Heat Transfer Coefficient for Internal Flow
- The heat transfer rate for laminar flow can be calculated using the following equation:
Q = A * h*ΔT
- Q is the heat transfer rate, k is the thermal conductivity of the fluid
- A is the cross-sectional area of the pipe
- ΔT = (Ts – Tref) is the mean temperature difference between the fluid and the pipe wall
- For laminar flow, the Nusselt number (Nu) can be calculated using the following equation:
Nu = 3.66
- Using the Nusselt number, the average heat transfer coefficient (h) can be calculated as:
h = (Nu * k) / h
where h is the hydraulic diameter of the pipe.
- Once the average heat transfer coefficient (h) is known, the heat transfer rate (Q) can be calculated using the first equation mentioned above.
- The assumption of laminar flow is only valid for low Reynolds numbers (less than 2300).
- The Dittus-Boelter equation is a widely used empirical equation that relates the average heat transfer coefficient for forced convection to the fluid properties, the flow conditions, and the geometric properties of the system.
- The equation for heat transfer coefficients is given by:
h = Nu * k/d
where h is the average heat transfer coefficient, k is the thermal conductivity and D is the hydraulic diameter of the pipe or duct.
- The Reynolds number is defined as:
Re =ρ* V * D/ μ
where, ρ is the fluid density, V is the fluid velocity, D is the hydraulic diameter, and μ is the dynamic viscosity of the fluid.
- The Prandtl number is defined as:
Pr = μ*Cp / k
- where Cp is the specific heat at a constant pressure of the fluid.
The assumption for Equations Dittus -Boelter equation
- The Dittus – Boelter equation is valid for
- Fully developed for thermal and fluid flows
- Turbulent flow in a circular pipe, Reynolds numbers > 2300
- Constant Wall Temperature.
- However, it has been found to give reasonably accurate results for a wide range of geometries and flow conditions.