{"id":48215,"date":"2019-11-08T16:35:02","date_gmt":"2019-11-08T15:35:02","guid":{"rendered":"https:\/\/www.thermal-engineering.org\/o-que-e-eficiencia-termica-do-ciclo-de-brayton-definicao\/"},"modified":"2020-01-28T08:26:30","modified_gmt":"2020-01-28T07:26:30","slug":"o-que-e-eficiencia-termica-do-ciclo-de-brayton-definicao","status":"publish","type":"post","link":"https:\/\/www.thermal-engineering.org\/pt-br\/o-que-e-eficiencia-termica-do-ciclo-de-brayton-definicao\/","title":{"rendered":"O que \u00e9 efici\u00eancia t\u00e9rmica do ciclo de Brayton – Defini\u00e7\u00e3o"},"content":{"rendered":"
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Os modernos motores de turbina a g\u00e1s e os motores a jato de respira\u00e7\u00e3o tamb\u00e9m seguem o ciclo de Brayton.\u00a0A efici\u00eancia t\u00e9rmica do ciclo de Brayton pode ser expressa em termos de temperaturas.\u00a0Engenharia T\u00e9rmica<\/div>\n<\/div>\n
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Efici\u00eancia t\u00e9rmica do ciclo de Brayton<\/h2>\n

Vamos assumir o\u00a0ciclo de Brayton ideal<\/strong>\u00a0que descreve o funcionamento de um\u00a0motor de calor com\u00a0<\/strong>press\u00e3o constante<\/strong>\u00a0.\u00a0Os modernos<\/strong>\u00a0motores de\u00a0turbina a g\u00e1s<\/strong>\u00a0e os motores a\u00a0jato de respira\u00e7\u00e3o<\/strong>\u00a0tamb\u00e9m seguem o ciclo de Brayton.\u00a0Esse ciclo consiste em quatro processos termodin\u00e2micos:<\/p>\n

    \n
  1. \n
    \"primeira<\/a>
    O ciclo ideal de Brayton consiste em quatro processos termodin\u00e2micos.\u00a0Dois processos isentr\u00f3picos e dois processos isob\u00e1ricos.<\/figcaption><\/figure>\n

    compress\u00e3o isentr\u00f3pica<\/strong>\u00a0– o ar ambiente \u00e9 aspirado para o compressor, onde \u00e9 pressurizado (1 \u2192 2).\u00a0O trabalho necess\u00e1rio para o compressor \u00e9 dado por\u00a0W\u00a0<\/strong>C<\/sub><\/strong>\u00a0= H\u00a0<\/strong>2<\/sub><\/strong>\u00a0– H\u00a0<\/strong>1<\/sub><\/strong>\u00a0.<\/strong><\/li>\n

  2. adi\u00e7\u00e3o de calor isob\u00e1rico<\/strong>\u00a0– o ar comprimido passa por uma c\u00e2mara de combust\u00e3o, onde o combust\u00edvel \u00e9 queimado e o ar ou outro meio \u00e9 aquecido (2 \u2192 3).\u00a0\u00c9 um processo de press\u00e3o constante, pois a c\u00e2mara est\u00e1 aberta para entrar e sair.\u00a0O calor l\u00edquido adicionado \u00e9 dado por\u00a0Q\u00a0<\/strong>add<\/sub><\/strong>\u00a0= H\u00a0<\/strong>3<\/sub><\/strong>\u00a0– H\u00a0<\/strong>2<\/sub><\/strong><\/li>\n
  3. expans\u00e3o isentr\u00f3pica<\/strong>\u00a0– o ar aquecido e pressurizado se expande na turbina, gasta sua energia.\u00a0O trabalho realizado pela turbina \u00e9 dado por\u00a0W\u00a0<\/strong>T<\/sub><\/strong>\u00a0= H\u00a0<\/strong>4<\/sub><\/strong>\u00a0– H\u00a0<\/strong>3<\/sub><\/strong><\/li>\n
  4. rejei\u00e7\u00e3o de calor isob\u00e1rica<\/strong>\u00a0– o calor residual deve ser rejeitado para fechar o ciclo.\u00a0O calor l\u00edquido rejeitado \u00e9 dado por\u00a0Q\u00a0<\/strong>re<\/sub><\/strong>\u00a0= H\u00a0<\/strong>4<\/sub><\/strong>\u00a0– H\u00a0<\/strong>1<\/sub><\/strong><\/li>\n<\/ol>\n

