Copy (2) of Mech26_05_04.qxd The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 ____________________________________________ *Corresponding author’s e-mail: jalaljalil@yahoo.com Heat Transfer Enhancement In a Ribbed Duct J.M. Jalil*, T.K. Murtadha, H.M. Kadom Educational Technology Department, University of Technology, P.O. Box 35010, Baghdad, Iraq Received 26 May 2004; accepted 21 March 2005 Abstract: The rib enhancement of heat transfer in a duct is studied numerically and experimentally, where hot air passes through a duct (0.04 x 0.16 x 1.15 m3) with different rib arrangement. The arranments are lower 12-rib arrangement; upper 12 rib arrangement and 24 rib staggered arrangement. The staggered arrangement gives better performance than the others. Also, the angle of attack was studied for lower arrangement, three different values were tested (45°, 60° and 90°). Angle of 60° gives better performance. Numerically, the three-dimension continuity, Navier-Stokes and energy equations are solved by finite volume method of flow of air through (0.04 x 0.16 x 0.6 m3). Validation of the code was performed by compar- ing the numerical result with the results obtained experimentally for staggered arrangement only. The agreement seems acceptable. The numerical studies were extended to study the case of cold air passing through hot ribbed duct. Keywords: CFD, Heat transfer enhancement, Finite volume ≥FGƒY hP iô› ‘ √QGô◊G ∫É≤àfG Ú°ù– ::áá°°UUÓÓNN iô› ∫ÓN øNÉ°S AGƒg QGôeG ¢SQO ~≤d ,É«∏ªY .É«∏ªY h ÉjO~Y ¢SQO ~b ≥FGƒ©dG ᣰSGƒH IQGô◊G ∫É≤àfG Ú°ù– ¿G3m).∞∏àfl Ö«JôJ ™e (1^15*0^16*0^04 ~Lh ~bh 90 ,60 ,45 ÉjGhõ∏d h »∏Ø°ùdG Ö«JÎ∏d Ωƒé¡dG ájhGR â°SQO ∂dòc .AGOG π° aG ≈£YG ∞dÉîàŸG Ö«JÎdG .24 ∞dÉîàeh ,12 …ƒ∏Y ,12 »∏Ø°S »g ≥FGƒ©dG äÉÑ«JôJ (0^04*0^16*1^15) iô› ∫ÓN AGƒg ¿Éjô÷ IO~ÙG Ωƒé◊G á≤jôW ᣰSGƒH ábÉ£dGh ºNõdGh á∏àµdG ßØM ä’OÉ©e πM ” ÉjO~Y .60 ∫ ¿Éc AGOG π° aG ¿G3m. .øNÉ°S iô› ∫ÓN OQÉÑdG AGƒ¡dG ¿ÉjôL ádÉM πª°ûàd ájO~©dG èFÉàædG äO~e ºK .ádƒÑ≤e áfQÉ≤ŸG âfÉc ~bh »Hƒ°SÉ◊G èeÉfÈdG ≥«b~àd ájO~©dG èFÉàædG ™e âfQƒb á«∏ª©dG èFÉàædG ::áá««MMÉÉààØØŸŸGG ääGGOOôôØØŸŸGGIO~ÙG Ωƒé◊G ,IQGô◊G ∫É≤àfG Ú°ù– ,™FGƒŸG ɵ«eÉæjO ‘ äÉbÉ«°ùdG Symbol Description aB,aE,aF,aN,aS,aW = coefficient s in general finite-volume equation Cµ, C1ε, C2ε = constants in turbulence model Dh = hydraulic diameter , m g = gravitational acceleration , m/s2 Hr,Wr = rib height and width, mm H = height of the d uct, m i,j,k = position indices for the x -, and y, and z -directions Iu = turbulence intensity Notation 11 The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 1. Introduction Rib turbulator has numerous applications in practical engineering systems where it is used in internal cooling passages of modern gas turbine blades that must be pro- tected from hot gas streams. Thus, the accurate prediction of the characteristics of the flow field in these regions is of great importance. Ribs on channel walls can increase heat transfer coefficients in the cooling passages. These ribs which are called turbulators, increase the level of mixing of the cooler air with the warmer air close to chan- nel wall, thereby enhancing the cooling capability of the passage through compressing the coolant flow rate. The rib turbulator was studied extensively in recent years experimentally and numerically. Experimentally, different techniques were used to evaluate the aerodynam- ic of fluid flow and heat transfer. Han (1988) studied the effect of the channel aspect ratio on the distribution of the local heat transfer coefficient in rectangular channels with two opposite ribbed walls (to simulate turbine airfoil cool- ing passages). Taslim, et al. (1996) used a liquid crystal technique to measure heat trsnsfer coefficients in twelve test sections with square and trapezoidal cross sectional areas representing blade mid-chord cooling