{"id":47563,"date":"2019-11-05T10:50:31","date_gmt":"2019-11-05T09:50:31","guid":{"rendered":"https:\/\/www.thermal-engineering.org\/o-que-e-o-numero-rayleigh-definicao\/"},"modified":"2020-01-23T20:13:00","modified_gmt":"2020-01-23T19:13:00","slug":"o-que-e-o-numero-rayleigh-definicao","status":"publish","type":"post","link":"https:\/\/www.thermal-engineering.org\/pt-br\/o-que-e-o-numero-rayleigh-definicao\/","title":{"rendered":"O que \u00e9 o n\u00famero Rayleigh – defini\u00e7\u00e3o"},"content":{"rendered":"
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O n\u00famero de Rayleigh \u00e9 um n\u00famero sem dimens\u00e3o, com o nome de Lord Rayleigh.\u00a0O n\u00famero de Rayleigh \u00e9 usado para expressar a transfer\u00eancia de calor em convec\u00e7\u00e3o natural.\u00a0Engenharia T\u00e9rmica<\/div>\n<\/div>\n
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O que \u00e9 o n\u00famero Rayleigh<\/h2>\n

O\u00a0n\u00famero de Rayleigh<\/strong>\u00a0\u00e9 um n\u00famero sem dimens\u00e3o, com o nome de Lord Rayleigh.\u00a0O\u00a0n\u00famero de Rayleigh<\/strong>\u00a0\u00a0est\u00e1 intimamente relacionado ao n\u00famero de Grashof e ambos os n\u00fameros s\u00e3o usados \u200b\u200bpara descrever a convec\u00e7\u00e3o natural (Gr) e a transfer\u00eancia de calor por convec\u00e7\u00e3o natural (Ra).\u00a0O\u00a0\u00a0n\u00famero de Rayleigh<\/strong>\u00a0\u00a0\u00e9 simplesmente definido como o produto do\u00a0n\u00famero de Grashof<\/a>\u00a0, que descreve a rela\u00e7\u00e3o entre flutuabilidade e viscosidade dentro de um fluido, e o\u00a0n\u00famero de Prandtl<\/a>\u00a0, que descreve a rela\u00e7\u00e3o entre difusividade de momento e difusividade t\u00e9rmica.<\/p>\n

Ra\u00a0x<\/sub>\u00a0= Gr\u00a0x<\/sub>\u00a0.\u00a0Pr<\/strong><\/p>\n

O\u00a0\u00a0n\u00famero de Grashof<\/strong>\u00a0\u00a0\u00e9 definido como a raz\u00e3o entre a for\u00e7a flutuante e a viscosa que atua sobre um fluido na camada limite de velocidade.\u00a0Seu papel na convec\u00e7\u00e3o natural \u00e9 o mesmo que o\u00a0\u00a0n\u00famero de Reynolds<\/a>\u00a0\u00a0na convec\u00e7\u00e3o for\u00e7ada.\u00a0A convec\u00e7\u00e3o natural ocorre se esse movimento e mistura s\u00e3o causados \u200b\u200bpor\u00a0varia\u00e7\u00f5es de\u00a0densidade<\/a>\u00a0resultantes de diferen\u00e7as de temperatura no fluido.\u00a0Normalmente, a densidade diminui devido a um aumento de\u00a0temperatura<\/a>\u00a0e faz com que o fluido suba.\u00a0Este movimento \u00e9 causado pela for\u00e7a flutuante.\u00a0A for\u00e7a principal que resiste ao movimento \u00e9 a for\u00e7a viscosa.\u00a0O n\u00famero de Grashof \u00e9 uma maneira de quantificar as for\u00e7as opostas.<\/p>\n

O\u00a0n\u00famero de Rayleigh<\/strong>\u00a0\u00e9 usado para expressar a transfer\u00eancia de calor em convec\u00e7\u00e3o natural.\u00a0A magnitude do n\u00famero de Rayleigh \u00e9 uma boa indica\u00e7\u00e3o sobre se a camada limite da convec\u00e7\u00e3o natural \u00e9 laminar ou turbulenta.\u00a0As correla\u00e7\u00f5es emp\u00edricas simples para o n\u00famero m\u00e9dio de Nusselt, Nu, em convec\u00e7\u00e3o natural, s\u00e3o da forma:<\/p>\n

Nu\u00a0x<\/sub>\u00a0\u00a0= C. Ra\u00a0x\u00a0<\/sub>n<\/sup><\/strong><\/p>\n

Os valores das\u00a0constantes C<\/strong>\u00a0e\u00a0n<\/strong>\u00a0dependem da geometria da superf\u00edcie e do\u00a0regime de vaz\u00e3o<\/a>\u00a0, que \u00e9 caracterizado pela faixa do\u00a0n\u00famero de Rayleigh<\/strong>\u00a0.\u00a0O valor de n \u00e9 geralmente n = 1\/4 para fluxo laminar en = 1\/3 para fluxo turbulento.<\/p>\n

