{"id":47519,"date":"2019-11-05T07:13:44","date_gmt":"2019-11-05T06:13:44","guid":{"rendered":"https:\/\/www.thermal-engineering.org\/o-que-e-numero-de-caracteristica-definicao\/"},"modified":"2020-01-23T15:19:40","modified_gmt":"2020-01-23T14:19:40","slug":"o-que-e-numero-de-caracteristica-definicao","status":"publish","type":"post","link":"https:\/\/www.thermal-engineering.org\/pt-br\/o-que-e-numero-de-caracteristica-definicao\/","title":{"rendered":"O que \u00e9 n\u00famero de caracter\u00edstica – defini\u00e7\u00e3o"},"content":{"rendered":"
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n\u00fameros de caracter\u00edstica s\u00e3o n\u00fameros sem dimens\u00e3o usados \u200b\u200bpara descrever um caractere do fluxo ou para descrever um caractere de transfer\u00eancia de calor.\u00a0N\u00fameros de caracter\u00edstica<\/div>\n<\/div>\n
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N\u00fameros de caracter\u00edstica<\/h2>\n

Na\u00a0din\u00e2mica de fluidos<\/a>\u00a0e na\u00a0transfer\u00eancia de calor<\/a>\u00a0, os\u00a0n\u00fameros das caracter\u00edsticas<\/strong>\u00a0s\u00e3o\u00a0n\u00fameros sem<\/strong>\u00a0dimens\u00e3o usados \u200b\u200bpara descrever um caractere do fluxo ou para descrever um caractere da transfer\u00eancia de calor.\u00a0Os n\u00fameros caracter\u00edsticos<\/strong>\u00a0podem ser usados \u200b\u200bpara comparar uma\u00a0situa\u00e7\u00e3o real<\/strong>\u00a0(por exemplo, fluxo de ar ao redor de um aerof\u00f3lio e fluxo de \u00e1gua em um tubo) com um\u00a0modelo de pequena escala<\/strong>\u00a0.\u00a0\u00c9 necess\u00e1rio manter os n\u00fameros das caracter\u00edsticas importantes iguais.\u00a0Os nomes desses n\u00fameros foram padronizados na\u00a0ISO 31-12<\/strong>\u00a0, que fornece nome, s\u00edmbolo e defini\u00e7\u00e3o para 25 n\u00fameros de caracter\u00edsticas selecionados usados \u200b\u200bpara a descri\u00e7\u00e3o de fen\u00f4menos de transporte.<\/p>\n<\/div>\n<\/div>\n

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N\u00famero de Reynolds<\/h2>\n

O\u00a0n\u00famero de Reynolds<\/a><\/strong>\u00a0\u00a0\u00e9 a propor\u00e7\u00e3o de\u00a0\u00a0for\u00e7as de in\u00e9rcia\u00a0<\/strong>\u00a0para\u00a0\u00a0as for\u00e7as viscosas<\/strong>\u00a0\u00a0e \u00e9 um par\u00e2metro conveniente para prever se uma condi\u00e7\u00e3o de escoamento vai ser\u00a0\u00a0laminar<\/a>\u00a0ou\u00a0turbulento<\/a><\/strong>\u00a0.\u00a0Pode-se interpretar que quando as\u00a0\u00a0for\u00e7as viscosas<\/strong>\u00a0\u00a0s\u00e3o dominantes (fluxo lento,\u00a0Re<\/strong>\u00a0baixo\u00a0) s\u00e3o suficientes o suficiente para manter todas as part\u00edculas de fluido alinhadas, ent\u00e3o o fluxo \u00e9 laminar.\u00a0Mesmo\u00a0Re<\/strong>\u00a0muito baixo\u00a0indica movimento de rastejamento viscoso, onde os efeitos da in\u00e9rcia s\u00e3o desprez\u00edveis.\u00a0Quando as\u00a0\u00a0for\u00e7as inerciais dominam<\/strong>\u00a0\u00a0sobre as for\u00e7as viscosas (quando o fluido est\u00e1 fluindo mais r\u00e1pido e Re \u00e9 maior), ent\u00e3o o fluxo \u00e9 turbulento.<\/p>\n

