experimental studies on heat transfer augmenatation ... - ethesis

TRANSFER ENLARGEMENT USING MODIFIED REDUCED WIDTH TWISTED TAPE (RWTT) AS INPUT FOR TUBE SIDE FLOW OF FLUID” submitted by. Gaurav Johar & Virendra Hasda in partial fulfillment of the requirements for the award of Bachelor of Technology in Chemical Engineering at National Institute of Technology, Rourkela (Honorable University) is an authentic work carried out by them under my supervision and guidance . This project report deals with the use of modified twisted tire inserts as Passive Heat Transfer Augmentation Device.

The effect of reduced width twisted tapes (RWTT), reduced width twisted tapes (BRWTT1) and reduced width twisted tapes with holes (BRWTT2) on the heat transfer and water heating friction factor for a range of Reynolds numbers was experimentally studied in a twin tube. heat exchanger. Based on the increase in heat transfer coefficient, a performance evaluation measure R1 and R3, it was found that the reduced width Baffled coiled tape and the reduced width Baffled coiled tape with holes perform much better than the reduced width coiled tape (RWTT) of the same ratio twisting. . Reynolds number for RRWTT, BRWTT1 and BRWTT2 44 5.4 Correlations for variation of friction factor with Reynolds number 45 5.5 Heat transfer coefficient vs.

R1 Performance evaluation criteria based on constant flow rate, dimensionless R3 Performance evaluation criteria based on constant pump power, dimensionless Ui Total heat transfer coefficient based on inner surface area, W/m2°C.

INTRODUCTION

Increases in heat exchanger performance can lead to more economical heat exchanger design that can help make energy, material and cost savings associated with a heat exchange process. The need to increase the thermal performance of heat exchangers and thereby achieve energy, material and cost savings has led to the development and use of many of the techniques mentioned. Use of heat transfer enhancement techniques results in an increase in heat transfer coefficient, but at the cost of an increase in pressure drop.

Therefore, heat transfer rate and pressure drop analysis must be performed while designing a heat exchanger using any of these techniques. In addition, issues such as long-term efficiency and detailed economic analysis of the heat exchanger should be considered. In order to achieve a high rate of heat transfer in an existing or new heat exchanger, while providing increased pumping power, several techniques have been proposed in recent years, which are discussed in the following sections.

Twisted tapes - a type of passive heat transfer technology - have shown significant results in previous studies.

LITERATURE REVIEW

CLASIFICATION OF ENHANCEMENT TECHNIQUES: [ 1, 2]

Heat transfer enhancement or augmentation techniques refer to improving thermohydraulic performance of heat exchangers. PASSIVE TECHNIQUES: These techniques usually use surface or geometric modifications to the flow channel by incorporating inserts or additional devices.

PASSIVE TECHNIQUES: These techniques generally use surface or geometrical modifications to the flow channel by incorporating inserts or additional devices. They

Coiled tubes: In these devices, secondary currents or eddies are created due to the curvature of the coils, which promotes a higher heat transfer coefficient in single-phase flows and in most boiling regions. ACTIVE TECHNIQUES: These techniques are more complex in terms of application and design as the method requires some external energy input to produce the desired flow change and improve the heat transfer rate. A low or high frequency is used to facilitate surface vibrations, resulting in higher convective heat transfer coefficients.

Injection: In this technique, the same or a different liquid is injected into the main bulk liquid through a porous heat transfer interface or upstream of the heat transfer section. Suction: This technique is used for both two-phase heat transfer and single-phase heat transfer. Jet impact: This technique is applicable for both two-phase and single-phase heat transfer processes.

In this method, the fluid is heated or cooled perpendicularly or obliquely to the heat transfer surface.

COMPOUND TECHNIQUES: A compound augmentation technique is the one where more than one of the above mentioned techniques is used in combination with the purpose of

PERFORMANCE EVALUATION CRITERIA: [1]
TREATED SURFACES: [1, 2]

Boiling: Some of the treated surfaces are as follows
Condensing: In condensation of vapours, treated surfaces promote drop wise condensation which is ideal for preventing surface wetting and break up the condensate film

ROUGH SURFACES: [ 1, 2]
EXTENDED SURFACES: [1, 2]
DISPLACED ENHANCEMENT DEVICES: [1, 2]
SWIRL FLOW DEVICES: [1, 2]
COILED TUBES : [1, 2]
ADDITIVES FOR LIQUIDS: [1, 2]
TWISTED TAPE IN LAMINAR FLOW: [6]
TWISTED TAPE IN TURBULENT FLOW: [6]

Twisted tape inserts increase heat transfer coefficients with relatively little increase in pressure drop. They are known to be one of the earliest eddy current devices used in the single-phase heat transfer processes. One of the early studies on heat transfer enhancement using twisted bands was conducted by Whitman [7].

8] concluded that short-length twisted bars perform better than full-length twisted bars because the torque created by the short-length twisted bar decays slowly downstream, which increases the heat transfer coefficient. heat with minimal pressure drop. The Stanton number is the ratio of the heat transfer rate to the enthalpy difference and gives a measure of the heat transfer coefficient. So the main objective of the twisted strip in the turbulent region is to reduce that resistance near the wall to promote better heat transfer.

Zozulya and Shkuratov [37] investigated the effect of the pitch of the twisted ribbons in the heat transfer process and reported the increase in the heat transfer coefficient for little reduction in the pitch. Full length double twisted tape provides higher heat transfer enhancement than regularly spaced twisted tapes. Twisted tape Smooth pitch drop of twisted tape has significant influence on heat transfer S.W.Chang &.

