Heat Transfer Investigation in a Circular Tube Fabricated from Nano-composite Materials Under a Constant Heat Flux

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 44 Heat Transfer Investigation in a Circular Tube Fabricated from Nano-composite Materials Under a Constant Heat Flux Prof. Dr. Qasim S. Mahdi, Mech. Eng. Department, Al Mustansirya University, Iraq. qasim66@yahoo.com. Asst. Prof. Dr. Fadhel Abbas, Mech. Eng. Department., Al Mustansirya University, Iraq. Fadhel975@yahoo.com. Hawraa Salih Mahdi, Mech. Eng. Department, Al Mustansirya University, Iraq. asj.waily89@gmail.com. Abstract-- Experimental and numerical investigation of the performance of heat exchanger fabricated from nano-composite material was carried out. In the experimental test rig the test section was fabricated from composite material. The composite material consists of polyester with nano-copper material. It's dimension 8 mm length, mm diameter and 1.5 mm thickness. Heat exchanger has been performed in the presented study for horizontal circular insulated tube under condition of constant heat flux (from 1244 W/m 2 to W/m 2 ) in laminar flow region (297<Re<167) to study the heat transfer behavior and friction factor. In order to improve the heat transfer twisted tape technology has been utilized. Experiments were carried out with three geometries of twisted tapes made from copper materials by varying their twist ratios (Y=H/di), thicknesses (t), and cut shapes along the tape edge. The experimental outcomes show well performance for increasing the average heat transfer coefficient with increasing s. The experimental outcomes show that the using of twisted tapes leads to a considerable increase in heat transfer enhancement by 15% for Y=2 at q=1244w/m 2 and by 77% for Y=6 at q=1244w/m 2 compered of plain tube at Re=167. Friction losses are decreases with increasing the. The twisted tape with triangular cut (TCTT) present well performance in heat transfer enhancement than other twisted tapes. Thus, the maximum amount of average Nusselt number (higher heat transfer enhancement) is for triangular cut twisted than case of plain tube. Comparison between the experimental result with reference result show a good agreement. Index Term-- Nanocomposite Materials, Twisted Tape, Heat Transfer Enhancement, Friction Losses. I. INTRODUCTION Most applications of heat transfer are found in the energy industry whereas different methods of it depend on cooling or heating fluid inside tubes. A comprehensive knowledge of heat transfer coefficients between the fluid flowing inside tube and the wall of tube is required for studying and designing such systems. An essential comprehension of heat transfer coefficients would contribute to increasing the efficiency of such systems. During the last decades many researchers have been experimentally, investigate the effects of nanofluid technology and the tabulators' passive techniques on heat transfer enhancement and pressure drop. Sivashanmugam and Suresh, 6 [1] experimentally examined circular stainlesssteel tube close-fitting with full-length helical screw part. The experimental results show when the twist ratio is raise the heat transfer coefficient raise, the twist is varied from (1.95 to 4.89) and with the twist ratio raise the friction factor increases. Wanga et al., 13 [2] evaluated the heat transfer of nanofluids containing carbon nanotubes in circular stainless tube. The heat transfer outcome shows an improvement in the of nano-fluid compared to the purified water. At of about 1 the heat transfer improvement is (7%) and (19%) for the nano-fluids with volumetric concentrically of (.5%) and (.24%). Esmaeilzadeh et al., 14 [3] an experimental examination of ɣ- Al 2O 3 /water nano fluid of.5% and 1% volumes concentrically through circular copper tube insert with twisted tape with different thicknesses, the twisted tape created of 1 m length and 7 mm width with (.5 mm,1 mm and 2 mm) thicknesses, and for all the tapes the twist ratio (H/d) were maintained constant at (3.41). The heat transfer coefficient outcomes specified that twisted tape inserts increased, as well, twisted tape increase in thickness is more improvements the convective heat transfer coefficient. As well, the maximum improvement was obtained at higher volume concentrically. The outcomes proofed that nano-fluids have much better heat transfer execution if it used with twisted tapes thicker. Sivakumar and Rajan,15 [4] experimental and numerical evaluation in concentric tube suited with copper twisted tape inside which have various twist ratio (Y=2.52, 3 and 3.2). It consists of an aluminum interior tube. The experimental record implemented to tube its exterior material is a mild steel. Heat transfer could be spotted with records of the smooth plain tube were improvement from (7% to 1%) of the tube insert with twist and transfer heat was suited in the circular concentric tube. The Nusselt numbers are increased with the increase of, and the improved heat transfer twisted tape is obtained because of the swirl flow action will be acquired with concentric tubes. Noor,16 [5] experimentally examined copper tube with CuO/ filtered-water nano- fluid (φ =.8% and.35%) volume concentrically moving through the inserted tube with varies geometries of copper twisted tape with Y=2.6 and 5.3 twist ratios, t=1 mm and 2 mm thickness and with triangular cuts and semicircular shape were utilized to examine their effects. The convective heat transfer outcomes safely grown with nano-fluids as operating fluid and inserting twisted

