Chapter 3. Experimental set up. 3.1 General

Transcription

1 Chapter 3 Experimental set up 3.1 General Experimental set up and various swirl flow generators such as full length twisted tapes, increasing and decreasing order of twist ratio sets and full length screw tape inserts have been fabricated. The water and ethylene glycol are used as test fluids in the present investigations. The test fluid flows in laminar region, through inner square duct and hot water at a high flow rate circulated through annular channel formed between a square duct and outer circular tube, to maintain nearly uniform wall temperature conditions. Main parts of experimental set up are Storage tanks, Calming section, a square duct, Mixing section, Riser, Valves, Rotameters, Resistance temperature detectors (RTD) and various swirl generators. Flow and temperature measuring devices are well calibrated for minimizing errors associated with experimental measurements. 3.2 Experimental set up Schematic diagram of the experimental set-up is shown in Fig.3.1. Experiments were carried out in a double-pipe heat exchanger. It has square duct (copper duct, W = 21.4 mm, D = 21.4 mm, L = 2000 mm, D e = 21.4 mm, AR =1.0, thickness (δ) = 2 mm) and outside circular annulus (stainless steel tube D 1 = 56 mm, D 2 = 62 mm, L = 2000mm). It consists of calming section, test section, Rotameters, storage tanks for test liquid and hot water having capacity of 0.3 m 3 each. Calming section of dimensions 1500 mm long, D = 21.41mm and D o = mm square stainless steel tube was used to eliminate entrance effects. The mixing chamber consisted of a rectangular box of stainless steel with wire mesh inserted inside in such a fashion that the working fluid moved in a tortuous path to give a uniform temperature along the cross section. Water and ethylene glycol were used as test fluids in the present study. Test liquid was flowing through the inner square duct and the hot water as heat transfer medium was passes, at very high flow rate in counter-flow, through an annular channel formed between a square duct and circular tube to attain nearly uniform wall temperature conditions. Hot water at constant flow rate of 30 x 10-3 m 3 /min (constant temperature of 60 o C when water as test fluid) and constant flow rate at 40 x 10-3 m 3 /min 27

2 (constant temperature of 70 o C, when ethylene glycol as test fluid) were used as the heat transfer medium in the annular channel. Two RTD PT 100 type temperature sensors one before the test section and other just after the mixing section are placed to measure the inlet and outlet temperature of test fluid. The hot water tank with the in built PID controller and heater (2 in nos. and 3 kw capacities each) are provided to supply hot water at constant temperature. The temperature of hot water is maintained at desired set point with the deviation of plus or minus 1 o C by Thirister controlled heating. Two more RTD PT 100 type temperature sensors are placed, one at inlet and other at the outlet of the annular channel to measure hot water temperatures. All the Calibrated RTD PT 100 type (Teflon coated) temperature sensors used to measure inlet, outlet temperatures of test liquid, hot water and wall temperatures were connected to a multipoint digital indicator (0.1 o C resolution). Local four thermometer pockets are also provided to measure inlet and outlet temperature of test liquid and hot water respectively. The wall temperature is obtained by taking average of 8 thermocouples installed in axial location along the wall of the duct. A groove of 1 mm depth is made on outer wall of the duct, temperature measuring probe inserted in to the groove and adhesive was applied around the groove to fix up the thermocouple to the wall. This is particularly essential to get inner wall temperature. The test section and connected piping were wound with asbestos insulation of approximately 30 mm thickness to minimize heat loss to the surrounding. The flanges (stainless steel, 30 mm thickness) are used to connect calming section to test section and mixing section at the other end. Mixing section ( stainless steel, square tube, 500 mm L and mm D e ), whose 100 mm length filled with wire mesh, just after the riser section, to ensure efficient mixing at the outlet. Riser section of 400 mm high kink provided at the outlet to ensure duct is fully filled with test liquid. Teflon gaskets of 8 mm thickness placed in between flanges to prevent heat conduction loss to calming section and mixing section. The pressure drop across the test section was measured by using vertical inclined U-tube manometer (θ = 45 o ) containing diethyl phthalate (specific gravity =1.117), for twisted tapes with water as working fluid. The U-tube vertical manometer containing carbon tetrachloride as manometer fluid (specific gravity = 1.59) was used for screw tapes with water as test fluid and twisted tapes with ethylene glycol as test liquids, respectively. Similarly, U-tube 28

3 manometer with mercury (specific gravity =13.6) as manometer fluid was employed for screw tapes with ethylene glycol as test fluid. Two calibrated Rotameters are provided to indicate the accurate flow rate of the test liquid and hot water respectively. Two calibrated Rotameters having flow ranges of (0.1 x 10-3 m 3 /min to 1.1 x 10-3 m 3 /min and 1.0 x 10-3 m 3 /min to 20 x 10-3 m 3 /min) are attached to cover full laminar range to the calming section to measure flow of test liquid, similarly Rotameters with flow ranges of (1 x 10-3 m 3 /min to 6 x 10-3 m 3 /min and 5 x 10-3 m 3 /min to 50 x 10-3 m 3 /min) are attached to annular channel side to measure hot water flow. The temperature in storage tanks of test liquid and hot water were maintained constant by circulating fluid within the tanks by opening bypass valves for circulation. Test liquid and hot water at constant temperature were drawn from inlet of tanks through centrifugal pumps. Flow rate of fluids were regulated using bypass valves. The details of experimental set up are shown in Photograph 3.1. Fig.3.1 Schematic diagram of experimental set up 29

