HEAT TRANSFER ANALYSIS IN A 3D SQUARE CHANNEL LAMINAR FLOW WITH USING BAFFLES 1 Vikram Bishnoi 2 Rajesh Dudi 1 Scholar and 2 Assistant Professor,Department of Mechanical Engineering, OITM, Hisar (Haryana) ABSTRACT Heat transfer and fluid flow characteristics in a square channel in the presence of baffle has been numerically investigated in the laminar flow regime. The computations are done for a Reynolds number of 100 and baffle height 5cm and 6cm and for without baffle. Channel length is taken as 100cm (L=100cm). The Navier Stokes equations along with the energy equation have been solved by using SIMPLE Technique. The results shows that the average surface Nusselt number increases by 2.3% and 3.63% in the case of baffle height 5cm and baffle height 6cm respectively with compared to no baffle case. The enhancement is due to the formation of vortices which travels long way in the downstream direction. It is further observed that the heat transfer increases with the increase in height of baffle and also by increasing the Reynolds number (Re). As we increase the baffle height from 5cm to 6cm the average surface Nusselt number increases by 1.3%. Further if we increase the Reynolds number from 100 to 200 the average surface number is increased by almost 35%. However the heat transfer enhancement is associated with pressure drop. 1. INTRODUCTION A device used for heat exchange between the two fluids at different temperatures, is known as heat exchanger. Enhancement of heat transfer through various means has been an intense area of research for many years. There are numerous applications where high performance heat exchange is desired. The cooling of electronics, such as microprocessors, in a car as radiator, in a refrigerator, in steam condensers, chemical plants, petrochemicals plant, power plants and petroleum refineries are the examples. In such type of applications the efficiency of the heat exchangers plays an important role in controlling the overall performance of the system. Heat exchangers are mainly used to transfer the heat from one fluid to the other efficiently, which are separated by a solid wall. An efficient heat exchanger could consume less energy resource, which benefits both economical and environmental aspects. By enhancing the heat transfer between the heat exchanger fluids the performance of a heat exchanger can be improved. The recent rise in the studies of heat exchange enhancement techniques has also lead to efficient and compact heat exchangers. This research involves the numerical analysis of heat exchange enhancement in a rectangular channel using baffles of different height. Few studies have shown that the heat transfer enhancement can also be achieved by including the baffles in the flow. Nasiruddin et al.[1] carried out heat transfer enhancement in a heat exchanger tube by installing a baffle. The effect of baffle size and orientation on the heat transfer enhancement was studied in detail. Three different baffle arrangements were considered. The results show that for the vertical baffle, an increase in the baffle height causes a substantial increase in the Nusselt number but the pressure loss was also very significant. For the inclined baffles, the results show that the Nusselt number enhancement was almost independent of the baffle inclination angle, with the maximum and average Nusselt number 120% and 70% higher than that for the case of no baffle, respectively. For a given baffle geometry, the Nusselt number enhancement was increased by more than a factor of two as the Reynolds number decreased from 20,000 to 5000.Simulations were conducted by introducing another baffle to enhance heat transfer. The results show that the average Nusselt number for the two baffles case is 20% higher than the one baffle case and 82% higher than the no baffle case. The above results suggest that a significant heat transfer enhancement in a heat exchanger tube can be achieved by introducing a baffle inclined towards the down streamside, with the minimum pressure loss. Yue-Tzu Yang et al [2] studied the numerical predictions on the turbulent fluid flow and heat transfer characteristics for rectangular channel with porous baffles which were arranged on the bottom and top channel walls in a periodically staggered way. The turbulent governing equations were solved by a control volume-based finite difference method with power-law scheme and the k e turbulence model associated with wall function to describe the turbulent structure. The velocity and pressure terms of momentum equations were solved by SIMPLE (semi-implicit method for pressure-linked equation) method. The parameters studied include the entrance Reynolds number Re (1* 10 4 5 * 10 4 ), the baffle height (h = 10, 20 and30 mm) and kind of baffles (solid and porous); whereas the baffle spacing S=H were fixed at 1.0 and the working medium was air. The numerical calculations of the flow field indicated ISSN 2278-5787 Page 12