cavities in a modern gas turbine. Gao and Sunden (2001) used liquid crystal thermography in the heat transfer experiment to demonstrate detailed temperature distribution between a pair of ribs on ribbed surfaces. Lee and Moneim (2001) examined heat transfer and flow behavior on a horizontal surface with a two-dimensional transverse rib using a CFD model. Kim and Kim (2001) performed numerical optimization coupled with Reynolds-averaged Navier- Stokes analysis of flow and heat transfer for the design of rib-roughened surface. Raisee and Bolhasani (2003) investigated the fluid flow and heat transfer through ducts roughened by arrays of either attached or detached ribs. the numerical approach used in this work is the finite-volume method. Hadhrami and Han (2003) studied the effect of various 45o angled- rib turbulator arrangements. Chang, et al. (2004) present- ed an experimental study of heat transfer in a rectangular channel with two opposite walls roughened by 45o stag- gered ribs swinging about two orthogonal axes under sin- gle and compound modes of pitching and rolling oscilla- tions. Tanda (2004), used repeated ribs on heat exchange surfaces to promote turbulence and enhance convective k = turbulent kinetic energy = ( )222 wvu 2 1 ′+′+′ , m 2/s2 L = length of test section , m P = mean statis pressure, P a, N/m 2 Pe = Peclet number Pr = Prandtl number Re = Reynolds number í hav DU . = Sö = source term T, Tb, Tin, Tw = mean inlet, bulk and wall temperatures , oC U,V,W = velocities in x ,y,z-directions , m/s Uav = average velocity , m/s x,y,z = cartesian coordinates, m ε = dissipation rate of turbulent kinetic energy ρ = density of air, kg/m3 σk, σε = turbulent Prandtl numbers for k and ε µ = dynamic viscosity , N.s/m2 ν = kinematic viscosity, m2/s νt = eddy or turbulent viscosity , m 2/s νe = effective kinemat ic viscosity , m 2/s Γ = diffusion coefficient, ,/ σµ=Γ N.s/m2 Γe = effective diffusion coefficient, ee / σµ=Γ , N.s/m 2 φ = general dependent variable Greek Symbols 12 The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 heat transfer. Onbasioglu and Huseyin (2004) used a liq- uid crystal based experimental investigation of heat trans- fer enhancement by ribs on a vertical plate. In this work, a duct roughened with ribs was investi- gated numerically and experimentally. In both cases, the effect of rib turbulator on fluid flow and heat transfer was studied and analyzed. 2. Mathematical Model The governing equations are: (Patanker, 1980) Continuity, (1) Navier Stokes, (2) (3) (4) Also the energy equation: (5) The standard k-ε model (Launder and Spalding) has two equations, one for k and one for ε. It uses the follow- ing transport equations for k and ε. (6) (7) (8) where ε, is the dissipation term. The parametric values in this study are given in Table 1. The general partial differential equation has the form: (9) where the three terms on the lefthand side are convection terms and the four terms on the righthand side are diffu- sion and source terms. The source term appearing in the above governing equation is given in Table 2. 3. Experimental Investigation In this study, flow in rectangular channel with rib tur- bulator was investigated experimentally for different air bulk velocities of 4.2, 5.8, 8.2 and 10 m/s. Figure 1 shows the rib locations along the duct for five arrangements. Figure 2 shows a schematics of the experimental appara- tus. The test duct has a cross section of 160x40 mm2 with a corresponding hydraulic diameter, Dh = 64 mm, and a length of 18 Dh . The side walls of the entire test duct were made of plexiglas plates to provide optical access for measurements. 