CH8-2.pptVIP

HEAT TRANSFER Internal Flow Heat Transfer Where we’ve been …… Introduction to internal flow, basic concepts, energy balance. Where we’re going: Developing heat transfer coefficient relationships and correlations for internal flow Convection correlations: laminar flow in circular tubes 1. The fully developed region from the energy equation,we can obtain the exact solution. for constant surface heat flux for constant surface temperature Note: the thermal conductivity k should be evaluated at . Convection correlations: laminar flow in circular tubes 2. The entry region for the constant surface temperature condition thermal entry length Convection correlations: laminar flow in circular tubes 2. The entry region(cont’d) for the combined entry length For values of Convection correlations: turbulent flow in circular tubes A lot of empirical correlations are available. For smooth tubes, the fully developed flow Heating: Cooling: For rough tubes, coefficient increases with wall roughness. For fully developed flows Consider the entry length For liquid metals, see textbook p461. Internal convection heat transfer coefficient(summary) For laminar and fully developed flow (§8.4.1): q” constant:  Ts constant: For laminar flow in entry region (before fully developed flow, §8.4.2: Ts constant :  Combined entry length with full tube: For turbulent and fully developed (§8.5) Heating Cooling Example: Oil at 150℃ flows slowly through a long, thin-walled pipe of 30-mm inner diameter. The pipe is suspended in a room for which the air temperature is 20 ℃ and the convection coefficient at the outer tube surface is 11W/m2.K. Estimate the heat loss per unit length of tube. Internal Flow Heat Transfer(summary) If constant heat flux, mean fluid temperature can be computed directly from the pipe area and inlet temperature For constant wall temperature (such as if phase change occurs on outer pipe surface),