# Nusselt number

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Nusselt number

In heat transfer at a boundary (surface) within a fluid, the Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary. Named after Wilhelm Nusselt, it is a dimensionless number. The conductive component is measured under the same conditions as the heat convection but with a (hypothetically) stagnant (or motionless) fluid.

A Nusselt number close to one, namely convection and conduction of similar magnitude, 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.

The convection and conduction heat flows are parallel to each other and to the surface normal of the boundary surface, and are all perpendicular to the mean fluid flow in the simple case. $\mathrm{Nu}_L = \frac{hL}{k_f} = \frac{\mbox{Convective heat transfer coefficient}}{\mbox{Conductive heat transfer coefficient}}$

where:

Selection of the characteristic length should be in the direction of growth (or thickness) of the boundary layer. Some examples of characteristic length are: the outer diameter of a cylinder in (external) cross flow (perpendicular to the cylinder axis), the length of a vertical plate undergoing natural convection, or the diameter of a sphere. For complex shapes, the length may be defined as the volume of the fluid body divided by the surface area. The thermal conductivity of the fluid is typically (but not always) evaluated at the film temperature, which for engineering purposes may be calculated as the mean-average of the bulk fluid temperature and wall surface temperature. For relations defined as a local Nusselt number, one should take the characteristic length to be the distance from the surface boundary to the local point of interest. However, to obtain an average Nusselt number, one must integrate said relation over the entire characteristic length.

Typically, for free convection, the average Nusselt number is expressed as a function of the Rayleigh number and the Prandtl number, written as: Nu = f(Ra, Pr). Else, for forced convection, the Nusselt number is generally a function of the Reynolds number and the Prandtl number, or Nu = f(Re, Pr). Empirical correlations for a wide variety of geometries are available that express the Nusselt number in the aforementioned forms.

The mass transfer analog of the Nusselt number is the Sherwood number.

## Derivation

The Nusselt Number may be obtained by a non dimensional analysis of the Fourier's law since it is equal to the dimensionless temperature gradient at the surface: $q = -k \nabla T$, where q is the heat flux, k is the thermal conductivity and T the fluid temperature.

Indeed if: $\nabla' = -L \nabla$, and $T' = \frac{T_h-T_0}{T_h-T_c}$

we arrive at  : $-\nabla'T' = -\frac{L}{k(T_h-T_c)}q=\frac{hL}{k}$

then we define  : $Nu_L=\frac{hL}{k}$

so the equation become  : $Nu_L=-\nabla'T'$

By integrating over the surface of the body: $\overline{Nu}=-{{1} \over {S'}} \int_{S'}^{} Nu \, dS'\!$, where $S' = \frac{S}{L^2}$

## Empirical Correlations

### Free convection

#### Free convection at a vertical wall

Cited as coming from Churchill and Chu: $\overline{\mathrm{Nu}}_L \ = 0.68 + \frac{0.67 \mathrm{Ra}_L^{1/4}}{\left[1 + (0.492/\mathrm{Pr})^{9/16} \, \right]^{4/9} \,} \quad \mathrm{Ra}_L \le 10^9$

#### Free convection from horizontal plates

For the top surface of a hot object in a colder environment or bottom surface of a cold object in a hotter environment $\overline{\mathrm{Nu}}_L \ = 0.54 \mathrm{Ra}_L^{1/4} \, \quad 10^4 \le \mathrm{Ra}_L \le 10^7$ $\overline{\mathrm{Nu}}_L \ = 0.15 \mathrm{Ra}_L^{1/3} \, \quad 10^7 \le \mathrm{Ra}_L \le 10^{11}$

For the bottom surface of a hot object in a colder environment or top surface of a cold object in a hotter environment $\overline{\mathrm{Nu}}_L \ = 0.27 \mathrm{Ra}_L^{1/4} \, \quad 10^5 \le \mathrm{Ra}_L \le 10^{10}$

