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The present paper investigates the thermal mixing and cooling processes in a passive micromixer, which is applicable for the cooling of electronic devices. Employing a porous block and testing different configurations for the mixing channel is considered to enhance the mixing process and cooling performance. A 2D lattice Boltzmann thermal model is utilized to investigate the thermal performance of a T-micromixer with a porous block. Two different types of mixing channel configurations, including a step-shaped and a zigzag-shaped channel, are considered, and the obtained results are compared with those of the simple mixing channel. The thermal mixing and cooling of two miscible fluids, at 50 and 25°C entering the micromixer, are investigated. The results show that changing the mixing channel configuration may create a chaotic laminar flow, which enhances the heat transfer rate between the mixed flow and the channel wall. Whatever the Reynolds number, the step-shaped mixing channel exhibits better mixing performance than the zigzag-shaped one. For the T-micromixer with a zigzag-shaped and step-shaped mixing channel, the cases with h/H = 0.5 and h/H = 0, respectively, exhibit better thermal mixing and cooling performance. [ABSTRACT FROM AUTHOR]

In this paper, the thermal performance improvements of a heat removal system like an electronic system have been analyzed. The studied case is a horizontal channel in which two partially inclined baffles are attached with variable height and number. The channel is crossed by a forced convective flow of a cooling fluid (air). This numerical work evaluates the influences of the height and number of the baffles on the enhancement of the heat transfer rate. The mathematical model of this system is composed of nonlinear partial equations that the analytical solution for them is very complex, hence the need for numerical analysis is mandatory with the aid of a finite volume method. Accordingly, The numerical results are presented in axial and transverse velocity, temperature, local and average Nusselt number, local friction coefficient, pressure drop, heat transfer rate, and turbulence kinetic energy. The results revealed that it is possible to improve the thermal performance of the considered system by adopting designs that allow the maximum heat transfer rate with the minimum energy loss. In addition, results show that at the lowest Reynolds number (Re = 10,000), as the height of baffles rises from 0.01 to 0.03 m (growth by 200%), the heat transfer rate augments about 59.09%. Moreover, at the highest evaluated Reynolds number (Re = 87,300), by increasing the height of baffles up to 200%, the heat transfer rate increases by approximately 50.53%. Furthermore, employing a higher number of baffles leads to more heat transfer rates and a significant pressure drop. [ABSTRACT FROM AUTHOR]

Summary: Frequent power outage in developing countries has created many problems for the people living in these regions, one of the most important of which is food spoilage due to the rise of refrigerator temperature. Ice storage systems are one of the promising techniques for handling this difficulty. Computational simulations are done here to influence the effects of dimensionless parameters on the charging rate of a shell and dual coil ice storage unit equipped with connecting plates as heat transfer enhancers. The ice storage unit is intended to be used as a backup cooling source for refrigerators in these regions. The studied parameters include the helical pitch length/storage height ratio (α1), the helical coil distance/storage diameter ratio (α2), the helical coil diameter/storage diameter ratio (α3), the connecting plate length/storage height ratio (α4), the connecting plate thickness/tube diameter ratio (α5), the modified Stefan number of the refrigerant flow (Ste*), and refrigerant flow Reynolds number (Re). The results suggest that the geometrical optimization of the proposed ice storage with α1, α2, and α3 parameters can improve the charging process up to 16.69%, 7.25%, and 18.84%, respectively. Also, the presence of full‐length connecting plates can enhance the charging rate by up to 12%. While the influence of the Ste* on the charging rate is considerably high (25.56%), the Re does not exhibit a noticeable effect (0.95%). Moreover, the influence of natural convection on the process was considered, however, it was found that it does not have a considerable effect on the ice formation. [ABSTRACT FROM AUTHOR]

