NUMERICAL INVESTIGATION OF THE BLADES EFFECT ON FLUID VELOCITY AND TEMPERATURE FIELDS IN SCRAPED SURFACE HEAT EXCHANGER

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1 ISSN : (Print) ISSN : (Online) NUMERICAL INVESTIGATION OF THE BLADES EFFECT ON FLUID VELOCITY AND TEMPERATURE FIELDS IN SCRAPED SURFACE HEAT EXCHANGER HOSSEIN OKHOVATI a1, AMIN NAMJOO b AND MORTEZA ABDOLZADEH c a Department of mechanical Eng., Sirjan science and research branch, Islamic azad university, Sirjan, b Department of mechanical Eng., Sirjan science and research branch, Islamic azad university, Sirjan, c Department of mechanical Eng., Sirjan science and research branch, Islamic azad university, Sirjan, ABSTRACT The scraped surface heat exchangers are consisting of two concentric cylinders, that inner one is rotating and outer one is stationary. The fluid flows in the annular region between two concentric cylinders. The inner cylinder is also equipped with blades. Since the quality of product under process at the devices is strongly influenced by the velocity field and the fluid temperature, the aim of present work is to investigate the effect of blade on the velocity and the temperature fields numerically. The results show comparable agreement with previous works. Among the important results of the research, can cited to reduce the difference between the maximum and minimum fluid pressure with increasing the number of blades. In the upper blade region of SSHE from rotor to blade, the fluid velocity increases with increasing number of blades,but the intensity of velocity gradients will be reduced. From blade to stator, little change in the fluid velocity profiles can be seen. Also in the blade region by increasing the number of blades, the fluid temperature increases and the heat transfer characteristics of flow will be improved. Keywords: Heat Exchanger, Scraped surface, Taylor Couette flow,simulation The viscous flow in the annular region between two concentric cylinders, when the inner one is rotating and the outer one is stationary, is known as Taylor Couetteflow. Beyond a certain critical speed of the rotating cylinder, the flow will become unstable which results appearance of vortices in the flow.these vortices strongly affect the velocity and temperature fields. Different flow regimes can be seen depend on the speed rate of the rotating cylinder. One of the usage of the above flow is in scraped surface heat exchangers (SSHE), which inner rotor was equipped with blades along the length. Scraped surface heat exchangers are commonly used in the food and chemical industries. Existence of rotational blades is used to reduce the fouling and improve the heat transfer which will finally results a uniform product.figure 1, shows a schematic of a SSHE. Figure1.SSHE schematic This flow regimes and behavior characterized by adimensionless numbercalled the Taylor number (Ta) or the rotational Reynolds number, which defined as: = (1),,h, represent the fluid density, tangential speed of inner cylinder and the gap distance between two cylinders, respectively. Tangential speed of inner cylinder is obtained as follow. = (2) Where is the radius of the inner cylinder and is the angular velocity of the rotor. Compare to the Couette-Taylor flow, research on SSHE are not too much. This is due to the complexity of existence of Taylor instability together with rotating scraper blades. Dumont and coworkers extensively studied the effect of blade on shear rate incouette-taylor vortex regimes. The shear rates were measured electro chemically in simple annular flow and in SSHE with twoblades. The measured wall shear rates using electro chemical method for the annular flow 1Corresponding author

