The effect of refrigerant blends 410A and 507 on the operation of an ejector cooling system -ECS-is theoretically studied with the aid of a validated multi-geometry ejector mathematical model. For a system cooling capacity of 1 kW, a set of possible design conditions were obtained by means of a parametric study varying the generator, condenser and evaporator temperatures from 50ºC to 70ºC; 30ºC to 40ºC and 5ºC to 15ºC. The ejector's lower U and values are obtained when R410A is employed, meaning higher primary fluid mass flow rates and smaller ejectors, respectively. ECS's upper COP s values are achieved at higher T GE and T EV with lower T CO . An ECS operating with these blends has a higher COP s using R410A, around 0.53 for T GE , T CO and T EV of 70ºC, 30ºC and 10ºC, respectively. The system using R507 has a similar but slightly lower performance. Therefore, the systems employing R410A and R507 are good options when a maximum generation temperature of 70ºC is available from a thermal source, either solar or industrial. As well, an ECS working with R410A and R507 experiences higher pressures and has a robust construction.
NomenclatureCOP coefficient of performance (dimensionless) d diameter (m) ECS ejector cooling system F f friction factor (dimensionless) h specific enthalpy (kJ/kg) l distance, length (m) m mass flow rate (kg/s) p pressure (MPa) Q heat flow rate (kW) T temperature (ºC) U entrainment ratio, 2 m / 1 m , (dimensionless) W mechanical power (kW) ejector area ratio, (d m /d t ) 2 , (dimensionless) efficiency (dimensionless) Subscripts CO condenser d diffuser e main nozzle exit EV evaporator GE generator m mixing chamber n main nozzle, between main nozzle exit and mixing chamber inlet p r reversible pump s system t main nozzle throat 1…6 thermodynamic cycle states Jorge I. Hernandez et al. / Energy Procedia 57 ( 2014 ) 3021 -3030 3023
Ligado a la crisis económica de Estados Unidos que estalló a finales de 2007, el Valle del Mezquital ha experimentado un alto retorno migratorio. En esta región, la migración internacional había definido parte fundamental de la dinámica económica desde finales del siglo pasado. El autor plantea la interrogante: ¿De qué manera se relacionan con la actividad campesina los migrantes internacionales retornados desde Estados Unidos al Valle del Mezquital? En su respuesta, concluye la emergencia de un proceso de recampesinización forzada por las condiciones de la agricultura y el mercado de trabajo local.
En este artículo se busca esbozar una antropología de lo contemporáneo que fundamente el planteamiento del tiempo presente y la imaginación del pasado y el futuro de una comunidad rural e indígena del centro de México. Mediante un acercamiento etnográfico presentado en cuatro escenas que tienen a la música como argumento, se abordan las dinámicas de construcción sociotemporal de la contemporaneidad. Con ello, el artículo hace una aportación a la escasa literatura en español sobre la antropología del tiempo y, en específico, sobre la antropología de lo contemporáneo en comunidades rurales, intentando ir más allá de los abordajes que se abocan a la búsqueda de la anterioridad pura o que, por el contrario, se concentran en la exaltación de lo nuevo entendido como ruptura con el pasado. De ahí que se concluya que lo contemporáneo no es el empate entre lo viejo y lo nuevo. Es la disrupción del tiempo que permite precisamente aprehender el tiempo vivo, el de la situación, el del transcurrir que siempre tiene un pie en el “no más” y otro en el “no aún”. Lo contemporáneo es el tiempo sociohistórico de la inteligibilidad, el tiempo en común, aquel presente compartido, que otorga viabilidad al presente etnográfico y que permite la imaginación histórica y la del futuro en la teoría antropológica.
In the socio-anthropological literature on the social change linked to the migration of Mexican indigenous populations, first country-city and then international, a kind of gray area is perceived, in which the linearity of the process seems to be decoupled. In particular, I refer to the contrasting narratives that at one pole have the indigenous atavistic and apathetic to change, presented by the indigenous ethnographies of the last century; and, on the other, the indigenous person who in contemporary anthropologies appears as appealing to novelty, consumption, overcoming poverty, educational improvement and everything involved in the search for well-being. In this paper, I propose that an alternative to investigate this gray area is to pay attention to the temporal substrates involved in said changes, to the ways in which indigenous communities manage to articulate the old and the new unevenly and discontinuously in the construction of their contemporaneities and , above all, to the multiple ways in which the search for ways to earn a living linked to constant mobility, were awakening humble aspirations and dreams among migrant workers and their communities that, committed to these images of the future, foreshadowed and foreshadowed the to come in your present practice.
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