5 Highlights for A 3D-CFD model thermal analysis within a scraped surface heat exchanger
In this paper, the coupling of fluid flow and heat transfer in scraper surface heat exchanger is numerically studied. Finite volume fluent ™ 6.3 the code is used to solve the continuity, momentum and energy equations in the actual SSHE geometry using the multi rotation coordinate system formula. The mesh of real SSHE geometry is realized by gambit ™ 2.2.3 to consider geometric singularity and its influence on heat transfer performance in SSHE. The steady-state laminar ion thermal flow of pure glycerol, 2% CMC solution and 0.2% Carbopol solution was studied. The phase change free cooling process of SSHE was studied. The thermal correlation is established by numerical method and verified by the given thermal correlation in the literature. The numerical results are in good agreement with the thermal correlation given in the literature. The thermal performance of SSHE is then examined in more detail using a numerical model. The effect of rotation speed on SSHE thermal performance was studied. For pure glycerol, the increase of rotation speed significantly reduces the cooling process due to viscous heating. Considering the non-Newtonian fluid, the increase of rotating speed improves the thermal efficiency of SSHE. The temperature distribution at the blade tip shows that blade fixation has an important impact on the thermal performance in shhe. The viscous heating of Newtonian viscous fluid is studied. The numerical results show that in the case of glycerol, when the rotating speed is x = 9 Rev S1, the total energy can be 25% higher than that without viscous heating. Finally, the study of mixing time shows that when the mixing time is equal to Tmix = 6.72 s, the thermal performance of mixing time is the best.
► A 3D numerical model was achieved in order to study SSHE flow and thermal behavior.
► The analysis was based on the thermal study of an SSHE.
► Three rheological fluids behaviors were tested.
► Thermal behavior of an SSHE was examined in real geometry.
► The mixing time was optimized to reach desired product texture.
Successful 3D-numerical model was achieved to examine the thermal performance of a scraped surface heat exchanger device. The geometry mesh was achieved with a hexahedral mesh for better discretizations of momentum and energy equations. Newtonian and non-Newtonian fluids were examined. The convective heat coefficient was numerically computed in several operating conditions of SSHE, in order to calculate the Nusselt number in the correlation. Most of the general form of the correlation in SSHE given in the literature was Nu ¼ aReb r Pr0:33. The numerical results showed a good agreement in comparison with the literature-given heat correlations. The numerical model was used to examine the heat performance of the exchanger, with varying blades speed and mass flow rate. It was shown that in the case of 2 wt.% CMC solution and 0.2% carbopol solution, increasing the rotating velocity improves the heat transfer. The best average temperature at the exit of SSHE was obtained when the rotating velocity was increased. In the case of the Newtonian fluid, the increase of the rotating velocity dramatically decreases the heat performance of the SSHE, due to viscous heating. The plotting of temperature profiles in axial direction has shown asymptotic behavior, and temperature profiles distortion is observed in the wake of attachment points of blades.
Analysis of the mass flow rate on the heat performance of SSHE was undertaken; it was shown that the increase of the mass flow rate reduces the efficiency of the SSHE in the case of 2 wt.% CMC. The numerical model developed allows to solve energy equation by and without taking viscous heating term in o account. This can give the real effect of viscous heating on the thermal performance of the exchanger. To demonstrate the effect of viscous heating on the heat performance of SSHE, we have solved the energy equation with and without the viscous term in Eq. (9). For the Newtonian Glycerin fluid, and in the case of rotational speed of the rotor of x = 9 rev s1, the dissipated energy is about 25% higher than without considering viscous heating. It was shown that viscous heating increases with rotating velocity. The developed numerical model will be used to examine in more details the crystallization phenomena in an SSHE using population balance. Geometrical optimization of the device is in progress, in order to improve the heat treatment received by the fluid. Finally, the study of the residence time effect on the axial temperature evolution has clearly shown that for a given value of tmix = 6.72 mn the treated product reached the desired cooling temperature.
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