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

\u00c9 um n\u00famero adimensional<\/strong>\u00a0\u00a0composto pelas caracter\u00edsticas f\u00edsicas do fluxo.\u00a0Um n\u00famero crescente de Reynolds indica uma crescente turbul\u00eancia do fluxo.<\/p>\n

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\u00c9 definido como:<\/span>
\n\"N\u00famero<\/a><\/p>\n

em que:<\/span>
\nV \u00e9 a velocidade do fluxo,<\/span>
\nD \u00e9 uma\u00a0<\/span>\u00a0dimens\u00e3o linear caracter\u00edstica<\/span><\/strong>\u00a0(comprimento percorrido do fluido;\u00a0\u00a0<\/span>di\u00e2metro hidr\u00e1ulico<\/span><\/a>\u00a0\u00a0etc.)<\/span>
\n\u03c1 densidade do fluido (kg \/ m\u00a0<\/span>3<\/span><\/sup>\u00a0),<\/span>
\n\u03bc viscosidade din\u00e2mica (Pa.s),<\/span>
\n\u03bd viscosidade cinem\u00e1tica ( m\u00a0<\/span>2<\/span><\/sup>\u00a0\/ s);\u00a0\u03bd = \u03bc \/ \u03c1.<\/span><\/p>\n<\/div>\n<\/div>\n

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N\u00famero Prandtl<\/span><\/h2>\n

O\u00a0<\/span>n\u00famero Prandtl<\/span><\/strong><\/a>\u00a0\u00e9 um n\u00famero sem dimens\u00e3o, nomeado em homenagem ao seu inventor, um engenheiro alem\u00e3o\u00a0<\/span>Ludwig Prandtl<\/span><\/strong>\u00a0, que tamb\u00e9m identificou a\u00a0<\/span>camada limite<\/span><\/a>\u00a0.\u00a0O\u00a0<\/span>n\u00famero de Prandtl<\/span><\/strong>\u00a0\u00e9 definida como a\u00a0<\/span>raz\u00e3o<\/span><\/strong>\u00a0de\u00a0<\/span>difusividade impulso<\/span><\/strong>\u00a0a\u00a0<\/span>difusividade t\u00e9rmica<\/span><\/strong>\u00a0.\u00a0A\u00a0<\/span>difusividade do momento<\/span><\/strong>\u00a0, ou como \u00e9 normalmente chamada, viscosidade cinem\u00e1tica, indica a resist\u00eancia do material aos fluxos de cisalhamento (diferentes camadas do fluxo viajam com velocidades diferentes devido, por exemplo, a diferentes velocidades das paredes adjacentes) em rela\u00e7\u00e3o \u00e0 densidade.\u00a0Ou seja, o\u00a0<\/span>n\u00famero Prandtl<\/span><\/strong>\u00a0\u00e9 dado como:<\/span><\/p>\n

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

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

\u03bd<\/span><\/strong>\u00a0\u00e9\u00a0<\/span>difusividade do momento<\/span><\/strong>\u00a0(viscosidade cinem\u00e1tica) [m\u00a0<\/span>2<\/span><\/sup>\u00a0\/ s]<\/span><\/p>\n

\u03b1<\/span><\/strong>\u00a0\u00e9\u00a0<\/span>difusividade t\u00e9rmica<\/span><\/a><\/strong>\u00a0[m\u00a0<\/span>2<\/span><\/sup>\u00a0\/ s]<\/span><\/p>\n

