Pre-research on Enhanced Heat Transfer Method for Special Vehicles at High Altitude Based on Machine Learning

Experimental investigation of effect of different tube row-numbers, fin pitches and operating conditions on thermal and hydraulic performances of louvered and wavy finned heat exchangers

The present study numerically examines the impact of zigzag fin surface pattern structures in a micro-channel heat exchanger on heat transfer efficiency using a commercial computational fluid dynamics program. The three newly designed louvered fin structures that comprise four triangular zigzag patterns, alternately spaced structures on both fin sides, a flat or zigzag fin structure on either side, and nine triangular zigzag patterns on both fin sides are compared with the reference flat fin. The air inlet velocity and temperature are 5 m/s and 30 °C, respectively, whereas the pressure at the outlet is atmospheric pressure. The wall temperature remains constant at 10 °C. The heat transfer performance is evaluated using the JF factor value, which reflects a performance comparison with the reference. The results show that the zigzag pattern changes the airflow direction, and the time air is trapped inside a fin domain increases. Air also tends to flow toward the zigzag pattern center, increasing the velocity and the heat transfer. The nine triangular zigzag fin pattern structures with the largest heat transfer area provide the best performance with the biggest JF factor increase of 3.24%. This result is obtained despite the comparison of the biggest friction factor values of the three new fin structures, suggesting that the impact of the JF factor far outweighed that of the friction factor.

A mathematical model has been developed to simulate mass and heat transfer of humid air flows at the airside of automobile air conditioning evaporators. The phenomenon of water condensation and water shedding in louvered fins are modeled based on extension of the existing Eulerian two-fluid method which treats water and air as two continuous media. A species transport equation for the mass fraction of water vapor in humid air is solved. The condensation of water from the air mixture translates into a sink term for the vapor transport equation and an equal source in the continuity equation for the volume fraction of liquid water in the two-fluid system of equations. A critical element in modeling the condensate transport in louvered fins calls for a surface tension force sub-model, since the surface tension force is the primary resistance against water shedding and draining. A formulation is proposed to evaluate the surface tension force based on searching for the most probable liquid-air interface where sharp gradient of water volume fraction exists. Numerical aspect of the implementation is discussed. In order to validate the model and demonstrate the applicability of the present methodology, a six-louver two-dimensional test case is established. The relative influence of the stopping force on liquid distribution pattern and flow was illustrated. The simulation is then carried out on a full-scale 2-D louvered fin design for different operating conditions. This study has demonstrated the feasibility of modeling “wet” heat transfer and water shedding in evaporator fins with an Eulerian two-fluid based method.

Chapter 3 - CFD simulation, design, and optimization for biohydrogen systems

Analysis of the effect of turbulence promoters in hollow fiber membrane distillation modules by computational fluid dynamic (CFD) simulations

For applications in the relatively low temperature refrigeration systems with large constant temperature bath, the present work performed the experimental studies on the airside performances of the staggered finned eight-tube heat exchangers with large fin pitches. The airside heat transfer coefficients and pressure drops for three fin types and two fin pitches are obtained and analyzed. The heat transfer enhancement with louver fins is 11–16 % higher than the flat fins and that with sinusoidal corrugated fins is 1.1–3.4 % higher than the flat fins. Higher Re brings larger enhancement for various fins. Fin pitches show weak influence on heat transfer for eight tube rows. However, effects of fin pitch on heat transfer for both the sinusoidal corrugation and the louvered fin are larger than the flat fins and they are different from those for N ≤ 6. Airside Colburn j factor are compared with previous and it could be concluded that the airside j factor is almost constant for finned tube heat exchangers with eight tubes and large fin pitches, when Re is from 250 to 2500. The results are different from previous studies for fewer tube rows.