4 Photograph 3.1 Experimental set up 3.3 Technical details of twisted tape inserts Twisted tapes of different twist ratio were made of stainless steel strips of thickness (δ) 1.0 mm and width 19 mm. Linear distance between two successive tape(h) were 56mm, 76mm, 86mm, 110 mm for tape of y = 2.66, y = 3.55, y = 4.01 and y = 5.10 respectively. Twist ratio (y) as defined by the ratio of linear distance of 180 o rotation of tape to the equivalent diameter of duct. The twisted tapes were made in the laboratory from a 1 mm thick stainless-steel strip. The two ends of a strip were held on a lathe, one at the headstock end and the other at the tailstock end by special devices made in the laboratory. The strip was then subjected to twist by turning the chuck manually. The twisted-tape was made red hot to relieve stress and again subjected to twist. By alternately heating and subjecting it to a certain amount of twist, the desired twist ratios were obtained. During the process of making twisted tapes, failure due to buckling and distortion had occasionally observed. Twisted tapes were fabricated in three different geometries: (a) Full-length twisted tape (y = 2.66, y = 3.55, y = 4.01, y = 5.1) of length 4500 mm each; 30

5 (b) The increasing order of twist ratio set was fabricated from three twisted tapes having equal length of 1500 mm and tapes with different twist ratios 2.66, 3.55 and 4.01, were connected in series, to give a full length of 4500 mm. (c) The decreasing order of twist ratio set was fabricated from three twisted tapes having equal length of 1500 mm and tapes with different twist ratios 4.01, 3.55 and 2.66, were connected in series, to give a full length of 4500 mm. The twisted tapes are shown in Figs.3.3 to Fig.3.4. Similarly twisted tapes photographs are shown in photograph 3.2. Fig. 3.2 Schematic diagram of full length twisted tapes Fig. 3.3 Schematic diagram of increasing order of twist (A) and decreasing order of twist (B) twisted tapes y =

6 y = 3.55 y = 4.01 y = 5.10 Decreasing order of twist ratio set Increasing order of twist ratio set Photograph 3.2 Twisted tape inserts 3.4 Technical details of screw tape inserts The geometrical configuration of helical screw-tape inserts is shown in Fig The helical screw-tape inserts with various twist ratio is made by winding uniformly a stainless steel strip thickness 1.0 mm (δ) and 9 mm width over 8 mm stainless steel rod. The twist ratio Y defined as the ratio of length of one twist (Linear distance of 360 o rotation of tape) to the diameter of the twist is varied from 1.44 to 4.66.The helical screw twist inserts are used in test section of the twist ratio values of 1.44, 2.55, 3.66, and 4.66 respectively, and details are shown in photograph

7 Y=1.44 Y=2.55 Y=3.66 Y=4.66 Photograph 3.3. Helical screw tape inserts Y=1.44 Y=2.55 Y=3.66 Y=4.66 Fig.3.4. Schematic diagram of helical screw tape inserts 33

8 3.5 Experimental procedure In the experiments, heater was switched on for hot water tank and water heated until the temperature reached 60 o C for water as test liquid and 70 o C for ethylene glycol as test liquid,. Then, centrifugal pump was switched on, and test liquid to the test section was adjusted using by-pass valve. The test liquid flow rate were varied through calibrated Rotameters from (0.1 x 10-3 m 3 /min to 2.5 x 10-3 m 3 /min) for water and (1 x 10-3 m 3 /min to 15 x 10-3 m 3 /min) for ethylene glycol respectively. Test liquid flows through inner square duct of a double pipe heat exchanger. The hot water as heating liquid at constant rate of 30 x 10-3 m 3 /min and 40 x 10-3 m 3 /min was pumped respectively through annular channel formed between inner square duct and outer circular tube to attain nearly uniform wall temperature conditions, for both water and ethylene glycol as test liquids. After ensuring steady state, temperature of the inlet and outlet of test liquid, hot water and wall temperature were recorded throughout the experiments and isothermal pressure drop was also measured by U-tube inclined or U tube vertical manometers depending upon type of test liquid and test insert with appropriate manometer fluid such as diethyl phthalate, carbon tetrachloride and mercury etc. Before pressure drop measurements were taken, the test section was freed of air bubbles by venting them through the riser section at end of the test section. The mass flow rate was measured by collecting the test fluid in a specified time with the help of weighing machine Then, the experiments were conducted for a plain square duct, duct fitted with full length twisted tapes of different twist ratios (y), increasing and decreasing order of twist ratio sets and full length screw tape inserts of different twist ratios( Y ). Twisted tapes were inserted within the duct diagonally for maximum utilization of swirl effect. The tapes were inserted diagonally for maintaining uniform effect of swirl flow on both sides of the tape. When tape is inserted diagonally, it divides a square duct in to two equal halves. Hence, the effect of swirl flow is uniform on both sides of tape and flow becomes periodically fully developed with the distance of periodicity equal to 90 o rotation of the tape. There is no possibility of generation of natural convection currents in the duct fitted with twisted tape because the fluid is influenced by swirl flow only. Therefore, a higher thermo hydraulic performance can be expected from a square duct fitted with twisted tape inserts. In the next chapter, calibration and standardization of experimental set up are discussed. 34