that the flow patterns around the porous and solid type baffles were entirely different due to different transport phenomena and it significantly influences the local heat transfer coefficient distributions. Relative to the solid-type baffle channel, the poroustype baffle channel had a lower friction factor due to less channel blockage. Concerning the heat transfer effect, both the solid-type and porous-type baffles walls enhanced the heat transfer relative to the smooth channel. It is further found that at the higher baffle height, the level of heat transfer augmentation was nearly the same for the porous-type baffle, the only difference being the Reynolds number dependence. As expected, the centerline-averaged Nusselt number ratio increases with increasing the baffle height because of the flow acceleration. Asif at el [3] investigated the mixed convective two dimensional flows in a vertical enclosure with heated baffles on side walls. All walls were assumed to be adiabatic, but baffles were considered as isothermally heated. Thus, cold flow was imposed through an opening at the bottom of the left wall and by taking heat from the baffles the fluid becomes heated and exits through outlet at the top of the right wall of the enclosure. Heated baffles were placed both at the left and right wall of the enclosure. The present study simulates a practical system such as a silencer. The consequent mathematical model was governed by the coupled equations of mass, momentum and energy and these equations were discritisized. The discritisized equations with specified boundary conditions were sought by Successive under Relaxation (SUR) method. A wide range of pertinent parameters such as Reynolds number 50 Re 300, Richardson number 0 Ri 10 and Prandtl number 0.01 Pr 2.0 were considered in the present study. Various results such as the streamlines, isotherms, heat transfer rates in terms of the average Nusselt number and temperature and also heating efficiency in the enclosure were presented for different parameters. It was observed that maximum heating efficiency was found at higher value of Reynolds and Richardson number. Majumdar et el [4] Heat transfer augmentation techniques refer to different methods used to increase rate of heat transfer without affecting much the overall performance of the system. In this Study the overall performance of suitably designed rectangular channel was analyzed with passive heat transfer augmentation technique. In the rectangular channel, different types of perforated baffle with different diameter were used. This experimental study investigates the local heat transfer characteristics and the associated fictional head loss in a rectangular channel with inclined solid and perforated baffle. A constant surface heat flux was applied from the top surface, but the bottom and the side surface were maintained at an adiabatic condition. The flow Reynolds number for this experimental study was varied between 7600 and 54000.Experimental results show that the heat transfer characteristics were strongly depended on the position, orientation and geometry of the perforated baffle plate. 2. NUMERICAL MODELING Figure 3.1 represents a three dimensional computational domain of square cross sectional area. The channel width and height is taken as 15cm and length, L=100cm. The baffle is based at 50cm from the inlet end of the channel. The baffle height (h) is taken as 5cm and 6cm in different cases. The thickness and width of the baffle is taken as 2cm and 15cm respectively in each case. Air has been taken as working fluid. Without baffle With baffle Figure 2.1 The flow is taken incompressible with constant properties. The governing flow equations i.e. continuity equation, momentum equation and energy ISSN 2278-5787 Page 13
