Performance analysis of micro-fin tubes compared to smooth tubes as a heat transfer enhancement technique for flow condensation

Heat transfer enhancement techniques are accompanied by pressure drop amplification, detrimentally affecting their performance; entropy generation analysis is an effective approach to assess heat transfer enhancement along with resulting pressure drop. Current study investigates and compares the performance of micro-fin (as a passive enhancement technique) and smooth tubes during flow condensation (for R134a refrigerant) through conducting entropy generation analysis. First, the impact of geometrical and operating variables on pressure losses and heat transfer contributions to entropy generation and total generated entropy inside both types of tubes is examined. Then, the conditions at which the application of micro-fin tubes in lieu of smooth ones is justifiable and of superior performance are identified utilizing entropy generation number. The simulation results indicate that entropy generation enhances in the micro-fin tubes as tube diameter, mass velocity, vapor quality, and wall heat flux rise, and saturation temperature declines. The same is observed in the smooth tube except for the mass velocity; an increase in this parameter leads to a decreasing-increasing trend in entropy generation. Moreover, the entropy generation number results indicate that applying micro-fin tubes rather than smooth ones is justifiable, i.e., has better performance, at lower mass velocities and vapor qualities, but higher saturation temperatures and wall heat fluxes

Exergy and environmental assessments of the performance of a molten carbonate fuel cell cogeneration plant: External steam reforming against internal steam reforming

Two different configurations of a system including a cogeneration plant based on molten carbonate fuel cell (MCFC) with internal steam reforming (IR-MCFC) and external steam reforming (ER-MCFC) for producing power and hot water are modeled and investigated thermodynamically. Energetic, exergetic and environmental analyses are performed for the proposed systems and compared with each other from different viewpoints. Effects of various parameters, namely after-burner emissions recycling, fuel utilization ratio, operating temperature of stack and current density are investigated on the output potential voltages and any kind of voltage losses, net generated power, CO2 emission and energy and exergy efficiencies of two proposed cogeneration plants. The main sources of irreversibility are introduced for each system as well. The comparative analysis revealed that energy efficiencies of the IR-MCFC and IR-MCFC based cogeneration systems are about 14.85% and 4.82% larger than those of the ER-MCFC and ER-MCFC-based cogeneration systems, respectively. Also, exergy efficiencies of the IR-MCFC and IR-MCFC based cogeneration systems are about 14.46% and 11.08% more than exergy efficiencies of their external reforming types, respectively. The results indicated that CO2 emission of ER-MCFC system (0.18 kg/MW) is almost two times of IR-MCFC system (0.36 kg/MW)

Performance Evaluation of Helical Coils as a Passive Heat Transfer Enhancement Technique under Flow Condensation by use of Entropy Generation Analysis

Current research focuses on the performance of helically coiled tubes as a passive heat transfer enhancement technique for R134a flow condensation from an entropy generation perspective. Similar to other enhancement techniques, helical coils, are accompanied by pressure drop as a penalty, diminishing their performance, so that these coils are of lower performance compared to straight tubes where the increase in entropy generation due to pressure drop overcomes the decrease in entropy generation due to enhanced heat transfer. Unlike previous studies that have largely investigated heat transfer and pressure drop characteristics of helical coils regardless of their performance, this study employs entropy generation analysis as an effective method to distinguish flow and geometrical conditions at which helical coils are of lower entropy, i.e. higher performance, compared to straight tubes. The findings reveal that, for both helical and straight tubes, entropy generation is enhanced as tube diameter, mass velocity, vapor quality, and wall heat flux increase and saturation temperature decreases. Additionally, applying helical coils within wider ranges of mass velocities can be justified at lower values of tube (≤8.3 mm) and coil diameters (≤200 mm), saturation temperatures (≤40 °C), and vapor quality (≤0.6), and at higher values of wall heat flux (≥15 kW/m2). These results substantiate that employing helical coils in lieu of straight tubes is not justifiable always (for all flow and geometrical conditions) although they are of superior heat transfer performance compared to straight tubes