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Summary: Present work encompasses developing a new methodology to incorporate the heat transfer characteristics of the evacuated space (vacuum) in the annulus region between the absorber tube and glass cover and the effect of selective coatings on the steel absorber tube for a 3‐dimensional computational fluid dynamics model of a parabolic trough solar collector (PTSC) receiver. In the present work, glass cover has been modeled semitransparent to the incoming solar flux, which has been applied on the glass cover. Wavelength‐dependent absorption properties of glass cover material also has been modeled. In the current work, the direct normal irradiation and the mass flow rate of heat transfer fluid (HTF) has been varied to check their individual effect on the performance of PTSC absorber tube. Circumferential temperature distribution of absorber tube and glass cover has been found out; heat transfer to HTF and pressure drop of HTF in the absorber tube have also been calculated. A comparative study has been undertaken to highlight the relative contribution of the vacuum and selective coating on the receiver performance. The value of circumferential temperature difference at mid‐length of absorber tube was found to be 14.78 K for 750 W/m2, which increases to 19.70 K for direct normal irradiation of 1000 W/m2. When vacuum is replaced by air in the annulus region, PTSC absorber performance decreases by 4.82%. Similarly, because of use of selective coating, the thermal efficiency of the receiver increased by 34.59%. Present work encompasses development of a new methodology to incorporate heat transfer characteristics of the evacuated space (vacuum) in the annulus region between the absorber tube and glass cover and selective coatings on the steel absorber tube for a 3‐dimensional computational fluid dynamics model of a parabolic trough solar collector receiver. Glass cover has been modeled semitransparent to the incoming solar flux, which has been applied on the glass cover. [ABSTRACT FROM AUTHOR]

In this paper, the forced convective heat transfer enhancement with nanofluids in a 90° pipe bend has been presented. Numerical investigation is carried out for the turbulent flow through the pipe employing finite volume method. The governing differential equations are discretized using hexahedral cells, and the resulting algebraic equations are solved using Commercial solver Fluent 6.3. In order to close the time averaged Navier-Stokes equations, the two-equation κ-ε turbulence model with a standard wall function have been used. The duct Reynolds number is varied in the range of 2,500-6,000. It is observed that the heat transfer is enhanced significantly by varying the volume fraction of the nanofluid. It is also found that the heat transfer is increased with Reynolds number. A strong secondary flow is observed due to the presence of the wall. Turbulent kinetic energy near outer wall is found to be higher than the inner wall of the bend. A comparative assessment for the heat transfer enhancement with different types of nanofluids is also presented. The computed results of area weighted average Nusselt numbers are validated with some of the existing literature. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the analysis of heat losses from the trapezoidal cavity receiver used in linear Fresnel reflector (LFR) system. The experimental studies are conducted under laboratory conditions that are specially designed for this purpose. The effects of parameters such as the temperatures of the tube surface, depth of receiver, number of tubes, and emissivity of tubes are investigated. The loss of heat is taking place from the tube outer surface to glass cover, below the receiver and then glass cover to ambient. As part of this investigation, the system is modelled and simulated using computational fluid dynamics (CFD). After validation, contribution of convection and radiation to the total heat transfer are found out using CFD. Computational predictions are shown to be consistent with the experimental observations which show that the CFD model is a reliable tool for predicting heat loss and overall heat loss coefficient. It was found that losses due to convection are between 5 and 18% of the total heat losses. [Copyright &y& Elsevier]

• Effect of the tilt angle on the limiting value of heat flux on TEG is established. • Effect of wind direction on the limiting input heat flux for TEGs is investigated. • Sizing of Fresnel lens concentrator is carried out for effective TEG. • Effect of ambient air temperature on the limiting input heat flux for TEG is found. This paper focuses on establishing the dependence of limiting value of input heat flux for solar assisted thermoelectric generators (TEG) on Angle of Attack (AOA) and wind direction of heat sink of TEG. Current study investigates the limiting input heat flux for TEGs with allowable hot side temperature of 150 °C. A fin block with eight fin configuration and fin length of 60 mm is chosen as heat sink configuration for TEG. Computational Fluid Dynamics (CFD) model is developed and analyzed in this work after validation with published experimental results. CFD model consists of four TEGs encapsulated within a target block and a finned block, placed within a domain which simulates low speed wind tunnel. Air flow in the wind tunnel simulates the outdoor wind conditions. Concentrated solar flux is applied on the face of target block. Effect of ambient air temperature on limiting input heat flux is also studied. Dependence of limiting input heat flux on Angle of Attack (AOA) and wind direction, which arises due to the two axis tracking of Sun by Fresnel lens concentrator has been quantified and it is observed that maximum limiting input heat flux occurs at 30 ° AOA. Maximum limiting heat flux reaches 24,000 W/m2 for 30 ° AOA under 5 m/s wind velocity. It is also observed that maximum input heat flux varies by 42.49% with change in wind velocity from 1 to 5 m/s. Maximum limiting input heat flux occurs at 45 ° for horizontal rotation. Area of square array Fresnel lens concentrator varies by 6.29% for 5 m/s wind velocity when AOA is considered to when AOA is not considered for calculation of Fresnel lens area. [ABSTRACT FROM AUTHOR]