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Measures the ratio of internal thermal resistance within a body to the external thermal resistance across the boundary layer. Important in estimating temperature gradients in a solid object during heat transfer. Defined as $Bi = \frac{hL}{k}$, where $h$ is the heat transfer coefficient, $L$ is the characteristic length, and $k$ is the thermal conductivity of the solid.

Represents the ratio of momentum diffusivity (kinematic viscosity) to mass diffusivity, it is more commonly used in mass transfer but can be relevant for coupled heat and mass transfer scenarios. Defined by $Sc = \frac{\nu}{D}$, where $\nu$ is the kinematic viscosity and $D$ is the mass diffusivity.

Expresses the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity and helps in analyzing forced and natural convection problems. Defined by $Pr = \frac{\nu}{\alpha}$ where $\nu$ is the kinematic viscosity and $\alpha$ is the thermal diffusivity.

Quantifies the ratio of buoyancy to viscous forces in a fluid and is used in the context of natural (free) convection. Defined as $Gr = \frac{gL^3\beta\Delta T}{\nu^2}$, where $g$ is the acceleration due to gravity, $\beta$ is the thermal expansion coefficient, $\Delta T$ is the temperature difference, and $\nu$ is the kinematic viscosity.

Represents the ratio of the rate of advection of a physical quantity by the flow to the rate of diffusion of the same quantity driven by an appropriate gradient. For heat transfer, it is given by $Pe = Re \times Pr = \frac{uL}{\alpha}$, where $u$ is the flow velocity, and $\alpha$ is the thermal diffusivity.

A combination of Grashof and Prandtl numbers that represents the ratio of buoyancy and thermal diffusivity forces in natural convection. Defined as $Ra = Gr \times Pr = \frac{g\beta\Delta TL^3}{\nu\alpha}$, where $g$ is the gravitational acceleration, $\beta$ is the thermal expansion coefficient, $\Delta T$ is the temperature difference, $L$ is the characteristic length, $\nu$ is the kinematic viscosity, and $\alpha$ is the thermal diffusivity.

Indicates the ratio of heat transferred into a fluid to the heat capacity of the fluid flowing. It can be described by $St = \frac{Nu}{Re \times Pr}$ or $St = \frac{h}{\rho u c_p}$, where $h$ is the heat transfer coefficient, $\rho$ is the density, $u$ is the velocity, and $c_p$ is the specific heat at constant pressure.

Represents the ratio of inertial forces to viscous forces and predicts the transition from laminar to turbulent flow which affects convective heat transfer rates. Defined by the equation $Re = \frac{\rho u L}{\mu}$, where $\rho$ is density, $u$ is velocity, $L$ is characteristic length, and $\mu$ is dynamic viscosity.

Describes the enhancement of heat transfer through a fluid layer as a result of convective motion of the fluid compared with conductive heat transfer across a stagnant fluid layer. Defined by $Nu = \frac{hL}{k}$, where $h$ is the convective heat transfer coefficient, $L$ is characteristic length, and $k$ is the thermal conductivity.