CHAPTER-3METHODOLOGY3.1 METHOD OF FABRICATION OF TUBE WALL INSERTS3.1.1 Fabrication of Twisted TapesTwisted tapes are made from Aluminium sheet of length 120 cm and width 20 mm. A twist ratio of 5 is maintained constant. Twist ratio is defined as the ratio of length between two consecutive points on a twist to the width of the tape.
At first, the CAD model is made which is shown in the fig. 3.1 below. Then, the strip from aluminium sheet is cut in the required dimensions. Twists are made by holding one end and rotating the other while maintaining the twist ratio constant and the final image is as shown in the fig. 3.2. Fig.
3.1 – CAD model of Twisted Tape Fig. 3.2 – Twisted Tape3.
1.2 Fabrication of FinsFins are also made from Aluminium sheet of 120 cm length and 90 mm width. Triangular Fins of 10 mm height and 10 mm base with a 30 mm longitudinal pitch and 20 mm transverse pitch in a staggered arrangement are made. The CAD model is made first as shown in the fig. 3.3 and fig. 3.
4 and then the fins are cut at two sides and are raised perpendicularly upright. Then the sheet containing fins is wound on the inner pipe. Gas Metal Arc Welding is done longitudinally for Aluminium-Aluminium material, and brazing is done circumferentially at the two ends for Aluminium-MS material. Fig. 3.5 shows the actual image of the fins. Fig. 3.
4 – 2-D CAD model of fins3.2 EXPERIMENTAL SETUPThe experimental setup consists of a double pipe heat exchanger, water tank with heater, thermocouples, data logger, rotameter, pressure transducer and a pump. The internal diameter of the inner pipe is 28 mm, and for outer pipe, it is 66 mm, both are of mild steel and 4 mm thick. The inner tube has two passive enhancements; the first is the twisted tape inserts on the inner side and the fins on the outer side of the tube. The test section is 1.2 m long and is insulated with asbestos and glass tape. Another pipe of 20 mm inner diameter and 80 cm length is attached at the entrance of the inner tube to eliminate the entrance effects or for getting a fully developed flow at the entrance. Hot fluid flows through the inner pipe, whereas the cold fluid flows through the annular space.
Hot fluid flows in a closed loop and is heated in the tank by autotransformer controlled heater whose temperature is controlled using a Selec TC303A thermostat. The outer walls of the tank are also perfectly insulated to prevent heat losses. Six K-type thermocouples are used for temperature measurements. One thermocouple is mounted at the entrance and the other at the exit of the inner tube.
Four thermocouples are mounted on the outer tube at different longitudinal positions with equal spacing. All the thermocouples are calibrated using standard procedure and are connected to Masibus 85XX+ Data Acquisition unit. The flow rate of the hot fluid is controlled by adjusting the valves. Cold water is directly taken from the constant temperature water tank, and the flow rate is measured by a rotameter. Two ABB 2600T series pressure transmitters are used to measure the pressure of the hot fluid at inlet and outlet so that the pressure drop can be calculated. At first, the CAD model of the experimental setup was developed which is as shown in the fig. 3.6.
The photographic view of the setup is shown in the fig. 3.7.
Fig. 3.7 – Experimental Setup3.2.1 NanofluidsNanofluid is the mixture of fluid, called base fluid, which is in larger quantity and nanoparticles, which are dispersed in the base fluid. The two-step method is followed for the nanofluid preparation. Fig.
3.8 – SEM image of iron oxide nanoparticleNanoparticles are procured from Nano Research Labs, India. ?- type iron oxide (20-30nm, 99.5% purity) is used as a nanoparticle in the experiment.
Fig. 3.8 show SEM analysis of Iron oxide.
