 5 OMITTED]

Conclusion

From the present investigation on double pipe heat exchanger with and without twisted tapes inserts at different mass flow rate of oil, it was found that:

(1) As compared to conventional heat exchanger, the augmented (with turbulator) heat exchanger has shown a significant improvement in heat transfer coefficient by 60% for full length twisted tape.

(2) On equal mass flow rate basis the heat transfer performance of full-length twisted tape is maximum followed by smooth tube.

(3) On unit pressure drop basis, the heat transfer performance of smooth tube is maximum followed by full-length twisted tape. It has been observed that thermal performance of smooth tube is better than full length twisted tape by 1.7-2.1.

References

[1] Bhatia, R.M., Kumar P., and Sud, Y.C., "Contribution to swirl flow heat transfer and friction factor calculations", Inst. Mech. Engrs. (India), Mech. Engg. Div. Vol. 48, pp34, 1967

[2] Kumar, P. and Sud, Y.C., "Heat transfer with coiled wire turbulence promoters", The Canadian journal of chem. Engg. Pp.378, Aug.1970.

[3] Hong, s.w. and Bergles, A.E., "Augmentation of laminar flow Heat Transfer in tube by means of twist tape insert", Journal of Heat Transfer, Trans. ASME, 98 (2), 251-256, 1976

[4] Saha, S.K., Gaitonde, U.N., and Date A.W., "Heat transfer and pressure drop characteristics of Laminar flow through a circular tube fitted with Regularly spaced twisted tape elements with multiple twists", Exp.Therm. Fluid. Sci., Vol 2 (3), pp 310-322, 1989

[5] Saha, S.K., and Chakraborty, D., "Heat transfer and pressure drop characteristics of Laminar flow through a circular tube fitted with Regularly spaced twisted tape elements with multiple twists", National conference on heat and mass transfer, pp 313-318, 1998

[6] Saha, S.K., and Dutta A., "Thermodynamic study of Laminar swirl flow through a circular tube fitted with twisted tape elements", Journal of Heat Transfer, ASME, Vol. 123, No.3 pp. 417-427, 2001.

[7] Saha, S.K., and Langile, P., "Heat transfer and pressure drop characteristics of Laminar flow through a circular tube with longitudinal strip inserts under uniform wall heat wall" Journal of Heat Transfer, ASME, Vol. 124, pp421-432, June-2002.

[8] Smith, F.J., and Meyer, J.P., "R-22 and Zeotropic R-22/R-142b Mixture condensation in Micro fin, High fin and Twisted tape insert tube", Transactions of the ASME Vol. 124, pp 912-920, Oct. 2002.

[9] Hsies, S.S. and Huang, I.W., "Heat transfer and pressure drop characteristics of Laminar flow through a circular tube with/without longitudinal strip inserts", ASME J. Heat Transfer, 122, pp.465-475, 2000.

[10] Solanki, S.C., Prakash, S. and Gupta, C.P. "forced convection heat transfer in doubly connected duct." Int. J. heat Fluid Flow, 8, pp. 107-110, 1987.

[11] Bergles, A.E., "Techniques to augment Heat Transfer, Handbook of Heat Transfer Applications", Chapter 3, McGraw Hill, New York.

Anil Singh Yadav

Assistant Professor, Department of Mechanical Engineering, IPS College of Technology and Management, Gwalior-474001, Madhya Pradesh, India. Email: [email protected]

Nomenclature

A                        Area of heat transfer [m.sup.2]
di                       Inside tube diameter, m
[d.sub.0]                Outside tube diameter, m
Di                       Inside diameter of annulus pipe, m
[D.sub.0]                Outside diameter of annulus pipe, m
Y                        Twist ratio =Half Pitch/tube diameter = P/di
L                        Length of tube
[delta]p                 Pressure drop, kg/[cm.sup.2]
M                        Volume rate of flow, LPM
M                        Mass flow rate, kg/sec
Q                        Rate of heat transfer, W
[C.sub.p]                Specific heat of fluid, KJ/kg-K
[Th.sub.i], [Th.sub.0]   Inlet and outlet temperature of hot fluid,
                           [degrees]C
[Tc.sub.i], [Tc.sub.0]   Inlet and outlet temperature of cold fluid,
                           [degrees]C
[h.sub.h]                Inside heat transfer coefficient for oil,
                           W/[m.sup.2]-k
[h.sub.c]                Water side heat transfer coefficient for oil,
                           W/[m.sup.2]-k
[theta]                  Log mean temperature difference, K
Re                       Reynolds No.
Pr                       Prandt1 No.
[N.sub.u]                Nusselt No.
K                        Thermal conductivity of fluid, W/m-k
f                        Friction factor
U                        Overall heat transfer coefficient,
                           W/[m.sup.2]-k

Greek Symbols

[alpha]                  Thermal diffusivity, [m.sup.2]/sec
[mu]                     Dynamic viscosity of fluid, N-sec/[m.sup.2]
[rho]                    Density, kg/m3
                         Kinematic viscosity of fluid, [m.sup.2]/sec

Subscripts

h                        For hot fluid (oil)
c                        For cold fluid (water)
i                        Inlet
o                        Outlet