Modelo unidimensional de transferência de calor em um absorverdor trapezoidal multi-tubos para concentradores Fresnel lineares

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMO: Este artigo descreve o modelo matemático unidimensional de transferência de calor em um absorvedor multi-tubos com cavidade trapezoidal utilizado em um concentrador solar do tipo Fresnel Linear. Todas as taxas de transferência de calor foram modeladas analiticamente e o modelo foi validado experimentalmente. Os experimentos para a validação do modelo foram realizados em um Fresnel com 60 m² de área localizados na cidade de Florianópolis. Nas condições analisadas, com diferenças de temperatura entre o fluido e o ambiente de até 200°C, aproximadamente 70% das perdas térmicas ocorrem por convecção das chapas metálicas com o ar externo e 30% por radiação e convecção pela janela de vidro. Na comparação dos resultados do modelo com os dos testes experimentais foi observado uma diferença média de 10%, sendo superior no modelo. As trocas de calor pelo ar no interior da cavidade foram consideradas como por difusão apenas, desconsiderando efeitos advectivos e, esta consideração não implicou em erros significativos.ABSTRACT: This article describes the one-dimensional heat transfer mathematical model of a multi-tube trapezoidal cavity absorber used in a linear Fresnel concentrator. All heat transfer rates were modeled analytically and the model was experimentally validated. The experiments for the validation of the model were carried out in a Fresnel with 60 m² of area located in the city of Florianópolis. Under the conditions analyzed, with temperature differences between the fluid and the environment of up to 200 ° C, approximately 70% of the thermal losses occur by convection of the metal plates with the external air and 30% by radiation and convection through the glass window. When comparing the results of the model with those of the experimental tests, an average difference of 10% was observed, being higher in the model. The heat exchanges for air inside the cavity were considered to be by diffusion only, disregarding advective effects and, this consideration did not imply significant errors.info:eu-repo/semantics/publishedVersio

A cosmological concordance model with dynamical vacuum term

We demonstrate that creation of dark-matter particles at a constant rate implies the existence of a cosmological term that decays linearly with the Hubble rate. We discuss the cosmological model that arises in this context and test it against observations of the first acoustic peak in the cosmic microwave background (CMB) anisotropy spectrum, the Hubble diagram for supernovas of type Ia (SNIa), the distance scale of baryonic acoustic oscillations (BAO) and the distribution of large scale structures (LSS). We show that a good concordance is obtained, albeit with a higher value of the present matter abundance than in the \Lambda CDM model. We also comment on general features of the CMB anisotropy spectrum and on the cosmic coincidence problem.Comment: Revised version. Accepted for publication in Physics Letters