Heat exchangers with periodic cellular metals (PCM) have become a substitute for those having louvered fins since they present good thermal performance and high mechanical strength for a given weight. Accordingly, heat transfer of a Wire-woven Bulk Kagome (WBK) sandwich panel was investigated in this study under constant heat flux conditions for various values of helical pitch lengths and wire diameters. The simulations for the flow, convection, and conduction were conducted by using the k-epsilon turbulence model. The channel was extended to avoid any entrance or exit effects. The numerical results showed that the friction factor and heat transfer increased by around 50% and 30%, respectively, with the wire diameter. Additionally, increasing the wire diameter increases heat transfer performance by around 30% for identical pumping power conditions. Friction factor and heat transfer increase up to 35% with decreasing helical pitch length. Heat transfer performance decreases with the change of helical pitch length from 12.5 mm to 17.5 mm by around 35% for a given pumping power. Also, high temperature differences seen in the WBK structure show the potential for further heat transfer enhancement by other possible configurations.

Cooling towers are required in building HVAC systems that use water as the cooling condenser fluid. Cooling towers used in this study are of the forced draft, counter flow, indirect/closed evaporative type. This study sought to demonstrate the performance characteristics of a closed system cooling tower by its effectiveness value, Number of Transfer Units (NTU), cooling capacity, and overall heat transfer and mass coefficient of the cooling tower. Experiments were performed on a heat exchanger coil intercrossed with ⅜ inch diameter intersections on parallel lines. Results of the experiment were then compared with the heat and mass transfer correlations taken from previous studies, and also combined with Computational Fluid Dynamics (CFD) simulations to examine the physical processes that occur in the cooling towers. All the experimental results, theoretical calculations and CFD simulations used variations of warm water mass, cold air, and water spray to present a clear description of the performance characteristics of a closed system cooling tower. The results of this study have shown that an increase in the amount of water spray mass flow causes an increase in the effectiveness value, heat transfer and overall mass transfer, as well as the cooling capacity of the cooling tower. The waste heat typically utilizes up to 80% of latent evaporation heat, and 20% of sensible air heat; however, waste heat in the closed system cooling tower utilizes 100% of latent evaporation heat. The mass transfer coefficient rate tends to be stable for a small mass of water spray.

Fluid dynamic simulations of EG-W (ethylene glycol–water)​ mixtures to predict nanofluid heat transfer coefficients

In order to predict and manage the thermal behavior of microelectronic packages cooled principally by natural convection, a two-way coupling method was developed between a accurate conduction model and a computational fluid dynamics (CFD) simulation. An iterative solution loop was performed by applying as a boundary condition the local convection coefficients obtained from the CFD to the conduction model, and the temperature field at the solid-fluid interface obtained from the solid conduction model to CFD simulation. For the first iteration, the conduction heat transfer was solved by considering an initial guess of a uniform convection coefficient (e.g. from empirical formulas) for each solid-fluid interface. The procedure was repeated until the convergence of the solution was reached. The comparison between the total convection coefficients including the radiation obtained with the CFD iterative procedure and those from the conventional empirical methods showed important differences, thus demonstrating the usefulness of the CFD approach to obtain accurate thermal results. The numerical results were compared to measurements from a test vehicle, carried out under the natural convection Jedec JESD51 standards in a still air chamber. Radiation plays an important role in heat transfer in this setting because of the large temperature difference between the walls of the box and the test vehicle. The junction temperatures obtained with the numerical simulation for different operating powers were in good agreement with the experimental measurements, with an error of less than 1°C, showing that the proposed methodology allows the accurate simulation of the natural heat transfer for microelectronic packages mounted on PCB in the horizontal orientation.

This study aims to investigate the thermal behavior and aerodynamic phenomena in a heated channel with varied rib configurations using computational fluid dynamics (CFD) simulations. Incorporating ribs in such systems enhances heat transfer and increases flow resistance and manufacturing costs. Understanding heat exchanger theory, measurement methods, and numerical calculations are crucial for creating efficient heat exchangers. The current research employs numerical analysis to assess the impact of hybrid ribs on heat transfer enhancement in forward-facing contracting channels (FFS). A two-dimensional forced convection heat transfer simulation under turbulent flow conditions was performed, considering the presence and absence of ribs with dimensions of 1 cm by 1 cm and spaced 11 cm apart. The arrangement of the ribs causes symmetrical temperature and flow distribution after and before each rib. The results demonstrate that the use of hybrid ribs outperforms the use of individual rib configurations in terms of thermal performance. This is due to the distinct flow patterns generated as the fluid passes through each rib. The triangle ribs had a more significant impact on the pressure drop than other rib configurations, while the cross ribs showed a lesser effect. The ribs improve the heat transfer coefficient while increasing the pressure drop, and the values of the Reynolds number were found to be directly proportional to the heat transfer coefficient and the pressure drop. The study concludes with a qualitative and quantitative analysis demonstrating the accuracy and coherence of the obtained computational results.

