The presence of a bend in a tube during two-phase flow boiling can improve heat transfer while also causing a pressure drop. The goal of this paper is to achieve an optimal geometry of the U-bend ...tube heat exchanger. The bend induces a high-pressure drop as well as increases heat transfer coefficient, striking a balance between pressure drop and heat transfer coefficient to obtain an optimal design can be achieved by minimizing entropy generation due to heat transfer and pressure drop. Hence, an entropy generation minimization during two-phase flow boiling of R134a was conducted using the constructal design method and the entropy generation techniques based on the second law of thermodynamics. The geometric optimization was subjected to the following constraints: the volume of the tube was constant with bending radius, and the tube's outer surface area was subjected to a uniform constant heat flux of 10 kW/m2. Geometric parameters such as total length Lt, internal diameter di and external diameter do of the tube were free to morph with respect to the degree of freedom until an optimal geometrical configuration was obtained at minimum entropy generation. The bend's curvature ratio was taken as a function of internal (hydraulic) diameter. The numerical computations were conducted with the mass fluxes from 100 to 500 kg/m2s and quality of 0.2 at the tube's inlet for downward-oriented flow. The results obtained were consistent with those found in the open literature. Furthermore, the optimal parameters of the tube heat exchanger were found to vary slightly as the minimum entropy generation rises with an increase in mass flux. The stability was achieved as minimum entropy generation increased which demonstrates the robustness of the optimized U-bend tube.
This paper shows the performance of a cylindrical
micro-pin fins with multiples-arrays structures for
maximum heat transfer. The structures has a varying geometric
sizes (diameter, height and ...spacing). The effects
of Reynolds number and thermal conductivity ratio on
the optimized geometric configurations and the maximum
heat transfer rate is documented. Two design configuration
were considered. Scales and computational fluid dynamics
analysis shows that the benefits of varying fin
height is minimal. Results show that performace is increased
when three rows of micro pin fin heat sinks with a
reduced degree of freedom (fixed height) when compared
to two rows of micro pin fins heat sink for the same amount
of material. The optimized diameters of the fins seems to
have greatest effect on perfomance of the heat sink.
There is an urgent and compelling need to develop innovative and more effective ways to integrate sustainable renewable energy solutions into the already existing systems or, better yet, create new ...systems that all together make use of renewable energy. This study aims to establish the optimum working conditions of a geothermal preheater in a power plant that makes use of both renewable and nonrenewable energy resources, where renewable (geothermal) energy is used to boost the power output in an environmentally sustainable way. Hence, two models, one, a simplified model of a Rankine cycle with single reheat and regeneration, and another, with a geothermal preheater substituting the low‐pressure feedwater heater (LPFWH), were compared. The Engineering Equations Solver software was used to perform an analysis of the thermodynamic performance of the two models designed. An analysis was done to evaluate the energetic and exergetic effects of replacing a LPFWH with a geothermal preheater sourcing heat from a low temperature geothermal resource (100°C‐160°C). Results from the thermodynamic analysis reveal that the hybridization boosts the power output by approximately 4% and it is superior in terms of the second law. Entropy generation minimization analysis was then employed to establish optimal working conditions of the hybrid system (ie, the geothermal preheater modeled as a downhole coaxial heat exchanger).
This study presents flow boiling evaporation heat transfer and flow patterns development of R134a in a U-bend tube with the curvature ratio of 2.55 for the downward flow orientation. The volume of ...fluid (VOF) multiphase model in the computational fluid dynamics (CFD) code Ansys Fluent® was utilized and incorporated with phase change model for volume fraction tracking of each of the phases. The simulations were conducted under these operating conditions: (mass fluxes from 200 to 600 kg/m2s, vapour qualities from 0.01–0.9 and 10 kW/m2 of heat flux). The relationships of mass flux with respect to vapour quality on heat transfer and frictional pressure drop in the U-bend tube were analysed. The effect of mass flux with respect to vapour quality on heat transfer coefficient showed that heat transfer coefficient increases as mass flux increases and with increase in vapour qualities at constant mass flux which later decreases at the higher vapour qualities due to dry-out. The frictional pressure drop was found to increase as mass flux and vapour quality increases as a result of increase in an interfacial shear stress of liquid and vapour phases. At higher vapour qualities, frictional pressure drops decrease due to drastic reduction of the liquid film. The influence of the bend was determined by the development of flow pattern map before and after the bend. The numerical computation results agrees with what is obtained in the open literature.
