{"id":47531,"date":"2019-11-05T08:11:58","date_gmt":"2019-11-05T07:11:58","guid":{"rendered":"https:\/\/www.thermal-engineering.org\/o-que-e-o-numero-prandtl-de-agua-e-ar-definicao\/"},"modified":"2020-01-23T20:04:48","modified_gmt":"2020-01-23T19:04:48","slug":"o-que-e-o-numero-prandtl-de-agua-e-ar-definicao","status":"publish","type":"post","link":"https:\/\/www.thermal-engineering.org\/pt-br\/o-que-e-o-numero-prandtl-de-agua-e-ar-definicao\/","title":{"rendered":"O que \u00e9 o n\u00famero Prandtl de \u00e1gua e ar – Defini\u00e7\u00e3o"},"content":{"rendered":"
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N\u00famero Prandtl de \u00c1gua e Ar.\u00a0O ar \u00e0 temperatura ambiente tem um n\u00famero Prandtl de 0,71 e para a \u00e1gua a 18 \u00b0 C \u00e9 de cerca de 7,56, o que significa que a difusividade t\u00e9rmica \u00e9 mais dominante para o ar do que para a \u00e1gua.\u00a0Engenharia T\u00e9rmica<\/div>\n<\/div>\n
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N\u00famero Prandtl de \u00e1gua e ar<\/h2>\n

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

N\u00famero\u00a0<\/strong>\u00a0Prandtl\u00a0<\/strong>de\u00a0<\/strong>\u00a0\u00e1gua e ar<\/strong><\/p>\n

O ar \u00e0 temperatura ambiente tem um\u00a0n\u00famero Prandtl<\/strong>\u00a0de\u00a00,71<\/strong>\u00a0e para a\u00a0\u00e1gua<\/a>\u00a0a 18 \u00b0 C \u00e9 de cerca de\u00a07,56<\/strong>\u00a0, o que significa que a difusividade t\u00e9rmica \u00e9 mais dominante para o ar do que para a \u00e1gua.\u00a0Para um n\u00famero Prandtl de unidade, a difusividade do momento \u00e9 igual \u00e0 difusividade t\u00e9rmica e o mecanismo e a taxa de transfer\u00eancia de calor s\u00e3o semelhantes aos da transfer\u00eancia de momento.\u00a0Para muitos fluidos, o\u00a0Pr<\/strong>\u00a0est\u00e1 na faixa de 1 a 10. Para os gases, o\u00a0Pr<\/strong>\u00a0geralmente \u00e9 de cerca de 0,7.<\/p>\n

\u00c1gua e vapor<\/strong>\u00a0\u00a0s\u00e3o um fluido comum usado para\u00a0\u00a0a troca de calor<\/strong>\u00a0\u00a0no circuito prim\u00e1rio (da superf\u00edcie das barras de combust\u00edvel ao fluxo do l\u00edquido refrigerante) e no circuito secund\u00e1rio.\u00a0\u00c9 utilizado devido \u00e0 sua\u00a0\u00a0disponibilidade<\/strong>\u00a0\u00a0e\u00a0\u00a0alta capacidade de calor,<\/strong>\u00a0\u00a0tanto para refrigera\u00e7\u00e3o quanto para aquecimento.\u00a0\u00c9 especialmente eficaz para transportar calor atrav\u00e9s da\u00a0\u00a0vaporiza\u00e7\u00e3o<\/strong>\u00a0\u00a0e\u00a0\u00a0condensa\u00e7\u00e3o<\/strong>\u00a0\u00a0da \u00e1gua devido ao seu\u00a0\u00a0calor latente muito grande de vaporiza\u00e7\u00e3o<\/strong>\u00a0.<\/p>\n

Uma desvantagem \u00e9 que os reatores moderados a \u00e1gua precisam usar\u00a0\u00a0o circuito prim\u00e1rio de alta press\u00e3o<\/strong>\u00a0\u00a0para manter a \u00e1gua no\u00a0\u00a0estado l\u00edquido<\/strong>\u00a0\u00a0e para obter efici\u00eancia termodin\u00e2mica suficiente.<\/p>\n<\/div>\n<\/div>\n

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N\u00famero Prandtl<\/h2>\n

O\u00a0n\u00famero Prandtl<\/strong>\u00a0\u00e9 um n\u00famero sem dimens\u00e3o, nomeado em homenagem ao seu inventor, um engenheiro alem\u00e3o\u00a0Ludwig Prandtl<\/strong>\u00a0, que tamb\u00e9m identificou a\u00a0camada limite<\/a>\u00a0.\u00a0O\u00a0n\u00famero de Prandtl<\/strong>\u00a0\u00e9 definida como a\u00a0raz\u00e3o<\/strong>\u00a0de\u00a0difusividade impulso<\/strong>\u00a0a\u00a0difusividade t\u00e9rmica<\/strong>\u00a0.\u00a0A\u00a0difusividade do momento<\/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\u00a0n\u00famero Prandtl<\/strong>\u00a0\u00e9 dado como:<\/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<\/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\u00famero Prandtl de \u00c1gua e Ar.\u00a0O ar \u00e0 temperatura ambiente tem um n\u00famero Prandtl de 0,71 e para a \u00e1gua a 18 \u00b0 C \u00e9 de cerca de 7,56, o que significa que a difusividade t\u00e9rmica \u00e9 mais dominante para o ar do que para a \u00e1gua.\u00a0Engenharia T\u00e9rmica N\u00famero Prandtl de \u00e1gua e ar N\u00famero\u00a0\u00a0Prandtl\u00a0de\u00a0\u00a0\u00e1gua … 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 Prandtl de \u00e1gua e ar - Defini\u00e7\u00e3o<\/title>\n<meta name=\"description\" content=\"N\u00famero Prandtl de \u00c1gua e Ar. 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