In the present study, two various passive methods for heat transfer enhancement, including conical ring and wire coil are placed in a tube as turbulators. Four conical rings with four side holes are utilized with the same distance. The wire coil is employed at the center of the tube. The considered Reynolds numbers are between 4000 and 10,000. The studied geometrical parameters contain the pitch and diameter of a wire coil. Four different pitches of wire coil, including 10, 12, 14, and 16 mm, are evaluated. Furthermore, four values of wire coil diameter such as 2, 4, 6, and 8 mm are certain. The obtained numerical results displayed that by declining the pitch of a wire coil (37.5%), the average Nusselt number increases by about 143%. Also, augmentation in wire coil diameter by 300% leads to a growth in average Nusselt number by about 131%. Moreover, owing to utilizing two various turbulators, the pressure drop is significantly high in comparison with the bare tube. At Re = 10,000, growth in the inner diameter of the wire coil by 300% leads to an increase in thermal performance by about 36.12%. Moreover, as the pitch of the wire coil rises by 60%, the thermal performance declines by about 35.71%. [ABSTRACT FROM AUTHOR]

In this paper, turbulence heat transfer and nanofluid flow in a shell and corrugated coil tube heat exchanger are evaluated numerically. The three‐dimensional numerical simulations have been done by finite volume method using a commercial computational fluid dynamics code. The spatial discretization of mass, momentum, turbulence dissipation rate, and turbulence kinetic energy equations has been achieved by a second‐order upwind scheme. A SIMPLE algorithm has been used for velocity–pressure coupling. To calculate gradients, Green‐Gauss cell‐based method has been utilized. The cross‐section of the coil tube is lobe shaped. First, the impact of corrugated tube cross‐section type and then, the impact of utilizing different types of nanofluid on thermal performance are investigated. The outcomes indicate that at high Reynolds number, utilizing a five‐lobe cross‐section causes augmentation in Nusselt number and pressure drop by about 4.8% and 3.7%, respectively. However, the three‐lobe type shows the highest thermal performance. Moreover, water/CuO has the most thermal performance. As the volume concentration of the nanofluid increases, the thermal performance declines. [ABSTRACT FROM AUTHOR]

Utilizing passive methods is an efficient way in order to improve the thermal performance of heat exchangers. The novelty of the present paper is that two kinds of passive techniques for heat transfer enhancement, including a corrugated tube equipped with a coiled wire as turbulator, are combined in the present study. Heat transfer and pressure drop in a corrugated tube with a wire coil are analyzed numerically by a commercial CFD software. The range of the considered Reynolds number is 5000–20,000, which guarantees that the flow regime is fully turbulent. The effects of two different geometrical parameters and of two flow operating parameters, namely the Reynolds and Prandtl numbers, are numerically investigated. The conservative equations for momentum are closed using the k-ε realizable model. Results indicate that the geometrical and fluid flow parameters have significant effects on the thermal performance of the system. When one increases the pitch ratio of the wire coil (P/d), the heat transfer enhancement ratio NuER decreases. The maximum decrease is obtained for P/d = 5 with − 7.78% at Re = 20,000 in comparison with the baseline case P/d = 2. The maximum increase in the performance evaluation criterion (PEC) is observed at Re = 5000: + 11.16% and + 17.4% for P/d = 3 and P/d = 5, respectively. The configuration with a roughness height equal to e = 3 mm exhibits the best thermal performance with a maximum enhancement in terms of NuER of + 3.3% at Re = 10,000 in comparison with the baseline case e = 0.5 mm. By considering the PEC number, increasing the parameter e has a negative effect on the thermal performance of the proposed system. [ABSTRACT FROM AUTHOR]

In this numerical study, 4 types of hybrid nanofluid, including Ag-MgO/water, TiO2-Cu/water, Al2O3-CuO/water, and Fe3O4-multi-wall carbon nanotube/water, have been considered potential working fluid in a single U-tube borehole heat exchanger. The selected hybrid nanofluid is then analyzed by changing the volume fraction and the Reynolds number. Based on the numerical results, Ag-MgO/water hybrid nanofluid is chosen as the most favorable heat carrier fluid, among others, considering its superior effectiveness, minor pressure drop, and appropriate thermal resistance compared to the pure water. Moreover, it was indicated that all cases of Ag-MgO/water hybrid nanofluid at various volume fractions (from 0.05 to 0.20) and Reynolds numbers (from 3200 to 6200) could achieve better effectiveness and lower thermal resistances, but higher pressure drops compared to the corresponding cases of pure water. Nevertheless, all the evaluated hybrid nanofluids present lower coefficient of performance (COP)-improvement than unity which means that applying them as working fluid is not economically viable because of having higher pressure drop than the heat transfer enhancement. [ABSTRACT FROM AUTHOR]