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Abstract: In stably and unstably stratified fluid layers there are often highly anisotropic and counter gradient heat fluxes occurring. Standard heat flux models as the isotropic k–ε–σ t model need to be improved for representing such behaviour. More complex algebraic models or even in some cases the full transport equations for the turbulent heat fluxes are therefore required. There, a triple correlation appears as an important closure term in the turbulent diffusion. Usually, this is modelled following Daly and Harlow, which has already been found to be not sufficiently accurate in buoyant flows. In this paper, some of the salient features of an internally heated fluid layer (IHL) and of Rayleigh–Bénard convection (RBC) are discussed basing on direct numerical simulation (DNS) data. In IHL a counter gradient heat flux occurs over a wide region. The transport equation for the triple correlation is analyzed using the DNS data. Based on this study a Reynolds-Averaged Navier Stokes (RANS) model for this closure term is derived which covers the influence of the fluid Prandtl number (Pr) and of buoyancy. The model is validated using the DNS data of both RBC and IHL for different Rayleigh and Prandtl numbers. [Copyright &y& Elsevier]

• Transport of an isolated, dust particle in a circular, straight, absorber pore is analyzed. • The effect of dust accumulation, in an absorber pore, on the heat transfer is analyzed. • Analysis show that dust deposition, in a straight pore, may be mitigated by applying heat-flux. • Dust accumulation is found to be detrimental, for heat transfer in an absorber pore. The deposition of dust in the absorber pores of an open volumetric air receiver (OVAR) offers a significant challenge in arid deserts. In this paper, to address this problem, two aspects are investigated, (i) the transport of an isolated dust particle in a straight, circular absorber pore, for a steady, thermally-hydraulically developing, laminar flow, and (ii) the consequences on heat transfer due to dust particles accumulation, in a straight, circular, absorber pore. The first problem is investigated with a semi-analytical approach and the second problem makes use of computational fluid dynamics simulations. The finding shows (a) a heat flux may be beneficial, for mitigating the dust deposition in a, straight, circular, clean absorber pore, (b) dust deposition in a straight, circular, absorber pore is detrimental to heat transfer, which may lead to its failure, (c) the thermal development length will be reduced for the dust deposited pore in comparison to a clean pore. The findings reveal a way of mitigating dust deposition in a clean pore, and the need for a cleaning strategy of dust deposited absorber pore, for sustainable operation of an open volumetric air receiver. [ABSTRACT FROM AUTHOR]

It is necessary to investigate the relative effect of various phenomena before neglecting the same for simulating the single-phase natural circulation loop to obtain an acceptable prediction. Hence, the present study focuses on the validation and effect of different assumptions related to Boussinesq approximation, property variation, bend effect, heat loss, axial fluid and wall conduction on the transient and steady-state characteristics. Effect of assumptions for different working fluids (water, brines and hybrid nanofluids), and assess the effect of different heat flux distributions, like uniform, linear, non-linear, sinusoidal and Gaussian, applied to heater are also explored. Mass flow rate (predicts heat transport capability), effectiveness (predicts heat transfer ability) and entropy generation rate (predicts irreversibility) are evaluated. The results disclose that the Boussinesq approximation with constant other properties under-predicts the steady-state mass flow rate and effectiveness and over-predicts the entropy generation rate. This assumption cannot be applied for all working fluids. The fluid and solid conduction is the crucial parameter for simulating transient characteristics. Gaussian heat flux distribution shows the highest mass flow rate and non-linear increasing heat flux shows the highest effectiveness and entropy generation rate. This study will help to consider reasonable assumptions for the numerical simulation. [ABSTRACT FROM AUTHOR]