Heat Transfer最新文献

This study examines the steady flow of an electrically conducting fluid through a rotating porous channel bounded by stationary, impermeable horizontal plates at constant temperature. The primary aim is to explore the combined effects of a magnetic field, wall slip conditions, and viscous dissipation. The channel rotates at a constant angular velocity, with slip conditions applied at the walls. A pressure gradient drives the primary flow, while rotation generates the secondary flow. Analytical solutions for velocity profiles and volumetric flow rates are obtained, and the temperature distribution is calculated using MATLAB's “bvp4c” function. The research offers novel insights into the behavior of primary and secondary flow velocities under different Hartmann and Taylor numbers, emphasizing the impact of slip conditions. Additionally, the influence of the Eckert number on temperature is analyzed in conjunction with these parameters. These findings contribute valuable theoretical perspectives for enhancing cooling systems in rotating machinery using conductive fluids in porous channels. This study opens avenues for future research to investigate unsteady flow conditions and the effects of variable magnetic fields and rotational speeds on fluid behavior in rotating porous channels.

In the current work, the study examines the flow patterns and heat transfer capabilities of tubes with various spherical dimple configurations. A numerical analysis, supported by experimental validation on a reference model, was conducted on a circular tube with an alternating flow path. The primary goal was to enhance the thermal performance of circular tubes by inducing mixing and vortex flows. The impact of three design factors on thermal–hydraulic performance was investigated, dimple pipe diameter (DPD), dimple group number (DGN), and number of dimples (NODs). Dimpled tubes consistently outperformed smooth tubes in heat transfer due to increased flow mixing and separation. Both increasing the Reynolds number and decreasing the design factors led to the formation of mixing and vortex patterns. The performance evaluation factor (PEF) varied across different dimple configurations. For DPD, PEF ranged from 1.14 to 1.33; for DGN, it ranged from 1.15 to 1.28; for NOD, it ranged from 0.95 to 1.21, all within a Reynolds number range of 4000–15,000. At a Reynolds number of 6000, all three configurations of DPD, DGN, and NOD outperformed the smooth pipe in terms of the Nusselt number. For DPD, Nusselt number improvements ranged from 25.7% to 30.8%, and friction factors increased by 21% to 67%. DGN configurations exhibited a wider range of Nusselt number enhancement from 25% to 49.6%, and friction factor increase from 37% to 72%. NOD configurations also demonstrated consistent improvements, with Nusselt number increases ranging from 27.35% to 31% and friction factor increases from 42% to 74%. Spherical dimples can significantly enhance the thermal–hydraulic performance of tubes, so the best configuration depends on the specific application, and the highest performance, with a 1.33 increase in PEF, was achieved with a dimple diameter of 2 mm (DPD = 2 mm) and a dimple density of four dimples per unit area (NOD = 4).

Effective heat distribution in electronic circuitry is essential to improve the performance and life of electronic components such as chips. This study presents a numerical analysis of heat transfer on a substrate board populated with an array of discrete heat sources, assumed to be placed in a horizontal air channel for forced convection cooling. The analysis of electronic packages is performed, taking into consideration the effect of thermal contact conductance (TCC) between the heat source (chip) and the substrate board. The dependence of the temperature distribution on the Reynolds number of air at the inlet and the heating power from the heat source is investigated for inlet velocities ranging from 0.6 to 1.4 m/s and observed to be significant. Temperature and heat transfer coefficient are observed to systematically increase with the increase in the heat dissipation from the heat source. Two configurations—inline and staggered—are analyzed, with the staggered configuration showing superior cooling performance. This improvement is attributed to the fact that staggered arrangements expose fewer heat sources to pre-heated air before it exits the system. Additionally, the location of the heat source reaching the highest temperature is found to be highly dependent on the TCC of the bonding material between the heat source and the substrate. A hybrid optimization strategy is employed, by combining Artificial Neural Network (ANN) and Genetic Algorithm (GA) for optimizing the location of heat sources. ANN is used for predicting the temperature distribution, subsequently followed by GA to minimize the maximum temperature attained by the heat generating source by varying other control variables like TCC thickness, inlet velocity, and heat generation. The thickness of the bonding layer is varied from 0.225 to 0.271 mm and the heat generation is varied from 1000 to 2000 W/m². Among them, TCC is observed to be an important parameter controlling the optimum location of heat generating sources. The results obtained from the proposed hybrid optimization strategy are compared with the simulation results and observed to be reasonably close.