Table 2.1 Performance Evaluation Criteria [1]

CHAPTER 3

PRESENT EXPERIMENTAL WORK

SPECIFICATIONS OF HEAT EXCHANGER USED
TYPES OF INSERTS USED
FABRICATION OF TWISTED TAPES
EXPERIMENTAL SETUP
EXPERIMENTAL PROCEDURE

Room temperature water was allowed to flow through the inner tube while hot water (set point 60°C) flowed through the annulus side in the countercurrent direction. For experiments, three types of twisted tape inserts made of stainless steel strips with a thickness of 1.80 mm were used. Reduced Width Twisted Tape (RWTT): Twisted tapes with a width of 16 mm and a thickness of 1.80 mm were used in the inner tube with an inner diameter of 22 mm, as shown in Figure 3.1.

Reduced Width Twisted Tape (BRWTT1): Barriers in the form of rectangular strips of size 16 mm × 10 mm × 1.80 mm were attached so that they protruded at right angles on each side to the surface of the twisted tape as shown in Figure Twisted Tape reduced width with holes (BRWTT2): These twisted strips were drilled with 6 mm diameter holes in the middle of two consecutive BRWTT1 strips as shown in Figure 3.4. One end was attached to the spindle of the lathe, and the other end was given a slow rotary motion by turning.

The end portions of the fabricated tapes were cut and 3mm holes were drilled to connect the two tapes. To make baffled twisted tape (BRWTT1) small cuts were made on original twisted tape and on the 16mm x 10mm strips which were then fitted together. Baffle strips protruded perpendicular to the surface of twisted tape on each side, as shown in Figure 3.2 and Figure 3.3.

Similarly, a rotameter is provided to control the flow rate of hot water from the hot water inlet tank. Temperature displayed by one of the RTDs (T1) was taken as reference and corrections were made to other RTD's values (ie T2-T4) accordingly. Before starting the experimental study of friction & heat transfer in heat exchanger using inserts, standardization of the experimental setup is done by obtaining the friction factor & heat transfer results for the smooth tube &.. comparing them with the available standard equations.

Air bubbles are removed from the manometer so that the liquid levels in both legs are equal when the flow stops. c. Water at room temperature can flow through the inner tube of the heat exchanger. d. The tank is equipped with a centrifugal pump and a bypass valve to recirculate hot water into the tank and into the experimental setup.

Wilson Chart

STANDARD EQUATIONS USED

PRECAUTIONS

CHAPTER 4

SAMPLE CALCULATIONS

ROTAMETER CALIBRATION

PRESSURE DROP & FRICTION FACTOR CALULATIONS

HEAT TRANSFER COEFFICIENT CALCULATION

CHAPTER 5

RESULTS & DISCUSSION

FRICTION FACTOR RESULTS

As shown in fig.5.1, except at low Re, the difference between fexp and ftheo is limited to ±10%, so we can easily assume that the friction factor equations are true for our experimental setup. The higher deviation between fexp and ftheo for low Re is due to limitations of the experimental setup. Since the ∆H values were very small (0.1-0.8 cm) for low Re and the smallest manometer reading was 0.1 cm, so we could not measure those low pressure drops with higher accuracy.

As the twist ratio decreases, a higher degree of swirl is created, resulting in a higher pressure drop and thus a higher friction factor. In the case of BRWTT1 and BRWTT2, a much higher friction factor is observed due to the increase in the degree of turbulence generated by the respective strips. As we can see from the correlation, it is quite clear that the friction factor increases as the twist ratio decreases.

For a given torque ratio, the friction factor also increases in the sequence RWTT

Fig 5.2 shows the variation of friction factor (f a ) with Reynolds Number for Smooth tube, Reduced width twisted tape (RWTT), Baffled Reduced width twisted tape (BRWTT 1 ), Baffled Reduced width twisted tape with holes (BRWTT 2 ) for different twist rat

HEAT TRANSFER COEFFICIENT RESULTS

Heat transfer coefficient(W/m2°C)

Reynolds Number

As the twist ratio decreases, a higher rotation rate is created which increases turbulence and thus the heat transfer coefficient increases as the twist ratio decreases. In the case of BRWTT1 & BRWTT2, a much higher heat transfer coefficient is observed due to the increase in the secondary flow rate generated which disturbs the entire thermal boundary layer and thus the heat transfer coefficient increases with the decrease in the ratio of twisting.

Heat Transfer Coefficient(W/m2°C)

TESTING OF EXPERIMENTAL DATA FOR REPEATABILITY

Heat transfer experiments were again conducted for BRWTT2 in the Reynolds number range Table Nos.A.4.1-A.4.3) to verify the previously obtained results. While repeating the experiment, the values for heat transfer coefficient were found to be well within.

CHAPTER 6

CONCLUSION

CHAPTER 7

SCOPE FOR FUTURE WORK

Experimental work can be done at low Reynolds number using viscous liquids, as the tapes have shown relatively better results at low Reynolds number. The distance between two successive baffles and holes can be varied to see their effect on the heat transfer and friction factor. Baffles can be held at an angle to the fluid flow rather than perpendicular to the fluid flow.

CALIBRATION

SMALL ROTAMETER CALIBRATION

Observation 2 Observation 3 Rotameter

FRICTION FACTOR RESULTS

STANDARDISATION OF SMOOTH TUBE (f vs. Re)

HEAT TRANSFER RESULTS

STANDARDISATION OF SMOOTH TUBE (h i vs. Re)

A.4EXPERIMENTAL DATA FOR REPEATABILITY