The nanofluid is prepared by dispersing these nanoparticles in DI water and Ethylene Glycol in the volume proportion of 60:40, which is the base fluid. To obtain stable nanofluid against sedimentation, Ultrasonication is used. The volume concentration (?) and mass of nanoparticle can be related to the following equation (1). (1)The required amount of nanoparticles to form a particular concentration is weighed using a high accuracy (resolution of 0.1mg) electronic weighing machine (fig. 3.9). Then, the nanoparticles and DI water and Ethylene Glycol in the volume proportion of 60:40 are mixed using a magnetic stirrer (REMI) to form a homogeneous mixture (fig.
3.10). After 30 minutes of magnetic stirring, nanofluid is sonicated for one-hour duration using Ultrasonicator (Oscar) shown in fig. 3.11.
There is no sedimentation observed in the prepared nanofluid for more than two days, which is sufficient to carry on the experiment. The volume concentration is stopped at 0.2% Fe2O3 because above such concentrations the problem of agglomeration occurred.
3.2.2 PumpCentrifugal pump was used to circulate hot fluid through the tube side. The specifications of the pump are given in the following Table 3.1:Table – 3.
1: Specifications of PumpPump SpecificationsPower 0.5 HPRated Speed 2800 rpmRated Head 20 mRated Discharge 30 LPMType Centrifugal 3.2.
3 Data Acquisition SystemMasibus 85XX+ Data Acquisition Unit as shown in fig. 3.14 is used for measuring temperatures. The detailed specifications are given in Table-22.214.171.124.4 Pressure TransmitterABB 2600T series 266 model pressure transmitters (fig.
3.13) were used for measuring the pressures at inlet and outlet. The range of pressure transmitter is 0-250 mBar. The power supply to the transmitter is 24V.
Table – 3.2: Specifications of Data Acquisition SystemModel 85XX+Input Type RTD PT-100(3W)Range -199.9 to 850.0 °CNo. of Channels 24Power 85-265V(AC)Frequency 45-66 Hz3.2.5 AutotransformerThe power supplied to the heater block can be adjusted by the auto transformer. The autotransformer (fig.
3.15) used in this experimental setup is having specifications of 240 V and 20A.3.2.6 ThermostatThe thermostat maintains the temperature of nanofluid inside the heater tank at 70°C.
It has an automatic switch/switch off function which switches on the heater when the temperature falls below 70°C and switches off when the temperature reaches 70°C. The thermostat used in this setup is shown in the fig. 3.16. The thermostat used is Selec TC303A with a supply voltage of 85-270 V (AC/DC).
3.2.7 ThermocoupleSix K-type thermocouples of diameter 5 mm are mounted at six different equidistant locations to measure the temperatures of fluid. The specifications are given in Table 3.3.
Table – 3.3: Specifications of ThermocoupleMaterials used Chromel/AlumelTemperature range -270 to 1260°CAccuracy(standard) ±2.2°CMelting point 1400°CVoltage output(over maximum temperature range) -6.4 to 54.
9 mV 3.3 EXPERIMENTAL PROCEDURETwo set of the experiments were conducted by varying the flow rate in inner tube and annular space, they are 1. The tube side flow rate is varied and annular region flow rate is constant: Annular region flow rate is maintained at 12 LPM and tube side flow rate is varied as 4, 6, 9, 12, and 15 LPM (Re 4600 to 18000). Again, the inlet temperature of the hot fluid is kept constant at 70?C.
Initially, DI water and Ethylene Glycol in the volume proportion of 60:40 is used as the hot fluid. After this, 0.05, 0.1 and 0.2% concentrations by volume of iron oxide nanofluids are used as the hot fluid.2.
The tube side flow rate is constant and annular region flow rate is varied: The tube side flow rate is maintained at 12 LPM, and the annular side flow rate is varied as 4, 6, 9, 12, and 15 LPM (Re 900 to 5500). Again, the inlet temperature of the hot fluid is kept constant at 70?C. Initially, DI water and Ethylene Glycol in the volume proportion of 60:40 is used as the hot fluid.
After this, 0.05, 0.1 and 0.2% concentrations by volume of iron oxide nanofluids are used as the hot fluid.