الفهرس 
1.3. Techniques of Heat Transfer Enhancement
CHAPTER 3: NUMERICAL METHOD AND VALIDATIONOnly 14 pages are availabe for public view
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Abstract
In the present work, the enhancement of the performance of double pipe counter flow heat exchanger by both swirl flow and nanofluids is numerically and experimentally investigated. The swirl flow is generated using 90º-one, two, and four tangential slots at the inlet of the outer pipe. Different slot angles (10, 15, 30, 45, 60, and 90º) are also investigated. The heat transfer enhancement using nanofluid mainly depends on the type, size, and concentration of nanoparticles in the base fluid. The used nanofluids in this study are formed by suspending nanoparticles in water (bass fluid). The volume concentrations of four different types of nanoparticles, Al2O3, CuO, TiO2 and ZnO in the range of 0% to 3% and nanoparticle diameters in the range of 20 nm to 50 nm are considered in this study. The present numerical results are obtained using the simulation of steady
three-dimensional, incompressible, Reynolds-Average Navier-Stokes (RANS)
equations, which are solved by employing ANSYS Fluent 15. Eight turbulence
models, including standard k–ε, realizable k–ε, RNG k–ε, SST k–ω, non-linear k–ε, v2 -f, RSM (Quadratic Pressure-Strain Model), and RSM (Stress-Omega Model), are tested. All these turbulence models are available directly in the ANSYS Fluent except the non-linear k–ε model which is implemented in the solver using User Defined Functions (UDF). Validation of the numerical methodology is performed using the available experimental data of swirl flow from the literature. The results
show that the RSM (Quadratic Pressure-Strain Model) performs better than the other models. Therefore, the present computations are carried out using RSM (Quadratic Pressure-Strain Model). The experimental study is conducted to provide experimental data for validation of the mathematical model. The present numerical prediction for heat transfer and pressure DROP gives acceptable results compared with the available empirical correlation and experimental data from literature as well as
the present measurements. The obtained for Nusselt number, effectiveness, pressure loss coefficient, and enhancement factor are presented and compared with the classical cases (conventional and axial flow cases). It is found that as the number of
tangential slots increases, the heat transfer and pressure DROP decrease, but the enhancement factor is greater than unity for all values of Reynolds number. The slot with tangent angle 30º has the highest heat transfer and enhancement factor compared to the other tested cases. The highest values of the enhancement factors are about 1.165, 1.168, 1.21, 1.164, 1.05, and 0.96 for the tangent angle of 10, 15, 30, 45, 60, and 90º, respectively. Therefore, the slot with a tangent angle of 30º is the best angle that achieves the highest enhancement factor. All swirl flow cases conducted in this research lead to energy saving and can be suggested as effective methods except the slot with tangent angle 90º. On the other hand, all nanofluid types achieve better heat transfer enhancement compared to pure water, but with slight increase in the pressure drop. The Al2O3 nanomaterial has better thermal enhancement characteristics followed by CuO, ZnO, and TiO2, respectively. The heat transfer and pressure DROP increase with increasing volume concentration of
nanofluid. As the particle diameter decreases, the heat transfer as well as the effectiveness increase and there is a slight variation in the pressure drop. The use of nanofluids can be suggested as being effective method for enhancing the performance of heat exchanger. Finally, the correlations for the Nusselt number, pressure loss coefficient and enhancement factor of the heat exchanger are provided,
based on the present results.