• Energy Research

Abstract Heat pipes have been extensively applied in the domain of energy conversion and heat recovery application. A novel radial heat pipe with internally finned condenser has been proposed to recover waste heat from air conditioning units. Full mathematical description and computational fluid dynamics model were developed to predict the loop flow and thermal performance in a single radial heat pipe, respectively. The research focuses mainly on the effects of input power, filling ratio, pipe diameter, and velocity of the cooling water on the thermal performance and entropy generation rate of this novel heat pipe. Numerical results illustrate that average reductions in total thermal resistance with utilization of fins approximately achieved 8.69% and 14.78% for fins nf = 4 and nf = 8, respectively. Similarly, entropy generation rate shows an average decrease of 19.4% and 37.5% for identical operations. Additionally, the effect of internally finned condenser on the multiphase fluid flow has been comprehensively analyzed, including the break of the condensed film, generation of bubbles and nucleation boiling. The results further indicate that incorporation of internal fins has a significant influence on enhancing thermal homogenization and waste thermal recovery efficiency of a radial heat pipe. Case results agreed well with experimental data within average deviation being no more than 10%.

Abstract Two-Phase Closed Thermosyphons (TPCTs) is a high-efficiency heat transfer technology being widely utilized in the fields of built and solar energy exploitations. In present work, a 2-D steady-state model is innovatively proposed to investigate thermal performance and flow dynamics of a single TPCT containing nanofluids. Conservation equations for mass, momentum, and energy were fully solved by the dichotomy algorithm. The effects of input powers, nanoparticles materials (Al2O3, Fe2O3 and Cu), and concentration levels (φ = 0–12 wt%) on the thermodynamics and entropy generation of the thermosyphon were numerically investigated and the results were evaluated through those from the pure water. A substantial change in the liquid film thickness, flow velocity profile, interfacial shear force, local heat transfer coefficient, temperature distribution, entropy generation rate and thermal resistance were subsequently obtained when using a nanofluid. Our numerical results revealed that nanofluids in thermosyphon could effectively enhance thermal performance by decreasing the evaporator temperature, thereby reducing overall entropy generation by about 41%, 32% and 29% for Cu, Fe2O3 and Al2O3 in comparison with pure water, respectively. Additionally, entropy generation significantly decays when nanoparticle concentration levels promote. Numerical results further indicate that the flow resistance of working fluid increases, maximum friction entropy generation increases by approximately 36.37%, 15.15%, and 9.09% for Cu, Fe2O3, Al2O3 of φ = 9 wt%, respectively. Moreover, the existence of an optimum concentration level for nanoparticles in maximizing the heat transfer limit was theoretically achieved for all nanofluids. Current results agreed well with experimental data within average deviation being no more than 10%. In conclusions, results suggest that TPCT-solar collect using nanofluid has a low simple payback period to absorb solar radiation to convert thermal energy compared to the conventional one charged with pure water.

Abstract In the present work, a novel heat transfer device, radial heat pipe (RHP) has been investigated due to lower flow resistance. Mathematical and computational fluid dynamics (CFD) models have been developed to investigate their thermal performance and two phases flow process inside RHP. The steady-state theoretical model of RHP has been employed to investigate the influence of various filling ratio range of (0–60%) of water and three different values of heat input (3267 W, 4004 W and 4817 W) on the thermal performance of a RHP at different operating conditions. CFD simulations further demonstrate that present built VOF model could effectively reproduce phases-change fluid flow and heat transfer inside RHP. Phenomena such as nucleation boiling, coalescence of bubbles, formation of the liquid film were observed in the heat pipe. Moreover, numerical modelling results were well validated by the experimental tests for various conditions, and maximum deviation falls within 10%.