0= ∂ ∂ + ∂ ∂ + ∂ ∂ z W y V x U ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ∂ ∂ ρ −= ∂ ∂ + ∂ ∂ + ∂ ∂ z U zy U y x U xx P1 z UW y UV x U ee e 2 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ∂ ∂ ρ −= ∂ ∂ + ∂ ∂ + ∂ ∂ z V zy V y x V xy P1 z VW y V x VU ee e 2 ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + y W zy V yy U x eee ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ∂ ∂ ρ −= ∂ ∂ + ∂ ∂ + ∂ ∂ z W zy W y x W xz P1 z W y VW x WU ee e 2 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ + z W zz V yz U x eee ννν ( ) ( ) ( ) ε−+⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ σ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ σ∂ ∂ + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ σ∂ ∂ = ∂ ∂ + ∂ ∂ + ∂ ∂ G z kv zy kv y x kv x kW z kV y kU x k t k t k t ( ) ( ) ( ) kkz v zy v y x v x W z V y U x 2 C2GC1 tt t ε − ε +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ε∂ σ∂ ∂ +⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ∂ ε∂ σ∂ ∂ + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ε∂ σ∂ ∂ =ε ∂ ∂ +ε ∂ ∂ +ε ∂ ∂ εε εε ε ⎥ ⎥ ⎦ ⎤ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ +⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ∂ ∂ + ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ = ν 222 222 t y W z V x W z V x U y V z W 2 y V 2 x U 2G ( ) ( ) ( ) φφφφ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ φ∂ Γ ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ φ∂ Γ ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ φ∂ Γ ∂ ∂ =φρ ∂ ∂ +φρ ∂ ∂ +φρ ∂ ∂ S zzyyx x W z V y U x Cµ C1ε C2ε σk σε 0.09 1.44 1.92 1.00 1.30 Table 1. Empirical constants in the k-ε ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ Γ ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ Γ ∂ ∂ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ Γ ∂ ∂ = ∂ ∂ + ∂ ∂ + ∂ ∂ z T zy T y x T xz WT y VT x UT ee e To solve the governing Eqs. (1)-(5), mathematical expression s for effective kinematic viscosity, eν , and effective diffusion coefficient , Γe, are required through use of a turbulence model. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + x W zx V yx U x eee 13 The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 Figure 1. Rib locations along the duct Figure 2. Schematic drawing of the overall experimental system 1. Blower 2. Heater 3. Mesh 4. Test Section 5. Pitot Tube 6. Manometer 7. Control System 8. Interface 9. Computer 10. Temp. Control (Thermostat) 11. Thermocouples 12. Ribs (a) Lower Ribs at α = 90o (b) Lower Ribs at α = 60o (c) Lower Ribs at α = 45o (d) Staggerd Ribs at α = 90o (e) Upper Ribs at α = 90o 25 cm 30 cm 193 cm 14 The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 The top and bottom are made from steel sheets. The plexiglas rectangular ribs of 5.2-mm width, 6.8-mm height were positioned along the duct at twelve stations. Five arrangements were tested: 1. Lower wall ribs with α = 45°. 2. Lower wall ribs with α = 60°. 3. Lower wall ribs with α = 90°. 4. Upper wall ribs with α = 90°. 5. Staggered arrangement where the ribs are positioned in lower and upper walls with α = 90°. Temperature distribution at various locations in longi- tudinal and lateral directions was measured using insulat- ed copper-constantan thermocouple wire type T (0.27 mm). The rib turbulators were placed on opposite walls with angles of 45°, 60°, 90°. Twelve rectangular ribs were tested in this investigation where rib height to duct hydraulic diameter ratio (e/H) is 0.106 and rib width to hydraulic diameter is 0.078. The rib turbulators were rec- tangular cross section rods (Hr = 6.8 mm and Wr = 5 mm). The inlet flow to the duct was heated to specified temper- atures and the local temperatures were measured in later- al and longitudinal directions using fifty thermocouples: twenty-five thermocouples for measuring the temperature of the flow and twenty-five for measuring the surface tem- perature of the channel. 