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Evaluation and prediction methods for heat exchangers have significantly developed over the years where the equal flow Reynolds number, equal flow velocity, and Wilson plot techniques are commonly used. However, equal flow Re and equal flow velocity techniques impose restrictions on the flow conditions and the flow passage configuration. Similarly, Wilson plot technique is applicable under a known exponent of Re . Using these techniques, it is difficult to obtain generalized correlations even for single passage because many unknowns are needed to be solved. Therefore, a simplified and handy method to develop heat transfer correlation is required to design new equipment or predict their performance. A general variance minimization method is presented to avoid the need for determination of various coefficients and exponents used in regression process. This method is validated by comparing its results with the experimental data. [ABSTRACT FROM AUTHOR]

The flow fields and heat transfer characteristics in a parallel-plate channel with a transversely placed spiral coil insert were investigated by three-dimensional numerical simulation. The structure of multi-longitudinal-vortices (MLVs) induced by the spiral coil and the effects of MLVs on velocity and temperature fields were studied. The three-dimensional spiral coil induces a series of longitudinal vortices in the channel including leading longitudinal vortex, mainstream longitudinal vortices, near-wall longitudinal vortices, and rear central longitudinal vortex. Transport by the longitudinal vortices can increase the mass exchange between the boundary layer and the mainstream, which speeds up the heat migration from the channel walls and enhances the heat diffusion in the mainstream. [ABSTRACT FROM AUTHOR]

Abstract A numerical study of three-dimensional (3D) MHD laminar flow in a short horizontal cylindrical annulus has been performed to characterize the impacts of a uniform axial magnetic field on both hydrodynamic and thermal behaviors. This configuration corresponds to a horizontal annular mold containing molten iron or low carbon steel in electromagnetic casting (EMC). Complex nonlinear governing equations are solved numerically by means of the finite volume method based on artificial compressibility algorithm. The induced electric potential caused by the magnetic field is considered. Results show that a typical 3D steady spiral flow arises and the symmetry breaking occurs under a weak magnetic field of Ha < 10. As the Hartmann number increases, the spiral flow is distinctly suppressed, and then it changes to a helical flow with two transverse cells adjacent to each of the end walls. Correspondingly, the flow patterns and isotherms of fluid become symmetric with respect to the mid-axial plane of the annulus. Further increasing the Hartmann number, the fluid particle trajectories essentially lie in the cross-section of the annulus, as for two-dimensional solutions, with an extremely small axial dependency. In addition, both the values of local Nusselt number at the isothermal walls and its axial dependency, mainly in the upper region (− 3 8 π ≤ φ ≤ 3 8 π), are significantly affected by the magnetic field. The results of the present work can be useful in producing high-quality products by magnetic control. Highlights • Axial-symmetry breaking occurs under a weak magnetic field of Ha < 10. • Magnetic field leads to the transition of flow from asymmetric spiral to symmetric helical, and finally quasi two-dimensional. • Heat transfer rates at the inner and outer cylinder walls are significantly affected by the magnetic field. [ABSTRACT FROM AUTHOR]

In this study, condensation of air–steam mixture bubble with mass fraction of steam above 0.5 was investigated first by a visual experiment at atmosphere. Then a 3-D numerical model based on volume of fluid (VOF) model and species model was developed to simulate the bubble condensation. In order to model steam condensation, mass and energy transfer between phases were modeled as source terms of conservation equations, using a correlation obtained from the experiment to predict the condensation heat transfer coefficient (HTC). After validation of the numerical model with the experimental results, influences of steam fraction and diameter of bubble on condensation characteristics are studied numerically. Moreover, steam concentration distribution in bubble was also analyzed along the time series of bubble condensation process. It was found that condensation HTC decreases with the increase of bubble diameter. With increase of steam fraction in bubble, the bubble volume shrinks more quickly due to the increased condensation rate, and the bubble accelerates more quickly reaching a higher terminal velocity. Aggregation of non-condensable air inside the bubble near the gas and water interface deteriorates the condensation heat and mass transfer. [ABSTRACT FROM AUTHOR]

Abstract: Experimental and theoretical investigations of water-side fouling have been performed inside four corrugated plate heat exchangers. They have different geometric parameters, such as plate height, plate spacing, and plate angle. Fouling experiments including particulate fouling tests and composite particulate and precipitation fouling tests have been performed. The tests are primarily focused on the effects of concentration and average velocity. Scanning electron microscope (SEM) was used to investigate the microscopic structures of composite fouling. Heat transfer coefficients and friction factors have been obtained in clean tests. The plate heat exchanger with the largest d e and height to pitch ratio shows the best anti-fouling performance. The Von-Karman analogy was used to obtain mass transfer coefficient to develop appropriate semi-theoretical fouling correlations; The wall shear stress is obtained from the pressure drop. The model predictions agree well with experimental data. [Copyright &y& Elsevier]