Table of Contents

## What is the Nusselt number of air?

The greater the air velocity (V)<0.9 the Nusselt number (Nu) increases with a small increase in magnitude, but the air velocity (V)>0.9 Nusselt number (Nu) also increases with a large increase gradient.

**What is the Rayleigh number used for?**

Rayleigh number is a dimensionless number that represents the ratio of buoyancy and thermal diffusivity. Difference between solid surface temperature and free stream temperature can be used as the characteristic temperature difference.

**What does the Rayleigh number of the mantle tell us?**

In Earth’s mantle, the Rayleigh number, a measure of the vigour of convection, is estimated to be 5 × 106 to 5 × 107. For this range, time-dependent convection, with instabilities originating at thermal boundary layers, is expected.

### What is significance of Nusselt number in convection?

A Nusselt number of value one represents heat transfer by pure conduction. A value between one and 10 is characteristic of slug flow or laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the 100–1000 range.

**What is the Prandtl number of air?**

0.72

9.9. 3 Prandtl Number

Fluid | Temp. °C | Prandtl number |
---|---|---|

air | 30 | 0.72 |

carbon disulfide | 25 | 2.36 |

chloromethane | 25 | 4.41 |

methanol | 25 | 6.83 |

**What is the thermal conductivity of air?**

0.000057 0.024

Thermal Conductivity

Material | Thermal conductivity (cal/sec)/(cm2 C/cm) | Thermal conductivity (W/m K)* |
---|---|---|

Air at 0° C | 0.000057 | 0.024 |

Helium (20°C) | … | 0.138 |

Hydrogen(20°C) | … | 0.172 |

Nitrogen(20°C) | … | 0.0234 |

#### How does Rayleigh number calculate Nusselt number?

The Nusselt number with large aspect ratio and higher RL formula is defined as the ratio of convective to conductive heat transfer across a boundary is calculated using Nusselt number = 0.046*Rayleigh number^0.333.

**Can the Rayleigh number be negative?**

It can’t. It will be negative if the Ts is lower than Tinf, and simply change the order for them.

**Why is Nusselt number constant for laminar flow?**

In laminar flow in a tube with constant surface temperature, both the friction factor and the heat transfer coefficient remain constant in the fully developed region. Therefore, for fully developed laminar flow in a circular tube subjected to constant surface heat flux, the Nusselt number is a constant.

## How do you find the thermal conductivity of air?

Thermal conductivity units conversion:

- 1 Btu(IT)/(ft h°F) = 1/12 Btu(IT)/(in h°F) = 0.08333 Btu(IT)/(in h°F) = 12 Btu(IT)in/(ft2 h°F) = 1.488 kcal/(m h°C) = 0.01731 J/(cm s K) = 1.731 W/(m K)
- 1 Btu(IT)/(in h°F) = 12 Btu(IT)/(ft h°F) = 144 Btu(IT)in/(ft2 h°F) = 17.858 kcal/(m h°C) = 0.20769 J/(cm s K)= 20.769 W/(m K)

**What is the K value of air?**

In most of the cases, K is referred to as the ratio of specific heats. Here, the value of K for air is 1.4.

**What is the R value of air space?**

Insulation R-Value

Material | R value(ft^2 °F h/Btu |
---|---|

Insulating glass(0.25 in space) | 1.54 |

Air space (3.5 in. thick) | 1.01 |

Free stagnant air layer | 0.17 |

Drywall (0.5 in. thick) | 0.45 |

### Does air have a high thermal conductivity?

However, the U and R values also are dependent on the thickness of the material whereas thermal conductivity does not account for this….Values for Common Materials.

Material | Conductivity at 25oC |
---|---|

Air | 0.024 |

Aluminum | 205 |

Bitumen | 0.17 |

Brass | 109 |

**What does a high Rayleigh number mean?**

turbulent flow

In fluid mechanics, the Rayleigh number (Ra) for a fluid is a dimensionless number associated with buoyancy-driven flow, also known as free or natural convection. It characterises the fluid’s flow regime: a value in a certain lower range denotes laminar flow; a value in a higher range, turbulent flow.

**What does a low Rayleigh number mean?**

For Rayleigh number values lower than a certain critical value, the system is in steady state. For values higher than that value, the system becomes unstable and natural convection movements take place.

#### What is the Nusselt number for laminar flow?

If the Nusselt number is about 1, it represents that the heat transfer is conduction only, but if the value is between 1 and 10, then it shows laminar of slug flow. If the range is more, it is active convection with turbulence in the 100–1000 range.

**What is the specific conductivity of air?**

Specific heat capacity (Cv) air at 0°C and 1 bara: 0.7171 kJ/kgK = 0.17128 Btu(IT)/(lbm °F) or kcal/(kg K) Thermal conductivity at 0°C and 1 bara: 24.35 mW/(m K) = 0.02094 kcal(IT)/(h m K) = 0.01407 Btu(IT)/(h ft °F)