4. Results and Discussions 4.1 Experimental Results Forty tests were carried out. Four values of inlet veloc- ities are studied (4.2, 5.8, 8.2 and 10 m/s) with two differ- ent inlet temperatures of 40 and 50ºC. In the experimental work, the hot air with controlled temperature enters the duct with ribs. The temperature of the air decreases all the way to the end of the duct because of the heat transfer from hot air to the duct wall which will be enhanced due to turbulence effect of ribs. This enhancement in heat transfer, usually, is accompanied with additional pressure drop. To study the effect of different angles of attack, three angles were tested 45o, 60o and 90o. In 90o, higher pres- sure drops are expected. The differences in pressure drop are explained as follows, in 90o the flow of air will be stopped completely in front of rib while in 60o, the rib allows the air in front of the rib to turn and take side pas- sage, parallel to the rib. In 45o, easier passage for air is allowed parallel to the rib. The high pressure drops at 90o are not necessarily accompanied by higher heat transfer. In 60o and 45o angles, the heat transfer will be higher due to longer ribs. The rib lengths are 16 cm for lower 90o, 18.5 cm for lower 60o and 22.7 cm for lower 45o. The longer ribs with angle of attack will have two advantages over the short 90o rib. First it causes lower pressure drop, which is the main parameter in fluid flow design. the sec- ond advantage is that, the larger rib means higher turbu- lence effect of consequently higher heat transfer enhance- ment. The only disadvantage is that longer rib means more material and higher cost. The effect of different angles of attack on heat transfer performance was evaluat- ed through the temperature difference. The high tempera- ture difference means high advantage gain from rib arrangement. Angle 60o seems to be the best value. Han (1988) also mentions the same for optimum value of angle of attack for rib arrangement. Four different inlet air velocities were tested: 4.2, 5.8, 8.2 and 10 m/s. This is achieved through changing the power supply to the fan. Due to limitation of heating equipment, only two dif- ferent inlet air temperatures of 40o and 50o were used. This was done by changing the power supply to the heater working elements. Clearly, the higher inlet air tempera- tures lead to higher air and wall temperatures along the duct. Equation φ Γφ Sφ Continuity 1 0 0 U-momentum U νe ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ∂ ∂ − x W zx V yx U xx P eee V-momentum V νe ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ∂ ∂ − y W zy V yy U xy P eee W-momentum W νe ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ +⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ν ∂ ∂ + ∂ ∂ − z W zz V yz U xz P eee Temperature T Γe 0 Kinetic energy k Γk G-ε Dissipation rate ε Γε k CG k C 2 21 ε − ε εε Table 2. Source term in the governing (PDES) 15 The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 4.2 Validation of the Code The first run of the computer program was for the con- dition similar to the experimental arrangement where hot air going through the duct and the wall temperature was calculated in addition to air temperature by conduction. One case was tested to compare the results with experi- mental part. The test case is staggered ribs with 90º, inlet air velocity 4.2 m/s, inlet air temperature 40 ºC. Only half length of the duct was simulated by computer program, with 12 ribs in staggered arrangement for half duct only (L = 0.6 m) with same cross section area as in experimen- tal part (0.4 m x 0.16 m). Figure 3a shows the velocity vector field, Figure 3b shows the isotherm contours. For isotherm contours, the high temperature is shown at inlet and it decreases with very low rate toward the walls and rib. For comparison with experimental