The Film/Convection Coefficient Calculator uses empirical correlations to calculate the convection coefficient, h. This is used because most industrial problems are too complex to calculate an h value using its boundary layer definition. Also these correlations are a quick way to estimate the convective heat transfer.
Several dimensionless numbers will be used to calculate h. These are the Grashof number (Gr), Nusselt number (Nu), Prandtl number (Pr), Reynolds number (Re) and the Rayleigh number (Ra). The calculation for these numbers will depend on the option selected in the Type of convection drop-down box. The Forced external option should be used when flow is forced over an exterior surface. The Forced internal option should be used when flow is forced through a part. The Buoyancy option should be selected if the flow is generated by a temperature difference (through buoyancy effects as natural convection).
After selecting a convection type, you must further describe the type of flow in the External flow, Internal flow or Buoyancy drop-down box. The options are mostly self explanatory. The [Q] refers to a model where the surface is heated by a constant heat flux. The [T] refers to a model where the surface is kept at a constant temperature. Refer to the following page, Theoretical Description of the Convection Coefficient Calculation for the equations used.
Depending on the type of flow selected, you will have to specify several properties. If you have selected internal or external flow, you will need to specify the Flow speed in the General tab. This is not the velocity on the actual surface (which is usually assumed to be 0), but is the flow close to the surface or the characteristic flow speed. For internal flow, the characteristic flow speed may be approximated by dividing the flow rate by the cross sectional area. If you have selected buoyancy flow, you will need to specify the Ambient temperature, Wall temperature and Gravity value for buoyancy. Since the wall temperature is usually unknown, it may be necessary to make an educated guess of the wall temperature, calculate the convection coefficient, run the analysis, and confirm whether the calculated wall temperature changes the convection coefficient or not. Alternatively, use a temperature-dependent convection coefficient and calculate a range of coefficients based on different wall temperatures. The analysis will then iterate until the appropriate convection coefficient is obtained.
The material properties for the fluid must be specified in the Fluid Properties tab. For more information on these properties, refer to the Material Properties section.
The relevant dimensions must be specified in the Geometry tab.
Once all the necessary information is input, press the Calculate film/convection coefficient button to view the convection coefficient. A dialog window will appear reporting the convection coefficient, the Nusselt, Reynolds, and Prandtl numbers, and the characteristic length. After dismissing this dialog, the calculated value will be shown in both the Value calculated for film/convection coefficient and the User-defined value for film/convection coefficient. The user-defined value can be altered if desired, such as to round off the calculation to fewer significant figures. When the OK button is pressed, the user-defined value will be entered in the Convection Coefficient field in the Surface Convection Object dialog.
All values entered in the calculator can be saved to the Surface Film/Convection Coefficient Library. See the following page, Film/Convection Library for details.