Heat Transfer Coefficient for Internal Flow
Laminar Flow
 The heat transfer rate for laminar flow can be calculated using the following equation:
Q = A * h*ΔT
where


 Q is the heat transfer rate, k is the thermal conductivity of the fluid
 A is the crosssectional 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).
Turbulent Flow
 The DittusBoelter 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.