results, the air and wall temperatures are compared and good agreement was achieved between the two. 4.3 Numerical Results The numerical part will be devoted to different physi- cal conditions. Usually in most papers, cold air passes through isothermal higher temperature duct walls which we chose to follow. In this part, air with different velocities (2.5, 5, 7.5 and 10 m/s) are passed through a duct similar to experimental part where the cross section is 0.16 x 0.04 m2 with a length of 0.6 m (half the experimental duct). The air inlet temperature was fixed at 20ºC and the wall plus rib tem- peratures were fixed at 40ºC. Number of ribs are 6 for lower and upper arrangements, and 12 for staggered arrangement. Three arrangements were tested, staggered, lower and upper. Five heights of ribs were tested, 3, 4, 5, 6 and 7 mm. Figure 4 presents the distribution of three - dimension- al flow field in staggered case for Re = 25000 and Hr = 7 mm. It can be seen from the figure that the introduction of ribs locally reduces the cross section resulting in a main flow acceleration around these obstacles. The sudden expansion downstream of the ribs leads to a separation zone behind them, after reattachment the entrained flow builds up a new boundary layer. The latter is accelerated by the mainstream through shear forces and impinges on the next rib. The flow deflection around the ribs is respon- sible for high - velocity vectors, finally indicating the existence of third small vortex on the top of the ribs. In contour figures, the three-dimensional plot of isotherm contours are plotted for only three levels in z- direction, one near the right wall and second at mid of the duct and the third near the left wall. The air enters with temperature of 20oC and the temperatures of the walls of the duct and ribs are isothermal at 40oC. Temperature of the air increases through the duct from 20oC at inlet to 40oC at the walls. The increase in air temperature depends on many factors. The main one is the existence of ribs will work in two different ways; first as surface area with temperature of 40oC. This point was eliminated in experimental part where the ribs are made of Plexiglass; but in numerical calculation, the ribs will work as surface area. Second the turbulence effect of ribs will increase the heat transfer. The other parameter that increases the air temperature was the rib height. As the height increases, increased area and work and more turbulence is expected. The third parameter is the inlet air velocity. As the inlet velocity increases, less time is available for heat transfer. Then the decrease in inlet air velocity will increase the temperature of air through the duct. Finally, the kind of arrangement of ribs will affect the process of heat transfer. Figure 5 shows the contours for lower arrangement at con- stant velocity 2.5 m/s with different rib heights 3, 4, 6 and 7 mm. The effect of rib height is clear as the lead to high- er air temperatures. The air bulk temperatuee increase as ribs, ribs with higher air velocity decreases. Figure 6 shows the varia- tion of air bulk temperature at constant air velocity of 5 m/s and different rib heights of 3, 4, 5, 6 and 7 mm: the higher temperatures at higher heights. The average Nusselt number for each of four walls at each x position, can be calculated by, right wall, (10) For lower ribs with 90o, Fig. 7 shows plots for four- inlet air velocities. Each plot shows the variation of Nusselt number with duct length. Higher Nu results with high rib height. The final correlation equation for air (at constant Prandtl number) for each arrangement is: Nu = a Reb xc Hrd (11) The values of constants are given in Table 3: 5. Conclusions and Recommendations The following are concluded: 1. Increasing rib height enhances the Nusselt number. 