The design of flow structures plays a crucial role in enhancing natural convective heat transfer within enclosures. By optimizing the geometry of enclosures to influence flow structures, we can significantly improve their natural convective heat transfer performance. Specifically, the dome-shaped wall can alter flow direction, improving flow circulation and natural convection. The current study conducts a numerical investigation of the laminar flow and natural convective heat transfer of air within a dome-shaped enclosure, while also considering the impact of a magnetic field. The analysis encompasses the interaction between magnetic field and buoyancy-driven flow. Governing equations for momentum, energy, and angular momentum are formulated, integrating the influence of the Lorentz force. The working fluid Pr = 0.71 is considered in this study. The equations are transformed into dimensionless form using key parameters, such as the buoyancy number (Ra) and Hartmann number (Ha). The modeled partial differential equations were carried out with a vorticity-stream function algorithm to explore the influence of magnetic field strength on the flow and thermal characteristics. Results indicate significant alterations in flow patterns and temperature distribution behavior under varying magnetic field and Rayleigh number. The interaction between buoyancy and magnetic fields plays a critical role in determining the heat transfer characteristics of an incompressible fluid, with Ra enhancing, and Ha suppressing, convective efficiency. Heat transfer enhancement of 82.84% is noticed for a Rayleigh number ranging from 10³ to 10⁴, while a 48.316% decrement is found for a Hartmann number ranging from 0 to 10 with Ra = 10⁵. The transition from a magnetically dominated regime (high Ha) to a thermally driven regime (low Ha) leads to a shift from a uniform temperature field to one with more complex thermal layering and mixing, which is reflected in the varying shapes and amplitudes of the Nusselt number distributions. At higher Ha values, magnetic forces dominate, significantly suppressing buoyancy-driven convection, and reducing the intensity of thermal mixing.

The evacuated tube solar air heaters (ETSAHs) are gaining popularity today because of their reduced heat loss capability. A performance evaluation of the evacuated tube solar collector (ETSC) using a thermal energy storage (TES) facility was carried out during the study. A new heat pipe (HP) system with a TES unit directly integrated into it was used in the study along with ETSAH. The novel HP system directly stores the thermal energy and is capable of heating the air uniformly from all directions. Therminol-55 (T-55) was the sensible heat storage material integrated inside a common condenser HP system. The solar collector's performance was evaluated at air flow rates ranging from 0.003 to 0.02 kg/s. The HP and T-55 exhibited a maximum temperature of 121°C and 127°C. The highest temperature of the air leaving ETSAH-TES was 128°C. ETSAH-TES delivered hot air over 100°C continuously for 7 h. The results depict that even when solar energy was erratic, the HP developed in this research supplied a continuous hot air flow. The average energy and exergy efficiency achieved was 37.87% and 2.8%, respectively. T-55 can store a maximum of 1920 kJ of energy. With a standard deviation ranging from 0.2% to 1.87%, the ETSC-TES numerical analysis is in excellent agreement with experimental results.

Numerical estimation for the impacts of the conductive partitions having different positions on the mixed laminar convection of air in a 2D vented enclosure was examined. The variable parameters are accepted as Reynolds number (Re = 10–1000), Richardson number (Ri = 0–5), and size of the partition (0.25H, 0.5H, and 0.75H). Twelve cases having several partition arrangements were analyzed. It was observed that excellent convection control can be obtained by using conductive partitions depending upon the Re and Ri combinations. Generally, at small Re, the mean Nu was not affected by the variation of geometry and Ri at small Re. The highest Nu is achieved in Case 12, a cavity with two partitions having a length of 0.75. At Re = 1000, the rate of increase in Nu at Ri = 0, 1, and 5 are obtained at 2.085, 1.868, and 1.43 according to the bare cavity, respectively. In addition, the effect of the solid–fluid thermal conductivity ratio (K = 0.002, 0.2, 1, 5, and 40) on heat transfer was investigated for Case 12. Empirical power-law Nusselt number correlation was derived for a 2D vented cavity with/without conductive partitions. In conclusion, the maximum heat transfer enhancement rate is obtained in the vented cavity with two length partitions of 0.75. At Re = 1000, the increases in heat transfer rate (Nu/Nu₀) for Ri = 0, 1, and 5 are 2.085, 1.87, and 1.43 times higher, respectively, compared with the bare cavity. In terms of effectiveness, Case 12 is the optimum case after Case 0.