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 45 tape to be compared with nano-fluids or DI-water in smooth tube event and these improvements are increased in both volume concentrically and Reynold number. Twisted tape with triangular cut (TCTT) at Y=2.6 and t= 2 mm with nano-fluid of CuO at (.35%) proofed the greatest performance of heat transfer improvement among additional twisted tapes where growed up by (73%) than case smooth tube with DI-water, while growed up by (62%) for the friction factor. Saud,17[6] studied the heat transfer numerically inside circular tube inserted with adjusted and multiple plain twisted tapes. several inserts design for triple, dual and quadruple twisted tapes internal the test tube have been tested numerically. These layouts including plain twisted tapes and also a set of circulars, semicircular, rhomboidal and triangular cuts in in adding to layout integrated set by methods of cylindrical connecting columns. Numerical outcome of methods in regime of turbulent flow the forced convection proofed that tube close-fitting with dual linked twisted tapes (TDCTT) is the best reading to its thermal performance when comparing it to other layouts design, the Nusselt number amount attained 7.37% and thermal performance factor (49.11%)over the plain tube. Therefore, in this study the effect of using new material of tube and changing the twist ratio, thickness and cutting shape of twisted tape with the flowing water through horizontal uniform heated tube will be investigated in order to get to the desired efficiency for heat transfer enhancement with less friction losses. 2. EXPERIMENTAL APPARATUAS AND PROCEDURE 2.1 Nano-Composites and Structural Composites Tube Achievement The dimension of the test tube is (8 mm length, mm diameter and 1.5mm thickness) was constructed by using three types of material (Fiber glass, polyester resin with catalyst, and nanoparticles copper oxide) and the properties of it is (k=295, ρ=398, c p=563.2) The mixture (polyester-nano) was mixed perfectly by implementing ultrasonic mixing device. The 75% weight nano copper was add to polyester to enhance the conductivity of the mixture. The total mixing time was 5 min to obtain a homogenous polyester-nano mixture with equal spaces between the nanoparticles inspected by implementing microscope to visualize the sample prototype after grinding and polishing the sample. Winding the fiber filament bundle after perfect saturated with polyster-nano mixture over the PVC tube with certain angle (45 ) forward and backward alternately until the last layer. Tubes are kept at room temperature for 7 days for maturation. After that the tube are grinding and polishing as shown in plate (1) figure (1) show the final shape of test section k= e [TA+2 r {d A+ ( 1 4 )ds}t A+ ( 1 2r )dst B] T B T A ds 2.2 Test Rig Description The straight composite nano copper tube with mm inner diameter, 1.5 mm thickness and 8 mm length are implemented as the test tube. Six thermocouples were glued to the external upper surface tube along the test tube with the same distance in between and thermocouples heads were well insulated to thermally isolate it from the heater tape. A thermal compound paste was covered the outside of the test section to ensure high level of heat transfer from the copper sheet that will be covered in term around the test section to get efficient conductivity. A self-glued teflon was implemented to fix, cover, and isolate the copper sheet. A.5 mm thickness teflon heat resistance insulator was wrapped around the thermocouples tip to thermally isolate it from the heater tape. Electrical heater tape with rating 57 W, resistivity 2.8 ohm/meter, (2*.16 mm) cross section, and 1.83 m length are coiled around the tube tightly to heat the tube with the wanted heat flux by joining it to voltage transformer (variance) that supplies an electric power AC across the heater tape to regulator the input voltage and give boundary condition of constant heat flux along the length of test tube. To prevent heat losses through the heater system and tube so, they covered with a layer of fire resistance asbestos insulation (3 mm width and 5 mm thickness) and other layer of fiber glass insulation with 5 mm thickness as shown in plate (2). At the inlet and exit of the test section two 4 mm pressure taps were inserted. The tube has an entrance length before section part and it is in long enough to make sure that the flow is hydro-dynamically fully developed when it is entering the heated tube. Figure (2) shows the experimental apparatus scheme of experimental test rig. 2.3 Twisted Tapes Geometries Twisted tapes inserts are used during the experimental study by changing their twist ratio, thickness and cutting shape. Twisted tapes were made from copper material with length.8 m and width 17 mm. all the types and dimensions of twisted tapes used during the experimental work are demonstrated in table (1). Twisted tape manufacturing is done by fixing one end of the tape and twisting the other end carefully to reach the desired twist ratio, and then these tapes are inserted inside the core tube along the test section by moving passage (flange) equipment at the end of the test tube. Figure (3) shows the geometrical details of twisted tapes. 2.4 Data Deduction Thermal conductivity of specimens is computed by using Lee s disk method [8]. (1)