    Como pode ser visto, podemos descrever e calcular (por exemplo, efici\u00eancia termodin\u00e2mica) esses ciclos (da mesma forma para o\u00a0ciclo de Rankine<\/strong>\u00a0) usando\u00a0entalpias<\/a>\u00a0.<\/p>\n

    Para calcular a\u00a0efici\u00eancia t\u00e9rmica<\/strong>\u00a0do\u00a0ciclo de Brayton<\/strong>\u00a0(compressor \u00fanico e turbina \u00fanica), os engenheiros usam a\u00a0primeira lei da termodin\u00e2mica<\/strong>\u00a0em termos de entalpia e n\u00e3o em energia interna.<\/p>\n

    A\u00a0primeira lei em termos de entalpia<\/a>\u00a0\u00e9:<\/p>\n

    dH = dQ + Vdp<\/strong><\/em><\/p>\n

    Nesta equa\u00e7\u00e3o, o termo\u00a0Vdp<\/em><\/strong>\u00a0\u00e9 um\u00a0trabalho de processo de fluxo.\u00a0<\/strong>Este trabalho, \u00a0\u00a0Vdp<\/em><\/strong>\u00a0, \u00e9 usado para\u00a0sistemas de fluxo aberto<\/strong>\u00a0como uma\u00a0turbina<\/strong>\u00a0ou uma\u00a0bomba<\/strong>\u00a0na qual existe um\u00a0“dp”<\/strong>\u00a0, ou seja, mudan\u00e7a de press\u00e3o.\u00a0N\u00e3o h\u00e1 altera\u00e7\u00f5es no\u00a0volume de controle<\/a>\u00a0.\u00a0Como pode ser visto, essa forma de lei\u00a0simplifica a descri\u00e7\u00e3o da transfer\u00eancia de energia<\/strong>\u00a0.<\/p>\n

    Existem express\u00f5es em termos de vari\u00e1veis \u200b\u200bmais familiares, como\u00a0<\/span>temperatura<\/span><\/a>\u00a0e\u00a0<\/span>press\u00e3o<\/span><\/a>\u00a0:<\/span><\/p>\n

    dH = C\u00a0<\/span><\/em><\/strong>p<\/span><\/sub><\/em><\/strong>\u00a0dT + V (1-aT) dp<\/span><\/em><\/strong><\/p>\n

    Onde\u00a0<\/span>C\u00a0<\/span><\/strong>p<\/span><\/sub><\/strong>\u00a0\u00e9 a\u00a0<\/span>capacidade de calor<\/span><\/a>\u00a0a uma press\u00e3o constante<\/span><\/strong>\u00a0e\u00a0<\/span>\u03b1<\/span><\/em><\/strong>\u00a0\u00e9 o coeficiente de expans\u00e3o t\u00e9rmica (c\u00fabico).\u00a0Para\u00a0<\/span>o g\u00e1s ideal<\/span><\/strong>\u00a0\u03b1T = 1 e, portanto:<\/span><\/p>\n

    dH = C\u00a0<\/span><\/em><\/strong>p<\/span><\/sub><\/em><\/strong>\u00a0dT<\/span><\/em><\/strong><\/p>\n

    A press\u00e3o constante<\/span><\/strong>\u00a0, a\u00a0<\/span>mudan\u00e7a de entalpia<\/span><\/strong>\u00a0\u00e9 igual \u00e0\u00a0<\/span>energia<\/span><\/strong>\u00a0transferida do ambiente atrav\u00e9s do aquecimento:<\/span><\/p>\n

    Processo isob\u00e1rico (Vdp = 0):<\/span><\/strong><\/p>\n

    dH = dQ \u2192 Q = H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong>\u00a0\u00a0\u00a0\u2192 H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong>\u00a0=\u00a0<\/span>C\u00a0<\/span><\/em><\/strong>P<\/span><\/sub><\/em><\/strong>\u00a0(T\u00a0<\/span><\/em><\/strong>2<\/span><\/sub><\/em><\/strong>\u00a0– T\u00a0<\/span><\/em><\/strong>1<\/span><\/sub><\/em><\/strong>\u00a0)<\/span><\/em><\/strong><\/p>\n