An even-span greenhouse groundnuts dryer having a reflective north wall has been developed and tested under passive and active modes. The effects of varying sample mass (580, 880, and 1180 g) on the thermal and enviro-economic performance indicators of the dryer have been investigated. The Nusselt number expression, embodied energy (EE), and capital cost (C_c) were used for thermal, environmental, and economic assessment of the dryer. The drying behavior of the groundnuts samples has also been estimated using the moisture ratio in drying models. Thermal indicators were observed to increase with increased sample mass and were found higher under active mode. The values of EE and C_c of the dryer under passive and active modes were 145.24 and 158.42 kWh, and INR 1257.11 and INR 2157.11, respectively. Enviro-economic indicators were also observed to improve with increased sample mass and were comparable for both the passive and active modes. The moisture ratio for all the samples shows an excellent fit with the Midilli–Kucuk model. An average Midilli–Kucuk model has been recommended to predict the drying behavior of groundnuts in the developed greenhouse dryer.

The exact solution is obtained for the unsteady fluid–particle suspension model of Rabinowitsch fluid through a vertical tube having ciliated walls. The flow inside the tube is produced by the metachronal waves of cilia. The lubricant approach is used to produce the solution of fluid phase velocity, particle phase velocity, stream function, and temperature. The pressure rise is calculated through a numerical analytical technique. The graphs are constructed to explore the characteristics of the velocity of both phases, thermal analysis, trapping phenomena, pressure gradient, and pressure rise. It is known that the slip parameter diminishes the velocity of both phases. The thermal slip parameter and density number upsurge the thermal profile. The particle phase velocity is less than the fluid phase. The present analysis can be useful in biomedical engineering to construct the heart and lung machines that are used to pump blood in arteries.

Solar energy is still commonly used to produce clean drinking water due to its simple construction, low maintenance, and ecofriendliness. This work aims to experimentally investigate the yield upgrade and the thermal performance of a novel concentrated single-axis tracking trough tubular solar still (TSS). This tubular still is identified by three baffles that generate four interrupted sections in the U-receiver, which is inserted with copper mesh and fitted in a hexagonal-shaped glass cover. Two identical TSS models were side-by-side outdoor tested in Baghdad-Iraq 33.3° N and 43.3° E from January to March 2024. The first is inserted with black copper mesh (Model I), and the other has no insertion (Model ll). The effect of the inserted copper mesh (60, 120, and 180 g) and the receivers' tilt angle (5°, 10°, and 15°) on the still performance are involved. The still thermal performance is analyzed per heat transfer coefficients, energy, and exergy efficiencies. The results revealed that the accumulated daily yield is enhanced for Model I by 78.9%–194.8% while the thermal efficiency is enhanced by 68.3%–206.4% when it is tilted at 15° with the insertion of 60–180 g copper mesh, respectively, compared with Model II. It is concluded that an effective improvement in the solar still yield is obtained by using copper mesh.

The present study investigates pool boiling heat transfer (PBHT) of R-134a on Cu─Al₂O₃ composite-coated patterned surfaces (CPS_I, CPS_II, CPS_III, and CPS_IV). Using a wire EDM method, four different types of copper patterned surfaces (PS_I, PS_II, PS_III, and PS_IV) were manufactured. Comparing the heat transfer coefficients (HTCs) of the Cu─Al₂O₃ composite-coated patterned surfaces to the uncoated Cu surfaces, a notable enhancement was observed. The maximum HTC improvements of 162%, 178%, 189%, and 211% were observed for CPS_I, CPS_II, CPS_III, and CPS_IV, respectively, when compared with bare Cu surfaces. These results demonstrate the effectiveness of these treatments in enhancing heat transfer compared to bare copper surfaces. The enhancement in PBHT is mainly due to the integration of porous Cu─Al₂O₃ composite coating with patterned surfaces which resulted in a larger heat transfer area, improved capillary action, and a substantial increase in active nucleation sites.