O\u00a0\u00a0n\u00famero de Rayleigh<\/strong>\u00a0\u00e9 definido como:<\/p>\n

\"n\u00famero<\/a><\/p>\n

Onde:<\/span><\/p>\n

g \u00e9 acelera\u00e7\u00e3o devido \u00e0 gravidade da Terra<\/span><\/p>\n

\u03b2 \u00e9 o coeficiente de expans\u00e3o t\u00e9rmica<\/span><\/p>\n

Parede<\/span><\/sub>\u00a0T\u00a0\u00e9 a temperatura da parede<\/span><\/p>\n

T\u00a0<\/span>\u221e<\/span><\/sub>\u00a0\u00e9 a temperatura a granel<\/span><\/p>\n

L \u00e9 o comprimento vertical<\/span><\/p>\n

\u03b1 \u00e9\u00a0\u00a0<\/span>difusividade t\u00e9rmica<\/span><\/a><\/p>\n

\u03bd \u00e9 a viscosidade cinem\u00e1tica.<\/span><\/p>\n

Para gases \u03b2 = 1 \/ T onde a temperatura est\u00e1 em K. Para l\u00edquidos \u03b2 pode ser calculado se for conhecida a varia\u00e7\u00e3o da densidade com a temperatura a press\u00e3o constante.\u00a0Para uma placa plana vertical, o fluxo fica\u00a0<\/span>turbulento<\/span><\/a>\u00a0pelo valor de:<\/span><\/p>\n

Ra\u00a0<\/span>x<\/span><\/sub>\u00a0\u00a0= Gr\u00a0<\/span>x<\/span><\/sub>\u00a0\u00a0.\u00a0Pr> 10\u00a0<\/span>9<\/span><\/sup>\u00a0<\/strong><\/p>\n

Como na convec\u00e7\u00e3o for\u00e7ada, a natureza microsc\u00f3pica das correla\u00e7\u00f5es de fluxo e convec\u00e7\u00e3o s\u00e3o distintamente diferentes nas regi\u00f5es\u00a0<\/span>laminar<\/span><\/a>\u00a0e\u00a0<\/span>turbulenta<\/span><\/a>\u00a0.<\/span><\/p>\n

Exemplo: N\u00famero Rayleigh<\/span><\/strong><\/h2>\n

\"N\u00famero<\/a>Uma placa vertical \u00e9 mantida a 50 \u00b0 C em 20 \u00b0 C de ar.\u00a0Determine a altura em que a camada limite ficar\u00e1 turbulenta se a turbul\u00eancia se estabelecer em Ra = Gr.Pr = 10\u00a0<\/span>9<\/span><\/sup>\u00a0.<\/span><\/p>\n

Solu\u00e7\u00e3o:<\/span><\/p>\n

Os valores da propriedade necess\u00e1rios para este exemplo s\u00e3o:<\/span><\/p>\n

\u03bd = 1,48 x\u00a0<\/span>10-5<\/span><\/sup>\u00a0m\u00a0<\/span>2<\/span><\/sup>\u00a0\/ s<\/span><\/p>\n

\u03c1 = 1,17 kg \/ m\u00a0<\/span>3<\/span><\/sup><\/p>\n

Pr = 0,700<\/span><\/p>\n

\u03b2 = 1 \/ (273 + 20) = 1\/293<\/span><\/p>\n

Sabemos que a circula\u00e7\u00e3o natural se torna turbulenta em aproximadamente Ra = Gr.Pr> 10\u00a0<\/span>9<\/span><\/sup>\u00a0, que \u00e9 cumprida na seguinte altura:<\/span><\/p>\n

\"n\u00famero<\/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":"

O n\u00famero de Rayleigh \u00e9 um n\u00famero sem dimens\u00e3o, com o nome de Lord Rayleigh.\u00a0O n\u00famero de Rayleigh \u00e9 usado para expressar a transfer\u00eancia de calor em convec\u00e7\u00e3o natural.\u00a0Engenharia T\u00e9rmica O que \u00e9 o n\u00famero Rayleigh O\u00a0n\u00famero de Rayleigh\u00a0\u00e9 um n\u00famero sem dimens\u00e3o, com o nome de Lord Rayleigh.\u00a0O\u00a0n\u00famero de Rayleigh\u00a0\u00a0est\u00e1 intimamente relacionado ao n\u00famero … 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 o n\u00famero Rayleigh - defini\u00e7\u00e3o<\/title>\n<meta name=\"description\" content=\"O n\u00famero de Rayleigh \u00e9 um n\u00famero sem dimens\u00e3o, com o nome de Lord Rayleigh. 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