\u03bc<\/span><\/strong>\u00a0\u00e9\u00a0<\/span>viscosidade din\u00e2mica<\/span><\/strong>\u00a0[Ns \/ m\u00a0<\/span>2<\/span><\/sup>\u00a0]<\/span><\/p>\n

k<\/span><\/strong>\u00a0\u00e9\u00a0<\/span>condutividade t\u00e9rmica<\/span><\/strong><\/a>\u00a0[W \/ mK]<\/span><\/p>\n

c\u00a0<\/span>p<\/span><\/sub><\/strong>\u00a0\u00e9\u00a0<\/span>calor espec\u00edfico<\/span><\/strong><\/a>\u00a0[J \/ kg.K]<\/span><\/p>\n

\u03c1<\/span><\/strong>\u00a0\u00e9\u00a0<\/span>densidade<\/span><\/a><\/strong>\u00a0[kg \/ m\u00a0<\/span>3<\/span><\/sup>\u00a0]<\/span><\/p>\n

Pequenos valores do\u00a0<\/span>n\u00famero de Prandtl<\/span><\/strong>\u00a0,\u00a0<\/span>Pr <<1<\/span><\/strong>\u00a0, significam que a difusividade t\u00e9rmica domina.\u00a0Enquanto que com valores grandes,\u00a0<\/span>Pr >> 1<\/span><\/strong>\u00a0, a difusividade do momento domina o comportamento.\u00a0Por exemplo, o valor t\u00edpico de merc\u00fario l\u00edquido, que \u00e9 de cerca de 0,025, indica que a\u00a0<\/span>condu\u00e7\u00e3o de calor<\/span><\/strong><\/a>\u00a0\u00e9 mais significativa em compara\u00e7\u00e3o com a\u00a0<\/span>convec\u00e7\u00e3o<\/span><\/strong>\u00a0, portanto a difusividade t\u00e9rmica \u00e9 dominante.\u00a0Quando Pr \u00e9 pequeno, significa que o calor se difunde rapidamente em compara\u00e7\u00e3o com a velocidade.<\/span><\/p>\n

Em compara\u00e7\u00e3o com o\u00a0<\/span>n\u00famero de Reynolds<\/span><\/a>\u00a0, o\u00a0<\/span>n\u00famero de Prandtl<\/span><\/strong>\u00a0n\u00e3o depende da geometria de um objeto envolvido no problema, mas depende apenas do estado do\u00a0<\/span>fluido<\/span><\/a>\u00a0e do fluido.\u00a0Como tal, o\u00a0<\/span>n\u00famero de Prandtl<\/span><\/strong>\u00a0\u00e9 freq\u00fcentemente encontrado em tabelas de propriedades, ao lado de outras propriedades, como viscosidade e condutividade t\u00e9rmica.<\/span><\/p>\n

Veja tamb\u00e9m:\u00a0<\/span>N\u00famero Prandtl<\/span><\/a><\/p>\n<\/div>\n<\/div>\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":"

n\u00fameros de caracter\u00edstica s\u00e3o n\u00fameros sem dimens\u00e3o usados \u200b\u200bpara descrever um caractere do fluxo ou para descrever um caractere de transfer\u00eancia de calor.\u00a0N\u00fameros de caracter\u00edstica N\u00fameros de caracter\u00edstica Na\u00a0din\u00e2mica de fluidos\u00a0e na\u00a0transfer\u00eancia de calor\u00a0, os\u00a0n\u00fameros das caracter\u00edsticas\u00a0s\u00e3o\u00a0n\u00fameros sem\u00a0dimens\u00e3o usados \u200b\u200bpara descrever um caractere do fluxo ou para descrever um caractere da transfer\u00eancia de calor.\u00a0Os n\u00fameros … 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 n\u00famero de caracter\u00edstica - defini\u00e7\u00e3o<\/title>\n<meta name=\"description\" content=\"n\u00fameros de caracter\u00edstica s\u00e3o n\u00fameros sem dimens\u00e3o usados para descrever um caractere do fluxo ou para descrever um caractere de transfer\u00eancia de calor. 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