I am looking of value heat transfer coefficient to assign in Ansys steady state thermal simulation. But I'm not clear about heat transfer coefficient is for material or air? Because in engineering tool box from internet source, the range is 10 to 1000W/m2K. In my understanding and reading it is between air and material. Which is I'm using aluminium conductor and air. Thank you.

The heat transfer coefficient needs to be set for the boundary/surface between object and air. The value of the heat transfer coefficient depends on several parameters: air velocity, air temperature, geometry of the surface in contact with the air, etc.

For natural/free convection the value of heat transfer can be between 5 - 25 (W/(m^2 K) and for forced convection from 25 to 250 units (W/ (m^2 K).

An approximated value of the heat transfer coefficient for natural convection can be calculated using this site: https://quickfield.com/natural_convection.htm

You can check those references for more information: Article Wind-chill-equivalent temperatures: Regarding the impact due...

Article SUMMERTIME COOLING WITH VENTILATION RADIATORS

What you are looking is for the Heat Transfer coefficient of air. So it is the air the medium involved in this Heat Transfer Coefficient, not the solid material. In fact, it is called the Convection Heat Transfer Coefficient because it takes into account the part of the Heat Transferred by Convection ir our problems ... hconv

In your case, this could be a problem of (Natural of Forced) Heat Convection onto a (vertical or horizontal) flat surface. So these details are important to select the correct expression for hconv ... the information whether you have natural or forced Convection and the details of your geometry.

Next, you have to look for a expression of the Heat Transfer Coefficient, depending on the range of air velocity and the air Temperature you have. I recommend you to go to a general Heat Transfer textbook to search for this. I like the Cegel, and I know the expression you are looking is in there :

HEAT TRANSFER; A Practical Approach. YUNUS A. CENGEL

Then, what you would have to do is to compute the physical properties for the air (density, viscosity and Prandtl number) inside the corresponding range of temperature and pressure. If your problem is of Natural Convecction, surely you will have to involve the Rayleigh and Grashof numbers in the calculations of hconv

dear Alexandru-Ionel Constantin . Thank you for you explanation. After my reading i agreed with you. I should assign h at the surface coefficient. But, i found new uncertainty, which is, the h cannot assign at the surface of my drawing. The wire automatic couple with the air . Even though, i have tried to assign many way in name selection and do the subtract method, it also coupled between surface body of ware and air. But, if i simulate my wire without fluid domain(in box) the h is appear on the surface horizontal of my wire. Please assist me as I am not clear cfd rules. Thank in advanced