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 46 The heat flow (e) per unit area and through the unit time (second) is computed by the following formula: e = I V / [πr 2 (T A +T B ) +2πr {d A T A +1/2 ds (T A +T B ) + d B T B +dctc}] (2) To achieve heating effect the electric power was applied at tube wall can be determined by: Q(electric power) = I V (3) Assumed that the external surface of nano-composite tube is well-insulated (no heat losses) so, the heat energy transfer to remain absorbed by the fluid: (electric power) = Qa (absorbed heat energy) = m c p (T fo T fi ) (4) Experimentally, the inside average heat transfer coefficient is calculated using the Newton s law [7,8]. Q ( absorbed heat energy) hi = A s (Ts Tm) (5) Where: As = π d i L (6) T m = T fi+ T fo 2 Surface temperature of tube can be calculated by [9]; T s = T s1+t s2 +T s3 +T s4 4 Then, the average inner Nusselt's number (Nu in )is intended as: (7) (8) Nu in = hi d i k The internal flow of heated tube is laminar fully developed flow and the values ranging from (297 to 167) and its estimate by the relation: (9) Re = 4 m π d i μ (1) where: μ: Dynamic viscosity (kg/m.sec) based on Tm(mean bulk fluid temperature). The thermal resistant [R thermal = ln(d o / d i) /(2πkl)] value across tube walls is too small thus, where the inner surface temperature is approximately equal to the outer surface temperature [Tso = Tsi], it is neglected during the surface temperature calculation. The friction factor coefficient (f) which is associated to the pressure drop (Δp) across the heated tube length. The pressure drop was determined from the differences in the level of manometer fluid can be calculated by equation [8]: f = 2 p d i L ρ v 2 (11) where: v = m ρ A c (12) Ac = π d i 2 4 ρ: density (kg/m 3 ) based on Tm (mean bulk fluid temperature). (13) 3. RESULTS AND DISCUSSION The experimental outcomes of local Nusselt number and friction factor have comparison with the well-known Shah's