2. High Reynolds number enhances the Nusselt number. 3. The optimum angle of attack was 60o for heat transfer enhancement. ( ) dy dx )k,j,i(T1 Nu L 0 − ∫= Rib arrange - ment a b c d Lower 1.478088 0.243154 -0.296225 0.004205 Upper 1.974745 0.329244 -0.240705 0.033619 Stagge Red 4.061650 0.272817 -0.203860 0.118233 Table 3 The values of constants in Eq. (11) 16 The Journal of Engineering Research III (2005) 10-18 (3D ) ⏐ 22 May 2005 ⏐ ZX Y (3D ) ⏐ 22 May 2005 ⏐ 20 30 40 30 30 20 20 20 20 30 30 20 40 40 ZX Y(3D ) ⏐ 22 May 2005 ⏐ Figure 3. Velocity vector field and isotherm contours along the duct with staggered ribs, v = 7.5 m/s and Hr = 3 mm (3D) ⏐ 23 May 2005 ⏐ X Y Z (3D) ⏐ 23 May 2005 ⏐ Figure 4. Velocity vector along the duct with staggered ribs for v = 10 m/s and Hr = 7 mm 17 The Journal of Engineering Research III (2005) 10-18 (3D) ⏐ 23 M ay 200 5 ⏐ 20 20 20 30 303030 30 30 40 40 40 40 40 3030 20 20 ZX Y (3 D) ⏐ 23 May 2005 ⏐ 20 20 203030 3030 30 30 30 40 40 40 4040 40 20 20 ZX Y (3D) ⏐ 23 May 2005 ⏐ 20 20 20202030 30 30 30 30 30 4040 40 40 ZX Y(3D) ⏐ 23 May 2005 ⏐ Figure 5. Isotherm contours along duct with lower ribs at v = 2.5 m/s, a) Hr = 3 mm, b) Hr = 5 mm, c) Hr = 7 mm Figure 6. Bulk temperature variation with duct length for different rib heights for v = 5 m/s and lower arrangement 3 mm 4 mm 5 mm 6 mm 7 mm 28 26 24 22 20 0.1 0.2 Duct length (m) 0.3 0.4 B ul k te m pe ra tu re ( C ) 18 The Journal of Engineering Research Vol. 3, No.1 (2006) 10-18 The following future work is recommended: 1. Studying further rib geometries like pitch and width. 2. Experimental study of cold air through ribbed duct. References Chang, S.W., Su, L.M. and Yang, T.L., 2004, "Heat Transfer in a Swinging Rectangular Duct with two Opposite Walls Roughened by 45º Staggered Ribs," Int. J. of Heat and Mass Transfer, Vol. 44(11), pp. 287-305. Gao, X. and Sunden, B., 2001, "Heat Transfer and Pressure Drop Measurements in Rib-roughened Rectangular Ducts," Experimental Thermal and Fluid Science, Vol. 24(1-2), pp. 25-34. Han, J.C., 1988, "Heat Transfer and Friction Characteristics in Rectangular Channels with Ribs Turbulators," J. Heat Transfer, Vol. 110. pp. 321-328. Hadhrami, L. A. and Chin Han, J.C., 2003, "Effect of Rotation on Heat Transfer in Two-pass Square Channels with Five Different Orientations of 45o Angle Rib Turbulator," Int. J. of Heat and Mass Transfer, Vol. 46(4), pp. 653-669. Kim, K. and Kim, S., 2001, "Numerical Optimization of Rib Shape to Enhance Turbulent Heat Transfer," Proceedings of the 2nd Int. Conference on Computational Heat and Mass Transfer. Lee, C.K. and Moneim, S.A., 2001, "Computational Analysis of Heat Transfer in Turbulent Flow Past a Horizontal Surface with Two - Dimensional Ribs," Int. Communications in Heat and Mass Transfer, Vol. 28(2), pp. 161-170. Onbasioglu, S. U. and Huseyin Onba, H., 2004, "On Enhancement of Heat Transfer with Ribs," Applied Thermal Engineering, Vol. 24(1), pp. 43-57. Patanker, S. V., 1980, Numerical Heat Transfer and Fluid Flow, McGraw-Hill, New York. Raisee, M. and Bolhasani, M.R., 2003, "Computation of Turbulent Flow and Heat Transfer in Passages with Attached and Detached Rib Array," Dept. of Mech. Engi., Faculty of Engineering, University of Tehran. Taslim, M. E., Li, T. and Spring, S. D., 1998, "Measurements of Heat Transfer Coefficient and Friction Factors in Passages Rib-Roughened on All Walls," Transactions of ASME, Vol.120, pp. 564-570. Tanda, G., 2004, "Heat Transfer in Rectangular Channels with Transverse and V-Shaped Broken Ribs," Int. J. of Heat and Mass Transfer, Vol. 47(2), pp. 229-243. Figure 7. Variation of Nusselt number with duct length for lower arrangement and v = 10 m/s and different ribs height 36 34 32 30 28 26 24 22 20 18 16 N us se lt n um be r 10 20 30 40 50 60 Duct Length (m)