### Flat plate in turbulent flow

The local Nusselt number for a turbulant flow is given by $\overline{\mathrm{Nu}}_x\ = St \mathrm{Re}_x\ Pr$ 

### Forced convection in turbulent pipe flow

#### Gnielinski correlation

Gnielinski is a correlation for turbulent flow in tubes: $\mathrm{Nu}_D = \frac{ \left( f/8 \right) \left( \mathrm{Re}_D - 1000 \right) \mathrm{Pr} } {1 + 12.7(f/8)^{1/2} \left( \mathrm{Pr}^{2/3} - 1 \right)}$

where f is the Darcy friction factor that can either be obtained from the Moody chart or for smooth tubes from correlation developed by Petukhov: $f= \left( 0.79 \ln \left(\mathrm{Re}_D \right)-1.64 \right)^{-2}$

The Gnielinski Correlation is valid for: $0.5 \le \mathrm{Pr} \le 2000$ $3000 \le \mathrm{Re}_D \le 5 \times 10^{6}$

#### Dittus-Boelter equation

The Dittus-Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. It is easy to solve but is less accurate when there is a large temperature difference across the fluid. It is tailored to smooth tubes, so use for rough tubes (most commercial applications) is cautioned. The Dittus-Boelter equation is: $\mathrm{Nu}_D = 0.023 \mathrm{Re}_D^{4/5} \mathrm{Pr}^{n}$

where:

D is the inside diameter of the circular duct
Pr is the Prandtl number
n = 0.4 for heating of the fluid, and n = 0.3 for cooling of the fluid.

The Dittus-Boelter equation is valid for $0.7 \le \mathrm{Pr} \le 160$ $\mathrm{Re}_D \gtrsim 10,000$ $\frac{L}{D} \gtrsim 10$

Example The Dittus-Boelter equation is a good approximation where temperature differences between bulk fluid and heat transfer surface are minimal, avoiding equation complexity and iterative solving. Taking water with a bulk fluid average temperature of 20 °C, viscosity 10.07×10¯⁴ Pa·s and a heat transfer surface temperature of 40 °C (viscosity 6.96×10¯⁴, a viscosity correction factor for (μ / μs) can be obtained as 1.45. This increases to 3.57 with a heat transfer surface temperature of 100 °C (viscosity 2.82×10¯⁴ Pa·s), making a significant difference to the Nusselt number and the heat transfer coefficient.

#### Sieder-Tate correlation

The Sieder-Tate correlation for turbulent flow is an implicit function, as it analyzes the system as a nonlinear boundary value problem. The Sieder-Tate result can be more accurate as it takes into account the change in viscosity (μ and μs) due to temperature change between the bulk fluid average temperature and the heat transfer surface temperature, respectively. The Sieder-Tate correlation is normally solved by an iterative process, as the viscosity factor will change as the Nusselt number changes. $\mathrm{Nu}_D = 0.027\mathrm{Re}_D^{4/5} \mathrm{Pr}^{1/3}\left(\frac{\mu}{\mu_s}\right)^{0.14}$

where:

• μ is the fluid viscosity at the bulk fluid temperature
• μs is the fluid viscosity at the heat-transfer boundary surface temperature

The Sieder-Tate correlation is valid for $0.7 \le \mathrm{Pr} \le 16,700$ $\mathrm{Re}_D \gtrsim 10,000$ $\frac{L}{D} \gtrsim 10$

### Forced convection in fully developed laminar pipe flow

For fully developed internal laminar flow, the Nusselt numbers are constant-valued. The values depend on the hydraulic diameter.

For internal Flow: $\mathrm{Nu} = \frac{h D_h}{k_f}$

where:

#### Convection with uniform surface heat flux for circular tubes

From Incropera & DeWitt, $\mathrm{Nu}_D =\frac{48}{11} \simeq 4.36$

#### Convection with uniform surface temperature for circular tubes

For the case of constant surface temperature,

NuD = 3.66

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