The temperature of the sun was modeled in this study using two transient solar temperature equations for sunrise and sunset that were developed for designing a latent heat thermal energy storage ...(LHTES) system for a concentrated solar power (CSP) plant. The derivation of the equations was based on the existing solar hour angle and the fundamental periodic function equations. Ansys' computational fluid dynamics code was used to investigate numerically the transient response of the conjugate melting and solidification of a phase change material (PCM) in a cylindrical shell and helical heat pipe (HHP) thermal system. The models for both equations were applied as user-defined functions (UDFs). The heat transfer medium was air. The predicted liquid and solid fractions provide quantitative data on the temperature and the stored solar energy. The effectiveness of both models in enhancing heat transfer and their suitability as real-time transient solar temperature models in heat transfer engineering is demonstrated by comparisons between their outputs. With high agreement, experimental data from the open literature was used to validate the numerical model's predictions. The solar temperature models aim to contribute to heat transfer enhancement for a reduced PCM energy storage time in designing a high-temperature solar thermal storage that is adequate to maintain a steady supply of electricity and energy for domestic and commercial applications and to accelerate the global transition to low-carbon energy.
In this research paper, the entropy generation was used as a criterion to measure the quality level of energy transfer for two-phase flow boiling of R134a in the elliptical U-bend tube. The ...eccentricities of the elliptical cross-section ranging from 0 to 0.904, were constrained to the same hydraulic diameter of 8 mm. The input parameters utilized in the numerical computations were mass velocities of 100 and 200 kg/m
2
s for the inlet vapor qualities of 0.1 and 0.2 and a constant uniform heat flux of 10 kW/m
2
imposed on the outer surface of the tube. The calculations were performed using the volume of fluid multiphase model coupled with the phase change model. The findings demonstrated that when the eccentricity of the U-bend tube increases, the quality of energy transfer in the heat exchanger improves as entropy generation reduces. The numerical calculations were validated and found to be consistent with what has been published in the open literature. The results of this paper are important for the development of heat exchangers for air conditioners and refrigerators.
Constructal design of high heat flux two-stacked horizontal microchannel heat sinks are presented for cooling of electronic devices. Using ANSYS Computational Fluid Dynamics (CFD) code for subcooled ...nucleate boiling, fixed volume constraints were invoked for the heat sinks and microchannels to achieve optimal flow velocities and pressure drops at global minimum thermal resistances of the microchannel heat sinks; this is in line with constructal design principle. Validation of the CFD code was done by comparing its results with those of experimental data in the open literature. The closeness of the two results showed the CFD code to be accurate in predicting subcooled flow boiling for cooling of microelectronic devices. Heat fluxes of 1100 and 1200 W/cm2 (1.1 × 107 W/m2 and 1.2 × 107 W/m2) were used in the simulations and optimizations. The thermal resistance, pumping power and Bejan number were based on microchannels that an optimal heat sink of 1 cm width could accommodate. The thermal resistances and base temperatures for optimal microchannel heat sinks at these high heat fluxes (the focus of the study) are low; the pumping power requirements are also low, which show that they are useable in the cooling of electronic devices and other similar applications. Also, this is an indication that optimal two-stacked microchannel heat sinks operating in subcooled flow boiling could be considered as a viable alternative in the cooling of electronics. Optimal two-stacked microchannel heat sink operating at 1200 W/cm2 and in counterflow and parallel flow arrangements, was used for the critical heat flux study and the results show that both have good CHF performance, although counterflow arrangement was better.
•Heat transfer enhancement of a parabolic trough receiver with perforated plate inserts is studied.•Effect of insert geometrical parameters on receiver thermal performance is ...investigated.•Correlations for Nusselt number and friction factor performance are derived and presented.•Performance evaluation using enhancement factors and collector modified thermal efficiency was demonstrated.•Thermodynamic performance is investigated using the entropy generation minimization method.
In this paper, a numerical investigation of thermal and thermodynamic performance of a receiver for a parabolic trough solar collector with perforated plate inserts is presented. The analysis was carried out for different perforated plate geometrical parameters including dimensionless plate orientation angle, the dimensionless plate spacing, and the dimensionless plate diameter. The Reynolds number varies in the range 1.02×104⩽Re⩽7.38×105 depending on the heat transfer fluid temperature. The fluid temperatures used are 400K, 500K, 600K and 650K. The porosity of the plate was fixed at 0.65. The study shows that, for a given value of insert orientation, insert spacing and insert size, there is a range of Reynolds numbers for which the thermal performance of the receiver improves with the use of perforated plate inserts. In this range, the modified thermal efficiency increases between 1.2% and 8%. The thermodynamic performance of the receiver due to inclusion of perforated plate inserts is shown to improve for flow rates lower than 0.01205m3/s. Receiver temperature gradients are shown to reduce with the use of inserts. Correlations for Nusselt number and friction factor were also derived and presented.
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