There are three modes of convection heat transfer: forced convection, natural convection, and mixed convection. Forced convection occurs when fluid motion is generated by external forces like pumps or fans. Natural convection occurs when fluid motion is driven by gravity due to temperature gradients. Mixed convection occurs when external forces and gravitational forces are both present. The heat transfer coefficient depends on parameters like geometry, flow rate, flow conditions, and fluid type. It can range from around 10 W/m2°C for air in natural convection to over 100,000 W/m2°C for water in pool boiling.