4 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 47 correlation and Hagen-Poiseuille's laminar flow equation for the fully developed laminar flow under constant heat flux condition in tube to confirm the accuracy of the experimental work results as shown in figure (4) and (5). 3.1 Effect of on the convective heat transfer The variation of average Nusselt number at different water (297 to 167) are clarified in figure (6). From the outcome curve, it presents the improvement of average Nusselt number of water flow inside tube for five Reynolds numbers. It can be verified that the average Nusselt number improvement increases with increment in. The physical causes for this behavior are the boundary layer evolves smoothly and flows are hydro-dynamically fully developed and they are thermally fully developed. The theoretical boundary layer thickness is zero, hence, the heat transfer coefficient approaches infinity. A minimization in thermal boundary layer's thickness, cause the convective heat transfer coefficient to increases. at the same time the laminar flow boundary layer will be thinner, the thermal access and hydrodynamic regions are longer and in these regions the heat transfer coefficient is high. 3.2 Effect of twisted tape insert inside tube In this section, the effects of changing twisted tape geometries on heat transfer and friction factor have been discussed in the following paragraphs. a) Effect of twist ratio The variation of the average Nusselt number at different water for twisted tape ratios (Y= 2 and 6) are clarified in figure (7) at q=1244 W/m 2. From the outcome curve, it can be realized that the heat transfer improvement in relations of average Nusselt number is increased by reducing the twist ratio of twisted tapes and maximizing the Reynolds number value. The physical causes behind this behavior is, when the typically twisted tapes (TTT) are replaced at the tube core, a swirl flow will be induced from the twist in the tape that disrupts the fluid boundary layer at tube surface due to the enhancement in tangential speed nearby the tube surface which comes from the repeated changes in surface geometries. The disruption of the fluid boundary layer leads to a thinner boundary layer that growing the heat transfer operation. In general, twisted tape generates two rotating swirls appearing on every side of the twisted tape at the first vortex large, but at weak counter rotating and the second vortex are smaller and stronger co-rotating. Therefore, at higher, the stronger relative vortices are increased in size where the other one is decreased in size thus main flow below effect of the tangential component of, the co-rotating vortex is accelerated and expanded where the counter-rotating vortex shrinks and slows down [1]. The aforementioned causes become more affected when decreasing the twist ratio that produces a stronger vortex of swirling that rises the turbulent intensity of the main flow to improve the viscous boundary layer mixing near the internal tube wall to augment the heat transfer process. Nusselt number of this work is improved by 97% for Y=6 and by 15% for Y=2 at Re=167 than for smooth tube case. Figure (8) shows the variation of friction factor for twisted tape ratios (Y= 2 and Y= 6). In basic case and for smooth tube, the friction factor is decreased when is increased because of the incremental in pressure drop. With inserting twisted tape. It is found that, there is a considerable augment in friction losses and whenever decrease the twist ratio cause increases in friction factor and will be so higher than for smooth plain tube at the same values for. In friction losses, the main cause for this incremental is the twisted tape vortex mixing will effect on the tangential and axial viscous boundary layers that enhance the shear forces near the tube surface. The friction factor for the present results increase by 41.5% for Y=2 and by 31.8% for Y=6 than those for smooth tube case. b) Effect of twisted tape thickness Figure (9) at q=1244 W/m 2 present the heat transfer variation improvement in relations of average Nusselt number for thicknesses of twisted tapes (t = 1 mm and 2 mm) at different five. It is clear from outcome figure that thickness of twisted tape has a significant influence on the heat transfer process as the tape is thickness is increased when the Nusselt number is increased. Incremental in the tape thickness tightens the swirling path flow that improves the tangential velocities for well mixing of the viscous boundary layer near the tube surface region. Also, the tape edge effects act for dissipates heat like a fin from the surface of the tube to the working fluid thus, the incremental in this area improves the convective heat transfer. Nusselt number improved by 154% for t =2 mm and by 15% for 1 mm respectively than for smooth tube respectively. while the friction losses were increasing by 25% for the twisted tape at t = 2 mm comparing to twisted tape at t=1 as shown in figure (1). c)the cut shape effect of twisted tape The influences of twisted tape with triangular cut (TCTT) on the variation of average Nusselt number enhancement and friction factor are depicted in figure (11), and figure (12) respectively. As shown at the same process condition, the heat transfer enhancement rate is much higher for TCTT by 3.1% than typical twisted tape and by 157% than case of smooth tube. In general, the typical twisted tape (TTT) is created not only in a swirling flow but also in case of gathering cuts along the twisted tape edge which creates several local swirls at each cutting piece that provides an excellent mixing for the sticky boundary layers in all the directions along the tube which shows higher enhancement in heat transfer improvement. Also, the heat transfer enhancement rate depends on the cutting form which controls the power of vortex created that means the vortices performed behind the triangular cut are stronger and efficient. On the other side, the friction factor is enhanced by 28.4% with TCTT than TTT and due to the addition of these local vortices elevates an additional shear stress due to increases in flow mixing between the viscous boundary layers