    Na entropia constante<\/span><\/strong>\u00a0, ou seja, no processo isentr\u00f3pico, a\u00a0<\/span>mudan\u00e7a de entalpia<\/span><\/strong>\u00a0\u00e9 igual ao\u00a0<\/span>trabalho do processo de fluxo<\/span><\/strong>\u00a0realizado no ou pelo sistema:<\/span><\/p>\n

    Processo isentr\u00f3pico (dQ = 0):<\/span><\/strong><\/p>\n

    dH = Vdp \u2192 W = H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u2192 H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong>\u00a0=\u00a0<\/span>C\u00a0<\/span><\/em><\/strong>P<\/span><\/sub><\/em><\/strong>\u00a0(T\u00a0<\/span><\/em><\/strong>2<\/span><\/sub><\/em><\/strong>\u00a0– T\u00a0<\/span><\/em><\/strong>1<\/span><\/sub><\/em><\/strong>\u00a0)<\/span><\/em><\/strong><\/p>\n

    \"primeira<\/a>
    O ciclo ideal de Brayton consiste em quatro processos termodin\u00e2micos.\u00a0Dois processos isentr\u00f3picos e dois processos isob\u00e1ricos.<\/span><\/figcaption><\/figure>\n

    A\u00a0<\/span>entalpia<\/span><\/strong>\u00a0pode ser transformada em uma\u00a0vari\u00e1vel\u00a0<\/span>intensiva<\/span><\/strong>\u00a0ou\u00a0<\/span>espec\u00edfica<\/span><\/strong>\u00a0dividindo-se pela\u00a0<\/span>massa<\/span><\/a>\u00a0.\u00a0<\/span>Os engenheiros usam mais a\u00a0<\/span><\/strong>entalpia espec\u00edfica<\/span><\/strong>\u00a0na an\u00e1lise termodin\u00e2mica do que a pr\u00f3pria entalpia.\u00a0A efici\u00eancia t\u00e9rmica desse ciclo simples de Brayton, para o g\u00e1s ideal e em termos de entalpias espec\u00edficas, agora pode ser expressa em termos de temperatura:<\/span><\/p>\n

    \"efici\u00eancia<\/a><\/p>\n<\/div>\n<\/div>\n

    ……………………………………………………………………………………………………………………………….<\/p>\n

    Este artigo \u00e9 baseado na tradu\u00e7\u00e3o autom\u00e1tica do artigo original em ingl\u00eas. Para mais informa\u00e7\u00f5es, consulte o artigo em ingl\u00eas. Voc\u00ea pode nos ajudar. Se voc\u00ea deseja corrigir a tradu\u00e7\u00e3o, envie-a para: translations@nuclear-power.com ou preencha o formul\u00e1rio de tradu\u00e7\u00e3o on-line. Agradecemos sua ajuda, atualizaremos a tradu\u00e7\u00e3o o mais r\u00e1pido poss\u00edvel. Obrigado.<\/p>\n","protected":false},"excerpt":{"rendered":"

    Os modernos motores de turbina a g\u00e1s e os motores a jato de respira\u00e7\u00e3o tamb\u00e9m seguem o ciclo de Brayton.\u00a0A efici\u00eancia t\u00e9rmica do ciclo de Brayton pode ser expressa em termos de temperaturas.\u00a0Engenharia T\u00e9rmica Efici\u00eancia t\u00e9rmica do ciclo de Brayton Vamos assumir o\u00a0ciclo de Brayton ideal\u00a0que descreve o funcionamento de um\u00a0motor de calor com\u00a0press\u00e3o constante\u00a0.\u00a0Os … Ler mais<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[14],"tags":[],"yoast_head":"\nO que \u00e9 efici\u00eancia t\u00e9rmica do ciclo de Brayton - Defini\u00e7\u00e3o<\/title>\n<meta name=\"description\" content=\"Os modernos motores de turbina a g\u00e1s e os motores a jato de respira\u00e7\u00e3o tamb\u00e9m seguem o ciclo de Brayton. 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