2 revealed that at Taylornumber 45 the transition could occur from laminar to vortex flow. It was found that the presence of blades could increase the shear rate times higher than the present in the same geometry without blades. Also, the transition from laminar to turbulent regime occurred at relatively higher Taylor number 90 for the two blades SSHE. High shear rate in the tip of the blades can cause mechanical damage to food particles(dumont et al., 2001). Dumont and co- workers (2000) studiedthe influence of the blades and rotational speed on the thermal characteristics of the products treated.they have shown somethermal modification of products can be achieved at low Taylor number (particularlyin the birth of vortices near the outlet)(dumont et al., 2000).Mabit et al. (2003) studied the effect of floating bladesin SSHE and also they carried out the experiments in absence of blades andstudied the flow patterns, including laminarcouette flow laminartaylor flow, turbulent Taylor flow and vortextaylor flow. The effect ofblades on the starch suspension in SSHE was studied experimentally by Mabit et al. (2003). Mechanical swelling of starch was used to quantify the volume fraction of processed product undergoing high shear rate. Soos et al (2007) studied the Taylor-Couette flow geometry with lobed inner cylinder instead of circular one. The specific design was made to reduce or deform the Taylorvortices and therefore to improve the mixing in the core of thevortices. The particle motion and their adherence on heated surface in SSHE was studied by Rodriguez et al. (2009). Their aim was a design for scraper geometry in order to preventing deposit on the heat exchanger surface. Yataghene et al. (2011) studied the flow patterns inside scraped surface heat exchangers under isothermal and continuous flow Continuity equation:. =0(3) Momentum equation: Numerical Simulation In figure 2 the geometry of the problem is shown. The Rotor and stator radiuses are considered 4 and 5 cm, respectively. The blade conditions experimentally. Axial, radial and tangential velocity components have been measured for Newtonian and non-newtonian shear thinning fluids. The analysis of the axial velocity inside SSHE showed the presence of very large axial heterogeneities and tangential velocity dominates around blades, and the maximum is reached near the tip of the blade. The results showed that under certain experimental flow conditions of rotating velocity and axial flow rate, a more effective flow can be obtained. They have also shown the mass flow rate does not affect the tangential velocity components. The axial heterogeneities were reduced by increasing mass flow rate (Yataghene et al., 2011). It can be seen that most of the investigation on SSHEs are based on experiments and a few numerical works can be found in the literatures. Due to this fact, the present work focus on the blade effect of the temperature and velocity field in the SSHEs numerically.numerical modeling has been based on the assumption that the mass flow rate and flow velocity along the axis of exchanger is low than the inner cylinder rotation speed,so the flow can be considered two dimensional. Also rotational speed range is intended which instabilities that are causing three dimensional flows, are not present. Governing Equations Flow and heat transfer are described with continuity, momentum and energy equations (Izadi et al., 2009). The flow assumed to be laminar, steady, two dimensional and incompressible. Also the viscous dissipation term in the energy equation can be neglected. By these assumption the governing equation simplify as follows.. = + V(4) Energy equation:. = (5) angle which is shown in figure 3 is considered to have the fixed valueof 147 degree and a distance of 0.25 cm from the rotor. Figure 4 shows the grid around the blade with triangular elements.

3 Figure2.The SSHE geometry Figure 4.The created grid around blade Figure 3.Blade angle Rotor and connected blades to it are rotating clock wise with constant angular velocity. The reason for triangular elements for the grid is sharp corners blades, that the created grid with these elements has better uniformity than tetrahedral elements. Finite volume method is used to solve the governing equations. Fluid properties are given in table 1. Fluid Density Table1.The physical properties of fluid Specific heat capacity. Thermal conductivity. Viscosity. Engine-oil In order to prevent using sliding mesh due to rotating blade, the problem has been solved by using the rotating reference frame (FLUENT 6.3 User's Guide, 2006). Through this, the rotor and blades areassumeto be stationary and fluid rotates between rotor and stator with angular velocity in the opposite direction. RESULTS AND DISCUSSIONS Mesh study In order to investigate the independency of the results from the grid size, the desired grid was refined in four steps. The selected geometry for this purpose is a SSHE with two blades, 150 degree blade angle.the rotational Reynolds number is equal to 10. Figure5 showsvelocity profiles in blade region with various number of meshess where the largest gradients occur.

4 Figure 5.Results of the independent review of the created grid Increasing the number of grid nodes from 6000 nodes to nodes, creates 4.62% change in velocity profile around the blade and with increasing the number of grid nodesfrom nodesto nodes, the percentage difference is reduced to 1.96% and with increasing the number of gridnodes from to 36000, this percentage difference is about1%. So the grid consists of nodes is selected because of accuracy and computational cost. The accuracy of numerical method In order to verify the results accuracy, the velocity profile in the blade section for the rotational Reynolds number 10 and angle of blade 147 degree obtained and the results are compared withnumerical and experimental results of Stranzinger s work(stranzinger et al., 2001). Figure6 shows this comparison and very good agreement can be seen. The error is less than 1%. Figure 6.Comparison of present work with reference work

5 The effects of the blades number Figures 7A to 7E show the stream function for various numbers of blades. In all case, the rotational Reynolds number is considered to be 10 and the blade angle is 150 degree.in addition to the considered conditions, is consider the temperature of rotor 75 C and the temperature of stator 50 C. Figure 7A.Stream function with two bladefigure 7B.Stream function with three blades. Figure 7C. Stream function with four blades. blades. Figure 7C. Stream function with five Figure 7E.Stream function with six blades. As can be seen in figures7a to 7E, increasing the number of blades will increases the number of created vortices and the number of vortices is equal to number of SSHE's blades. Figures8A to 8E showspressure contours for the various numbers of blades.the results show apart from other parts of blade, pressure distribution is almost uniform.

6 Figure 8A. Pressure contour with two blades. Figure 8B.Pressure contourwith three blades. Figure 8C. Pressure contour with four blades. Figure 8D. Pressure contour with five blades. Figure 8E.Pressure contour with six blades. The effect of blades on pressure distribution in blade region can be seen in figure 9. As can be seen, heat exchanger with three blades has largest absolute pressure and the one with two blades has the lowest absolute pressure.

