Hii, I am trying to simulate rc slab subjected to fire and then cooled it by water, the question is what will be the convection coefficient is it the same when concrete is cooled by air??
Heat transfer coefficient is not a property of a substance but its a parameter that varies with nature of the fluid flow (laminar or turbulent), surface geometry, temperature difference etc. So, the values of heat transfer coefficient for water on rc slab differs from air on rc slab.
Assuming the fluid velocity, temperature difference and surface geometry for both water on rc slab and air on rc slab, considering your problem as the forced convection (where you give certain velocity to the fluid), water on rc slab will have the higher heat transfer coefficient. The reasons are,
Water has the higher specific heat,so higher the heat absorbing capability
The Prandtl number value for water is higher than that of the air at same temperature. So, the Nusselt number value will be more for water.
as the mentioned previously in the answers. the heat transfer coefficient depends on many parameters. if you consider your flow as over a flat plate, you can use one of the correlations presented in the following table. I am pretty sure you can find it in your heat transfer book.