• Energy Research

In this study, a numerical investigation of the solar parabolic trough collector with an area of 0.856 m2 by combination of Monte Carlo Ray‐Trace (MCRT) and finite volume method (FVM) has been carried out using the FLUENT software. The purpose of this study is to investigate the effect of metal foams and nanofluids on the thermal performance of a direct absorption parabolic trough collector (DAPTC). For this purpose, three‐dimensional fully developed laminar convection heat transfer of Al2O3 and TiO2/water nanofluids is examined by 0.1, 0.2, and 0.3 vol %. Furthermore, 64 cm copper‐filled porous media in a volumetric absorbent tube are applied. The obtained results of numerical simulations are compared with the experimental data. The results demonstrate a good agreement between the numerical simulations and experimental data. The results indicate that the maximum efficiency was 34.51% for TiO2/water at 0.3 vol %. Furthermore, using porous media with high absorption coefficient and scattering coefficient that could absorb more incoming radiation and transfer to heat transfer fluid can result in increasing the efficiency of the collector. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 727–740, 2019

The impact of corrugated absorber tubes on the performance of a parabolic trough collector (PTC) is experimentally analyzed. For this purpose, nine different helically corrugated absorber tubes were applied. The impacts of the pitch length and roughness height on the performance of the PTC were examined under the ASHRAE standard 93 (2010). The Reynolds number varied between 5000 and 10,000. The results demonstrate that the thermal performance and heat transfer are considerably increased in the corrugated absorber tubes. The average heat transfer rate increases between 85.7 and 107.2% in corrugated tubes, which is related to the roughness height and pitch ratio. The thermal performance has a maximum value of about 2.29 for the pitch length and roughness height of 3 mm and 1.5 mm. The average friction factor increases between 1.25 and 1.85 times in corrugated tubes. Furthermore, the variations of the Nusselt number and friction factor versus pitch-to-diameter ratio, roughness height-to-diameter ratio, Prandtl number, and Reynolds number are presented as correlations by using the best fit to the series of experimental data.

During the last years, many studies have been performed in the field of parabolic trough collectors (PTCs) to achieve thermal performance enhancement. The use of turbulators can help to increase the fluid mixing and as a result, the decrease in wall temperature and risk of tube deformation. In this paper, the impact of conical strip inserts on the thermal performance of a PTC is numerically investigated. Reynolds number varies from 8000 to 40,000. Working fluid temperature at the inlet is 300 K. The results demonstrate that the Nusselt number increases up to 91.949%. Furthermore, the friction factor in the PTC with conical strip inserts is 5.21 more than the plain tube. Performance evaluation was used for analysing the benefits of this kind of turbulator. The overall thermal–hydraulic performance (PEC) varied from 1.107 to 0.679.

Effects of variation of the thermal conductivity on forced convection in a parallel-plates channel heat exchanger occupied by a fluid saturated porous medium are investigated analytically based on the perturbation methods. Walls of the channel are kept at a constant heat flux. Thermal conductivity of the medium is assumed to be a linear function of temperature (due to moderate radiation heat transfer in cellular foams or temperature dependent conductivity of the material). The Brinkman–Forchheimer–extended Darcy model for the flow field is used. Relations representing the temperature profile and Nusselt number as functions of porous medium shape parameter and thermal conductivity variation parameter are derived. Obtained Nusselt number and temperature profile are studied parametrically. No analytical investigation based on a variable conductivity approach for Brinkman–Forchheimer–extended Darcy model has been previously performed. Results show that a linear increase in the thermal conductivity of the medium results in a semi-linear increase in the Nusselt number.

Abstract The use of flat plate solar collectors (FPSCs) is the most common technology to employ solar energy. FPSCs enhance thermal efficiency and, as a result, can reduce both the size and the costs of a system. Surface modification techniques are extensively used to improve the thermal performance of systems. Among such techniques, helically corrugated tubes play an important role in industrial applications. Therefore, an experimental study was conducted in accordance with ASHRAE 93 (2010) to examine the effects of roughness height and pitch length of corrugated risers on the heat transfer in an FPSC in a turbulent flow. Nine corrugated risers with roughness height to diameter ratios ( e / D H ) of 0.05, 0.1, and 0.15, and pitch ratios ( P / D H ) of 0.3, 0.5, and 0.7 were manufactured and examined. The results demonstrated that the maximum efficiency of the FPSC with corrugated risers ( e / D H =0.15 and P / D H = 0.3) was about 61.59%. The friction factor increased by about 59.3–97.6%. The thermal performance factor had the maximum value of 2.3 at the Reynolds number of 5153. Based on the experimental results, the helically corrugated risers in the FPSC have a potential for being used in thermal applications.

Abstract Vortex tube is a mechanical device that operates as a refrigerating machine. This device separates a compressed gas stream into hot and cold streams. In this paper a series of experiments has been carried out to investigate the influence of uniform curvature of main tube on the performance of the vortex tube. Results show that the curvature in the main tube has different effects on the performance of the vortex tube depending on inlet pressure and cold mass ratio. It was found that the maximum temperature difference (ΔTc,max) belonged to straight vortex tube, type A, however the maximum refrigeration capacity ( Q ˙ c , max ) belonged to curved vortex tube, type C. Non-dimensional cold temperature difference ( Δ T c / Δ T c , max ) and non-dimensional refrigeration capacity ( Q ˙ c / Q ˙ c , max ) are found to be independent of the curvature of the vortex tube.

Abstract A Solar Chimney Power Plant (SCPP) consists of three cardinal elements – collector, chimney, and turbine(s). In collector, there can be natural and artificial roughness such as the surface of the earth and the energy storage system, respectively. In this paper, using a three-dimensional simulation, the performance of SCPP with and without artificial roughness has been investigated for the weather conditions of Semnan. Also, the dimensions of Manzanares power plant were considered, and the simulated SCPP, without roughness, was validated based on it. Simulation results reveal that, Semnan (53.23oE, 35.34oN), has a good potential for using SCPP. Without roughness, the maximum power of SCPP was 50.81 kW for April. It is found that the artificial roughness in SCPP improves heat transfer, but reduces velocity; therefore, there is an optimal dimension and location for it. Geometrically, the location of this roughness near the collector entrance has a better performance than other locations, but this change of locations has little impact on the performance of SCPP.