Internal flow, turbulent flow The Dittus-Bölter correlation (1930) is a common and particularly simple correlation useful for many applications. This correlation is applicable when forced convection is the only mode of heat transfer; i.e., there is no boiling, condensation, significant radiation, etc. The accuracy of this correlation is anticipated to be ±15%. For a fluid flowing in a straight circular pipe with a Reynolds number between 10,000 and 120,000 (in the turbulent pipe flow range), when the fluid's Prandtl number is between 0.7 and 120, for a location far from the pipe entrance (more than 10 pipe diameters; more than 50 diameters according to many authors) or other flow disturbances, and when the pipe surface is hydraulically smooth, the heat transfer coefficient between the bulk of the fluid and the pipe surface can be expressed explicitly as: where: is the hydraulic diameter is the thermal conductivity of the bulk fluid is the fluid viscosity mass flux isobaric heat capacity of the fluid is 0.4 for heating (wall hotter than the bulk fluid) and 0.33 for cooling (wall cooler than the bulk fluid). The fluid properties necessary for the application of this equation are evaluated at the bulk temperature thus avoiding iteration. Forced convection, external flow In analyzing the heat transfer associated with the flow past the exterior surface of a solid, the situation is complicated by phenomena such as boundary layer separation. Various authors have correlated charts and graphs for different geometries and flow conditions. For flow parallel to a plane surface, where is the distance from the edge and is the height of the boundary layer, a mean Nusselt number can be calculated using the Colburn analogy. Thom correlation There exist simple fluid-specific correlations for heat transfer coefficient in boiling. The Thom correlation is for the flow of boiling water (subcooled or saturated at pressures up to about 20 MPa) under conditions where the nucleate boiling contribution predominates over forced convection. This correlation is useful for rough estimation of expected temperature difference given the heat flux: where: is the wall temperature elevation above the saturation temperature, K q is the heat flux, MW/m2 P is the pressure of water, MPa Note that this empirical correlation is specific to the units given. Heat transfer coefficient of pipe wall The resistance to the flow of heat by the material of pipe wall can be expressed as a "heat transfer coefficient of