equation are used to simulate the incompressible steady flow in the given computational domain. The continuity equation in three dimensions for an incompressible steady state flow is given by: transfer rate will increases rapidly. The heat transfer is shown in the figure given below by making two different planes in the channel. The momentum equations are: The energy equation is: The following data is considered for the calculation: temperature contour without baffle Reynolds number = 100 inlet temperature =300K Outlet pressure = 0 Pa wall temperature = 400K 3. RESULTS Comparison of Flow Characteristics and Heat Transfer Characteristics for Re=100 at baffle height =5cm, baffle height =6cm and without baffle. 3.1 Temperature Contours and Heat Transfer Characteristics Comparison of temperature contour Figure 3.1 shows the temperature variation of fluid in the channel in different cases (without baffle, with baffle height 5cm and 6cm) at Re=100. These plots show that the heat increases at a fast rate in the channel with baffle of 6cm height as compared to baffle of 5 cm height and at very fast rate with compare to channel without baffle. This will show that the heat transfer rate increases in the channel as we induce the baffle in the channel and with the increase in the height of baffle in the channel the heat temperature contour with baffle height 5cm ISSN 2278-5787 Page 14
Further we can see that as the baffle height is increased from 5cm to 6cm the peak in the average Nusselt number line is also increases at fast rate. This will show that the Nusselt Number will increases rapidly at the wall with the existence of baffles and also with the increase in the height of baffle. The percentage increase in the value of average Nusselt number for the channel with baffle as compared to the plane channel is shown below: temperature contour with baffle height 6cm figure 3.1 Reynolds number Baffle height(h) Cm % increase in average surface nusselt number 100 5 2.3 100 6 3.63 3.2 Surface Nusselt number variation Figure 4.2 shows the Surface Nusselt Number variation at the wall of the cannel in different cases (without baffle, with baffle height 5cm and 6cm) at Re=100.We can see clearly from the plots that a peak is generated in the average surface Nusselt number line in the plot where the baffle is placed in the channel compared with the Nusselt number line of channel with no baffle case. 3.3 Pressure Characteristics The enhancement of heat transfer achieved by using baffle is associated with an increase in the pressure loss. Surface Nusselt Number Plot Figure 3.2 Pressure distribution plot Figure 3.3 Figure 3.3 shows the pressure variation along the channel length with a baffle height 5cm and 6cm at ISSN 2278-5787 Page 15
Reynolds Number 100. The results show that the pressure loss increases as the baffle height is increased. The figure shows that maximum pressure drop occurs just downstream of baffle because of the formed drag and then pressure is recovered and approaches a stabilized value. Percentage error= (0.01947-0.0192/0.01947)*100=1.3% 4. VALIDATION OF RESULT Fig. 4.2 velocity variation at the outlet along the REFRENCES channel height Fig.4.1 velocity vector plot For validation of result the approximate results are compared with the theoretical results. Theoretically the horizontal velocity for a 3-D laminar flow is given by: U=2*V inlet (1-(r 2 /R 2 )) U=horizontal velocity V inlet = inlet velocity R=0.075 r=0 for velocity at centerline of channel and 0.075 at the wall of channel For finding the outlet velocity at the centerline of the channel put r=0 and R=0.075 and inlet velocity=0.009738m/sec U cl =2*0.009738 (1-(0 2 /0.075 2 ))=0.019476m/sec Form fig.5.1 the outlet velocity at the centerline is 0.0192 m/sec 1. Nasiruddin, M.H. Kamran Siddiqui, HEAT TRANSFER AUGMENTATION IN A HEAT EXCHANGER TUBE USING A BAFFLE, International Journal of Heat and Fluid Flow 28 (2007) 318 328. 2. Yue-Tzu Yang, Chih-Zong Hwang, CALCULATION OF TURBULENT FLOW AND HEAT TRANSFER IN A POROUS-BAFFLED CHANNEL, International Journal of Heat and Mass Transfer 46 (2003) 771 780 3. M. R. Asif, M. S. Hossain and K. A. Hossain, HEAT TRANSFER IN A RECTANGULAR ENCLOSURE WITH BAFFLES, ARPN Journal of Engineering and Applied Sciences VOL. 6, NO. 4, APRIL 2011 4. R.M. Majumdar and V.M. Kriplani, INTERNAL COOLING AUGMENTATION USING INCLINED PERFORATED BAFFLES IN RECTANGULAR CHANNEL, International Journal of Mechanics and Thermodynamics. Volume 2, Number 2 (2011), pp. 113-126 5. S.R. Hiravennavar, E.G. Tulapurkara, G. Biswas, A NOTE ON THE FLOW AND HEAT TRANSFER ENHANCEMENT IN A CHANNEL WITH BUILT-IN WINGLET PAIR, International Journal of Heat and Fluid Flow 28 (2007) 299 305. 6. A. Sohankar, HEAT TRANSFER AUGMENTATION IN A RECTANGULAR CHANNEL WITH A VEE- SHAPED VORTEX GENERATOR, International Journal of Heat and Fluid Flow 28 (2007) 306 317. 7. S. Tiwari, D. Chakraborty, G. Biswas, P.K. Panigrahi, NUMERICAL PREDICTION OF FLOW AND HEAT TRANSFER IN A CHANNEL IN THE PRESENCE OF A BUILT-IN CIRCULAR TUBE WITH AND WITHOUT AN INTEGRAL WAKE SPLITTER, International Journal of Heat and Mass Transfer 48 (2005) 439 453. 8. S.P.WALDE, V.M.KRIPLANI, REVIEW OF HEAT TRANSFER ENHANCEMENT IN DIFFERENT TYPES OF BAFFLES AND THEIR ORIENTATIONS, S.P.Walde et al. / International Journal of Engineering Science and Technology (IJEST). ISSN 2278-5787 Page 16