7 Figure 9.investigation of the effect of the number of blades on the pressure profiles in blade region Table 2 shows the effect of the number of blades on maximum and minimum pressure and difference between the maximum and minimum pressure in SSHE.The results show the difference between the maximum and minimum fluid pressure decreases, with increasing the number of blades. Table 2.Investigation of the effect of number of blades on maximum and minimum pressure and the pressure difference Number of blades Maximum pressure(kpa) Minimum pressure(kpa) Pressure difference(kpa) The effect of blades number on velocity contours is shown in figures 10A to 10E. According to figures 10A to 10E, it can be seen that increasing number of blades only affects the number of created vortices and does not change the shape of vortex. So the created vortex between the two blades in SSHE with two blades is quite similar to the vortex between the two blades in SSHE with six blades.

8 Figure 10A.Velocity contour with ttwo blades.figure 10B.Velocity contourwiththree blades. Figure 10C.Velocity contour with four blades. Figure 10D.Velocity contour with five blades. Figure 10E.Velocity contour with six blades. The fluid velocity field in blade region with different number of blades is shown in the figure 11. As shown, in the left part of diagram, which is from the rotor toward blade, increasing the number of blades increases the velocity but the intensity of velocity gradients is reduced. While in the right part of diagram that is from blade toward stator. We are not observed significant change in the fluid velocity.

9 Figure 11.Investigation of the effect of number of blades on the fluid velocity field in blade region Table 3 shows maximum velocity for various numbers of blades. It can be seen that number of blade does not have significant effect on the maximum velocity in SSHE. Table 3.Maximum velocity respect to different number of blades The figures 12A to 12E show temperature contours for various numbers of blades. As the Number of blades 2 3 number 4 of 5 blades 6 increases the heat transfer improve specially at blade region. Maximum velocity (m/s) Figure 12A.Temperature contour with two blades.figure 12B.Temperature contour with three blades.

10 Figure 12C.Temperature contour with four blades blades. Figure 12D.Temperature contour with five Figure 12E.Temperature contour with six blades Fluid temperature profiles in blade region with the different number of blades are shown in figure 13.As can be seen in figure 13, with increasing the number of blades in blade region will increase temperature and the heat transfer also will be improved.

11 Figure 13.investigation of the effect of the number of blades on the fluid temperature field in blade region CONCLUSION In recent study, the effect of the number of blades on the flow and heat transfer of fluid in SSHE by using finite volume method is investigated. Comparing the results with pervious research shows good agreements. In the upper blade region of SSHE from rotor to blade, the fluid velocity increases with increasing number of blades. But the intensity of velocity gradients is reduced. From blade to stator, significant change is not found in the fluid velocity. Increasing the number of blades REFERENCES Dumont, E, Fayolle, F., Legrand, J.; Flow regimes and wall shear rates determination within a scraped surface heatexchanger. J. Food Eng,vol. 45, pp Dumont,E., Della Valle, D., Fayolle, F.; Influence of flowregimes on temperature heterogeneities within a scrapedsurface heat exchanger. J. Food Process Eng,Vol.23, pp Mabit,J., Fayolle, F., Legrand, J.; Shear rate investigation in a scraped surface heat exchanger. Chem. Eng. Sci,Vol.58,pp , Mabit, M., Loisel, C., Fayolle, F., Legrand, J.; Determinationof the volume fraction submitted to high shear in a Izadi, M., Behzadmehr, A., Jalali-Vahid, D.; Numerical study of developing laminar forced convection of a nanofluid in an annulus.international Journal of Thermal Sciences,vol. 48,pp FLUENT 6.3 User's Guide, Fluent Inc, September Stranzinger, K. Feigl, E. Windhab.; Non- Newtonian flow behaviour in narrow annular gap reactors.chemical Engineering Science,vol. 56,pp reducethe difference between the maximum and minimum fluid pressure.however, apart from the blades, pressure distribution is almost uniform. Heat exchanger with three blades have the largest absolute pressure while, the lowest absolute pressure is found in the heat exchanger with two blades. Increasing the number of blades will cause temperature increases in the blade region and so will improve the heat transfer process. scrape surface heat exchanger. J. FoodEng,Vol.57, pp Soos, M., Wu, H., Morbidelli, M.; Taylor Couette unit with a lobed inner cylinder cross section. AIChEJ. Vol.53, pp Rodriguez Pascual, M., Ravelet, F., Delfos, R., Witkamp, G.J.; Large eddy simulations and stereoscopic particle image velocimetry measurements in a scraped heat exchanger crystallizer geometry. Chemical Engineering Science,vol.64, pp Yataghene, M., Fayolle, F., Legrand, J.; Flow patterns analysis using experimental PIV technique inside scraped surface heat exchanger in continuous flow condition. Chemical Engineering and Processing: Process Intensification,vol.47, pp