Both conduction and convection happen in fluids. As heat transfer through both processes reduces the temperature difference, they can be considered as competing against each other in transferring heat. There are many different types of fluids, such as air, water, oil, or mercury. The rates of conduction and convection vary in different fluids. Sometimes, conduction dominates. Other times, convection dominates. The Prandtl number is a parameter that can be used to roughly determine which process will "win":
where ν is the kinematic viscosity and α is the thermal diffusivity. If a fluid is more viscous (stickier), the Prandtl number is greater and the heat transfer will be less convective.
The following simulation shows how fluids with different Prandtl numbers transfer heat.