The increasing demand to enhance the efficiency of heat exchangers has sparked numerous investigations aimed at increasing heat transfer rates while simultaneously reducing the size and cost of industrial equipment. Among the various apparatus utilized in different industries, the double-pipe heat exchanger has garnered significant attention due to its simplicity and versatile applications. Over recent years, numerous meticulous and invaluable studies have delved into double-pipe heat exchangers. This review meticulously analyzes the developmental trajectory of this heat exchanger type while extensively discussing methods for enhancing heat transfer within these systems. In striving to present a comprehensive overview, the authors have meticulously gathered information on various enhancement methods, including active and passive. Recent studies exploring passive heat transfer augmentation methods in double-pipe heat exchangers have been summarized. These methods are summarized under surface modification (like dimples, vortex generators, and protrusion), inserts (like twisted tapes and helical coil), and extended surfaces (like fins and baffles). The prime objective of the current study is to organize the literature related to combining different heat transfer augmentation methods. An additional section on alternating cross-sectional tubes used in double-pipe heat exchangers has been summarized, exhibiting increased vorticity without vortex generator devices. Longitudinal vortices are created throughout the tube's length, leading to a notable improvement in its thermal efficiency. Furthermore, detailed discussions on using Nanofluids in these heat exchangers are provided. Additionally, correlations, primarily focusing on the Nusselt number and pressure drop coefficient, are presented within this review. This comprehensive review is anticipated to offer valuable insights for future investigations in this field.
The impact of a magnetic field on the convective heat transfer of ferrofluids from a heated sphere immersed in a porous medium is investigated. The dimensional governing boundary layer equations are initially transformed into a convenient non-dimensional form utilizing the non-dimensional variables. The resulting nonlinear systems of equations are then numerically solved inside the computing domain into a regular rectangle using the effective Finite Difference Method (FDM). Numerical outcomes are then represented in terms of local Nusselt number, velocity, temperature profile, and skin friction coefficient, respectively for a range of porosity parameters, ϵ = 0.4, 0.6, 0.8, magnetic effect parameter or Hartmann number, Ha = 0.0, 1.0, 3.0, 5.0 and the ferroparticle volume fraction coefficients, ϕ = 0%, 2%, 4%, 6%. It is thought that the base fluid’s Prandtl number, Pr=6.8733, is constant. In addition, the flow pattern inside the boundary layer region is shown using streamlines and isotherms, and the underlying physics of the flow behavior is then explored. There is a graphical presentation of the data. The findings show that velocity decreases with increasing value Ha, ϕ and ϵ. An increase in the Hartmann number Ha causes the temperature to rise. The local N_u and C_(f )are decreasing as Ha and ϕ values increase. With an increase in the porosity parameter ϵ and ϕ, the temperature profile rises and the C_f and local N_u decrease. For increasing Ha, the figure of streamlines seems to depict functions with more gradual changes, and for isotherms, it represents functions with sharper, exponential-like increases. While many works focus on ferrofluids or porous media individually, combining the study of heat transfer in ferrofluids within porous structures can represent a distinct focus. The problem is crucial for developing advanced heat transfer technologies for more efficient energy management in various engineering applications.
Fluidized bed apparatuses are widely used in chemical engineering. Description of the hydrodynamic state of the apparatus is the starting point for predicting most technological operations in gas-solid flows. The object of this study is the gas and solids distributions in a fluidized bed column. The key aim of the study is to develop a simple yet informative mathematical model of the migration of gas and particulate solids in a fluidized bed column. The model is developed to solve the problem in a two-dimensional formulation. The phase migrations of the fluidized bed along the height of the column are described on the basis of the mathematical apparatus of the theory of Markov chains, and an explicit difference scheme is used for the mathematical model of particle transfer in the radial direction. A cell of small but finite size acts as a representative volume of the simulated system. The representative volume of such geometry is apparently used for modeling the motion of fluidized bed phases for the first time. At the same time, it is precisely this model structure that corresponds to the tradition of identifying the radial and axial coefficients of particle macrodiffusion. Parametric identification of the model is carried out on the basis of the empirical relationships known from the literature. The numerical experiments performed in the study showed the qualitative consistency of the proposed model. A comparison of calculations with the results of a natural experiment also confirmed the presence of predictive capabilities in the model. Thus, the proposed model can be considered as a reliable scientific basis for computer methods for calculating fluidized bed devices.
Considering the excellent properties of hybrid nanofluids, it is a good idea to use them to improve the performance of solar collectors. The performance of a parabolic trough solar collector equipped with an internally finned absorber tube (IFAT) has been numerically studied based on the environmental, economic, exergy, and energy perspectives. The working fluid of the collector is hybrid nanofluid (HNF), consisting of of MWCNT and Fe3O4 nanoparticles and Therminol VP-1 as base fluid (BF). The volume fraction of nanoparticles is 3%, the operating fluid temperature is 600 K, and the Reynolds number is in the range of 2×104-2×105. The fluid flow is simulated using Ansys-Fluent software. Environmental analysis was done using the life cycle method, and economic analysis was based on the levelized cost of energy. Investigation of the results showed that the energy and exergy efficiencies of the collector when using the IFAT is higher than the smooth absorber tube (SAT), and using HNF is higher than base fluid. The maximum energy efficiency enhancement of the PTC using IFAT-BF, SAT-HNF, and IFAT-HNF compared to SAT-BF is 4.93, 1.49, and 5.35%, respectively, and the maximum exergy efficiency enhancement is 6.09, 1.6 and 6.6%, respectively. Based on the economic approach, it was observed that for the Re lower than 5×104, the use of PTC with IFAT is more economical than the PTC with SAT. The environmental analysis showed that the use of the PTC with SAT compared to PTC with IFAT and also the use of HNF instead of the BF produces less CO2 in the life cycle of the collector.
Improving heat transfer in thermal systems has become a focus of many research studies due to the critical need for efficient waste of residual heat. The regulation of heat transfer between components in thermal systems has a direct impact on their efficiency and performance. As a result, effective heat management is critical to improving the efficiency of thermal systems and extending the life of their components. It should be noticed that baffles are important structural components widely used in various industrial applications like heat exchangers, solar collectors, electronic cooling, etc. In addition, baffles enhance fluid mixing and heat transfer behaviors. Most industrial systems do not operate in a steady state. In particular, transient phases occur during start-up and shut-down or during the control phase of controlled systems. Thus, in laminar flow, baffles induce flow unsteadiness or help the flow to bifurcate from a steady state to an unsteady flow. This paper treated the effects of different baffle shapes incorporated in channels on heat transfer rate, efficiency and friction factor in mixed and forced convection cases. Various experimental and numerical studies have been carried out on this topic to examine heat transfer enhancement compared to the flow energy. It was noticed that increasing the Reynolds number, blockage ratio and decreasing the Grashof number can achieve an increase in heat transfer. The maximum heat transfer enhancement was obtained for higher blockage ratio and higher Reynolds number in forced convective flow. The highest heat transfer improvement was obtained for the 45° angled baffles (between 150% and 850%). In mixed convective flow, the highest rate of heat transfer was obtained for transverse baffles (2.8 times compared to a similar channel with no baffles). Finally, This comprehensive review is beneficial for researchers focused on flow and heat transfer applications to use other baffle designs and fluids beyond air.