5 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 48 in fluid at the tube wall and twisted tape edge. at all events, the nature of the physical flow of fluid inside the tube controlled by the significance of inertia and sticky forces that is the definition of (Re). Therefore, as it could be visible in the figures of heat transfer enhancement in terms of average Nusselt number (Nu ) and in the above sections, the (Nu ) is increased where increased. For higher values, the impact of inertia force becomes more important and the viscosity damping becomes less efficient which leading to thinner viscous boundary layer due to the increment in turbulence degree. A better heat transfer is occurred between the inner surface tube and working fluid. For these experimental results, the performance of twisted tapes aforesaid previously is more efficient in heat transfer enhancement with an increment in values through the companied action of the original inertia force and the enhancement due to the swirling flow by twisted tapes. 3.3 Effect of heat flux on the convective heat transfer At three different heat fluxes (1244, and 3) W/m 2, the variation of average Nusselt number with are depicted in figure (13). From the outcome curve, it presents the improvement of average Nusselt number for water flow inside the tube for five values of with increase in heat flux. INSERT SET Typical twisted tape (TTT) Typical twisted tape (TTT) Typical twisted tape (TTT) Triangular cut twisted tape (TCTT) THICKN ESS MM (T) Table I Typical dimensions of the twisted tapes insert inside tube PITCH (H) NUMBER OF REVOLUTION WIDTH MM TWISTED RATIO (Y) METAL CUT DIMENSION Copper Copper Copper Copper width cut (we=6mm) depth cut (de=8mm) (a) (b) (c) (d) (e) (f) (g) Plate (1) The mixing prosses, winding and polishing.

6 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 49 (a) (b) (c) (d) (e) (f) (g) Plate (2) Show the steps of thermocouple fixation and tube insulation. 8mm Fig. 1. Final shape of test section. Fig. 2. Schematic diagram of exp. test rig, 1)Water tank,2)valves,3)flowmeter,4) Manometer, 5) Test section, 6) Thermocouples,7) Thermometer, 8) Variac,9) Water pump.

7 Average Nusselt number AVerage Nusselt number Local Nusselt Number Friction factor International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:2 5 Fig. 3. Typical twisted tape types and geometries Nu shah eq. [11] Nu Exp. Work 4 Axial distance ratio (Z/d) hagen Poiseuille Exp. Work Reynolds Number Fig. 4. Comparison between Shah equation and experimental work at q"=1244 W/m 2, Re=1689. Fig. 5. Comparison between Hagen equation and experimental work at q"=1244 W/m tube with Y=2 tube with Y=6 1 Fig. 6. Variation of average Nusselt number with Reynolds number for smooth horizontal tube at q"= 1244 W/m 2. Fig. 7. Variation of average Nusselt number with for twisted tapes at different twist ratio at q"= 1244 W/m 2.

8 Friction factor Average Nusselt number Friction factor Average Nusselt number International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No: Y=2 Y= tube with t=1 tube with t=2 1 Fig. 8. Variation of friction factor with for twisted tapes at different twist ratios at q"= 1244 W/m 2. Fig. 9. Variation of average Nusselt number with for twisted tapes at different thicknesses for typical twisted tape at q"= 1244 W/m t=1 t= Fig. 1. Variation of friction factor with for different thicknesses for typical twisted tape at q"= 1244 W/m tube with TTT tube with V-cut Fig. 11. Variation of average Nusselt number coefficient with for different cut shape of twisted tape at q"= 1244 W/m 2.

9 Friction factor Average Nusselt number International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No: TTT V-cut q=1244 q= q=3 1 Fig. 12. Variation of friction factor with Reynolds number for different cut shape of twisted tape at q"= 1244 W/m 2 Fig. 13. Variation of average Nusselt number with for different heat fluxes(w/m 2 ) for smooth horizontal tube.