The Peclet Number ($Pe$) is a dimensionless parameter used in fluid mechanics to characterize the ratio of convective to diffusive mass transfer in a fluid flow. It helps in understanding the dominance of convective or diffusive transport processes. The Peclet Number is defined as the ratio of convective mass transfer rate to diffusive mass transfer rate in a fluid flow. The Peclet Number is dimensionless. The formula for the Peclet Number is given by:

Where,

• $Pe$ is the Peclet Number,
• $L$ is a characteristic length,  
• $v$ is the velocity of the fluid, 
• $\alpha$ is the thermal diffusivity of the fluid.