In this paper, a novel Kenics static mixer applied in a direct-contact heat exchanger (DCHE) was proposed in order to enhance the heat transfer efficiency. The computational fluid dynamics (CFD) simulation method was adopted to investigate the effect of the continuous phase and dispersed phase on direct-contact heat transfer (DCHT) in the Kenics static mixer. The perforation index (PI) was introduced to depict the performance of the Kenics static mixer, and the effect of different numbers of openings at three initial heat transfer temperature differences (at the inlet of the continuous and dispersed phases) was studied. The simulation results showed that under the same working conditions, a larger initial temperature difference would result in greater differences in the fluid uniformity index (UI), turbulence intensity, and time evolution of a gaseous working substance. The increase of hole numbers and decrease of hole diameters can optimize the DCHT. Under three initial temperature differences, the heat transfer efficiency for the 6-hole and 9-hole Kenics DCHEs was about 20% higher than the Kenics DCHE without holes.

To improve the comprehensive performance of the solar photovoltaic-thermal (PV/T) integrated system and address the issue of reduced conversion efficiency caused by increased PV temperature, this study establishes a three-dimensional transient model of a PV/T-phase change material (PCM) system with bionic fractal fins. Paraffin RT-42 (phase change temperature: 311–316 K) is selected as the PCM, and computational fluid dynamics (CFD) is employed to systematically investigate the effects of the presence or absence of PCM, fin structure, solar radiation intensity, and cooling water flowrate on system performance. The results show that the integration of PV/T and PCM significantly enhances the temperature uniformity of the PV, reducing the maximum temperature difference from 6 °C to 4 °C. Compared with the PV/T system, the thermal efficiency is improved by 24.82% and the electrical efficiency by approximately 5%. The bionic fractal fins outperform traditional straight fins in heat transfer enhancement by increasing the heat exchange area and constructing a multipath heat transfer network. When the solar radiation intensity increases from 600 W/m2 to 1000 W/m2, the system's thermal efficiency rises from 54.96% to 80.36%, while the electrical efficiency decreases from 14.38% to 13.39% due to PV temperature increase. The cooling water flowrate has a significant effect on system temperature regulation: at a flowrate of 0.1 m/s, the average PV temperature is 9.22 K lower than that at 0.01 m/s, and the electrical efficiency is improved by 5.2%. Thus, optimizing the thermal and electrical efficiency of the system can be achieved by adjusting the flowrate. This research provides a new technical path and theoretical support for improving the performance of PV/T systems.

The present study investigated the enhancement of convective heat transfer in horizontal tubes using twisted tape inserts, particularly emphasizing the influence of modifications to their geometrical configurations. Three twisted tape models were designed using SolidWorks: a plain tape, a tape with curved edges, and a tape with curved edges and holes. Two twist ratios were considered, namely TR1 = 6.666 and TR2 = 5. Computational Fluid Dynamics (CFD) simulations were performed to evaluate the velocity and temperature fields, Nusselt number, and friction factor over a Reynolds number range of 15,000–25,000. The results indicate that employing twisted tape with curved edges and perforations leads to the greatest enhancement in heat transfer. Specifically, the Nusselt number increases from 410 for curved edges without perforations to 460, whereas the plain tape achieves 380 at Re = 25,000 and TR2. The tape with curved edges (without holes) demonstrated the most favorable improvement in friction factor, attaining a value of 0.024 versus 0.029 for the plain tape, corresponding to a 20.8% reduction. As a result, the performance evaluation factor achieved a peak value of 1.24, indicating a 30.5% improvement. These findings demonstrated that optimized twisted tape geometries significantly enhanced heat transfer performance while maintaining reasonable pressure drop penalties. This highlights their potential applicability in thermal system design, particularly in compact and energy-efficient heat exchangers.

A review on application and heat transfer enhancement of supercritical CO2 in low-grade heat conversion